U.S. patent application number 13/682004 was filed with the patent office on 2014-05-22 for waveguide illumination system.
The applicant listed for this patent is Sergiy Victorovich Vasylyev. Invention is credited to Sergiy Victorovich Vasylyev.
Application Number | 20140140091 13/682004 |
Document ID | / |
Family ID | 50727778 |
Filed Date | 2014-05-22 |
United States Patent
Application |
20140140091 |
Kind Code |
A1 |
Vasylyev; Sergiy
Victorovich |
May 22, 2014 |
WAVEGUIDE ILLUMINATION SYSTEM
Abstract
An illumination system employing a waveguide. Light received
from an edge or an end of a waveguide is propagated in response to
transmission and total internal reflection. Light deflecting
elements distributed along the propagation path of light
continuously change the out-of-plane propagation angle of light
rays and cause decoupling of portions of the propagated light from
the core of the waveguide at different distances from the light
input edge or end. Light escapes from the waveguide into an
intermediate layer at low out-of-plane angles and is further
redirected by light extraction features out of the system. In one
embodiment, the illumination system is configured to emit
collimated light. In one embodiment, the illumination system
includes shallow surface relief features. In one embodiment, the
light deflecting elements include forward-scattering particles
distributed throughout the volume of the waveguide. Additional
collimating and non-collimating illumination units and methods are
also disclosed.
Inventors: |
Vasylyev; Sergiy Victorovich;
(Elk Grove, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Vasylyev; Sergiy Victorovich |
Elk Grove |
CA |
US |
|
|
Family ID: |
50727778 |
Appl. No.: |
13/682004 |
Filed: |
November 20, 2012 |
Current U.S.
Class: |
362/606 ;
362/551; 385/31 |
Current CPC
Class: |
G02B 6/0045 20130101;
G02B 6/0055 20130101; G02B 6/0041 20130101; G02B 6/0085 20130101;
G02B 6/0065 20130101; G02B 6/0043 20130101; G02B 6/005 20130101;
G02B 6/0053 20130101; G02B 6/001 20130101; G02B 6/0038 20130101;
G02B 6/0061 20130101 |
Class at
Publication: |
362/606 ; 385/31;
362/551 |
International
Class: |
F21V 8/00 20060101
F21V008/00 |
Claims
1. An illumination system, comprising: an optical waveguide having
at least one major surface and configured to guide light towards at
least one predetermined direction along said major surface in
response to a total internal reflection; a plurality of light
deflecting elements distributed within said waveguide along said
direction and configured to incrementally deflect light rays from
said direction in at least one plane perpendicular to said major
surface; and light extracting means optically coupled to said major
surface and configured for progressively extracting light from said
waveguide in response to light incidence onto said major surface at
an angle lower than the angle of a total internal reflection.
2. An illumination system as recited in claim 1, wherein said
waveguide has a planar configuration having two opposing broad-area
surfaces extending parallel to each other.
3. An illumination system as recited in claim 1, wherein said
waveguide has a cylindrical configuration having a longitudinal
dimension substantially greater than a transversal dimension.
4. An illumination system as recited in claim 1, wherein said
waveguide comprises at least one light input edge or end and
further comprises at least one light source optically coupled to
said at least one light input edge or end.
5. An illumination system as recited in claim 1, further comprising
at least one light emitting diode configured to inject light into
said optical waveguide.
6. An illumination system as recited in claim 1, further comprising
at least one light source configured for inputting light into an
edge or end of said optical waveguide, wherein said light source is
selected from the group of elements consisting of light emitting
diodes, compact fluorescent lamps, cold-cathode fluorescent lamps,
halogen lamps, mercury-vapor lamps, sodium-vapor lamps, metal
halide lamps, electroluminescent lamps, and lasers.
7. An illumination system as recited in claim 1 further comprising
at least one light source and at least one collimating element
associated with said at least one light source.
8. An illumination system as recited in claim 1, further comprising
a reflective layer adjacent to a major surface of said
waveguide.
9. An illumination system as recited in claim 1, wherein said light
extracting means comprises an optical element selected from the
group of elements consisting of light turning films, light
scattering layers or surfaces, screens, image prints, and textured
surfaces.
10. An illumination system as recited in claim 1, wherein said
light extracting means is configured to direct light generally
towards a normal to said major surface.
11. An illumination system as recited in claim 1, further
comprising a buffer layer disposed between said optical waveguide
and said light extracting means and having a lower refraction index
than said optical waveguide.
12. An illumination system as recited in claim 1, wherein each of
said light redirecting elements comprises at least one surface
relief feature.
13. An illumination system as recited in claim 1, wherein each of
said light redirecting elements is selected form the group of
surface relief features having a relatively low aspect ratio and
consisting of prisms, lenses, prismatic grooves, corrugations,
undulations, depressions, indentations, recesses, protrusions, and
extensions.
14. An illumination system as recited in claim 1, wherein each of
said light redirecting elements comprises a surface facet making a
sufficiently low dihedral angle with respect to the prevailing
plane of said optical waveguide, said dihedral angle being below a
predetermined maximum angle.
15. An illumination system of claim 1 in a front light
configuration, wherein said waveguide has the form of a planar
panel and is made substantially transparent at least along a
transversal direction with respect to a prevailing plane of said
panel.
16. An illumination system of claim 1 in a backlight configuration,
wherein said waveguide has the form of a planar panel.
17. An illumination system of claim 1 within day lighting device,
wherein said system is configured for a generally unimpeded
transversal light passage and is further configured to emit
artificial light from a broad-area surface.
18. An illumination system as recited in claim 1, wherein said
system is configured for emitting at least a substantial part of
the emitted light in the form of a collimated beam.
19. An illumination system, comprising: a planar optical waveguide
having a first broad-area surface and an opposing second broad-area
surface extending generally parallel to said first broad-area
surface; a buffer layer of an optically transmissive material
attached to said first surface, said buffer layer having a
refractive index lower than the refractive index of said waveguide;
a plurality of light deflecting elements within said waveguide
configured to extract light into said buffer layer; and a light
extracting layer attached to said buffer layer and configured to
extract light from said buffer layer.
20. A directional edge-lit lighting fixture, comprising: a planar
sheet of an optically transmissive material configured to guide
light along a predetermined direction in response to optical
transmission and a total internal reflection; at least one strip of
light emitting diodes attached to an edge of said sheet; and means
for extracting light from a major surface of said sheet and for
collimating the extracted light at least in a plane parallel to
said predetermined direction; whereby said directional edge-lit
lighting fixture can emit light within a pre-defined angular range
being substantially smaller than 180 degrees in said plane.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from U.S. provisional
application Ser. No. 61/563,018 filed on Nov. 22, 2011,
incorporated herein by reference in its entirety, and from U.S.
provisional application Ser. No. 61/648,236 filed on May 17, 2012,
incorporated herein by reference in its entirety.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not Applicable
INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT
DISC
[0003] Not Applicable
NOTICE OF MATERIAL SUBJECT TO COPYRIGHT PROTECTION
[0004] A portion of the material in this patent document is subject
to copyright protection under the copyright laws of the United
States and of other countries. The owner of the copyright rights
has no objection to the facsimile reproduction by anyone of the
patent document or the patent disclosure, as it appears in the
United States Patent and Trademark Office publicly available file
or records, but otherwise reserves all copyright rights whatsoever.
The copyright owner does not hereby waive any of its rights to have
this patent document maintained in secrecy, including without
limitation its rights pursuant to 37 C.F.R. .sctn.1.14.
BACKGROUND OF THE INVENTION
[0005] 1. Field of the Invention
[0006] The present invention relates to light emitting waveguides
such light pipes, optical fibers or planar waveguides provided with
a series of light-deflecting features distributed along the optical
path and configured to create a controlled illumination pattern
emitted from a major waveguide's surface. This invention also
relates to optical illuminators and light distribution systems
employing such waveguides, for example, panel luminaires,
side-emitting optical fibers, edge-lit LED front lights and
backlights, lighting panels, LCD display backlights, daylighting
luminaires, diffusers, computer screens, advertising displays, road
signs, and the like, as well as to a method for redistributing
light from various-type light sources.
[0007] 2. Description of Background Art
[0008] Conventionally, light emitting devices employing a waveguide
include a series of optical features distributed along the light
propagation path in the waveguide and configured to extract light
from the waveguide in a perpendicular direction. The optical
features are conventionally formed by small cuts, notches or
grooves in the waveguide surface which extract light by means of
reflection, refraction and/or scattering.
[0009] FIG. 1 depicts an example of the prior-art planar slab
waveguide which employs an array of surface microstructures formed
in a major surface of the waveguide. FIG. 2 depicts a prior-art
planar slab waveguide having an array of linear V-grooves formed in
the waveguide's surface. FIG. 3 depicts an example of the
conventional side-emitting optical fiber which includes a series of
notches formed in the fiber's surface along its longitudinal
axis.
[0010] FIG. 4 generalizes the optical operation of the prior-art
devices shown in FIG. 1, FIG. 2 and FIG. 3 and depicts a
cross-section of the waveguide and an individual light extracting
feature formed in the waveguide's surface. The light extracting
feature commonly has the cross-sectional shape of a prismatic
cavity and employs a reflective face inclined at a considerable
slope angle to the surface of the waveguide, usually about
45.degree. or so. Since the light is confined within the waveguide
by means of a total internal reflection (TIR) from waveguide's
longitudinal walls, this slope angle generally needs to be
sufficiently high in order to overcome TIR and allow the reflected
light to exit from the light emitting surface opposing the light
extracting feature. As illustrated FIG. 4, at least some light rays
may be reflected from the reflective face of the light extracting
feature and then exit from the opposing surface of the waveguide.
Accordingly, when a large array of such light extracting feature is
distributed along a major surface of the waveguide, the waveguide
can emit luminous flux from the opposing surface in a broad angular
range.
[0011] The reflective face usually has an optically transmissive
surface and provides reflection by means of TIR when the incidence
angles are greater than the TIR angle with respect to a normal to
the reflective face. However, since the light rays propagating in
the waveguide have essentially random angular distribution within
the acceptance angle of the waveguide, at least a portion of the
rays can strike the reflective face at angles being lower than the
TIR angle. In this case, TIR will not occur and the respective rays
may exit from the waveguide through the unwanted face (see FIG. 4),
resulting in light loss and reduced system efficiency. The
reflective faces can be selectively mirrored to eliminate this
light spillage. However, doing so will introduce reflection losses
at the mirrored surface compared to the lossless TIR and will also
add fabrication steps, such as fabricating a mask, applying the
mask to the waveguide surface with precision alignment, vacuum
metallization, etc.
[0012] In conventional edge-lit waveguide illumination devices, the
light extracted from the waveguide by the sloped reflective faces
generally has a high angular dispersion from the waveguide's
surface normal. Particularly, the divergence of light emitted by
prior-art waveguide-based devices often approximates that of a
lambertian source with a full 180.degree. angular spread. The lack
of beam directionality hampers the utility of conventional
waveguide illumination systems in the applications requiring at
least some degree of light collimation.
[0013] Furthermore, the sloped reflective surfaces of light
extracting features refract light propagating along the line of
sight perpendicular to the waveguide's surface. This makes the
conventional devices ill suited for the front light applications in
which an edge-lit lighting panel is positioned in front of a
viewable screen or image print. Each of the light extracting
features alters the light path from the viewer to the print and
bends the light towards other portions of the print compared to the
neighboring smooth areas of the front light panel. As a result, the
visual appearance and resolution of the print may deteriorate.
Considering than at least some light propagating in the waveguide
may also escape from the waveguide toward the viewer, the print
contrast may also be affected.
[0014] Also, when the front and rear surfaces of the planar
waveguide generally have the same optical properties, e.g., being
characterized by the same stepped drop in the refractive index, the
light ray which obtains a non-TIR propagation angle within the
waveguide may escape from either surface. Various mechanisms may
contribute to such light leakage. The rays which propagate at
less-than-TIR angles (with respect to surface normal) may include
high-incidence-angle portions of the initial light beam injected at
the waveguide's edge, light scattered by impurities in the
waveguide, stray light from light extraction elements, stray light
resulting from natural divergence or leakage from the waveguide,
light which propagation angles are altered by the non-parallelism
of waveguide walls, light reflected from the opposing wall by means
of a Fresnel reflection, etc. Since such light has about equal
chance to escape through either front or rear surface, at least a
substantial portion of it will exit from the unwanted side of the
waveguide resulting in energy loss and considerable glare. The
prior-art lighting panels employing light extraction features based
on light scattering rather than on reflection typically introduce
even more unwanted glare and light spillage due to the uncontrolled
nature of light scattering mechanism. Additionally, such lighting
panels are usually characterized by a relatively high level of
opacity and either substantially opaque or can be translucent at
best, but not fully transparent, which inhibits the basic light
guiding function of the panel. Yet further, when the conventional
planar waveguide employing light extracting features (such as
surface microstructure or scattering elements) is lit from an edge,
at least a portion of light extracted by these features is emitted
towards the viewer which substantially degrades the contrast and
visibility of the bodies or images disposed behind the waveguide.
This prevents using these panels for front lights where the
perceptible quality of the background to be lit is important.
[0015] Besides sometimes being characterized by reduced optical
qualities or light spillage, the conventional systems employing
relatively deep cuts, notches or grooves may also be affected by at
least some loss of structural strength and rigidity compared to a
smooth-surface panel having no such microstructures.
[0016] It is therefore an object of this invention to provide an
improved waveguide illumination system providing an efficient light
extraction with a minimum light loss and without using excessively
deep (relatively to the transversal size) microstructures in the
waveguide's surface. It is another object of this invention to
eliminate or at least substantially reduce the light spillage
through the unwanted side of the waveguide. It is yet another
object of this invention to provide an improved waveguide
illumination system which can be configured for enhanced light
collimation and controlled directionality of the emitted beam. It
is yet another object of this invention to provide a waveguide
illumination system capable of distributing light from a compact
source over a large area and emitting the distributed light from
said area in the form of a collimated beam with a prescribed
angular spread or pattern. It is yet another object of this
invention to provide an improved waveguide illumination system
which can effectively used as a backlight or a front light panel
that will not substantially alter the light paths and apparent
image fidelity for the viewer. Other objects and advantages of this
invention will be apparent to those skilled in the art from the
following disclosure.
BRIEF SUMMARY OF THE INVENTION
[0017] Accordingly, the present invention is directed to waveguide
illumination systems which may be employed to emit directional
light beams or uniformly illuminate a designated area with a very
low light loss. More particularly, at least some embodiments of
this invention are directed to planar light-emitting waveguides and
at least some embodiments are directed to cylindrical side-emitting
waveguides and optical fibers. This invention is also directed to
directional (collimating) illumination systems which employ
light-emitting waveguides, such as lighting luminaires, backlights,
front lights and the like.
[0018] The present invention solves a number of light distribution
and illumination problems within a compact optical system which is
not hindered by the limitations of conventional waveguides
employing various kinds of light extraction features used to
decouple light from the waveguide mode.
[0019] An advantage of the present system is to provide controlled
light extraction through a designated surface of the waveguide
while minimizing light loss and controlling the angular
distribution of the extracted light. Light is extracted from the
waveguide into an intermediate layer by means of incremental
deflections from the prevailing propagation direction after which
it is further redirected out of the waveguide. A two-stage light
extraction process enables the directionality of the emitted light
and minimizes light spillage into non-functional directions.
[0020] In at least one embodiment, the invention features a
multi-layer optical structure having a waveguide layer, an
intermediate buffer layer and a light extraction layer. Various
implementation of the invention include a planar configuration of
the waveguide and a cylindrical configuration of the waveguide.
[0021] The buffer layer has a lower refractive index than the
waveguide layer and preserves the waveguiding function of the
waveguide layer at least for a range of incidence angles. The
waveguide includes light deflecting elements distributed along the
intended path of light propagation configured to incrementally
deflect light rays by relatively small angles upon each
interaction. According to an aspect of the present invention, the
differential between the refractive indices at the opposing
surfaces or sides of the waveguide and the smallness of the
deflection angles ensure light extraction into the buffer layer
while generally preventing light escape through the surface portion
which is not disposed in optical contact with buffer layer. The
light extraction layer further extracts and redirects light out of
the illumination system.
[0022] In at least one embodiment, the light deflecting elements
include relatively low-profile surface relief features deflecting
light by means of a total internal reflection (TIR). Each surface
relief feature may have at least one facet which forms a relatively
low dihedral angle with a surface plane. According to an aspect,
the dihedral angle may be sufficiently low to prevent premature
light leakage from waveguide through the respective facet.
According to another aspect, each interaction of light with the
facet results in light deflection from its original propagation
path by means of TIR and bends further away from a prevailing plane
or axis of the waveguide. According to a further aspect, this
process may continue until TIR is suppressed at least at one
surface of the waveguide and light exits from the waveguide and may
be further redirected by the light extraction layer. In at least
one implementation, the surface relief features include low-profile
prismatic surface relief features.
[0023] In at least one implementation, the surface relief features
include shallow surface undulations or corrugations.
[0024] In at least one implementation, the dihedral angle of the
facets varies across the surface as a function of a distance from a
light input area of the waveguide. In at least one implementation,
the waveguide includes two symmetrically disposed segments each
having a light input edge or end and each provided with an array of
light-deflecting surface relief features.
[0025] In at least one implementation, the waveguide includes a
linear array of surface relief features extending parallel to a
reference line. In at least one implementation, the waveguide
includes a two-dimensional array of discrete surface relief
features.
[0026] In at least one implementation, the waveguide illumination
system is configured for a generally unimpeded transversal light
passage through its body. In at least one implementation, the
waveguide illumination system is substantially transparent at least
along a direction normal to its waveguiding surface.
[0027] In at least one embodiment, the light deflecting elements
include an internal corrugated boundary between two optically
transmissive materials having different refractive indices.
[0028] In at least one embodiment, the light deflecting elements
include light scattering particles distributed throughout the body
of the waveguide and configured to continuously change the light
propagation direction by means of forward scattering.
[0029] In at least one embodiment, the light deflecting elements
include an internal corrugated boundary between two optically
transmissive materials having different refractive indices.
[0030] In at least one embodiment, the waveguide illumination
system of this invention includes at least one light source
configured to input light into the waveguide. In at least one
embodiment, the light source is optically coupled to a light input
edge or a light input end of the waveguide. Various implementations
of the light source include light emitting diodes (LEDs), LED
arrays, fluorescent lamps, incandescent lamps, cold-cathode or
compact fluorescent lamps, halogen, mercury-vapor, sodium-vapor,
metal halide, electroluminescent lamps or sources, lasers, etc. In
at least one embodiment, the light source may include various
light-collimating or beam shaping elements.
[0031] In at least one implementation, the light extraction layer
includes a light turning film or structure. In further
implementations, the light turning film or structure may include
one or more microstructured surfaces, one or more optically
transmissive layers, one or more inter-layer corrugated boundaries
or a reflective layer. In at least one implementation, the light
turning film or structure includes a plurality of reflective or
refractive facets inclined at an angle to the layer's surface. The
reflective or refractive surfaces may be formed by prismatic
grooves, notches, undercuts or other type of surface modification.
Alternative implementations of the light extraction layer include a
screen comprising a scattering layer or image print.
[0032] In at least one embodiment, the waveguide illumination
system of this invention is incorporated into a daylighting system.
According to an aspect, the waveguide illumination system is
configured and used as a hybrid luminaire combining natural and
artificial illumination. Such waveguide illumination system has a
layered panel structure which transmits sunlight delivered from a
skylight in a transversal direction with respect to the prevailing
plane of the panel and distributes and emits light received from an
array of LEDs coupled to an edge of the waveguide.
[0033] In at least one embodiment, the waveguide illumination
system of this invention is configured as an edge-lit front light
for an image screen which provides high optical transparency and
image fidelity while efficiently illuminating the underlying
image.
[0034] In at least one embodiment, the waveguide illumination
system of this invention is configured as an edge-lit
backlight.
[0035] In at least one embodiment, the waveguide illumination
system of this invention is incorporated into a lighting luminaire
with improved beam directionality. According to an aspect, the
edge-lit waveguide panel distributes and emits light from a
broad-area surface of the panel in the form of a collimated
beam.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
[0036] The invention will be more fully understood by reference to
the following drawings which are for illustrative purposes
only:
[0037] FIG. 1 is a schematic perspective view of a conventional
light emitting waveguide employing surface microstructures for
light extraction.
[0038] FIG. 2 is a schematic perspective view of a conventional
light emitting waveguide employing V-shaped grooves for light
extraction.
[0039] FIG. 3 is a schematic perspective view of a conventional
side-emitting optical fiber employing cuts or notches formed in the
fiber surface for light extraction.
[0040] FIG. 4 is a schematic cross-sectional view of a conventional
light-emitting waveguide portion, showing the principles of light
extraction.
[0041] FIG. 5 is a schematic perspective view of a waveguide
illumination system, according to at least one embodiment of the
present invention.
[0042] FIG. 6 is a schematic cross-sectional view and raytracing of
a waveguide illumination system portion, according to at least one
embodiment of the present invention.
[0043] FIG. 7 is a schematic cross-sectional view and raytracing of
a waveguide illumination system portion, showing light reflection
by a surface relief feature, according to at least one embodiment
of the present invention.
[0044] FIG. 8 is a schematic cross-sectional view and raytracing of
a waveguide illumination system portion, showing light interaction
with an opposing surface of a waveguide, according to at least one
embodiment of the present invention.
[0045] FIG. 9 is a schematic cross-sectional view of and raytracing
of a waveguide illumination system portion, showing the operation
of a light turning film attached to a buffer layer, according to at
least one embodiment of the present invention.
[0046] FIG. 10 is a schematic view of a waveguide illumination
system portion, illustrating the principles of light collimation,
according to at least one embodiment of the present invention.
[0047] FIG. 11 is a schematic view of a waveguide illumination
system, showing a buffer layer having a corrugated boundary with a
light extraction layer, according to at least one embodiment of the
present invention.
[0048] FIG. 12 is a schematic cross-sectional view and raytracing
of an illumination system portion, showing surface microstructure
formed in a light extraction layer, according to at least one
embodiment of the present invention.
[0049] FIG. 13 is a schematic cross-sectional view of and
raytracing of a waveguide illumination system portion, showing a
light scattering layer attached to a buffer layer, according to at
least one embodiment of the present invention.
[0050] FIG. 14 is a schematic cross-sectional view of a waveguide
illumination system portion in a front light configuration, showing
an exemplary light path between a light scattering layer and a
viewer, according to at least one embodiment of the present
invention.
[0051] FIG. 15 is a schematic cross-sectional view of a waveguide
illumination system portion, showing a translucent light scattering
layer, according to at least one embodiment of the present
invention.
[0052] FIG. 16 is a schematic cross-sectional view of a waveguide
illumination system portion, showing a smooth corrugated surface of
a waveguide, according to at least one embodiment of the present
invention.
[0053] FIG. 17 is a schematic cross-sectional view of a waveguide
illumination system portion, showing a cladding layer attached to a
corrugated surface of a waveguide, according to at least one
embodiment of the present invention.
[0054] FIG. 18 is a schematic view of a waveguide illumination
system having a cylindrical configuration, according to at least
one embodiment of the present invention.
[0055] FIG. 19 is a schematic view and raytracing of a waveguide
illumination system, showing multiple reflections from surface
relief features, according to at least one embodiment of the
present invention.
[0056] FIG. 20 is a schematic view and raytracing of a waveguide
illumination portion, showing a light collimator associated with a
light input edge of a waveguide, according to at least one
embodiment of the present invention.
[0057] FIG. 21 is a schematic view of a waveguide illumination
system including a wedge-shaped waveguide and a light turning film,
according to at least one embodiment of the present invention.
[0058] FIG. 22 is a schematic view of a waveguide illumination
system including a wedge-shaped waveguide and a light scattering
layer, according to at least one embodiment of the present
invention.
[0059] FIG. 23 is a schematic view of a waveguide illumination
system, showing a light extraction layer having a mirrored surface,
according to at least one embodiment of the present invention.
[0060] FIG. 24 is a schematic view of a waveguide illumination
system, illustrating a light collimation function of the system,
according to at least one embodiment of the present invention.
[0061] FIG. 25 is a schematic view of a waveguide illumination
system used in conjunction with a daylighting device, according to
at least one embodiment of the present invention.
[0062] FIG. 26 is a schematic view of a waveguide illumination
system portion, showing a buffer layer attached to a corrugated
surface of a waveguide, according to at least one embodiment of the
present invention.
[0063] FIG. 27A through FIG. 27I show various configurations of
surface relief features, according to at least some embodiments of
the present invention.
[0064] FIG. 28 is a schematic elevation view of a waveguide
illumination system, showing a linear arrangement of parallel
surface relief features formed in a waveguide surface, according to
at least one embodiment of the present invention.
[0065] FIG. 29 is a schematic elevation view of a waveguide
illumination system, showing a two dimensional array of surface
relief features, according to at least one embodiment of the
present invention.
[0066] FIG. 30 is a schematic view of a waveguide illumination
system portion, showing a buffer layer and a back-scattering light
extraction layer attached to a waveguide surface comprising surface
relief features, according to at least one embodiment of the
present invention.
[0067] FIG. 31 is a schematic view of a waveguide illumination
system portion, showing a light diffusing layer, according to at
least one embodiment of the present invention.
[0068] FIG. 32 is a schematic view of a waveguide illumination
system portion, showing light redirecting prismatic grooves within
a light diffusing layer, according to at least one embodiment of
the present invention.
[0069] FIG. 33 is a schematic view of a waveguide illumination
system portion, showing light redirecting slits within a light
diffusing layer, according to at least one embodiment of the
present invention.
[0070] FIG. 34 is a schematic view of a waveguide illumination
system, showing surface relief features having a variable slope,
according to at least one embodiment of the present invention.
[0071] FIG. 35 is a schematic view of a waveguide illumination
system, showing two opposing arrays of surface relief features and
two light sources coupled to opposing edges or ends of a waveguide,
according to at least one embodiment of the present invention.
[0072] FIG. 36 is a schematic exploded view of a front light
implementation of a waveguide illumination system, according to at
least one embodiment of the present invention.
[0073] FIG. 37 is a schematic cross-sectional view and raytracing
of a waveguide illumination system, showing light scattering
features within a waveguide, according to at least one embodiment
of the present invention.
[0074] FIG. 38 is a schematic cross-sectional view and raytracing
of a waveguide illumination system, showing light scattering
features within a waveguide and a buffer layer positioned between
the waveguide and a light extraction layer, according to at least
one embodiment of the present invention.
[0075] FIG. 39 is a schematic cross-sectional view and raytracing
of a waveguide illumination system in a two-sided backlight
implementation, according to at least one embodiment of the present
invention.
[0076] FIG. 40 is a schematic cross-sectional view and raytracing
of an illumination system portion illustrating the light extracting
operation of scattering particles in conjunction with a low-index
layer, according to at least one embodiment of the present
invention.
[0077] FIG. 41 is a schematic cross-sectional view and raytracing
of an illumination system portion illustrating the light extracting
operation of forward-scattering particles in conjunction with a
low-index layer, according to at least one embodiment of the
present invention.
[0078] FIG. 42 is a schematic cross-sectional view and raytracing
of a waveguide further illustrating a forward-scattering operation
of light scattering features, according to at least one embodiment
of the present invention.
[0079] FIG. 43 is a schematic view of a scattering pattern
characterizing an exemplary forward-scattering particle, according
to at least some embodiments of the present invention.
[0080] FIG. 44 is a schematic view of a scattering pattern
characterizing another exemplary forward-scattering particle,
according to at least some embodiments of the present
invention.
[0081] FIG. 45 is a schematic view of an illumination system in an
axisymmetrical configuration, showing a light source in the center,
according to at least one embodiment of the present invention.
[0082] FIG. 46 is a schematic view of an illumination system in an
alternative axisymmetrical configuration, showing a plurality of
light sources in a central ring area, according to at least one
embodiment of the present invention.
[0083] FIG. 47 is a schematic cross-sectional view and raytracing
of an illumination system portion, showing a reflective layer
adjacent to a waveguide and a right-angle turning film adjacent to
the same waveguide, according to at least one embodiment of the
present invention.
[0084] FIG. 48 is a schematic elevation view of a waveguide
illumination system, showing an annular arrangement of parallel
surface relief features formed in a waveguide surface, according to
at least one embodiment of the present invention.
[0085] FIG. 49 is a schematic cross-sectional view and raytracing
of a waveguide portion, showing a corrugated boundary between two
transmissive materials having different refractive indices,
according to at least one embodiment of the present invention.
[0086] FIG. 50 is a schematic perspective view of a directional
lighting fixture employing a collimating illumination system,
according to at least one embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0087] Referring more specifically to the drawings, for
illustrative purposes the present invention is embodied in the
apparatus generally shown in the preceding figures. It will be
appreciated that the apparatus may vary as to configuration and as
to details of the parts without departing from the basic concepts
as disclosed herein. Furthermore, elements represented in one
embodiment as taught herein are applicable without limitation to
other embodiments taught herein, and in combination with those
embodiments and what is known in the art.
[0088] A wide range of applications exist for the present invention
in relation to the collection of electromagnetic radiant energy,
such as light, in a broad spectrum or any suitable spectral bands
or domains. Therefore, for the sake of simplicity of expression,
without limiting generality of this invention, the term "light"
will be used herein although the general terms "electromagnetic
energy", "electromagnetic radiation", "radiant energy" or exemplary
terms like "visible light", "infrared light", or "ultraviolet
light" would also be appropriate.
[0089] The present invention seeks to provide waveguide
illumination systems capable of progressively extracting at least a
substantial portion of light propagating in a waveguide and
emitting the extracted light from a waveguide's side wall towards
one or more predetermined directions in a controlled manner and
without substantial changing of the waveguide surface smoothness
and/or continuity, such as introducing high-aspect-ratio cuts,
notches or grooves.
[0090] According to the present invention, there is provided an
illumination system employing a waveguide. The waveguide is
configured to guide light toward a predetermined direction by means
of a Total Internal Reflection (TIR) from its opposing walls having
substantially smooth surfaces and defining a waveguide core. The
waveguide core should preferably be made from a material having
good broadband optical clarity and transmission. When the
illumination system is designed to operate in a specific spectral
range, the material should be highly transmissive at least in that
spectral range. The waveguide core may be manufactured from glass
or a suitable polymeric material including but not limited to
optical quality PMMA (acrylic), silicone, polycarbonate, PET
(polyethylene terephthalate), polystyrene, polyolefin, polyesters,
APET, PETG, or PVC, as well as any optically clear resin which is
obtainable by polymerization and curing of various compositions.
The waveguide may be formed by a single layer of the appropriate
optically transmissive material or it may also include any number
of additional layers made from the same or different materials
having sufficient optical clarity for light guiding purposes.
[0091] In one exemplary case of an edge-lit lighting waveguide
panel, the waveguide may be configured to receive light from a
light source on one edge and guide the light toward an opposing
terminal end or edge. In another exemplary case of a planar
waveguide having an axisymmetrical or free-form configuration,
light may me input through an opening in the central area of the
waveguide and subsequently propagate radially from the input area
towards the outer edge. In a further exemplary case of an optical
fiber or a cylindrical-configuration light pipe, the waveguide may
be configured to guide light from a first terminal end to an
opposing second terminal end. Once light is input into the
waveguide and its propagation angles permit for TIR to occur at
waveguide's one or more major surfaces, it becomes trapped within
the core boundaries and can propagate considerable distances until
it is extracted, absorbed or it reaches the opposing edge, outer
edge or terminal end of the waveguide.
[0092] The present invention is generally directed to edge-lit
planar waveguides emitting from a broad-area surface and to
end-lit, side-emitting cylindrical waveguides. Accordingly, when
the waveguide has a planar configuration with parallel walls, each
of the opposing major surfaces of the waveguide as well as the body
of waveguide may be characterized by a plane which may be referred
to as a prevailing plane of the respective element. Likewise, the
illumination system based on the waveguide may also have a well
defined planar shape and may thus also be characterized by a
prevailing plane. It will be appreciated that, in case of the
parallelism of the opposing broad-area waveguide surfaces, the
prevailing planes of the surfaces and the prevailing plane of the
waveguide will be generally parallel to each other. Thus, when a
particular plane or a reference line makes an angle with respect to
one of those prevailing planes, it will also make the same angle
with the other parallel prevailing planes. Particularly, when the
term like "out-of-plane angle" is used to describe the angular
relationship a light ray or reference geometry object such as plane
or axis with one of the above reference planes, such term may also
be generally applied to the other parallel reference planes without
limitations.
[0093] A cylindrical waveguide may be characterized by a prevailing
axis, such as a longitudinal axis of the cylindrical body forming
the waveguide. In the context of the present inventions, as well as
for the purpose of illustrating the operation of the presently
preferred embodiments, a side-emitting waveguide having a
cylindrical configuration may also be characterized by a prevailing
plane. However, unlike the case of a planar waveguide having well
defined planes based on geometrical dimensions, the prevailing
plane of a cylindrical waveguide may be defined as a plane which is
extending parallel to the prevailing axis and which is also
generally separating the emitting and non-emitting sides of the
waveguide.
[0094] The waveguide of the illumination system includes a light
emitting region associated with light deflecting elements
distributed throughout the waveguide's body or throughout at least
one of its major surface along the intended light propagation path.
The function of the light deflecting elements is to cause a
continuous change of the out-of-plane angle of a ray with the
distance which the ray has propagated along the waveguide. The
light deflecting elements distributed along the optical path
gradually deflect light from the original propagation direction in
an incremental manner and eventually communicate such light a
greater angle with respect to a surface normal than the critical
TIR angle thus causing the light to exit from the waveguide at
different locations along the extent of the waveguide. It is
preferred that the deflecting elements are substantially
non-absorbing so that the repetitive interaction of light rays with
such elements does not cause perceptible ray attenuation along the
propagation path. Suitable lossless or near-lossless mechanism
which may be employed for deflecting light rays include but are not
limited to TIR and/or refraction at a boundary between two
dielectric materials having different refractive indices.
[0095] The distance which a light ray may travel within the
waveguide before it is extracted by overcoming TIR at the waveguide
surface largely depends on the initial propagation angle.
Generally, rays having greater initial out-of-plane (in case of a
planar waveguide) or also out-of-axis (in case of a cylindrical
waveguide) angle will travel shorter longitudinal distances than
rays initially propagating at more oblique angles with respect to
the waveguide's prevailing plane or axis, as they require less
interactions with the light deflecting elements to overcome TIR.
However, it should be understood that the actual light path of each
ray as well as its distance traveled within the waveguide may
depend on other factors as well, particularly in view of the random
character of light propagation within the waveguide and random ray
interactions with deflecting elements.
[0096] In at least some embodiments of the present invention, the
illumination system includes a buffer layer or cladding layer
attached to a major waveguide surface with a good optical contact.
The buffer layer has a lower refractive index than the waveguide
but higher than that of the outside medium. The buffer layer
provides a differential in refractive index drop at the opposing
major surfaces or sides of the waveguide and suppresses TIR at
least for some uttermost incidence angles compared to the opposing
boundary contacting with low-index medium (such as air). This
causes the controlled leakage of the deflected out-of-plane rays
primarily through the designated major surface of the waveguide
contacting with the buffer layer rather than through both opposing
surfaces of the waveguide. Suitable materials for the buffer layer
can advantageously be selected for low-n fluoropolymers or resins,
such as, for example, FEP, ETFE (both having a refractive index of
1.34-1.35), PFTE AF 1600 (n.apprxeq.1.31), PFTE AF 2400
(n.apprxeq.1.29), certain silicones, and the like.
[0097] According to at least some embodiments, the light deflecting
elements include shallow surface relief features formed in the
waveguide surface. The surface relief features are configured to
slightly alter the angular distribution of light upon each
interaction and cause small portions of light to overcome TIR and
leak out of the waveguide's core into the buffer layer while the
main portion of can remain trapped within the waveguide.
[0098] The surface relief features may be formed by shallow (low
aspect ratio) recesses or depressions in the light guiding surface
of the waveguide. Each surface relief feature may comprise a
reflective face inclined to the surface plane at a sufficiently low
dihedral angle .alpha.nd configured to reflect light by means of
TIR. The TIR surface of the reflective face should generally face
the light input edge or end of the waveguide so that it can be
illuminated by a light source attached to that edge or end. The
dihedral angle or slope of each reflective face with respect to the
waveguide surface should be substantially less than 45.degree. and
may ordinarily be less than 20.degree. and, more preferably, less
than 10.degree.. According to at least one embodiment, the dihedral
angle may take an angular value between 1.degree. and 3.degree..
According to at least one embodiment, the dihedral angle may be
between 0.1.degree. and 1.degree..
[0099] The upper practical limit for the dihedral angle of the
reflective face may be selected from various considerations
depending on the intended application of the illumination system.
According to some embodiments employing the buffer layer, each
reflective face should preferably be configured to reflect, by
means of TIR, substantially the entire light beam impinging onto
the reflective face back into the waveguide. This means that the
dihedral angle of the reflective face should be smaller than a
predetermined value defined by the differential between the
refractive indices of the waveguide and the outside medium so as
not to cause considerable light leakage through the reflective
faces of respective surface relief features. According to some
embodiments configured for emitting collimated light from a major
surface of the waveguide, the dihedral angle of reflective faces
may be selected based on the desired degree of collimation.
[0100] The lower practical limit for the dihedral angle of the
reflective faces may be selected, for example, based on the desired
rate of light extraction that would ensure that most light injected
into the waveguide can be removed along the propagation path.
Various other factors may also be considered, such as the spacing
between individual light extracting elements, the refractive index
of the waveguide the adjacent layers, and whether or not the
uniformity of light emission from the waveguide's surface is
needed.
[0101] While each surface relief feature may have a very low aspect
ratio (e.g., the ration between the feature's depth or height to
its width or base at the surface, it should be understood that the
light deflecting portions of each reflective should still be
essentially non-parallel to the light guiding surface of the
waveguide at least in a cross-section perpendicular to the
prevailing direction of light propagation. Therefore, it will be
appreciated that each of the reflective faces of respective light
deflecting features will alter the light propagation angles upon
each interaction with the guided light. Particularly, each
interaction of light with the reflective face will broaden the
angular distribution of light propagating in the waveguide and
cause at least a portion of reflected light to obtain greater
out-of-plane angles and thus smaller incidence angles with respect
to the TIR surface(s) of the waveguide.
[0102] For the purpose of this discussion, the term "incidence
angle" of a light ray in relation to a surface generally refers to
an angle that this ray makes with respect to a normal to that
surface. It will be appreciated by those skilled in the art of
optics that, when referring to light or other waves passing through
a boundary formed between two different refractive media, such as
air and glass, for example, the ratio of the sines of the angles of
incidence and of refraction is a constant that depends on the ratio
of refractive indices of the media (the Snell's law of refraction).
The following relationship can describe light bending property of
an interface between two refractive media: n.sub.I sin
.phi..sub.I=n.sub.R sin .phi..sub.R, where n.sub.I is the
refractive index of the material where the light is incident from,
n.sub.R is the refractive index of the material where the light
refract to, and .phi..sub.I and .phi..sub.R are the angle of
incidence and the angle of refraction, respectively. It will be
further appreciated that such optical interface can also be
characterized by the angle of a Total Internal Reflection (TIR)
which is the value of .phi..sub.I for which .phi..sub.R equals
90.degree.. Accordingly, for a surface characterized by a stepped
drop in refractive index along the ray propagation path, the
incidence angle may be less than, equal to, or greater than the TIR
angle at the given surface.
[0103] A TIR angle .phi..sub.TIR can be found from the following
expression:
[0104] .phi..sub.TIR=(n.sub.R/n.sub.Isin
90.degree.)=arcsin(n.sub.R/n.sub.I). In an exemplary case of the
interface between acrylic with the reflective index n.sub.I of
about 1.49 and air with n.sub.R of about 1, .phi..sub.TIR is
approximately equal to 42.degree..
[0105] Since each reflective face of the surface relief features
broadens the angular distribution of light propagating in the
waveguide, at least some uttermost out-of-plane rays may obtain
incidence angles with respect to the boundary between the waveguide
core and the buffer layer which are less than the TIR angles for
the respective boundary. Therefore, these rays will refract into
the buffer layer and thus exit from the waveguide core.
[0106] It will be understood that, due to the nature of incremental
deflection of light rays by relatively small angles using the light
deflecting features, the rays escaping from the waveguide due to
less-than-TIR incidence angles will generally have a relatively
narrow angular spread. Additionally, the rays emerging from the
major surface of the waveguide and refracting into a smaller-index
medium at near-TIR incidence angles will have relatively low angles
with respect to the waveguide surface and relatively high
refraction angles. The angular spread of the emerging rays can be
controlled by the appropriate configuration of the light deflecting
means and their distribution density along the prevailing light
path in the waveguide. Particularly, by way of example and not
limitation, the dihedral angle of reflective faces may be
advantageously selected to result in the refracted rays emerging at
grazing angles with respect to the waveguide surface (corresponding
to refraction angles close to 90.degree.) and having a fairly
narrow angular spread. Accordingly, the subsequent surface relief
featured distributed along the waveguide will respectively deplete
the remaining light from the waveguide and eventually extract it at
grazing angles with respect to the surface plane. As the maximum
deviation angle from the plane of the waveguide can be limited to a
sufficiently low value, the resulting light beam may have a high
degree of collimation while propagating nearly parallel to the
longitudinal axis of the waveguide.
[0107] The shallow surface relief features may be formed by a
variety of suitable means and may comprise any surface
irregularities, undulations or corrugations that slightly alter the
reflection properties of the surface and cause light reflection at
different angles compared to a perfectly flat or straight surface.
When the surface relief features have a linear or cylindrical
geometry, the longitudinal axis of each such feature should
generally extend perpendicular to the prevailing direction of light
propagation on the waveguide.
[0108] The surface relief features may be fabricated together with
the waveguide where the waveguide core may be cast or molded using
a negative replica of the surface relief features. Alternatively,
the waveguide may be fabricated first and the surface relief
features may be formed in it by any suitable method for structural
surface modification. Suitable methods may include laser ablation,
chemical etching, embossing, grinding, polishing, molding,
extrusion, material expansion or contraction, bending, etc. The
surface relief features may also be formed in an external layer of
optically transmissive material which may be then attached to a
major surface of the main waveguide's body with a good optical
contact.
[0109] Suitable surface relief features may also be formed by
corrugations or bends of the waveguide as a whole, as well as by
any other means causing portions of the surface to reflect light at
a greater out-of-plane angle compared to an ideally-flat, smooth
surface.
[0110] According to at least some embodiments of the present
invention, the light deflecting elements are formed by light
scattering features distributed throughout the body of the
waveguide at least in its light emitting region. Particularly, the
light scattering features can be formed by small dielectric
particles or by any other form of optical irregularities in the
otherwise homogenous body of the waveguide. By way of example, the
light scattering features may be formed by very small proportions
of imbedded, finely divided, spherical or aspherical particles made
from a transparent plastic or glass material which refractive index
differs from that of the waveguide by a predetermined amount.
[0111] The light scattering particles can be homogenously
distributed in the volume of the waveguide's body. Each particle
can have a plain structure or a core-shell structure such as, for
example that of known core-shell particles obtainable by emulsion
polymerization. The light scattering features are configured to
deflect light propagating in the waveguide from the original
propagation path by means of forward scattering and communicate
greater angles to said light with respect to a normal to the
waveguide surface. Each light scattering feature should be designed
to limit the scattering angle within a relatively narrow cone.
Thus, a series of light scattering features on the optical path may
thus provide function of incremental light deflection somewhat
similar to the function of shallow surface microstructures
explained above. Single or multiple interactions of light
propagating by means of TIR in the waveguide with such light
scattering features will result in the extraction of relatively
small portions of light from the waveguide core at different
locations across the waveguide's surface. The structure and optical
properties of the light scattering features should be preferably
selected to result in light out coupling from the waveguide's core
at oblique angles with respect to the surface. When the buffer
layer is employed, light should primarily leak out of the waveguide
into this layer rather than exit from the opposing surface of the
waveguide.
[0112] According to at least some embodiments of the present
invention, the illumination system may be further provided with a
light extraction or light distribution layer adjacent to a major
surface of the waveguide. The main function of the light extraction
or distribution layer is to further direct or distribute the light
extracted by the light deflecting elements. When the buffer or
cladding layer is employed, the light extraction or distribution
layer may be externally attached to the buffer layer with a good
optical contact or made an integral part of the buffer layer.
[0113] By way of example and not limitation, the light extraction
layer may comprise a light turning film or structure. The light
turning film may redirect the collimated light beam emerging from
the waveguide towards a perpendicular of the waveguide surface thus
providing a useful source of directional light emitted along the
entire length of the light emitting region of the waveguide. At
least some types of light turning or light redirecting films that
may be incorporated into the light extraction layer of the present
invention are disclosed in co-pending, co-owned application Ser.
No. 13/662,311 which is incorporated by reference in its entirety
herein.
[0114] In a non-limiting example, the light turning film may be of
a refractive type configured for turning the collimated light
emerging from the waveguide by approximately 90.degree. away from
the waveguide surface. In an alternative non-limiting example, the
light turning film may be of a reflective type and may direct light
through the waveguide and also perpendicular to its surface.
[0115] According to at least some embodiments, the light extraction
layer may include a screen comprising a scattering layer or image
print. The image print may be printed or otherwise deposited
directly on the external surface of the buffer layer.
Alternatively, the image print may be provided on a transparent
substrate which can be attached to the buffer layer. The screen may
reflect and/or scatter light propagating in the buffer layer toward
an observer located at a distance from the waveguide. According to
different variations of this invention, the light scattering screen
may be configured to provide forward scattering, back scattering or
any combination of the two. Particularly, the screen may be made
opaque with back-scattering properties only, in which case the
illumination system may be used as a front light. Alternatively,
the screen may be made at least partially transmissive with the
forward scattering function, in which case the illumination system
may be used as a backlight. Furthermore, the transmissive
properties of the screen may be adjusted so that the screen can be
lit by the light emerging from the waveguide and made visible from
both sides, thus forming a two-sided illumination system. Useful
examples of the light extraction or distribution layer may also
include light scattering surfaces or films, phosphorescent or
fluorescent films, light filtering films or layers, diffusers, and
the like.
[0116] When the light extraction layer includes a viewable screen
and the waveguide is positioned between the screen and the viewer
in a front-light configuration, the surface relief features may be
made substantially smooth and shallow so that they will not
substantially alter the smoothness and continuity of the waveguide
surface and will not notably bend the path of light propagating at
near-normal angles with respect to the waveguide surface.
[0117] Various layers employed in the waveguide illumination
system, may be attached to each other or to the respective surfaces
using any suitable method providing a good optical contact. For
example, any two layers may be simply laminated onto each other
with no air gaps. Alternatively, any intermediate layers may be
used such as optical adhesives or two-sided transparent adhesive
films to promote optical and physical contact. The respective
layers of the illumination system may also be attached to each
other by chemical bonding, heat bonding, ultrasonic bonding,
welding, etc.
[0118] According to at least some embodiments of this invention,
the waveguide illumination system can be made optically
transmissive in a transversal direction and configured for a
generally unimpeded transversal light passage through its body. In
addition to that, the system may be configured to emit collimated
light from a selected broad-area surface or its portion and limit
light emission from the opposing surface. In other words, the
waveguide illumination system may be configured to provide
directional illumination from one side with the prescribed degree
of collimation of the emitted beam while precluding or at least
substantially reducing light escape from the opposing side and
preserving the transversal optical transmissivity. In contrast to
the prior art illumination systems, the waveguide illumination
system of this invention may be configured to not require using any
opaque layers to prevent light decoupling from an unwanted side or
surface.
[0119] According to at least some embodiments of this invention,
the waveguide can be made both highly transmissive and transparent
along the light path perpendicular to the waveguide's prevailing
plane or axis. Particularly, the waveguide can be configured to
have a very high visual transparency at least in the direction
along a normal to its broad surface. In other words, in addition to
having high light transmissivity, the waveguide of this invention
may have the property of transmitting light without appreciable
scattering along a normal viewing direction so that bodies or
images lying beyond can be seen clearly. While the light deflecting
elements of this invention are used to deviate light from the
waveguiding light paths by means of multiple incremental
deflections, they can still be configured to not appreciably alter
the propagation angles of light propagating perpendicular to the
prevailing plane or axis of the waveguide. This is generally in
contrast to the prior art illumination systems employing waveguides
with other types of light extraction microstructures or scattering
features which cause deterioration of either one or both the
transmissivity or transparency.
[0120] In operation, the light deflecting elements alter the
propagation angles of light with respect to the prevailing plane or
longitudinal axis of the waveguide by means of continuous
incremental deflections along the propagation path. Each deflection
alters the propagation angle by a relatively small amount which
allows most rays to propagate a considerable distance in a
waveguide. Multiple interactions of light rays with the light
deflecting elements continues until at least the uttermost
out-of-plane rays obtain less-than-TIR incidence angles with
respect to the waveguide surface and exit from the waveguide core
at relatively low angles with respect to the prevailing plane of
the waveguide. When the buffer layer is employed, it creates a
differential in the refractive index drop at the opposing surfaces
or longitudinal sides of the waveguide. In turn, it creates a
preference for light rays to exit from the waveguide through the
side or surface to which the buffer layer is attached. The light
extraction layer intercepts the light emerging from the waveguide
and directs it further at higher angles with respect to the
prevailing plane of the waveguide, thus finally extracting light
from the illumination system and directing it towards one or more
predetermined directions.
[0121] The present invention will now be described by way of
example with reference to the accompanying drawings.
[0122] FIG. 5 depicts an embodiment of a waveguide illumination
system 2 in accordance with the invention. System 2 includes a
waveguide 4 exemplified by a rectangular, planar slab waveguide
having generally smooth-surface walls configured to conduct light
by means of TIR. Waveguide 4 is configured to have a first major
broad-area surface 10 and an opposing major broad-area surface 12
extending generally parallel to surface 10. Waveguide 4 also has
four edges, one of the edged being designated as a light input edge
and the opposing edge being designated as a terminal edge. As
waveguide 4 may be associated with additional layers attached to
either of its major surfaces or edges, the main waveguide body
defined by its major surfaces 10 and 20 and by the four edges is
also hereinafter referred to as a waveguide core.
[0123] A light source 400 is provided on the light input edge so
that waveguide 4 can guide light from the light input edge towards
the opposing terminal edge by mean of TIR which involves bouncing
light from at least surfaces 10 and 12. The prevailing direction of
light propagation in waveguide 4 defines a longitudinal axis 200 of
the waveguide. The waveguide edges extending parallel to the
longitudinal axis (the longitudinal edges) may also be made smooth
and polished in order to be able to guide light by means of TIR.
Waveguide 4 preferably has a refractive index sufficiently greater
than the refractive index of the outside medium to provide for the
TIR light guiding properties in a predetermined acceptance
angle.
[0124] Light source 400 may include any suitable single or multiple
light sources of any known type. According to one embodiment, light
source 400 may include one or more light emitting diodes (LEDs).
Multiple LEDs may be arranged in a linear strip or a
two-dimensional array. When high-brightness LEDs are employed, as
may be the case, for example, when system 2 is employed in an
overhead lighting panel, other type of wide-area luminaire, the
LEDs may also be provided with a heat sink to remove excess heat
generated by the LED chips. A power supply (driver) may also be
provided which electrical characteristics may be matched with those
of the LED light source 400. Furthermore, a suitable support frame
or housing may also be provided to hold waveguide 4 and source 400
with the associated components together and/or encase all or at
least some parts of system 2.
[0125] It is noted that light-emitting devices suitable for light
source 400 are not limited to LEDs and may also include fluorescent
lamps, incandescent lamps, cold-cathode or compact fluorescent
lamps, halogen, mercury-vapor, sodium-vapor, metal halide,
electroluminescent lamps or sources, lasers, etc. Each light source
may have any suitable shape, including compact two-dimensional or
elongated one-dimensional shapes.
[0126] Light source 400 may have integrated optics such as
collimating or light-redistributing lenses, mirrors, lens arrays,
mirror arrays, light diffusers, waveguides, optical fibers and the
like. When light source 400 includes a series of compact light
sources, such as LEDs, each LED may be provided with individual
collimating optics or, alternatively, a single collimating optical
element may be supplied to inject light from all of the LEDs into
the edge of waveguide 4. Numerous applications of system 2 exist
where waveguide 4 may have a planar slab configuration and where
light source 400 may be associated with a strip of high-power LEDs
optically coupled to the light input edge of the waveguide.
[0127] Surface 10 of waveguide 4 has at least one light emitting
region comprising light deflecting elements exemplified by surface
relief features 8 formed in surface 10. More particularly, surface
relief features 8 are represented by repetitive shallow depressions
formed in a stepped arrangement in surface 10. The depressions can
be characterized by alternating peaks and valleys connected by
smooth, sloped surface portions. Surface relief features 8
preferably have a linear geometry with a common longitudinal axis
extending generally perpendicular to the longitudinal axis 200 of
waveguide 4. The shallow depressions forming surface relief
features 8 slightly alter the structure of surface 10 yet allowing
the surface to remain generally smooth and planar. Since surface 10
is optically transmissive, the shallow surface relief features 8
formed in this surface generally preserve both longitudinal and
transversal transmissivity of waveguide 4. Furthermore, surface 10
can be characterized by a first TIR angle .phi..sub.TIR1 which can
be found from the expression:
.phi..sub.TIR1=arcsin(n.sub.0/n.sub.1), where n.sub.0 is the
refractive index of the outside medium and n.sub.1 is the
refractive index of the core of waveguide 4. Ordinarily, the
outside medium can be air with n.sub.0.apprxeq.1, in which case
.phi..sub.TIR1.apprxeq. arcsin(1/n.sub.1).
[0128] Waveguide 4 further comprises a buffer layer 6 disposed in a
good optical and physical contact with surface 12 at least in the
light emitting region.
[0129] Buffer layer 6 has a refractive index lower than the
refractive index of waveguide 4 and may also have a function of a
cladding layer for the core of waveguide 4. Accordingly, the
interface between the core of waveguide 4 and buffer layer 6 may be
characterized by a second TIR angle .phi..sub.TIR2 which can be
found from the following expression:
.phi..sub.TIR2=arcsin(n.sub.2/n.sub.1), where n.sub.2 is the
refractive index of buffer layer 6. The system may also be
characterized by a critical TIR angle .phi..sub.TIRC which is the
greater of the first and second TIR angles .phi..sub.TIR1 and
.phi..sub.TIR2 respectively. When features 8 are sufficiently
shallow and make very low dihedral angles with the plane of surface
10, the critical TIR angle .phi..sub.TIRC generally defines a
minimum incidence angle that the light propagating in waveguide 4
must have with respect a normal to the longitudinal walls of the
waveguide's core in order to remain being guided through the core
by means of TIR.
[0130] Referring further to the embodiment illustrated in FIG. 5,
n.sub.0<n.sub.2<n.sub.1 and, consequently,
.phi..sub.TIR2>.phi..sub.TIR1 and .phi..sub.TIRC=.phi..sub.TIR2.
It will be appreciated, as long as the incidence angles of light
rays onto surfaces 10 and 12 remain above .phi..sub.TIRC, light can
propagate within the core in a waveguide mode.
[0131] As it will be described in detail below, surface relief
features 8 are configured to redirect light within waveguide 4 so
that predefined portions of light are eventually communicated
incidence angles which are less than .phi..sub.TIR2 but still
substantially greater than .phi..sub.TIR1. Maintaining the
incidence angle generally above .phi..sub.TIR1 ensures that surface
10 and its portions formed by features 8 will continue to be
reflective by means of TIR. On the other hand, making the incidence
angle less than .phi..sub.TIR2 for a small portion of light causes
the extraction of the respective portion from the waveguide core
into layer 6. In other words, a major function of surface relief
features 8 is to cause a controlled leakage of light from the core
of waveguide 4 into buffer layer 6 through surface 12 along the
propagation path, yet preventing light escaping through surface
10.
[0132] While the light-bending properties of surface relief
features 8 of the above-illustrated embodiment are selected to be
insufficient to extract light from system 2 without additional
means, they nevertheless play an important role in system 2
operation. Surface relief features 8 act cooperatively with layer 6
to preliminary extract light from the waveguide core into the
buffer layer so that light can be further directed and distributed
with improved efficiency using additional light extraction
features. The refractive index of buffer layer 6 and the properties
of surface relief features 8 can be configured to recover light
from the core of waveguide 4 through surface 12 at any desirable
rate along the propagation path and without causing light loss
through surface 10.
[0133] Accordingly, system 2 further includes a light extraction
layer 20 exemplified by a light turning film disposed in contact
with an external surface 14 of buffer layer 6. Surface 14 is
opposing the broad surface of buffer layer 6 that is contacting
waveguide 4. The light turning film comprises two transparent
layers having different refractive indices and separated by a
corrugated boundary between the layers. The film is configured to
extract light from buffer layer 6 and redirect it toward a
designated direction which may be advantageously selected to be
normal to the plane of waveguide 4.
[0134] An optional specularly reflective or diffusively reflective
layer (not shown) may be provided and positioned adjacent to
surface 10 of waveguide 4 to reflect any stray light that may
escape through surface 10 outside of the waveguide. The stray light
may include, for example, rays that are scattered by impurities in
the materials of waveguide 4, layer 6 or layer 20, as well as by
imperfections of surface relief features 8 or the corrugated
boundary within the light turning film.
[0135] FIG. 6 illustrates further details and operation of system 2
depicted in FIG. 5. Each surface relief feature 8 comprises a first
face 16 generally facing light source 400 and a second face 18
generally turned away from source 400. Both faces 16 and 18 are
ordinarily planar and each form non-zero dihedral angles with the
prevailing plane of surface 10. These dihedral angles may also be
referred to as slope angles of the respective faces to surface
10.
[0136] The slope of each face 16 is preferably selected to be
fairly low and substantially lower than the slope of the respective
face 18 so that faces 16 have considerably greater surface area
than faces 18. Accordingly, this translates into a considerably
longer cross-sectional profile of face 16 compared to the profile
of face 18. The slope of each face 18 may be advantageously
selected to ensure that the face is completely shaded from source
400 by the respective face 16. Particularly, the angle which face
18 makes with a normal to the prevailing plane of surface 10 is
preferably smaller than .phi..sub.TIRC. This can ensure that light
propagating in the waveguide mode within the core of waveguide 4
will never strike any of faces 18 and will interact only with faces
16. Suitable angles for faces 18 may include normal or near-normal
angle with respect to surface 10.
[0137] In operation, source 400 illuminates the light input edge of
waveguide 4 with a divergent beam which causes at least a
substantial part of the beam to enter the waveguide core at angles
permitting for TIR. Waveguide 4 further guides light toward the
opposing terminal edge by bouncing said light from the opposing
parallel surfaces 10 and 12. Since surface relief features 8 are
sufficiently shallow and the slopes of faces 16 are low, the change
in the propagation angle with respect to axis 200 is also low.
Therefore, most light reflected by each face 16 continues its
propagation in the waveguide by means of TIR while incrementally
obtaining a slightly broader angular distribution with respect to
axis 200 at each interaction with surface 10 in the light emitting
region.
[0138] It will be appreciated that, provided that there is a
sufficient optical path along the waveguide's longitudinal axis,
the incremental deviation of a light ray from axis 200 will
eventually result in said ray reaching surface 12 at an incidence
angle which is less than .phi..sub.TIRC. This, in turn, will
ultimately cause ray extraction into buffer layer 6. Obviously,
light rays having relatively small out-of-plane angles will
generally undergo morel bounces from faces 16 before reaching
sub-TIR angles and before being extracted from the core of
waveguide 4 than rays having larger out-of-plane angles. For
example, a ray 72 strikes face 16 of one of the surface relief
features 8 and is losslessly reflected by TIR back into waveguide
4. While the reflection from face 16 increases the out-of-plane
angle of ray 72, the incidence angle with respect to surface 12
still remains greater than the TIR angle .phi..sub.TIR2 at the
interface between the waveguide 4 and buffer layer 6. Therefore,
ray 72 undergoes TIR from surface 12 and continues to be guided by
means of TIR. Accordingly, longer optical path and additional
interactions with surface relief features 8 will be needed to
extract ray 72 into buffer layer 6.
[0139] In contrast, light propagating in waveguide 4 at angles
close to the critical TIR angle .phi..sub.TIRC can be extracted
into buffer layer 6 near the light input edge of the waveguide, as
illustrated by the path of a ray 74. Ray 74 is emanated by the same
light source 400 but has a greater initial out-of-plane angle than
ray 72. Ray 74 strikes face 16 of an individual surface relief
feature 8 at an incidence angle which is greater than
.phi..sub.TIR1. Therefore, ray 74 is reflected from face 16 by
means of TIR and is directed toward opposing surface 12 at a
greater out-of-plane angle than before striking feature 8. When the
slope of face 16 is sufficient to result in an less than
.phi..sub.TIR2 incidence angle of ray 74 onto surface 12, no TIR
will occur at surface 12 and ray 74 will refract into buffer layer
6.
[0140] Ray 74 further propagates in buffer layer 6 towards opposing
surface 14 where it enters the light turning film of light
extraction layer 20. Layer 20 is preferably configured to have a
good optical contact with buffer layer 6. Although it may be simply
laminated onto surface 14 with no air bubbles, an adhesion
promoting layer may also be used, such as a layer of optical
adhesive of double-sided adhesive tape or film, for example. The
refractive index of the inner layer of the light turning film
contacting surface 14 should preferably be not less than the
refractive index of buffer layer 6. Likewise, the refractive index
of the adhesion promoting layer, is any, should also be no less
than the refractive index of layer 6. According to some
embodiments, the above refractive indices may be matched to each
other in order to substantially reduce or eliminate the Fresnel
reflections.
[0141] The outer layer of the light turning film may have a
refractive index greater than its inner layer. The corrugated
boundary between the inner and the outer layer acts as a prismatic
array and redirects light at a different angle with respect to the
surface or its normal. The redirection mechanism may involve
refraction and/or TIR. Accordingly, the internal boundary
corrugations of the light redirecting film may be configured to
intercept rays propagating at near-grazing angles in buffer layer 6
and redirect them towards a normal to the plane of waveguide 4, as
illustrated in FIG. 6 referring to ray 74.
[0142] FIG. 7 depicts, in a cross-section parallel to the light
propagation path, a portion of waveguide 4 including an individual
prismatic depression in surface 10 and illustrates the light
redirecting operation of surface relief feature 8 formed by said
surface depression.
[0143] For the purpose of clarity and explaining the principles of
light redirection by surface 10, the individual surface relief
feature 8 is shown surrounded by flat portions of surface 10 which
are parallel to longitudinal axis 200. However, it should be
understood that system 2 may have any number of surface relief
features 8 which can be spaced apart, contacting each other,
overlapping, or otherwise distributed with any prescribed density
along the intended propagation path.
[0144] Surface relief features 8 may have a constant pitch or
spacing. Alternatively, the spacing between adjacent features 8 can
be made variable along the propagation path. Particularly, it may
be advantageous to provide some initial spacing between surface
relief features 8 near the light input edge and gradually increase
the density of the features as the distance from the light input
edge increases. This may help improve the uniformity of light
emission from surface 12 as the increasing density of surface
relief features 8 will compensate the depletion of light by the
preceding features 8.
[0145] Referring further to FIG. 7, light propagates in waveguide 4
left to right within an angular cone having an angular aperture of
.+-..beta., where .beta. is an out-of-plane propagation angle
counted from longitudinal axis 200. An uttermost ray 174 having the
propagation angle of -.beta. is shown striking face 16 of surface
relief feature 8 at a point 90.
[0146] Face 16 is inclined at a dihedral angle .alpha., hereinafter
also referred to as a slope angle .alpha., with respect to the
prevailing plane of surface 10. Angle .alpha. is selected to be
sufficiently low in order to preserve TIR at face 16 and to not
cause decoupling of ray 174 through surface 10. 16 and result in
TIR back to the core of waveguide 4. In will be appreciated that
when light propagates in a waveguide mode, an angle complementary
to angle .beta. should generally exceed .phi..sub.TIRC. Considering
that, when low-n buffer layer 6 is employed (not shown in FIG. 7),
.phi..sub.TIRC=.phi..sub.TIR2 and .phi..sub.TIR2>.phi..sub.TIR1,
the acceptable range of angle .alpha. may be generally defined by
the following relationship:
0<.alpha.<.phi..sub.TIR2-.phi..sub.TIR1.
[0147] Accordingly, the uttermost ray 174 having the propagation
angle -.beta. upon entering point 90 reflects from face 16 by means
of TIR and obtains a new propagation angle of .beta.+2.alpha., as a
matter of optics. Therefore, the individual surface relief feature
8 causes widening the light propagation cone in the core waveguide
4 by angle 2.alpha. and also causes temporary angular asymmetry of
the cone by the same angle.
[0148] FIG. 8 depicts an upper portion of waveguide 4 and
illustrates the interaction of light with opposing surface 12 of
waveguide 4 after its passing surface relief feature 8 of FIG. 7.
Due to the smallness of slope .alpha. and the resulting smallness
of the deflection angle caused by surface relief feature 8, a
substantial part of light continues to propagate in the core of
waveguide 4 by means of TIR. Particularly, all rays having the
incidence angles with respect to surface 12 less than .beta. are
reflected from surface 12 by means of TIR. In other words, all
light remaining in the angular range .+-..beta., with respect to
longitudinal axis 200, remains also confined in a waveguide
mode.
[0149] However, the uttermost rays from the broadened angular
propagation cone may now have incidence angles which no longer
exceed second TIR angle .phi..sub.TIR2. As further illustrated in
FIG. 8, those rays may refract into buffer layer 6, generally at
relatively low acute angles .gamma. with respect to surface 12.
[0150] In an illustrative example where
90.degree.-.beta..apprxeq..phi..sub.TIR2, substantially all of the
light having the propagation angles greater than .beta. will exit
from the core of waveguide 4 into buffer layer 6. Thus, it will be
appreciated that, after interacting with surface 12, the light beam
propagating within waveguide 4 will shed a relatively narrow cone
of 2.alpha. into buffer layer 6 and again obtain the prior
.+-..beta. angular range. In other words, the optical interface
formed by surface 12 and separating waveguide 4 from
smaller-refractive-index buffer layer 6 is "shaving off" rays
having propagation angles in excess of the critical angles that
still permit TIR at surface 12. The escaping cone 2.alpha.
represents a fixed portion of the angular distribution of light in
waveguide 4. Thus, the slope a of face 16 determines the amount of
light extracted by each feature 18 into buffer layer 6 along the
optical path. Accordingly, the rate of light extraction can be
accurately controlled by varying the slope of TIR faces in the
light extraction area for a given waveguide 4 geometry and relative
refractive indices of the waveguide core and buffer layer 6.
[0151] It should be noted that a certain portion of light may
undergo reflection from surface 12 even when the incidence angle
onto said surface is less than .phi..sub.TIR2, owing to the
so-called Fresnel reflection from the optical interface between two
layers having different refractive indices. Light reflected from
surface 12 by means of Fresnel reflection will thus return back
into waveguide 4 and can be recycled.
[0152] When waveguide 4 has planar slab geometry, the shallow
depressions of in surface 10 that form surface relief features 8
may be made by sheet casting or extrusion from a suitable
transparent polymer, such as acrylic, polystyrene or polycarbonate,
for example. Alternatively, surface relief features 8 may be formed
in a flat sheet of glass or polymer by any suitable methods for
micro-replication or material removal. For example, grinding,
milling or fly-cutting may be used with subsequent polishing of
surface 10. In a more specific example, surface relief features 8
may be formed using a sharp diamond-tipped bit or cutter in which
case the sufficient surface finish may be obtained without the need
of subsequent polishing. In an exemplary implementation, the
diamond cutting tool having the appropriate shape and slope of the
cutting surface can be dragged across the surface of waveguide 4
leaving a shallow groove. In an alternative exemplary
implementation, the diamond cutting tool can be used in a
"fly-cutting" mode as it can be spun in a spindle at a high speed
(preferably at speeds of 20000 to 100000 RPM), plunged to the
appropriate depth into the surface of waveguide 4 and moved across
the waveguide's surface. The rotation axis the tool should be
preferably inclined at an angle with respect to a normal to the
prevailing plane of waveguide 4 corresponding to the desired slope
angle of the face 16 to be formed. Features 8 may be formed
directly in the surface of waveguide 4 or they can also be formed
in a separate optically transparent film or plate which can be
attached to surface 10.
[0153] The structure and operation of light extraction layer 20
including a light turning film is illustrated in FIG. 9 by way of
example.
[0154] The light turning film of layer 20 comprises a first layer
104 and a second layer 106 disposed in contact with each other and
having a corrugated boundary between the two layers. Corrugations
108 forming the boundary have a linear triangular configuration in
a cross-section with peaks and valleys extending perpendicular to
the plane of drawing. Corrugations 108 also define a plurality of
alternating interface facets making predetermined angles with the
prevailing plane of system 2 so that the corrugated boundary
between layers 104 and 106 comprises a plurality of facets 124
alternating with facets 126. Facets 124 are characterized by a
first dihedral angle 162 with respect to the prevailing plane of
system 2 and facets 126 are characterized by a second dihedral
angle 164 with respect to that plane. Planes parallel to the
prevailing plane of system 2 are indicated by reference lines 142
and 148 in FIG. 9. The light turning film is configured so that a
refractive index n.sub.3 of layer 106 contacting buffer layer 6 is
smaller than a refractive index n.sub.4 of layer 104 facing
outwardly from layer 6.
[0155] The light turning film of layer 20 is configured to accept
light propagating at low angles along surface 14 in buffer layer 6
and redirect said light at a greater angle with respect to the
surface so as to result in light decoupling through surface 110.
Referring further to FIG. 9, an angle 160 represents the low angle
that incident ray 74 makes with surface 14 when it enters layer 20.
In the illustrated embodiment, the light turning film may be
configured to accept light emerging from layer 6 at angles between
0.degree. and 20.degree. from surface 14 (corresponding to
90.degree. and 70.degree. incidence angles with respect to a
surface normal, respectively) and communicate said light a smaller
angle with respect to the surface normal so that TIR can be
overcome at surface 110. When system 2 is configured for light
collimation, angle 160 should preferably be within a predefined
narrow range of angles from the prevailing plane of waveguide 4,
said range being primarily defined by the desired angular cone of
the collimated light. While angle 160 may represent very sharp,
near-grazing angles with respect to surface 14, it should also be
understood that this angle may take any other suitable angular
values provided that system 2 has the same basic operation.
[0156] Facets 124 are configured to have a generally smaller
dihedral angle with respect to the prevailing plane of system 2
than facets 126. Furthermore, the dihedral angle 162 of facets 124
is preferably selected to be less than an angle which is
complementary to angle 160 in order to provide refraction towards a
normal to that plane.
[0157] The dihedral angle 164 of each facet 126 is preferably made
greater than a maximum designed value of the angle that light can
make with surface 14 in layer 104 after refracting at facet 124. At
the same time, the dihedral angle 164 of each facet 126 should
preferably be selected so that the facet 126 can intercept light
refracted by a preceding adjacent facet 124 and reflect it by means
of TIR.
[0158] Referring yet further to FIG. 9, light ray 74 deflected by
the respective surface relief feature 8 (not shown) and receiving a
sub-TIR angle with respect to surface 12 is extracted from the core
of waveguide 4 (not shown) and enters buffer layer 6. Ray 74
further crosses surface 14 and enters layer 106 of layer 20. The
refractive index n.sub.2 of layer 6 can be matched to the
refractive index n.sub.3 of layer 106 so that ray 74 can make about
the same refraction angle as the incidence angle with respect to a
surface normal 140.
[0159] Ray 74 entering layer 106 at a sharp angle with respect to
surface 14 propagates in layer 106 until it strikes facet 124 of
the corrugated boundary with layer 104. Depending on the angle 162,
ray 74 may slightly bend toward a normal 144 by means or refraction
at the interface between the lower refractive index layer 106 and
the higher refractive index of layer 104, after which it may strike
the next adjacent facet 126. The slope of facet 126 defined by the
dihedral angle 164 is selected to result in TIR at the interface
between high-index layer 104 and low-index layer 106. Upon TIR,
facet 126 communicates an additional bend angle to ray 74, this
additional bend angle being twice the angle between ray 74 and
facet 126. As a result, ray 74 may exit from layer 20 nearly
perpendicular to surface 110. It should be understood that light
turning film may also be configured to result in the emergence
angles other than normal. However, it should also be understood
that, ordinarily, ray 74 will be communicated an exit angle with
respect to waveguide's 4 prevailing plane which is substantially
greater than angle 160. According to at least some embodiments of
this inventions, when system 2 is used for light distribution and
improved collimation, it may be preferred that the slopes of
surface relief features 8 and other parameters of waveguide and the
respective outer layers are selected so that the light beam emitted
from a major broad area surface of system 2 has a divergence which
is at least less than the initial divergence of light emitted by
source 400.
[0160] FIG. 10 illustrates yet further details and operational
aspects of embodiments of system 2 shown in the preceding drawing
figures and depicts an exemplary case when system 2 is configured
for emitting collimated light from its broad-area surface.
[0161] A fan of rays 802 exemplifies the angular distribution of
light initially propagating in waveguide 4 before interacting with
surface relief features 8. It will be appreciated that the maximum
half-angle of fan of rays 802 is defined by the acceptance angle of
the core of waveguide 4. The acceptance angle is primarily defined
by the refractive indices of waveguide 4 and buffer layer 6.
[0162] As light passes the first surface relief feature 8 along its
propagation path, a portion of its rays obtains a greater
out-of-plane angle thus widening the angular distribution of light,
as exemplified by a fan of rays 804. In order to enable this
widening of the angular distribution, face 16 of feature 8 is
inclined at low slope angle .alpha. with respect to prevailing
plane 202 of surface 10.
[0163] The slope of face 16 is sufficiently small and
90.degree.-.alpha.<<.phi..sub.TIR1. This prevents refraction
at face 16 and light escape from waveguide 4 through surface 10.
Therefore, the interaction of light propagating in waveguide 4 with
face 16 will result in TIR back into the waveguide. Moreover, the
slope angle .alpha. is also low so that the increment in the
angular distribution it produces is substantially less than the
angular span of fan of rays 802.
[0164] Face 16 has a substantially planar shape and smooth
surface.
[0165] Accordingly, light rays striking face 16 will obtain an
increment in their out-of-plane angles which is twice the slope
angle .alpha.. It will be appreciated that the range of directions
represented by fan of rays 802 is at least partially overlapping
with the range of directions represented by fan of rays 804.
[0166] As a result of TIR from face 16 of an the first feature 18,
at least some of the uttermost rays in fan of rays 804 may form
incidence angles with respect to a normal to surface 12 greater
than second TIR angle .phi..sub.TIR2. Therefore, upon reaching
surface 12, these uttermost rays will cross said surface and
refract into buffer layer 6, as illustrated by a fan of rays 808.
It will be understood that this escaping light represents a small
portion of light guided through waveguide 4 and is generally
characterized by relatively low emergence angles in layer 6 (or
high refraction angles with respect to a surface normal).
Furthermore, it will be understood that, when angle .alpha. is
sufficiently low, the angular span of fan of rays 808 will also be
relatively low. Moreover, the divergence of fan of rays 808 can be
easily controlled by varying slope angle .alpha. of the respective
face 16. For example, when a high degree of light collimation is
desired, the fan of rays 808 may be provided with a very low
divergence by making angle .alpha. very low, accordingly.
[0167] As the extracted light emerges from the core of waveguide 4,
it further crosses buffer layer 6 and eventually strikes its outer
surface 14. Surface 14 representing the optical interface between
layer 6 and light extracting layer 20 is configured for an
unimpeded light passage into layer 20. Particularly, the inner
layer 106 of the light turning film exemplifying layer 20 may be
provided with the refracting index approximately matching the
refractive index of layer 6, in which case TIR and Fresnel
reflections can be substantially suppressed.
[0168] Referring further to FIG. 10, a fan of rays 810 represents
light which is extracted from waveguide 4 and which further
propagates into layer 106. Obviously, when the refractive indices
of layers 6 and 106 are matched, fan of rays 810 can have
essentially the same narrow angular span and low slope with respect
to the prevailing plane of system 2 as fan of rays 808.
[0169] Referring yet further to FIG. 10, the corrugated boundary
154 between layer 106 and 104 comprises a plurality of asymmetric
corrugations 108 defined by two different slopes of the opposing
facets forming each of said corrugations. As explained in the above
examples, corrugations 108 redirect light propagating at low angles
towards a normal to the prevailing plane of system 2, as
illustrated by a fan of rays 812.
[0170] It will be appreciated that fan of rays 812 may have a
slightly different angular span than fan of rays 810 due to the at
least one refraction occurring at boundary 154. However, it will
also be appreciated that corrugations 108 may be designed to result
in the angular span of fan of rays 812 still being sufficiently
narrow.
[0171] As light redirected by the light turning film of layer 20
exits from system 2 along a normal to surface 110, it remains
confined within a relatively narrow angular cone, as illustrated by
a fan of rays 814. When exiting from surface 110, the out-of-normal
rays may undergo some refraction further away from the surface
normal since the refractive index of the outside medium is lower
than that of layer 104. However, when the angular distribution of
fan of rays 812 is sufficiently narrow, the angular distribution
within the emergent fan of rays 814 will also be relatively
narrow.
[0172] Thus, system 2 can emit highly collimated light from its
frontal surface without employing traditional collimating elements
such as lenses or mirrors. Accordingly, it can be shown that the
process of light extraction and collimation can continue along the
light propagation path in waveguide 4. As illustrated by a fan of
rays 806 representing light propagating at greater-than-TIR angles
and reflected from surface 12, layer 6 depletes light from
waveguide 4 in a controlled manner by "shaving-off" only a narrow
cone of the uttermost rays. The rays escaping into buffer 6 obtain
their sub-TIR angles with respect to surface 12 due to TIR from
features 8. Since features 8 are distributed along the longitudinal
axis 200 of waveguide 4, they will continue providing additional
angular bias to the guided light and thus result in continuous
light extraction from system 2 through surface 110.
[0173] In view of the above description, it will be appreciated
that the collimating function of system 2 was achieved using
simple, non-collimating and non-focusing elements for
light-deflection, such as shallow surface microstructures having
planar surfaces. The prior-art devices used for directing light
into a relatively narrow emission cone ordinarily use various
complex-shape collimating optical elements such as spherical or
aspherical lenses, parabolic or spherical mirrors, as well as
arrays of such optical elements in various combinations. In
contrast, the above illustrated embodiments of system 2 use no such
complex shapes of elements.
[0174] Furthermore, the conventional devices employing lenses,
mirrors or their arrays and commonly require precise positioning of
the light emitting features with respect to the collimating
elements. Particularly, each light emitting feature should
typically be positioned along the optical axis and in focal area of
the respective collimating element. Contrary to that, the layers or
individual light deflecting or redirecting features of system 2 do
not necessarily require any special positioning or alignment with
respect to each other except the very basic alignment or
positioning of the layers with respect to each other. Thus, the
relatively simple and manufacturing-friendly structure of system 2
can be advantageously selected for a number of illumination
applications requiring at least some degree of collimation, such
as, for example, directional wide-area illuminators, LED panel
luminaires for general or special lighting, spotlights, accent
lights, flashlights, backlights with a limited emission angle, and
the like.
[0175] FIG. 11 illustrates an alternative light-collimating
variation of system 2 in which buffer layer 6 has corrugated
external surface 14 and light extracting layer 20 is disposed in
contact with layer 6 conforming to the relief of surface 14.
Additionally, light extracting layer 20 has a higher refractive
index than layer 6 so that the pair of layers 6 and 20 forms a
light turning structure similar to the light turning film described
in the above examples. Accordingly, corrugations 108 are now formed
by the corrugated boundary between layers 6 and 20.
[0176] Each corrugation 108 includes facet 124 configured for
refracting light towards surface 110 and adjacent facet 126
configured for reflecting light by means of TIR generally along a
normal to surface 110. Thus, facets 124 and 126 may be configured
to provide nearly 90.degree. light bending by two stages: the first
stage being the refraction at facet 124 and the second stage being
TIR at facet 126. It will be appreciated that the slope of facets
124 may be advantageously selected to intercept and bend
substantially all of the light escaping from waveguide 4 into
buffer layer 6.
[0177] Similarly to the above-described embodiments and examples,
since buffer layer 6 has a lower refractive index than the core of
waveguide 4, it provides the required asymmetry in refractive
indices at the optical interfaces formed by surfaces 10 and 12 so
that light escapes from waveguide 4 primarily through layer 6 and
the light-turning structure formed by layers 6 and 20.
[0178] Accordingly, each face 16 of surface relief features 8
formed in surface 10 has slope angle .alpha. which is low enough to
prevent light leakage through surface 10 but is sufficient to
eventually extract at least a substantial part of light propagating
in waveguide 4. As explained above, this requirement may be
generally satisfied by limiting angles .alpha. to less than
.phi..sub.TIR2-.phi..sub.TIR1, where .phi..sub.TIR1 and
.phi..sub.TIR2 are the TIR angles at the waveguide 4 boundaries
formed by surfaces 10 and 12, respectively. Furthermore, angle
.alpha. may be further restricted to even smaller angles to
minimize the fan-out angle of the light escaping into layer 6
and/or reducing or eliminating the unwanted light leakage resulting
from Fresnel reflections at surface 12.
[0179] It will be appreciated by those skilled in the art that the
Fresnel reflection generally occurs at each light passage from one
refractive medium into another if there is a difference in
refractive indices between the media. Although the Fresnel
reflections usually account for a small fraction of light energy
refracting into the other medium, especially when the difference of
refractive indices is relatively small, the relative amount of
reflected light increases at high incidence angles. Particularly,
Fresnel reflection increases when light travels from a higher
refractive index medium into a lower refractive index medium at an
angle of incidence closely approaching the TIR angle at the optical
interface between the two media. Therefore, referring to the
optical interface formed by the core of waveguide 4 and buffer
layer 6, some light may still reflect from surface 12 back into the
waveguide core even when the incidence angle is lower that the
second TIR angle .phi..sub.TIR2.
[0180] In order to minimize the chance for such rays to exit
through surface 10, angle .alpha. may be limited to a reduced
allowable angular range of
0<.alpha.<<.phi..sub.TIR2-.phi..sub.TIR1. In this case,
sub-TIR rays reflected from surface 12 back towards surface 10 by
means of Fresnel reflection will strike the respective face 16 at
an incidence angle which is still less than .phi..sub.TIR1.
Accordingly, the respective face 16 will reflect said rays towards
surface 12 by means of TIR which will prevent premature light
escaping from waveguide 4 through surface 10 and still result in
light decoupling from surface 12. Thus, the sufficiently low angles
.alpha. may provide a sufficient cushion for light recycling in
waveguide 4 and maintaining its intended operation even in the
presence of unwanted reflections from surface 12.
[0181] Referring further to FIG. 11, the light exit portion of
system 2 may be provided with a light diffusing layer. For example,
the external surface 110 of layer 20 may be patterned to provide
diffusing properties. Alternatively, a light diffusing film or
sheet may be attached to surface 110. Such diffusing layer may be
used to soften the angular distribution of collimated light emitted
by system 2. It may also have a function of masking the intensity
irregularities across the light-emitting surface. The surface
features of the light diffusing layer may be configured to limit
the diffusion angle to a desired beam spread and generally preserve
the directionality of the emitted beam.
[0182] FIG. 12 shows an embodiment of system 2 in which layer 20
has a microstructured surface configured for extracting light from
system 2 by means of refraction and/or TIR. The microstructured
surface of layer 20 represents a linear prism array where each
linear prism extends perpendicular to axis 200 and generally
parallel to linear surface relief features 8 of waveguide 4.
[0183] Similarly to the embodiment of FIG. 6, light source 400
provided on the waveguide's light input edge illuminates the edge
and injects light into waveguide 4. The light injected into the
waveguide propagates in a waveguide mode by bouncing from opposing
surface 10 and 12 of the waveguide. The repetitive pattern of
surface relief features 8 along axis 200 ensures that most light
rays undergo multiple interactions with features 8 along the
propagation path. Surface relief features 8 deflect light from the
original propagation direction by incrementally communicating the
respective light rays a greater out-of-plane angle at each
interaction eventually resulting in light decoupling from waveguide
4 into buffer layer 6. The dihedral angles of the shallow prismatic
corrugations representing features 8 are so selected as to result
in extracting at least a substantial part of light from waveguide 4
by means of incremental deflections.
[0184] Layer 20 preferably having the same or greater refractive
index than layer 6 receives light emerging from waveguide 4 and
layer 6 and further redirects it out from system 2. For this
purpose, the facets of each linear prism of light extraction layer
20 should be positioned to prevent light reflection back into layer
20. In one embodiment, such system 2 may be utilized as a
broad-area luminaire emitting light at an angle with respect to a
surface normal. In one embodiment, such system 2 may be utilized as
a front-light in which case, for example, an image print or
painting (not shown in FIG. 12) may be externally attached to layer
20 or disposed in an immediate proximity to said layer.
[0185] Each linear prism of layer 20 may also be configured to
intercept light rays propagating at a first angle with respect to a
surface normal and redirect them at a greater angle with respect to
the same normal so that light emitted by system 2 is collimated at
least in a plane perpendicular to the longitudinal axis of the
array of prisms. As illustrated in FIG. 12 by example of ray 74, at
least some rays may be emitted from system 2 in a perpendicular
direction with respect to the surface plane.
[0186] The prismatic layer 20 may be made by a variety of means. In
a non-limiting example such layer 20 may be made in the form of a
microstructured sheet or film and then laminated onto layer 6.
Alternatively, layer 20 may be deposited onto layer 6 first and the
prismatic array may be embossed in a subsequent step. In a further
non-limiting example, the fabrication of system 2 may include
initially forming a complete layered structure (including waveguide
4 and layers 6 and 20) with smooth external surfaces and
subsequently providing microstructures in waveguide 4 and layer 20
in a single step.
[0187] FIG. 13 illustrates a front-light implementation of system 2
where layer 20 comprises a screen having at least one light
scattering surface to be illuminated by the light propagating in
waveguide 4. Such layer 20 may be formed, for example, by providing
an opaque or semi-opaque scattering layer on top of surface 14. The
opaque light scattering layer may be provided by a variety of means
and may include, for example, white paint or pigment, colored paint
or pigment, back-scattering film, surface texture, ink,
phosphorescent or fluorescent substance, liquid crystals, etc. In a
further non-limiting example, layer 20 may comprise a screen
containing any print, image, logo, text, symbols, pattern, or the
like features. When the screen includes an image print, the print
may be formed directly on surface 14 using screen printing, digital
printing, offset printing, ink spraying, hand painting, machine
painting or the like processes. Alternatively, the print may be
formed on the surface of an external film, paper or any other
suitable substrate which can be laminated or otherwise attached to
surface 14. Such external film can be made of an opaque material in
which case the printed surface should face layer 6. Alternatively,
the external film with an image print can be made from an optically
transparent material in which case the print may face either layer
6 or away from layer 6.
[0188] Referring further to FIG. 13, rays 72 and 76 having
sufficiently high out-of-plane angles exit from waveguide 4 into
buffer layer 6 at different locations along axis 200 where they are
scattered by layer 20 generally towards a normal to axis 200. In
contrast, ray 72 illustrating the main bulk of light rays
propagating in waveguide 4 at lower out-of-plane propagation
angles, continues to be guided by means of TIR from surfaces 10 and
12 until it obtains the sufficient out-of-plane angle to exit into
layer 6 and to be extracted from system 2 by layer 20. Layer 20 may
be configured to scatter light primarily towards surface 10 and may
optionally incorporate a reflective layer to reflect any stray
light escaping towards the opposing direction.
[0189] According to at least some embodiments, the slope of the
light reflecting faces 16 of surfaces relief features 8 can be made
low enough in order not to perceptibly affect the visual appearance
of surface 10 or the light-scattering screen of layer 20 compared
to the case when surface 10 is perfectly smooth and flat.
Additionally, the surface depressions forming features 8 may be
made substantially shallow so as not to significantly deflect light
rays propagating at low angles with respect to a normal to the
prevailing plane of system 2.
[0190] FIG. 14 further illustrates the operation of system 2 of
FIG. 13, where system 2 has a planar front-light configuration. For
the purpose of illustrating the front-light operation of system 2,
it is assumed that an observer is viewing system 2 from a normal
viewing angle.
[0191] Obviously, each sloped face 16 will slightly alter the light
propagation path between the viewer and the light scattering screen
compared to the case where surface 10 would be perfectly smooth and
planar. Slope angle .alpha. of faces 16 with respect to plane 202
of surface 10 will define how much light will deviate from "an
ideal" path along the surface normal. Accordingly, at any non-zero
angle .alpha., an actual light path 402 from a light
emitting/scattering point at surface 14 to a viewer's eye 660 will
be different from a hypothetical light path coinciding with a
normal 800 to plane 202. Particularly, path 402 will deviate by a
deviation angle .delta. from normal 800 and result in the observer
viewing a different area of layer 20 which is offset from the
respective "on-axis" area by an offset distance 406. This offset
distance 406 depends on angle .alpha., as well as on the refractive
indices and thicknesses of waveguide 4 and layer 6. If layer 20
comprises a high-fidelity image print and angle .alpha. is high,
the observer may experience seeing the neighbouring image pixels
compared to the case of viewing the same print through a perfectly
flat transparent plate.
[0192] However, it will be appreciated that angle .alpha. may be
selected to be sufficiently low so that deviation angle .delta.
will also be low resulting in a negligibly small offset distance
406 so that the observer will not experience a perceptible change
in the visual image quality. By way of example and not limitation,
angle .alpha. may take particular values of 1 angular degree or
less. It can be shown that at such slope angles of faces 16 and
with using some common transparent materials for waveguide 4 and
buffer layer 6, the deviation angle .delta. will also be about 1
degree or less, in which case the offset distance 406 will
generally not exceed 1.5-2% of the combined thickness of waveguide
4 and layer 6. Particularly, if the thickness of the respective
transparent layers of system 2 is about 5 mm, offset distance 406
will generally be less than 100 microns at near-normal viewing
angles.
[0193] Furthermore, the slopes of faces 16 can be made identical to
each other in which case the light path deviations caused by the
plurality of individual features 8 will simply translate the entire
image, as viewed by the observer, perpendicularly to the surface
normal by a small distance and thus will also not cause the loss of
perceptive image fidelity. In the illustrated front light
configuration of system 2, faces 18 can be made perpendicular or
near-perpendicular to surface 10 so that the visible aperture of
faces 18 will be negligibly small, also being substantially smaller
than the visible aperture of faces 16. This should ensure that
faces 18 do not notably interfere with image viewing.
[0194] As substantially all of the light propagating through system
2 can be distributed along waveguide 4 and emitted towards the
image print along the propagation path, the efficiency of system 2
as a front light can be made fairly high. As explained above,
surface relief features 8 preliminary extract light into buffer
layer 6 where the extracted light illuminates layer 20. Layer 20,
in turn, scatters light towards the observer and permanently
extracts at least a substantial portion of light from system 2.
Since the unwanted light leakage through surface 10 is eliminated
or at least substantially reduced by providing sufficiently low
slopes of faces 16 and by providing an asymmetry in refractive
indices of the media adjacent to surfaces 10 and 12 of waveguide 4,
a front light employing system 2 may be used for displaying images
in higher fidelity, bright illumination, and improved contrast
compared to conventional edge-lit front lights.
[0195] FIG. 15 illustrates a variation of light scattering layer 20
which is configured to scatter light into both hemispheres from its
plane. Such layer 20 may be exemplified by a textured matte-finish
surface of an optically transmissive plate or film. Such layer 20
may also be exemplified by a semi-transparent layer of white or
colored paint, ink, phosphors or dye deposited onto an optically
transmissive surface. The appropriate light scattering features of
layer 20 may be formed directly on surface 14 or they may be
provided on a transparent or translucent substrate which can be
attached to surface 14.
[0196] In a backlight variation of this invention, layer 20 may be
provided with light diffusing features which diffuse and
forward-scatter light emerging from layer 6 toward the viewer. By
way of non-limiting example, the light diffusing layer 20 of FIG.
15 can be configured to provide a bright uniform glow from its
surface. It may also be associated with a translucent image print,
see-through LCD screen, colored film and the like, making system 2
suitable for edge-lit signage and general illumination
applications.
[0197] FIG. 16 shows an embodiment of system 2 in which surface
relief features 8 are exemplified by smooth and shallow linear
undulations or corrugations extending generally perpendicular to
longitudinal axis 200 of waveguide 4 and providing surface waviness
in a cross-section parallel to axis 200. In the illustrated
embodiment, faces 16 and 18 both have smooth curved surfaces which
smooth conjugates between each other.
[0198] Similarly to the sharp-corner surface relief features 8
discussed above, the smooth linear undulations may be formed by
casting or extrusion of waveguide 4 from an optically transmissive
polymeric material or formed in a flat sheet of glass or polymer by
micro-replication or material removal. In another example
applicable to both the planar and cylindrical geometries of
waveguide 4, the smooth undulations or corrugations of surface
relief features 8 may be formed by laser ablation or thermal
evaporation. In the case of waveguide 4 made from acrylic, a
CO.sub.2 laser with the operating wavelength of about 10 microns
may be used to selectively ablate the surface material and produce
the required features. Optional polishing may include, for example,
buffing, flame polishing or thermal annealing. In further examples,
the smooth surface 10 may be subjected to any other suitable
surface modification process such as embossing, imprinting or
etching in order to produce the suitable surface relief features 8.
In yet further examples, various processes involving heat sources
may be used to modify surface 10 accordingly by means of material
melting softening, thinning, stretching, etc.
[0199] The surface undulations may be made periodic and having a
constant pitch and/or slope. Alternatively, the width or slope of
each undulation may be made variable in a cross-section along the
propagation path. The amplitude of the undulations may also be made
constant or variable. Particularly, if a constant pitch is
employed, the amplitude or surface slope may be made increasing
along the propagation path in order to compensate the gradual light
depletion in waveguide 4. A useful variation of surface relief
features 8 may include shallow surface undulations having a
variable slope of faces 26 which increases along the intended
optical path. It will be appreciated that the increase of the
slopes of undulations or corrugations along waveguide 4 will
increase the rate of light extraction from the waveguide along the
optical path thus compensating the light depletion and resulting in
an improved uniformity across the waveguide's surface.
Particularly, the individual slopes of faces 16 may be selected to
provide light uniformity within 20-30% across the light emitting
surface of system 2.
[0200] Furthermore, undulations or corrugations forming features 8
may be made essentially random within predefined ranges of width,
height and/or slopes. The distribution of surface relief features 8
along axis 200 may also be made random or ordered. The
randomization or quasi-randomization of features 8 may have a
particular advantage for simplifying the fabrication process as
well as for reducing the glare from surface 10 in the end products
employing system 2. The light extracting properties of waveguide 4
essentially equivalent to making smooth undulations in surface 10
may also be provided by making the thickness of waveguide 4
variable along the propagation path.
[0201] Accordingly, each surface relief feature 8 represented by
smooth surface undulations or corrugations may be configured to
have a reflective face 16 facing the light source and an opposing
face 18 facing away from the light source. Each face 16 may be
shaped so that at least a portion of its surface is generally
inclined at the appropriate angle .alpha. with respect to the
prevailing plane 202 of surface 10. In FIG. 16, slope angle .alpha.
is illustrated by the angle between plane 202 and a tangent 30 to
face 16 of surface relief feature 18. Similarly to at least some of
the above described embodiments, the acceptable range of angle
.alpha. may be selected from the following relationship:
0<.alpha.<.phi..sub.TIR2-.phi..sub.TIR1.
[0202] Each face 16 is designed to introduce an additional
out-of-plane angle to light propagating in waveguide 4 and extract
light propagating at near-critical TIR angles into layer 6. Layer
20 is provided to finally extract light from system 2 and scatter
or direct the extracted light out of the illumination system.
[0203] Also, in a continuing similarly to the above described
embodiments employing sharp-cornered shallow depressions and planar
faces 16 and 18, the depth of each corrugation or undulation can of
features 18 can be made sufficiently small relatively to the width
so as to result in very low slope angles that faces 16 and 18 make
with plane 202. On one hand, it allows for distributing light along
a considerable length of waveguide 4 since each feature 18
redirects only a small fraction of light propagating in waveguide 4
and extracts only a portion of light striking its surface allowing
the rest to be guided further through the waveguide by means of
TIR. On the other hand, low slope angles provide for low deviation
angles .delta. and small offset distances 406 for the viewer, which
makes system 2 particularly suitable for low-distortion edge-lit
front lights.
[0204] FIG. 17 shows an embodiment of waveguide illumination system
2 in which an external cladding layer 22 is provided on top of
surface 10. Layer 22 may be made from the same or similar material
that buffer layer 6 and may have an external boundary 122 with the
outside medium such as air. Cladding layer 22 may provide
protection of surface 10 from abrasion, scratches, contamination or
optical contacting with other bodies or substances which may
adversely impact the light guiding properties. Ordinarily, the
refractive index of cladding layer 22 should not exceed the
refractive index of buffer layer 6 to provide for maximum isolation
of light in waveguide 4. However, it should be understood that
layer 22 may also have any other suitable refractive index. It will
be appreciated that even when the refractive index of layer 22 is
the same lower than that of layer 6, system 2 may still operate in
the manner described above. Although at least some of the light
rays propagating in waveguide 4 and obtaining angles lower than
critical TIR angle .phi..sub.TIRC with respect to the surface
normal may transiently exit into layer 22, the boundary 122 of
layer 22 with the even lower-index outside medium will ensure that
these rays will reflect from surface 122 by means of TIR. The rays
reflected from surface 122 can then return back to waveguide 4
where they can ultimately escape through buffer 6 and can be
further extracted from system 2 by layer 20.
[0205] FIG. 18 illustrates an embodiment of waveguide illumination
system 2 where system 2 has a cylindrical configuration. By way of
example, waveguide 4 may be represented by a large-core polymer
optical fiber (LCPOF). LCPOF may also include an optional cladding
layer (not shown in FIG. 18) surrounding the fiber core. Surface
relief features 8 are formed in the side of the cylindrical
waveguide 4 opposite to the intended light emission direction. The
opposing side of cylindrical waveguide 4 is provided with buffer
layer 6 having the refractive index lower than the refractive index
of the fiber's core. Light extraction layer 20 is provided on top
of layer 6 and may comprise, by way of example, a light turning
film designed to accept light emerging at low angles from the fiber
and redirect it at a higher angle with respect to the fiber's
longitudinal axis. Each feature 8 may have face 16 facing the light
source and adjacent face 18 facing away from the light source. The
slope of each face 16 with respect to the longitudinal axis 200 of
waveguide 4 is sufficiently low in order to prevent light escape
through that face and in order to cause the extraction of
relatively small portions of light into layer 6 along the
propagation path according to the principles discussed above.
[0206] In operation, light rays initially propagate in the fiber's
core of waveguide 4 at propagation angles permitting for TIR from
the longitudinal walls of the fiber, that is at the incidence
angles with respect to a surface normal greater than critical TIR
angle .phi..sub.TIRC. Each ray having a sufficient out-of-plane
propagation angle eventually strikes one or more faces 16 which
progressively communicate greater out-of-plane propagation angles
to the ray at each interaction. As any ray reaches the minimum
out-of-plane angle sufficient for suppressing TIR at the boundary
with buffer layer 6, it can escape into layer 6 and can be further
directed by the light turning film of layer 20. Particularly, the
light turning film may be configured to emit light in a relatively
narrow range towards a perpendicular to the fiber's axis at least
in a cross-sectional plane parallel to said axis. Additionally,
since the fiber ordinarily has a circular or elliptical transversal
cross-section, the cylindrical configuration of the waveguide 4 may
also provide at least some light collimation in the plane
perpendicular to the longitudinal axis 200. Therefore, it will be
appreciated that system 2 having a cylindrical configuration may be
configured to collimate light in one or two dimensions and emit the
collimated light perpendicular to the fiber along its entire length
thus providing an efficient side-emitting fiber illuminations
system.
[0207] It should be understood, however, that the application of
cylindrical configurations of waveguide 4 is not limited to the
side emitting fibers but also includes various light pipes, edge
illuminators or any suitable illumination systems which may benefit
from the elongated shape of the light distributing waveguide. In a
cylindrical configuration, waveguide 4 may have any suitable shape
in a cross-section perpendicular to the longitudinal axis 200 of
the waveguide. Suitable cross-sectional shapes may include but are
not limited to: circular, elliptical, square, rectangular,
hexagonal, trapezoidal or other shapes having any number of sides
each having straight or curved profiles. The cross-sectional shape
may also be formed by a profile which contour can be made variable
along the longitudinal axis of waveguide 4.
[0208] FIG. 19 depicts an embodiment of waveguide illumination
system 2 illustrating the incremental increasing the out-of-plane
angle of the guided light by multiple TIR reflections from
respective surface relief features 8. Referring to FIG. 19, a light
ray emitted by edge-coupled light source 400 initially propagates
in waveguide 4 at an incidence angle with respect to surface 12 far
exceeding the critical TIR angle .phi..sub.TIRC. Surface relief
features 8 formed in surface 10 are distributed along the
longitudinal axis of the waveguide.
[0209] The plurality of features 8 alters a generally planar
cross-sectional outline of surface 10. Each feature 8 is formed by
a shallow recess or depression in surface 10 and has two opposing
adjacent faces, 16 and 18. Face 16 is facing the light source and
has a low slope angle with respect to the prevailing plane of
surface 10. Face 18 is facing away from the light source and has a
generally higher slope with respect to the prevailing plane of
surface 10.
[0210] As the light ray randomly encounters features 8 on its path,
it strikes the respective faces 16 and reflects from them by means
of TIR. Since the angle of reflection is equal to the angle of
incidence with respect to a normal to face 16, the ray
incrementally obtains a greater out-of-plane angle at each
interaction with features 8 and continues to propagate along the
longitudinal axis of waveguide 4. This process is continuing until
the angle of incidence to surface 12 exceeds the TIR angle at that
surface in which case the ray escapes into buffer layer 6.
[0211] It will be appreciated that, depending on the initial
propagation angle, light may propagate different distances in
waveguide 4 until it exits into buffer layer 6, even if the slopes
of reflective faces are kept constant. Considering that the
conventional light sources have at least some beam divergence, at
least a substantial portion of light input through the waveguide's
edge can be effectively distributed along the longitudinal axis of
waveguide 4 and extracted from surface 12 at generally low
emergence angles with respect to that surface.
[0212] As the light ray decoupled from the core of waveguide 4
further propagates through layer 6, it reaches light extracting
layer 20. The light turning film of layer 20 intercepts light
emerging from waveguide 4 and redirects it at a normal angle with
respect to the prevailing plane of system 2. Thus, light becomes
effectively extracted from system 2 with collimation. This
operation makes system 2 particularly suitable for making
directional illumination systems, such as, for example,
side-emitting large-core fibers and planar edge-lit LED panels.
[0213] Various parameters of surface relief features 8 may be
varied to fine tune the light distribution and emission from the
surface of waveguide 4. These parameters include but are not
limited to: width, height or slope of reflective faces 26, general
shape and distribution of features 8 along the propagation path,
etc. An optional mirrored surface may be provided along surface 10
of waveguide 4 to reflect any stray light that may escape from the
waveguide towards a direction opposing to layer 20.
[0214] It should be understood that the differential between the
stepped drop in refractive indices outwardly at surface 10 and 12
of the waveguide 4 is important to force light to escape from the
waveguide's core generally through surface 12 and not through
surface 10. As illustrated above, such differential can be easily
obtained by providing buffer layer 6 having a lower refractive
index than the core of waveguide 4 but higher than that of the
outside medium. Since the addition of the buffer layer can
generally lower the acceptance angle of waveguide 4 compared to the
bare waveguide core surrounded by low-n air on both sides, a light
collimating feature may be associated with light source 400 or with
the light input edge of the waveguide in order to narrow the
natural divergence of light beam emanated by the light source.
[0215] FIG. 20 shows a collimating element 440 attached to the
light input edge of waveguide 4. Element 440 can be made from
transparent material and may have any optical configuration
suitable for coupling light from source 400 into waveguide 4 at a
limited range of propagation angles permitting for TIR from both
surfaces 10 and 12. Element 440 may be attached to the light input
edge using an optical adhesive. Alternatively, collimating element
440 may be provided as an integral part of waveguide 4 and formed
by tapering the light input edge of waveguide 4 accordingly.
Collimating element 440 is preferably configured to have opposing
concave walls reflecting light by means of TIR. However,
particularly when the divergence of light from source 400 is too
high for TIR and/or when source 400 is coupled to the light input
edge using a refractive medium such as optical adhesive or
encapsulant, the concave walls of element 400 may be mirrored for
increasing the acceptance angle of the collimating element.
[0216] Considering that waveguide 4 will only effectively conduct
light that enters its edge within a certain acceptance cone, let's
define an acceptance angle .theta..sub.max of waveguide 4 being the
half-angle of this acceptance cone. It will be appreciated by those
skilled in the art that acceptance angle .theta..sub.max can be
found from the following expression:
sin .theta. max = n 1 2 - n 2 2 n 0 , ##EQU00001##
where n.sub.1 is the refractive index of the core of waveguide 4,
n.sub.2 is the refractive index of buffer layer 6 and n.sub.0 is
the reflective index of the medium light is traveling through
before entering waveguide 4. When source 400 is coupled to the
light input edge of waveguide 4 through a layer of air
(n.sub.0.apprxeq.1), sin .theta..sub.max= {square root over
(n.sub.1.sup.2-n.sub.2.sup.2)}. Accordingly, a numerical aperture
(NA) of waveguide 4 can be defined as NA=n.sub.0 sin
.theta..sub.max, or, in the case of source to waveguide coupling
through air, NA=sin .theta..sub.max= {square root over
(n.sub.1.sup.2-n.sub.2.sup.2)}.
[0217] As illustrated in FIG. 20, collimating element 440 may be
useful to inject far off-axis rays into waveguide 4 so that they
can propagate by means of TIR in the waveguide's core and can also
be distributed and emitted through surface 14 as explained in the
examples above. Collimating element 440 should be preferably
configured to provide at least some initial divergence of light
upon entering into waveguide 4. Particularly, when it is desired
that system 2 can begin emitting light within a short distance from
the light input edge, collimating element 440 may be configured to
provide the initial divergence approximating the acceptance cone
with the half angle being close to the acceptance angle
.theta..sub.max of waveguide 4.
[0218] FIG. 21 shows an embodiment of waveguide illumination system
2 comprising light source 400, waveguide 4, buffer layer 6 and
light extraction layer 20. Waveguide 4 has a wedge shape with the
light input edge being wider than the opposing terminal edge and
with the thickness of waveguide 4 gradually decreasing along the
light propagation path. The taper of waveguide 4 defined by slope
angle .alpha. is relatively low so that waveguide 4 still has a
generally planar configuration. Surface 10 is generally smooth and
may have one or more flat portions and at least one inclined
surface portion defining an extended surface relief feature 8 and
its reflective face 16 facing the light guided in waveguide 4.
Buffer layer 6 is provided on opposing side waveguide 4 and is
attached to surface 12 with a good optical contact. Layer 6 is
further followed by light extraction layer 20.
[0219] The refractive index of buffer layer 6 is lower than the
refractive index of waveguide 4 which creates a differential in the
refractive index drop at surfaces 10 and 12 and enables the
preference for light escaping through surface 12 when the light is
bent to sufficiently high out-of-plane angles. Light extraction
layer 20 is exemplified by a light turning film which turns light
by almost 90 degrees so that the light rays emerging from layer 6
at low angles with respect to surface 14 can be directed generally
towards a normal to the prevailing plane of system 2. Layer 20 may
also comprise a light scattering surface, screen, image print,
etc., as discussed above.
[0220] Similarly to the above-described principles, light rays
propagating in a waveguide mode and having different out-of-plane
angles will emerge from system 2 at different locations along the
propagation path, depending on the slope of face 16, resulting in
light distribution and extraction along the extent of the light
emitting region of waveguide 4. It will be appreciated that slope
angle .alpha. can be made sufficiently small (about one degree or
less) which will result in a small angular divergence of light
emerging into buffer layer 6. Accordingly, the light turning film
of layer 20 can be configured to turn light emerging from buffer
layer 6 by up to 90.degree. away from the surface plane while
preserving the small divergence. As the light beam turned by the
light turning film overcomes TIR and emerges from layer 20, it will
have a well defined directionality and due to being confined within
a finite angular range. Thus, system 2 depicted in FIG. 21 can be
configured to emit light from the entire light emitting region with
improved light collimation.
[0221] FIG. 22 illustrates a front-light variation of system 2
employing a wedge-shaped waveguide 4 in which layer 20 comprises a
screen that back-scatters light generally towards a normal to the
prevailing plane of waveguide 4, so that at least a substantial
part of the scattered light can pass through waveguide 4 and exit
from system 2 through surface 10. Screen 2 may be formed by a
bright, light scattering paint or film for illumination purposes.
Alternatively, screen 2 may comprise an image print viewable
through layer 6 and waveguide 4 and deposited onto surface 14 or
onto an intermediate substrate using any suitable printing
process.
[0222] Referring to FIG. 22, light can propagate considerable
distances along longitudinal axis 200 in a waveguide mode while
undergoing multiple TIR interactions with opposing surfaces 10 and
12. Upon each interaction with a sloped portion of surface 10,
light is progressively communicated a broader angular spread with
respect to axis 200, as illustrated by the example of ray 72. Upon
reaching a sub-TIR angle with respect to surface 12, at least the
uttermost out-of-plane rays, as exemplified by rays 76 and 72, will
exit from the core of waveguide 4 at different locations along axis
200, depending on the initial propagation angles. Accordingly,
light extraction layer 20 extracts the emergent light by means of
scattering which causes at least portions of the scattered light to
overcome TIR at surface 10 and exit from system 2 towards a normal
to the surface.
[0223] FIG. 23 illustrates a further variation of system 2 in which
layer 6 has corrugated outer surface 14 and layer 20 conforms to
this corrugated shape thus also forming a corrugated boundary
between the two layers. Additionally, light extracting layer 20 has
a specularly reflective surface which can be obtained, for example,
by mirroring surface 14 or by depositing a specularly reflective
film or foil onto layer 6. Corrugations 18 can be made identical to
each other and having a constant pitch.
[0224] Referring to FIG. 23, facets 124 facing the light source may
be advantageously configured to reflect light emerging from
waveguide 4 at angles approximately perpendicular to the prevailing
plane of system 2, as illustrated by ray 74, thus providing an
efficient light turning structure of a reflective type. According
to the principles discussed above, waveguide 4 may be configured to
emit light into buffer layer 6 within a narrow angular cone. In
turn, such reflective light turning structure may be configured to
redirect the emitted light towards surface normal while preserving
the narrow beam divergence. It will be appreciated that this will
result in system 2 emitting a highly collimated beam from surface
10.
[0225] It will be understood that system 2 may include any other
variations of light extracting layer 20 which may be configured to
permanently extract light from system 2 and direct and distribute
it according to the specific application. Light extraction layer 20
may also incorporate any other light directing structures which
change the propagation path of light emerging from waveguide 4 at
low angles with respect its major surface. Various modifications of
layer 20 may include lens arrays, prism arrays, mirrors, diffusers,
retroreflective elements, scattering elements, color changing
elements or layers, etc.
[0226] FIG. 24 explains the light collimation function of certain
embodiments of system 2 in further detail. In FIG. 24, light source
400 emits light having a relatively broad angular distribution 900.
When such light enters the light input edge or end of waveguide 4,
it propagates within the waveguide along its longitudinal axis 200
undergoing multiple reflections from surfaces 10 and 12 by means of
TIR. The interactions of light with surface 10 include reflections
from surface relief features 8 (not shown) which gradually
increases the out-of-plane angle of light propagation and
eventually results in light escape from waveguide 4 into low-n
buffer layer 6.
[0227] As explained above, when the characteristic slope angle
.alpha. of surface relief features is sufficiently low, the angular
distribution of light escaping into layer 6 is also very narrow as
it generally subtends an angular range from zero to a small angle
which depends on slope angle .alpha.. Accordingly, when light
extraction layer 20 turns the emerging light by up to 90 degrees,
the angular distribution of light emitted from the broad surface of
system 2 will also be relatively narrow. It will be appreciated
that the light beam emerging from layer 2 may experience some
broadening of the angular distribution compared to its propagation
in the bulk materials of system 2 generally having refractive
indices considerably greater than a unity. Nevertheless, it will
also be appreciated that the angular distribution of light emitted
by system 2 can be made substantially narrower than that of source
400.
[0228] This is illustrated in FIG. 24 by reference to angular
distributions 912, 914, 916, and 918 exemplifying collimated light
emerging from the broad-area surface of system 2 at different
distances from source 400. Each the angular distributions 912, 914,
916, and 918 may be made narrower than the broad distribution 900
of light source 400 at least in a plane in which the reflection by
surface relief features 8 occurs within waveguide 4. Therefore,
system 2 may be configured to not only distribute light from a
compact source over a large area and emit such light from said
large area, thus resulting in a reduced glare, but also provide an
improved collimation and/or directionality of the emitted light
compared to the light beam emitted by the source.
[0229] It will be appreciated that the light-collimating
embodiments of system 2 illustrated above do not generally require
applying any opaque or mirror layer onto surface 10 in order to
prevent light escaping through that surface. Due to the combined
function of surface relief features 8, which provide incremental
out-of-plane deflection of light rays propagating along axis 200,
buffer layer 6, which preliminary extracts only the uttermost
deflected rays from waveguide 4, and light extracting layer 20,
which finally extracts the pre-extracted light from system 2,
virtually no light may be allowed to exit through surface 10 even
though said surface can have a very high optical
transmissivity.
[0230] This is in a sharp contrast to the conventional illumination
systems employing a waveguide, such as edge-lit backlights or
lighting luminaires, for example. In such conventional systems, a
substantial portion of light escapes through an unwanted side of
the waveguide (usually at least 25% and up to 50%) which requires
using a special diffuse or specular reflector to be attached to
that surface. The reflector typically includes a sheet of highly
reflective material which redirects (with some reflection loss) the
escaping light towards the other side of the waveguide. It will be
appreciated that the use of an opaque reflector layer introduces
additional losses (compared to TIR) and precludes the possibilities
of transmitting light in a transversal direction or using the
system as a front light.
[0231] On the contrary, according to at least some embodiments of
the present invention, waveguide illumination system 2 can maintain
high transversal transmissivity and allow for a generally unimpeded
light passage along in a perpendicular direction with respect to
its major surfaces. Therefore, such system 2 may be used for
transmitting light from a different light source in a transversal
direction.
[0232] By way of example and not limitation, system 2 can be
implemented as a light-collimating edge-lit luminaire which may
also be positioned horizontally in the light path of a daylighting
system, such as a skylight located above the luminaire. In such
configuration, system 2 can be configured to provide illumination
by distributing and emitting light emanated by one or more LEDs
attached to the edge of waveguide 4 and, additionally, to transmit
light from the skylight perpendicularly through its body thus
forming a combined solar/electric lighting luminaire.
[0233] FIG. 25 illustrates an exemplary embodiment of system 2
which is used at the light exit end of a skylight 502. Skylight 502
can be ordinarily designed for rooftop installation and
illumination of the building interior with the natural sunlight
through an opening in the roof or ceiling underneath the skylight's
light-collecting aperture.
[0234] Referring to FIG. 25, skylight 502 includes a dome-shaped
diffuser sheet 260, and optional reflective side walls 512 and 514.
Diffuser sheet 260 can be made from an optically clear plastic
material and may have at least one microstructured surface to
improve light diffusion. Walls 512 and 514 are preferably covered
with a sheet or film of secularly reflective material to aid in
light channeling from sheet 260 to the light-emitting opening
below. The light emitting opening of skylight 502 may have
approximately the same aperture as the light-receiving aperture of
sheet 260. Alternatively, skylight 502 may be made in a tapered
version, in which case the above apertures may have different
transversal dimensions with respect to each other.
[0235] Referring further to FIG. 25, waveguide illumination system
2 is implemented in the form of a, optically transmissive planar
panel and includes waveguide layer 4, low-n buffer layer 6 and
light extracting layer 20 formed by a light turning film. Light
source 400 including a strip of high-brightness LEDs is positioned
adjacent to the light input edge of waveguide 4. At least a
substantial portion of surface 10 is provided with sufficiently
shallow surface relief features 8 each including low-slope face 16
and adjacent face 18. The orientation of surface relief features 8
is such that each face 16 faces source 400 and each face 18 is
turned away from source 400. System 2 having the above layered
structure and the form factor of a relatively thin panel is
positioned below skylight 502 in the respective light-emitting
opening of the skylight so that the prevailing plane of waveguide 4
extends perpendicularly to a vertical axis 44.
[0236] In electric lighting operation, waveguide 4 receives light
from the linear array of LEDs at its light input edge and guides
said light by means of TIR. Accordingly, light injected through the
light input edge propagates in a waveguide mode along axis 200
towards the opposing terminal end of waveguide 4. According to the
principles described above, surface relief features extract light
into buffer layer 6 along the propagation path. In turn, the light
turning film of light extracting layer 20 finally extracts light
downwards. Optionally, system 2 may be configured to provide a
prescribed degree of collimation and emit directional light along
vertical axis 44. Accordingly, since light escape through surface
10 is minimized or eliminated, substantially all of the light
emitted by source 400 and distributed along waveguide 4 is emitted
into the building interior. With the exception of light which is
absorbed or scattered during propagation in waveguide 4 or layers 6
and 20, practically no additional light is lost in the system.
Importantly, no light is directed back towards sheet 260 which
would otherwise constitute a major energy loss in the case when
waveguide 4 would employ conventional microstructures or other
types of prior-art light extraction features.
[0237] In daylighting operation, system 2 receives a diffuse beam
of sunlight emerging from sheet 260 and transmits it further
downwards through its body. Since all respective layers and
surfaces of system 2 are optically transmissive, the sunlight
transversally passes through the panel without undergoing
substantial reflection, backscattering or attenuation. When
additional diffusing of daylight is necessary or when masking the
portions of skylight disposed above the light emitting opening is
desired, system 2 may further comprise one or more light diffusing
layers operating in the transmissive mode. In view of the
above-described operation within a skylight, it will be appreciated
that system 2 can emit artificial light from its broad-area surface
and also doubles as a skylight luminaire by allowing the daylight
into the building interior and optionally providing enhanced light
diffusion.
[0238] It will be appreciated that a unique operation of system 2
is obtained, at least in part, by employing a two-stage light
extraction mechanism. The first stage includes incremental light
deflection by surface relief features 8 and the second stage
includes beam turning by light extracting layer 20. Buffer layer 6
separates these stages from each other and provides the functional
differential in the refractive index drop at the opposing sides of
waveguide 4. By employing these features, in combination, system 2
suppresses light extraction through the unwanted side of waveguide
4 and can be configured to emit light primarily through the
designated side or surface. When desired, as illustrated above, it
may also be configured to almost completely shut-off light emission
from the unwanted side despite being optically transmissive and
allowing light to pass transversely through its body.
[0239] FIG. 26 shows an alternative arrangement of the layers
around waveguide 4, where surface 10 is configured to be
substantially flat but surface 12, instead, has surface relief
features formed by smooth and shallow corrugations or undulations.
Accordingly, buffer layer 6 and light extraction layer 20 adjacent
to surface 12 may conform to the shallow relief features of surface
12 or may, alternatively, have flat external surfaces.
[0240] Accordingly, the refractive index of buffer layer 6 being
greater than that of the outside medium and lower than that of
waveguide 4 provides the functional difference between TIR angles
.phi..sub.TIR1 and .phi..sub.TIR2 at surfaces 10 and 12,
respectively. Therefore, the incremental angular bias of light
propagation caused by the undulations of surface relief features 8
along the propagation path in the waveguide 4 will result in light
escaping primarily through surface 12 while surface 10 will
continue to reflect substantially all light by means of TIR.
[0241] By way of example and not limitation, layer 20 of the
embodiment of FIG. 26 may be formed by a scattering layer or a
print formed directly on surface 14 of layer 6 or formed on a
separate substrate which can be laminated onto surface 14. When
laminating layer 20 onto surface 14, a soft roller may be used for
pressing layer 20 against surface 14 in order to fill the shallow
depressions in surface 14 and avoid forming of air gaps.
[0242] The amplitude of the undulations forming surface relief
features 8 may ordinarily be very small so that the relief of layer
20 will be virtually unnoticeable when viewed from a distance.
Additionally, the shallow surface relief features 8 will deflect
light propagating between the visible surface of layer 20 and
viewer's eye 660 by only a small amount causing no perceptible
visual distortions.
[0243] Ray 72 illustrates a light path in waveguide 4 at relatively
low out-of-plane propagation angles allowing for TIR at both
surfaces 10 and 12. Each TIR of ray 72 from face 16 of the
respective feature 8 will generally result in a greater
out-of-plane angle thus introducing additional angular bias and
widening the angular distribution of light guided in waveguide 4.
Accordingly, ray 72 may eventually obtain an incidence angle less
than the TIR angle with respect to surface 12 when striking one of
the successive faces 16 and exit into layer 6 where it will be
scattered by layer 20 towards the viewer. Ray 74 illustrates a ray
path of the light being extracted from system 2 as it already
obtained a sufficiently high angle with respect to the longitudinal
axis of waveguide 4. As ray 74 exits from waveguide 4 into buffer
layer 6, it strikes the light scattering surface of layer 20 and
can be directed towards the viewer thus providing the illumination
function of an edge-lit front light.
[0244] FIG. 27A through FIG. 27I illustrate various shapes of
surface relief features 8. In FIG. 27A, feature 8 is shown to
include face 16 inclined at a low slope angle .alpha. to the
prevailing plane of surface 10 and opposing face 18 inclined at a
substantially greater slope angle to said plane. In FIG. 27B, face
18 is shown being perpendicular to the prevailing plane of surface
10. It should be understood that the illustrative case of FIG. 27B
may also include variations of face 18 having angles which are
close to 90.degree. but not exactly normal. Particularly, when the
respective feature 8 is formed in surface 10 by replication
processes like injection molding, compression molding, embossing,
or extrusion processes, face 18 may have a suitable draft angle of
3-4.degree. with respect to a surface normal. Such draft angle may
be necessary, for example, to facilitate mold release.
[0245] In FIG. 27C, surface relief feature 8 is shown to
additionally comprise a flat top portion 860 which extends parallel
to the plane of surface 10 and may also be configured to reflect by
means of TIR. In FIG. 27D the top portion 860 has a curved shape in
a cross-section.
[0246] FIG. 27E shows feature 8 in which faces 16 and 18 are
disposed symmetrically and have about the same slope angle with
respect to the plane of surface 10. Such a symmetric configuration
of features 8 may be selected, for example, for the case where
waveguide 4 is illuminated from both opposing edges or terminal
ends.
[0247] In FIG. 27F, surface relief feature 8 is formed by a smooth
undulation or shallow recess in surface 10, where faces 16 and 18
are formed by the opposing side surfaces of the undulation. FIG.
27G shows surface relief feature 8 formed by a funnel-shaped
shallow cavity in surface 10. It should be understood that a
relatively small portion of face 16 facing light source 400 (not
shown in FIG. 27G) may have an arbitrary high slope angle. Such
portion may include, for example, the small area immediately
adjacent to the vertex of the funnel. However, it is generally
preferred that at least a major portion of face 16 still has
relatively low slope angle .alpha. with respect to the prevailing
plane of surface 10.
[0248] It will be appreciated that there is a great variety of
possible shapes that can be used for surface relief features 8.
Accordingly, any suitable profile or any suitable perturbation or
irregularity of otherwise smooth and flat surface 10 may be used to
form individual surface relief features 8, including any ordered or
random surface relief structures, bumps, recesses, grooves,
corrugations, surface waviness, etc., provided that they can
introduce the required additional angular bias for light
propagating in waveguide 4 along its path and cause controlled
light leakage into layer 6.
[0249] FIG. 27H shows surface relief feature 8 formed by a textured
micro-relief portion of surface 10. The textured portion may be
surrounded by non-textured, flat portions of surface 10 thus
creating a distinct, well defined microstructured area of the
respective feature 8. The texture may be formed in a random or
ordered manner, provided that each elementary micro-relief feature
has a low aspect ratio (low height to width) so that the respective
surface slope angles do not exceed a certain maximum angle which
would cause light leakage from waveguide 4 through surface relief
feature 8. According to one embodiment, considering that the walls
of micro-relief features of the surface texture may have a variable
slope from top to bottom, the walls of each micro-relief feature
should have a sufficiently low prevailing slope to provide the
above discussed operation. Additionally, each micro-relief feature
should have a generally smooth surface on the scale which is
comparable to the wavelength of light of source 400. This
requirement is aimed at minimizing or preventing light scattering
and/or reflecting at higher than the prescribed angles with respect
to the longitudinal axis of waveguide 4 or the plane of surface
10.
[0250] It should be understood that surface relief feature 8 are
not limited to recesses or undulation-type surface microstructure
but may also be formed by light-deflecting surface protrusions or
indentations of the appropriate shape and even by varying the
thickness of waveguide 4 along the light propagation path, provided
the prevailing slope of the respective surface structures is less
than the prescribed maximum angle.
[0251] By way of example and not limitation, the surface of the
each textured area of FIG. 27H may include an array of microlenses.
According to the principles discussed above, each of the microlens
should have a sufficiently low profile ensuring that the slope of
the microlens walls with respect to surface 10 does not exceed the
maximum allowed angular value so that system 2 may operate in the
manner described in the above embodiments.
[0252] In FIG. 27I, an alternative configuration of surface relief
feature 8 is illustrated which includes a low-aspect-ratio,
smooth-walled protrusion in surface 10. Referring to a projection
of surface relief feature 8 onto a plane parallel to surface 10,
such protrusion may have an elongated shape, a round shape or any
other suitable shape or outline. In a non-limiting example, a
two-dimensional protrusion may be formed by depositing a micro
droplet of UV-curable polymer onto surface 10. The polymer should
preferably have a low surface tension allowing for the droplet to
obtain the desired low-profile shape before curing. Alternatively,
surface 10 of waveguide 4 may be specially treated or coated to
increase its surface tension and obtain the desired surface
wettability.
[0253] Referring to FIG. 27I and to the example of forming surface
relief feature by a UV-curable droplet, slope angle .alpha. may be
associated with the angle at which the liquid contacts the surface.
This contact angle is commonly known as the wetting (or dihedral)
angle that a liquid droplet makes to a solid surface. Accordingly,
in the illustrated example, the wetting angle of the droplet with
respect to surface 10 should be kept sufficiently low in order to
form a low-profile surface relief feature 8 which could deflect
light by a sufficiently small angle with respect to its original
propagation direction.
[0254] It is noted that surface relief features 8 may be arranged
in surface 10 in a variety of ways. For example, surface relief
features 8 may be formed in a parallel array of strips or bands
extending perpendicular to longitudinal axis 200 of waveguide 4.
The respective strips or bands can be made substantially straight.
Alternatively, they may have a constant curvature or even some
waviness.
[0255] FIG. 28 shows an embodiment of system 2 in which waveguide 4
has a shape of a rectangular plate or slab and in which surface
relief features 8 are arranged in a parallel linear array which
longitudinal axis is perpendicular to axis 200. FIG. 28 also shows
an exemplary configuration of light source 440 which includes a
strip of LEDs where each LED is provided with individual
collimating element 440. Each collimating element 440 intercepts
light emitted by the respective LED and collimates said light
towards axis 200. Collimating element 440 may be configured to
provide light collimation in at least one plane or dimension. By
way of example and not limitation, collimating element 440 of FIG.
28 may have a linear geometry and may be characterized by a
longitudinal axis and an optical plane both of which extending
perpendicular to the plane of the drawing. It will be appreciated
that such linear collimating element 440 will provide light
collimation in a plane which is parallel to the respective plate or
slab. In a further non-limiting example, collimating element 440
may have a round aperture and may be configured to collimate light
in two orthogonal dimensions such as those parallel and
perpendicular to the prevailing plane of the rectangular plate or
slab.
[0256] FIG. 29 shows an alternative exemplary arrangement of
surface relief features 8 across surface 10 and also shows an
alternative configuration of a plurality of collimating elements
440. Referring to FIG. 29, each surface relief feature 8 is formed
by a textured area as explained in relation to FIG. 27H and is
separated from the adjacent surface relief features 8 by a spacing
area in an ordered array pattern. The size of each surface relief
feature 8 may be varied. Particularly, the size may increase with
the distance from light source 400 to compensate the light
depletion in waveguide 4 along axis 200. Alternatively, the density
of surface relief features 8 in the array may increase with the
distance from source 400 for the same reason. Referring further to
FIG. 29, an array of light collimating elements 440 is arranged on
a single transparent substrate and form a solid piece structure
which can be placed between the LED strip and waveguide 4.
Accordingly, light collimating elements 440 of FIG. 29 may also be
configured to emit light along axis 200 and provide light
collimation in one or two dimensions.
[0257] FIG. 30 depicts an embodiment of illumination system 2 in
which buffer layer 6 is provided on the microstructured surface 10
of waveguide 4. Light extracting layer 20 provided on the external
surface of the buffer layer 6, accordingly so that surface 12 is
exposed to the outside medium. Layer 6 should provide a good
optical contact between waveguide 4 and light extracting layer 20.
It may be formed as a conformal thin coating (conforming to the
shape of surface relief features 8) of a constant thickness.
Alternatively, as shown in FIG. 30, layer 6 can be made of a
sufficient thickness to fill the depressions in surface 10 and
provide a smooth external surface. The light extracting layer 20 of
FIG. 30 can be configured to provide strong backscattering of light
emerging from waveguide 4 with a highly diffuse reflectivity in
which case system 2 can be advantageously used, for example, for
general lighting applications. Layer 20 may also be associated with
an image print in which configuration the system 2 can be used, for
example, as a front light, decorative light or as an illumination
cover for signage or painting/printing arts.
[0258] In operation, referring to FIG. 30, ray 72 propagating in
waveguide 4 strikes surface 10 where it is reflected from one of
surface relief features 8 by means of TIR due to the relatively low
propagation angle with respect to axis 200. The slope of face 16
with respect to the prevailing plane of surface 10 is sufficiently
low so as to result in the incremental increase in the out-of-plane
angle of ray 74 without causing it to exit through surface 10. As a
result of TIR from face 16, ray 72 obtains slightly lower incidence
angle with respect to normal 800 and further propagates towards the
opposing surface 12 of waveguide 4.
[0259] Since the drop in refractive index outwardly from waveguide
4 at its surface 12 is greater than that at surface 10, the
incidence angle of ray 72 into surface 12 is not sufficient to
overcome TIR at that surface. Therefore, ray 72 continues to
propagate in the waveguide mode through the core of waveguide
4.
[0260] In contrast, rays 74 and 76, deflected by the preceding
surface relief features 8 (not shown), strike surface 10 at smaller
angles with respect to normal 800, said angles being less than the
critical angle of TIR at the interface between waveguide 4 and
low-n layer 6. As a result, rays 74 and 76 exit from the core of
waveguide 4 and strike layer 20 which scatters the extracted rays
back towards the viewer's eye 660.
[0261] FIG. 31 depicts an embodiment similar to that of FIG. 30
except that light extracting layer 20 is configured to transmit and
diffuse light emerging from waveguide 4 towards the opposing side.
In such configuration, system 2 can be used, for example, for
backlights, transmissive sign displays or area illumination.
[0262] Referring to FIG. 30 and FIG. 31, exemplary rays 74 and 76
illustrate light emerging from waveguide 4 and illuminating layer
20. As it is further illustrated, the emerging light can be further
directed by layer 20 to provide either front light or back light
operation depending on the configuration of the light
scattering/diffusing layer. Furthermore, the surface of layer 20
can be illuminated from one or more additional directions and by
one or more additional light sources such as, LEDs, lighting
luminaires or sunlight. By way of example and not limitation, when
layer 20 comprised a transmissive light diffuser, it can be
illuminated from the appropriate edge of waveguide 4 by a strip of
LEDs and by a beam of sunlight incident onto layer 20 perpendicular
to its surface, in which case system 2 of FIG. 31 may have
operation similar to FIG. 25
[0263] Referring to FIG. 31, a distant light source 890 is
illustrated which illuminates system 2 perpendicularly to its
plane. Ray 892 passes through the transparent layers of waveguide 4
and buffer layer 6 after which it strikes layer 20. In turn, layer
20 scatters ray 892 similarly to rays 74 and 76, all of which are
extracted from the waveguide toward a viewer 600. When light source
890 is exemplified by sunlight, such as that delivered from a
skylight or a similar device, system 2 can be used as a luminaire
for hybrid lighting and can provide combination of natural and
artificial illumination. In this case, light from source 400 can be
advantageously made dimmable in response to the change in the
sunlight intensity so that constant lighting in the room can be
maintained. The corresponding layers of system 2 may be configured
to provide illumination pattern which is generally symmetrical with
respect to no normal 800.
[0264] FIG. 32 depicts an embodiment of system 2 in which light
extracting layer 20 is made from an optically transmissive material
and includes V-shaped prismatic grooves 380 which slope is selected
to redirect light emerging from waveguide 4 into a perpendicular
direction with respect to the prevailing propagation path of light
in the waveguide. It is preferred that a refractive index n.sub.6
of layer 20 is at least equal or greater than that of buffer layer
6 to prevent TIR at the boundary between the two layers.
[0265] Accordingly, rays 74 and 76 emerging first into buffer layer
6 enter layer 20 and strike respective sloped faces of grooves 380
where said rays are reflected by beans of TIR towards along a
normal to the prevailing plane of waveguide 4. Ray 78 strikes an
interface with the air pocket formed by groove 380 and is reflected
by TIR back into waveguide 4 and towards surface 10. However, since
surface 10 represents a boundary with the outside medium (air)
which has substantially lower index than waveguide 4, ray 78 will
eventually reflect from surface 10 and can thus have a further
chance of being fully extracted from system 2 by other grooves 380
along the propagation path.
[0266] It will be appreciated that the arrangement of FIG. 32 can
be configured to efficiently collimate light at least in the plane
of the drawing that is a plane which is parallel to longitudinal
axis 200 of waveguide 4 and which is also perpendicular to the
prevailing plane of waveguide 4. Accordingly, when waveguide 4 of
system 2 is illuminated by a non-collimated or weakly collimated
beam of light at its light input edge or end, the illustrated
embodiment of system 2 can emanate highly collimated light at least
in the above-mentioned plane. It will also be appreciated that the
degree of such collimation can be adjusted in a broad range, for
example, by adjusting the slopes of faces 16 of features 8 and/or
the slopes or configuration of grooves 380. Moreover, additional
angular distribution effects for the emitted light can be achieved
by varying the respective slopes of faces 16 and/or grooves 380
along the longitudinal axis 200 of waveguide 4.
[0267] It will be further appreciated that system 2 may include
various collimated elements attached to the input edge or end of
waveguide 4 which can provide additional means for controlling the
angular spread or distribution of light emitted from the respective
broad surface of system 2. Particularly, when system 2 employs a
planar configuration of waveguide 4 and discrete light sources such
as LEDs, collimating elements 440 such as those illustrated in FIG.
28 and FIG. 29 may be used to additionally collimate the incident
beam in a plane parallel to axis 200 and also parallel to the
prevailing plane of waveguide 4. In this case, system 2 can emit
light which is collimated in two orthogonal planes or dimensions
rather than just in a single plane or dimension. Additionally, it
is noted that the degree of collimation and angular distribution of
light emitted by system 2 may be independently controlled in each
of the orthogonal planes or dimensions thus providing a virtually
unlimited number of combinations for the emitted beam parameters.
In a non-limiting example, when used as an area illumination device
such as ceiling-mounted panel luminaire, system 2 may be configured
to illuminate a well defined area below the luminaire while
emitting much fewer or no light towards non-functional
directions.
[0268] According to one embodiment, slope angle .alpha. of each
face 16 may be limited to about three angular degrees in order to
maximize the collimation power of system 2 and/or minimize the
light leakage through surface relief features 8. More particularly,
according to certain embodiments, slope angle .alpha. of each face
16 may be less than two and a half degrees, less than two degrees,
less than one and a half degrees, and less than one degree.
According to one embodiment, the minimum slope angle .alpha. of
each face 16 may be about one half of a degree.
[0269] The slope of the reflecting walls of grooves 380 may be
selected to redirect light emerging from waveguide 4 at any
suitable angle with respect to a normal to the prevailing plane or
longitudinal axis of system 2. Particularly, grooves 380 of light
extraction layer 20 may be configured to redirect the emergent
light so as to result in system 2 generally emitting light at an
off-normal angle from its major broad-area surface. Such an
off-normal angle may take particular values of, for example, thirty
degrees, forty five degrees or sixty degrees. Considering that
system 2 may be configured to emit a major portion of collimated
light in a limited range of angles with respect to the surface
normal, such range may be between zero and thirty degrees, between
fifteen degrees and forty five degrees, between thirty degrees and
sixty degrees, for example. Such off-normal illumination may be
useful for a number of applications of system 2. By way of example
and not limitation, system 2 emitting off-normal collimated beam
may be used to illuminate a portion of a wall from a planar
lighting fixture mounted flush with a ceiling, in an application
like accent illumination of wall-mounted fine art drawings and the
like.
[0270] It should be understood that grooves 380 may be substituted
by any other features capable of redirecting light emerging from
waveguide 4 toward a prescribed direction. FIG. 33 shows another
non-limiting example of light redirecting features associated with
layer 20 and exemplified by sharp-angle slits 382 in the surface of
layer 20. Each slit 382 has a relatively narrow base and extends
deep into the body of layer 20 at an angle with respect to surface
normal. Such configuration of light redirecting features may be
more advantageous from the point of view of enhancing the light
extraction efficiency. As it can be seen from FIG. 33, ray 78 is
extracted from system 2 in the first pass as opposing to the
configuration illustrated in FIG. 32. The deep and narrow slits 382
may be made by any suitable means, including but not limited to
laser ablation, slitting with a sharp blade, material cracking,
etc. The process of forming of such slits should preferably allow
for forming optically smooth walls which can efficiently reflect
light by means of TIR. It is noted that, while straight profiles of
the slit walls are illustrated in FIG. 33, such walls may also have
any curvilinear or stepped profile as well.
[0271] In a further non-limiting example, grooves 382 may be
replaced by narrow undercuts made in a surface of layer 20. Each
undercut may have parallel or nearly-parallel walls. and may be
formed, for instance, by laser ablation, a sharp blade or by any
other means of material removal or cutting. Similarly, to the above
example of slits 382, at least the light-redirecting wall of each
undercut should preferably have an optically smooth surface
allowing for TIR. The light-redirecting wall of each undercut
should make a dihedral angle with the surface of layer 20 which is
suitable for light extraction towards a normal direction with
respect to the prevailing plane of system 2.
[0272] FIG. 34 illustrates an embodiment of system 2 in which
surface relief features have a variable slope with respect to the
prevailing plane of waveguide 4. Accordingly, system 2 of FIG. 34
includes waveguide 4 defined by opposing TIR surfaces 10 and 12,
low-index buffer layer 6 attached to surface 12, light extraction
layer 20 attached to the buffer layer, and light source 400 coupled
to an edge or terminal end of the waveguide. Surface 10 is provided
with an array of micro-stepped surface relief features 8. Each
feature 8 has a TIR face facing the light source and inclined at a
sufficiently low angle with respect to the prevailing plane of
surface 10 and/or waveguide 4 so that light leaks out of waveguide
4 primarily through buffer layer 6 and light extraction layer 20.
The slope of individual TIR faces increases along the light
propagation path as a function of the distance from light source
400. This can be useful, for example for compensating of light
depletion by the previous features 8 along the waveguide and for
providing a uniform light output from the light emitting surface of
layer 20. According to one embodiment, the slope angle of the TIR
faces varies between approximately one half of a degree and three
degree. According to one embodiment, the slope angle of the TIR
faces increase with the distance from source 400 linearly.
According to one embodiment, the slope angle of the TIR faces
increase with the distance from source 400 according to a
non-linear function in which the rate of slope increase accelerates
with the distance from source 400.
[0273] It will be appreciated that system 2 of FIG. 33 is designed
to receive light from one side and emit it from the respective
broad-area surface. Such configuration exhibits certain geometrical
asymmetry with respect to a normal to the prevailing plane of the
system, which, in turn, my potentially result in an unwanted
asymmetry in light distribution in the prevailing plane of light
extraction and collimation. In order to eliminate such asymmetry,
system 2 may be configured in a symmetrical configuration which is
also illuminated from the opposing sides by symmetrically disposed
light sources.
[0274] FIG. 35 shows a two-sided implementation of system 2 which
includes two symmetrical segments along axis 200. Accordingly,
waveguide 4 also has two symmetrically disposed segments and two
opposing light input edges or ends which are illuminated with the
respective light sources 400 and 410. Each of the waveguide
segments is provided with an array of surface relief features 8
formed in surface 10.
[0275] Each of surface relief features 8 is represented by a
low-profile, linear asymmetric micro-prism protruding from a base
at surface 10 outwardly from waveguide 4. The tips or ridges of the
micro-prisms may have sharp corners or they may also be slightly
rounded or even have small flat portions.
[0276] The larger-area facets of the micro-prisms are tilted
outwardly from the central portion of waveguide 4 and are
configured for incremental deflection of propagating in a waveguide
mode. Each of the larger-area facets should preferably have an
optically smooth, polished surface to preserve TIR at surface 10
and minimize light loss.
[0277] The dihedral angle of the larger-area facets of the
micro-prisms is variable along axis 200, increasing along the
intended light propagation path from the opposing edges or ends of
waveguide 4 towards the mid-portion of waveguide 4. This ensures
that the rate of light deflection increases as a function of the
distance from the respective light source and at least partially
compensates light depletion in waveguide 4 by the preceding
micro-prisms. In will be appreciated that the dihedral angles of
the microprisms may be particularly tailored to provide for a
nearly-constant rate of light extraction from the surface of
waveguide 4 along axis 200. Additionally, the rate of progressive
light extraction along axis 200 by the prismatic surface relief
features 8 should preferably be sufficient to ensure that
substantially all or at least a substantial portion of light
injected into waveguide 4 may be extracted.
[0278] Accordingly, the opposing lateral sides of light extracting
layer 20 may be configured to receive light from the respective
portions of waveguide 4 and further direct the extracted light
towards surface normal.
[0279] Referring further to FIG. 35, each of the opposing segments
of system 2 may be configured to emit a slightly asymmetric beam
with respect to a normal to the emitting broad-area surface.
However, since the opposing segments of system 2 are symmetrical
and emit beams that mirror each other with respect to a surface
normal, the resulting light distribution representing a
superposition of the respective light beams will also become
symmetrical with respect to the plane of symmetry of system 2
regardless of the distance from the emitting surface.
[0280] Accordingly, by varying the angular distribution and
directionality of the individual beams emitted by the opposing
system 2 segments of FIG. 35, the resulting light beam may be
focused by pointing the respective beams toward converging
directions or a target. Alternatively, the individual beams may
also be defocused by pointing them beams toward diverging
directions or away from a certain target.
[0281] It will thus be appreciated that the structure and operation
of system 2 allow for an unprecedented control of the beam emitted
from its broad-area surface compared to the conventional art
employing waveguide illumination systems. In accordance with at
least some embodiments and the principles of light collimation
described above in reference, for example, to FIG. 9 and FIG. 10,
the angular distribution of light emerging from waveguide 4 can be
made very narrow at least in one plane or dimension. In turn, light
extraction layer 20 may be configured to direct the
narrow-distribution light emerging from waveguide 4 towards a
plurality of distinct directions. Such directions may be parallel,
convergent or divergent. The divergent or convergent directions may
also follow a particular angular pattern. Alternatively, the
direction of light emission from the light emitting region of
system 2 may be randomized within a limited range thus providing an
improved light diffusion without sending light into non-functional
directions.
[0282] In an illustrative example, each of angular distributions
912, 914, 916, and 918 of FIG. 24 may be directed towards a common
area located at a distance from system 2 along a normal to axis
200. As a result, particularly when the respective distributions
are sufficiently narrow, the intensity of the light beam in the
respective area can be made greater than in the intensity of the
surrounding areas. Thus, system 2 may be configured to additionally
focus light emitted from its surface.
[0283] Such focusing can be achieved by the appropriate configuring
the light extraction layer 20. For instance, when layer 20 employs
light redirecting features such as prismatic grooves 380 of FIG. 32
or slits 382 of FIG. 33, the slope of the respective features may
be varied accordingly so as to provide such focusing of the emitted
light. In an exemplary case, referring to FIG. 32, the TIR walls of
respective grooves 380 redirecting rays 74 and 76 can make slightly
different dihedral angles to the prevailing plane of waveguide 4 so
as to result in said rays converging at a certain distance from
system 2.
[0284] FIG. 36 shows an embodiment of illumination system 2 in a
front light configuration in which there is provided a
light-guiding layer formed by planar waveguide 4 and light
extraction layer 20 positioned immediately adjacent to the
broad-area surface 12 of the waveguide. Waveguide 4 has high
optical transmissivity of the material and also high transparency
at least in a direction perpendicular to the waveguide's plane. The
waveguide is configured for transmitting light from edge to edge by
means of TIR from at least the opposing surfaces 10 and 12 both of
which are made smooth (polished) and highly transparent. The side
walls of waveguide 4 can also be made smooth and capable of
reflecting light by means of TIR. Light source 400 is coupled to
the light input edge of the waveguide. The opposing (terminal) edge
can be provided with a mirrored surface to reflect light back into
the waveguide and recycle rays that have not been extracted from
the waveguide in a single pass.
[0285] Layer 20 comprises a screen containing an image print which
is opaque for the incident light and includes at least one region
which has good light scattering or reflective properties. The image
may contain any text, graphics, symbols or patterns and can be
exemplified, for example, by a front-illuminated display that can
be found in the signage industry.
[0286] Waveguide 4 and layer 20 of FIG. 36 should be preferably
positioned very close to each other, including the case when they
are disposed in a physical contact by at least portions of their
surfaces. However, a direct optical contact without any
intermediate layer (such as air) should be avoided in order to
prevent the suppression of TIR of surface 12 and unwanted premature
light leakage from waveguide 4.
[0287] Waveguide 4 is exemplified by a transparent plastic matrix
with a plurality of very fine optically transmissive glass or
plastic particles which are incorporated into the matrix in very
small proportions by volume and have forward-scattering properties.
The forward-scattering particles are meant to mean such scattering
particles that scatter light substantially in a forward direction
and have a very low or negligible scattering far sideways and in
the reverse direction.
[0288] The forward-scattering particles are made from an optically
transmissive material which refractive index differs from that of
the main bulk of waveguide 4 by a predetermined amount. A minimum
difference in refractive indices is required for the particles to
effectively scatter light rays that propagate sufficiently close to
them. Particularly, it is preferred that the refractive index of
the particles differs by more than 0.02 but less than 0.4 from the
refractive index of the body of waveguide 4. The forward-scattering
particles should preferably be finely separated from each other by
considerable distances in order to provide for consistent and
predictive light scattering as well as maintain high visual
transparency and transmissivity of the waveguide.
[0289] By way of example and not limitation, the core of waveguide
4 can be made from PMMA (n.sub.core=1.49) and the particles can be
made from polystyrene (n.sub.part.apprxeq.1.59) or FEP
(n.sub.part.apprxeq.1.34). The particles can have a simple,
single-component structure or they can have a more complex
morphology and may be composed by a core and a shell made from
different materials. Depending on the other parameters involved,
the particles can have a mean diameter less than, equal to or
greater than the wavelength of visible light.
[0290] The concentration of forward-scattering particles should be
sufficiently low to keep waveguide 4 optically thin along the
perpendicular to the prevailing plane of the waveguide. At the same
time, the particle concentration should be high enough to make
waveguide 4 optically thick along longitudinal axis 200 and yet
highly transmissive.
[0291] The term "optical depth" or "optical thickness" is commonly
directed to mean the quantity of light removed from a light beam by
scattering or absorption during its path through a medium. In the
context of this invention, as applicable to waveguides including
non-absorbing, forward-scattering particles, this term can be more
narrowly directed to mean the quantity of light that has been
perceptibly scattered from the original propagation path of the
light beam. The original propagation path is the path of the light
beam in the homogeneous medium of the waveguide in the absence of
the scattering particles.
[0292] Particularly, if I.sub.0 is the reference intensity of
radiation in a homogeneous, weakly-absorbing medium of waveguide 4
and I is the observed intensity of light propagating along the same
optical path, then an optical depth .tau. of the medium in the
presence of forward-scattering particles can be defined by the
following expression: I=I.sub.0e.sup.-.tau..
[0293] In the optically thin case, that is referring to the case of
light propagating along the perpendicular to the plane of waveguide
4, .tau.<<1 and e.sup.-.tau..apprxeq.1.apprxeq..tau., so that
the above expression can be simplified as follows:
I=I.sub.0(1-.tau.). Accordingly, it at least some embodiments of
the present invention, waveguide 4 can be configured to perceptibly
scatter less than 5% of light propagating along the waveguide's
normal and more preferably scatter less than 2% of said light. In
other words, the column density of light scattering particles
should be low enough so that .tau. is less than 0.02-0.05 along the
normal to the waveguide's prevailing plane and so that the high
transparency of the waveguide is maintained.
[0294] At the same time, the concentration and light scattering
parameters of forward-scattering particles should be selected to
ensure that at least a substantial part of light is removed from
waveguide 4 by means of scattering as such light propagates through
the column of material along longitudinal axis 200. According one
embodiment, it is preferred that at least approximately half of the
light beam input into waveguide 4 through an edge is scattered as
it propagates towards the opposing terminal edge. According to one
embodiment, it can be preferred that at least 80% or light is
scattered along its longitudinal propagation in waveguide 4, which
corresponds to an optically thick case of .tau..gtoreq.1.6.
[0295] An important relevant parameter for estimating the required
concentration of light scattering particles is an effective
cross-section .sigma. which defines the area around the particle
where light is likely to be scattered. In general, the scattering
cross-section is different from the geometrical cross-section of a
particle, and it depends upon the wavelength of light and the
permittivity, shape and size of the particle. Another important
parameter defining the type of scattering and the effective
cross-section of dielectric particles of diameter d in a refractive
medium is the so-called size parameter x.
x = .pi. d n med .lamda. , ##EQU00002##
where n.sub.med is the refractive index of the refractive medium
and .lamda. is the wavelength of the propagating light.
[0296] The general case for an arbitrary value of x is called Mie
scattering in the relevant art. In the case of relatively large
particles compared to the wavelength (x>>1), the total
cross-section tends toward a geometric limit of
.sigma.=.pi.d.sup.2/4. As the particles become smaller, the forward
scattering diffraction peak can be observed, in which case the
cross section will be become twice the geometric limit.
[0297] In the case of very small particles (x<<1), the so
called Rayleigh limit, the total cross-section is given by
.sigma. = 2 / 3 .pi. d 2 x 4 ( m 2 - 1 m 2 - 1 ) 2 ,
##EQU00003##
where m is the ratio between the refractive index of the particle
and the refractive index of the medium.
[0298] Considering light propagating in waveguide 4 having sparsely
distributed scattering particles and assuming that the particles do
not shadow each other, the optical depth can be related to the
scattering cross-section through the following expression:
.tau.=NL.sigma., where L is the propagation path length and N is
the number density of light scattering particles.
[0299] The relationship between an optical depth along the
waveguide's longitudinal axis .tau..sub.81 and an optical depth
perpendicular to the prevailing plane of the waveguide
.tau..sub..perp. defines the relative difference in waveguide
transparency between the respective directions. It can also be used
to define the relationship between the thickness and the length of
the waveguide for the given scattering efficiency of the particles
and particle concentration.
[0300] The scattering particles should be particularly configured
to be substantially invisible to the naked eye from a distance and
should not introduce a perceptible blur or haze to the bodies or
images behind waveguide 4. Additionally, the scattering particles
should be configured to not introduce a substantial glare or loss
in image contrast when waveguide 4 is lit by source 400 or by any
other external light source.
[0301] In a non-limiting example, the core of waveguide 4 may
include an Acrylite.RTM. Endlighten acrylic sheet (e.g. available
from CYRO Industries, Rockaway N.J.) which combines visual
transparency and light-scattering properties. In a further
non-limiting example, waveguide 4 may include a Plexiglas.RTM. ELiT
acrylic sheet. The sheets can be made using extrusion, casting or
any other suitable process and can be configured to provide 90-92%
light transmission along surface normal.
[0302] In operation, referring further to FIG. 36, the light input
edge of waveguide 4 receives light emanated by source 400 and
refracts light into the body of waveguide allowing at least a
substantial part of the emitted light to propagate in a waveguide
mode. Such light injected through the light input edge of waveguide
4 propagates towards the opposing terminal edge by means of
transmission and TIR from surfaces 10 and 12 until it is
incrementally scattered by a the light scattering particles to a
sufficient angle to overcome TIR at surface 12 and exit towards
light extracting layer 20. The image print of layer 20
back-scatters the out coupled light into all directions so that at
least a portion of the scattered light can reach the viewer's eye
660. Thus, the observer can clearly see the image print which is
brightly lit by the highly transparent top layer represented by
waveguide 4.
[0303] FIG. 37 illustrates the structure and operation of the
embodiment of FIG. 36 in further detail by depicting an exemplary
path of ray 74 emanated by source 400. Waveguide 4 comprises a
large plurality of forward-scattering particles 36 made from a
transparent refractive material that has different refractive index
than waveguide 4. Each scattering particle 36 is configured to
provide strong forward scattering propertied so that each
interaction of light ray with such particle can change its
propagation direction by up to a relatively small, predetermined
amount. The bend angle introduced by each particle 36 should be low
enough to generally prevent light exiting from waveguide 4 at high
angles with respect to the prevailing plane of the waveguide.
Instead, each scattering particle 36 should be configured to
provide incremental deviation of rays along the propagation path
and eventually result in light rays exiting from surface 12 at low
angles with respect to the surface, thus providing means for
illuminating layer 20 by the decoupled light.
[0304] Particles 36 are preferably finely distributed through the
volume of waveguide 4 in a concentration low enough to maintain
high transparency of the waveguide along normal 800. At the same,
time, the concentration should be sufficient to extract most of the
light from waveguide 4 by means of forward scattering and by means
of eventual communicating incidence angles greater then TIR at
surface 12.
[0305] Due to the probabilistic nature of scattering and particle
encounter, ray 74 may undergo multiple scattering events until it
reaches the incidence angle greater than the TIR angle at surface
12. It will be appreciated that at certain concentrations of light
scattering particles 36, a portion of light beam may remain
non-extracted upon reaching the terminal edge of waveguide 4.
Therefore, the terminal edge may be provided with reflective
surface 602 which will reflect the above portion of light back into
the waveguide and promote a more complete light utilization. In
certain cases, employing reflective layer 602 may also result in an
improved uniformity of light emitted by system 2.
[0306] It will be appreciated by those skilled in the art that when
waveguide 4 is surrounded by air, the smooth waveguide/air
interfaces at surfaces 10 and 12 may appear functionally identical
with respect to light propagating by TIR within the waveguide.
Accordingly, with the absence of TIR suppression or at one of the
surfaces, the propagating light has essentially equal probability
of exiting through either surface. As a result, about 50% of light
may exit from waveguide 4 without illuminating the image print of
layer 20. Although the emergence angle of light exiting through
surface 10 can be made sufficiently low so as not to interfere
significantly with viewing the illuminated image print from a
normal direction, the loss of the respective portion of light may
be unwanted for a variety of reasons.
[0307] In order to eliminate or at least substantially reduce this
light loss and enhance the brightness of the image print, a
suitable intermediate outcoupling layer can be provided between
waveguide 4 and layer 20, as illustrated in FIG. 38.
[0308] Referring to FIG. 38, the intermediate outcoupling layer is
exemplified by highly transparent buffer layer 6 disposed in a good
optical contact with both waveguide 4 and layer 20. Similarly to at
least some of the embodiments discussed above in reference to, for
example, FIG. 5 and FIG. 6, buffer layer 6 has a lower refractive
index than waveguide 4. At the same time, the refractive index of
layer 6 should be greater than the refractive index of the
surrounding air or otherwise of the material adjacent to surface
10.
[0309] Buffer layer 6 allows for TIR at surface 12 at sufficiently
high incidence angles, that is when the propagation angle is
sufficiently low with respect to longitudinal axis 200. Similarly
to the embodiments described in reference to FIG. 5 and FIG. 6,
surface 10 can be characterized by the first TIR angle
.phi..sub.TIR1 and surface 12 can be characterized by the second
TIR angle .phi..sub.TIR2.
[0310] Accordingly, when the incidence angle onto surface 12
becomes lower than .phi..sub.TIR2 as a result of scattering by
particles 36 and incremental increase of the out-of-plane angle,
light can escape from waveguide 4 through the respective surface
and illuminate the image surface of layer 20. Since the refractive
index of air is lower than that of layer 6, the TIR angle
.phi..sub.TIR1 surface 10 is lower than the second TIR angle
.phi..sub.TIR1. This results in the prevailing light escape paths
through layer 6 and no light or at least much less light leaking
through surface 10.
[0311] Referring further to FIG. 38, system 2 may also include
collimating element 440 associated with the light input edge of
waveguide 4. Collimating element 440 may be attached to the light
input edge of waveguide 4 with or without good optical contact or
it can be spaced apart from the edge by a small air gap or by an
intermediate optically transmissive layer. For maximizing the light
input, collimating element 440 may be coupled to the waveguide edge
using an optical adhesive or encapsulant. Collimating element 440
may be configured to narrow the angular light distribution emitted
by light source 400. This may ensure that the second TIR
.phi..sub.TIR2 is not exceeded in waveguide 4 prematurely for at
least some rays and that no substantial portion of light exits from
the waveguide 4 near the light input edge. Since the embodiment of
FIG. 38 may be configured to admit up to about twice as much light
onto the print of layer 20 and eliminate or at least substantially
suppress light loss through the opposing surface 10 compared to the
embodiment of FIG. 37, it may be advantageously selected for at
least some applications where more complete light utilization and
improved image contrast are important.
[0312] FIG. 39 depicts an embodiment of system 2 configured as a
backlight and more particularly as a two-sided backlight. An
additional pair of the buffer layer and the light extraction layer
is provided on surface 10, as indicated by reference numerals 506
and 520, respectively.
[0313] Similarly, the layers 506 and 520 are laminated to surface
10 with a good optical contact with or without intermediate
adhesive layers. Both light extraction layers 20 and 520 may be
configured as transmissive diffusers and can have any suitable
color, tint, light transmissive and scattering characteristics.
Either one of the layers 20 and 520 may also be configured to
display an images or any suitable pattern or text.
[0314] Accordingly, light propagating through waveguide 4 and
scattered by particles 36 can be evenly distributed along the
surfaces 10 and 12 and will illuminate both layers 20 and 520 thus
providing the desired operation of a two-sided backlight having
waveguide 4 sandwiched between the opposing light extracting
layers. Ray 74 of FIG. 39 exemplifies light exiting from waveguide
4 through surface 12 and ray 76 exemplifies light emerging from
surface 10.
[0315] FIG. 40 illustrates the effect of adding layer 6 between
waveguide 4 and light extraction layer 20 for an exemplary case of
an isotropic scattering particle 536 embedded into the material of
the waveguide. A ray 720 exemplifies light propagating in a
waveguide mode through waveguide 4.
[0316] Referring to FIG. 40, ray 720 is scattered into all
directions by the particle 536. While surfaces 10 and 12 receive an
about equal number of scattered rays, the low-n layer 6 enables the
asymmetry in TIR angles between surfaces 10 and 12 and results in
more light extracted through surface 12 towards layer 20 compared
to the amount of light escaping from waveguide 4 through surface
10.
[0317] FIG. 41 illustrates the advantage of employing anisotropic
scattering particles by example of a single forward-scattering
particle 36 operating in conjunction with the low-n layer 6. As
shown in FIG. 41, ray 720 is forward-scattered by particle 36 at
scattering angles not exceeding the complementary angle to the
first TIR angle .phi..sub.TIR1. At the same time, the maximum
scattering angle is greater than the complementary angle to the
second TIR angle .phi..sub.TIR2 which causes at least some
uttermost scattered rays to obtain sub-TIR incidence angles with
respect to surface 12 and exit from waveguide into layer 6. Since
layer 6 is optically transmissive, all light rays exiting into
layer 6 reach light extraction layer 20 which, in turn may redirect
or redistribute said rays and direct them into the prescribed
directions. Accordingly, by limiting the scattering angle by a
predetermined value, the unwanted light escape through surface 10
can be eliminated or at least substantially reduced compared to the
case of FIG. 40 where isotropic particle 536 is employed.
[0318] A scattering angle .delta. can be defined as an angle
between the original propagation direction that a light ray has
before encountering scattering particle 36 and the direction of a
scattered ray resulting from ray interaction with the particle.
Accordingly, referring to FIG. 42, if ray 720 has out-of-plane
propagation angle .beta. before scattering, it may obtain a new
out-of-plane propagation angle .beta.+.delta. thus broadening the
angular distribution of light propagating in waveguide 4.
[0319] In an operational aspect of this invention,
forward-scattering particles 36 may be configured to provide
function similar to surface relief features 8 of at least some
embodiments discussed above (see, e.g., the discussion on
broadening the angular distribution of propagating light beam in
reference to FIG. 7 and FIG. 8). More particularly, it may be
appreciated that an analogy may be drawn between the scattering
angle .delta. of FIG. 42 and the angular increment 2.alpha. of FIG.
8. Although angle .delta. of FIG. 42 is obtained by using a light
scattering mechanism and a specially configured particle 36
embedded into the bulk of waveguide 4 material while angle 2.alpha.
is obtained using surface relief feature 8 having a characteristic
slope .alpha. with respect to the waveguide surface, both of these
angular values represent a relatively small incremental deviation
of the respective light rays from the original propagation
directions. Either one of the above-compared light deviation
mechanisms may be configured directed to provide a controlled
leakage of light through the designated major surface of waveguide
4. Additionally, it is noted that by selecting the appropriate
parameters of surface relief features 8 and forward-scattering
particles 36 of the respective embodiments, the angular spread and
angular/spatial distribution of light extracted from waveguide 4
can also be controlled, which may be used, for example, for
providing a light collimating function of system 2.
[0320] It will be appreciated by those skilled in the art that,
although each individual interaction of a light ray with
forward-scattering particle 36 has a random character, by selecting
the material, refractive index, size, concentration and/or other
parameters of particles 36, the light scattering pattern of the
particles may be tailored to provide a controlled magnitude and
rate of spreading of the light beam along the propagation path in
waveguide 4. As layer 6 frustrates TIR for at least those rays that
have incidence angle into surface 12 greater than the critical TIR
angle .phi..sub.TIRC, light will primarily exit from waveguide 4
toward layer 20 and not toward the opposing side of the
waveguide.
[0321] By applying Mie scattering calculations to dielectric
spheres in refractive medium and considering an exemplary case of
PMMA waveguide 4 (n.apprxeq.1.49) and polystyrene particles
(n.apprxeq.1.59) a scattering phase function can be obtained for
various particle sizes. The scattering phase function, or phase
function, gives the angular distribution of scattered light
intensity at a given wavelength.
[0322] FIG. 43 shows a scattering phase function plot for an
exemplary spherical particle 36 having a 5 .mu.m in diameter,
calculated according to Mie theory for 532 nm wavelength. As it can
be seen from an angular distribution 972 of scattered light
intensity, the scattering angle .delta. generally does not exceed
5.degree. for the most part of light beam. More particularly, about
90% of scattered light will be deviated from the original
propagation path by 5.degree. or less. Therefore, each interaction
of light beam with such a narrowly-scattering particle 36 will
broaden the angular distribution by a small increment which can be
sufficient for useful light extraction through surface 12 and the
adjacent layer 20 but generally insufficient for light escape
through opposing surface 10.
[0323] FIG. 44 shows a similar plot calculated for 0.9 .mu.m
diameter of particle 36 where the scattered light intensity has a
different angular distribution 974. The analysis of angular
intensity distribution 974 indicates that about 90% of the
scattered light beam is confined within .+-.17.degree. scattering
cone and that 95% of the beam is contained within .+-.20.degree.
scattering cone, respectively. Thus, statistically, scattering
angles .delta. will generally be greater for the case of 0.9 .mu.m
diameter of particle 36 than for the case a larger, 5 .mu.m
particle 36. Accordingly, light rays having angular distribution
974 will be extracted from waveguide 4 much faster and will travel
much shorter distances along axis 200 than light rays having
angular distribution 972. It will be thus appreciated that the
particle size can be used for controlling the rate of light
extraction and overall light output for a given area of system 2.
The size of particles 36 therefore represents an important system
parameter that generally needs to be factored into the system
design along with the other parameters discussed above for the
respective embodiments.
[0324] According to at least some embodiments of the present
invention, the size and other parameters of forward-reflecting
particles 36 can be selected to result in the scattering angles
.delta. that generally do not exceed the difference in TIR angles
at surfaces 10 and 12. In other words,
.delta.<.phi..sub.TIR2-.phi..sub.TIR1, where .phi..sub.TIR2 is
the critical TIR angle at the interface of waveguide 4 with buffer
layer 6 (e.g., surface 12 in FIG. 38) and .phi..sub.TIR1 is the
critical TIR angle at the interface of waveguide 4 with the outside
medium (e.g., surface 10 in FIG. 38). This can minimize light
escape into air through the waveguide surface opposing the
interface between waveguide 4 and buffer layer 6.
[0325] It should be understood that this invention is not limited
to the cases where system 2 has the shape of a rectangular plate,
sheet or film or an elongated rod and may be applied to the case
when system 2 has any other suitable shape. Particularly, system 2
can have any basic geometric form, a free-form or any combination
thereof. Additionally, any two-dimensional planar shape of system 2
can be bent in any suitable way to form a three-dimensional shape.
This can be used, for example, to provide a broader illumination
pattern or create artistic or decorative effects. Similarly, system
2 having an elongated or rod-like geometry can be bent or formed to
create a two-dimensional or three-dimensional shape.
[0326] FIG. 45 depicts an example of system 2 in an axisymmetrical
configuration where light source 400 is positioned at the central
area of waveguide 4 and surface relief features 8 are represented
by nested radial rings. Waveguide 4 may have an area 290 which is
clear and void of any features 8.
[0327] FIG. 46 shows an alternative round configuration of system 2
in which there is a central opening in waveguide 4 where a ring of
multiple light sources 400 is attached to the inner edge of the
waveguide. In configurations depicted in FIG. 45 and FIG. 46, light
injected into waveguide 4 in the central area and propagating
radially away from the center is emitted from the broad surface of
the waveguide by the combined function of surface relief features
8, buffer layer 6 and light extraction layer 20, according to the
principles described above. In view of the foregoing description of
the invention, it should be understood that the above-described
forward-scattering particles 36 can also be used in place or in
addition to surface relief features 8.
[0328] It should be understood that at least some of the
configurations of system 2 shown in a cross-section in the
foregoing embodiments may also be implemented in an axisymmetrical
form obtained by the revolution of the respective cross-section
around an axis. It should also be understood that this invention is
not limited to the light input through an edge into waveguide 4,
but can also be applied the case where light can be input at any
suitable location of waveguide 4, including an arbitrary location
across surfaces 10 or 12. The light can be input by embedding light
source 400 into the envelope of waveguide. Light source 400 can be
alternatively attached to a broad-area surface of waveguide 4 or
spaced apart from the body of the waveguide. When source 400 is
externally attached to waveguide 4, a suitable collimating or
non-collimating light coupler can be used to inject light into
waveguide's core.
[0329] At least some of the foregoing embodiments were described
upon the case where a difference between the refractive indices at
the opposing walls or surfaces of waveguide 4 was used to force
light escape towards a designated side of the waveguide. However,
this invention is not limited to this and may be applied to the
case when any other suitable means are used for suppressing light
leakage through the unwanted side of the waveguide. Particularly, a
specularly reflective layer, such as a mirrored surface, may be
provided on the side of the waveguide opposing to the light
emitting side to return any stray light back to the waveguide.
[0330] FIG. 47 shown an embodiment of system 2 comprising planar
waveguide 4 sandwiched between a light turning film 620 and a
sheet-form specular reflector 630. Waveguide 4 has generally planar
opposing broad-area surfaces 10 and 12. Surface 12 has a plurality
of shallow surface relief features 8 represented by shallow steps
formed in said surface.
[0331] Light source 400, which may be represented by one or more
LEDs, cold cathode fluorescent lamp (CCFL) or any other source, is
positioned near the light input edge of waveguide 4. Collimating
element is provided to collect light propagating from source 400
away from the light input edge and inject such light into the
waveguide.
[0332] Each surface relief feature has a planar face 16 which is
configured to reflect light propagating at relatively low
out-of-plane angles by means of TIR, as illustrated by a. The slope
angle .alpha. of the respective faces 16 is sufficiently low so as
to generally result in multiple interactions of light rays before
they can obtain the sufficient out-of-plane angle to overcome TIR
at either surface 10 or 12. This is illustrated by the example of a
light path of a ray 372 in FIG. 47. The slope of face 16 of the
respective surface relief feature 8 is insufficient to communicate
a sub-TIR incidence angle with respect to either surface 10 or 12.
Accordingly, ray 372 can continue propagating in the waveguide mode
until its interactions with the subsequent surface relief features
8 result in reaching the minimum out-of-plane angle to suppress
TIR.
[0333] Reflector 630 is positioned adjacent to surface 10 in such a
way that there is at least minimal air gap between the two.
Reflector 630 has a high specular reflectivity and is configured to
reflect light emerging from surface 10 at high exit angles with
respect to surface normal back towards waveguide 4.
[0334] Light turning film 620 has a prismatic structure facing
waveguide 4. The grooves of the prismatic structure are aligned
parallel to each other in an linear array which longitudinal axis
extends generally perpendicular to longitudinal axis 200 of
waveguide 4.
[0335] By way of example and not limitations, light turning film
620 may be exemplified by the Transmissive Right Angle Film (TRAF)
which is commercially available from 3M. The TRAF film has a
polyester backing substrate with the prismatic structure made from
modified acrylic resin. It has the acceptance angle of 0.degree. to
20.degree. with respect to the plane of the film and a nominal
thickness of about 155 .mu.m. Accordingly, surface relief features
8 may be configured to gradually increase the out-of-plane angle of
light propagating in the waveguide mode and eventually result in
light exiting from the waveguide core towards TRAF at such an
out-of-plane angle which will be within the acceptance angle of the
TRAF. In turn, TRAF may intercept and further redirect the emerging
light away from the emitting surface by an angle of approximately
70.degree. thus resulting in light emission from the broad-area
surface of system 2 in a perpendicular direction.
[0336] It will be appreciated that surface relief features 8 may be
configured to match any other acceptance angle of light turning
film 620, simply by adjusting slope angle .alpha. of the respective
faces 16. Particularly, each face 16 may have such a slope angle
.alpha. which will ensure that most light will emerge from
waveguide 4 also at an out-of-plane angle which is within the
acceptance angle of film 620.
[0337] It will be appreciated that, in order to achieve the desired
operation, slope angle .alpha. will typically be much lower than
the acceptance angle waveguide 4. Most light rays propagating in
waveguide 4 will require multiple interactions with surface relief
features 8 along the propagation path in order to obtain the
minimum out-of-plane angle to overcome TIR at surface 12 and yet to
enter film 620 at the prescribed low out-of-plane angles. It will
thus be appreciated that most light will be extracted from
waveguide 4 by deviating from the original propagation path in an
incremental fashion and that the increments to the out-of-plane
angle communicated by each surface relief feature 8 will be
relatively low due to the smallness of slope angle .alpha..
[0338] A light ray 374 of FIG. 47 exemplifies the final portion of
the light path according to the above scenario. Ray 374 emitted by
source 400 and injected into waveguide 4 has a relatively high
out-of-plane angle with respect to the waveguide's prevailing plane
as a result of its multiple interactions with surface relief
features 8 along the propagation path. Particularly, the angle
which is complementary to the out-of-plane angle of ray 374 is
lower than the critical TIR angle at surface 12 by a relatively
small difference which may be overcome by an additional interaction
with a single surface relief feature 8. As ray 374 strikes face 16
of the respective surface relief feature 8, it is reflected by
means of TIR and obtains an increment in the out-of-plane angle. It
will be appreciated by those skilled in the art that the increment
in the out-of-plane angle will be twice the slope angle .alpha. of
face 16.
[0339] Accordingly, as ray 374 is directed towards the opposing
surface 12, its new incidence angle with respect to a normal to
surface 12 may become less than the critical TIR angle at that
surface resulting in ray decoupling from waveguide 4. Upon exiting
from a greater-index material of waveguide 4 into a low-n outside
medium (such as air), ray 374 will bend substantially towards the
plane of waveguide 4 due to the refraction angle being greater than
the angle of incidence. Therefore, even if ray 374 was making a
relatively high out-of-plane angle when propagating within
waveguide 4, its respective out-of-plane angle outside of waveguide
4 would generally be substantially lower, in accordance with the
Snell's law, including near-zero angles. Accordingly, the maximum
allowable angle that ray 374 can make with respect to surface 12
can be set to the acceptance angle of film 620 in which case
substantially all rays emerging from waveguide 4 may be turned
towards a perpendicular direction. Since film 620 may be configured
to generally preserve the angular distribution of light that it
redirects and since most rays emerging from waveguide 4 may be
distributed within a relatively narrow angular cone, a highly
collimated beam may thus be obtained.
[0340] Referring further to FIG. 47, the maximum allowable slope
angle .alpha..sub.MAX of face 16 may be defined from the following
reasoning for a given acceptance angle .phi..sub.ACC of light
turning film 620. It is noted that this reasoning is provided by
way of an illustrative example to assist the reader in
understanding of the operation of this invention and should not be
construed as limiting the scope of the invention.
[0341] Since the out-of-plane angle of ray 374 should preferably be
equal to or lower than the acceptance angle .phi..sub.ACC upon
exiting from waveguide 4, the minimum refraction angle
.phi..sub.RMIN of ray 374 counted off from a normal to surface 12
should be complementary to angle .phi..sub.ACC, that is
.phi..sub.RMIN=90.degree.-.phi..sub.ACC.
[0342] By using the Snell's law, it can be shown that the minimum
angle of incidence .phi..sub.IMIN corresponding to .phi..sub.RMIN
can be found from the following relationship: n.sub.1 sin
.phi..sub.IMIN=n.sub.0 sin(90.degree.-.phi..sub.ACC) or n.sub.1 sin
.phi..sub.IMIN=n.sub.0 cos(.phi..sub.ACC), where n.sub.1 is the
refractive index of the material of waveguide 4 and n.sub.0 is the
refractive index of the outside medium. Accordingly, assuming the
outside medium being the air with n.sub.0.apprxeq.1, sin
.phi..sub.IMIN=cos(.phi..sub.AAC)/n.sub.1.
[0343] Since the angle of reflection is equal to the angle of
reflection in respect to a normal to face 16, TIR from face 16 will
decrement the incidence angle or ray onto surface 12 by 2.alpha..
Now, considering that the minimum incidence angle that ray 374 may
take with respect to a normal to surface 12 while propagating in a
waveguide mode is the critical TIR angle .phi..sub.TIRC and that
the incidence angle of reflected ray should not exceed
.phi..sub.IMIN, obtain
.alpha..sub.MAX=(.phi..sub.TIRC-.phi..sub.IMIN)/2.
[0344] For the case illustrated in FIG. 47 and in view of the above
assumptions, .phi..sub.TIRC=arcsin(n.sub.0/n.sub.1).apprxeq.
arcsin(1/n.sub.1). Accordingly, slope angle .alpha. of face 16 may
be selected to not exceed .alpha..sub.MAX, where
.alpha. MAX = 1 2 ( arcsin ( 1 / n 1 ) - arcsin ( cos ( .phi. ACC )
/ n 1 ) ) ##EQU00004##
[0345] For instance, if waveguide 4 of FIG. 47 is made from PMMA
(acrylics) having refractive index n.sub.1.apprxeq.1.49 and light
turning film 620 includes TRAF having the acceptance angle
.phi..sub.ACC of 20.degree., it will give
.alpha..sub.MAX.apprxeq.1.5.degree.. It is noted however, that
greater slope angles may also be selected for faces 16 in which
case at least some rays may emerge from waveguide 4 at out-of-plane
angles beyond the acceptance angle of film 620. Accordingly, a
portion of light emitted from system 2 may have a prescribed degree
of collimation and another portion of the emitted light may have a
more diffuse angular distribution. It will be appreciated that,
when the primary function of system 2 of FIG. 47 is to emit a
collimated light beam while minimizing light scattering, slope
angle .alpha..alpha.shall either not exceed the maximum angle
.alpha..sub.MAX or exceed it only by a relatively small amount.
Particularly, the slope angle .alpha. of each face 16 with respect
to prevailing plane of waveguide 4 may be made less than two
angular degrees. It can be shown that, at such low angle, at least
a major portion of light escaping through surface 12 will still
have out-of-plane angles less than 20 degrees, which is within the
acceptance angle of the TRAF film. Accordingly, when film 620 is
exemplified by the TRAF film, practically all of the emerging light
may be redirected towards the surface normal while generally
preserving the narrow, 20-degrees angular spread and resulting in
system 2 emission of a collimated beam perpendicularly to its
plane.
[0346] Light turning film 620 may also be provided with light
scattering properties or associated with an external light
scattering layer. This may be useful, for example, for smoothing
out the irregularities in light distribution that may be present in
the collimated beam. In another example, the light scattering
features associated with light turning film 620 may also be useful
for additional spreading of the collimated beam emitted by system 2
over a broader angular range.
[0347] At least some of the foregoing embodiments were described
upon the case where surface relief features 8 are arranged in
parallel rows in a surface of waveguide 4. However, this invention
is not limited to this and may be applied to the case where surface
relief features are arranged in an array which can have any other
configuration. For example, as illustrated in FIG. 48, surface
relief features may be formed in surface 10 in an annular stepped
arrangement radially extending from source 400 and being
symmetrical with respect to axis 200. However, it is noted that any
other suitable pattern may also be used for arranging feature 8 in
surface 10, which may also include symmetric, asymmetric,
intermittent, ordered or random patterns. For instance, any of the
surface distribution patterns of light extracting features of the
prior art devices, also including those shown in FIG. 1 through
FIG. 3, may be used to distribute surface relief features 8 across
a major surface of waveguide 4.
[0348] The foregoing embodiments were described upon the case where
the light deflecting elements are exemplified by either surface
relief features 8 formed in a major surface of waveguide 4 or
forward-scattering particles 36 distributed though the body of the
waveguide. However, this invention is not limited to this and may
be applied to the case where the light deflecting elements have any
other type provided that system 2 has the same basic operation.
[0349] For instance, the light deflecting elements may include
diffraction or holographic structures distributed along either one
or both of surfaces 10 and 12. Such structures may be configured to
deflect light by small angles in an incremental fashion along the
propagation path in waveguide 4 and to cause controlled light
leakage from the waveguide at relatively low out-of-plane
angles.
[0350] In a further instance, the light deflecting elements that
can provide the incremental light deflection along the optical path
in waveguide 4 may also be formed by a corrugated boundary between
two different transmissive materials incorporated into the body of
the waveguide. An illustrative example of such waveguide is shown
in FIG. 49. Accordingly, waveguide 4 of FIG. 49 has a planar slab
shape and is formed by two optically transmissive dielectric layers
642 and 644 disposed in optical contact with each other. The
refractive index of layer 642 is n.sub.1 and the refractive index
of layer 644 is n.sub.31 which is different than n.sub.1. The
difference in the refractive indices creates a refractive optical
interface between layers 642 and 644 which bends all rays entering
it at any non-zero incidence angles. In the case illustrated in
FIG. 49, n.sub.31<n.sub.1.
[0351] The boundary between layers 642 and 644 is not planar and
includes prismatic corrugations each formed by a pair of planar
facets 670 and 672. The corrugations may be adjacent to each other
and form a continuous corrugated boundary between the respective
layers. It is noted, however, that the corrugations may also be
alternated with flat portions of the boundary and may also be
distributed along axis 200 according to any suitable pattern. Facet
670 makes a dihedral angle 766 with the prevailing plane of
waveguide 4 and facet 673 makes a dihedral angle 768 with the same
plane. According to one embodiment, angle 766 is greater than angle
768 and may take particular values up to 90 degrees. According to
one embodiment, angles 766 and 768 may be identical or nearly
identical. According to one embodiment, angle 766 may be less than
angle 768.
[0352] In operation, a ray 722 propagating in layer 642 of
waveguide 4 at an out-of-plane angle .beta. strikes facet 670 of
the inter-layer boundary and refracts into the lower-index layer
644. Ray 722 further strikes surface 12 of waveguide 4 and reflects
from the surface by means of TIR. Ray 722 further refracts at facet
672 and enters layer 642 again. When dihedral angle 768 of facet
672 is lower than the dihedral angle 766 of facet 670, the
consecutive refraction of ray 722 at facets 670 and 672 will result
in a new out-of-plane propagation angle .beta.+.omega. which. It
will be appreciated that the angular increment .omega. to the
out-of-plane angle is a function of the difference in refractive
indices between layers 642 and 644 and the difference between
dihedral angles 766 and 768. Accordingly, it will also be
appreciated that such waveguide 4 with a corrugated inter-layer
boundary may be configured to provide an incremental increase of
the out-of-plane angle .alpha.long the propagation path. In turn,
this may eventually result in ray 722 exiting from waveguide 4 and
being intercepted and redirected by the light extraction layer (not
shown) according to the principles described above.
[0353] Various surface treatment techniques other than
microstructuring or replication may also be used to produce
suitable light deflecting elements which could incrementally bend
light in small angular increments along the propagation path. For
example selective UV exposure or chemical processing may be used to
cause repetitive variations in the refractive index along the
waveguide's surface or through the body of the waveguide. The
variations of the refractive index along the propagation path may
create a number of optical interfaces which can bend light by a
relatively small angle upon each interaction with the propagated
light. As light rays accumulate a sufficient increment to the
out-of-plane angle to overcome TIR, they can exit from waveguide 4
where they can be further redirected by light extracting layer
20.
[0354] System 2 may also incorporate any number of auxiliary layers
serving various purposes, such as, for example, providing
additional mechanical strength, environmental resistance, peel
resistance, improved visual appearance, color, etc. Any optical
interface between a layer formed by a lower refractive index
transmissive medium and a layer formed by a higher refractive index
transmissive medium may also be provided with an intermediate
optically transmissive layer, for example, for promoting the
optical contact or adhesion between the layers. The intermediate
layer should preferably have a refractive index which is greater
than the lower of the two refractive indices at the given optical
interface.
[0355] It may be appreciated that the system 2 may be implemented
in a sheet form and may provide efficient light distribution across
a large-area and emission of a collimated beam from its entire
surface, which may find utility in various lighting devices.
Particularly, system 2 may be employed for making directional
lighting luminaires and fixtures having a wide emission area,
compact form and reduced glare.
[0356] FIG. 50 shows an exemplary embodiment of a low-profile
directional luminaire 1050 which incorporates system 2 including at
least edge-lit planar slab waveguide 4 and light extraction layer
20 configured for light collimation. Such luminaire 1050 may be
configured in the form of a panel installable in a similar manner
to the fluorescent troffers or non-collimating direct-lit and
edge-lit LED lighting panels. By way of a non-limiting example,
luminaire 1050 may be designed for drop in ceilings in commercial
and institutional buildings and have dimensions common for such
applications. For instance, similarly to the drop-in tiles of the
common suspended grid ceilings, the outer dimensions of luminaire
1050 may be 2'.times.2', 2'.times.4', or 1'.times.4' so that the
luminaire may directly replace the tile panels in the grid.
[0357] Referring further to FIG. 50 luminaire 1050 includes two
linear arrays of high-brightness LEDs 878 and 880, respectively,
attached to the opposing edges of waveguide 4. Each LED array (or
LED strip) may be mounted on a heat sink (indicated as 452 and 454)
to dissipate the heat generated by LEDs 878 and 880. Each of the
heat sinks 452 and 454 may be made from aluminum in the form of a
rectangular block, a hollow rectangular pipe, a channel or any
structural profile having the appropriate cross-section. Various
conventional finned or finless heat sinks may also be employed.
[0358] Each of LEDs 878 and 880 may be provided with some kind of
individual collimating or beam shaping optics. For instance, each
LED may include a dome shaped lens which can aid in light input
into waveguide 4 and reducing light spillage.
[0359] Luminaire 1050 may include a metal or plastic housing 462
configured to hold various luminaire components together
particularly including waveguide 4, light extracting layer 20, LED
strips, heat sinks, wiring (not shown), etc. When housing 462 is
made from a plastic material, the material may be opaque or
transmissive/translucent. Housing 462 may enclose just the
perimeter area of system 2 or may also cover the non-luminous back
surface of the system. In either case, especially when housing 462
is made from an opaque or poorly transmitting material, it should
have an opening corresponding to the light emitting area of system
2. The dimensions of the opening should preferably be at least
slightly smaller than the dimensions of waveguide 4 to ensure that
system 2 and any of its components can be sored securely inside
housing 462. In other words, an outline 1002 of the waveguide
should be sufficiently large compared to the opening so that the
waveguide won't fall through the opening under normal operation and
use of luminaire 1050. Alternatively, or in addition to that, the
opening may be covered by an optically transmissive sheet of
plastic material. The transmissive sheet may also be provided with
light diffusing properties and may be made a part of the optical
assembly of system 2.
[0360] Luminaire 1050 may be provided with any number or light
diffusing layers or otherwise beam-shaping layers. For example, an
opaque light diffusing sheet of a reflective type may be provided
on the back of waveguide 4 to recycle and homogenize stray light. A
light diffusing sheet of a transmissive type may be provided on the
opposing (light emitting) side of the waveguide to smooth out
possible non-uniformities of the emitted beam and/or further
control beam spread.
[0361] Luminaire 1050 of FIG. 50 is also shown to include an LED
driver 554 representing a self-contained power supply that has
outputs matched to the electrical characteristics of the strips of
LEDS 400 and 880. LED driver 554 may be attached to housing 462 or
it may also be incorporated into the housing. Alternatively, LED
driver 554 may be located at a distance from the main body of
luminaire 1050 and may be electrically connected to the LED arrays
via a power cord of a suitable length.
[0362] LED driver 554 may ordinarily be current-regulated and
configured to deliver a consistent current over a range of load
voltages. LED driver 554 may also be configured to provide dimming
of the LEDs by means of pulse width modulation (PWM) circuits or by
any other suitable means. The LED driver may also have more than
one channel for separate control of the opposing LED arrays or for
separate control of individual LEDs or LED groups within the
arrays. The respective LEDs or LED groups, in turn, may be
configured to emit light in different color or different color
temperatures thus allowing for obtaining various static or dynamic
illumination effects and/or for just varying the color of light
emitted by luminaire 1050.
[0363] The structure of system 2 which is incorporated into
luminaire 1050 of FIG. 50 may be selected from any of the foregoing
embodiments of any their variations. Particularly, by employing a
light-collimating embodiment of system 2 with planar waveguide 4,
luminaire 1050 may be configured to emit a highly directional light
beam into a prescribed angular range or towards a well defined area
to be illuminated.
[0364] Depending on the application, the beam angle of luminaire
1050 may be limited to any particular value which is considerably
less than a full 180-degree in any plane. It will be appreciated
that since system 2 may provide light collimation at least in a
longitudinal plane which is parallel to axis 200 and perpendicular
to the prevailing plane of waveguide 4, the directionality of such
luminaire 1050 can be substantially enhanced in comparison to the
conventional edge-lit lighting panels which typically emit light
according to a highly diffuse, lambertian pattern. As it has been
illustrated in reference to the above-described embodiments of
system 2, beam collimation may also be provided in a transversal
plane (a plane perpendicular to axis 200), for example by using and
appropriately configuring the collimating elements which may be
associated with individual LEDs (see, e.g., FIG. 28 and FIG. 29).
Accordingly, the edge-lit luminaire 1050 employing system 2 and
fewer LEDs may be configured to provide highly directional,
wide-area beam patterns normally attainable only by using
direct-lit LED panels having large two-dimensional arrays of
discrete light sources with individual optics. It is essential that
system 2 may be configured to emit substantially all of the light
extracted from waveguide 4 into functional directions with minimum
or no light rejection.
[0365] Considering that, in many practical applications, the light
beam emitted by system 2 and/or luminaire 1050 may not have sharply
defined boundaries, the beam angle may be defined as two times the
vertical angle at which the intensity is 90% of the maximum beam
intensity. In turn, the vertical angle may be defined as an angle
between the center of the emitted directional beam and the
direction in which the beam intensity is evaluated. For example,
with the luminaire 1050 pointed downward, a vertical angle of
0.degree. may thus describe the center of a directional beam
emitted along the surface normal.
[0366] By way of example and not limitation, the beam angle of
directional luminaire 1050 may be limited at least in one plane to
less than 120 degrees. By way of further non-limiting examples, the
beam angle may be limited to 90 degrees, 75 degrees, 60 degrees, 45
degrees, 30 degrees, or 15 degrees. It is noted, however, that any
other practical limits for the beam angle may be also established
depending on the desired illumination pattern and system 2 can be
configured accordingly. It will also be understood that the beam
angle may be controlled independently in each of the longitudinal
and transversal planes which are orthogonal with respect to each
other. In one embodiment, the beam angle may be made the same or
similar in both planes. In one embodiment, the beams angles may be
made different in the respective planes. In one embodiment, the
collimation may be provided in only one of the above planes. For
example, the beam angle in the longitudinal plane may be 60 degrees
and the beam angle in the transversal plane may be up to a full 180
degrees with a diffuse lambertian or gaussian pattern.
[0367] Further details of operation of waveguide illumination
system 2 shown in the drawing figures as well as its possible
variations and further applications will be apparent from the
foregoing description of preferred embodiments. Although the
description above contains many details, these should not be
construed as limiting the scope of the invention but as merely
providing illustrations of some of the presently preferred
embodiments of this invention. Therefore, it will be appreciated
that the scope of the present invention fully encompasses other
embodiments which may become obvious to those skilled in the art,
and that the scope of the present invention is accordingly to be
limited by nothing other than the appended claims, in which
reference to an element in the singular is not intended to mean
"one and only one" unless explicitly so stated, but rather "one or
more." All structural, chemical, and functional equivalents to the
elements of the above-described preferred embodiment that are known
to those of ordinary skill in the art are expressly incorporated
herein by reference and are intended to be encompassed by the
present claims. Moreover, it is not necessary for a device or
method to address each and every problem sought to be solved by the
present invention, for it to be encompassed by the present claims.
Furthermore, no element, component, or method step in the present
disclosure is intended to be dedicated to the public regardless of
whether the element, component, or method step is explicitly
recited in the claims. No claim element herein is to be construed
under the provisions of 35 U.S.C. 112, sixth paragraph, unless the
element is expressly recited using the phrase "means for."
* * * * *