U.S. patent application number 14/639083 was filed with the patent office on 2015-06-25 for light directing films.
The applicant listed for this patent is SERGIY VASYLYEV. Invention is credited to SERGIY VASYLYEV.
Application Number | 20150176798 14/639083 |
Document ID | / |
Family ID | 50547009 |
Filed Date | 2015-06-25 |
United States Patent
Application |
20150176798 |
Kind Code |
A1 |
VASYLYEV; SERGIY |
June 25, 2015 |
LIGHT DIRECTING FILMS
Abstract
Light directing films employing at least two layers forming a
continuous corrugated boundary between major surfaces of the film.
Light received by a major surface of the film is internally
redirected by interacting with the facets of the corrugated
inter-layer boundary and is emitted from the opposing major surface
towards a new propagation direction which is different from the
original propagation direction by up to 90 degrees. Light directing
films may have additional layers and/or layer boundaries which may
provide additional bend angles to light rays or otherwise diffuse
the emitted beam.
Inventors: |
VASYLYEV; SERGIY; (ELK
GROVE, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
VASYLYEV; SERGIY |
ELK GROVE |
CA |
US |
|
|
Family ID: |
50547009 |
Appl. No.: |
14/639083 |
Filed: |
March 4, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13662311 |
Oct 26, 2012 |
9004726 |
|
|
14639083 |
|
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61552332 |
Oct 27, 2011 |
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Current U.S.
Class: |
362/327 ;
362/331 |
Current CPC
Class: |
G02B 5/0278 20130101;
G02B 6/001 20130101; G02B 6/0053 20130101; G02B 5/0236 20130101;
F21V 5/02 20130101 |
International
Class: |
F21V 5/02 20060101
F21V005/02; F21V 8/00 20060101 F21V008/00 |
Claims
1. A light directing structure defined by two opposing broad-area
surfaces, comprising: a first layer of an optically transmissive
material having a first refractive index; a second layer of an
optically transmissive material having a second refractive index
different than the first refractive index; and a corrugated optical
interface formed between said first and second layers and
characterized by a stepped change in a refractive index, wherein
said corrugated optical interface comprises a plurality of facets
adjacent to each other and forming different dihedral angles
between 0.degree. and 90.degree. with respect to a prevailing plane
of said first layer, wherein at least one of said facets is
configured to redirect light by means of at least refraction or a
total internal reflection.
2. A light directing structure as recited in claim 1, wherein said
corrugated optical interface represents a boundary between said
first and second layers.
3. A light directing structure as recited in claim 1, wherein each
of said plurality of facets is planar.
4. A light directing structure as recited in claim 1, wherein at
least one of said plurality of facets has a curved surface.
5. A light directing structure as recited in claim 1, wherein said
corrugated optical interface comprises a repeating pattern of
prismatic structures.
6. A light directing structure as recited in claim 1, wherein said
plurality of facets forms a continuous prismatic pattern.
7. A light directing structure as recited in claim 1, wherein said
first layer of an optically transmissive material comprises
prismatic surface structures.
8. A light directing structure as recited in claim 1, wherein said
corrugated optical interface comprises linear prismatic structures
extending parallel to a reference line.
9. A light directing structure as recited in claim 1, wherein a
surface of said first layer of an optically transmissive material
is mirrored.
10. A light directing structure as recited in claim 1, wherein a
surface of said first layer of an optically transmissive material
is configured for an unimpeded transversal light passage.
11. A light directing structure as recited in claim 1, wherein a
surface of said first layer is made attachable to a surface of an
optical waveguide.
12. A light directing structure as recited in claim 1, wherein a
surface of said first layer comprises a light diffusing surface
texture.
13. A light directing structure as recited in claim 1, wherein
outer surfaces of said first and second layers are generally smooth
and transparent.
14. A light directing structure as recited in claim 1, wherein the
material of one of said first and second layers is selected from
low-n fluoropolymers or resins.
15. A light directing structure as recited in claim 1, wherein the
lower of the two refractive indices of said first and second layers
is less than 1.49.
16. A light directing structure as recited in claim 1, further
comprising a third layer of an optically transmissive material
adjacent to said first layer of an optically transmissive material
and optically coupled to said first layer.
17. A light directing structure as recited in claim 1, wherein said
first and second layers are stacked on top of another structure
having a corrugated optical interface formed between layers of
different refractive index materials.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of application Ser. No.
13/662,311, filed 26 Oct. 2012. This application claims priority
from U.S. provisional application Ser. No. 61/552,332 filed on Oct.
27, 2011, 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 directing films and
light turning films used to redirect light. The present invention
also relates to illumination and backlighting devices employing
light directing and light turning films, such as wide-area lighting
panels, LCD backlights, computer screens, advertising displays,
road signs, and the like.
[0007] 2. Description of Background Art
[0008] Conventionally, light directing films employed in
backlighting systems include a structured major surface with an
array of parallel prisms extending along a direction perpendicular
to the plane in which the film is designed to bend the incident
light. The prisms are usually arranged side by side and form a
plurality of peaks and grooves thus forming a corrugated surface of
the film. Applications exist where surface corrugations are
unwanted since they are prone to soiling and also complicate
surface cleaning. Furthermore, in multi-layer transmissive
structures, the corrugations create air pockets between layers
which may be unwanted in at least some applications such as those
where a monolithic structure is desired. Furthermore, the
distributed air pockets represent a layer having a very low
refractive index which creates a strong total internal reflection
interface. Such interface may prevent certain light rays to pass
through the transmissive structure without being reflected,
particularly in the applications involving light rays propagating
at low angles with respect to the layer surfaces.
[0009] It is therefore an object of this invention to provide an
improved light directing film having one or more smooth surfaces
and eliminating the air spaces associated with surface corrugations
of the prior art devices. 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
[0010] Accordingly, the present invention is directed to light
directing and light turning films which may be employed to redirect
light at various angles with respect to the original propagation
direction. More particularly, at least some embodiments of this
invention are directed to right angle light turning films and light
collimating films.
[0011] The present invention solves a number of light redirection
and distribution problems within a film-thickness optical system
which is not hindered by the limitations of conventional light
directing films solely relying upon external surface
microstructures to provide light bending function. An advantage of
the present system is to provide light redirection within the film
body by employing one or more corrugated internal boundaries formed
by film layers having different refractive indices. In at least
some embodiments, it enables employing a generally smooth light
input surface of the film and allows for laminating the light
redirecting films onto light distribution objects such as planar
and cylindrical optical waveguides.
[0012] In at least one embodiment, the invention features an
optical film structure having two layers with different refractive
indices and a generally smooth light input surface. The continuous
boundary between the two layers is shaped in the form of an array
of linear corrugations extending along a pre-selected reference
line. Each corrugation has at least two facets which form non-zero
dihedral angles with a plane which is parallel to the film surface.
Light passes through at least one facet and is redirected from its
original propagation path by means of refraction or total internal
reflection (TIR). The dihedral angles of the facets are selected to
generally result in light bending away from the light input
surface.
[0013] In at least one embodiment, the light redirecting facets of
the corrugated inter-layer boundary are configured to provide a
two-stage redirection, the first stage by means of refraction and
the second stage by means of TIR. The net result is an improved
overall light bending capabilities of the film for the common
materials available for forming the respective layers of the light
directing film.
[0014] In at least one embodiment, the light directing film
comprises a multilayer structure where two or more two-layer films
are stacked on top of each other. Each of the stacked two-layer
films may have its own corrugated boundary between two
different-index optically transmissive materials. The first film in
the stack bends light by a certain angle towards the surface
normal. The next film in the stack receives such light and provides
additional bend angle. The number of elementary two-layer films in
the stack and the configuration of the respective layers may be
selected to provide the required overall bend angle of the
resulting light directing film.
[0015] In at least some embodiments, the light output surface of
the light directing film is provided with surface relief features.
In one embodiment, the surface relief features comprise
light-diffusing texture. In another embodiment, the surface relief
features comprise prismatic microstructures. According to various
aspects of the invention, the prismatic microstructures may provide
additional bending angles for light redirected by the film and may
further provide brightness enhancement and/or light collimating
functions.
[0016] In one embodiment, the light redirecting film is wrapped
around a portion of the longitudinal wall of a cylindrical
waveguide, such as a large-core optical fiber, rod light pipe or
the like. In one embodiment, the inter-layer linear corrugations of
the light directing film follow a circular path. In one embodiment,
the linear corrugation follows a curvilinear path across the
surface of the film.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
[0017] The invention will be more fully understood by reference to
the following drawings which are for illustrative purposes
only:
[0018] FIG. 1 is a schematic perspective view of a light directing
film, according to at least one embodiment of the present
invention.
[0019] FIG. 2 is a schematic cross-sectional view of a light
directing film portion, according to at least one embodiment of the
present invention.
[0020] FIG. 3 is a schematic view and ray tracing for light
directing film portion, showing a boundary between two transmissive
layers comprising isosceles prismatic facets, according to at least
one embodiment of the present invention.
[0021] FIG. 4 is a schematic view and ray tracing for light
directing film portion configured for turning light entering the
film at near-grazing angles, according to at least one embodiment
of the present invention.
[0022] FIG. 5 illustrates the operation of a light directing film
of FIG. 4 in further detail.
[0023] FIG. 6 is a schematic view and ray tracing for a light
directing film in a stacked configuration, according to at least
one embodiment of the present invention.
[0024] FIG. 7 is a schematic view showing a light directing film
wrapped around a portion of a cylindrical waveguide, according to
at least one embodiment of the present invention.
[0025] FIG. 8 is a schematic view of a portion of light directing
film showing a light diffusing output surface, according to at
least one embodiment of the present invention.
[0026] FIG. 9 is a schematic view of a portion of light directing
film showing a light output surface with prismatic microstructured
pattern, according to at least one embodiment of the present
invention.
[0027] FIG. 10 is a schematic top elevation view of a light
directing film showing a circular reference line, according to at
least one embodiment of the present invention.
[0028] FIG. 11 is a schematic top elevation view of a light
directing film showing a curvilinear reference line, according to
at least one embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0029] 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.
[0030] 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.
[0031] The present invention seeks to provide light directing films
capable of changing the light propagation direction and having a
monolithic structure with at least one smooth external surface.
[0032] According to the present invention, there is provided an
optically transmissive light directing film comprising at least two
layers of different materials having a joint boundary enclosed
between the two external parallel surfaces of the film. The light
directing film comprises at least a first layer having a first
refractive index and a second layer having a second refractive
index which is different from the refractive index of the first
layer. Both layers extend generally parallel to each other and
parallel to the opposing broad-area surfaces of the film. The
materials for both layers are preferably selected from solid
polymeric optical materials having good broadband optical clarity
and transmission. When the light directing film is designed to
operate in a specific spectral range, the materials should be
transmissive at least in that spectral range. It is also preferred
that there is no opaque material include between the first and
second layers of the light directing film.
[0033] The first layer and the second layer are disposed in contact
with each other along the entire width of the light directing film
and jointly form an internal corrugated boundary having a zig-zag
shape in a cross-section. The boundary corrugations, each having a
triangular cross sectional shape, extend parallel to each other and
define a first array of optical interface facets inclined at a
first dihedral angle with respect to the surface of the film and a
second array of optical interface facets inclined at a second
dihedral angle with respect to the surface of the film. Any of the
facets may have a curved surface, but ordinarily each facet is
planar.
[0034] The facets of the first array alternate with the facets of
the second array so that the boundary between the two layers has a
saw-tooth contour in a cross-section perpendicular to the
longitudinal axis of the corrugations. The cross-sectional contour
may be described by a continuous series of generally straight lines
connecting alternating peaks and valleys of the corrugations. The
height and width of the corrugations may be varied in a broad
range, but ordinarily all of the corrugations may have identical,
uniform dimensions and constant pitch across the extent of the
light directing film.
[0035] The first dihedral angle may be the same as the second
dihedral angle, in which case the corrugated boundary will be
formed by adjacent pairs of identical facets where the facets are
disposed symmetrically, within each pair, with respect to a plane
perpendicular to the surface of the light directing film. However,
the first and second dihedral angles may also be made different
from each other, taking any angular value between 0.degree. and
90.degree..
[0036] The respective layers of the light directing film may be
made from 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.
Suitable materials for the layer having a lower refractive index of
the two may advantageously be selected for low-n fluoropolymers or
resins, such as, for example, FEP, ETFE (both having a refractive
index of 1.3-1.35), PFTE AF 1600 and PFTE AF 2400, certain
silicones and the like.
[0037] Each of the facets of the corrugated boundary forms an
optical interface characterized by a step change in the refractive
index. As a result, each of the facets will bend the light striking
the facet at any incidence angle other than 0.degree. or
90.degree.. It will be appreciated by those skilled in the art
that, depending on the orientation of the facets with respect the
incident light and depending on whether the refractive index
increases or decreases along the light path, the facets may
primarily bend light by means of refraction or a total internal
reflection (TIR).
[0038] It will be further appreciate that, when referring to light
or other waves passing through a boundary formed between two
different refractive media, such as air and glass, 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
a 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 and n.sub.R are the refractive indices of the material
where the light is incident from and of the material where the
light refract to, respectively, 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.. A TIR angle .phi..sub.TIR can be found from the
following expression:
[0039] .phi..sub.TIR arcsin(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..
[0040] A preferred method of forming the layers of the light
directing film involves forming one of the layers first from a film
of the respective transmissive material. This first-formed layer is
preferably provided with surface corrugations which accurately
replicate the corrugations of the layer boundary to be incorporated
internally into the light directing film.
[0041] The surface corrugations of the first-formed layer may be
ordinarily formed using replication from a master mold or cylinder
carrying a negative replica of the intended surface microstructures
employing common processes such as hot embossing, casting or
extrusion. Masters for the replication or casting tools used for
manufacturing the light directing films described herein, may be
made by any known technique, such as diamond machining, for
example.
[0042] The surface corrugations of the first-formed layer may also
be formed one at a time by various direct surface modification
processes including, for example, micromachining, scribing by a
diamond tool, selective laser ablation, etching and the like
methods. When high fidelity of the corrugations is required and/or
when they are formed by extra fine microstructures, they may be
also fabricated by photolithography using gray scale photomasks and
thick photoresists.
[0043] The other layer may be deposited over the corrugations of
the first-formed layer in a liquid form with the subsequent curing
by means of irradiation UV or visible-spectrum radiation, heat,
moisture, precursor evaporation, etc. Alternatively, the
second-formed layer may be provided in the form of a sufficiently
thick, viscous resin evenly spread over the first-formed layer
placed on a leveled surface. The finished light directing film may
then be obtained by applying pressure on top of the second-formed
layer so that the viscous resin could conform to the shape of the
corrugations of the first-formed layer. The second-formed layer may
also be made in the form of a film which may then be heated to a
softening point and impressed onto the first-formed layer thus also
conforming to the shape of the corrugations. In the latter case,
the material of the second-formed layer should preferably have a
lower softening point than the material of first-formed layer.
[0044] The light directing films of the present invention utilize
the light bending capability of the corrugated interface between
the first layer and the second layer. The light directing film may
be configured so that at least some of the facets forming the
inter-layer corrugations redirect incident light by means of
refraction, TIR, or both. The surface corrugations may be
configured to include at least two types of facets, each type
making a different dihedral angle with the film surface than the
other one. Both types of the facets may be configured to direct
light by means of either refraction or TIR. Also, one of the types
of the facets may be configured to refract light and the other one
may be configured to reflect light by means of TIR.
[0045] The light directing film finds particular utility in LCD
backlights, advertising backlights and front lights, signage, panel
lighting, beam steering, and the like. In some more particular
examples, the light directing film may be used as a brightness
enhancement film or as a light turning film in the applications
employing planar waveguides, such as edge-lit lighting panels,
signs or LCD screens.
[0046] According to preferred embodiments of the present invention,
the light directing film is configured to have a generally smooth,
broad-area light input surface opposing to the broad-area light
output (light emitting) surface. Since the light directing film has
a smooth light input surface, it may be laminated onto another
optical structure such as an edge-lit planar waveguide or an
end-lit large-core optical fiber with a good physical and optical
contact. The good optical contact with no air gap substantially
reduces the drop in the refractive index outwardly from the
underlying optical structure and increases the effective TIR angle
within the structure. Accordingly, the lamination of the light
input surface onto the optical waveguide structure may provide for
a generally unimpeded light passage from such optical structure
into the light directing film in a broad range of incidence angles.
Thus, the light directing film over-laminated to the optical
waveguide structure may be configured to efficiently extract light
from such optical structure and further provide the intended light
redirection function.
[0047] The light output surface may also be made generally smooth
so that additional layers may be laminated on top of the light
directing film thus forming an intermediate light-bending layer in
the resulting monolithic, multi-layer optical structure. The top
layers disposed above the light directing film may have various
functions, such as protection from scratching, abrasion, forming a
chemical or radiation barrier, and the like. Furthermore, two or
more light directing films can be stacked on top of each other
forming a multi-layer light turning structure, for example, to
increase the bend angle of otherwise alter or complement the light
directing operation of a single film layer. According to at least
one embodiment, an external surface of the light directing film may
be optionally mirrored in order to provide a reflective light
bending film structure.
[0048] The present invention will now be described by way of
example with reference to the accompanying drawings.
[0049] FIG. 1 depicts a portion of light directing film 2 in
accordance with one embodiment of the invention. Film 2 is provided
with smooth external surfaces exemplified by a first broad-area
surface 10 and an opposing second broad-area surface 12 which
extends parallel to surface 10. Film 2 comprises at least two
optical layers disposed in contact with each other. The two optical
layers include a first layer 4 of a first transparent material and
a second layer 6 of a second transparent material.
[0050] The materials of layer 4 and layer 6 have different
refractive indices and form a joint boundary 20 which represents an
optical interface characterized by a stepped change of refractive
index between the respective transmissive materials. Boundary 20
has a structured, corrugated shape which is defined by a plurality
of prismatic corrugations 8 extending parallel to each other.
Corrugations 8 may extend generally parallel to a pre-determined
reference line 800 which may hereinafter also be referenced to as a
longitudinal axis of the respective corrugations. The orientation
of reference line 800 and, hence, the orientation of corrugations
8, may be selected based on the intended direction of propagation
of light incident onto the light directing film 2.
[0051] According to one embodiment, the orientation of reference
line 800 may be made generally perpendicular to a plane in which
film 2 is supposed to provide light redirection. When film 2 has a
well defined rectangular shape, the reference line 800 may be
selected to extend parallel to one of the planar dimensions of the
respective rectangle.
[0052] FIG. 2 depicts a schematic cross-section of a portion of
light directing film 2 in accordance with one embodiment. In the
case illustrated in FIG. 2, first layer 4 has a refractive index
n.sub.1 which is lower than a refractive index n.sub.2 of layer 6.
Each of the parallel surfaces 10 and 12 may be made smooth and
planar. However, the boundary 20 between layer 4 and layer 6
represents a continuous corrugated surface formed by an
uninterrupted sequence of linear prismatic corrugations 8.
[0053] Each of corrugations 8 has the shape of a triangular prism
in a cross-section perpendicular to the longitudinal axis of the
corrugation. Each corrugation 8 defines a pair of facets 24 and 26
so that corrugated surface of boundary 20 comprises a plurality of
facets 24 alternating with facets 26. Each faced 24 forms a
dihedral angle 62 with respect to a plane which is parallel to
surfaces 10 and 12. Each facet 26 is normal to either surface.
[0054] In operation, referring to FIG. 2, surface 12 may be
designated as the light input surface and surface 10 may be
designated as a light output surface with respect to the incident
light. Referring further to FIG. 2, an incident ray 102 enters film
2 through surface 12 at a normal angle with respect to the plane of
the surface.
[0055] It will be appreciated that each of the surfaces 10 and 12
may be characterized by a surface normal in a vicinity of a ray
entrance onto the surface of film 2. It will also be appreciated
that, in case of the parallelism of surface 10 and 12, a normal to
one of such surfaces will also be a normal to the other surface.
Additionally, at least the area of film 2 in the vicinity of light
ray entrance may be characterized by a prevailing plane which is
parallel to both surface 10 and 12. Accordingly, a normal 50 to
surface 12 will also be a normal to surface 10 and to the
prevailing plane along an optical path of a ray transversally
crossing film 2. In the optical terms, ray 102 entering into the
surface of film 2 at a normal angle will have a zero incidence
angle with respect to surface normal 50.
[0056] Ray 102 subsequently strikes the optical interface formed by
the boundary 20 between layer 6 and layer 4. Particularly, ray 102
strikes one of facets 24. Since facet 24 is inclined at an angle
with respect to light input surface 12, ray 102 undergoes
refraction which increases the ray's propagation angle with respect
to surface normal 50. At the same time, the refraction of ray 102
decreases an angle 62 which ray 102 makes with respect to a
reference line 42 being parallel to surfaces 10 and 12. Upon
leaving facet 24 and entering layer 4, ray 102 makes a non-zero
angle 34 with respect to normal 50.
[0057] The air surrounding film 2 has a refractive index or around
1, which is lower than the refractive index of layer 4. Therefore,
as ray 102 emerges from layer 4 at surface 10, it obtains an even
greater emergence angle 36 with respect to surface normal 50
compared to angle 34. It will be appreciated that in the
configuration of film 2 and light path geometry illustrated in FIG.
2, emergence angle 36 will always be greater than the zero
incidence angle of ray 102 onto surface 12. It will also be
appreciated that film 2 of FIG. 1 and FIG. 2 can also effectively
bend light in a reversed operation, e.g., when light enters film 2
through surface 10 instead of surface 12 and emerges from surface
12. This reversed operation can be understood, for example, by
reversing the path of ray 102 in FIG. 2. It should also be
understood that the illustrated structure of film 2 may also
refract and bend rays entering its surface at incidence angles
other than normal. Accordingly, film 2 may be configured to
redirect light rays incident onto its surface in a broad range of
incidence angles and redirect such rays at different angles thus
providing new propagation directions to each ray.
[0058] FIG. 3 depicts an embodiment of light directing film 2 which
is similar to that of FIG. 2 except that each of corrugations 8 is
shaped as an isosceles triangular prism where each of the facets 24
and 26 a makes the same dihedral angle 62 with respect to the
prevailing plane of film 2.
[0059] In operation, a ray 104 strikes external surface 12 of film
2 which is also a smooth external surface of layer 6. Ray 104 makes
an incidence angle 30 with respect to a surface normal 40 and
refracts into layer 6 at a refraction angle 32 with respect to the
same normal. Ray 104 further strikes facet 24 where it refracts
into layer 4 in response to the slope of facet 24 and to the
difference between refractive indices n.sub.1 and n.sub.2. As ray
104 strikes surface 10 of film 2, it makes incidence angle 34. The
light input surface 10 represents a smooth external surface of
layer 4 and is configured for an unimpeded transversal light
passage. Upon refracting at surface 10, ray 104 emerges from the
other side of film 2 at emergence angle 36 which is generally
different than angle 30. It will be appreciated that, when
n.sub.1<n.sub.2, angle 36 will generally be lower than angle 30.
Thus, film 2 in the exemplary configuration illustrated in FIG. 3
may be used for reducing the angular spread of light which finds
utility in devices like brightness enhancement films.
[0060] It is noted that the configuration of film 2 depicted in
FIG. 3 may also be operated in a reversed mode in which case the
film may be configured to increase the divergence of light and may
act as a light spreading or light diffusing film.
[0061] FIG. 4 illustrates an embodiment of light directing film 2
employing a two-stage redirection which may generally result in
greater attainable bend angles. In FIG. 4, film 2 has smooth
parallel surfaces 10 and 12 and is formed by two layers disposed in
contact with each other: layer 4 and layer 6. Each of the layers 4
and 6 may have smooth external surface and a corrugated interface
surface defined by corrugated boundary 20 between the layers.
Surface 12 is configured as the light input surface and the
opposing surface 10 is configured as the light output surface.
[0062] The corrugations 8 forming boundary 20 between surface 10
and surface 12 have a linear triangular configuration and extend
parallel to reference line 800 (not shown in FIG. 4). The
corrugations of boundary 20 also define a plurality of adjacent
interface facets. Each interface facet makes a different dihedral
angle from each of the adjacent interface facets so that the
corrugated surface of boundary 20 comprises a plurality of facets
24 characterized by a first dihedral angle alternating with facets
26 characterized by a second dihedral angle with respect to the
prevailing plane of film 2. The refractive index n.sub.1 of layer 4
is lower than refractive index n.sub.2 of layer 6 so that there is
provided a stepped drop in refractive index along the propagation
path of light within the body of film 2.
[0063] Film 2 of FIG. 4 is configured to accept light at near
grazing angles by its surface 12 internally redirect such light at
up to 90.degree. and emit the redirected light from surface 10.
Particularly, film 2 may be configured to accept light striking
surface 12 at angles between 0.degree. and 20.degree. from the
surface (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. An angle 60 represents the oblique angle that an incident
ray 116 makes with surface 12 in layer 6 when it enters layer 6 of
film 2. In the illustrated embodiment, angle 60 may take any
suitable angular values and, particularly, may include very low,
near-grazing angles.
[0064] Facets 24 are configured to have a smaller dihedral angle
with respect to the prevailing plane of film 2 than facets 26.
Furthermore, the dihedral angle of facets 24 is preferably selected
to be less than an angle which is complementary to angle 60 in
order to provide refraction and ray bending away from surface
12.
[0065] The dihedral angle of each of facets 26 should preferably be
greater than the maximum designed value of angle 60 that light can
make with surface 12 in layer 4. At the same time, the dihedral
angle of facets 26 should preferably be selected to enable
intercepting light refracted by a preceding adjacent facet 24 and
further enable reflecting the intercepted light by means of TIR
from facet 26.
[0066] In operation, referring to FIG. 4, light ray 116 enters
layer 6 of film 2 at a low out-of-plane angle and makes angle 60
with surface 12 while propagating within the layer 6. Ray 116
further strikes facet 24 where it bends slightly towards the light
output surface 10 by means or refraction at the interface between
the lower refractive index of layer 6 and the higher refractive
index of layer 4, after which it strikes the next adjacent facet
26. The slope of facet 26 defined by the dihedral angle with the
plane of film 2 is selected to result in TIR at the interface
between the high-index layer 4 and low-index layer 6. As a matter
of optics, facet 26 communicates an additional bend angle to ray
116, this additional bend angle being twice the angle between ray
116 and facet 26. As a result, ray 116 exits from film 2 nearly
perpendicularly to surface 10. It should be understood that film 2
may also be configured to emit light from surface 10 at emergence
angles other than normal to the surface.
EXAMPLE
[0067] In FIG. 5, an example of light directing film 2 is shown as
it is applied for extracting light from an optical waveguide and
turning such light generally towards a perpendicular direction with
respect to the film surface.
[0068] Referring to FIG. 5, an edge-lit planar waveguide 202 made
from PMMA (acrylic) with a refractive index of 1.49 is configured
to propagate light by means of TIR from its major surfaces. Film 2
including two transparent layers with corrugated inter-layer
boundary has a total thickness of 150 microns. The height of the
triangular prisms forming the boundary corrugations 8 is about 50
to 70 microns. Layer 4 of film 2 is made of optically clear
polycarbonate having a refractive index of 1.58. Layer 6 of film 2
is made of PFTE AF 2400 (an amorphous fluoropolymer commercially
available from Dupont.TM.) having a refractive index of
approximately 1.29. The dihedral angle 62 of each facet 24 is 28
degrees and the dihedral angle 64 of each facet 26 is 55.5
degrees.
[0069] The outer surface of layer 6 forming the light input surface
of film 2 is generally smooth and transparent. It may also be
specially treated to enhance the lamination and/or adhesion
properties. Film 2 is laminated onto a portion of planar waveguide
202 so that layer 6 is facing the waveguide and surface 12 is
disposed in a good optical contact with the waveguide. A thin layer
of optical adhesive (not shown) may be provided to promote the
optical contact and adhesion of film 2 to waveguide 202.
[0070] In operation, ray 126 initially propagates in waveguide 202
at an out-of-plane angle of 31 degrees. Since the angle of
incidence of ray 126 with respect to surface normal 40 is
complementary to the out-of-plane angle, the incidence angle of the
ray onto surface 12 is 59
degrees)(90.degree.-31.degree.=59.degree..
[0071] The boundary between waveguide 202 and film 2 represents an
optical interface between PMMA (n=1.49) and PFTE AF 2400 (n=1.29)
and may thus be characterized by a critical TIR angle of 60 degrees
as defined by the Snell's law. Since the 59-degree angle of
incidence of ray 126 onto such boundary is slightly lower than the
TIR angle of 60 degrees, TIR is suppressed and the ray may enter
layer 6 of film 2.
[0072] Upon entering layer 6, ray 126 makes a refraction angle of
about 81.9 degrees with normal 40, as defined by the Snell's law of
refraction. This corresponds to slope angle 60 of ray 126 being
about 8.1 degrees.
[0073] Ray 126 further strikes one of the facets 24 which is
inclined at angle 62 to reference line 42 being parallel to
surfaces 10 and 12. It can easily be found that ray 126 makes an
incidence angle of 53.9 degrees with respect to a normal 44 to the
facet surface and refracts at a refraction angle of about
41.3.degree. with respect to the same normal.
[0074] The refracted ray 126 further strikes the adjacent facet 26
which is inclined at an angle of 55.5 degrees with respect to a
reference line 48 being parallel to surfaces 10 and 12.
Accordingly, it can easily be found that ray 126 makes an incidence
angle of 55.2 degrees with respect to normal 46 to facet 26. This
incidence angle is greater than the 54.7.degree. TIR angle at the
interface between polycarbonate (layer 4) and PFTE AF 2400 (layer
6).
[0075] Therefore, ray 126 is reflected from the respective facet 26
by means of TIR and is further directed towards surface 10. Since
the angle or reflection is the same as the angle of incidence, it
can be shown that ray 126 striking facet 26 at 55.2-degree
incidence angle will be redirected at approximately 0.3-degrees
angle with respect to a normal to surface 10. Accordingly, as a
near-zero incidence angle onto surface 10 is obtained ray 126 may
exit from film 2 nearly perpendicular to that surface and to the
prevailing plane of film 2.
[0076] It will be appreciated that, in the above-illustrated
example, ray 126 entered into film 2 at a relatively low (about 8
degrees) out-of-plane angle and was turned by approximately 82
degrees towards the normal to the prevailing plane of the film. The
corrugated boundary between layers 6 and 4 light has provided most
of the right-angle turning function of light directing film 2.
[0077] Thus, as illustrated in the above example, light directing
film 2 may be coupled to a waveguide for extracting and collimating
at least the portion of light which propagates at near-TIR angles
in the waveguide. Film 2 may thus provide for a controlled leakage
of light into layer 6 at near-grazing angles and turn such light
towards a perpendicular direction.
[0078] It is noted that the angular spread of light leaking from
waveguide 202 into film 2 may be limited by appropriately selecting
the refractive indices of the waveguide and layer 6. Accordingly,
the light extracted and redirected by film 2 towards a surface
normal may also have a limited divergence resulting in the emission
of a collimated beam from the broad-area surface of the film.
[0079] FIG. 6 depicts an embodiment of light directing film 2 which
comprises multiple layers of optically transparent materials
disposed in contact with each other and represents two
single-interface light directing films stacked on top of each
other. The layers have alternating refractive indices and multiple
optical interfaces. At least two internal optical interfaces within
film 2 may have corrugated shapes. The multi-layer stacked film 2
may be laminated on top of optical waveguide 202 which may have,
for example, a planar slab configuration.
[0080] Referring to FIG. 6, light directing film 2 comprises two
opposing external surfaces 10 and 12 extending parallel to each
other. Both of the surfaces may be generally smooth and void of any
microstructures. Surface 12 is configured for light input into film
2 and surface 10 is configured for light output from the film. In
other words, the broad-area surface 12 is configured for receiving
light and the opposing parallel surface 10 is configured for
emitting light internally redirected in film 2.
[0081] The bottom portion of light directing film 2 comprises layer
4 and layer 6 having refractive indices n.sub.1 and n.sub.2,
respectively. Boundary 20 between layers 4 and 6 comprises a
plurality of corrugations 8 represented by alternating facets 24
and 26. Each of the facets 24 and 26 makes a relatively low
dihedral angle with the prevailing plane of film 2.
[0082] Similarly, the top portion of light directing film 2
comprises layers 84 and 86 with refractive indices of n.sub.3 and
n.sub.4, respectively. A boundary 80 between layers 84 and 86
comprises a plurality of corrugations 18 represented by alternating
facets 74 and 76. The dihedral angles of facets 74 and 76 with
respect to the prevailing plane of film 2 are generally greater
than the dihedral slope angles of facets 24 and 26 of the lower
film portion. A boundary 82 between the top and bottom films may be
generally and may also comprise a thin adhesive layer (not shown)
to promote adhesion between the respective layers.
[0083] According to one embodiment n.sub.1<n.sub.2 and
n.sub.3<n.sub.4. Layers 6 and 86 may be made from the same
material having lower refractive index than refractive indices of
layers 4 and 84. Similarly, layers 4 and 84 may also have the same
or similar refractive index. It is also noted that all of layers
may have different refractive indices and may be made from
different materials, provided however, that there if an outward
stepped drop in material refractive index at each of the boundaries
20 and 80 along the intended transversal propagation path of light
through film 2.
[0084] An advantage of the multi-layer, multi-stage film may be
that it may employ high-n and low-n materials with smaller
differences between the refractive indices than, for example, it
was illustrated in FIG. 5, and still provide high bend angles for
grazing-angle light.
[0085] In operation, referring further to FIG. 6, ray 136 exits
from waveguide 202 and enters layer 6 of film 2 at oblique
refraction angle 60. Ray 135 further refracts from one of the
facets 24 and reflects by means of TIR from adjacent facet 26.
While the bend angles at facets 24 and 26 may yet be insufficient
to turn ray 136 at a right angle, each of the interactions of such
ray with the boundary facets nevertheless results in a greater
out-of-plane propagation angle compared to the angle at which ray
136 enters the film. Accordingly, the respective bottom portion of
film 2 redirects ray 136 and turns it approximately halfway towards
the surface normal.
[0086] Ray 136 further crosses boundary 82 between the lower and
upper portions of film 2, with some refraction. Ray 136 further
refracts at facet 74 and is reflected by means of TIR from adjacent
facet 76. Accordingly, the corrugated boundary 80 further bends ray
136 and directs it towards surface 10 at a normal angle. Since
surface 10 is generally smooth and transparent ray 136 emerges from
surface 10 at a near-zero angle with respect to a surface
normal.
[0087] Thus, the corrugated boundaries 20 and 80 of film 2 act
cooperatively to bend light entering surface 12 at oblique angles.
Facets 24 and 26 of boundary 20 represent a first bend stage and
facets 74 and 76 represent a second bend stage for ray 136. The
greater angles of facets 74 and 76 compared to facets 24 and 26
reflect the fact that ray 136 enters the second bend stage at a
greater out-of-plane angle compared to the first stage and requires
a different geometry of the corrugated inter-layer boundary to
ensure that such ray is intercepted by the respective pair of
facets and redirected by means of refraction and TIR.
[0088] It should be understood that film 2 is not limited to be
used in a planar form and/or disposed on flat surfaces. Film 2 may
also be made conforming to irregular surfaces and it may also be
wrapped around objects such as, for example, cylindrical light
pipes or large-core optical fibers.
[0089] FIG. 7 depicts light directing film 2 wrapped around a
portion of a cylindrical waveguide 204. Film 2 is laminated onto
waveguide 204 so that the lower-index layer 6 faces the waveguide
by and the higher-index layer 4 faces away from the waveguide.
Optically smooth surface 12 should preferably have a good physical
and optical contact with the surface of waveguide 204. Surface 12
may be glued to the waveguide using a thin layer of optical
adhesive to promote such contact and eliminate any air gaps.
[0090] Linear corrugations 8 representing a boundary between
different-refractive-index layers extend parallel to each other and
along an arc-shaped reference line 820. Reference line 820 is
shaped in the form of an ark conforming to the circular
cross-section of cylindrical waveguide 204 and lies in a plane
which is generally perpendicular to the longitudinal axis of the
waveguide. In such a configuration, light directing film 2 may be
used, for example, for extracting light from waveguide 204 and emit
the extracted light in a collimated form from surface 10. In at
least one embodiment, film 2 may be configured to direct the
extracted light along a direction which is normal to the
longitudinal axis of the cylindrical waveguide 204.
[0091] In operation, ray 146 propagates in cylindrical waveguide
204 by means of TIR at an angle with respect to waveguide's surface
normal which is slightly greater than the TIR angle at the boundary
of waveguide with the surrounding air. Since layer 6 has a
refractive index greater than the air, TIR is suppressed in
waveguide 204 at least for the uttermost out-of-plane rays
propagating in the waveguide. In other words, the TIR angle at the
boundary between waveguide 204 and film 2 is greater than the TIR
angle at the interface between the material of the waveguide and
air.
[0092] According to one embodiment, the refractive index of layer 6
may be substantially lower than that of waveguide 204 and result in
ray 146 incidences into film 2 at a low out-of-axis angle which may
also include near-grazing angles. Similarly to the principles
described above in reference to FIG. 4 and FIG. 5, corrugated
boundary 20 of film 2 may be configured to turn ray 146 towards a
normal with respect to the prevailing propagation direction of
light in waveguide 204 thus resulting in light emission from a
longitudinal side of waveguide 204 to which film 2 is attached. For
example, similarly to the principles described in reference to FIG.
5, the cross-sectional shape of corrugations 8 may be configured to
redirect light rays by means of refraction from one subset of
boundary facets and further redirect such rays by means of TIR from
another subset of boundary facets. Since film 2 may be configured
to extract light propagating within a relatively narrow range of
out-of-plane angles in waveguide 204, the degree of light
collimation may be precisely controlled.
[0093] FIG. 8 shows an embodiment of film 2 similar to that
depicted in FIG. 5 except that the light output surface 10 has a
light-diffusing surface texture. According to one embodiment, the
surface texture may have a random character. However, it should be
understood that any other types of light-diffusing surface
microstructures may also be used, such as, for example, microprisms
or microlenses. The textured surface 10 may be configured to
diffuse light emerging from film 2, for example, in order to
provide a soft glow from the film surface and/or reduce intensity
irregularities in the emitted beam.
[0094] FIG. 9 illustrates a further variation of the light output
surface 10 where such surface is textured with a pre-defined
prismatic microstructured pattern. The microprisms may have a
function of further redirecting light emerging from surface 10.
Particularly, such function may include reducing the divergence of
emitted light rays from a surface normal. According to an aspect,
the function of the microprisms of surface 10 may be somewhat
similar to the function of the microprisms of brightness
enhancement film such as BEF film commercially available from 3M
Company.
[0095] The operation of the microstructured surface 10 is further
illustrated in FIG. 9 by example of a light ray 152. Ray 152
propagating in optical waveguide 202 enters optically transmissive
layer 6 having refractive index n.sub.2 which is lower than the
refractive index of the waveguide. Upon entering into layer 6, ray
152 makes an angle 160 with respect to light input surface 12 of
film 2. Ray 152 further undergo refraction at facet 24 of the
corrugated boundary 20 and TIR at adjacent facet 26 of the same
boundary subsequently entering layer 4 of film 2. The refractive
index n.sub.1 of layer 4 is preferably greater than n.sub.1. To
enable such two-stage bending of ray 152. However, at least in some
embodiments, the difference in refractive indices or the dihedral
angles of facets 24 and/or 26 may be insufficient to make a
complete turn of ray 154 towards a normal to the light output
surface 10. Accordingly, as illustrated in FIG. 9, the prismatic
corrugations or microstructured pattern of surface 10 may be
configured to provide an additional bend angle and result in ray
154 exiting at a right angle with respect to the surface plane.
[0096] It is noted that this invention is not limited to aligning
corrugations 8 along a straight reference line and may be applied
to the case when such corrugations extend along any other path
which may incorporate straight segments, curvilinear segments or
any combination thereof. Accordingly, film 2 may have any suitable
dimensions and may also be formed or cut to other than rectangular
shapes.
[0097] FIG. 10 and FIG. 11 illustrate different exemplary
configurations of film 2 and different shapes of reference line 800
which defines the path along which the linear corrugations 8 may
extend across the film surface. In FIG. 10, depicted with the light
emitting surface 10 facing towards the viewer, reference line 800
is a circle which is centered with respect to the circular outline
of film 2. In FIG. 11, reference line 800 is curvilinear and
extends generally perpendicular to a longer dimension of the
rectangular film 2. It is further noted that linear corrugations 8
may extend continuously along the pre-selected reference line or
they may also arranged in series where corrugations 8 may be
alternating with spacing areas along such reference line.
[0098] The height and width of corrugations 8 as well as the
dihedral angles formed by facets 24 and 26 may be varied in a broad
range across the area of film 2. By way of example and not
limitation, when film 2 is used for extracting and redirecting
light from an edge-lit planar waveguide or end-lit optical fiber,
the dihedral angles of facets 24 and/or 26 may increase or decrease
as a function of the distance from the respective light source.
Similarly, when the external light emitting surface 10 is provided
with prismatic corrugations for brightness enhancement or improved
light turning capabilities of film 2, the angles of the respective
facets of the prisms may also be varied as a function of the
distance from the light source.
[0099] Further details of operation of light directing film 2 shown
in the drawing figures as well as its possible variations 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."
* * * * *