U.S. patent application number 13/161279 was filed with the patent office on 2011-10-13 for optical element for lateral light spreading in edge-lit displays and system using same.
This patent application is currently assigned to 3M Innovative Properties Company. Invention is credited to Kenneth A. Epstein, Michael F. Weber.
Application Number | 20110249446 13/161279 |
Document ID | / |
Family ID | 37192543 |
Filed Date | 2011-10-13 |
United States Patent
Application |
20110249446 |
Kind Code |
A1 |
Epstein; Kenneth A. ; et
al. |
October 13, 2011 |
OPTICAL ELEMENT FOR LATERAL LIGHT SPREADING IN EDGE-LIT DISPLAYS
AND SYSTEM USING SAME
Abstract
An illumination light unit has at least one light source that
generates illumination light. The unit also includes a reflecting
cavity having one or more reflectors and a controlled transmission
mirror disposed at an output of the reflecting cavity. The
controlled transmission mirror includes an input coupling element,
an output coupling element and a first multilayer reflector
disposed between the input and output coupling elements. At least
some of the illumination light is reflected within the reflecting
cavity by the one or more reflectors and is transmitted out of the
reflecting cavity through the controlled transmission mirror. The
illumination light unit may be used for generating light for space
lighting, or for illuminating a display. For example, the unit may
be used in a backlight to illuminate a lightguide placed behind a
display panel.
Inventors: |
Epstein; Kenneth A.; (St.
Paul, MN) ; Weber; Michael F.; (Shoreview,
MN) |
Assignee: |
3M Innovative Properties
Company
|
Family ID: |
37192543 |
Appl. No.: |
13/161279 |
Filed: |
June 15, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11167003 |
Jun 24, 2005 |
|
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13161279 |
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Current U.S.
Class: |
362/296.09 |
Current CPC
Class: |
G02F 1/133615 20130101;
G02F 1/133617 20130101; G02F 1/13362 20130101 |
Class at
Publication: |
362/296.09 |
International
Class: |
F21V 7/00 20060101
F21V007/00 |
Claims
1. An illumination light unit, comprising: at least a first light
source capable of generating illumination light; and a reflecting
cavity having one or more reflectors and a controlled transmission
mirror disposed at an output of the reflecting cavity, the
controlled transmission mirror comprising an input coupling
element, an output coupling element, a transparent layer disposed
between the input and output coupling elements, and a first
multilayer reflector disposed between the input and output coupling
elements, at least some of the illumination light from the at least
a first light source being reflected within the reflecting cavity
by the one or more reflectors and being transmitted out of the
reflecting cavity through the controlled transmission mirror.
2. A unit as recited in claim 1, wherein the at least a first light
source comprises a light emitting diode (LED).
3. A unit as recited in claim 1, wherein the at least a first light
source comprises at least a first light source and a second light
source, the first light source generating light at a first
wavelength and the second light source generating light at a second
wavelength different from the first wavelength.
4. A unit as recited in claim 1, further comprising a light
wavelength converter disposed to convert the wavelength of the
illumination light output through the controlled transmission
mirror.
5. A unit as recited in claim 1, wherein the reflecting cavity is
elongated along a longitudinal axis and has a first end, and the
controlled transmission mirror is on a first side of the reflecting
cavity, substantially parallel to the longitudinal axis.
6. A unit as recited in claim 5, wherein the at least a first light
source is disposed at the first end of the reflecting cavity.
7. A unit as recited in claim 5, wherein the at least a first light
source is disposed on a second side of the reflecting cavity.
8. A unit as recited in claim 1, wherein the reflecting cavity
comprises at least one curved reflector on an optical path between
the one or more light sources and the controlled transmission
mirror.
9. A unit as recited in claim 8, wherein the at least one curved
reflector is curved in one dimension only.
10. A unit as recited in claim 8, wherein the at least one curved
reflector comprises at least two curved reflectors, each of the
curved reflectors curved in two dimensions.
11. A unit as recited in claim 1, wherein the transparent layer is
disposed between the first multilayer reflector and the output
coupling element.
Description
RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 11/167,003, filed on Jun. 24, 2005, published as U.S.
2006/0290844, now allowed.
[0002] This application is related to the following applications,
all of which are incorporated herein by reference: U.S. Pat. No.
7,903,194; U.S. Pat. No. 7,322,731; U.S. Patent Application
Publication No. 2006/0290843; and U.S. Patent Application
Publication No. 2006/0290845.
FIELD OF THE INVENTION
[0003] The invention relates to optical lighting and displays, and
more particularly to signs and display systems that are illuminated
by edge-lit backlights.
BACKGROUND
[0004] Liquid crystal displays (LCDs) are optical displays used in
devices such as laptop computers, hand-held calculators, digital
watches and televisions. Some LCDs, for example used in as laptop
computers, cell phones, and some smaller computer monitor and
television screens, are illuminated from behind using a backlight
that has a number of light sources positioned to the side of the
display panel. The light is guided from the light sources using a
light guide that is positioned behind the display. The light guide
typically includes some arrangement for extracting the light from
the light guide and directing the light towards the display panel.
This arrangement is commonly referred to as an edge-lit
display.
[0005] One important aspect of the backlight is that the light
illuminating the display panel should be uniformly bright.
Illuminance uniformity is particularly a problem when the light
sources used at the edge of the backlight are point sources, for
example LEDs. The light guide is typically designed to spread the
light within the light guide so that the display has no dark areas.
This problem is less acute when extended light sources are
employed, for example, fluorescent tubes, although it is still
necessary to ensure that the amount of light extracted per unit
area is uniform across the display.
SUMMARY OF THE INVENTION
[0006] One embodiment of the invention is directed to an optical
system that has an image-forming panel having an illumination side
and a viewing side, and a light guide disposed to the illumination
side of the image-forming panel. An illumination light unit has one
or more light sources capable of generating illumination light and
a controlled transmission mirror arranged to receive the
illumination light from the one or more light sources. The
controlled transmission mirror has an input coupling element, an
output coupling element and a first multilayer reflector disposed
between the input and output coupling elements. The input coupling
element redirects at least some of the illumination light incident
thereon in a direction substantially perpendicular to the first
multilayer reflector into a direction that is transmitted through
the multilayer reflector to the output coupling element.
Illumination light from the output coupling element is coupled into
the light guide.
[0007] Another embodiment of the invention is directed to an
illumination light unit, that has at least a first light source
capable of generating illumination light and a reflecting cavity
having one or more reflectors and a controlled transmission mirror
disposed at an output of the reflecting cavity. The controlled
transmission mirror includes an input coupling element, an output
coupling element and a first multilayer reflector disposed between
the input and output coupling elements. At least some of the
illumination light from the at least a first light source is
reflected within the reflecting cavity by the one or more
reflectors and is transmitted out of the reflecting cavity through
the controlled transmission mirror.
[0008] The above summary of the present invention is not intended
to describe each illustrated embodiment or every implementation of
the present invention. The following figures and detailed
description more particularly exemplify these embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The invention may be more completely understood in
consideration of the following detailed description of various
embodiments of the invention in connection with the accompanying
drawings, in which:
[0010] FIG. 1 schematically illustrates an edge-lit liquid crystal
display (LCD) device having a backlight that uses an illumination
light unit, according to principles of the present invention;
[0011] FIG. 2A schematically illustrates a cross section through an
exemplary embodiment of an illumination light unit according to
principles of the present invention;
[0012] FIG. 2B schematically illustrates a cross section through
another exemplary embodiment of an illumination light unit
according to principles of the present invention;
[0013] FIGS. 3A-3D schematically illustrate cross-sectional views
of different embodiments of input coupling elements for a
controlled transmission mirror, according to principles of the
present invention;
[0014] FIGS. 4A-4D schematically illustrate cross-sectional views
of different embodiments of output coupling elements for a
controlled transmission mirror, according to principles of the
present invention;
[0015] FIG. 5A schematically illustrates a cross-sectional view of
an embodiment of a polarization sensitive controlled transmission
mirror, according to principles of the present invention;
[0016] FIGS. 5B and 5C schematically illustrate different
embodiments of polarization-sensitive output coupling elements
according to principles of the present invention;
[0017] FIGS. 6A and 6B schematically illustrate an embodiment of an
illumination light unit having a controlled transmission mirror,
according to principles of the present invention;
[0018] FIG. 7A schematically illustrates another embodiment of an
illumination light unit having a controlled transmission mirror,
according to principles of the present invention;
[0019] FIG. 7B schematically illustrates an exemplary embodiment of
a backlight that uses an illumination light unit, according to
principles of the present invention;
[0020] FIGS. 8A and 8B schematically illustrate other embodiments
of illumination light units having a controlled transmission
mirror, according to principles of the present invention;
[0021] FIGS. 9A and 9B schematically illustrate another exemplary
embodiment of an illumination light unit having a controlled
transmission mirror, according to principles of the present
invention;
[0022] FIGS. 10A and 10B schematically illustrate another exemplary
embodiment of an illumination light unit having a controlled
transmission mirror, according to principles of the present
invention; and
[0023] FIGS. 11A-11C schematically illustrate more exemplary
embodiments of illumination light units having a controlled
transmission mirror, according to principles of the present
invention.
[0024] While the invention is amenable to various modifications and
alternative forms, specifics thereof have been shown by way of
example in the drawings and will be described in detail. It should
be understood, however, that the intention is not to limit the
invention to the particular embodiments described. On the contrary,
the intention is to cover all modifications, equivalents, and
alternatives falling within the spirit and scope of the invention
as defined by the appended claims.
DETAILED DESCRIPTION
[0025] The present invention is applicable to illuminated signs and
displays, such as liquid crystal displays (LCDs, or LC displays),
and is particularly applicable to LCDs that are illuminated using
light sources that are placed to the side of the display panel, for
example as are used in cell phones, portable DVD players, laptop
computer displays and the like.
[0026] A schematic exploded view of an exemplary embodiment of an
edge-lit display device 100 is presented in FIG. 1. In this
exemplary embodiment, the display device 100 uses a liquid crystal
(LC) display panel 102, which typically comprises a layer of LC 104
disposed between panel plates 106. The plates 106 are often formed
of glass, or another stiff material, and may include electrode
structures and alignment layers on their inner surfaces for
controlling the orientation of the liquid crystals in the LC layer
104. The electrode structures are commonly arranged so as to define
LC panel pixels, areas of the LC layer where the orientation of the
liquid crystals can be controlled independently of adjacent pixels.
A color filter may also be included with one or more of the plates
106 for imposing color on the displayed image.
[0027] An upper absorbing polarizer 108 is positioned above the LC
layer 104 and a lower absorbing polarizer 110 is positioned below
the LC layer 104. In the illustrated embodiment, the upper and
lower absorbing polarizers 108, 110 are located outside the LC
panel 102. The absorbing polarizers 108, 110 and the LC panel 102,
in combination, control the transmission of light from a backlight
112 through the display 100 to the viewer. In some exemplary
embodiments, when a pixel of the LC layer 104 is not activated, it
does not change the polarization of light passing therethrough.
Accordingly, light that passes through the lower absorbing
polarizer 110 is absorbed by the upper absorbing polarizer 108,
when the absorbing polarizers 108, 110 are aligned perpendicularly.
When the pixel is activated, on the other hand, the polarization of
the light passing therethrough is rotated, so that at least some of
the light that is transmitted through the lower absorbing polarizer
110 is also transmitted through the upper absorbing polarizer 108.
Selective activation of the different pixels of the LC layer 104,
for example by a controller 113, results in the light passing out
of the display at certain desired locations, thus forming an image
seen by the viewer. The controller 113 may include, for example, a
computer or a television controller that receives and displays
television images.
[0028] One or more optional layers 109 may be provided over the
upper absorbing polarizer 108, for example to provide mechanical
and/or environmental protection to the display surface. In one
exemplary embodiment, the layer 109 may include a hardcoat over the
absorbing polarizer 108.
[0029] Some types of LC displays may operate in a manner different
from that described above and, therefore, differ in detail from the
described system. For example, the absorbing polarizers may be
aligned parallel and the LC panel may rotate the polarization of
the light when in an unactivated state. Regardless, the basic
structure of such displays remains similar to that described
above.
[0030] The backlight 112 comprises one or more illumination light
units 114 that generate the illumination light and direct the
illumination light into a lightguide 120. The illumination light
units 114 include a number of light sources 116 to generate the
illumination light. The light sources 116 may be extended light
sources that emit light over an extended length. One example of an
extended light source is a cold cathode, fluorescent tube. The
light sources 116 may also be effective point light sources, for
example, light emitting diodes (LEDs). Other types of light sources
may also be used, such as organic LEDs (OLEDs). This list of light
sources is not intended to be limiting or exhaustive.
[0031] The light sources 116 may be operated within a light
reflecting cavity 118 that is used to collect and direct light to
the lightguide 120. The lightguide 120 guides illumination light
from the light sources 116 to an area behind the display panel 102,
and directs the light to the display panel 102. The light guide 120
may receive illumination light through a single edge, or through
multiple edges. In other embodiments, not illustrated, the light
may be coupled into the light guide 120 through a light coupling
mechanism other than the edge of the light guide 120.
[0032] A base reflector 122 may be positioned on the other side of
the light guide 120 from the display panel 102. The light guide 120
may include light extraction features 123 that are used to extract
the light from the lightguide 120 for illuminating the display
panel 102. For example, the light extraction features 123 may
comprise diffusing spots on the surface of the light guide 120 that
direct light either directly towards the display panel 102 or
towards the base reflector 122. Other approaches may be used to
extract the light from the light guide 120.
[0033] The base reflector 122 may also be useful for recycling
light within the display device 100, as is explained below. The
base reflector 122 may be a specular reflector or may be a diffuse
reflector. One example of a specular reflector that may be used as
the base reflector 122 is Vikuiti.TM. Enhanced Specular Reflection
(ESR) film available from 3M Company, St. Paul, Minn. Examples of
suitable diffuse reflectors include polymers, such as polyethylene
terephthalate (PET), polycarbonate (PC), polypropylene, polystyrene
and the like, loaded with diffusely reflective particles, such as
titanium dioxide, barium sulphate, calcium carbonate and the like.
Other examples of diffuse reflectors, including microporous
materials and fibril-containing materials, are discussed in
co-owned U.S. Patent Application Publication 2003/0118805 A1,
incorporated herein by reference.
[0034] An arrangement of light management layers 124 may be
positioned between the backlight 112 and the display panel 102 for
enhanced performance. For example, the light management layers 124
may include a reflective polarizer 126. The light sources 116
typically produce unpolarized light but the lower absorbing
polarizer 110 only transmits a single polarization state, and so
about half of the light generated by the light sources 116 is not
suitable for transmission through to the LC layer 104. The
reflecting polarizer 126, however, may be used to reflect the light
that would otherwise be absorbed in the lower absorbing polarizer
110, and so this light may be recycled by reflection between the
reflecting polarizer 126 and the base reflector 122. At least some
of the light reflected by the reflecting polarizer 126 may be
depolarized and subsequently returned to the reflecting polarizer
126 in a polarization state that is transmitted through the
reflecting polarizer 126 and the lower absorbing polarizer 110 to
the LC panel 102. In this manner, the reflecting polarizer 126 may
be used to increase the fraction of light emitted by the light
sources 116 that reaches the LC panel 102, and so the image
produced by the display device 100 is brighter.
[0035] Any suitable type of reflective polarizer may be used, for
example, multilayer optical film (MOF) reflective polarizers,
diffusely reflective polarizing film (DRPF), such as
continuous/disperse phase polarizers, wire grid reflective
polarizers or cholesteric reflective polarizers.
[0036] Both the MOF and continuous/disperse phase reflective
polarizers rely on the difference in refractive index between at
least two materials, usually polymeric materials, to selectively
reflect light of one polarization state while transmitting light in
an orthogonal polarization state. Some examples of MOF reflective
polarizers are described in co-owned U.S. Pat. Nos. 5,882,774,
incorporated herein by reference. Commercially available examples
of MOF reflective polarizers include Vikuiti.TM. DBEF-D200 and
DBEF-D400 multilayer reflective polarizers that include diffusive
surfaces, available from 3M Company, St. Paul, Minn.
[0037] Examples of DRPF useful in connection with the present
invention include continuous/disperse phase reflective polarizers
as described in co-owned U.S. Pat. No. 5,825,543, incorporated
herein by reference, and diffusely reflecting multilayer polarizers
as described in e.g. co-owned U.S. Pat. No. 5,867,316, also
incorporated herein by reference. Other suitable types of DRPF are
described in U.S. Pat. No. 5,751,388.
[0038] Some examples of wire grid polarizers useful in connection
with the present invention include those described in U.S. Pat. No.
6,122,103. Wire grid polarizers are commercially available from,
inter alia, Moxtek Inc., Orem, Utah.
[0039] Some examples of cholesteric polarizers useful in connection
with the present invention include those described in, for example,
U.S. Pat. No. 5,793,456, and U.S. Patent Publication No.
2002/0159019. Cholesteric polarizers are often provided along with
a quarter wave retarding layer on the output side, so that the
light transmitted through the cholesteric polarizer is converted to
linear polarization.
[0040] A polarization mixing layer 128 may be placed between the
backlight 112 and the reflecting polarizer 126 to aid in mixing the
polarization of the light reflected by the reflecting polarizer
126. For example, the polarization mixing layer 128 may be a
birefringent layer such as a quarter-wave retarding layer.
[0041] The light management layers 124 may also include one or more
prismatic brightness enhancing layers 130a, 130b. A prismatic
brightness enhancing layer is one that includes a surface structure
that redirects off-axis light into a propagation direction closer
to the axis 132 of the display device 100. This controls the
viewing angle of the illumination light passing through the display
panel 102, typically increasing the amount of light propagating
on-axis through the display panel 102. Consequently, the on-axis
brightness of the image seen by the viewer is increased.
[0042] One example of a brightness enhancing layer has a number of
prismatic ridges that redirect the illumination light through a
combination of refraction and reflection. Examples of prismatic
brightness enhancing layers that may be used in the display device
include the Vikuiti.TM. BEFII and BEFIII family of prismatic films
available from 3M Company, St. Paul, Minn., including BEFII 90/24,
BEFII 90/50, BEFIIIM 90/50, and BEFIIIT. Although only one
brightness enhancing layer may be used, two brightness enhancing
layers 130a, 130b may be used, with their structures oriented at
about 90.degree. to each other. This crossed configuration provides
control of the viewing angle of the illumination light in two
dimensions, the horizontal and vertical viewing angles.
[0043] One exemplary embodiment of the illumination light unit 199
is now described with reference to FIG. 2A. The figure shows part
of the illumination light unit 199, including some light sources
116a, 116b. A reflecting cavity 118 may be formed between at least
one reflecting surface 202 and a controlled transmission mirror 200
that are arranged so that at least some of the illumination light
produced by the sources 116a, 116b is reflected by both the
controlled transmission mirror 200 and the reflecting surface 202.
In the illustrated embodiment, the reflecting surface 202 is
positioned behind the light sources 116a, 116b. The reflecting
cavity 118 advantageously provides uniform edge illumination for
back-lit displays that use linear light sources, such as CCFLs, or
quasi-point light sources, such as LEDs, but may also be used with
other types of light sources.
[0044] The controlled transmission mirror 200 comprises a
multilayer reflector 204 that has a reflection spectrum such that
at least some of the light generated by the light sources 116a,
116b, when normally incident on the multilayer reflector 204, is
reflected.
[0045] An input coupling element 206 is disposed at the lower side
of the multilayer reflector 204, and an output coupling element 208
is disposed at the upper side of the multilayer reflector 204. The
input coupling element 206 and output coupling element 208 are used
to change direction of at least some of the light entering these
coupling elements 206, 208, so as to couple light through the
controlled transmission mirror 200. Exemplary embodiments of input
coupling elements 206 and output coupling elements 208 include
diffusers, both surface and bulk diffusers, and microreplicated
surfaces. Examples of suitable input coupling elements 206 and
output coupling elements 208 are described in greater detail below.
The output coupling element 208 may be the same type of coupling
element as the input coupling element 206, for example, the input
and output coupling element 206, 208 may both be bulk diffusers, or
may be different from the input coupling element 206. The input and
output coupling elements 206, 208 may be laminated or otherwise
formed integrally with the multilayer reflector 204.
[0046] The multilayer dielectric reflector 204 is generally
constructed of optical repeating units that form the basic building
blocks of a dielectric stack. The optical repeating units typically
include two or more layers of at least a high and a low refractive
index material. A multilayer reflector can be designed, using these
building blocks, to reflect infrared, visible or ultraviolet
wavelengths and one or both of a given orthogonal pair of
polarizations of light. In general, the stack can be constructed to
reflect light of a particular wavelength, .lamda., by controlling
the optical thickness of the layers according to the
relationship:
.lamda.=(2/M)*D.sub.r, (1)
where M is an integer representing the order of the reflected
light, and D.sub.r is the optical thickness of an optical repeating
unit. For the first order reflection (M=1), the optical repeating
unit has an optical thickness of .lamda./2. Simple quarter-wave
stacks comprise a number of layers that each have an optical
thickness of one quarter of the wavelength, .lamda./4. Broadband
reflectors can include multiple quarter-wave stacks tuned to
various wavelengths, a stack with a continuous gradation of the
layer thickness throughout the stack, or combinations thereof. A
multilayer reflector may further include non-optical layers. For
example, a coextruded polymeric dielectric reflector may include
protective boundary layers and/or skin layers used to facilitate
formation of the reflector film and to protect the reflector.
Polymeric optical stacks particularly suited to the present
invention are described in published PCT Patent Application WO
95/17303, entitled MULTILAYER OPTICAL FILM, and U.S. Pat. No.
6,531,230, incorporated herein by reference. In other embodiments,
the dielectric stack may be a stack of inorganic materials. Some
suitable materials used for the low refractive index material
include SiO.sub.2, MgF.sub.2 and CaF.sub.2 and the like. Some
suitable materials used for the high refractive index material
include TiO.sub.2, Ta.sub.2O.sub.5, ZnSe and the like. The
invention is not limited to quarter-wave stacks, however, and is
more generally applicable to any dielectric stack including, for
example, computer optimized stacks and random layer thickness
stacks.
[0047] Reflection by a dielectric stack of light at a particular
wavelength is dependent, in part, on the propagation angle through
the stack. The multilayer reflector 204 may be considered as having
a reflection band profile (e.g., band center and bandedges) for
light propagating in the stack at a particular angle. This band
profile changes as the angle of propagation in the stack changes.
The propagation angle in the stack is generally a function of the
incident angle and the refractive indices of the materials in the
stack and the surrounding medium. The wavelength of the reflection
bandedge changes as the propagation angle in the stack changes.
Typically, for the polymeric materials under consideration, the
bandedge of the reflector for light at normal incidence shifts to
about 80% of its normal incidence value when viewed at grazing
incidence in air. This effect is described in greater detail in
U.S. Pat. No. 6,208,466, incorporated herein by reference. The
bandedge may shift considerably further when the light is coupled
into the reflector using a medium having a refractive index higher
than air. Also, the shift in the bandedge is typically greater for
p-polarization light than for s-polarization light.
[0048] The angular dependence of the reflection band profile, e.g.,
bandedge shifting with angle, results from a change in the
effective layer thickness. The reflection band shifts towards
shorter wavelengths as the angle increases from normal incidence.
While the total path length through a given layer increases with
angle, the change in band position with angle does not depend on
the change in the total path length through a layer with angle,
.theta., where the angle is measured relative to an axis 230
perpendicular to the layers of the reflector 204. Rather, the band
position depends on the difference in path length between light
rays reflected from the top and bottom surfaces of a given layer.
This path difference decreases with angle of incidence as shown by
the familiar formula n.d.cos.theta., which is used to calculate the
wavelength, .lamda., to which a given layer is tuned as a .lamda./4
thick layer.
[0049] The above description describes how the bandedge of the
reflection band profile changes as a function of angle. As used
herein, the term bandedge generally refers to the region where the
multilayer reflector changes from substantial reflection to
substantial transmission. This region may be fairly sharp and
described as a single wavelength. In other cases, the transition
between reflection and transmission may be more gradual and may be
described in terms of a center wavelength and bandwidth. In either
case, however, a substantial difference between reflection and
transmission exists on either side of the bandedge.
[0050] As light at the particular wavelength propagates in the
stack at increasing propagation angles (measured from an axis
normal to the interface of the repeating units), the light
approaches the bandedge. In one example, at high enough propagation
angles, the stack will become substantially transparent to that
particular wavelength of light and the light will transmit through
the stack. Thus, for a given wavelength of light, the stack has an
associated propagation angle below which the stack substantially
reflects the light and another propagation angle above which the
stack substantially transmits the light. Accordingly, in certain
multilayer stacks, each wavelength of light may be considered as
having a corresponding angle below which substantial reflection
occurs, and a corresponding angle above which substantial
transmission occurs. The sharper the bandedge, the closer these two
angles are for the associated wavelength. For the purposes of the
present description, the approximation is made that these two
angles are the same and have a value of .theta..sub.min.
[0051] The above description describes the manner in which
monochromatic light in a given stack shifts from reflection to
transmission with increasing angle of propagation. If the stack is
illuminated with light having a mixture of components at different
wavelengths, the angle, .theta..sub.min at which the reflective
stack changes from being reflective to transmissive is likely to be
different for the different wavelength components. Since the
bandedge moves to shorter wavelengths with increasing angle, the
value of .theta..sub.min is lower for light at longer wavelengths,
potentially allowing the more light at longer wavelengths to be
transmitted through the multilayer reflector than at shorter
wavelengths. In some embodiments it is desired that the color of
the light passing out of the controlled transmission mirror be
relatively uniform. One approach to balancing the color is to use
an input and output coupling element that couples more light at
shorter wavelengths than at longer wavelengths into the controlled
transmission mirror.
[0052] One example of such a coupling element is a bulk diffuser
that contains scattering particles dispersed within a polymer
matrix, as is discussed below with regards to FIGS. 3A and 4A. The
scattering particles have a refractive index different from the
surrounding matrix. The nature of diffusive scattering is that, all
else being equal, light at shorter wavelengths is scattered more
than light at longer wavelengths.
[0053] In addition, the degree of scattering is dependent on the
difference between the refractive indices of the particles and the
surrounding matrix. If the difference in refractive index is
greater at shorter wavelengths, then even more short wavelength
light is scattered. In one particular embodiment of a diffusive
coupling element, the matrix is formed of biaxially stretched PEN,
which has an in-plane refractive index of about 1.75 for red light
and about 1.85 for blue light, where the light is s-polarized,
i.e., has high dispersion. The in-plane refractive index is the
refractive index for light whose electric vector is polarized
parallel to the plane of the film. The out-of-plane refractive
index, for light polarized parallel to the thickness direction of
the film, is about 1.5. The refractive index for p-polarized light
is lower than that of the s-polarized light, since the p-polarized
light experiences an effective refractive index that is a
combination of the in-plane refractive index and the out-of-plane
refractive index. The particles in the matrix may have a high
refractive index, for example titanium dioxide (TiO.sub.2)
particles have a refractive index of about 2.5. The refractive
index of TiO.sub.2 varies by approximately 0.25 over the range 450
nm-650 nm, which is greater than the approximately 0.1 refractive
index variation for PEN over a similar wavelength range. Thus, the
refractive index difference between the particles and the matrix
changes by about 0.15 across the visible spectrum, resulting in
increased scattering for the blue light. Consequently, the
refractive index difference between the particles and the matrix
can vary significantly over the visible spectrum.
[0054] Thus, due to the wavelength dependence of the diffusive
scattering mechanism and the large difference in the refractive
index difference over the visible spectrum, the degree to which
blue light is scattered into the multilayer reflector is relatively
high, which at least partially compensates for the larger value of
.theta..sub.min at shorter wavelengths.
[0055] Other embodiments of input and output coupling elements, for
example those described below with reference to FIGS. 3B-3D and
4B-4D, rely primarily on refractive effects for diverting the
light. For example, a coupling element may be provided with a
surface structure or holographic features for coupling the light
into or out of the multilayer reflector. Normal material dispersion
results in greater refractive effects for shorter wavelengths.
Therefore, input and output coupling elements that rely on
refractive effects may also at least partially compensate for the
larger value of .theta..sub.min at shorter wavelengths.
[0056] With the understanding, therefore, that the light entering
the controlled transmission mirror may have a wide variation in the
value of .theta..sub.min, the following description refers to only
a single value of .theta..sub.min for simplicity.
[0057] Another effect that the system designer can use to control
the amount of light passing through the multilayer reflector is the
selection of a Brewster's angle, i.e., the angle at which
p-polarized light passes through the multilayer reflector without
reflective loss. For adjacent isotropic layers 1 and 2 in the
multilayer reflector, having refractive indices n1 and n2
respectively, the value of Brewster's angle in layer 1,
.theta..sub.B, for light passing from layer 1 to layer 2, is given
by the expression tan .theta..sub.B=n2/n1. Thus, the particular
materials employed in the different layers of the multilayer
reflector may be selected to provide a desired value of Brewster's
angle.
[0058] The existence of the Brewster's angle for a multilayer
reflector provides another mechanism for allowing light to pass
through the reflector other than relying on the input and output
coupling layers to divert the light through large angles. As the
angle within the controlled transmission mirror is increased for
p-polarized light, the reflection band substantially disappears at
Brewster's angle. At angles above the Brewster's angle, the
reflection band reappears and continues to shift to shorter
wavelengths.
[0059] In certain embodiments, it may be possible to set the value
of .theta..sub.B for blue light to be less than .theta..sub.min,
but have .theta..sub.B be greater than .theta..sub.min for red
light. This configuration may lead to an increased transmission for
blue light through the multilayer reflector, which compensates at
least in part for the higher value of .theta..sub.min for shorter
wavelength light.
[0060] Returning to FIG. 2A, at least some of the light from the
light source 116a propagates towards the controlled transmission
mirror 200. A portion of the light, exemplified by light ray 210,
passes through the input coupling element 206 and is incident on
the multilayer reflector 204 at an angle greater than
.theta..sub.min and is transmitted through the reflector 204.
Another portion of the light, exemplified by light ray 212, is
incident at the input coupling element 206 at an angle less than
.theta..sub.min, but is diverted by the input coupling element 206
to an angle of at least .theta..sub.min, and is transmitted through
the multilayer reflector 204. Another portion of light from the
light source 116a, exemplified by light ray 214, passes through the
input coupling element 206 and is incident at the multilayer
reflector 204 at an angle that is less than .theta..sub.min.
Consequently, light 214 is reflected by the multilayer reflector
204 to the reflecting surface 202. The light 214 may be reflected
at the reflecting surface 202 either specularly or diffusely.
[0061] In some embodiments it may be desired that the multilayer
reflector 204 is attached to the output coupling element 208 in a
manner that avoids a layer of air, or some other material of a
relatively low refractive index, between the multilayer reflector
204 and output coupling element 208. Such close optical coupling
between the multilayer reflector 204 and the output coupling
element 208 reduces the possibility of total internal reflection of
light at the multilayer reflector 204.
[0062] The maximum angle of the light within the controlled
transmission mirror 200, .theta..sub.max, is determined by the
relative refractive indices of the input coupling element 206,
n.sub.i, and the refractive index of the particular layer of the
multilayer reflector 204, n.sub.1, n.sub.2, where the subscripts 1,
2 refer to the alternating layers in the multilayer reflector 204.
Where the input coupling element 206 is a surface coupling element,
the value of n.sub.i is equal to the refractive index of the
material on which the coupling surface is formed. The effective
refractive index of the multilayer reflector 204 is the average of
the refractive indices of the high index and low index layers.
Propagation from the input coupling element 206 into multilayer
reflector 204 is subject to Snell's law. The value of
.theta..sub.max in each alternate layer of the multilayer reflector
204 is given by the expression:
.theta..sub.max=sin.sup.-1 (n.sub.i/n.sub.1,2). (2)
where either n.sub.1 or n.sub.2 is used. Where n.sub.i>n.sub.1
and n.sub.i>n.sub.2, then .theta..sub.max can be up to
90.degree..
[0063] The output coupling element 208 is used to extract at least
some of the light out of the controlled transmission mirror 200.
For example, some of light 212 may be diffused by the output
coupling element 208 so as to pass out of the controlled
transmission mirror 200 as light 220.
[0064] Other portions of the light, for example ray 222, may not be
diverted by the output coupling element 208. If light 222 is
incident at the upper surface of the output coupling element 208 at
an angle greater than the critical angle of the output coupling
element, .theta..sub.c=sin.sup.-1 (1/n.sub.0), where n.sub.0 is the
refractive index of the output coupling element, then the light 222
is totally internally reflected within the output coupling element
208 as light 224. The reflected light 224 may subsequently be
totally internally reflected at the lower surface of the input
coupling element 206. Alternatively, the light 224 may subsequently
be diverted by the input coupling element 206 and pass out of the
controlled transmission mirror 200 towards the reflecting surface
202.
[0065] If the light that passes into the multilayer reflector 204
with an angle of at least .theta..sub.min is incident at the output
coupling element 208 with an angle greater than .theta..sub.c, then
that light which is not diverted out of the output coupling element
208 is typically totally internally reflected within the output
coupling element 208. If, however, the light that passes into the
multilayer reflector 204 with an angle of at least .theta..sub.min
reaches the output coupling element 208 at a propagation angle less
than .theta..sub.c then a fraction of that light may be transmitted
out through the output coupling element 208, even without being
diverted by the output coupling element 208, subject to Fresnel
reflection loss at the interface between the output coupling
element 208 and the air. Thus, there are many possibilities for the
light to suffer multiple reflections and for its direction to be
diverted within the cavity 114. The light may also propagate
transversely within the substrate 202 and/or within the space
between the controlled transmission mirror 120 and the base
reflector 118. These multiple effects combine to increase the
likelihood that the light is spread laterally and extracted to
produce a backlight illuminance of uniform brightness.
[0066] Except for the possibility that the multilayer reflector has
a value of Brewster's angle, .theta..sub.B, that is lower than
.theta..sub.min, there is a forbidden angular region for light
originating at the light source 116a. This forbidden angular region
has a half-angle of .theta..sub.min, and is located above the light
source 116a. Light cannot pass through the multilayer reflector 204
within the forbidden angular region. Light 232 from a neighboring
light source 116b, however, may be able to escape from the
controlled transmission mirror 200 at a point perpendicularly above
light source 116a, at the axis 230, and so the illumination light
unit 199 is effective at mixing light from different light sources
116a, 116b.
[0067] In view of the description of the controlled transmission
mirror 200 provided above, it can be seen that the function of the
input coupling element 206 is to divert at least some light, that
would otherwise be incident at the multilayer reflector 204 at an
angle less than .theta..sub.min, so as to be incident at the
multilayer reflector 204 at an angle of at least .theta..sub.min.
Also, the function of the output coupling element 208 is to divert
at least some light, that would otherwise be totally internally
reflected within the multilayer reflector 204, so as to pass out of
the controlled transmission mirror 200.
[0068] The controlled transmission mirror 200 may include a
transparent layer 250 disposed between the output coupling element
208 and the multilayer reflector 204, as is schematically
illustrated in FIG. 2B. In other embodiments, the transparent layer
250 may be between the input coupling element 206 and the
multilayer reflector 204. The transparent layer 250 may be formed
of any suitable transparent material, organic or inorganic, for
example polymer or glass. Suitable polymer materials may be
amorphous or semi-crystalline, and may include homopolymer,
copolymer or blends thereof. Example polymer materials include, but
are not limited to, amorphous polymers such as poly(carbonate)
(PC); poly(styrene) (PS); acrylates, for example acrylic sheets as
supplied under the ACRYLITE.RTM. brand by Cyro Industries,
Rockaway, N.J.; acrylic copolymers such as isooctyl
acrylate/acrylic acid; poly(methylmethacrylate) (PMMA); PMMA
copolymers; cycloolefins; cylcoolefin copolymers; acrylonitrile
butadiene styrene (ABS); styrene acrylonitrile copolymers (SAN);
epoxies; poly(vinylcyclohexane); PMMA/poly(vinylfluoride) blends;
atactic poly(propylene); poly(phenylene oxide) alloys; styrenic
block copolymers; polyimide; polysulfone; poly(vinyl chloride);
poly(dimethyl siloxane) (PDMS); polyurethanes; and semicrystalline
polymers such as poly(ethylene); poly(propylene); poly(ethylene
terephthalate) (PET); poly(carbonate)/aliphatic PET blends;
poly(ethylene naphthalate)(PEN); polyamides; ionomers; vinyl
acetate/polyethylene copolymers; cellulose acetate; cellulose
acetate butyrate; fluoropolymers; poly(styrene)-poly(ethylene)
copolymers; PET and PEN copolymers, and clear fiberglass panels.
Some of these materials, for example PET, PEN and copolymers
thereof, may be oriented so as to change the material refractive
index from that of the isotropic material.
[0069] The transparent layer 250 may be used to allow more lateral
spreading of the light from the light sources 116 before extracting
the light from the controlled transmission mirror 200 using the
output coupling element 208.
[0070] One or more of the edges of the transparent layer 250 may be
covered by a reflector 252. Thus, light 254 that might otherwise
escape from the transparent layer 250 is reflected back into the
transparent layer 250 and may be extracted from the illumination
light unit 114 as useful illumination light. The reflector 254 may
be any suitable type of reflector, including a multilayer
dielectric reflector, a metal coating on the edge of the
transparent layer 250, a multilayer polymer reflector, a diffuse
polymer reflector, or the like. In the illustrated embodiment, the
reflector 252 at the side of the lower reflector 202 may be also
used as a side reflector for the reflecting cavity 118, although
this is not intended to be a limitation of the invention.
[0071] In some other embodiments, the controlled transmission
mirror 200 may be provided with two multilayer reflectors
positioned on either side of the transparent layer 250. The
multilayer reflectors may have the same value of .theta..sub.min,
although this is not required. The use of a transparent layer is
described further in U.S. patent application Ser. No. ______,
titled "OPTICAL ELEMENT FOR LATERAL LIGHT SPREADING IN BACK-LIT
DISPLAYS AND SYSTEM USING SAME" filed on even date herewith, and
having attorney docket no. 60499US002, incorporated herein by
reference.
[0072] Exemplary embodiments of different types of input coupling
elements are now discussed with reference to FIGS. 3A-3D. In other
exemplary embodiments, not illustrated, a transparent layer may be
provided between the multilayer reflector and either of the input
and output coupling elements.
[0073] In FIG. 3A, an exemplary embodiment of a controlled
transmission mirror 320 comprises an input coupling element 326, a
multilayer reflector 304 and an output coupling element 308. In
this particular embodiment, the input coupling element 326 is a
bulk diffusing layer, comprising diffusing particles 326a dispersed
within a transparent matrix 326b. At least some of the light
entering the input coupling element 326 at an angle less than
.theta..sub.min, for example light rays 328, is scattered within
the input coupling element 326 at an angle greater than
.theta..sub.min, and is consequently transmitted through the
multilayer reflector 304. Some light, for example ray 330, may not
be scattered within the input coupling element 326 through a
sufficient angle to pass through the multilayer reflector 304, and
is reflected by the multilayer reflector 304. Suitable materials
for the transparent matrix 326b include, but are not limited to,
polymers such as those listed as being suitable for use in a
transparent layer above.
[0074] The diffusing particles 326a may be any type of particle
useful for diffusing light, for example, transparent particles
whose refractive index is different from the surrounding polymer
matrix, diffusely reflective particles, or voids or bubbles in the
matrix 326b. Examples of suitable transparent particles include
solid or hollow inorganic particles, for example glass beads or
glass shells, solid or hollow polymeric particles, for example
solid polymeric spheres or polymeric hollow shells. Examples of
suitable diffusely reflecting particles include particles of
titanium dioxide (TiO.sub.2), calcium carbonate (CaCO.sub.3),
barium sulphate (BaSO.sub.4), magnesium sulphate (MgSO.sub.4) and
the like. In addition, voids in the matrix 426b may be used for
diffusing the light. Such voids may be filled with a gas, for
example air or carbon dioxide.
[0075] Another exemplary embodiment of a controlled transmission
mirror 340 is schematically illustrated in FIG. 3B, in which the
input coupling element 346 comprises a surface diffuser 346a. The
surface diffuser 346a may be provided on the bottom layer of the
multilayer reflector 304 or on a separate layer attached to the
multilayer reflector 304. The surface diffuser 346a may be molded,
impressed, cast or otherwise prepared.
[0076] At least some of the light incident at the input coupling
element 346, for example light rays 348, is scattered by the
surface diffuser 346a to propagate at an angle greater than
.theta..sub.min, and is consequently transmitted through the
multilayer reflector 304. Some light, for example ray 350, may not
be scattered by the surface diffuser 346a through a sufficient
angle to pass through the multilayer reflector 304, and is
reflected.
[0077] Another exemplary embodiment of a controlled transmission
mirror unit 360 is schematically illustrated in FIG. 3C, in which
the input coupling element 366 comprises a microreplicated
structure 367 having facets 367a and 367b. The structure 367 may be
provided on the bottom layer of the multilayer reflector 304 or on
a separate layer attached to the multilayer reflector 304. The
structure 367 is different from the surface diffuser 346a of FIG.
3B in that the surface diffuser 346a includes a mostly random
surface structure, whereas the structure 367 includes more regular
structures with defined facets 367a, 367b.
[0078] At least some of the light incident at the input coupling
element 366, for example rays 368 incident on facets 367a, would
not reach the multilayer reflector 304 at an angle of
.theta..sub.min but for refraction at the facet 367a. Accordingly,
light rays 368 are transmitted through the multilayer reflector
304. Some light, for example ray 370, may be refracted by facet
367b to an angle less than .theta..sub.min, and is, therefore,
reflected by the multilayer reflector 304.
[0079] Another exemplary embodiment of a controlled transmission
mirror 380 is schematically illustrated in FIG. 3D, in which the
input coupling element 386 has surface portions 382 in optical
contact with the multilayer reflector 304 and other surface
portions 384 that do not make optical contact with the multilayer
reflector 304, with a gap 388 being formed between the element 386
and the multilayer reflector 304. The presence of the gap 388
provides for total internal reflection (TIR) of some of the
incident light. This type of element may be referred to as a TIR
input coupling element.
[0080] At least some of the light incident at the input coupling
element 386, for example rays 390 incident on the non-contacting
surface portions 384 would not reach the multilayer reflector 304
at an angle of .theta..sub.min but for internal reflection at the
surface 384. Accordingly, light rays 390 may be transmitted through
the multilayer reflector 304. Some light, for example ray 392, may
be transmitted through the contacting surface portion 382 to the
multilayer reflector 304. This light is incident at the multilayer
reflector 302 at an angle less than .theta..sub.min, and so is
reflected by the multilayer reflector 304.
[0081] Other types of TIR input coupling elements are described in
greater detail in U.S. Pat. No. 5,995,690, incorporated herein by
reference.
[0082] Other types of input coupling elements may be used in
addition to those described in detail here, for example input
coupling elements that include a surface or a volume hologram.
Also, an input coupling element may combine different approaches
for diverting light. For example, an input coupling element may
combine a surface treatment, such as a surface structure or surface
scattering pattern, or surface hologram, with bulk diffusing
particles.
[0083] It may be desired in some embodiments for the refractive
index of the input coupling element and output coupling element to
have a relatively high refractive index, for example comparable to
or higher than the effective refractive index (the average of the
refractive indices of the high index and low index layers) of the
multilayer reflector 304. A higher refractive index for the input
and output coupling elements helps to increase the angle at which
light may propagate through the multilayer reflector 304, which
leads to a greater bandedge shift. This, in turn, may increase the
amount of short wavelength light that passes through the controlled
transmission mirror, thus making the color of the backlight
illumination more uniform. Examples of suitable high refractive
index polymer materials that may be used for input and output
coupling elements include biaxially stretched PEN and PET which
depending on the amount of stretch, can have in-plane refractive
index values of 1.75 and 1.65 respectively for a wavelength of 633
nm.
[0084] Commensurate with the choice of materials for the input and
output coupling elements, the substrate should be chosen to have an
index that does not cause TIR that would block prohibitive amounts
of light entering or exiting at large angles. Conversely, a low
index for the substrate would result in high angles of propagation
in the substrate after injection from the input coupler having a
higher index than the substrate. These two effects can be chosen to
optimize the performance of the system with respect to color
balance and lateral spreading of the light.
[0085] Similar approaches may be used for the output coupling
element. For example, a controlled transmission mirror unit 420 is
schematically illustrated in FIG. 4A having an input coupling
element 406, a multilayer reflector 404 and an output coupling
element 428. In this particular embodiment, the output coupling
element 428 is a bulk diffusing layer, comprising diffusing
particles 428a dispersed within a transparent matrix 428b. Suitable
materials for use as the diffusing particles 428a and the matrix
428b are discussed above with respect to the input coupling element
326 of FIG. 3A.
[0086] At least some of the light entering the output coupling
element 428 from the multilayer reflector 404, for example light
ray 430, may be scattered by the diffusing particles 428a in the
output coupling element 408 and is consequently transmitted out of
the light output coupling element 428. Some light, for example ray
432, may not be scattered within the output coupling element 428
and is incident at the top surface 429 of the output coupling
element 428 at an incident angle of .theta.. If the value of
.theta. is equal to or greater than the critical angle,
.theta..sub.c, for the material of the matrix 428b, then the light
432 is totally internally reflected at the surface 429.
[0087] Another exemplary embodiment of controlled transmission
reflector 440 is schematically illustrated in FIG. 4B, in which the
output coupling element 448 comprises a surface diffuser 448a. The
surface diffuser 448a may be provided on the upper surface of the
multilayer reflector 404 or on a separate layer attached to the
multilayer reflector 404.
[0088] Some light propagating within the multilayer reflector 404,
for example light 450, is incident at the surface diffuser 448a and
is scattered out of the light mixing layer 440. Some other light,
for example light 452, may not be scattered by the surface diffuser
448a. Depending on the angle of incidence at the surface diffuser
448a, the light 452 may be totally internally reflected, as
illustrated, or some light may be transmitted out of the controlled
transmission mirror 440 while some is reflected back within the
multilayer reflector 404.
[0089] Another exemplary embodiment of controlled transmission
mirror 460 is schematically illustrated in FIG. 4C, in which the
output coupling element 466 comprises a microreplicated structure
467 having facets 467a and 467b. The structure 467 may be provided
on a separate layer 468 attached to the multilayer reflector 404,
as illustrated, or be integral with the top surface of the
multilayer reflector 404 itself. The structure 467 is different
from the surface diffuser 448a of FIG. 4B in that the surface
diffuser 448a includes a mostly random surface structure, whereas
the structure 467 includes more regular structures with defined
facets 467a, 467b.
[0090] Some light propagating within the multilayer reflector 404,
for example light 470, is incident at the surface diffuser
structure 467 and is refracted out of the light mixing layer 460.
Some other light, for example light 472, may not be refracted out
of the light mixing layer 460 by the structure 467, but may be
returned to the multilayer reflector 404. The particular range of
propagation angles for light to escape from the controlled
transmission mirror 460 is dependent on a number of factors,
including at least the refractive indices of the different layers
that make up the controlled transmission mirror 460 and the layer
468 as well as the shape of the structure 467.
[0091] Another exemplary embodiment of a controlled transmission
mirror 480 is schematically illustrated in FIG. 4D, in which the
output coupling element 486 comprises a light coupling tape that
has surface portions 482 in optical contact with the multilayer
reflector 404 and other surface portions 484 that do not make
optical contact with the multilayer reflector 404, forming a gap
488 between the element 486 and the multilayer reflector 404.
[0092] At least some of the light incident at the output coupling
element 486, for example light ray 490, is incident at a portion of
the multilayer reflector's surface that is not contacted to the
output coupling element, but is adjacent to a gap 488, and so the
light 490 is totally internally reflected within the multilayer
reflector 404. Some light, for example ray 492, may be transmitted
through the contacting surface portion 482, and be totally
internally reflected at the non-contacting surface portion 484, and
so is coupled out of the controlled transmission mirror 480.
[0093] Other types of output coupling elements may be used in
addition to those described in detail here. Also, an output
coupling element may combine different approaches for diverting
light out of the controlled transmission mirror. For example, an
output coupling element may combine a surface treatment, such as a
surface structure or surface scattering pattern, with bulk
diffusing particles.
[0094] In some embodiments, the output coupling element may be
constructed so that the degree to which light is extracted is
uniform across the output coupling element. In other embodiments,
the output coupling element may be constructed so that the degree
to which light is extracted out of the controlled transmission
mirror is not uniform across the output coupling element. For
example, in the embodiment of output coupling element 428
illustrated in FIG. 4A, the density of diffusing particles 428a may
be varied across the output coupling element 428 so that a higher
fraction of light can be extracted from some portions of the output
coupling element 428 than others. In the illustrated embodiment,
the density of diffusing particles 428a is higher at the left side
of the output coupling element 428. Likewise, for the embodiments
of controlled transmission mirrors 440 460, 480 illustrated in
FIGS. 4B-4D, the output coupling elements 448, 468, 488 may be
shaped or designed so that a higher fraction of light can be
extracted from some portions of the output coupling elements 448,
468, 488 than from other portions. The provision of non-uniformity
in the extraction of the light from the controlled transmission
mirror, for example extracting a smaller fraction of light from
portions of the controlled transmission mirror that contain more
light and extracting a higher fraction of light from portions of
the controlled transmission mirror that contain less light, may
result in a more uniform brightness profile in the illumination
light propagating towards the display panel.
[0095] The number of bounces made by light within the controlled
transmission mirror, and therefore, the uniformity of the extracted
light, may be affected by the reflectivity of both the input
coupling element and the output coupling element. The trade-off for
uniformity is brightness loss caused by absorption in the input
coupling element, the multilayer reflector and the output coupling
element. This absorption loss may be reduced by proper choice of
materials and material processing conditions.
[0096] In some exemplary embodiments, the controlled transmission
mirror may be polarization sensitive, so that light in one
polarization state is preferentially extracted. A cross-section
through one exemplary embodiment of a polarization sensitive
controlled transmission mirror 520 is schematically illustrated in
FIG. 5A. The controlled transmission mirror 520 comprises an
optional transparent layer 502, a multilayer reflector 504, an
input coupling element 506 and a polarization sensitive output
coupling element 528. A three-dimensional coordinate system is used
here to clarify the following description. The axes of the
coordinate system have been arbitrarily assigned so that the plane
of the controlled transmission mirror 520 lies parallel to the x-y
plane, with the z-axis having a direction through the thickness of
the controlled transmission mirror 520. The lateral dimension shown
in FIG. 5A is parallel to the x-axis, and the y-direction extends
in a direction perpendicular to the drawing.
[0097] In some embodiments, the extraction of only one polarization
of the light propagating within the controlled transmission mirror
520 is effected by the output coupling element 528 containing two
materials, for example different polymer phases, at least one of
which is birefringent. In the illustrated exemplary embodiment, the
output coupling element 528 has scattering elements 528a, formed of
a first material, embedded within a continuous matrix 528b formed
of a second material. The refractive indices for the two materials
are substantially matched for light in one polarization state and
remain unmatched for light in an orthogonal polarization state.
Either or both of the scattering elements 528a and the matrix 528b
may be birefringent.
[0098] If, for example, the refractive indices are substantially
matched for light polarized in the x-z plane, and the refractive
indices of the first and second materials are n.sub.1 and n.sub.2
respectively, the condition holds that
n.sub.1x.apprxeq.n.sub.1z.apprxeq.n.sub.2x.apprxeq.n.sub.2z, where
the subscripts x and z denote the refractive indices for light
polarized parallel to the x and z axes respectively. If n.sub.1y #
n.sub.2y, light polarized parallel to the y-axis, for example light
530, may be scattered within the output coupling element 528 and
pass out of the controlled transmission mirror 520. The
orthogonally polarized light, for example light ray 532, polarized
in the x-z plane, remains substantially unscattered on passing
within the output coupling element 520 because the refractive
indices for this polarization state are matched. Consequently, if
the light 532 is incident on the top surface 529 of the output
coupling element 528 at an angle equal to, or greater than, the
critical angle, .theta..sub.c, of the continuous phase 528b, the
light 532 is totally internally reflected at the surface 529, as
illustrated.
[0099] To ensure that the light extracted from the output coupling
element 528 is well polarized, the matched refractive indices are
preferably matched to within at least .+-.0.05, and more preferably
matched to within .+-.0.01. This reduces the amount of scatter for
one polarization. The amount by which the light in the
y-polarization is scattered is dependent on a number of factors,
including the magnitude of the index mismatch, the ratio of one
material phase to the other and the domain size of the disperse
phase. Preferred ranges for increasing the amount by which the
y-polarized light is forward scattered within the output coupling
element 528 include a refractive index difference of at least about
0.05, a particle size in the range of about 0.5 .mu.m to about 20
.mu.m and a particle loading of up to about 10% or more.
[0100] Different arrangements of polarization-sensitive output
coupling element are available. For example, in the embodiment of
output coupling element 548, schematically illustrated in FIG. 5B,
the scattering elements 548a constitute a disperse phase of
polymeric particles within a continuous matrix 528b. Note that this
figure shows a cross-sectional view of the output coupling element
548 in the x-y plane. The birefringent polymer material of the
scattering elements 548a and/or the matrix 548b may be oriented,
for example, by stretching in one or more directions. Disperse
phase/continuous phase polarizing elements are described in greater
detail in co-owned U.S. Pat. Nos. 5,825,543 and 6,590,705, both of
which are incorporated by reference.
[0101] Another embodiment of polarization-sensitive output coupling
element 558 is schematically illustrated in cross-section in FIG.
5C. In this embodiment, the scattering elements 558a are provided
in the form of fibers, for example polymer fibers or glass fibers,
in a matrix 558b. The fibers 558a may be isotropic while the matrix
558b is birefringent, or the fibers 558a may be birefringent while
the matrix 558b is isotropic, or the fibers 558a and the matrix
558b may both be birefringent. The scattering of light in the
fiber-based, polarization sensitive output coupling element 558 is
dependent, at least in part on the size and shape of the fibers
558a, the volume fraction of the fibers 558a, the thickness of the
output coupling element 558, and the degree of orientation, which
affects the amount of birefringence. Different types of fibers may
be provided as the scattering elements 558a. One suitable type of
fiber 558a is a simple polymer fiber formed of one type of polymer
material that may be isotropic or birefringent. Examples of this
type of fiber 558a disposed in a matrix 558b are described in
greater detail in co-owned U.S. patent application Ser. No.
11/068,159, incorporated herein by reference. Another example of
polymer fiber that may be suitable for use in the output coupling
element 558 is a composite polymer fiber, in which a number of
scattering fibers formed of one polymer material are disposed in a
filler of another polymer material, forming a so-called
"islands-in-the-sea" structure. Either or both of the scattering
fibers and the filler may be birefringent. The scattering fibers
may be formed of a single polymer material or formed with two or
more polymer materials, for example, a disperse phase in a
continuous phase. Composite fibers are described in greater detail
in U.S. patent application Ser. Nos. 11/068,157 and 11/068,158,
both of which are incorporated by reference.
[0102] It will be appreciated that the input coupling element may
also be polarization sensitive. For example, where unpolarized
light is incident on the controlled transmission mirror, a
polarization-sensitive scattering input coupling element may be
used to scatter light of one polarization state into the controlled
transmission mirror, allowing the light in the orthogonal
polarization to be reflected by the multilayer reflector back to
the base reflector. The polarization state of the reflected light
may then be mixed before returning to the controlled transmission
mirror. Thus, the input coupling element may permit light in
substantially only one polarization state to enter the controlled
transmission mirror. If the different layers of the controlled
transmission mirror maintain the polarization of the light, then
substantially only one polarization of light may be extracted from
the controlled transmission mirror, even if a
non-polarization-sensitive output coupling element is used. Both
the input and output coupling elements may be polarization
sensitive. Any of the polarization sensitive layers used as an
output coupling element may also be used as an input coupling
element.
[0103] Some different exemplary embodiments of illumination light
unit that use a controlled transmission mirror are schematically
illustrated in FIGS. 6-8. In FIG. 6A, the illumination light unit
600 includes an elongated reflecting cavity 602 with one or more
light sources 606 at one end. FIG. 6B shows a cross section through
the light unit 600. There may be one or more light sources
positioned at the other end (not shown). The reflecting cavity 602
is provided with one or more reflective walls 612, so that light
from the light source is reflected within the reflecting cavity 602
and is extracted from the cavity 602 via a controlled transmission
mirror 604. In some exemplary embodiments, the interior of the
reflecting cavity 602 may be empty, in which case the reflective
walls 612 are formed on the inner surfaces of exterior walls. In
other exemplary embodiments, the reflecting cavity 602 may be
formed by providing reflectors on the outside surface of a solid,
transparent body.
[0104] In some embodiments, the reflective walls 612 may be
diffusely reflective and in other embodiments the reflective walls
612 may be specularly reflective. The reflective walls 612 may be,
for example, multilayer dielectric coatings, multilayer polymer
optical films, or metallic coatings. In some embodiments, the
reflective walls need not lie parallel to the controlled
transmission mirror 604. The reflective walls 612 need not
completely surround the reflecting cavity 602, but are arranged
instead so that at least some of the light is reflected between the
reflective walls and the controlled transmission mirror 604.
[0105] In some exemplary embodiment the ends 608 may be reflective.
In the illustrated embodiment, the reflecting cavity 602 has a
circular cross-section, although it will be appreciated that other
shapes of cross-sections may also be used, for example the
cross-section may be elliptical, triangular, square, rectangular,
or some other shape. The dimensions of the controlled transmission
mirror 604 may be set to match the dimensions of the edge of the
light guide that is being illuminated by the illumination light
unit 600.
[0106] The path of an exemplary light beam 614 is shown in FIG. 6B.
The light beam 614 is reflected by the reflective walls 612 within
the reflecting cavity 602 and is transmitted through the controlled
transmission mirror 604.
[0107] The light source(s) 606 may comprise one or more LEDs. The
LEDs may all produce light of the same color. In another
embodiment, the light source 606 may include a color converter,
such as a phosphor, for generating light of a different color from
that generated within the LED. For example, a phosphor may be used
to generate white light using a blue or UV LED. The light source
606 may be located within the reflecting cavity 602, for example,
if the light source 606 is an LED, then the end 608 may be provided
with a hole through which the LED is passed from the outside of the
cavity 602 to the inside. In another embodiment, the light source
606 may be located outside the reflecting cavity 602 and the light
from the light source 606 may pass through an aperture into the
reflecting cavity 602.
[0108] The extraction of light through the controlled transmission
mirror 604 may be graded along its length so that less light is
extracted from the reflecting cavity 602 closer to the light source
606, with increasing extraction further away from the light source
606, so that the brightness of the light extracted from the
illumination light unit 600 is relatively uniform along its
length.
[0109] Another embodiment of an illumination light unit 700 is
shown in FIG. 7A, in which a number of light sources 706 are
located at the end 710 of a reflecting cavity 702. In this
exemplary embodiment, there is more than one light source 706 and
the cross-sectional shape of the reflecting cavity 702 is
rectangular. The light sources 706 may each generate light of the
same color or of a different color. In the case where different
light sources 706 generate light of different colors, the light
from each light source 706 is mixed in the reflecting cavity 702
with the light from the other light sources 706 so that the light
emerging from the controlled transmission mirror 704 may be a mixed
color. For example, if there are three light sources 706 producing
red, green and blue light respectively, the light emerging from the
controlled emission mirror 704 may be a white color. The shade of
the mixed color output light depends, inter alia, on the relative
output powers of the different light sources and on the spectral
properties of the controlled transmission mirror 704.
[0110] The extraction of light through the controlled transmission
mirror 704 may be graded along the length of the controlled
transmission mirror 704 so that the brightness of the light
extracted from the illumination light unit 700 is relatively
uniform along its length.
[0111] An exemplary embodiment of a backlight 720 that uses the
illumination unit 700 is schematically illustrated in FIG. 7B. The
illumination unit 700 is at least partially surrounded by a
reflector 722 and is positioned so that the light 724 emitted from
the controlled transmission mirror 704 is directed towards a
lightguide 726. An optional brightness enhancing layer 728, for
example a prismatic brightness enhancing layer, may be positioned
between the illumination unit 700 and the lightguide 726. The
brightness enhancing layer 728 reduces the angular spread of the
light entering the lightguide 726 and may promote lateral spreading
in the lightguide 726. Some of the light, for example ray 730, may
be reflected by the brightness enhancing layer 728. The reflected
light 730 may be redirected back towards the lightguide 726 by the
controlled transmission mirror 704 or some other reflector in the
illumination unit 700, or by the reflector 722 that surrounds the
illumination light unit 700. It will be appreciated that other
embodiments of illumination light unit, for example illumination
unit 600, may also be used in such a configuration for illuminating
a light guide.
[0112] Another embodiment of an illumination light unit 800 is
shown in FIG. 8A, in light sources 806 are located on a face 808 of
a reflecting cavity 802 opposing a controlled transmission mirror
804. In this exemplary embodiment, there is more than one light
source 806 and the cross-sectional shape of the reflecting cavity
802 is rectangular. The light sources 806 may each generate light
of the same color or of different colors. The reflective cavity 802
may be used to mix the light from the different light sources 806
so that the intensity profile of the light output from the
controlled transmission mirror 804 is relatively uniform.
Furthermore, in the case where the light sources 806 produce light
of different colors, the different colored light is mixed so that
the light emerging from the controlled transmission mirror 804 is a
mixed color. For example, if there are three light sources 806
producing red, green and blue light respectively, the light
emerging from the controlled emission mirror 804 may be a white
color. The light from the light sources 806 may be mixed within the
reflecting cavity 802 so that the brightness of the light extracted
from the illumination light unit may be relatively uniform.
[0113] Another embodiment of an illumination unit 820 is
schematically illustrated in FIG. 8B, in which the controlled
transmission mirror 854 is positioned on the top of the reflecting
cavity 802. Additional light source 806 may be placed around the
edge of the reflecting cavity 802.
[0114] Another embodiment of an illumination light unit 900 is
schematically illustrated in FIGS. 9A and 9B. The unit 900 has a
reflecting cavity 902 that includes a reflector 908 and a
controlled transmission mirror 904. One or more light sources 906
are provided on a base 907. The base 907 may be reflective. The
base 907 may also provide electrical connections for driving the
light source 906 and provide a heatsink for removing heat from the
light source 906.
[0115] Light 920 from the light sources 906 is reflected by the
reflector 908 towards the controlled transmission mirror 904. The
reflector 908 may have any suitable shape and may be curved (as
illustrated) or flat. If the reflector 908 is curved, the curve may
be any suitable type of curve, for example elliptical or parabolic.
In the illustrated embodiment, the reflector 908 is curved in one
dimension. The reflector 908 may be any suitable type of reflector,
for example a metalized reflector, a multilayer dielectric
reflector or a multiple layer polymer film (MOF) reflector. Light
that is transmitted through the controlled transmission mirror 904
may be coupled into a light guide 912 for back-illuminating a
display device. The space 914 within the reflecting cavity 902 may
be filled, or may be empty. In embodiments where the space 914 is
filled, for example with a transparent optical body, then the
reflector 908 may be a reflective coating on the body. In
embodiments where the space is empty, then the reflector 908 may be
a front surface reflector. Different configurations of reflective
cavity are described further in U.S. patent application Ser. Nos.
10/701,201 and 10/949,892, incorporated herein by reference.
[0116] The light sources 906, for example LEDs, may all produce
light of the same color, or different LEDs may produce light of
different colors, for example red, green and blue. In some
exemplary embodiments, an optional wavelength converter 922 may be
used to change the color of at least some of the light 920. For
example, where the light 920 is blue or ultraviolet, the wavelength
converter 922 may be used to convert some of the light to green
and/or red light 924 (dashed lines). A low-pass reflector 926 may
be positioned between the controlled transmission mirror 904 and
the wavelength converter 922. The low-pass reflector 924 transmits
the relatively short wavelength light 920 from the light sources
906 and reflects light 924a from the wavelength converter 922
towards the light guide 912.
[0117] In another embodiment, the controlled transmission mirror
904 may use as an output coupling element a diffuser having a
matrix loaded with phosphor particles. In such a configuration,
some of the light transmitted through the multilayer reflector is
converted by the phosphor to light of a different wavelength. Light
that is not diffused or converted by the particles may be totally
internally reflected by the matrix layer so as to pass back through
the multilayer reflector.
[0118] Another embodiment of an illumination light unit 1000 is
schematically illustrated in FIG. 10. The unit 1000 includes a
number of reflecting cavities 1002 formed between curved reflectors
1008 and a controlled transmission mirror 1004. The controlled
transmission mirror 1004 may be provided as a single sheet common
to each of the reflecting cavities 1002, or may be provided as a
segmented mirror, each segment being associated with a respective
cavity 1002. One or more light sources 1006 are provided in each
reflecting cavity. The light sources 1006 may be mounted on a base
1007 that may be reflective. The base 1007 may also provide
electrical connections for driving the light sources 1006 and
provide a heatsink for removing heat from the light sources
1006.
[0119] Light 1020 from the light sources 1006 may be reflected by
the reflectors 1008 towards the controlled transmission mirror
1004. The reflectors 1008 may have any suitable shape and may be
curved (as illustrated). Any suitable type of curve shape may be
used for the curved the reflector 1008, for example ellipsoidal or
paraboloidal. In the illustrated embodiment, the reflectors 1008
are curved in two dimensions. The reflectors 1008 may be any
suitable type of reflector, for example a metalized reflector, a
multilayer dielectric reflector or an MOF reflector. Light that is
transmitted through the controlled transmission mirror 1004 may be
coupled into a light guide 1012 for back-illuminating a display
device.
[0120] The light sources 1006, for example LEDs, may all produce
light of the same color, or different LEDs may produce light of
different colors, for example red, green and blue. In some
exemplary embodiments, an optional wavelength converter 1022 may be
used to change the color of at least some of the light 1020 that
passes out of the controlled transmission mirror 1004. A low-pass
reflector 1026 may be positioned between the controlled
transmission mirror 1004 and the wavelength converter 1022.
[0121] Another embodiment of an illumination unit 1100 that may be
used as a backlight for a display device is schematically
illustrated in FIG. 11A. In this embodiment, one or more light
sources 1106 are disposed between first and second reflectors 1102,
1104. In some embodiments, the light sources 1106, which may be
LEDs, may emit light substantially away from the second reflector
1102, in which case an optional curved reflector 1108 may be
provided to direct the light 1110 along the space between the first
and second reflectors 1102, 1104. In other embodiments, not
illustrated, the light sources 1106 may substantially emit light
sideways in a direction along the space between the first and
second reflectors 1102, 1104.
[0122] The first and second reflectors 1102, 1104 may be specular
reflectors, for example ESR film available from 3M Company, St.
Paul, Minn. A folding reflector 1112 is positioned at each end to
fold the light 1110 into a reflecting cavity formed between the
second reflector 1104 and a controlled transmission mirror 1114.
The light 1110 is eventually directed out of the unit 1100 through
the controlled transmission mirror 1114. The first reflector 1102
may be mounted on a base 1116 that provides electrical power to the
light sources 1106 and may also operate as a thermal sink to remove
heat from the light sources 1106.
[0123] The light sources 1106 may be arranged in different patterns
on the first reflector 1102. In the arrangement illustrated in FIG.
11B, which shows a slice through the unit 1100 between the first
and second reflectors 1102, 1104, the light sources 1106 are
arranged in a linear pattern, with the light being directed towards
the edges 1120a, 1120b. In the arrangement schematically
illustrated in FIG. 11C, the light sources 1106 and reflector 1108
are arranged in a radial pattern, so that the light is directed
radially outwards to the folding reflector 1112 situated around the
periphery of the first reflector 1102.
[0124] An illumination light unit as described herein is not
restricted to use for illuminating a liquid crystal display panel.
The illumination light unit may also be used wherever discrete
light sources are used to generate light and it is desirable to
have uniform illumination out of a panel that includes one of more
of the discrete light sources. Thus, the controlled transmission
mirror may find use in solid state space lighting applications and
in signs, illuminated panels and the like.
[0125] The present invention should not be considered limited to
the particular examples described above, but rather should be
understood to cover all aspects of the invention as fairly set out
in the attached claims. Various modifications, equivalent
processes, as well as numerous structures to which the present
invention may be applicable will be readily apparent to those of
skill in the art to which the present invention is directed upon
review of the present specification. The claims are intended to
cover such modifications and devices.
* * * * *