Process for making optical structures for diffusing light

Optical diffusing structures can be fabricated from photopolymerizable material by directing light through a transparent or translucent substrate and then through the photopolymerizable material for a period of time sufficient to photopolymerize only a portion of the material. The resultant structure can be utilized as a diffuser, a viewing screen, and in other applications, and can be combined with other light-directing structures such as arrays of tapered optical waveguides.

BACKGROUND OF THE INVENTION 
Optical structures that scatter or diffuse light generally function in one 
of two ways: (a) as a surface diffuser utilizing surface roughness to 
refract or scatter light in a number of directions; or (b) as a bulk 
diffuser having flat surfaces and embedded light-scattering elements. 
A diffuser of the former kind is normally utilized with its rough surface 
exposed to air, affording the largest possible difference in index of 
refraction between the material of the diffuser and the surrounding medium 
and, consequently, the largest angular spread for incident light. However, 
a diffuser of this type suffers from two major drawbacks: a high degree of 
backscattering and the need for air contact. Backscattering causes 
reflection of a significant portion of the light back to the originating 
source when it should properly pass through the diffuser, lowering the 
efficiency of the optical system. The second drawback, the requirement 
that the rough surface must be in contact with air to operate properly, 
may also result in lower efficiency. If the input and output surfaces of 
the diffuser are both embedded inside another material, such as an 
adhesive for example, the light-dispersing ability of the diffuser may be 
reduced to an undesirable level. 
In one version of the second type of diffuser, the bulk diffuser, small 
particles or spheres of a second refractive index are embedded within the 
primary material of the diffuser. In another version of the bulk diffuser, 
the refractive index of the material of the diffuser varies across the 
diffuser body, thus causing light passing through the material to be 
refracted or scattered at different points. Bulk diffusers also present 
some practical problems. If a high angular output distribution is sought, 
the diffuser will be generally thicker than a surface diffuser having the 
same optical scattering power. If however the bulk diffuser is made thin, 
a desirable property for most applications, the scattering ability of the 
diffuser may be too low. 
Despite the foregoing difficulties, there are applications where an 
embedded diffuser may be desirable, where the first type of diffuser would 
not be appropriate. For example, a diffuser layer could be embedded 
between the output polarizer layer and an outer hardcoat layer of a liquid 
crystal display system to protects the diffuser from damage. Additionally, 
a diffuser having a thin profile, which will retain wide optical 
scattering power when embedded in other materials and have low optical 
backscatter and therefore higher optical efficiencies than conventional 
diffusers, would be highly desirable.

DESCRIPTION OF THE INVENTION 
A light diffuser can be fabricated from a film of photopolymerizable 
material by directing collimated or nearly-collimated light through a 
substrate of a transparent or translucent material and into the 
photopolymerizable material. Collimated light may be defined as that light 
where the divergence angle of the light rays is less than 0.5 degrees. By 
contrast, the divergence angle of the light rays in nearly-collimated 
light is less than .+-.10 degrees, preferably less than .+-.5 degrees, and 
more preferably less than .+-.3.5 degrees. In this application, whether 
collimated or nearly-collimated, the light is preferably incoherent, i.e., 
light that does not have a uniform phase. Most light sources (with the 
exception of laser light sources) such as arc lamps, incandescent lamps, 
or fluorescent lamps produce incoherent light, although coherent light may 
also be utilized. 
The photopolymerizable material is exposed to the light for a period of 
time sufficient to crosslink (or polymerize) only a portion of the 
material. After this has occurred, the non-crosslinked portion of the 
material is removed, leaving a highly-modulated surface on the 
photopolymerized portion. This remaining structure can be employed 
directly as a diffuser or it may used to create a metallic replica for 
embossing another material to create a diffuser. 
The Substrate Material 
Suitable materials for the substrate include (a) optically clear, 
transparent materials; (b) semi-clear, transparent materials with some 
haze or light scattering due to inhomogeneities in the composition or the 
structure of the material; and (c) translucent materials. Suitable 
materials for the substrates may also be classified by their crystallinity 
and include (a) amorphous materials; (b) semi-crystalline materials that 
contain crystalline domains interspersed in an amorphous matrix; and (c) 
purely crystalline materials. Although such materials can be organized 
according to the three preceding classifications, it should be noted that 
the crystallinity of many polymers suitable for this application can 
change depending on how the polymer is manufactured. Therefore, a given 
substance may fall within one or more of those classes. The substrate 
typically has two opposing flat surfaces generally parallel to each other, 
but other configurations could be employed. 
Materials meeting the criteria of the foregoing paragraph include inorganic 
glasses such as borosilicate glass and fused silica; amorphous polymers 
such as cellulose acetate, cellulose triacetate, cellulose butyrate, 
ethylene-vinyl alcohol copolymers such as polyvinyl alcohol, polymethyl 
methacrylate, and polystyrene; and semi-crystalline polymers include 
polyesters, nylons, epoxies, polyvinyl chloride, polycarbonate, 
polyethylene, polypropylene, polyimides, and polyurethanes. Of the 
foregoing semi-crystalline polymers, polyester in a film is preferable and 
polyethylene terephthalate (PET) (a polyester) was found to be the most 
preferable choice for the substrate. All of the materials set forth in 
this paragraph are commercially available. 
The Photopolymerizable Material 
The photopolymerizable material is comprised of at least three essential 
ingredients: a photopolymerizable component, a photoinitiator, and a 
photoinhibitor. The first essential ingredient, a photopolymerizable 
component, can be a photopolymerizable monomer or oligomer, or a mixture 
of photopolymerizable monomers and/or oligomers. Commercially-available 
photopolymerizable monomers and oligomers suitable for this application 
include (a) epoxy resins such as bisphenol A epoxy resins, epoxy cresol 
novolac resins, epoxy phenol novolac resins, bisphenol F resins, 
phenol-glycidyl ether-derived resins, cycloaliphatic epoxy resins, and 
aromatic or heterocyclic glycidyl amine resins; (b) allyls; (c) vinyl 
ethers and other vinyl-containing organic monomers; and (d) acrylates and 
methacrylates such as urethane acrylates and methacrylates, ester 
acrylates and methacrylates, epoxy acrylates and methacrylates, and 
(poly)ethylene glycol arylates and methacrylates. Acrylate monomers are 
described in U.S. Pat. No. 5,396,350, issued Mar. 7, 1995, to Beeson et 
al., for a Backlighting Apparatus Employing an Array of Microprisms, U.S. 
Pat. No. 5,428,468, issued Jun. 27, 1995, to Zimmerman et al., for an 
Illumination System Employing an Array of Microprisms, U.S. Pat. No. 
5,462,700, issued Oct. 31, 1995, to Beeson et al., for a Process for 
Making an Array of Tapered Photopolymerized Waveguides, and U.S. Pat. No. 
5,481,385, issued Jan. 2, 1996, to Zimmerman et al., for a Direct View 
Display with Array of Tapered Waveguides, all of which are incorporated 
herein by reference. 
The following mixtures for the first essential element of the 
photopolymerizable material have been found to yield acceptable results in 
increasing order of preference: (a) a mixture of acrylates and epoxy 
resins; (b) mixtures of aromatic diacrylates and bisphenol A epoxy resins; 
and (c) a mixture of ethoxylated bisphenol A diacrylate (EBDA) and Dow 
epoxy resin DER-362 (a polymer of bisphenol A and epichlorohydrin). An 
example of the last is a mixture of 70 parts by weight of EBDA and 30 
parts by weight of Dow epoxy resin DER-362. Other materials can also be 
used as will readily occur to those skilled in the art. A factor relevant 
to the selection of the photopolymerizable component is that the cure rate 
and shrinkage of epoxy resins may differ from that of the acrylate 
materials. 
The second essential ingredient of the photopolymerizable material, a 
photoinitiator, produces an activated species that leads to 
photopolymerization of the monomer or oligomer or the mixture of monomers 
and/or oligomers when it is activated by light. Preferred photoinitiators 
are disclosed in U.S. Pat. No. 5,396,350, U.S. Pat. No. 5,462,700, and 
U.S. Pat. No. 5,481,385, cited above. The most preferred photoinitiator is 
.alpha.,.alpha.-dimethoxy-.alpha.-phenyl acetophenone (such as 
Irgacure-651, a product of Ciba-Geigy Corporation). The photoinitiator has 
been successfully used at a loading level of 2 parts photoinitiator per 
hundred parts monomer or oligomer material. Preferably, the photoinitiator 
should be used at a loading level of 0.5-to-10 parts photoinitiator per 
hundred parts of the monomer or oligomer material, and more preferably at 
a loading level of 1-to-4 parts photoinitiator per hundred parts monomer 
or oligomer material. 
The third essential ingredient of the photopolymerizable material, an 
inhibitor, prevents photopolymerization at low light levels. The inhibitor 
raises the threshold light level for polymerization of the photopolymer so 
that there will be a distinct boundary between the crosslinked and the 
non-linked photopolymerizable material instead of a gradient. Various 
inhibitors are known to those skilled in the art, as described in U.S. 
Pat. No. 5,462,700 and U.S. Pat. No. 5,481,385, cited above. Oxygen is a 
preferred inhibitor and is inexpensive. It is readily available if the 
photopolymerization is performed in the presence of air. 
An Arrangement for Photopolymerization 
As illustrated in FIG. 1, a layer 10 of photopolymerizable material is 
deposited upon a substrate 20 by any convenient method, such as doctor 
blading, resulting in a layer of a generally uniform thickness of about 
0.02 mm to about 2 mm, preferably of about 0.12 mm to about 0.37 mm, and 
more preferably a thickness of about 0.2 mm to about 0.3 mm. Satisfactory 
results have been obtained with a layer of a generally uniform thickness 
of about 0.2 mm to about 0.3 mm. Optionally, a glass support layer 30 can 
be placed underneath the substrate 20. Preferably, the top surface 14 of 
the layer 10 is open to an atmosphere containing oxygen. It should be 
understood that the elements shown in FIG. 1 and the remaining figures are 
not to scale; actual and relative dimensions may vary from those shown. 
Referring to FIG. 2, collimated or nearly-collimated light is directed 
through the bottom surface 22 of the substrate 20 and through the 
photopolymerizable layer 10. (If a glass support layer 30 has been 
provided, the light first passes through the glass.) The light can be any 
visible light, ultraviolet light, or other wavelengths (or combinations of 
wavelengths) capable of inducing polymerization of the photopolymerizable 
material, as will readily occur to those skilled in the art. However, many 
of the commonly-used photoinitiators, including Irgacure-651, respond 
favorably to ultraviolet light in the wavelength range from about 350 nm 
to about 400 nm, although this range is not critical. Preferably, the 
intensity of the light ranges from about 1 mW/cm.sup.2 to about 1000 
mW/cm.sup.2, more preferably between about 5 mW/cm.sup.2 and about 200 
mW/cm.sup.2, and optimally about 30 mW/cm.sup.2, .+-. about 10 
mW/cm.sup.2. Satisfactory results have been obtained with a light 
intensity of approximately 30 mW/cm.sup.2. 
As light passes through the photopolymerizable layer 10, the molecules of 
the photopolymerizable material will begin to crosslink (or polymerize), 
beginning at the bottom surface 12 of the photopolymerizable layer 10 (the 
top surface 24 of the substrate 20). Before the entire thickness of the 
photopolymerizable layer 10 has had an opportunity to crosslink, the light 
is removed, leaving only the lower photocrosslinked polymer component 40 
of the photopolymerizable layer 10. 
The dosage of light required to achieve the desired amount of crosslinking 
depends on the photopolymerizable material employed. For example, if the 
photopolymerizable mixture of EBDA and Dow epoxy resin DER-362 material 
and the photoinitiator .alpha.,.alpha.-dimethoxy-.alpha.-phenyl 
acetophenone are used and applied in a thickness ranging from about 0.2 mm 
to about 0.3 mm, the total light dose received by the photopolymerizable 
layer 10 preferably ranges from about 5 mJ/cm.sup.2 to about 2000 
mJ/cm.sup.2, more preferably from about 20 mJ/cm.sup.2 to about 300 
mJ/cm.sup.2, and optimally from about 60 mJ/cm.sup.2 to about 120 
mJ/cm.sup.2. 
A satisfactory result was obtained using the photopolymerizable mixture of 
EBDA and Dow epoxy resin DER-362 material. It was applied in a thickness 
of approximately 0.2 mm to 0.3 mm, together with the photoinitiator 
Irgacure-651 at a loading level of 2 parts photoinitiator per hundred 
parts of the photopolymerizable mixture. The light source intensity was 
approximately 30 mW/cm.sup.2 and the dosage was between 60 mJ/cm.sup.2 and 
120 mJ/cm.sup.2. 
Removal of the Unphotopolymerized Portion 
A developer is then applied to the photopolymerizable layer 10 to remove 
the unpolymerized portion. The developer can be any material, usually 
liquid, that will dissolve or otherwise remove the unpolymerized material 
without affecting the crosslinked component 40. Suitable developers are 
organic solvents such as methanol, acetone, methyl ethyl ketone (MEK), 
ethanol, isopropyl alcohol, or a mixture of such solvents. Alternatively, 
one can employ a water-based developer containing one or more surfactants, 
as will readily occur to those skilled in the art. 
After the unpolymerized portion had been removed, the photocrosslinked 
component 40 remains on the substrate 20, as shown in FIG. 3. If desired, 
the photocrosslinked component 40 can be removed from the substrate 20. 
The Highly-Modulated Surface 
The surface 42 of the photocrosslinked component 40 is highly modulated, 
exhibiting smooth bumps ranging in size from about 1 micron to about 20 
microns in both height and width. The aspect ratios, i.e., the ratios of 
the heights to the widths, of the bumps on the highly modulated surface 42 
of the photocrosslinked component 40 are generally quite high. Since the 
substrate is optically clear or semi-clear to the unaided human eye and 
has no obvious masking features to block light transmission, one might not 
expect the highly-modulated surface 42. 
A highly modulated surface can be achieved with substrates fabricated from 
photopolymerizable material containing only one monomer or oligomer 
component, or a mixture of such components. These photocrosslinked 
materials will exhibit variations in the spatial uniformity of 
polymerization due to random fluctuations in the spatial intensity of the 
applied light and statistical fluctuations in the microscopic structure of 
the substrate 20. An example of the latter is the material PET, a 
semi-crystalline polymer material containing random microscopic crystals 
interspersed with amorphous polymer. The random microscopic crystals will 
refract light differently than the surrounding amorphous polymer if the 
refractive indexes of the two phases are slightly different. Internally, 
the polymerized component 40 will exhibit striations 44 running through 
the thickness of the layer. 
The dosage of light can be applied in a single exposure or in multiple 
exposures or doses, leaving the photopolymerizable material unexposed to 
light between exposures. Multiple exposures of light to achieve the same 
total dosage can result in a surface more highly modulated than would 
occur from a single exposure. 
The photopolymerized component 40 can be used in a number of ways. For 
example, it can be employed as a light diffuser in a projection viewing 
screen or as a component in a liquid crystal display (LCD) illumination 
system to hide the system's structural features. 
Replication of the Photocrosslinked Layer 
A conforming metal replica layer 50 can be formed on the highly-modulated 
surface 42 through electroforming, electroless deposition, vapor 
deposition, and other techniques as will readily occur to those skilled in 
the art, as illustrated in FIG. 4. The metallic layer 50 is then used to 
make embossed copies of the surface structure of the original 
photocrosslinked component 40. The metallic replica layer 50 may be used 
in a variety of known embossing methods such as thermal embossing into 
clear or translucent thermoplastic materials or soft-embossing or casting 
(i.e., photocure embossing) into a clear or translucent photoreactive 
material or mixture. 
As shown in FIG. 5, an embossable layer 60 of material, such as 
polycarbonate, acrylic polymer, vinyl polymer, or even photopolymerizable 
material, is placed on a substrate (e.g., of PET). The metallic replica 
layer 50 is then applied to the embossable layer 60, creating a mating 
surface as indicated by the dashed line 62. In the case of hard embossing 
or preferably thermal embossing, the metallic replica layer 50 is pushed 
into the surface of the embossable layer 60, simultaneously with the 
application of heat or pressure, or both. 
In the case of soft embossing or casting, the metallic replica layer 50 is 
placed in contact with a reactive liquid photopolymerizable material, and 
the latter is then photoexposed to form a solid polymeric film. Typically, 
the light used to expose the photopolymer in a soft embossing application 
is not collimated. Therefore, unless the embossable layer 60 was 
fabricated from photopolymerizable material exposed to collimated or 
nearly-collimated light, the embossable layer 60 will not have striations. 
By using any of the foregoing embossing techniques, a large number of 
pieces having the surface contour of the highly-modulated surface 42 of 
the original photocrosslinked component 40 can be made. The metallic 
replica layer 50 is removed leaving the resulting embossed layer 80 shown 
in FIG. 6. The embossed layer 80 may be employed as a light diffuser, with 
or without the underlying substrate 70. 
Applying a Fill Layer 
To reduce backscattering of light, the photocrosslinked component 40 of 
FIG. 3 can be coated with a transparent or translucent fill layer 152, as 
shown in FIG. 7. Similarly, as shown in FIG. 8, the fill layer 152 could 
be applied to the embossed layer 80 of FIG. 6. 
The index of refraction n.sub.2 of the fill layer 152 may differ from the 
index n.sub.1 of the photocrosslinked component 40. For example, if 
n.sub.1 =1.55, then n.sub.2 may range from about 1.30 to about 1.52, or 
from about 1.58 to about 1.80. The optimal refractive index is a function 
of the desired distribution of the light exiting the diffuser 150, i.e., 
for a given value for n.sub.1, the diffusing light pattern obtained when 
light passes completely through the diffuser 150 may be varied by changing 
n.sub.2. Of course, one may also vary n.sub.1 to suit the application. 
Suitable materials for the fill layer 152 having an index of refraction 
typically less than n.sub.1 include silicone, fluorinated acrylates or 
methacrylates, fluoro epoxies, fluorosilicones, fluororethanes, and other 
materials as will readily occur to those skilled in the art. Materials 
such as aromatic acrylates, having an index of refraction typically 
greater than n.sub.1, may also be employed for the fill layer 152. 
A variation of the arrangement of FIG. 8 is shown in FIG. 9. In lieu of an 
essentially homogenous material for the fill layer, a layer 160 containing 
light-scattering particles 162 having yet a third index of refraction 
n.sub.3 could be utilized. Alternatively, as depicted in FIG. 10, 
light-scattering particles 82 could be placed in the embossable layer 60. 
In either case, the light-scattering particles 162 or 82 could be made 
from an optically-transmissive material such as glass beads or polymer 
beads or polymer particles made from, for example, amorphous, 
optically-clear polymers such as polystyrene, acrylics, polycarbonates, 
olefins, or other materials as will readily occur to those skilled in the 
art. 
The various layers of the light diffusers of FIGS. 7-10, of differing 
indices of refraction, could be arranged with respect to the light source 
to alter the diffusion effect on the light. For example, light could pass 
through the diffuser by first passing through a layer having a higher 
index of refraction and then passing through a layer having a lower index 
of refraction, or vice versa. In addition, the reflectivity of the 
diffusing structures and the amount of backscattered light also can be 
altered by changing the direction of the light passing through the 
structures. Preferably, for diffuser applications demanding low 
backscattering of incident light (the optical loss that lowers the 
efficiency of the optical system), the light should pass through the layer 
with the lower refractive index before the higher refractive index layer. 
A Liquid Crystal Display System 
The diffusing structures of FIGS. 3 and 6-10 could be employed in a liquid 
crystal display (LCD) system, as illustrated in FIG. 11. The system has a 
light source 200 providing light to a waveguide 210. Microstructures or 
scattering elements (not shown) on waveguide 210 project light out of 
waveguide 210 and through a diffuser 220 and a liquid crystal modulator 
layer 230. The diffuser 220 may have an optional transparent or 
translucent fill layer 222, similar to the diffusing structures of FIGS. 
7-10. Preferably, the fill layer 222 has a lower refractive index than 
layer 220. 
The diffuser 220 can perform one or more of the following functions: (a) 
hide the structural features of the scattering elements on the waveguide 
210; (b) improve the uniformity of light transmitted from the waveguide 
210 to the liquid crystal modulating layer 230; (c) define the angular 
distribution of light transmitted from the waveguide 210 to the liquid 
crystal modulating layer 230, facilitating increased brightness or the 
same brightness at reduced power; and optionally (d) function as a 
transflective diffuser, i.e., an optical device utilizing both transmitted 
light and reflected light. In the latter case (d), under low ambient 
light, the display is illuminated with the light source 200 and waveguide 
210. However, in high ambient light, the light source 200 may be turned 
off and the display can be illuminated by sunlight that passes through 
liquid crystal modulating layer 230 and is reflected from the diffuser 
220. It should be understood that the diffuser 220 and the fill layer 222 
could be reversed with respect to the direction of light travel. 
In FIG. 12, the diffusing structure is employed as a viewing screen. In 
this embodiment, a light source 200 provides light to a waveguide 210 
having microstructures or scattering elements (not shown) that project 
light through a liquid crystal modulating layer 230 and then through a 
viewing screen 240 utilizing the one of the structures illustrated in 
FIGS. 3 and 6-10. The diffuser 240 may have an optional transparent or 
translucent fill layer 242, similar to the diffusing structures of FIGS. 
7-10. 
A Projection Display System 
The diffusing structures may also serve as a viewing screen in a projection 
display system. In FIG. 13, a light source 300 provides light to an 
image-forming device 310, such as a liquid crystal modulating layer. 
Optics 320 can be provided to focus the light from the light source 300 
and to project the image created by the image-forming device 310. The 
image is projected onto a viewing screen 330 incorporating one of the 
diffusing structures of FIGS. 3 and 6-10, optionally with a transparent or 
translucent fill layer 332. Optionally, a Fresnel lens 340 can be placed 
before the viewing screen 330. 
A Diffusing Structure with Tapered Optical Waveguides 
The diffusing structures of this invention discussed thus far may be 
combined with other optical structures to form multiple-layer, 
light-transmitting viewing screens. For example, the diffusers of FIGS. 3 
and 6-10 may be combined with an array of tapered-optical waveguides, by 
juxtaposing a diffusing structure with an array. Examples of arrays of 
tapered optical waveguides are shown in FIGS. 14-22. 
The cross-section of a tapered optical waveguide may assume any shape 
including a square, a rectangle, a polygon, a circle, or an oval. 
Alternatively, the waveguide could be a lenticular structure tapered in 
one direction and extending across the viewing screen in the perpendicular 
direction and having for example a rectangular cross section, as discussed 
below. Typical structures for arrays of tapered optical waveguides are 
discussed in detail in U.S. Pat. No. 5,462,700 and U.S. Pat. No. 5,481,385 
cited above. 
FIGS. 14 and 15 are perspective and elevation views, respectively, of an 
array 400 of tapered optical waveguides where each waveguide has a square 
cross-section. Each tapered optical waveguide 402 has an input surface 
404, an output surface 406 having an area less than the input surface 404, 
and sidewalls 408. Preferably, the interstitial regions 410 between the 
waveguides 402 are filled with an absorbing material having a refractive 
index lower than the refractive index of waveguides 402. 
The angular distribution of light leaving the waveguides can be altered by 
varying the relative dimensions and geometry of the waveguides, as 
discussed in the cited patents. Some of the light transmitted by the 
tapered optical waveguides 402 will undergo total internal reflection from 
the sidewalls 408 and then exit the tapered optical waveguides 402 at 
angles larger than the input angle. The shape of the tapered optical 
waveguides 402 and the refractive index difference between the tapered 
optical waveguides 402 and the interstitial regions 410 can be chosen such 
that light entering the tapered optical waveguides 402 at angles greater 
than the critical angle will intersect the sidewalls 408 at angles that 
will not support total internal reflection. That light will pass through 
the sidewalls 408 and be absorbed by the absorbing material in the 
interstitial regions 410. 
An arrangement combining a diffuser utilizing one of the structures of 
FIGS. 3 and 6-10 with an array of tapered optical waveguides is shown in 
FIG. 16. Light first enters an array 420 of tapered optical waveguides 422 
and then passes through a diffusing structure 430, having a diffuser 432 
and a fill layer 434, which acts as a viewing screen. Preferably, the 
refractive index of the fill layer 434 is less than that of the waveguides 
422 and the interstitial regions 424 are filled with an absorbing material 
having an index of refraction also less than that of than the index of 
refraction of the waveguides 422. 
An alternative configuration of the arrangement of FIG. 16 is illustrated 
in FIG. 17. There, light first enters a diffusing structure 440, having a 
diffuser 442 and a fill layer 444, and then an array 450 of tapered 
optical waveguides 452. Preferably, the diffuser 442 utilizes one of the 
diffusing structures of FIGS. 3 and 6-10 and the interstitial regions 454 
are filled with an absorbing material having an index of refraction also 
less than that of the waveguides 452. 
An Arrangement Utilizing Lenticular Waveguides 
The diffusing structures of FIGS. 3 and 6-10 may be combined with other 
types of optical structures such as, for example, arrays of tapered 
optical waveguides in which the output surface area of each tapered 
optical waveguide is less than the input surface area of each waveguide. 
Included within the class of tapered optical waveguides are arrays of 
lenticular tapered optical waveguides tapered only along one axis and 
which extend across the planar optical device in the perpendicular 
direction. Also included are stacked layers of tapered optical waveguide 
arrays. 
FIGS. 18 and 19 are perspective and elevation views, respectively, of an 
array 500 of tapered optical waveguides having a lenticular structure. 
Each lenticular tapered optical waveguide 502 has an input surface 504, an 
output surface 506 having an area less than the input surface 504, and 
sidewalls 508. Preferably, the interstitial regions 510 between lenticular 
waveguides 502 are filled with an absorbing material having a refractive 
index less than that of the waveguides 502. The discussion regarding 
angular distribution and critical angle with respect to the tapered 
optical waveguides 402 applies equally to the lenticular tapered optical 
waveguides 502. 
Two or more arrays of lenticular tapered optical waveguides can be stacked 
in layers, where the lenticular features of one layer 530 are oriented at 
an angle with respect to the lenticular features of the second layer 540, 
as shown in FIG. 20. Although the angular offset shown in FIG. 20 is 
90.degree., another angle more suitable to the application could be 
selected. 
The structure of FIG. 20 may be combined with one of the diffusing 
structures of FIGS. 3 and 6-10, as shown in FIG. 21. Light passes first 
through the lenticular waveguide arrays 600 and 610, and then through the 
diffuser 620. Again, the diffuser 620 has a fill layer 622 that preferably 
has a lower index of refraction with respect to the waveguide arrays 600 
and 610. Alternatively, as illustrated in FIG. 22, the diffuser 630 can be 
placed ahead of the waveguide arrays 640 and 650. 
While there has been described what is believed to be the preferred 
embodiment of the invention, those skilled in the art will recognize that 
other and further modifications may be made thereto without departing from 
the spirit of the invention, and it is intended to claim all such 
embodiments that fall within the true scope of the invention.