U.S. patent application number 13/331101 was filed with the patent office on 2012-04-12 for curved retarder based optical filters and eyewear.
This patent application is currently assigned to RealD, INC.. Invention is credited to DAVID A. COLEMAN, Gary D. Sharp.
Application Number | 20120086907 13/331101 |
Document ID | / |
Family ID | 40533927 |
Filed Date | 2012-04-12 |
United States Patent
Application |
20120086907 |
Kind Code |
A1 |
COLEMAN; DAVID A. ; et
al. |
April 12, 2012 |
CURVED RETARDER BASED OPTICAL FILTERS AND EYEWEAR
Abstract
Curved polarization filters and methods of manufacturing
thereof. One method includes laminating a planar polarization layer
to a planar retarder layer at a predetermined orientation and
bending the laminate to create a curved filter. The strain on the
retarder layer results in stress-induced birefringence, and the
predetermined orientation of the retarder substantially compensates
for the stress-induced birefringence.
Inventors: |
COLEMAN; DAVID A.; (Boulder,
CO) ; Sharp; Gary D.; (Beverly Hills, CA) |
Assignee: |
RealD, INC.
Beverly Hills
CA
|
Family ID: |
40533927 |
Appl. No.: |
13/331101 |
Filed: |
December 20, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12249876 |
Oct 10, 2008 |
|
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13331101 |
|
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60979326 |
Oct 11, 2007 |
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Current U.S.
Class: |
351/159.56 ;
264/1.7; 359/489.07 |
Current CPC
Class: |
Y10T 156/10 20150115;
Y10T 156/1007 20150115; B29D 11/00644 20130101; B29D 11/0073
20130101; G02B 5/3033 20130101; G02B 30/25 20200101; B32B 38/1866
20130101; Y10T 156/1744 20150115; B29D 11/00634 20130101; B32B
2307/40 20130101; B29K 2995/0034 20130101; B32B 37/12 20130101 |
Class at
Publication: |
351/159.56 ;
359/489.07; 264/1.7 |
International
Class: |
G02C 7/02 20060101
G02C007/02; B29D 11/00 20060101 B29D011/00; G02B 5/30 20060101
G02B005/30 |
Claims
1. A lens configured to decode three dimensional content
comprising: a polarizing layer laminated with a polymeric material
layer on one or both sides; a retarder layer laminated to a front
of the polarizer layer directly or to the polymeric material layer
to form a sheet, said retarder layer aligned to decode a desired
circular polarization; and wherein the lens is curved utilizing a
thermoforming process to form said lens configured to decode three
dimensional content.
2. The lens of claim 1 wherein said polarizing layer has a
polarizing efficiency of 99.9% and a luminous transmittance of
38.8%.
3. The lens of claim 1 wherein the polymeric material layer is over
the retarder layer.
4. The lens of claim 1 wherein said polymeric material layer
includes cellulous acetate.
5. The lens of claim 1 wherein said polarizer layer is tinted with
iodine crystals.
6. The lens of claim 1 wherein said retarder layer is polyvinyl
alcohol or polycarbonate.
7. The lens of claim 1 further comprising a polarization angle set
with respect to the retarder such that the retarder and the
polarizer block light of one or the other of left-circular
polarization or right-circular polarization.
8. The lens of claim 1 further comprising a retardation angle
suitable for left-circular polarization.
9. The lens of claim 1 further comprising a retardation angle
suitable for right-circular polarization degrees.
10. The lens of claim 1 further comprising a lens thickness
suitable for stereoscopic eyewear.
11. The lens of claim 1 further comprising a designed retardation
suitable for the retarder layer.
12. A method of fabricating a curved lens configured to decode
three dimensional content comprising: forming filters from sheets
of material comprising: a polarizing layer laminated with a
polymeric material layer on one or both sides; a retarder layer
laminated to a front of the polarizer layer directly or the
polymeric material, said retarder layer aligned to decode a desired
circular polarization, and wherein said filters have a
predetermined alignment of a polarizing axis associated with said
sheet; heating the filters to a deformation temperature; curving
the filters using pressure; and cooling the curved filters.
13. The method of claim 12 further comprising utilizing a
polarizing material having a polarizing efficiency of 99.9% and a
luminous transmittance of 38.8%.
14. The method of claim 12 further comprising using a polymeric
material layer over the retarder layer.
15. The method of claim 12 further comprising heating the filters
to a deformation temperature.
16. The method of claim 12 further comprising a polarization angle
set with respect to the retarder such that the retarder and the
polarizer block light of one or the other of left-circular
polarization or right-circular polarization.
17. The method of claim 12 further comprising fabricating the lens
to have a retardation angle suitable for left-circular
polarization.
18. The method of claim 12 further comprising fabricating the lens
to have a retardation angle suitable for right-circular
polarization.
19. The method of claim 12 further comprising fabricating the lens
to have a transmittance percentage of 38.8%.
20. The method of claim 12 further comprising fabricating the lens
to have a lens thickness suitable for stereoscopic eyewear.
21. The method of claim 12 further comprising fabricating the lens
to have a designed retardation suitable for the retarder layer.
22. The method of claim 12 further comprising utilizing a polymeric
layer as the retarder layer.
23. The method of claim 22 further comprising utilizing cellulous
diacetate or polycarbonate-for the polymeric material layer.
24. Eyeglasses comprising: a frame configured to retain a left lens
and right lens; said left lens comprising; a polarizing layer
laminated with a polymeric material layer on one or both sides; a
retarder layer laminated to a front of the polarizer layer directly
or the polymeric material to form a sheet, said retarder layer
aligned to decode a desired left handed circular polarization; and
wherein a filter is curved utilizing a thermoforming process to
form said lens configured to decode three dimensional content; and
said right lens comprising; a polarizing layer laminated with a
polymeric material layer on one or both sides; a retarder layer
laminated to a front of the polarizer layer directly or the
polymeric material to form a sheet, said retarder layer aligned to
decode a desired right handed circular polarization; and wherein a
filter is curved utilizing a thermoforming process to form said
lens configured to decode three dimensional content.
25. The eyeglasses of claim 24 wherein said left lens utilizes a
retardation angle suitable for right-circular polarization.
26. The eyeglasses of claim 24 wherein said right lens utilizes a
retardation angle suitable for left-circular polarization.
27. A lens configured to decode three dimensional content
comprising: a polarizing layer laminated with a polymeric material
layer on one or both sides; a retarder layer laminated to a front
of the polarizer layer directly or to the polymeric material to
form a sheet, said retarder layer aligned to decode a desired
circular polarization; and wherein a filter is curved using a press
into a lens configured to decode three dimensional content.
28. The lens of claim 27 wherein said polarizing layer, polymeric
layer and retarder layer have a combined thickness suitable for
stereoscopic eyewear.
29. A lens configured to decode three dimensional content
comprising: a polarizing layer laminated with a polymeric material
layer on one or both sides; a retarder layer laminated to a front
of the polarizer layer directly or to the polymeric material to
form a sheet, said retarder layer aligned to decode a desired
circular polarization; wherein a filter is curved using a press
process into an optical element configured to decode three
dimensional content.
30. The lens of claim 29 wherein said polarizing layer, polymeric
layer and retarder layer have a combined thickness suitable for
stereoscopic eyewear.
31. The lens of claim 30 wherein said polarizing layer, polymeric
layer, retarder layer and one or more thickness layers have a
combined thickness suitable for stereoscopic eyewear.
32. A method of fabricating a curved lens configured to decode
three dimensional content comprising: forming filters from sheets
of material comprising, a polarizing layer laminated with a
polymeric material layer on one or both sides; a retarder layer
laminated to a front of the polarizer layer directly or the
polymeric material, said retarder layer aligned to decode a desired
circular polarization, and wherein said filters have a
predetermined alignment of a polarizing axis associated with said
sheet; curving said filters into lenses by: applying high pressure
with controlled heating from both sides of a polished mold.
33. The method of claim 32 further comprising fabricating said lens
with said polarizing layer, polymeric layer and retarder layer
having a combined thickness suitable for stereoscopic eyewear.
34. The method of claim 32 further comprising fabricating said lens
with said polarizing layer, polymeric layer and retarder layer
having a combined thickness suitable for stereoscopic eyewear.
35. The method of claim 32 further comprising adding material to a
thickness of a lens.
36. The method of claim 32, further comprising fabricating said
lens with said polarizing layer, polymeric layer, retarder layer
and one or more injection molding layers having a combined
thickness suitable for stereoscopic eyewear.
37. Glasses configured to decode three dimensional content
comprising: two lenses having: a polarizing layer laminated with a
polymeric material layer on one or both sides; a retarder layer
laminated to a front of the polarizer layer directly or to the
polymeric material to form a sheet, said retarder layer configured
to decode a desired circular polarization; wherein a first filter
is curved into a first lens and a second filter is curved into a
second lens wherein together the first and second lens are able to
decode three dimensional content; and wherein the first and second
lenses are configured such that light that is left-circular
polarized is blocked by one of the first and second sheets and
light that is right-circular polarized is blocked by the other of
the first and second sheets.
38. The glasses of claim 37 wherein said polarizing layer,
polymeric layer and retarder layer have a combined thickness
suitable for stereoscopic eyewear.
39. The glasses of claim 37 wherein said first and second lens in
combination are configured to decode three dimensional content
displayed on a computer or television monitor.
40. Glasses configured to decode three dimensional content
comprising: two optical elements having: a polarizing layer
laminated with a polymeric material layer on one or both sides a
retarder layer laminated to a front of the polarizer layer directly
or to the polymeric material to form a sheet, said retarder layer
configured to decode a desired circular polarization; wherein a
first filter is curved into a first optical element and a second
filter is curved into a second optical element; wherein the first
and second sheet are configured such that: light that is
left-circular polarized is blocked by one of the first and second
sheets and light that is right-circular polarized is blocked by the
other of the first and second sheets.
41. The glasses of claim 40 wherein said polarizing layer,
polymeric layer and retarder layer have a combined thickness
suitable for eyeglasses.
42. The glasses of claim 41 wherein said polarizing layer,
polymeric layer, retarder layer and one or more thickness layers
have a combined thickness suitable for eyeglasses.
43. The glasses of claim 40 wherein said pair of lenses is
configured to decode three dimensional content displayed on a
computer monitor.
44. A method of fabricating curved lenses configured to decode
three dimensional content comprising: providing filters from
material comprising: a polarizing layer laminated with a polymeric
material layer on one or both sides; a retarder layer laminated to
a front of the polarizer layer directly or the polymeric material,
said retarder layer aligned to decode a desired circular
polarization, wherein a first and second filter used to form a left
and right lens are formed from first and second sheets configured
such that light that is left-circular polarized is blocked by one
of the first and second sheets and light that is right-circular
polarized is blocked by the other of the first and second sheets,
curving said first and second filters into lenses by applying high
pressure with controlled heating from both sides.
45. The method of claim 44 further comprising fabricating said lens
with said polarizing layer, polymeric layer and retarder layer
having a combined thickness suitable for stereoscopic eyewear.
46. The method of claim 45 further comprising adding a material to
increase the thickness to said lenses.
47. The method of claim 45 further comprising fabricating said lens
with said polarizing layer, polymeric layer and retarder layer and
additional layers having a combined thickness suitable for
stereoscopic eyewear.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. Ser. No.
12/249,876 filed Oct. 10, 2008 and claims priority to U.S.
Provisional Patent Application No. 60/979,326, entitled "Method and
Apparatus for Curved Retarder-based Optical Polarization Filters"
filed Oct. 11, 2007, the entire contents of each application are
incorporated herein by reference.
TECHNICAL FIELD
[0002] This disclosure generally relates to optical filters, and
more particularly, this disclosure relates to curved optical
filters for viewing stereoscopic or non-stereoscopic images.
BACKGROUND
[0003] Stereoscopic imaging involves recording three-dimensional
(3-D) visual information or creating the illusion of depth in an
image. One easy way to create depth perception in the brain is to
provide the eyes of the viewer two different images, representing
two perspectives of the same object, with a minor deviation similar
to the perspectives that both eyes naturally receive in binocular
vision. Many optical systems display stereoscopic images using this
method. The illusion of depth can be created in a photograph,
movie, video game, or other two-dimensional (2-D) image.
BRIEF SUMMARY
[0004] Stereoscopic and non-stereoscopic eyewear may include a
low-cost optical filter manufactured by laminating a retarder film
(e.g., a quarter wave plate ("QWP")) and a polarizer film from
separate roll stock. Laminating a retarder film from roll stock to
a polarizer film from roll stock involves cutting, aligning, and
laminating the films at a suitable orientation that allows for the
desired optical property. The end result of this process is a
planar optical filter.
[0005] Disclosed in the present application is an optical filter in
a curved configuration. The optical filter includes a polarization
layer and a retarder layer laminated to the polarization layer at a
predetermined angle, wherein the retarder and polarization layers
are bent to the curved configuration. The retarder layer has a
stress-induced birefringence, and the predetermined angle
substantially compensates for the stress-induced birefringence and
disposes the optical axis of the retarder film in a desired
orientation. In one embodiment, the retarder layer is a QWP. In
another embodiment, the retarder layer is a multi-layer retarder
stack. In another embodiment, the polarizer layer is disposed on
the inner layer of the retarder layer and an additional
non-birefringent layer is laminated to the outer surface of the
retarder layer. Other configurations are possible.
[0006] The present disclosure also provides a method for
manufacturing a curved filter having a retardation axis in a
desired orientation. The method includes providing a planar
retarder film having an optical axis and a planar polarization
film. The method also includes predetermining an angle at which the
planar retarder film would be laminated to the planar polarization
film, and laminating the planar retarder film and the planar
polarization film at the predetermined angle. The method further
includes bending the laminated retarder and polarization films,
whereby a stress is exerted on the retarder film, the stress
causing a stress-induced birefringence in the retarder film. The
predetermined angle substantially compensates for the
stress-induced birefringence and allow the optical axis of the
retarder film to be in the desired orientation.
[0007] The present disclosure further provides an apparatus for
laminating curved optical films, which includes a curved drum
operable to provide a curved surface for bonding first and second
optical films. The apparatus also includes at least two feeders
operable to feed the first and second optical films onto the curve
drum, a coating apparatus operable to apply adhesive to a surface
of the first or second optical film, and a press roller operable to
apply pressure against the first and second optical films and
laminate the first and second optical films. Methods of
manufacturing a filter using such an apparatus are also provided
herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1(A) is a perspective schematic view of a planar
polarizer filter;
[0009] FIG. 1(B) is a perspective schematic view of a curved
polarizer filter;
[0010] FIG. 1(C) is a perspective schematic view of a curved
polarizer filter having an optical axis in a desired
orientation;
[0011] FIG. 2 is a chart correlating cylindrical curve radius with
retarder angle adjustment for a curved filter;
[0012] FIG. 3 is a chart illustrating the optical performance of
various filters;
[0013] FIG. 4 is a perspective schematic view of another embodiment
of a curved polarizer filter;
[0014] FIG. 5 is a schematic diagram of an embodiment of a
lamination apparatus; and
[0015] FIG. 6 is a display system incorporating curved filters in
eyewear for viewing stereoscopic images.
DETAILED DESCRIPTION
[0016] One technique to provide 3-D images is to encode light bound
for each eye with different polarizations. Such a scheme may
involve using orthogonal linearly polarized states, or circularly
polarized states of opposite handedness. To present a stereoscopic
picture using a circularly polarized 3-D image system, two images
are projected and superimposed onto the same screen through
circular polarizing filters of opposite handedness. The viewer
wears eyeglasses that include a pair of circular polarizers ("CPs")
of opposite handedness, which function as analyzers. Light that is
of left-circular polarization is blocked by the right-handed
analyzer while right-circularly polarized light is blocked by the
left-handed analyzer. The result is similar to that of stereoscopic
viewing with linearly polarized glasses, except the viewer can tilt
his or her head and not compromise the quality of the polarization
encoding and decoding.
[0017] Commonly-assigned U.S. Pat. No. 4,792,850 by Lenny Lipton,
which is hereby incorporated by reference, discloses electronically
driven CPs that alternate between left and right handedness in
synchronization with the left and right images being displayed by
the image projector. Direct view displays may also be used to
encode the polarization states for different eyes; for example,
alternate pixels of a direct view display may provide light of
different polarization states. Another way to provide alternate
right/left eye images is using a single display or projector that
actively encodes the images using a polarization switch. Examples
of such a technique are disclosed in the commonly-assigned U.S.
patent application Ser. No. 11/424,087, entitled "Achromatic
Polarization Switches," filed Jun. 14, 2006, which his hereby
incorporated by reference.
[0018] Instead of encoding the 3-D information with polarization,
the left and right eye images may be encoded on distinct wavelength
bands as disclosed by Maximus et al. in U.S. patent application
Ser. No. 10/576,707. Eyewear is then used to selectively filter the
wavelengths appropriate for each eye. As described by Maximus et
al., this filtering may be performed using dichroic filters;
however, it is also possible to perform this filtering using
polarization interference.
[0019] Additionally, the selective filtering of light by
polarization interference may be used to enhance vision and/or
protect eyes from harmful light rays. For example, such filtering
may be used in sunglasses, color corrective eyewear, or protective
eyewear. The selective filtering of incident light may provide any
desired spectral transmission (including visible light and light
not visible to the eye). The filtering structure may include
multi-layer polarizing structures and may be formed by fabricating
sheet laminates that are dye-cut to form inexpensive laminates. One
embodiment of the apparatus operable to provide selective filtering
includes a pair of polarizing elements that sandwich a retarder
stack. Further details of the design of such eyewear are described
by Sharp in commonly-assigned U.S. Pat. No. 7,106,509, which is
hereby incorporated by reference.
[0020] In order to watch a motion picture using polarization
encoding, the viewer wears a pair of paper frame or plastic frame
glasses with polarization filters. The filters in such glasses are
generally produced by laminating a retarder film to a polarizing
film encapsulated by triactyl cellulose ("TAG") using adhesives. In
some embodiments, the retarder film may be a QWP and the adhesive
used for lamination is a pressure sensitive adhesive ("PSA"). In
other embodiments, the retarder film may be a half-wave plate
("HWP"). Such viewing eyewear uses a planar filter.
[0021] A retarder, as described in the present application, may
comprise N linear retarders that have been designed using Finite
Impulse Response (FIR) filter techniques, wherein the impulse
response of the N retarders generates at least N+1 output impulses
from a polarized impulse input. As such, placing retarder stacks
between neutral polarizers forms FIR filters, and these FIR filters
can be designed using standard signal processing methods. The
amplitude of each responsive output impulse is determined by the
orientations of the retarders and the analyzing polarizer. Further
details of the design approaches for the described retarder stacks
are described in the commonly assigned U.S. Pat. No. 7,106,509 and
by U.S. patent application Ser. No. 09/754,091, which is also
incorporated by reference herein.
[0022] In general, the component stock films for retarders and
polarizers have planar geometry. However, it is desirable both for
optical improvement and for cosmetic enhancement to produce eyewear
with curved filters. Shaping the filter to form a structure with
either cylindrical or compound curvature subsequent to the
lamination of component stock films can induce unacceptable strain
in the component films. This strain either induces birefringence or
modifies the desired intrinsic birefringence and thus degrades the
performance of the filter.
[0023] In U.S. Pat. No. 5,051,309, Kawaki et al. disclose a method
of forming polarizing glasses employing linear polarization
elements. U.S. Pat. No. 5,051,309 shows an example of a polyvinyl
alcohol film (kuraray Vinylon #7500 produced by Kuraray Inc.) which
was stretched monoaxially to 4 times its original size in an
aqueous solution containing 0.03% of iodine, 0.43% of potassium
iodide and 0.71% of boric acid, and then dried. Polycarbonate
sheets in this '309 patent having a thickness of 0.4 mm and a
retardation value of 3290 nm were laminated by a polyurethane
adhesive onto both surfaces of the polarizing layer obtained above
so that their directions of orientation agreed with each other to
obtain a polarizing plate having a single plate transmittance of
38.8% and a degree of polarization of 99.9%. Kawaki's method
involves either annealing the birefringent polycarbonate layers
that are in front of the polarizing element to eliminate the
birefringence or by "super-stretching" these layers to have a very
large retardance parallel to the polarizer axis. The polarizer
effectively hides the birefringence of the polymer. In U.S. Pat.
No. 6,177,032, Smith et al. disclose a method of pre-forming the
individual functional layers and then assembling the final filter.
Again, birefringence of the layers preceding the polarizer is
altered by annealing.
[0024] When a specific retardation and retarder angle are required,
such as when CPs are used in combination with retarder stack
filters, neither of these methods are acceptable. Thus, there
remains a need for curved filters incorporating retarder-based
polarization filters, in which the stress-induced birefringence is
either compensated or minimized.
[0025] Exemplary embodiments of a curved optical filter and methods
of manufacturing such a filter will be discussed below with
references to FIGS. 1 to 6. Specifically, various methods and
apparatus to compensate for or minimize strain-induced changes in
the retarder layer of a filter will be disclosed.
[0026] FIG. 1(A) shows a planar filter 100 having a retarder layer
104 laminated to a polarizer 106. The retarder layer 104 has an
optical axis 112 along axis 120 and the polarizer layer 106 has an
optical axis 114 along axis 122. In the illustrated embodiment, the
retarder layer 104 is a QWP, but in other embodiments, the retarder
layer 104 may be a half-wave plate ("HWP") or any retarders
described in the present application. In some embodiments
additional optical components may be added to the planar filter
100. For example, planar filter 100 may further include a substrate
layer to provide better structural support. In some other
embodiments, the ordering of the retarder and polarizer layers 104
and 106 may be inverted. It is to be appreciated that the structure
of the filter 100 may be varied to satisfy various design
needs.
[0027] In the embodiment in FIG. 1(B), the filter 100 is bent
around a first bend axis (not shown) to form a curved filter 101 by
pulling on ends 102 of the filter 100. The cylindrical filter 101
can be mounted on a glasses frame to retain the desired curved
configuration. It is to be appreciated that, in other embodiments,
the filter 101 can be bent around multiple bent axes to have a
compound curvature. In some embodiments, thermoforming processes
known in the art may be used to retain the desired curved
configuration. In one embodiment, the filter 101 is bent around
first and second bend axes, and undergoes a thermoforming process
to retain a compound curvature.
[0028] Because of the finite thickness of the filter 100, a
compressive stress 110 is exerted on the polarizer layer 106 while
a tensile stress 108 is exerted on the retarder layer 104. In both
layers, the directions of stresses 108 and 110 are tangent to the
curvature of the cylindrical filter, and the magnitude of each
stress is uniform across the surface of the curved filter 101.
[0029] The optical effect of stretching or bending a material that
displays strain birefringence can be conveniently described using
the strain tensor, . The eigenvectors ({right arrow over
(.epsilon.)}.sub.1,{right arrow over (.epsilon.)}.sub.2, {right
arrow over (.epsilon.)}.sub.3) of are parallel to the principle
axes of the dielectric tensor (often erroneously called optic
axes). The birefringence, .DELTA.n , is proportional to the
difference in the magnitudes of the eigenvalues of , and the
retardance is equal to the integral of An across the thickness of
the film; .DELTA.nd, in the case of uniform films of thickness d.
For materials exhibiting linear elasticity, sequential stretching
operations are equivalent to adding the strain tensor for each
individual operation:
='+''. (1)
[0030] The strain tensor of a stretched polymer retarder film with
retardance F and slow axis oriented parallel to the x-axis can be
written:
' = 1 t K [ .GAMMA. 0 0 0 0 0 0 0 0 ] , ( 2 ) ##EQU00001##
where K is the strain optic coefficient and d is the film
thickness. For convenience we neglect strain in the orthogonal
directions, i.e., we choose a Poisson ratio of 0 although it can be
shown that the derivation is general. It is trivially seen that
additional stretching parallel to the x-axis increases the
retardance while stretching perpendicular to the x-axis decreases
the retardance. However, if the stretching is performed at a 45
degree angle to the x-axis, the additional strain tensor is:
'' = 1 2 [ '' - '' 0 - '' '' 0 0 0 0 ] . ( 3 ) ##EQU00002##
[0031] The eigenvalues of are then:
1 = 1 2 ( .GAMMA. d K + '' - ( .GAMMA. d K ) 2 + ( '' ) 2 ) 2 = 1 2
( .GAMMA. d K + '' + ( .GAMMA. d K ) 2 + ( '' ) 2 ) ( 4 )
##EQU00003##
and the orientation of {right arrow over (.epsilon.)}.sub.1 is
.theta. = - tan - 1 ( 2 .GAMMA. d K - ( .GAMMA. d K ) 2 + ( '' 2 )
2 '' ) . ( 5 ) ##EQU00004##
[0032] When equation 5 is expanded for
.epsilon.''>>.GAMMA./(dK) it can be shown that for small
additional stretching at 45 degrees from the optic axis, the change
in the retarder orientation is linear in .epsilon.''. In contrast,
equation 4 can be solved for .DELTA.n to show that the change in
retardation is quadratic in .epsilon.'' in the same limit.
[0033] Based on the above discussion, it is apparent that
stretching parallel or perpendicular to the retarder optic axis
only changes the magnitude of the retardance while the optic axis
orientation is stable. In contrast, stretching at 45 degrees to the
optic axis rotates the optic axis for relatively small strains.
[0034] Referring back to FIG. 1(B), the polarizer axis 122 is
parallel to the strain direction, and accordingly, its orientation
remains stable. The tensile stress 108, however causes an in-plane
rotation of the retarder axis 112 by .theta. (equation 5) as well
as an increase in the retardance according to equation 4. The
retarder axis 112 is now oriented along axis 124. Due to the
in-plane rotation of the retarder axis 112, the filter 101 now
produces elliptical instead of circularly polarized light, and the
discrimination between left and right eye images is
compromised.
[0035] Referring now to FIG. 1(C), when the stretching is uniform
in both magnitude and direction across the surface of the film, it
is possible to predict the change in both retardance and optic axis
orientation and to compensate for it by adjusting the initial
retardance and orientation of the film. Therefore, even materials
such as polycarbonate with a large strain-optic coefficient may be
used. In the embodiment illustrated in FIG. 1(C), the retarder and
polarizer layers are laminated at a predetermined angle to
compensate for the stress-induced birefringence in the retarder
film and to dispose the retarder axis in the desired orientation as
indicated by axis 120.
[0036] In order to produce retardance F parallel to the x-axis in
the presence of uniform strain of magnitude c with principle strain
direction oriented at .theta. degrees to the x-axis, it can be
shown that an initial retardance of
.GAMMA..sub.0= {square root over
(.GAMMA..sup.2+K.sup.2d.sup.2.epsilon..sup.2-2Kd.epsilon..GAMMA.
Cos 2.theta.)}, (6)
should be oriented at:
.theta. 0 = - tan - 1 ( ( - .GAMMA. + K d cos 2 .theta. + .GAMMA. 2
+ K 2 d 2 2 - 2 K d .GAMMA. cos 2 .theta. ) csc 2 .theta. K d ) . (
7 ) ##EQU00005##
[0037] Equations (6) and (7) are applied individually for each
layer in a multi-layer stack to account for differences in strain,
retardance, and retardance orientation. The trivial solution occurs
when the desired retardance is zero. Equations (6) and (7) reduce
to the intuitive result that the initial retardance is chosen to
exactly cancel the retardance induced by strain.
[0038] These conclusions can also be extended to polarizing
components. The polymer matrix of a polarizer film may be highly
stretched such that the assumptions about additive strain from the
previous section are unlikely to apply. However, the symmetry
remains the same: the polarization axis orientation will remain
stable under parallel (and perpendicular) stretching, but may be
rotated if the stretching occurs at 45 degrees to the polarizer
axis. Consequently it is desirable to orient the polarizer axis
parallel or perpendicular to the principle component of any
expected strain.
[0039] It is possible to either analytically or numerically model
the mechanical structure of the cylindrical filter 101 that would
allow the tensile stress 108 in the outer retarder layer 104 to
balance the compressive stress 110 on the inner polarization layer
106. By knowing the elastic constants for the layers of the filter
101, it is then possible to solve for the tensile stress 108 and
then use equations (6) and (7) to compute and predetermine the
initial retardance and retarder orientation that would compensate
for the stress-induced birefringence. By laminating the retarder
and polarizer layers at a predetermined off-set angle 128, the
optic axis 112 of the retarder layer 104 would be rotated from axis
126 to the desired axis 120 as illustrated in FIG. 1(C) due to the
bending of the curved filter 101 and the stress-induced
birefringence.
[0040] In practice, it is often more convenient to simply measure
the change in the retarder film at different radii of curvature and
then generate a look-up table or chart for the off-set angle that
would compensate for the stress birefringence. Due to the quadratic
correction to the retardance for small strain, the same retardation
film may be used for a broad range of curvatures while only
adjusting the film orientation during lamination.
[0041] An exemplary lookup chart for a CP filter is shown in FIG.
2. Plotted along the x-axis are the various radii of the CP filter
and plotted along the y-axis are the corresponding off-set angles
that would compensate for the stress-induced birefringence. The
data were obtained by measuring the ellipticity of light exiting CP
filters with varying radii of curvature. The analytic expression
for the optic axis orientation was then used to determine the optic
axis orientation in the bent filter and thus the offset angle for
compensating it. For large radius curves, the offset angle goes to
zero. For very small radius curves, the relation eventually breaks
down as changes in retardance become increasingly important. It is
to be appreciated that similar lookup charts can be created for
other types for filters in accordance to the principles disclosed
herein.
[0042] FIG. 3 is a chart illustrating the optical performance of
various filters. The improved performance of a filter that has been
adjusted to compensate for stress-induced birefringence relative to
that of a filter that has not been pre-adjusted is demonstrated in
FIG. 3. FIG. 3 shows a set of spectra obtained for two different
filters that were crossed with a reference filter. The first filter
is manufactured with a 45 degree optic axis orientation (without
pre-adjustment) for the retarder; the second filter was
pre-adjusted for an 8 cm radius of curvature using the cutting
angle offset in FIG. 2. In the planar configuration, the first
filter produced leakage below 0.2% over much of the visible
spectrum whereas the second filter had much higher leakage as
expected. When bent to a radius of curvature of 8 cm, the first
filter produced leakage greater than 0.4% over the entire
spectrum--highlighting the need for correction. The second filter
performed nearly as well with a radius of 8 cm as the first filter
did in the planar configuration.
[0043] In addition to adjusting the initial retardance and retarder
orientation, stress-induced birefringence can also be reduced by
using retarder films with optimal material properties. Irrespective
of the stretching direction, the effect of induced strain decreases
as the magnitude of the initial retardance increases. Therefore, it
is advantageous to use as large of an initial retardance as
possible relative to the quantity Kd.epsilon.'', in order to
minimize the effect of induced strain on the final structure.
Accordingly, it is desirable to minimize the thickness, d, of the
film and/or the strain optic coefficient, K. Specifically, it is
preferred that the ratio
K d '' .GAMMA. ##EQU00006##
is less than 0.01 in order to maintain approximately a 1%
uniformity in the retardance and optic axis orientation.
[0044] Satisfying the above prescription for any significant strain
is difficult with polycarbonate-based retardation films because of
the relatively high strain-optic coefficient of polycarbonate. When
using such films, it is therefore preferable to choose thinner film
stock. In the case in which the polycarbonate retarder film is
approximately 60 .mu.m thick, decreasing the thickness to 12 .mu.m
only increases the strain-optic stability by a factor of 5. Such
thin films may be difficult to work with in the manufacturing
setting and may be susceptible to damage during any solvent welding
or other lamination processes. Improved strain resistance can be
obtained by using optical plastics such as cellulose di-acetate or
cyclic olefin copolymer ("COC") retarder film. The strain-optic
coefficient of these materials is approximately 10 times smaller
than that of polycarbonate, and thus, a thicker substrate may be
used to ease manufacturability while still minimizing
stress-induced birefringence. An exemplary embodiment is the
COC-based film manufactured under the brand name Arton.RTM.. This
material has excellent optical clarity, uniform birefringence, and
a sufficiently small strain optic coefficient to enable
thermoforming in a variety of applications.
[0045] FIG. 4 is a curved filter 200 that was bent around a
plurality of bend axes and configured to have a compound curvature.
The filter 200 further includes a substrate layer 206. The filter
200 includes a retarder layer 202 laminated to a polarizer layer
204. The compound curvature may be retained using a thermoforming
process. In one embodiment, the thermoforming process includes
applying heat to the substrate layer. To minimize the effect of
stress birefringence resulted from the bending of the filter 200,
the retarder layer 202 is preferably made of materials having
strain optic coefficient ranging from 0.001 to 0.025. The thickness
of the retarder layer is preferably between 50 to 120 microns. In
an exemplary embodiment, the retarder layer 202 is made of cyclic
olefin block copolymer. It is to be appreciated that the retarder
layer can be made of other materials and have various thicknesses
so
K d '' .GAMMA. ##EQU00007##
[0046] long that the ratio is approximately less than 0.01.
[0047] FIG. 5 is a perspective schematic view of another embodiment
of a curved polarizer filter 300. Curved filter 300 includes a
polarizer layer 314 having optic axis 306, a retarder layer 316
having an optic axis 304, and a non-birefringent mechanical layer
318. The mechanical layer 318 is added to sandwich the retarder
layer 316 to balance the stain on the retarder layer 316. In some
embodiments, the mechanical layer 318 may include a negative
c-plate. The thickness and elastic modulus of the mechanical layer
318 is preferably chosen so that it exerts a tensile stress 312
that balances compressive stress 310, which would minimize the
stress-induced birefringence in the retarder layer 316.
Furthermore, the strain optic coefficient of the mechanical layer
318 preferably is sufficiently small that the tensile stress 312
imparts minimal additional in-plane birefringence. In an exemplary
embodiment, the polarizer layer 314 may include a TAC encapsulated
polarizer, and it would be sufficiently balanced mechanically by an
equal thickness of TAC in the mechanical layer 318. As result, the
orientation of the optic axis 304 of the retarder layer 316 would
not be affected by stress-induced birefringence.
[0048] FIG. 6 is a schematic diagram of an embodiment of a
lamination apparatus 400 for manufacturing a curved filter while
minimizing the strain on the components of the curved filter. The
lamination apparatus 400 includes a curved drum 406 operable to
provide a curved surface for bonding first and second optical films
402 and 404. It is preferable that the radius of the drum 406
matches the radius of the finished eyewear. In an exemplary
embodiment, the first optical film 402 is a polarizer film, and the
second optical film 404 is a retarder film, such as a QWP film. In
another exemplary embodiment, the second film 404 is a QWP oriented
at 45 degrees to the first film 402. The lamination apparatus 400
further includes at least two feeders 412 operable to feed the
first and second optical films onto the curved drum 406 and a
coating apparatus 414 operable to apply adhesive to a surface of
the first or second optical film. A press roller 410 of the
lamination apparatus 400 is operable to apply pressure against the
first and second optical films 402 and 404 to laminate them.
[0049] In operations, to laminate the first and second films 402
and 404, each film is fed onto the drum at different rates to
accommodate the different radius of curvature of each film. The
films 402 and 404 travel on the drum surface for a finite time
through region 408 in order to allow the internal stress to be
relieved. Press roller 410 completes the lamination process by
exerting pressure on the films 402 and 404 and causing the adhesive
to form bonds between the films 402 and 404.
[0050] It is to be appreciated that the curved filters disclosed in
the present application can be incorporated into various eyewear
for viewing stereoscopic or non-stereoscopic images displayed by
any imaging systems described herein. For example, a display system
600 as illustrated in FIG. 7 can include a projection screen 602
and polarization filtering eyewear 604 that incorporates two curved
filters 606 and 608. Stereoscopic 3-D imagery is observed using a
single polarization-preserving screen 602 sequentially displaying
left and right perspective imagery, with polarization filtering
eyewears 604. In some embodiments, the curved filters 606 and 608
are of alternately orthogonal polarization. In some particular
embodiments, the curved filters 606 and 608 are operable to provide
circularly polarized light of opposite handedness. In an exemplary
embodiment, the polarization-preserving screen 602 is a direct-view
screen.
[0051] It will be appreciated by those of ordinary skill in the art
that the invention(s) can be embodied in other specific forms
without departing from the spirit or essential character thereof.
Any disclosed embodiment may be combined with one or several of the
other embodiments shown and/or described. This is also possible for
one or more features of the embodiments. The steps herein described
and claimed do not need to be executed in the given order. The
steps can be carried out, at least to a certain extent, in any
other order.
[0052] Further, it will be appreciated by one of ordinary skill in
the art that various retardance and optic axis values depend
sensitively on the mechanical properties of all of the layers and
adhesives in the filter. It will also be appreciated that the
circular polarizer disclosed herein may be combined with various
other display components to perform similar results. The presently
disclosed embodiments are therefore considered in all respects to
be illustrative and not restrictive. The scope of the invention is
indicated by the appended claims rather than the foregoing
description, and all changes that come within the meaning and
ranges of equivalents thereof are intended to be embraced
therein.
[0053] Additionally, the section headings herein are provided for
consistency with the suggestions under 37 C.F.R. .sctn.1.77 or
otherwise to provide organizational cues. These headings shall not
limit or characterize the invention(s) set out in any claims that
may issue from this disclosure. Specifically and by way of example,
although the headings refer to a "Technical Field," the claims
should not be limited by the language chosen under this heading to
describe the so-called technical field. Further, a description of a
technology in the "Background" is not to be construed as an
admission that technology is prior art to any invention(s) in this
disclosure. Neither is the "Brief Summary" to be considered as a
characterization of the invention(s) set forth in the claims found
herein. Furthermore, any reference in this disclosure to
"invention" in the singular should not be used to argue that there
is only a single point of novelty claimed in this disclosure.
Multiple inventions may be set forth according to the limitations
of the multiple claims associated with this disclosure, and the
claims accordingly define the invention(s), and their equivalents,
that are protected thereby. In all instances, the scope of the
claims shall be considered on their own merits in light of the
specification, but should not be constrained by the headings set
forth herein.
* * * * *