U.S. patent application number 10/603286 was filed with the patent office on 2004-12-30 for display apparatus.
This patent application is currently assigned to Eastman Kodak Company. Invention is credited to Cobb, Joshua M., Kurtz, Andrew F., Mi, Xiang-Dong.
Application Number | 20040263989 10/603286 |
Document ID | / |
Family ID | 33418652 |
Filed Date | 2004-12-30 |
United States Patent
Application |
20040263989 |
Kind Code |
A1 |
Cobb, Joshua M. ; et
al. |
December 30, 2004 |
DISPLAY APPARATUS
Abstract
A display apparatus (10) comprises a light source (20) for
forming a beam of light. Illumination optics shapes and directs the
beam of light. A splitter for splits the beam of light into at
least three color beams of light. A modulation optical system (120)
for each of the three color beams of light comprises a
pre-polarizer (160), a wire grid beamsplitter (170), a reflective
spatial light modulator (30), and a polarization analyzer (165). An
imaging relay lens (130) in each color provides an intermediate
image of the reflective spatial light modulator from the modulated
light for that color. A dichroic combiner (26) recombines the
modulated light for each given color, such that the multiple color
beams form the respective intermediate images along a common
optical axis to form a combined intermediate image. A projection
lens (32) images the combined intermediate image to a display
screen. An imager field lens (140) provides nominally telecentric
light to the spatial light modulators.
Inventors: |
Cobb, Joshua M.; (Victor,
NY) ; Kurtz, Andrew F.; (Rochester, NY) ; Mi,
Xiang-Dong; (Rochester, NY) |
Correspondence
Address: |
Milton S. Sales
Patent Legal Staff
Eastman Kodak Company
343 State Street
Rochester
NY
14650-2201
US
|
Assignee: |
Eastman Kodak Company
|
Family ID: |
33418652 |
Appl. No.: |
10/603286 |
Filed: |
June 25, 2003 |
Current U.S.
Class: |
359/634 ;
348/E9.027 |
Current CPC
Class: |
H04N 9/315 20130101;
G02B 5/3058 20130101; G02B 13/22 20130101; G03B 21/208 20130101;
G02B 27/283 20130101; G03B 33/12 20130101; G03B 21/2073
20130101 |
Class at
Publication: |
359/634 |
International
Class: |
G02B 027/14 |
Claims
What is claimed is:
1. A display apparatus comprising: (a) a light source for forming a
beam of light; (b) illumination optics for shaping and directing
said beam of light; (c) a splitter for splitting said beam of light
into at least three color beams of light; (d) a modulation optical
system for each of said three color beams of light, comprising: (1)
a pre-polarizer for polarizing said beam of light to provide a
polarized beam of light of a given color; (2) a wire grid
polarization beamsplitter for receiving said polarized beam of
light, for transmitting said polarized beam of light having a first
polarization, and for reflecting said polarized beam of light
having a second polarization orthogonal to said first polarization,
wherein subwavelength wires on said wire grid polarization
beamsplitter face a reflective spatial light modulator; (3) an
imager field lens that provides nominally telecentric light to said
reflective spatial light modulators (4) a reflective spatial light
modulator wherein said reflective spatial light modulator receives
said polarized beam of light, having either a first polarization or
a second polarization, and selectively modulates said polarized
beam of light to encode data thereon, providing both modulated
light and unmodulated light which differ in polarization; (5)
wherein said reflective spatial light modulator reflects back both
said modulated light and said unmodulated light to said wire grid
polarization beamsplitter; (6) wherein said wire grid polarization
beamsplitter separates said modulated light from said unmodulated
light; and (7) a polarization analyzer that receives said modulated
light, and which further removes any residual unmodulated light
from said modulated light; (8) an imaging relay lens in each color
that provides an intermediate image of the reflective spatial light
modulator from the modulated light for that color; (e) a dichroic
combiner for re-combining the modulated light for each given color,
such that the multiple color beams form the respective intermediate
images along a common optical axis to form a combined intermediate
image; and (f) a projection lens for imaging said combined
intermediate image to a display screen.
2. The display apparatus as in claim 1 wherein said imager field
lens is low stress or low absorption optical glass.
3. The display apparatus as in claim 1 wherein said imager field
lens is fabricated from amorphous fused silica.
4. The display apparatus as in claim 1 wherein said imager field
lens has uniform residual birefringence.
5. The display apparatus as in claim 1 wherein said imager field
lens is constructed from two lens elements with crossed
polarization axes to cancel residual retardances.
6. The display apparatus as in claim 1 wherein said imager field
lens is mounted with a compliant adhesive.
7. The display apparatus as in claim 1 wherein said dichroic
combiner is located in proximity to the intermediate images.
8. The display apparatus as in claim 1 wherein said dichroic
combiner is a v-prism.
9. The display apparatus as in claim 1 wherein said dichroic
combiner is an x-prism.
10. The display apparatus as in claim 1 wherein said dichroic
combiner is a Philips prism.
11. The display apparatus as in claim 1 wherein said imaging relay
lenses operate at a magnification greater than unity
magnification.
12. The display apparatus as in claim 1 wherein said imaging relay
lenses operate at a nominal 2.times. magnification.
13. The display apparatus as in claim 1 wherein said imaging relay
lens is a double gauss type lens.
14. The display apparatus as in claim 1 wherein said imaging relay
lens is double telecentric.
15. The display apparatus as in claim 1 wherein said illumination
optics are constructed with an integrating bar and an internal
intermediate image of said integrating bar.
16. The display apparatus as in claim 1 wherein said modulation
optical system has at least one polarization compensator located
between said wire grid polarization beamsplitter and said
reflective liquid crystal device for conditioning oblique rays of
said modulated beam.
17. The display apparatus as in claim 1 wherein said modulation
optical system has two polarization compensators with said imager
field lens located between them.
18. The display apparatus as in claim 17 wherein said polarization
compensator or compensators provides corrective retardances for at
least one of a group comprises of said wire grid polarization beam
splitter, spatial light modulator, and imager field lens.
19. The display apparatus as in claim 1 wherein said pre-polarizer
is a wire grid polarizer.
20. The display apparatus as in claim 1 wherein said polarization
analyzer is a wire grid polarizer.
21. The display apparatus as in claim 1 wherein said spatial light
modulator is a LCD.
22. The display apparatus as in claim 1 wherein said spatial light
modulator is a vertically aligned LCD.
23. The display apparatus as in claim 1 wherein said imager field
lens is part of a Ramsden eyepiece.
24. The display apparatus as in claim 1 wherein said dichroic
combiner is located in proximity to the internal aperture stops of
the imaging relay lenses.
25. A modulation optical system for providing modulation of an
incident light beam comprising: (a) a prepolarizer for
pre-polarizing said beam of light to provide a polarized beam of
light; (b) a wire grid polarization beamsplitter for receiving said
polarized beam of light, for transmitting said polarized beam of
light having a first polarization, and for reflecting said
polarized beam of light having a second polarization orthogonal to
said first polarization, wherein subwavelength wires on said wire
grid polarization beamsplitter face a reflective spatial light
modulator; (c) wherein said reflective spatial light modulator
receives said polarized beam of light, having either a first
polarization or a second polarization, and selectively modulates
said polarized beam of light to encode data thereon, providing both
modulated light and unmodulated light which differ in polarization;
(d) wherein said reflective spatial light modulator reflects back
both said modulated light and said unmodulated light to said wire
grid polarization beamsplitter; (e) wherein a polarization
compensator, located between said wire grid polarization
beamsplitter and said reflective liquid crystal device, is provided
for conditioning oblique light rays; (f) wherein said wire grid
polarization beamsplitter separates said modulated light from said
unmodulated light; (g) a polarization analyzer receives said
modulated light, and which further removes any residual unmodulated
light from said modulated light; and wherein said modulation
optical system further comprises an imager field lens prior to each
of said reflective spatial light modulators to provide nominally
telecentric light to said spatial light modulators.
26. The modulation optical system as in claim 25 wherein said
imager field lens is a low stress or low absorption optical
glass.
27. The modulation optical system as in claim 25 wherein said
imager field lens is fabricated from amorphous fused silica.
28. The modulation optical system as in claim 25 wherein said
imager field lens has uniform residual birefringence.
29. The modulation optical system as in claim 25 wherein said
imager field lens is constructed from two lens elements with
crossed polarization axes to cancel residual retardances.
30. The modulation optical system as in claim 25 wherein said
imager field lens is mounted with a compliant adhesive.
31. The modulation optical system as in claim 25 wherein said
modulation optical system has two of said polarization compensators
with said imager field lens located between them.
32. The modulation optical system as in claim 25 wherein said
polarization compensator or compensators provides corrective
retardances for at least one of said wire grid polarization beam
splitter, said spatial light modulator, or said imager field
lens.
33. The modulation optical system as in claim 25 wherein said
pre-polarizer is a wire grid polarizer.
34. The modulation optical system as in claim 25 wherein said
polarization analyzer is a wire grid polarizer.
35. The modulation optical system as in claim 25 wherein said
modulation optical system is used in an image projection or an
image printing device.
36. The modulation optical system as in claim 25 wherein said
spatial light modulator is a LCD.
37. The modulation optical system as in claim 25 wherein said
spatial light modulator is a vertically aligned LCD.
38. A modulation optical system as in claim 25 wherein said
reflective spatial light modulator receives said polarized beam of
light having a first polarization state transmitted through said
wire grid polarization beamsplitter.
39. A modulation optical system as in claim 25 wherein said
reflective spatial light modulator receives said polarized beam of
light having a second polarization state reflected from said wire
grid polarization beamsplitter.
40. A modulation optical system for providing modulation of an
incident light beam comprising: (a) polarization optics including
at least two polarization devices, where at least one of said
polarization devices is a wire grid polarization beamsplitter,
wherein said wire grid polarization beamsplitter receives said
incident beam of light, and transmits a polarized beam of light
having a first polarization, and reflects a polarized beam of light
having a second polarization nominally orthogonal to said first
polarization, wherein subwavelength wires on said wire grid
polarization beamsplitter face a reflective spatial light
modulator; (b) wherein said reflective spatial light modulator
receives said polarized beam of light, having either a first
polarization or a second polarization, and selectively modulates
said polarized beam of light to encode data thereon, providing both
modulated light and unmodulated light which differ in polarization;
(c) wherein said reflective spatial light modulator reflects back
both said modulated light and said unmodulated light to said wire
grid polarization beamsplitter; (d) wherein a polarization
compensator, located between said wire grid polarization
beamsplitter and said reflective spatial light modulator, is
provided for conditioning oblique light rays; (e) wherein said wire
grid polarization beamsplitter separates said modulated light from
said unmodulated light; and (f) wherein said modulation optical
system further comprises an imager field lens prior to said
reflective spatial light modulator.
41. The modulation optical system as in claim 40 wherein said
imager field lens provides nominally telecentric light to the
spatial light modulator.
42. The modulation optical system as in claim 40 wherein said
imager field lens is a low stress or low absorption optical
glass.
43. The modulation optical system as in claim 40 wherein said
imager field lens is fabricated from amorphous fused silica.
44. The modulation optical system as in claim 40 wherein said
imager field lens has uniform residual birefringence.
45. The modulation optical system as in claim 40 wherein said
imager field lens is constructed from two lens elements with
crossed polarization axes to cancel residual retardances.
46. The modulation optical system as in claim 40 wherein said
imager field lens is mounted with a compliant adhesive.
47. The modulation optical system as in claim 40 wherein said
modulation optical system has two compensators with said imager
field lens located between them.
48. The modulation optical system as in claim 40 wherein said
polarization compensator or compensators provides corrective
retardances for at least one of the wire grid PBS, the spatial
light modulator, and the imager field lens.
49. The modulation optical system as in claim 40 wherein said
modulation optical system further comprises a pre-polarizer.
50. The modulation optical system as in claim 49 wherein said
pre-polarizer is a wire grid polarizer.
51. The modulation optical system as in claim 40 wherein said
modulation optical system further comprises a polarization
analyzer.
52. The modulation optical system as in claim 51 wherein said
polarization analyzer is a wire grid polarizer.
53. The modulation optical system as in claim 40 wherein said
modulation optical system receives an incident light beam that is
pre-polarized.
54. A modulation optical system as in claim 40 wherein said
reflective spatial light modulator receives said polarized beam of
light having a first polarization state transmitted through said
wire grid polarization beamsplitter.
55. A modulation optical system as in claim 40 wherein said
reflective spatial light modulator receives said polarized beam of
light having a second polarization state reflected from said wire
grid polarization beamsplitter.
56. The modulation optical system as in claim 40 wherein said
modulation optical system is used in an image projection or an
image printing device.
57. The modulation optical system as in claim 40 wherein said
spatial light modulator is an LCD.
58. The modulation optical system as in claim 40 wherein said
spatial light modulator is a vertically aligned LCD.
59. A modulation optical system for providing modulation of an
incident light beam comprising: (a) polarization optics including
at least two polarization devices, where at least one of said
polarization devices is a polarization beamsplitter, wherein said
polarization beamsplitter receives said incident beam of light, and
transmits a polarized beam of light having a first polarization,
and reflects a polarized beam of light having a second polarization
nominally orthogonal to said first polarization; (b) wherein a
reflective spatial light modulator receives said polarized beam of
light, having either a first polarization or a second polarization,
and selectively modulates said polarized beam of light to encode
data thereon, providing both modulated light and unmodulated light
which differ in polarization; (c) wherein said reflective spatial
light modulator reflects back both said modulated light and said
unmodulated light to said polarization beamsplitter; (d) wherein a
polarization compensator, located between said polarization
beamsplitter and said reflective spatial light modulator, is
provided for conditioning oblique light rays; (e) wherein said
polarization beamsplitter separates said modulated light from said
unmodulated light; and (f) wherein said modulation optical system
further comprises an imager field lens prior to said reflective
spatial light modulator.
60. The modulation optical system as in claim 59 wherein said
polarization beamsplitter is a MacNeille type prism.
61. The modulation optical system as in claim 59 wherein said
polarization beam splitter is a wire grid.
62. The modulation optical system as in claim 59 wherein said
imager field lens provides nominally telecentric light to the
spatial light modulator.
63. The modulation optical system as in claim 59 wherein said
imager field lens is a low stress or low absorption optical
glass.
64. The modulation optical system as in claim 59 wherein said
imager field lens is fabricated from amorphous fused silica.
65. The modulation optical system as in claim 59 wherein said
imager field lens has uniform residual birefringence.
66. The modulation optical system as in claim 59 wherein said
imager field lens is constructed from two lens elements with
crossed polarization axes to cancel residual retardances.
67. The modulation optical system as in claim 59 wherein said
imager field lens is mounted with a compliant adhesive.
68. The modulation optical system as in claim 59 wherein said
modulation optical system has two compensators, with the imager
field lens located between them.
69. The modulation optical system as in claim 59 wherein said
polarization compensator or compensators provides corrective
retardances for at least one of the wire grid PBS, the spatial
light modulator, and the imager field lens.
70. The modulation optical system as in claim 59 wherein said
modulation optical system further comprises a pre-polarizer.
71. The modulation optical system as in claim 70 wherein said
pre-polarizer is a wire grid polarizer.
72. The modulation optical system as in claim 59 wherein said
modulation optical system further comprises a polarization
analyzer.
73. The modulation optical system as in claim 70 wherein said
polarization analyzer is a wire grid polarizer.
74. The modulation optical system as in claim 59 wherein said
modulation optical system receives an incident light beam that is
pre-polarized.
75. A modulation optical system as in claim 59 wherein said
reflective spatial light modulator receives said polarized beam of
light having a first polarization state transmitted through said
polarization beamsplitter.
76. A modulation optical system as in claim 59 wherein said
reflective spatial light modulator receives said polarized beam of
light having a second polarization state reflected through said
polarization beamsplitter.
77. The modulation optical system as in claim 59 wherein said
modulation optical system is used in an image projection or an
image printing device.
78. The modulation optical system as in claim 59 wherein said
spatial light modulator is an LCD.
79. The modulation optical system as in claim 59 wherein said
spatial light modulator is a vertically aligned LCD.
80. A display apparatus comprising: (a) a light source for forming
a beam of light; (b) illumination optics for shaping and directing
said beam of light; (c) a splitter for splitting said beam of light
into at least three color beams of light; (d) a modulation optical
system for each of said three color beams of light, comprising: (1)
a pre-polarizer for polarizing said beam of light to provide a
polarized beam of light of a given color; (2) a transmissive
spatial light modulator wherein said transmissive spatial light
modulator receives said polarized beam of light, and selectively
modulates said polarized beam of light to encode data thereon,
providing both modulated light and unmodulated light which differ
in polarization; (3) an imager field lens that provides nominally
telecentric light to said transmissive spatial light modulators (4)
wherein a polarization analyzer separates said modulated light from
said unmodulated light; and (5) an imaging relay lens in each color
that provides an intermediate image of the transmissive spatial
light modulator from the modulated light for that color; (e) a
dichroic combiner for re-combining the modulated light for each
given color, such that the multiple color beams form the respective
intermediate images along a common optical axis to form a combined
intermediate image; and (f) a projection lens for imaging said
combined intermediate image to a display screen.
81. A display apparatus as in claim 80 wherein said polarization
analyzer is a wire grid polarizer or a wire grid polarization beam
splitter.
82. A display apparatus as in claim 80 that further comprises a
polarization compensator.
83. A display apparatus as in claim 80 wherein said transmissive
spatial light modulator is a Liquid Crystal Display (LCD).
84. A display apparatus comprising: (a) a light source for forming
a beam of light; (b) illumination optics for shaping and directing
said beam of light; (c) a splitter for splitting said beam of light
into at least three color beams of light; (d) a modulation optical
system for each of said three color beams of light, comprising: (1)
an angle sensitive optic for directing light into a digital
micromirror device; (2) an imager field lens that provides
nominally telecentric light to said digital micromirror device; (3)
a digital micromirror device wherein said digital micromirror
device receives said beam of light, and selectively modulates said
beam of light to encode data thereon, providing both modulated
light and unmodulated light which differs in angular directionality
over light; (4) wherein said digital micromirror device reflects
back both said modulated light and said unmodulated light to said
angle sensitive optic; (5) wherein said angle sensitive optic
separates said modulated light from said unmodulated light; and (6)
an imaging relay lens in each color that provides an intermediate
image of the digital micromirror device from the modulated light
for that color; (e) a dichroic combiner for re-combining the
modulated light for each given color, such that the multiple color
beams form the respective intermediate images along a common
optical axis to form a combined intermediate image; and (f) a
projection lens for imaging said combined intermediate image to a
display screen.
85. A display apparatus as in claim 84 wherein said angle sensitive
optic is a Philips prism.
86. A modulation optical system for providing modulation of an
incident light beam comprising: (a) polarization optics including
at least two polarization devices, where at least one of said
polarization devices is a polarization beamsplitter, wherein said
polarization beamsplitter receives said incident beam of light, and
transmits a polarized beam of light having a first polarization,
and reflects a polarized beam of light having a second polarization
nominally orthogonal to said first polarization; (b) wherein a
reflective spatial light modulator receives said polarized beam of
light, having either a first polarization or a second polarization,
and selectively modulates said polarized beam of light to encode
data thereon, providing both modulated light and unmodulated light
which differ in polarization; (c) wherein said reflective spatial
light modulator reflects back both said modulated light and said
unmodulated light to said polarization beamsplitter; (d) wherein
said polarization beamsplitter separates said modulated light from
said unmodulated light; and (e) wherein said modulation optical
system further comprises an imager field lens prior to said
reflective spatial light modulator.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] Reference is made to commonly-assigned copending U.S. patent
application Ser. No. 09/813,207, filed Mar. 20, 2001, entitled A
DIGITAL CINEMA PROJECTOR, by Kurtz et al.; U.S. patent application
Ser. No. 10/040,663, filed Jan. 7, 2002, entitled DISPLAY APPARATUS
USING A WIRE GRID POLARIZING BEAMSPLITTER WITH COMPENSATOR, by Mi
et al.; U.S. patent application Ser. No. 10/050,309, filed Jan. 16,
2002, entitled PROJECTION APPARATUS USING SPATIAL LIGHT MODULATOR,
by Joshua M. Cobb; U.S. patent application Ser. No. 10/131,871,
filed Apr. 25, 2002, entitled PROJECTION APPARATUS USING SPATIAL
LIGHT MODULATOR WITH RELAY LENS AND DICHROIC COMBINER, by Cobb et
al.; U.S. patent application Ser. No. 10/237,516, filed Sep. 9,
2002, entitled COLOR ILLUMINATION SYSTEM FOR SPATIAL LIGHT
MODULATORS USING MULTIPLE DOUBLE TELECENTRIC RELAYS, by Joshua M.
Cobb; and U.S. patent application Ser. No. 10/392,685, filed Mar.
20, 2003, entitled PROJECTION APPARATUS USING TELECENTRIC OPTICS,
by Cobb et al., the disclosures of which are incorporated
herein.
FIELD OF THE INVENTION
[0002] This invention generally relates to a projection apparatus
that forms a color image from digital data using a spatial light
modulator. More particularly, this invention relates to a
projection apparatus that provides intermediate imaging optics that
relay an image of a spatial light modulator to a projection lens,
wherein the intermediate imaging relay optics include one or more
field lenses located in proximity to the spatial light
modulator.
BACKGROUND OF THE INVENTION
[0003] In order to be considered as suitable replacements for
conventional film projectors, digital projection systems must meet
demanding requirements for image quality. In particular, to provide
a competitive alternative to conventional cinematic-quality
projectors, an electronic or digital projection apparatus must meet
high standards of performance, providing high resolution, wide
color gamut, high brightness, and frame-sequential contrast ratios
exceeding 1,000:1.
[0004] The most promising solutions for multicolor digital cinema
projection employ, as image forming devices, one of two basic types
of spatial light modulators. The first type of spatial light
modulator is a digital micromirror device (DMD), developed by Texas
Instruments, Inc., Dallas, Tex. DMD devices are described in a
number of patents, for example U.S. Pat. Nos. 4,441,791; 5,535,047;
5,600,383 (all to Hornbeck); and U.S. Pat. No. 5,719,695
(Heimbuch). Optical designs for projection apparatus employing DMDs
are disclosed in U.S. Pat. No. 5,914,818 (Tejada et al.); U.S. Pat.
No. 5,930,050 (Dewald); U.S. Pat. No. 6,008,951 (Anderson); and
U.S. Pat. No. 6,089,717 (Iwai). DMDs have been employed in digital
projection systems. However, although DMD-based projectors
demonstrate some capability to provide the necessary light
throughput, contrast ratio, and color gamut, inherent resolution
limitations (with current devices providing only 1024.times.768
pixels) and high component and system costs have restricted DMD
acceptability for high-quality digital cinema projection.
[0005] The second type of spatial light modulator used for digital
projection is a liquid crystal device (LCD). The LCD forms an image
as an array of pixels by selectively modulating the polarization
state of incident light for each corresponding pixel. LCDs appear
to have advantages as spatial light modulators for high-quality
digital cinema projection systems. These advantages include
relatively large device size and favorable device yields. Among
examples of electronic projection apparatus that utilize LCD
spatial light modulators are those disclosed in U.S. Pat. No.
5,808,795 (Shimomura et al.); U.S. Pat. No. 5,798,819 (Hattori et
al.); U.S. Pat. No. 5,918,961 (Ueda); U.S. Pat. No. 6,010,221 (Maki
et al.); and U.S. Pat. No. 6,062,694 (Oikawa et al.).
[0006] In an electronic projection apparatus using spatial light
modulators, individual colors, conventionally red, green, and blue
(RGB), are separately modulated in a corresponding red, green, or
blue portion of the optical path. The modulated light of each color
is then combined in order to form a composite, multicolor RGB
image. There are two basic approaches for projection optics that
combine the modulated color light. The first approach, which can be
characterized as a convergent approach, is adapted from earlier,
conventional projection systems. Using the convergent approach, the
component red, green, and blue light have separate axes which are
converged by separate projection optics that effectively direct and
focus each light path as necessary in order to form a composite,
multicolor color image at some focal plane. As an illustrative
example, U.S. Pat. No. 5,345,262 (Yee et al.) discloses a
convergent video projection system. Significantly, the disclosure
of U.S. Pat. No. 5,345,262 illustrates one of the major problems
with the convergent projection approach: namely, that the separate
color images must be properly registered on the projection surface.
Misregistration or poor focus along any one of the color light
projection paths can easily result in an unsatisfactory image. It
is instructive to observe that, using this approach, the image
paths are converged only at the focus plane (screen).
[0007] U.S. Pat. No. 5,907,437 (Sprotbery et al.) discloses an
attempt to simplify design complexity and alleviate some of the
light path alignment and registration problems inherent to
multicolor projection systems using the convergent approach
described above. In the U.S. Pat. No. 5,907,437 disclosure, a light
valve projection system is described in which a converging optical
system converges the red, green, and blue modulated light paths in
order to form an internal converged image, which is then re-imaged
to the screen by the projection lens. The design strategy outlined
in U.S. Pat. No. 5,907,437 thus simplifies the projection lens
design task for a system using the convergent approach. However,
other problems inherent to a convergent approach remain.
[0008] One notable problem with approaches similar to that
disclosed in U.S. Pat. No. 5,907,437 is a relatively high etendue.
As is well known in the optical arts, etendue relates to the amount
of light that can be handled by an optical system. Potentially, the
larger the etendue, the brighter the image. Numerically, etendue is
proportional to the product of two factors, namely the image area
and the square of the numerical aperture. Increasing the numerical
aperture, for example, increases etendue so that the optical system
captures more light. Similarly, increasing the source image size,
so that light originates over a larger area, increases etendue and,
therefore, brightness. As a general rule, increased etendue results
in a more complex and costly optical design. Using an approach such
as that outlined in U.S. Pat. No. 5,907,437, for example, lens
components in the optical system must be designed for large
etendue. The source image area for the light that must be converged
through system optics is the sum of the combined areas of the
spatial light modulators in red, green, and blue light paths;
notably, this is three times the area of the final multicolor image
formed. That is, for the configuration disclosed in U.S. Pat. No.
5,907,437, optical components handle a sizable image area,
therefore a high etendue, since red, green, and blue color paths
are separate and must be optically converged. Moreover, although
the configuration disclosed in U.S. Pat. No. 5,907,437 handles
light from three times the area of the final multicolor image
formed, this configuration does not afford any benefit of increased
brightness, since each color path contains only one-third of the
total light level. In particular, the second relay lens and the
projection lens of a convergent optics system such as that
disclosed in U.S. Pat. No. 5,907,437 are inherently constrained by
a large etendue, which adds cost and complexity to such a solution.
Moreover, the second relay lens must be color corrected over the
full visible spectrum. At the same time, different segments of the
relay lens and of the projection lens handle different wavelengths,
so that localized lens imperfections, dust, or dirt not only affect
the projected image, but can impact the color quality. In light of
etendue constraints, of color correction requirements, of dust and
dirt sensitivity, and of the need for maximizing brightness levels
for digital projection, there appear to be significant inherent
limitations that hamper the convergent approach exemplified in U.S.
Pat. No. 5,907,437.
[0009] An alternative approach to projection optics can be
characterized as a coaxial approach. In contrast to the convergent
approach in which component red, green and blue light beams are
bent to converge at a focal plane, the coaxial approach combines
the component red, green, and blue modulated light beams along a
common axis. In order to do this, the coaxial approach employs a
dichroic combining element, such as an X-cube or Philips prism.
X-cubes or X-prisms and related dichroic optical elements, such as
those disclosed in U.S. Pat. No. 5,098,183 (Sonehara) and U.S. Pat.
No. 6,019,474 (Doany et al.) are well known in the optical imaging
arts. The dichroic combining element combines modulated light from
each color path and folds the color paths together along a common
axis in order to provide the combined color image to a projection
lens. Referring to FIG. 1, there is shown a simplified block
diagram of a conventional digital projection apparatus 10 using the
coaxial approach. Each color path (r=Red, g=Green, b=Blue) uses
similar components for forming a modulated light beam. Individual
components within each path are labeled with an appended r, g, or
b, appropriately. For the description that follows, however,
distinctions between color paths are specified only when necessary.
Following any of the three color paths, a light source 20 provides
unmodulated light, which is conditioned by uniformizing optics 22
to provide a uniform illumination. A polarizing beamsplitter 24
directs light having the appropriate polarization state to a
spatial light modulator 30 which selectively modulates the
polarization state of the incident light over an array of pixel
sites. The action of spatial light modulator 30 forms an image. The
modulated light from this image, transmitted along an optical axis
O.sub.r, O.sub.g, O.sub.b through polarizing beamsplitter 24, is
directed to a dichroic combiner 26, typically an X-cube, Philips
prism, or combination of dichroic surfaces in conventional systems.
Dichroic combiner 26 combines the red, green, and blue modulated
images from separate optical axes O.sub.r, O.sub.g, O.sub.b to form
a combined, multicolor image for a projection lens 32 along a
common optical axis O for projection onto a display surface 40,
such as a projection screen.
[0010] In contrast to the convergent approach outlined above with
reference to U.S. Pat. No. 5,907,437, the coaxial approach, as
shown in the block diagram of FIG. 1 and as exemplified in U.S.
Pat. No. 5,808,795 has a number of advantages. With respect to
light throughput, the coaxial approach, because it combines light
paths along a common axis, does not increase the etendue of the
optical system. Instead, with respect to projection lens 32,
dichroic combiner 26, by folding the appropriate optical axes
O.sub.r and O.sub.b to join with optical axis O.sub.g and form a
common optical axis O, optically overlaps the areas of spatial
light modulators 30r, 30g, 30b. Thus, the etendue has no increase
whether one, two, three, or more spatial light modulators are
combined in this way. Since each light color is separately
modulated, then combined and provided to projection lens 32 along a
common optical axis O, no optical system is required between
dichroic combiner 26 and projection lens 32.
[0011] A Philips prism, such as that disclosed in U.S. Pat. No.
3,202,039 (DeLang et al.) could alternately be employed as dichroic
combiner 26. Familiar to those skilled in the digital image
projection arts, Philips prisms have been employed as chromatic
separator or combiner components in projector designs such as those
disclosed in U.S. Pat. Nos. 6,280,035 and 6,172,813 (both to
Tadic-Galeb et al.); U.S. Pat. No. 6,262,851 (Marshall); and U.S.
Pat. No. 5,621,486 (Doany et al.), for example.
[0012] While digital projection apparatus 10 designed using the
basic model of FIG. 1 are able to provide good levels of image
quality, there is room for improvement. Constraints imposed by
dichroic coatings are a key consideration. Dichroic coatings used
within dichroic combiner 26 can be expensive and difficult to
design and fabricate for suitable performance with incident light
over a wide range of angles, particularly in projection
applications where high brightness levels and a broad color gamut
are needed. Dichroic coatings reflect and transmit light as a
function of incident angle and wavelength. As the incident angle
varies, the wavelength of light that is transmitted or reflected
also changes. Where a dichroic coating is used with an optical
system having a low F# and a broad spectrum, the typical result is
a variable efficiency versus both angle and wavelength for both
transmitted and reflected light. Misalignment or wedge of the
interior surfaces of an X-prism type dichroic combiner can also
cause image separation, image blur, and color shading. The "seam"
at which dichroic surfaces are combined tends to appear as one or
more linear shadow artifacts in the displayed image. Fabrication of
a high-quality X-cube is further complicated by the requirement
that individual component prisms have identical refractive indices;
in practice, this is best accomplished when the same glass melt is
used for all prism components Generally these and other various
problems that effect X-prisms can be overcome, and X-prisms are
widely used in projection systems. However, obtaining high quality
custom designed X-prisms can be a problem for prototype or low
manufacturing volume applications. Moreover, high brightness
applications such as occur in digital cinema systems, can impose
high heat levels, which can damage adhesives and coating surfaces
of the X-prism. Admittedly, the problems experienced with the
dichroic separator (which sees intense light loads) and the
dichroic combiner (which sees lower light loads, but through which
the final image is formed) do not have the same sensitivities, but
similar solutions may be needed by both. The design and fabrication
of both the dichroic separator and combiner can be helped if the F#
can be increased. In particular, if a larger F# light cone can be
used, surface tolerance requirements in a dichroic combiner can be
relaxed, thereby reducing cost and alignment complexity. However,
conventionally, a light cone having a smaller F# is used in
projection systems, since system designs are directed to maximizing
brightness.
[0013] As is another well known principle applied in the design of
projection apparatus, it is beneficial to minimize the retrofocus
distance of projection lens 32, thus minimizing the working
distance requirements and cost of projection lens 32. It would be
preferable to avoid the cost and complexity requirements of a
projection lens having a long back focal length relative to its
effective focal length, such as the solution disclosed in U.S. Pat.
No. 6,008,951 (Anderson), for example.
[0014] U.S. Pat. No. 5,944,401 (Murakami et al.) discloses, as an
alternative to X-cube dichroics, a V-prism optical block comprising
dichroic surfaces within plastic prisms. This solution provides
some relief for back working distance requirements, since the
refractive index of plastics exceeds that of air. To minimize back
working distance, transmissive spatial light modulators are
employed, allowing image-formation as close to the combining
optical block as possible. However, this arrangement would not be
well-suited for projector apparatus using reflective spatial light
modulators, since back working distance requirements are still
excessive. In terms of back working distance, the solution of U.S.
Pat. No. 5,944,401 is not advantaged over conventional X-cube
designs. A sizable projection lens would be required for full-scale
cinema projection. Moreover, the solution disclosed in U.S. Pat.
No. 5,944,401 does not address the inherent angular limitations of
dichroic surfaces described above. Thus, brightness levels are
constrained with this type of design solution.
[0015] U.S. Pat. No. 5,597,222 (Doany et al.) discloses, for use in
a digital projector, an optical relay lens system that alleviates
some of the difficulties noted above that relate to inherent
tolerance problems and projection lens working requirements. U.S.
Pat. No. 5,597,222 discloses the use of a single 1.times.,
double-telecentric relay lens to relay the combined image from
individual RGB color paths to a MacNeille polarizing beamsplitter
(PBS), also termed a polarization beamsplitter. In U.S. Pat. No.
5,597,222 spatial light modulators are disposed very near a
dichroic combiner X-cube, to minimize thereby some of the potential
adverse effects of imperfections in outer surface flatness and
tolerance errors in inner surface fabrication. The system disclosed
in U.S. Pat. No. 5,597,222 is advantaged in that the design of its
projection lens is simplified when compared with similar designs.
The working distance requirements for the projection lens are
significantly reduced using the design approach of U.S. Pat. No.
5,597,222. The single 1.times. double telecentric relay provides
the necessary working distance to allow insertion of the MacNeille
PBS prior to the intermediate internal combined image in the image
path. The projection lens can then re-image this internal image to
the screen without the requirements for long working distance that
are typically required when using a PBS and/or a dichroic color
combiner, such as an X-prism.
[0016] The solution presented in U.S. Pat. No. 5,597,222, however,
falls far short of what is needed to compensate for inherent
problems with X-cube coatings and surfaces so that both image
brightness and color gamut can be maintained. For example, the
design noted in U.S. Pat. No. 5,597,222 fails to address inherent
angular dependencies in the dichroic coating response, so that it
remains difficult to support a large color gamut while maintaining
image brightness at the same time. Moreover, the projection lens
must also use a high numerical aperture with this design, which
implies added cost over designs with lower numerical aperture.
Because of the scale of spatial light modulator components, the
design of U.S. Pat. No. 5,597,222 is still very dependent on
high-quality X-cube design. Further, the arrangement disclosed in
U.S. Pat. No. 5,597,222 employs a relatively large number of
optical components between a polarizing beamsplitter and its
modulating LCD. With a large number of optical components in the
path of a polarized illumination source, some unavoidable stress
birefringence would necessarily alter the polarization states of
both unmodulated and modulated light traveling in both directions,
resulting in loss of image contrast.
[0017] U.S. Pat. No. 5,357,289 (Konno et al.) discloses a system
that is similar to that disclosed U.S. Pat. No. 5,597,222, as it
uses a single 1.times. relay lens to present an internal
intermediate image to the projection lens, thereby significantly
reducing the working distance requirements imposed on projection
lens design. U.S. Pat. No. 5,357,289 provides an alternate
construction to that shown in U.S. Pat. No. 5,597,222 for using
polarization and color combining prisms. In the apparatus of U.S.
Pat. No. 5,357,289, both the polarizing and color-combining prism
are in the vicinity of the spatial light modulators, rather than
spaced well apart, as in the apparatus of U.S. Pat. No. 5,597,222.
Instead of the conventional X-prism, the apparatus of U.S. Pat. No.
5,357,289 uses a V-prism as a color combiner, where the V-prism is
similar to that disclosed in U.S. Pat. No. 5,944,401 described
above. The V-prism approach avoids some of the inherent problems
with X-cube fabrication and use. While the approach disclosed in
U.S. Pat. No. 5,357,289 eases the demands on projection lens
design, the imaging relay (first lens group) presents a challenge,
since it must provide a long working distance for the spatial light
modulators and associated PBS and color-combining V-prism. As with
the approach noted in U.S. Pat. No. 5,597,222, the approach shown
in U.S. Pat. No. 5,357,289 uses a single imaging relay lens for all
three colors (RGB), operating nominally at 1.times. magnification.
As was seen with the U.S. Pat. No. 5,597,222 apparatus, the U.S.
Pat. No. 5,357,289 approach requires a complex imaging relay lens
that is fully color corrected over a broad part of the visible
spectrum in order to form a white light image having minimal color
aberrations or color differences in the third order aberrations,
particularly with respect to distortion and defocus.
[0018] U.S. Pat. No. 6,247,816 (Cipolla et al.) discloses use of a
1.times. relay lens for relaying an intermediate image towards a
dichroic combiner in only one of the color paths. The solution in
U.S. Pat. No. 6,247,816 addresses a component packaging problem,
but does not alleviate any of the angular constraints imposed by
dichroic combiner response. Neither does the solution in U.S. Pat.
No. 6,247,816 provide any relief with respect to back working
distance requirements of the projection lens.
[0019] U.S. Pat. No. 4,836,649 (Ledebuhr et al.) discloses a
projector system that uses internal imaging in both the
illumination and imaging paths. The system utilizes nominally
1.times. imaging relays, with the portion of the relays used to
interface to the light valves is common to both the illumination
and imaging paths. The cited advantages of this architecture are to
minimize the size of polarization components and to help alleviate
back working distance constraints for the projection lens. While
this arrangement provides some advantages, the color-combining
dichroic surfaces must still handle light at low F# values,
resulting in reduced color gamut. Likewise, as the imaging relay
directly provides an internal white light image, the imaging relay
lens must be color corrected across the entire visible spectrum.
Finally, the projection lens must also operate at a low F# when
using this solution.
[0020] Thus, it can be seen that there is a need for improvement in
digital projection optics design that alleviates the inherent
angular limitations of dichroic coatings while providing maximum
brightness and color gamut, minimizes the working distance
requirements of projection optics, and allows a high F# for
projection optics.
[0021] An optical system described in pending U.S. patent
application Ser. No. 10/050,309 provides an optical configuration
that alleviates many of the problems described previously. This
system provides for a projection system that produces an internal
white light image, by means of combining three color (RGB)
intermediate images relayed by three optical systems and combined
by a color combining prism. This system provides working distance
advantages, as well as a reduced numerical aperture at the color
combining prism, which improves the manufacturability of the prism.
Although this system can be configured with various components, it
provides superior performance when the color combining prism is a
V-prism (similar to the V-prism described in U.S. Pat. No.
5,357,289) and the polarization beamsplitter is a wire grid
polarizer (see U.S. Pat. No. 6,243,199 (Hansen et al.)). Although
this system works remarkably well, including for high brightness
projection applications such as digital cinema, there are
opportunities both for improvements and extensions. In particular,
the system of the present invention provides opportunities to have
smaller and less expensive optical components, an easier
opto-mechanical package, and a less difficult optical design. Other
advantages will become apparent from the discussion of the system
of the present invention.
SUMMARY OF THE INVENTION
[0022] Briefly, according to one aspect of the present invention a
display apparatus comprises a light source for forming a beam of
light. Illumination optics shapes and directs the beam of light and
splitting means splits the beam of light into at least three color
beams of light. A modulation optical system for each of the three
color beams of light comprises a pre-polarizer, a wire grid
beamsplitter, a reflective spatial light modulator, and a
polarization analyzer. An imaging relay lens in each color provides
an intermediate image of the reflective spatial light modulator
from the modulated light for that color. A dichroic combiner
recombines the modulated light for each given color, such that the
multiple color beams form the respective intermediate images along
a common optical axis to form a combined intermediate image. A
projection lens images the combined intermediate image to a display
screen. The electronic projection further comprises a imager field
lens prior to each of the spatial light modulators to provide
nominally telecentric light to the spatial light modulators.
[0023] The invention and its objects and advantages will become
more apparent in the detailed description of the preferred
embodiment presented below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] While the specification concludes with claims particularly
pointing out and distinctly claiming the subject matter of the
present invention, it is believed that the invention will be better
understood from the following description when taken in conjunction
with the accompanying drawings, wherein:
[0025] FIG. 1 is a schematic block diagram showing major components
of a conventional, prior art digital projection apparatus;
[0026] FIG. 2 is a schematic block diagram showing the major
components of the modulation and imaging potions of the electronic
projection system of the present invention;
[0027] FIG. 3 is a schematic diagram of a portion of an
illumination optical system appropriate for use in the electronic
projection system of the present invention;
[0028] FIG. 4 is a schematic diagram of another portion of an
illumination system appropriate for use in the electronic
projection system of the present invention;
[0029] FIG. 5 is a schematic block diagram of an imaging relay lens
optical system in accordance with the present invention;
[0030] FIG. 6 is a schematic block diagram of a modulation optical
system in accordance with the present invention;
[0031] FIG. 7 is a schematic block diagram of a prior art
modulation optical system;
[0032] FIG. 8 is a perspective view showing the construction of a
polarization compensator;
[0033] FIGS. 9a-9d show the possible axial orientations of
birefringence;
[0034] FIG. 10 is a perspective illustration of the electronic
projection apparatus of the present invention;
[0035] FIGS. 11a and 11b are schematic block diagrams of alternate
modulation optical systems in accordance with the present
invention;
[0036] FIG. 12 is a schematic block diagram of an alternate
configuration for the imager field lens;
[0037] FIG. 13 is a schematic block diagram of a portion of an
alternate optical design for the imaging relay lens and imager
field lens provided for the electronic projection apparatus of the
present invention; and
[0038] FIG. 14 is a schematic block diagram of an alternate design
for an electronic projection system of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0039] The present description is directed in particular to
elements forming part of, or cooperating more directly with,
apparatus in accordance with the invention. It is to be understood
that elements not specifically shown or described may take various
forms well known to those skilled in the art.
[0040] Referring to FIG. 2, there is shown, in schematic form, a
preferred implementation of components used in the in the imaging
optical path of the electronic projection system 100 of the present
invention. In particular, FIG. 2 illustrates the basic elements of
the imaging system, including a modulation optical system 120,
imaging relay lens 130, dichroic combiner 155, and projection lens
150. The illumination system 110 is represented in FIG. 2 in
simplified form primarily by illumination lens 280. FIG. 2
illustrates these basic elements for the green color channel only,
and the presence of the red and blue color channels is indicated
only by the labeled arrows directed into dichroic combiner 155. In
each color channel, the optical system includes an imaging relay
lens 130 that provides an internal image 200. Internal image 200 is
preferably a real image (I.sub.g for the green channel) of the
corresponding spatial light modulator 175, that is also preferably
magnified at a magnification Rx. In order to form a magnified real
image I.sub.g, imaging relay lens 130 magnifies, as its optical
object, the image that is located on spatial light modulator 175
and reflected from wire grid polarization beamsplitter 170. Imaging
relay lens 130 is nominally double-telecentric, such that it
collects nominally telecentric light at the spatial light modulator
175, and outputs an image residing in nominally telecentric space.
The image light that emerges from imaging relay lens 130 is
directed along optical axis 290 and into dichroic combiner 155.
Because dichroic combiner 155 handles telecentric light, there is
minimal tendency for color shading across magnified real image
I.sub.g (or I.sub.r, and I.sub.b, for the respective red and blue
channels) due to angular variances. Dichroic combiner 155 is
preferentially a V-prism, as depicted in FIG. 2, but it could also
be an X-prism (or X-Cube), crossed dichroic plates, a Philips
prism, or other combination of dichroic surfaces. Dichroic combiner
155 may also be fabricated from amorphous fused silica or other low
stress glass, if the concerns for contrast loss, or color or
contrast shading, warrant the added cost.
[0041] The optical configuration of the projector 100 of FIG. 2 is
similar to the system described in pending U.S. patent application
Ser. No. 10/050,309. This new system can share many of the same
significant advantages that are provided by the system of the prior
application. For example, by magnifying the image formed on spatial
light modulator 175 with some magnification factor Rx>1.times.,
imaging relay lenses 130 also effectively focuses the respective
magnified real images I.sub.r, I.sub.g, or I.sub.b toward dichroic
combiner 155 at a larger F# than 1.times. relay operation would
provide. As an example, projection system 100 can be designed to
operate at F/2.3 at the spatial light modulator 175, and with
imaging relay lens 130 providing Rx=2.times. magnification, the
speed at the dichroic combiner 155 is reduced to F/4.6. As a
result, dichroic combiner 155 can be designed with internal
dichroic coatings on internal surfaces 157 with sharper cut-offs,
and therefore narrower spectral bands and a larger color gamut than
would be achievable under a lower F#. Moreover, with the use of
imaging relay lens 130, no light is lost even though a higher F# is
achieved at dichroic combiner 155, since a low F# is still used at
spatial light modulator 175. As a result, an improved magnified
real image I.sub.r, I.sub.g, or I.sub.b are provided, as the output
of dichroic combiner 155, along a common optical axis 290 and into
projection lens 150. These images are overlapped to form a
registered white light image.
[0042] Again, like the system discussed in the prior application,
the arrangement of FIG. 2 also provides advantages for lowering
cost and complexity requirements of projection lens 150. With the
arrangement of FIG. 2, projection lens 150 can advantageously work
at a higher F# in order to project the multicolor image (I)
combined from each magnified real image I.sub.r, I.sub.g, or
I.sub.b onto display surface 40 (not shown in FIG. 2). In addition,
projection lens 150 needs only a small working distance to project
the multicolor image (I) combined from each magnified real image
I.sub.r, I.sub.g, or I.sub.b onto display surface 40, as this
internal image can be placed near the exit face 158 of dichroic
combiner 155. The use of the imaging relay lens 130 separates the
polarization beamsplitter and the dichroic combiner, so that they
are not adjacent, as is common in many standard electronic
projectors. In such systems (such as FIG. 1), the projection lens
32 suffers a long working distance, as it must look through both
the polarization beamsplitter 24 and the dichroic combiner 26. By
comparison, the short working distance provided for the projection
lens 150 of the projector 100 of FIG. 2, means that the lens
elements within projection lens 150 can be small, despite the large
F#. The projection lens design can then be simplified, to have
comparable complexity to the everyday cinema projection lenses used
in motion picture film projectors. The projection lens 150 can
advantageously work at a higher F# in order to project a multicolor
image combined from magnified multicolor real image I than would
otherwise be possible. Projection lens 150 can be, for example, a
simple 5- to 7-element lens assembly that employs commonly
available optical glass and is comparable in cost and complexity to
commercially available cinema projection lenses used with motion
picture film projection apparatus. This is in contrast to
conventional digital cinema and large-scale electronic projection
systems that typically require complex and costly projection
lenses. The net savings for the projection lens can be
.about.10.times., when compared to the projection lenses designed
for competitive digital cinema projection systems. Similar cost
advantages can also be provided for any anamorphic attachment lens
used with projector 100.
[0043] Furthermore, if the imaging relay lenses 130 provide
enlarged images I.sub.r, I.sub.g, or I.sub.b to dichroic combiner
155, then dichroic combiner 155 is larger than it would be if
1.times. magnification had been used. However, even with 2.times.
magnification, dichroic combiner 155 can be both smaller and
cheaper than the comparable component used in standard prior art
projection systems (see again FIG. 1). Prior art implementations
for projection systems with intermediate images, such as those
disclosed in U.S. Pat. Nos. 5,597,222 and 6,247,816 that were cited
previously, do not provide systems with three imaging relays (one
per color) utilized to form a common image projected along a common
optical axis, as does the system of the present invention.
Likewise, these prior art patents also do not provide three
magnifying imaging relays operating at a greater than unity
magnification, such as 2.times..
[0044] The projector 100 of the present invention differs most
significantly from the system described in the copending
application (U.S. patent application Ser. No. 10/050,309) mentioned
previously, because of the inclusion of imager field lens 140.
Imager field lens 140 is provided as part of imaging relay lens
130, with the resulting principal advantage that the imaging relay
lens 130 is easier to design, has smaller lens elements, and
thereby has reduced cost and complexity compared to the equivalent
lens provided in the prior patent application. Preferentially, the
spatial light modulator (or imager) 175 resides in nominally
telecentric space, such that both the incident illumination light
and the reflected outgoing modulated light are telecentric (chief
rays parallel normal to the modulator). Imager field lens 140 is
then preferably placed in close proximity to the spatial light
modulator 175, such that imager field lens 140 directs the chief
rays collected from the off axis field points in a convergent
manner into the main portion of imaging relay lens 130. Imager
field lens 140 is also intrinsically part of the illumination
system 110, with the other optical elements of said illumination
system presenting a uniform field of light of the appropriate size
and aspect ratio towards the spatial light modulator 175, with
imager field lens 140 modifying this illumination light to be
telecentrically incident. This will be explained in greater detail
in the discussion of the illumination system (see FIG. 4). Finally,
imager field lens 140 is also an intrinsic part of modulation
optical system 120, as it can affect the polarization contrast
response both through the polarization beamsplitter and across the
field of the spatial light modulator. The prior art projection
systems with internal intermediate images, including the systems
disclosed in U.S. Pat. Nos. 5,597,222 and 6,247,816, do not
disclose the use of an imager field lens 140 or equivalent, nor do
they anticipate either the advantages or the problems and solutions
that result from the use of this component in a projection
system.
[0045] While FIG. 2 provided a reduced view of an illumination
system 110 appropriate for projector 100, depicting only imager
field lens 140 and a single illumination lens 280, the illumination
system 110 is actually more complicated than that. Referring to
FIG. 3, light from a polychromatic light source (not shown) is
directed as focused source light 260 into an integrating bar 250.
The light source is typically a lamp, such as a Xenon arc lamp, but
it could also be some other type of high-intensity light emitter.
In a typical lamp, the light emitter (arc, plasma, or filament) is
located within a reflector (typically elliptical or parabolic in
profile) and a consolidated light beam is provided as output. This
light beam is then incident, directly or indirectly, into the
uniformizing optics. In a preferred embodiment, the incident light
is provided as focused source light 260 into integrating bar 250,
which serves as the uniformizing optics. The focused source light
260 can be approximated as telecentrically incident (parallel to
the local optical axis 290) to integrating bar 250. Well-known in
the optical design art, integrating bars, also termed light-mixing
bars, use total internal reflection (TIR) effects to homogenize
incident light, thereby providing a spatially uniform plane of
illumination. Other options for uniformizing optics include a
diffusing screen, an integrating tunnel, a fiber optic faceplate,
an incoherent fiber optic bundle, or a lenslet array, such as a
fly's eye integrator assembly. In general, the uniformizing optics
provides a uniform plane of light, which for the integrating bar
250 of FIG. 3, would occur at its output at plane A. The definition
or tolerance for illumination uniformity is relative, and typically
a gradual fall-off in illumination intensity from center to edge of
10-15% is acceptable.
[0046] The illumination optics 110 can then be constructed a
variety of ways, to transfer the light from the exit face at plane
A of the integrating bar 250 to incidence at the imager plane 275.
The partial illumination system depicted in FIG. 3 shows the
integrating bar 250, and a base condenser lens 255, which presents
light to an illumination aperture stop 265 at plane B. The
illumination system is continued, as depicted in FIG. 4, with a
series of illumination lenses 280 (which may or may not be
identical), and imager field lens 140. (Although FIGS. 3 and 4 in
combination illustrate an illumination system more completely, the
two illustrations are not on the same scale.) In the system of FIG.
4, the exit face at plane A of integrating bar 250 is re-imaged to
an internal illumination image 270, which is subsequently re-imaged
to the imager plane 275. The illumination light is nominally
telecentric at both the internal illumination image 270 and imager
plane 275. This illumination system provides both an illumination
aperture stop 265 at plane B, and a re-imaged illumination aperture
stop 265a. A physical stop (such as an iris) can be placed at one
or both of these planes to control the numerical aperture of the
illumination light incident to the imager plane 275. The base
condenser 255 and the illumination lenses 280 in combination direct
an image of the nominally uniform light at exit face A of
integrating bar 250 towards the imager plane 275. Imager field lens
140 most importantly contributes to make this illumination light
nominally telecentric to the imager plane 275. Imager field lens
140 also makes a small contribution to the overall magnification
from the exit face A of the integrating bar 250 to the imager plane
275. Typically, the illumination light will slightly overfill the
active area (image area) of the spatial light modulator 175 placed
at imager plane 275.
[0047] The system of FIG. 4 is simplified to show a single color
channel, and does not depict a dichroic separator, except by the
dashed line labeled "D". A dichroic separator, which splits the
white light beam emerging from light uniformizing optics into three
color beams (nominally red, green, and blue light beams,
respectively) can be an X-prism (similar to dichroic combiner 26
depicted in FIG. 1) or a V-prism (similar to dichroic combiner 155
depicted in FIG. 2). In the partial illumination system of FIG. 4,
the dichroic separator can be located at or near the illumination
aperture stop 265. By locating a dichroic separator at plane B,
there is a potential problem that the inevitable angular response
variation of the separator will be expressed as color shading
across the pupil or aperture stop. Such color shading in angle
space can become a spatial variation in the field later, if angular
effects through the polarization beamsplitter or through the lens
system (vignetting) then become color variant. This effect can be
minimized by supplying the system with color filters 285 (nominally
one per color channel), which are nominally located in normally
incident or telecentric space, as shown in FIG. 4. Alternately, the
dichroic separator can be located at or near the internal
illumination image 270, which sees nominally telecentric light. As
such, the field point will see an angle averaged color response of
the dichroic separator. The illumination system 110 of FIG. 4 can
also include optics, such as waveplates and mirrors, which are
depicted generally as other optics 287, and which modify the
polarization orientation or propagation direction of the light per
the design. Likewise, the illumination system 110 interacts with
polarizers, polarization compensators, and the spatial light
modulator, but those components are not depicted in FIG. 4 for
simplicity.
[0048] An alternate illumination system can be understood by
considering FIG. 4 further. In the illumination system 110 of FIG.
4 as generally depicted, the dichroic separator is nominally
located at the illumination aperture stop 265, and the size of the
internal illumination image 270 is of secondary importance. As a
result, the size of the beam through the illumination aperture stop
265, and the angular spectrum of that beam, determine the design
parameters for the dichroic separator. In this system the
magnification N to the internal illumination image 270 in FIG. 4
can be small (N.about.1 to 2), so as to reduce the optical path
length. On the other hand, the illumination system 110 can be
configured differently, by changing the magnification to the
internal illumination image 270 and then moving the dichroic
separator. For example, the internal image of the exit face at
plane A of the integrating bar 250 can be magnified by Nx, where
for example, N.about.4.5. In this case, the spatial size and
angular extent of the beam at the internal illumination image 270
determine the design of the dichroic separator. The color filters
285 are preferably located in telecentric space, and may
immediately follow the dichroic separator. The key difference is
that the first version provides room to place the dichroic
separator at the aperture stop, while the second provides room to
place it at the telecentric image. Depending on the system color
tolerances and color gamut requirements, one or the other
location/design may be favored.
[0049] Considering again the illumination system of FIGS. 3 and 4,
a further variant illumination system is suggested. In particular,
the second and third illumination lenses 280 could be removed, and
the imager field lens 140 and imager plane 275 are then relocated
to the vicinity of the plane identified in FIG. 4 as the internal
illumination image 270. The illumination system is then more
compact and simplified with the elimination of two or more
illumination lens elements. In that case, it can however be
difficult to provide sufficient room for the other optics (287) and
mirror fold locations necessary to build a projector 100 that is
compact in an overall sense.
[0050] The design of the projector 100 of the present invention can
be better understood with reference to FIGS. 5 and 6, which show
more accurate renderings of the imaging relay lens 130 and the
modulation optical system 120 than is provided in FIG. 4.
Accordingly, FIG. 5 shows an imaging relay lens 130, which
comprises several lens elements, including the imager field lens
140. Letter markers "p", "d", "q" are provided to help track the
imaging paths through the portion of the system depicted in FIG. 5,
while "I" denotes the intermediate or internal image. FIG. 6 shows
an expanded view of the modulation optical system 120, which
includes the pre-polarizer 160, the polarization analyzer 165, the
wire grid polarization beamsplitter 170, the spatial light
modulator 175, the polarization compensators 180 and 185, and the
imager field lens 140. Spatial light modulator 175, which is
preferably an LCD, is nominally located at imager plane 275, such
that a sharp image of the appropriate size is presented to the
intermediate internal image I (see FIG. 5). Spatial light modulator
175 is shown mounted to a modulator package 177, which may include
a heat sink, cooling means, and electrical connections to drive
circuitry which provide the image data (all not shown). For
context, FIG. 6 also shows a portion of the imaging relay lens
130.
[0051] In the preferred embodiment, imaging relay lens 130 is
double-telecentric and forms a magnified intermediate (or internal)
real image I of the image plane 275 near or within dichroic
combiner 155. In that case, because dichroic combiner 155 handles
telecentric light, there is minimal tendency for color shading
across magnified real image I due to angular variances.
Significantly, by magnifying the image provided at the imager plane
275 with some magnification factor (Rx) greater than 1.times.,
imaging relay lens 130 also effectively focuses magnified real
image I at a higher F# than 1.times. relay operation would provide.
The design of imaging relay lens 130, including imager field lens
140, with a magnification Rx>1.times. is also preferable because
the field angle collected at the spatial light modulator 175 is
separate/different than the field angle at the display surface
(screen). This means the field supported at the modulator can be
chosen with consideration to the angular performance of the
polarization beamsplitter, rather than being dictated by the field
angle needed to the final projected image. The actual system
depicted in FIG. 5 is representative of an imaging relay lens 130
providing Rx=2.times. magnification, such that F/2.3 light at the
imager plane 275 is F/4.6 at the dichroic combiner 155.
[0052] The projector 100 and imaging relay lens 130 are still
significantly advantaged compared to the pending patent application
(U.S. patent application Ser. No. 10/050,309) due to the presence
of imager field lens 140. In that prior system, the lens elements
of the equivalent imaging relay lens were larger because image
light propagated telecentrically from the imager plane 275, through
the polarization compensators, off the polarization beamsplitter,
through the polarization analyzer, before the first lens element
was encountered. In one design of an imaging relay lens used in the
prior application, one or more aspheric lens elements were used to
reduce the lens aberrations experienced because of the large fast
optical beams involved. By comparison, an imaging relay lens 130
designed with an imager field lens 140 provides for a smaller
overall lens assembly, that does not require aspheric lens elements
to obtain the comparable performance. Although a projector 100 of
the present invention is enhanced by having the imaging relay
lenses 130 provide enlarged images of the imager planes 275 with
magnifications Rx>1.times. (for various reasons, as given
previously), improvements can still be provided to the projector
design even if it has unity magnification (Rx>1.times.). In
particular, as the overall projector is assembled with three
nominally identical imaging relay lenses 130 (one per color), the
reduced size and cost resulting from a design with an imager field
lens 140 provides for a more compact and less expensive
projector.
[0053] As shown in FIG. 5, imager field lens 140 has modest optical
power, and it directs the chief rays towards an imaging aperture
stop 210 located downstream of the wire grid polarization
beamsplitter 170. The precise location of the imaging aperture stop
210 is primarily determined by the optical design, relative to
minimizing optical aberrations that would degrade image quality and
also reducing the cost and complexity of the imaging relay lens
130. One natural form for the imaging relay lens 130 is a "double
gauss" lens type, as depicted in FIG. 5. Certainly, the location of
the imaging aperture stop 210 can be moved around by design. For
example, if the imager field lens 140 was provided with significant
optical power, the imaging aperture stop could be located in the
vicinity of the wire grid polarization beamsplitter 170. Such a
design would likely be of little benefit, as the imager field lens
140 would be quite fast, and the illumination system would be
required to work with an aperture stop in an awkward location.
Additionally, the angular response requirements imposed on the
polarization beamsplitter (in general, or for a wire grid
polarization beamsplitter in particular), would be significantly
more demanding.
[0054] It should be noted that as the electronic projection system
100 of the present invention utilizes a series of intermediate
internal images, both in the illumination and imaging systems, the
various image planes have accompanying aperture stop planes. These
include the illumination aperture stop 265, the re-imaged
illumination aperture stop 265a (see FIG. 4), the imaging aperture
stop 210 (see FIGS. 2 and 5), and an aperture stop (shown
un-numbered in FIG. 2) within projection lens 150. For example, the
aperture stop within the projection lens 150 can be the actual
limiting aperture stop for the entire optical system that sets the
imaging F#. One of the illumination aperture stops (265 or 265a)
may define a near-to-the-limit angular extent, allowing a little
angular overfilling of the spatial light modulator 175. In this
way, light that has emerged from the integrating bar 250 and is
traversing the optical path at angles beyond those chosen for
imaging, can be removed at location where any resulting heat can
also readily be removed.
[0055] As a design approach useful for electronic projection
systems, a specific design comprising imaging relay lenses 130,
that provide internal or intermediate images (I), and utilize a
field lens 140 adjacent to the spatial light modulator 175, is
novel. Notably however, the use of field lenses near the spatial
light modulators (imagers) in electronic projector designs has
generally been avoided. This is particularly true in systems that
utilize spatial light modulators, such as liquid crystal displays
(LCDs), which are polarization based in modulation. The
polarization response of the system, and the resulting frame
sequential contrast provided by the projector, are largely
determined by the response of the LCDs, the various polarizers, and
the polarization compensators. In such systems, it is good general
practice to minimize the number of optics in the optical path
between the LCD and the last polarization contrast component
(typically the polarization analyzer). This is because any stress
on such extra optics, whether mechanical mounting or fabrication
stress, or thermally induced stress, can cause stress
birefringence. As birefringence is a directional variation in
refractive index, and is a polarization sensitive phenomenon,
stress birefringence can alter polarization states and affect
contrast. Depending on the system configuration, uniform losses in
contrast and/or spatially variant losses in contrast can occur.
Also, as the use of field lenses near the imagers (spatial light
modulators) increases the range of angles through the polarizers,
the use of such field lenses has been generally avoided because of
the limited angular responses available from conventional visible
wavelength polarization beamsplitters.
[0056] The preferred embodiment for the modulation optical system
120 portion of projector 100 is depicted in FIG. 6, and includes
the pre-polarizer 160, the polarization analyzer 165, the wire grid
polarization beamsplitter 170, the spatial light modulator 175, the
polarization compensators 180 and 185, and the imager field lens
140. In constructing a modulation optical system 120 using an
imager field lens 140, it is obviously preferable to use a
polarization beamsplitter with a wide angular response, so at to
handle the combination of the imaging speed (F#) and the field
convergence angle introduced by the imager field lens 140. While
various polarization beamsplitter technologies can be considered
for high speed (small F#), high contrast, high optical efficiency
systems, the wire grid polarization beams splitter from Moxtek Inc.
of Orem, Utah is a superior candidate.
[0057] In several related pending patent applications cited
previously (U.S. patent application Ser. Nos. 09/813,207,
10/040,663, and 10/050,309) modulation optical systems 120 using
wire grid polarizers have been disclosed. The design and attributes
of the modulation optical system (also referred to as an "optical
core" in industry parlance) are critical to the projector design,
as it determines the frame sequential contrast, or the modulation
between the On and Off states from one image frame to the next. In
particular, the first application (Ser. No. 09/813,207) describes a
modulation optical system 120 similar to that shown in FIG. 7
comprising a pre-polarizer 160, a polarization analyzer 165, a wire
grid polarization beamsplitter 170, a spatial light modulator 175,
and a polarization compensator 180. Among other things, this
application teaches that to attain its goal of >1,000:1 frame
sequential contrast, the modulation optical system 120 is optimally
configured with the sub-wavelength wires 171 of the wire grid
polarization beamsplitter 170 facing the spatial light modulator
175. This application further teaches that for optimal contrast,
spatial light modulator 175 is a LCD employing vertically aligned
LC molecules. The second application (Ser. No. 10/040,663) teaches
the design, use, and need for the modulation optical system 120 to
be enhanced with a polarization compensator 180 optimized for
operation in a system utilizing wire grid polarizers. However,
neither of these applications anticipate the design of the
modulation optical system 120 of FIG. 6, which has an imager field
lens 140, and thus neither application anticipates the accompanying
issues and potential problems. Aspects of the actual design of
imager field lens 140 can depend on the polarization behavior of
the neighboring optics, as well as on the polarization attributes
of imager field lens 140 itself.
[0058] The contrast or polarization extinction properties of
modulation optical system 120 obviously depend in large part on the
transmitted and reflected responses of the constituent polarizers
for the "s" and "p" polarization states of the incident light.
Because the polarization response of all polarizers varies with
both incidence angle and polarization state, it is important to
provide adequate response over the range of angles (F#) used in the
system. Otherwise, the higher angle light will likely contribute
leakage from one polarization state into the other, and contrast
will be reduced. Contrast losses can also be significant for the
oblique and skew rays traversing the polarization optical system.
Oblique rays are those rays that fall in the four quadrants outside
the extinction axes defined by the crossed polarizers, but which
lie in planes that contain the local optical axis 290. The skew
rays are the rays that lie in planes that do not contain the local
optical axis 290.
[0059] In the original electronic projection systems that were
developed utilizing reflective liquid crystal displays, each LCD
was addressed from behind using a CRT. Today, state of the art
reflective LCDs are directly electronically addressed by means of a
silicon backplane. These modern devices, which are known as liquid
crystal on silicon (LCOS) displays, generally comprise a silicon
substrate, which is patterned with pixel addressing circuitry,
overcoated with reflective and light blocking layers, followed by
an LCD alignment layer, a thin (.about.3 .mu.m) layer of liquid
crystal, and an anti-reflection (AR) coated cover glass. The
optical performance of a LCD depends on many design parameters,
including the material properties of the liquid crystals, the
electrode structure, the pixel patterning and proximity, the ON
state and OFF state orientations of the liquid crystal molecules,
the use and construction of the alignment layers, the optical
properties of the reflective, anti-reflective, and light blocking
layers, etc. For example, while the liquid crystal molecules are
nominally vertical to the inside surfaces of the silicon substrate
and the cover glass, in actuality the surface adjacent molecules
are oriented with a residual tilt of 1-2 degrees from the normal.
If this residual tilt angle becomes larger, device contrast starts
to suffer. The net contrast provided by a modulation optical system
can be degraded by various subtle effects within the LCDs (large
tilt angles, bias voltages for the OFF state, thermally induced
stresses, and large incident angles (large NA's)), as well as by
the response variations of the polarizers themselves.
[0060] Certainly, polarization contrast can be potentially enhanced
by making design changes to the actual polarization devices (the
wire grid polarization beamsplitter and the LCDs) themselves.
However, as it is not always possible or easy to alter the
fundamental design, manufacturing, and performance limitations of
these devices, alternate methods of improving contrast have been
sought. In particular, many projection and display systems have
made use of polarization compensators of various designs.
[0061] Compensators and polarizers are constructed from
birefringent materials, which have multiple indices of refraction.
Comparatively, isotropic media (such as glass) have a single index
of refraction, and uniaxial media (such as liquid crystals) have
two indices of refraction. Optical materials may have up to three
principle indices of refraction. The materials with all three
different refractive indices are called biaxial, and are uniquely
specified by its principal indices nx.sub.0, ny.sub.0, nz.sub.0,
and three orientational angles as shown in FIG. 9a. FIG. 9b shows a
biaxial film with the axes of nx.sub.0, ny.sub.0, and nz.sub.0
aligned with x, y, and z axes, respectively. The materials with two
equal principal refractive indices are called uni-axial materials.
These two equal indices are ordinary index and referred as no. The
other different refractive index is called an extraordinary index
n.sub.e. The axis of n.sub.e is also referred to as an optical
axis. Uniaxial materials are uniquely characterized by n.sub.e,
n.sub.0, and two angles describing the orientation of its optical
axis. When all three principal indices are equal, the materials are
called isotropic.
[0062] Light sees varying effective indices of refraction depending
on the polarization direction of its electric field when traveling
through a uniaxial or biaxial material, consequentially, a phase
difference (retardance) is introduced between two eigen-modes of
the electric field. This phase difference varies with the
propagation direction of light, so the transmission of the light
varies with angle when uniaxial or biaxial materials are placed
between two crossed polarizers. These phase differences translate
into modifications of the local polarization orientations for rays
traveling along paths other than along or parallel to the optical
axis. In particular, a compensator modifies or conditions the local
polarization orientations for rays at large polar angles, which
also includes both oblique and skew rays. A liquid crystal material
is typically a uniaxial material. When it is sandwiched between two
substrates as in a liquid crystal display, its optic axis generally
changes across the thickness depending on the anchoring at the
substrates and the voltage applied across the thickness. A
compensator is constructed with one or more uniaxial and/or biaxial
films, which are designed to introduce angularly dependent phase
differences in a way to offset the angle dependence of phase
difference introduced by liquid crystals or other optical elements.
As is well known in the art, a uniaxial film with its optic axis
parallel to the plane of the film is called a A-plate as shown in
FIG. 9c, while a uniaxial film with its optic axis perpendicular to
the plane of the film is called a C-plate, as shown in FIG. 9d. A
uniaxial material with n.sub.e greater than n.sub.o is called
positively birefringent. Likewise, a uniaxial material with n.sub.e
smaller than n.sub.o is called negatively birefringent. Both
A-plates and C-plates can be positive or negative depending on
their n.sub.e and n.sub.o.
[0063] A more sophisticated multi-layer polarization compensator
180 has its optic axis or three principal index axes varying across
its thickness, as in FIG. 8, where a stack of compensation films
(birefringent layers 190a, 190b, and 190c) are used with a
substrate 195 to assemble the complete compensator. A detailed
discussed of stack compensation can be found in U.S. Pat. No.
5,619,352 (Koch et al.). As is well known in art, C-plates can be
fabricated by the use of uniaxially compressed polymers or casting
acetate cellulose, while A-plates can be made by stretched polymer
films such as polyvinyl alcohol or polycarbonate. For increased
robustness, polarization compensators can be fabricated with
inorganic materials rather than the more commonly used
polymers.
[0064] In U.S. patent application Ser. No. 10/040,663, a modulation
optical system 120 similar to that in FIG. 7 is discussed, wherein
a polarization compensator 180 is described for operation in
conjunction with a vertically aligned LCD and a wire grid
polarization beamsplitter 170. As was described in that
application, an exemplary compensator can have retardance designed
to optimize the performance of the VA-LCD, the wire grid
polarization beamsplitter, or both in combination. For example, the
compensator can include an A-plate with 0.02 .lambda.'s (.about.11
nm) retardance to correct for residual stress birefringence within
the VA LCD, and a negative C-plate (approx. -233 nm retardance) to
correct for incidence angle response variations when the LCD is
operated in fast optical systems (F/3.0 or below). Likewise, as was
discussed, a compensator can be provided for the wire grid
polarization beamsplitter 170 comprising a combination of an
A-plate and a positive C-plate having a retardation of +90 nm and
+320 nm respectively. It was noted that the compensators for the
wire grid polarization beamsplitter 170 and the LCD are co-located
between these two components, and can be combined into one packaged
compensator device. The combined compensator 180 then comprises the
+11 nm A-plate for the VA LCD (0.02.lambda.'s compensation), a +87
nm C-plate, and a +90 nm A-plate for the wire grid polarization
beamsplitter 170 in sequential order, with the +11 nm A-plate
located closest to the LCD (175). The two A-plates cannot be simply
combined, as the +11 nm A-plate needs to be rotatable, while the
+90 nm A-plate has a fixed orientation relative to the
sub-wavelength wires 171. However, previously stated, this prior
application does not anticipate the use of an imager field lens 140
in a modulation optical system 120, nor its potential impact on
contrast performance and the design of any neighboring polarization
compensators.
[0065] Considering once again the modulation optical system 120 of
the present invention, as depicted in FIG. 6, various techniques
can be employed to enhance the frame sequential contrast in
conjunction with the use of image field lens 140. As in the prior
applications, wire grid polarization beamsplitter 170 is preferably
oriented with its sub-wavelength wires 171 on substrate 172, facing
the spatial light modulator 175. Likewise, for high contrast, it is
preferable that spatial light modulator 175 is a liquid crystal
display utilizing vertically aligned LC molecules, although other
types of high contrast polarization modulators could be used.
However, with specific regards to a modulation optical system 120
incorporating an imager field lens 140, frame sequential contrast
can be readily maintained if image field lens 140 does not
introduce any stress birefringence. If that is the case, light can
traverse imager field lens 140 without incurring any rotation of
the polarization vectors. As a result, the polarization compensator
180 and second polarization compensator 185 shown in FIG. 6 can be
combined into one device, in a similar fashion to the compensator
described in U.S. patent application Ser. No. 10/040,663. While the
compensator could potentially be placed to either side of imager
field lens 140, for mounting and contrast reasons, it will likely
be located adjacent to the spatial light modulator 175. Although
imager field lens 140 has optical power and therefore introduces
phase change across its diameter, relative to the polarization
contrast performance of modulation optical system 120, it is
important that this lens introduces minimal phase retardation.
[0066] For most applications, including many polarization systems,
many glasses may be sufficiently stress free to be utilized in a
fashion similar to imager field lens 140. However, in an electronic
projection system seeking high contrast (1,000:1 or greater) and
high screen lumens, small amounts of de-polarization or
polarization rotation, whether originating from intrinsic
birefringence, or mechanically or thermally induced stress
birefringence, can degrade the contrast. Moreover, as stress
birefringence is frequently spatially non-uniform, a spatial
contrast variation could result from stress in imager field lens
140.
[0067] Most optical glasses are amorphous (isotropic) or
non-crystalline, and therefore lack intrinsic birefringence.
Mechanically induced stress birefringence can be avoided by using
symmetrical fabrication techniques and compliant mounting, which
could be accomplished with a flexible adhesive, such as RTV.
Thermally induced stress birefringence, as could occur from light
absorption, can be minimized by choosing optical glasses with a low
optical stress coefficient, a low absorption coefficient, or both.
For example, SF-57 glass has the lowest stress coefficient of any
optical glass, and has been used in electronic projection systems
for that reason. However, SF-57 glass is expensive, hard to work,
and has relatively high blue light absorption. Alternately,
amorphous fused silica glass has the lowest light absorption across
the visible spectrum, and has been used successfully in many
projection systems. Even though its stress optical coefficient is
not the lowest, the lack of heat from light absorption very
effectively minimizes stress birefringence. Therefore, the imager
field lens 140 used in the modulation optical system 120 of FIG. 6
is preferably made from a low stress or low absorption glass, and
in particular, from amorphous fused silica.
[0068] Of course, even constructed from a preferred optical glass,
imager field lens 140 may experience some residual birefringence
that could impact the performance of a high contrast electronic
projection system. In that case, it is preferable that this
residual birefringence be uniform across the imager field lens 140.
It is then possible to fabricate an imager field lens 140 where
this residual stress has been cancelled out. Accordingly, FIG. 12
depicts an imager field lens 140 comprising two imager field lens
elements 142 and 142'. In combination, these lenses would provide
the total optical power required for imager field lens 140 as
dictated by the optical design. The two imager field lens elements
142 and 142' are preferably fabricated from the same optical
material. Thereafter, the birefringent axis of each lens element is
independently determined, and then the lens elements are aligned
with the axes oriented orthogonally to each other so as to cancel
the residual retardances. The lens elements are then assembled to
create imager field lens 140, preferably with a low stress optical
adhesive. Optimally, the optimum retardance cancellation is
achieved at the operating temperature.
[0069] As another approach, if imager field lens 140 provides a
stable uniform amount of residual retardance, the polarization
compensators 180 and 185 can be designed with in-plane (XY) and
out-of-plane (Z) retardance A-plate and C-plate materials, to
correct for the lens residual retardances. As a worst case, if
imager field lens 140 has stable but non-uniform residual
retardances, then one or both of the polarization compensators 180
and 185, as a correction, could be designed with matching, opposite
sign, spatially variant retardances. However, producing spatially
variant or patterned polarization compensators is a non-trivial
complication.
[0070] In FIGS. 2 and 6, the modulation optical system of the
present invention is shown to include a pre-polarizer 160. In the
bright projection systems required for applications such as digital
cinema, the lamps employed, such as xenon arc lamps, emit
un-polarized light from a large emitting volume (large LaGrange or
etendue). Typically in such systems, little can be done to salvage
the light (50% of the total) of the polarization state rejected by
the pre-polarizer. However, in many projection systems using
smaller lamp sources, polarization converters can be used to
convert a rejected polarization state to the orthogonal state, with
the resulting polarized illumination light being directed at the
spatial light modulator. In such a case, the pre-polarizer 160
shown in modulation optical system 120 of the present invention is
effectively replaced, or substituted for, by the use of a
polarization converter. Many forms of polarization converters are
known in the art, including systems with large polarization beam
splitting prisms and systems with micro-prism arrays (U.S. Pat. No.
5,555,186 (Shioya) and U.S. Pat. No. 5,898,521 (Okada), for
example). It should also be understood that both the pre-polarizer
160 and the polarization analyzer 165 provided in modulation
optical system 120 of the present invention may be selected from a
variety of potential polarizer technologies, including wire grid
polarizers, dye or polymer polarizers, thin film polarizers, or
giant birefringence type polarizers.
[0071] As a means to assist in understanding the improved projector
100 of the present invention, FIG. 10 depicts a three dimensional
view of the system, with many of the critical components of the
imaging side of the system readily visible. In particular, this
figure shows a nominal layout with three imaging systems (red,
green, and blue), each partially comprising a spatial light
modulator 175, an imager field lens 140, and an imaging relay lens
130. The illustration also shows a dichroic combiner 155 of the
V-prism type, but the projection lens is not depicted. Portions of
the illumination system, and particularly the dichroic separator
34, are shown, but much of the illumination system is obscured by
the imaging optics shown in the foreground, or is not shown (such
as the light source and integrating bar). Certainly the overall
system can be configured many ways, and this illustration
represents a compact construction, but not necessarily an
configuration that is optimal overall.
[0072] The preferred embodiment for the modulation optical system
120, as shown in FIG. 6, utilizes a wire grid polarization
beamsplitter 170 and reflective spatial light modulator 175. In
this system, the illumination light is transmitted through the wire
grid polarization beamsplitter 170 before being incident on the
spatial light modulator 175. The modulated image light that emerges
from spatial light modulator 175 then reflects off of wire grid
polarization beamsplitter 170, before entering the main body of the
projection lens 130. The un-modulated image light preferably is
transmitted back through wire grid polarization beamsplitter 170
and is absorbed or rejected without returning as ghost or flare
light. This approach provides high contrast, high optical
efficiency, while incurring some manageable mechanically packaging
constraints. Alternately, the modulation optical system 120 could
be constructed as shown in FIG. 11a, where the illumination light
reflects off the wire grid polarization beamsplitter 170 before
being incident to the reflective spatial light modulator 175. In
this case, the modulated image light that emerges from spatial
light modulator 175 is transmitted through wire grid polarization
beamsplitter 170, before entering the main body of the projection
lens 130. Although this system provides an architecture with
somewhat easier opto-mechanics, the imaging light that is
transmitted through the wire grid polarization beamsplitter 170
suffers the classic aberrations induced by transmission through a
tilted plate. Although these aberrations can be corrected, both the
imaging relay lens 130 and the projection lens 150 may suffer
increased complexity and cost.
[0073] The modulation optical system 120 of FIG. 11a also lends
itself to an alternate configuration with a simplified
opto-mechanical construction. In particular, the spatial light
modulator 175 can be a transmissive device rather than a reflective
device. In that case, pre-polarized illumination light would
approach the spatial light modulator 175 from "behind" (from the
right side of the spatial light modulator 175), and modulated and
un-modulated light could be provided in accordance to the drive
signals directed to each pixel. For such a transmissive system, the
wire grid polarization beamsplitter 170 could be removed and
polarization analyzer 165 retained, to provide a simplified and
more compact optical path. Alternately, polarization analyzer 165
could be removed and wire grid polarization beamsplitter 170
retained, to provide a simplified system with potentially higher
contrast than the previous case. Imager field lens 140 would still
be used on the imaging side to help simplify the design of an image
relay lens or projection lens downstream. A second imager field
lens 140 could also be used on the illumination side of the
transmissive spatial light modulator 125, although that would not
be necessary.
[0074] As another alternative, modulation optical system 120 could
be constructed as shown in FIG. 11b, with a polarization
beamsplitter prism 173. The principal advantage of using a prism,
as compared to the tilted plate beamsplitter provided in FIG. 6, is
a reduced optical path length and therefore a reduced working
distance requirement. In this case, polarizing beamsplitter prism
173 could for example be a conventional MacNeille beamsplitter
(U.S. Pat. No. 2,403,731) or an embedded wire grid beamsplitter
(U.S. Pat. No. 6,288,840 (Perkins et al.)). Other types of
polarizers and polarization beamsplitters can be used in modulation
optical system 120, including the 3M multi-layer polymer sheet
polarization beamsplitter (U.S. Pat. No. 5,962,114 (Jonza et al.)),
provided that the polarization response and thermal stability are
adequate.
[0075] A variation of the design for the imaging relay 130 lens
depicted in greatest detail in FIG. 5 is to utilize a Ramsden
eyepiece 145 type design, as depicted in FIG. 13. The classical
Ramsden eyepiece is a two element design that provides an
accessible aperture stop and an accessible field along with a
modest angular field. For example, to use this in the projection
system, the polarizing beamsplitting prism could be placed between
the two lens elements of the Ramsden eyepiece, effectively
identifying the lens element closest to spatial light modulator 175
as the imaging field lens 140. Assuming the dichroic combiner is
located at or near a telecentric intermediate image, then an
imaging relay lens could be designed incorporating the Ramsden
eyepiece on one side of the imaging aperture stop 210, and other
lens elements on the other side of the stop. To obtain better
optical performance, other more complicated eyepiece designs, using
3 or more lens elements, can be designed. Some of these eyepiece
designs will steer the overall design of the imaging relay lens 130
to take on a double gauss construction similar to that in FIG. 5,
while other designs may steer the overall imaging relay lens 130
into a different solution space. A design using a Ramsden eyepiece,
or variation thereof, may also provide the free space to locate the
dichroic combiner at the imaging aperture stop 210, while the
system design still provides an imaging relay lenses 130 to create
an intermediate image (I). Again, the intermediate image (I) would
be imaged to a display surface by a projection lens. This
alternative may enable an even more compact projection system
design than that provided with imaging relay lenses employing a
double gauss configuration. The polarization beamsplitter could
also be placed after the two lens elements of the Ramsden, rather
than between them, but this further increases the number of
elements between the beamsplitter and the spatial light modulator,
which increases the potential for polarization contrast
degradation.
[0076] It has been mentioned previously that an electronic
projection system according to the present invention, utilizing
intermediate imaging relay lenses 130 and an imager field lens 140
can be constructed with a dichroic combiner other than the
preferred V-prism (dichroic combiner 155) shown in FIG. 2. The
alternate use of an X-prism or X-cube can easily be inferred by
comparing FIG. 1 (which has an X-- prism type dichroic combiner 26)
and FIG. 2. As another alternative, FIG. 14 depicts an electronic
projector 100 of the present invention in which a Philips prism 28
is used to combine light from the different color channels (RGB).
As before, the action of spatial light modulators 175 forms an
image for each color channel. The modulated light from these images
is transmitted along the respective optical axis O.sub.r, O.sub.g,
O.sub.b, through imager field lens 140, through a beamsplitter,
through imaging relay lens 130 and to Philips prism 28. Then,
Philips prism 28 combines the red, green, and blue modulated images
from separate optical axes O.sub.r, O.sub.g, O.sub.b to form a
combined, multicolor image I.sub.rgb near an exit face of the prism
assembly and along a common optical axis O. Projection lens 150
re-images the multicolor image I.sub.rgb to the display surface 40.
As previously, spatial light modulator 175 is preferably a
polarization modulator array such as an LCD, and the beamsplitter
is a polarization beamsplitter 24 such as a MacNeille type prism of
a wire grid polarization beamsplitter.
[0077] Alternately, the system could be configured with
polarization beam splitting prism and the dichroic combiner
switched. As an example, FIG. 5 illustrates a potion of the
projector, in which the wire grid polarization beamsplitter 170 is
located in proximity to imager plane 275 and the dichroic combiner
155 is located in proximity to the internal image (I). In this
alternate system, the dichroic combiner 155 is located in proximity
to imager plane 275 and the polarization beamsplitter is located in
proximity to the internal image (I). In this case, the polarization
beamsplitter preferably comprises a glass block with an internal
polarization splitting surface, such that it can be used in
transmission without the optical aberrations of a tilted plate. A
MacNeille type prism is one example of such a polarization
beamsplitter that would be viable for this alternative
configuration. This construction does have the potential
disadvantage that it places numerous glass elements between the
spatial light modulator (LCD) and the polarization beamsplitter,
any one of which could possess residual stress birefringence that
can degrade the polarization contrast.
[0078] As another alternative, it should be noted that the
projector of the present invention can be made to work with spatial
light modulators 175 that are other than LCDs. For example, in the
system of FIG. 14, the spatial light modulator 175 could be a DMD
type modulator instead of being an LCD. DMD devices do not modulate
the polarization state, but the light directionality, on a pixel by
pixel basis. In such a case, the system could be altered to
substitute a total internal reflection (TIR) beamsplitter or
another angularly sensitive optics, such as a Schlieren aperture
mirror, for polarizing beamsplitter 24, as is well known in the
digital projection art. The resulting electronic projector could
still be constructed to provide an internal multicolor image
I.sub.rgb at or near the dichroic combiner, which is the Philips
prism 28 shown in FIG. 14, but which could also be a V-prism (as in
FIG. 2) or an X-prism (as in FIG. 1). As previously, the imaging
relay lenses 130 could operate at unity or near unity
magnification, and provide a potentially advantaged system, with a
reduced working distance requirement on the projection lens 150,
resulting in a simplified, lower cost projection lens as compared
to the systems on the market today. Certainly, the use of the
imaging relay lenses 130 provides some offsetting costs that may
mitigate the advantages. Designing the projector 100 to include
imager field lenses 140 should reduce the cost and complexity of
the imaging relay lenses 130. The projector 100 can be further
designed with the imaging relay lenses 130 providing greater than
unity magnification (2.times., for example), such that the cost and
complexity of the combining prism could be reduced. The resulting
system may be further cost advantaged compared to the standard DMD
type systems available in the market today.
[0079] Thus, what is provided is an improved electronic projection
apparatus and method for image projection with an illumination
system for providing, from a white light source, color illumination
of high intensity and high efficiency, a modulation optical system
having a spatial light modulator in each color channel and
associated means to separate the modulated light from the
unmodulated light, and an intermediate imaging optics in each color
channel comprising imaging relay lenses for providing a suitable
images for projection onto a display surface; in which each color
channel employs an imager field lens between a beamsplitter and the
respective spatial light modulator, for providing telecentric light
at said spatial light modulators.
Parts List
[0080] 10 Projection apparatus
[0081] 20 Light source
[0082] 20r Light source, red
[0083] 20g Light source, green
[0084] 20g Light source, blue
[0085] 20b Uniformizing optics
[0086] 22 Uniformizing optics, red
[0087] 22r Uniformizing optics, green
[0088] 22g Uniformizing optics, blue
[0089] 22b Polarizing beamsplitter
[0090] 24 Polarizing beamsplitter, red
[0091] 24r Polarizing beamsplitter, green
[0092] 24g Polarizing beamsplitter, blue
[0093] 24g Dichroic combiner
[0094] 28 Philips prism
[0095] 30 Spatial light modulator
[0096] 30r Spatial light modulator, red
[0097] 30g Spatial light modulator, green
[0098] 30b Spatial light modulator, blue
[0099] 32 Projection lens
[0100] 34 Dichroic separator
[0101] 40 Display surface
[0102] 100 Projector
[0103] 110 Illumination system
[0104] 120 Modulation optical system
[0105] 130 Imaging relay lens
[0106] 140 Imager field lens
[0107] 142 Imager field lens element
[0108] 142' Imager field lens element
[0109] 145 Ramsden eyepiece
[0110] 150 Projection lens
[0111] 155 Dichroic combiner
[0112] 157 Internal surface
[0113] 158 Exit face
[0114] 160 Pre-polarizer
[0115] 165 Polarization analyzer
[0116] 170 Wire grid polarization beamsplitter
[0117] 171 Sub-wavelength wires
[0118] 172 Substrate
[0119] 173 Polarization beamsplitter prism
[0120] 175 Spatial light modulator
[0121] 177 Modulator package
[0122] 173 Polarization compensator
[0123] 185 Second polarization compensator
[0124] 190a Birefringent layer
[0125] 190b Birefringent layer
[0126] 190c Birefringent layer
[0127] 195 Substrate
[0128] 200 Internal image
[0129] 210 Imaging aperture stop
[0130] 250 Integrating bar
[0131] 255 Base condenser
[0132] 260 Focused source light
[0133] 265 Illumination aperture stop
[0134] 265a Re-imaged illumination aperture stop
[0135] 270 Internal illumination image
[0136] 275 Imager plane
[0137] 280 Illumination lens
[0138] 285 Color filter
[0139] 287 Other optics
[0140] 290 Optical axis
* * * * *