U.S. patent application number 09/862183 was filed with the patent office on 2002-02-07 for image projection system with a polarizing beam splitter.
This patent application is currently assigned to MOXTEK. Invention is credited to Gardner, Eric, Hansen, Douglas P., Perkins, Raymond T..
Application Number | 20020015135 09/862183 |
Document ID | / |
Family ID | 25337868 |
Filed Date | 2002-02-07 |
United States Patent
Application |
20020015135 |
Kind Code |
A1 |
Hansen, Douglas P. ; et
al. |
February 7, 2002 |
Image projection system with a polarizing beam splitter
Abstract
An image projection system has a wire grid polarizing beam
splitter which functions as both the polarizer and the analyzer in
the system. A light source produces a source light beam directed at
the beam splitter which reflects one polarization and transmits the
other. A liquid crystal array is disposed in either the reflected
or transmitted beam. The array modulates the polarization of the
beam, encoding image information thereon, and directs the modulated
beam back to the beam splitter. The beam splitter again reflects
one polarization and transmits the other so that the encoded image
is either reflected or transmitted to a screen. The beam splitter
can be an embedded wire grid polarizer with an array of parallel,
elongated, spaced-apart elements sandwiched between first and
second layers. The elements form a plurality of gaps between the
elements which provide a refractive index less than the refractive
index of the first or second layers.
Inventors: |
Hansen, Douglas P.; (Spanish
Fork, UT) ; Perkins, Raymond T.; (Orem, UT) ;
Gardner, Eric; (EagleMountain, UT) |
Correspondence
Address: |
Garron M. Hobson
THORPE, NORTH & WESTERN, L.L.P.
P.O. Box 1219
Sandy
UT
84091-1219
US
|
Assignee: |
MOXTEK
|
Family ID: |
25337868 |
Appl. No.: |
09/862183 |
Filed: |
May 21, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09862183 |
May 21, 2001 |
|
|
|
09363256 |
Jul 28, 1999 |
|
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|
6234634 |
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Current U.S.
Class: |
353/31 |
Current CPC
Class: |
G02B 5/3058 20130101;
G03B 21/14 20130101; G02B 27/283 20130101; G03B 21/2073
20130101 |
Class at
Publication: |
353/31 |
International
Class: |
G03B 021/00 |
Claims
What is claimed is:
1. An image projection system, comprising: a) a light source
capable of producing a visible light beam; b) a polarizing beam
splitter, located near the light source in the light beam and
oriented at an angle with respect to the light beam, the beam
splitter comprising: 1) a first transparent substrate having a
first surface located in the light beam with the light beam
striking the first surface at an angle, and having a refractive
index; 2) a second layer, separate from the first transparent
substrate, having a refractive index; and 3) a generally parallel
arrangement of thin, elongated, spaced-apart elements disposed
between the first transparent substrate and the second layer, and
forming a plurality of gaps between the elements, the gaps
providing a refractive index less than the refractive index of the
first transparent substrate or the second layer, the arrangement
being configured and the elements being sized to interact with
electromagnetic waves of the source light beam to generally (i)
transmit light through the elements which has a polarization
oriented perpendicular to a plane that includes at least one of the
elements and the direction of the incident light beam, defining a
transmitted beam, and (ii) reflect light from the elements which
has a polarization oriented parallel with the plane that includes
at least one of the elements and the direction of the incident
light beam, defining a reflected beam; c) a reflective array
located near the polarizing beam splitter in either the reflected
or transmitted beam, the array modulating the polarization of the
beam by selectively altering the polarization of the beam to encode
image information thereon and creating a modulated beam, the array
being oriented to direct the modulated beam back towards the
polarizing beam splitter; d) the beam splitter further being
located in the modulated beam and oriented at an angle with respect
to the modulated beam, and the arrangement of elements of the beam
splitter interacting with electromagnetic waves of the modulated
beam to generally (i) transmit light through the elements which has
a polarization oriented perpendicular to the plane that includes at
least one of the elements and the direction of the incident light
beam, defining a second transmitted beam, and (ii) reflect light
from the elements which has a polarization parallel with the plane
that includes at least one of the elements and the direction of the
incident light beam, defining a second reflected beam, to separate
out the unaltered polarization from the modulated beam; e) a screen
located in either the second reflected beam or the second
transmitted beam for displaying the encoded image information.
2. A system in accordance with claim 1, wherein the transparent
substrate has a thickness less than approximately 5
millimeters.
3. A system in accordance with claim 1, wherein the transparent
substrate has a flatness less than approximately 3 standard
wavelengths deviation per inch.
4. A system in accordance with claim 1, wherein the first
transmitted beam has a geometric distortion less than approximately
3 wavelengths deviation per inch.
5. A system in accordance with claim 1, wherein the gaps between
the elements include air.
6. A system in accordance with claim 1, wherein the gaps between
the elements have a vacuum.
7. A system in accordance with claim 1, wherein the gaps between
the elements include a material different from materials of the
first transparent substrate and the second layer.
8. A system in accordance with claim 1, wherein the gaps include a
material that is a same material as the second layer.
9. A system in accordance with claim 1, wherein the gaps include a
material that is a same material as the first transparent
substrate.
10. A system in accordance with claim 1, wherein the gaps between
the elements include water.
11. A system in accordance with claim 1, wherein the gaps between
the elements include magnesium fluoride.
12. A system in accordance with claim 1, wherein the gaps between
the elements include oil.
13. A system in accordance with claim 1, wherein the gaps between
the elements include hydrocarbon compounds.
14. A system in accordance with claim 1, wherein the gaps between
the elements include plastic.
15. A system in accordance with claim 1, wherein the gaps between
the elements include fluorinated hydrocarbon.
16. A system in accordance with claim 1, wherein the arrangement
has a configuration and the elements have a size which would
normally create a resonance effect in combination with one of the
layer or the substrate within the visible spectrum; and wherein the
gaps with a lower refractive index than the refractive index of one
of the layer or the substrate causes a shift of the normally
occurring resonance effect to a lower wavelength, thereby
broadening a band of visible wavelengths in which no resonance
effect occurs.
17. A system in accordance with claim 1, wherein the second layer
includes a film.
18. A system in accordance with claim 1, wherein the second layer
includes a plurality of films.
19. A system in accordance with claim 1, wherein the second layer
includes a vacuum deposited film selected from the group consisting
of: silicon dioxide, silicon nitride, magnesium fluoride, and
titanium oxide.
20. A system in accordance with claim 1, wherein the second layer
includes a sheet of glass.
21. A system in accordance with claim 1, wherein the second layer
includes a sheet of plastic.
22. A system in accordance with claim 1, wherein the second layer
includes a film of hexamethal disilazane.
23. A system in accordance with claim 1, wherein the beam splitter
is a generally planar sheet.
24. A system in accordance with claim 1, wherein the beam splitter
is oriented with respect to the light beam or the modulated beam at
an incident angle between approximately 0 to 80 degrees.
25. A system in accordance with claim 1, wherein the beam splitter
is oriented with respect to the light beam or the modulated beam at
incidence angles greater than 47 degrees or less than 43
degrees.
26. A system in accordance with claim 1, wherein the light beam has
a useful divergent cone with a half angle between approximately 10
and 25.degree..
27. A system in accordance with claim 1, wherein the beam splitter
is used at an F-number less than approximately f/2.5.
28. A system in accordance with claim 1, wherein the beam splitter
has a throughput of at least 50% defined by the product of the
fractional amount of p-polarization transmitted light and the
fractional amount of s-polarization reflected light; and wherein
the s-polarization transmitted light and p-polarization reflected
light are both less than 5%.
29. A system in accordance with claim 1, wherein the beam splitter
has a throughput of at least 50% defined by the product of the
fractional amount of s-polarization transmitted light and the
fractional amount of p-polarization reflected light; and wherein
the p-polarization transmitted light and s-polarization reflected
light are both less than 5%.
30. A system in accordance with claim 1, wherein the beam splitter
has a throughput for the light beam of at least 65%, defined by the
product of the fractional amount of reflected light and the
fractional amount of transmitted light; and wherein the percent of
reflected light or the percent of transmitted light is greater than
approximately 67%.
31. A system in accordance with claim 1, further comprising a
pre-polarizer disposed between the light source and the beam
splitter.
32. A system in accordance with claim 1, further comprising a
post-polarizer disposed between the beam splitter and the
screen.
33. A system in accordance with claim 1, wherein the array is
disposed in the reflected beam; and wherein the screen is disposed
in the second transmitted beam.
34. A system in accordance with claim 1, wherein the array is
disposed in the transmitted beam; and wherein the screen is
disposed in the second reflected beam.
35. A system in accordance with claim 1, wherein a) the arrangement
of elements has a period less than approximately 0.21 microns, b)
the elements have a thickness between approximately 0.04 to 0.5
microns, and c) the elements have a width of between approximately
30 to 76% of the period.
36. A system in accordance of claim 1, wherein the elements each
have a cross section with a base, a top opposite the base, and
opposite left and right sides; and wherein the sides form an angle
with respect to the base greater than approximately 68 degrees.
37. A method for projecting an image, the method comprising: a)
producing a source light beam having a wavelength in a range
between approximately 0.4 to 0.7 microns using a light source; b)
substantially separating polarizations of the source light beam
using a polarizing beam splitter disposed in the source light beam,
the polarizing beam splitter including: 1) a first layer having a
refractive index; 2) a second layer, separate from the first layer,
having a refractive index; 3) a generally parallel arrangement of
thin, elongated, spaced-apart elements, disposed between the first
and second layers, configured and sized to interact with
electromagnetic waves of the source light beam to generally (i)
transmit light through the elements which has a polarization
oriented perpendicular to a plane that includes at least one of the
elements and the direction of the incident light beam, defining a
transmitted beam, and (ii) reflect light from the elements which
has a polarization orientation that lies in the plane that includes
at least one of the elements and the direction of the incident
light beam, defining a reflected beam; 4) a plurality of gaps,
formed between the elements and the first and second layers,
configured to provide a refractive index less than the refractive
index of the first or second layers; c) modulating either the
transmitted or reflected beam and creating a modulated beam by
selectively altering the polarization of the beam using an array
disposed in either the transmitted or reflected beam; d)
substantially separating the polarizations of the modulated beam
using the polarizing beam splitter disposed in the modulated beam,
the elements interacting with electromagnetic waves of the
modulated beam to generally (i) transmit light through the elements
which has a polarization oriented perpendicular to plane that
includes at least one of the elements and the direction of the
incident light beam, defining a second transmitted beam, and (ii)
reflect light from the elements which has a polarization
orientation that lies in the plane that includes at least one of
the elements and the direction of the incident light beam, defining
a second reflected beam; and e) displaying either the second
transmitted beam or the second reflected beam on a screen.
38. An image display system for producing a visible image, the
system comprising: a) a light source configured to emit a source
light beam having a wavelength in a range between approximately 0.4
to 0.7 microns; b) a liquid crystal array positioned and oriented
to receive and modulate at least a portion of the source light beam
to create a modulated beam containing image information; c) a
screen positioned and oriented to receive and display at least a
portion of the modulated beam; and d) a polarizing beam splitter
positioned and oriented to receive both the source light beam and
the modulated beam, the polarizing beam splitter including: 1) a
first layer having a refractive index; 2) a second layer, separate
from the first layer, having a refractive index; 3) a generally
parallel arrangement of thin, elongated, spaced-apart elements,
disposed between the first and second layers, configured and sized
to interact with electromagnetic waves of the source light beam to
generally (i) transmit light through the elements which has a
polarization oriented perpendicular to a plane that includes at
least one of the elements and the direction of the incident light
beam, defining a transmitted beam, and (ii) reflect light from the
elements which has a polarization orientation that lies in the
plane that includes at least one of the elements and the direction
of the incident light beam, defining a reflected beam, and
interacts with the electromagnetic waves of the modulated beam to
generally (i) transmit light through the elements which has a
polarization oriented perpendicular to a plane that includes at
least one of the elements and the direction of the modulated light
beam, defining a second transmitted beam, and (ii) reflect light
from the elements which has a polarization orientation that lies in
the plane that includes at least one of the elements and the
direction of the modulated light beam, defining a second reflected
beam; and 4) a plurality of gaps, formed between the elements and
the first and second layers, configured to provide a refractive
index less than the refractive index of the first or second
layers.
39. A system in accordance with claim 38, wherein the first layer
is a substrate having a thickness less than approximately 5
millimeters.
40. A system in accordance with claim 38, wherein the first layer
is a substrate having a flatness less than approximately 3 standard
wavelengths deviation per inch.
41. A system in accordance with claim 38, wherein the either of the
first or second transmitted beams has a geometric distortion less
than approximately 3 wavelengths deviation per inch.
42. A system in accordance with claim 38, wherein the gaps between
the elements include air.
43. A system in accordance with claim 38, wherein the gaps between
the elements have a vacuum.
44. A system in accordance with claim 38, wherein the gaps between
the elements include a material different from materials of the
first transparent substrate and the second layer.
45. A system in accordance with claim 38, wherein the gaps include
a material that is a same material as the second layer.
46. A system in accordance with claim 38, wherein the gaps include
a material that is a same material as the first transparent
substrate.
47. A system in accordance with claim 38, wherein the gaps between
the elements include water.
48. A system in accordance with claim 38, wherein the gaps between
the elements include magnesium fluoride.
49. A system in accordance with claim 38, wherein the gaps between
the elements include oil.
50. A system in accordance with claim 38, wherein the gaps between
the elements include hydrocarbon compounds.
51. A system in accordance with claim 38, wherein the gaps between
the elements include plastic.
52. A system in accordance with claim 38, wherein the gaps between
the elements include fluorinated hydrocarbon.
53. A system in accordance with claim 38, wherein the arrangement
has a configuration and the elements have a size which would
normally create a resonance effect in combination with one of the
layers within the visible spectrum; and wherein the gaps with a
lower refractive index than the refractive index of one of the
layers causes a shift of the normally occurring resonance effect to
a lower wavelength, thereby broadening a band of visible
wavelengths in which no resonance effect occurs.
54. A system in accordance with claim 38, wherein the second layer
includes a film.
55. A system in accordance with claim 38, wherein the second layer
includes a plurality of films.
56. A system in accordance with claim 38, wherein the second layer
includes a vacuum deposited film selected from the group consisting
of: silicon dioxide, silicon nitride, magnesium fluoride, and
titanium oxide.
57. A system in accordance with claim 38, wherein the second layer
includes a sheet of glass.
58. A system in accordance with claim 38, wherein the second layer
includes a sheet of plastic.
59. A system in accordance with claim 38, wherein the second layer
includes a film of hexamethal disilazane.
60. A system in accordance with claim 38, wherein a) the
arrangement of elements has a period less than approximately 0.21
microns, b) the elements have a thickness between approximately
0.04 to 0.5 microns, and c) the elements have a width of between
approximately 30 to 76% of the period.
61. A system in accordance with claim 38, wherein the array is
disposed in the reflected beam, and wherein the screen is disposed
in the second transmitted beam.
62. A system in accordance with claim 38, wherein the array is
disposed in the transmitted beam, and wherein the screen is
disposed in the second reflected beam.
63. An image projection system, comprising: a) a light source
producing a visible light beam; b) a polarizing beam splitter
located near the light source in the light beam and oriented at an
angle with respect to the light beam, the beam splitter comprising:
1) a first transparent substrate having a first surface located in
the light beam with the light beam striking the first surface at an
angle; 2) the first transparent substrate having a thickness less
than approximately 5 millimeters; and 3) a generally parallel
arrangement of thin, elongated, spaced-apart elements, disposed on
the first transparent substrate, the arrangement being configured
and the elements being sized to interact with electromagnetic waves
of the source light beam to generally (i) transmit light through
the elements which has a polarization oriented perpendicular to a
plane that includes at least one of the elements and the direction
of the incident light beam, defining a transmitted beam, and (ii)
reflect light from the elements which has a polarization oriented
parallel with the plane that includes at least one of the elements
and the direction of the incident light beam, defining a reflected
beam; c) a reflective array located near the polarizing beam
splitter in either the reflected or transmitted beam, the array
modulating the polarization of the beam by selectively altering the
polarization of the beam to encode image information thereon and
creating a modulated beam, the array being oriented to direct the
modulated beam back towards the polarizing beam splitter; d) the
beam splitter further being located in the modulated beam and
oriented at an angle with respect to the modulated beam, and the
arrangement of elements of the beam splitter interacting with
electromagnetic waves of the modulated beam to generally (i)
transmit light through the elements which has a polarization
oriented perpendicular to the plane that includes at least one of
the elements and the direction of the incident light beam, defining
a second transmitted beam, and (ii) reflect light from the elements
which has a polarization parallel with the plane that includes at
least one of the elements and the direction of the incident light
beam, defining a second reflected beam, to separate out the
unaltered polarization from the modulated beam; and e) a screen
located in either the second reflected beam or the second
transmitted beam for displaying the encoded image information.
64. A system in accordance with claim 63, further comprising: a)
second layer, separate from the first transparent substrate; and b)
the arrangement of elements being disposed between the first
transparent substrate and the second layer; and c) a plurality of
gaps formed between the elements; and d) the gaps providing a
refractive index less than a refractive index of the first
transparent substrate or the second layer.
65. A system in accordance with claim 63, wherein the first
transparent substrate having a flatness less than approximately 3
standard wavelengths deviation per inch.
66. A system in accordance with claim 63, wherein either the first
or second transmitted beams have a geometric distortion less than
approximately 3 wavelengths deviation per inch.
67. An image projection system, comprising: a) a light source
producing a visible light beam; b) a polarizing beam splitter
located near the light source in the light beam and oriented at an
angle with respect to the light beam, the beam splitter comprising:
1) a first transparent substrate having a first surface located in
the light beam with the light beam striking the first surface at an
angle; 2) the first transparent substrate having a flatness less
than approximately 3 standard wavelengths deviation per inch; and
3) a generally parallel arrangement of thin, elongated,
spaced-apart elements, disposed on the first transparent substrate,
the arrangement being configured and the elements being sized to
interact with electromagnetic waves of the source light beam to
generally (i) transmit light through the elements which has a
polarization oriented perpendicular to a plane that includes at
least one of the elements and the direction of the incident light
beam, defining a transmitted beam, and (ii) reflect light from the
elements which has a polarization oriented parallel with the plane
that includes at least one of the elements and the direction of the
incident light beam, defining a reflected beam; c) a reflective
array located near the polarizing beam splitter in either the
reflected or transmitted beam, the array modulating the
polarization of the beam by selectively altering the polarization
of the beam to encode image information thereon and creating a
modulated beam, the array being oriented to direct the modulated
beam back towards the polarizing beam splitter; d) the beam
splitter further being located in the modulated beam and oriented
at an angle with respect to the modulated beam, and the arrangement
of elements of the beam splitter interacting with electromagnetic
waves of the modulated beam to generally (i) transmit light through
the elements which has a polarization oriented perpendicular to the
plane that includes at least one of the elements and the direction
of the incident light beam, defining a second transmitted beam, and
(ii) reflect light from the elements which has a polarization
parallel with the plane that includes at least one of the elements
and the direction of the incident light beam, defining a second
reflected beam, to separate out the unaltered polarization from the
modulated beam; and e) a screen located in either the second
reflected beam or the second transmitted beam for displaying the
encoded image information.
68. A system in accordance with claim 67, further comprising: a) a
second layer, separate from the first transparent substrate; and b)
the arrangement of elements being disposed between the first
transparent substrate and the second layer; and c) a plurality of
gaps formed between the elements; and d) the gaps providing a
refractive index less than a refractive index of the first
transparent substrate or the second layer.
69. A system in accordance with claim 67, wherein the first
transparent substrate has a thickness less than approximately 5
millimeters.
70. A system in accordance with claim 67, wherein either the first
or second transmitted beams have a geometric distortion less than
approximately 3 wavelengths deviation per inch.
71. An image projection system, comprising: a) a light source
producing a visible light beam; b) a polarizing beam splitter
located near the light source in the light beam and oriented at an
angle with respect to the light beam, the beam splitter comprising:
1) a first transparent substrate having a first surface located in
the light beam with the light beam striking the first surface at an
angle; and 2) a generally parallel arrangement of thin, elongated,
spaced-apart elements, disposed on the first transparent substrate,
the arrangement being configured and the elements being sized to
interact with electromagnetic waves of the source light beam to
generally (i) transmit light through the elements which has a
polarization oriented perpendicular to a plane that includes at
least one of the elements and the direction of the incident light
beam, defining a transmitted beam, and (ii) reflect light from the
elements which has a polarization oriented parallel with the plane
that includes at least one of the elements and the direction of the
incident light beam, defining a reflected beam; c) a reflective
array located near the polarizing beam splitter in either the
reflected or transmitted beam, the array modulating the
polarization of the beam by selectively altering the polarization
of the beam to encode image information thereon and creating a
modulated beam, the array being oriented to direct the modulated
beam back towards the polarizing beam splitter; d) the beam
splitter further being located in the modulated beam and oriented
at an angle with respect to the modulated beam, and the arrangement
of elements of the beam splitter interacting with electromagnetic
waves of the modulated beam to generally (i) transmit light through
the elements which has a polarization oriented perpendicular to the
plane that includes at least one of the elements and the direction
of the incident light beam, defining a second transmitted beam, and
(ii) reflect light from the elements which has a polarization
parallel with the plane that includes at least one of the elements
and the direction of the incident light beam, defining a second
reflected beam, to separate out the unaltered polarization from the
modulated beam; and e) a screen located in either the second
reflected beam or the second transmitted beam for displaying the
encoded image information; and f) the first and second transmitted
beams having a geometric distortion less than approximately 3
wavelengths deviation per inch.
72. A system in accordance with claim 71, further comprising: a) a
second layer, separate from the first transparent substrate; and b)
the arrangement of elements being disposed between the first
transparent substrate and the second layer; and c) a plurality of
gaps formed between the elements; and d) the gaps providing a
refractive index less than a refractive index of the first
transparent substrate or the second layer.
73. A system in accordance with claim 71, wherein the first
transparent substrate has a thickness less than approximately 5
millimeters.
74. A system in accordance with claim 71, wherein the first
transparent substrate having a flatness less than approximately 3
standard wavelengths deviation per inch.
Description
[0001] This application is a continuation-in-part of U.S. Ser. No.
09/363,256 filed Jul. 28, 1999.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an image projection system
operable within the visible spectrum which includes a polarizing
beam splitter which reflects one linear polarization of light and
transmits the other. More particularly, the present invention
relates to such an image projection system with a beam splitter
that is comprised of a plurality of elongated, reflective elements
which are disposed on a substrate in such a way to reduce geometric
distortions, astigmatism and/or coma in the resulting light beam,
and/or which are embedded or otherwise configured to protect the
elements.
[0004] 2. Related Art
[0005] Polarized light is necessary in certain applications, such
as projection liquid crystal displays (LCD). Such a display is
typically comprised of a light source; optical elements, such as
lenses to gather and focus the light; a polarizer that transmits
one polarization of the light to the liquid crystal array; a liquid
crystal array for manipulating the polarization of the light to
encode image information thereon; means for addressing each pixel
of the array to either change or retain the polarization; a second
polarizer (called an analyzer) to reject the unwanted light from
the selected pixels; and a screen upon which the image is
focused.
[0006] It is possible to use a single polarizing beam splitter
(PBS) to serve both as the first polarizer and the second polarizer
(analyzer). If the liquid crystal array is reflective, for example
a Liquid Crystal On Silicon (LCOS) light valve, it can reflect the
beam that comes from the polarizer directly back to the polarizer
after encoding the image by modifying the polarization of selected
pixels. Such a system was envisioned by Takanashi (U.S. Pat. No.
5,239,322). The concept was elaborated by Fritz and Gold (U.S. Pat.
No. 5,513,023). These similar approaches would provide important
advantages in optical layout and performance. Neither, however, has
been realized in practice because of deficiencies in conventional
polarizing beam splitters. The disadvantages of using conventional
polarizing beam splitters in projection liquid crystal displays
includes images that are not bright, have poor contrast, and have
non-uniform color balance or non-uniform intensity (due to
non-uniform performance over the light cone). In addition, many
conventional polarizing beam splitters are short-lived because of
excessive heating, and are very expensive.
[0007] In order for such an image projection system to be
commercially successful, it must deliver images which are
significantly better than the images provided by conventional
cathode ray tube (CRT) television displays because it is likely
that such a system will be more expensive than conventional CRT
technology. Therefore, the image projection system must provide (1)
bright images with the appropriate colors or color balance; (2)
have good image contrast; and (3) be as inexpensive as possible. An
improved polarizing beam splitter (PBS) is an important part of
achieving this goal because the PBS is a limiting component which
determines the potential performance of the display system.
[0008] The PBS characteristics which significantly affect the
display performance are (1) the angular aperture, or the f-number,
at which the polarizer can function; (2) the absorption, or energy
losses, associated with the use of the PBS; and (3) the durability
of the PBS. In optics, the angular aperture or f-number describes
the angle of the light cone which the PBS can use and maintain the
desired performance level. Larger cones, or smaller f-numbers, are
desired because the larger cones allow for more light to be
gathered from the light source, which leads to greater energy
efficiency and more compact systems.
[0009] The absorption and energy losses associated with the use of
the PBS obviously affect the brightness of the system since the
more light lost in the optics, the less light remains which can be
projected to the view screen. In addition, the amount of light
energy which is absorbed by the polarizer will affect its
durability, especially in such image projection systems in which
the light passing through the optical system is very intense, on
the order of watts per square centimeter. Light this intense can
easily damage common polarizers, such as Polaroid sheets. In fact,
the issue of durability limits the polarizers which can be used in
these applications.
[0010] Durability is also important because the smaller and lighter
the projection system can be made, the less expensive and more
desirable is the product. To accomplish this goal, however, the
light intensity must be raised even higher, further stressing the
PBS, and shortening its useful life.
[0011] A problematic disadvantage of conventional PBS devices is
poor conversion efficiency, which is the primary critical
performance factor in displays. Conversion efficiency is a measure
describing how much of the electrical power required by the light
source is translated into light intensity power on the screen or
panel that is observed by people viewing it. It is expressed as the
ratio of total light power on the screen divided by the electrical
power required by the light source. The conventional units are
lumens per watt. A high ratio is desirable for a number of reasons.
For example, a low conversion efficiency will require a brighter
light source, with its accompanying larger power supply, excess
heat, larger enclosures and cabinet, etc. In addition, all of these
consequences of low conversion efficiency raise the cost of the
projection system.
[0012] A fundamental cause of low conversion efficiency is poor
optical efficiency, which is directly related to the f-number of
the optical system. A system which has an f-number which is half
the f-number of an otherwise equivalent system has the potential to
be four times as efficient in gathering light from the light
source. Therefore, it is desirable to provide an improved
polarizing beam splitter (PBS) which allows more efficient
harvesting of light energy by offering a significantly smaller
potential f-number (larger angular aperture), and therefore
increases the conversion efficiency, as measured in
lumens/watt.
[0013] There are several reasons for the poor performance of
conventional polarizing beam splitters with respect to conversion
efficiency when they are used as beam splitters in projection
systems. First, current beam splitters work poorly if the light
does not strike them at a certain angle (or at least, within a
narrow cone of angles about this principal angle of incidence).
Deviation of the principal ray from this angle causes each type of
polarizing beam splitter to degrade the intensity, the purity of
polarization, and/or the color balance. This applies to the beam
coming from the light source as well as to the beam reflected from
the liquid crystal array. This principal angle depends upon the
design and construction of the PBS as well as the physics of the
polarization mechanism employed in these various beam splitters.
Currently available polarizing beam splitters are not capable of
operating efficiently at angles far from their principal polarizing
angles in the visible portion of the electromagnetic spectrum. This
restriction makes it impossible to implement certain promising
optical layouts and commercially promising display designs.
[0014] Even if the principal ray strikes the polarizer at the best
angle for separating the two polarizations, the other rays cannot
diverge far from this angle or their visual qualities will be
degraded. This is a serious deficiency in a display apparatus
because the light striking the polarizer must be strongly
convergent or divergent to make efficient use of the light emitted
by typical light sources. This is usually expressed as the f-number
of the optical system. For a single lens, the f-number is the ratio
of the aperture to the focal length. For optical elements in
general, the F-number is defined as
F/#=1/(2 n sin .THETA.)
[0015] where n is the refractive index of the space within which
the optical element is located, and .THETA. is the half cone angle.
The smaller the F-number, the greater the radiant flux,
.PHI..sub.c, collected by the lens, and the more efficient the
device will be for displaying a bright image. The radiant flux
increases as the inverse square of the F/#. In an optical train,
the optical element with the largest F/# will be the limiting
factor in its optical efficiency. For displays using traditional
polarizers, the limiting element is nearly always the polarizer,
and thus the PBS limits the conversion efficiency. It would clearly
be beneficial to develop a type of PBS that has a smaller F/# than
any that are currently available.
[0016] Because traditional polarizers with small F/#s have not been
available, designers typically have addressed the issue of
conversion efficiency by specifying a smaller, brighter light
source. Such sources, typically arc lamps, are available, but they
require expensive power supplies that are heavy, bulky, and need
constant cooling while in operation. Cooling fans cause unwanted
noise and vibration. These features are detrimental to the utility
of projectors and similar displays. Again, a PBS with a small F/#
would enable efficient gathering of light from low-power, quiet,
conventional light sources.
[0017] Another key disadvantage of conventional polarizing beam
splitters is a low extinction, which results in poor contrast in
the image. Extinction is the ratio of the light transmitted through
the polarizer of the desired polarization to the light rejected of
the undesired polarization. In an efficient display, this ratio
must be maintained at a minimum value over the entire cone of light
passing through the PBS. Therefore, it is desirable to provide a
polarizing beam splitter which has a high extinction ratio
resulting in a high contrast image.
[0018] A third disadvantage of conventional polarizing beam
splitters is a non-uniform response over the visible spectrum, or
poor color fidelity. The result is poor color balance which leads
to further inefficiency in the projection display system as some
light from the bright colors must be thrown away to accommodate the
weaknesses in the polarizing beam splitter. Therefore, it is
desirable to provide an improved polarizing beam splitter that has
a uniform response over the visible spectrum, (or good color
fidelity) giving an image with good color balance with better
efficiency. The beam splitter must be achromatic rather than
distort the projected color, and it must not allow crosstalk
between the polarizations because this degrades image acuity and
contrast. These characteristics must apply over all portions of the
polarizer and over all angles of light incidence occurring at the
polarizer. The term spathic has been coined (R. C. Jones, Jour.
Optical Soc. Amer. 39, 1058, 1949) to describe a polarizer that
conserves cross-sectional area, solid angle, and the relative
intensity distribution of wavelengths in the polarized beam. A PBS
that serves as both a polarizer and analyzer must be spathic for
both transmission and reflection, even in light beams of large
angular aperture.
[0019] A fourth disadvantage of conventional polarizing beam
splitters is poor durability. Many conventional polarizing beam
splitters are subject to deterioration caused by excessive heating
and photochemical reactions. Therefore, it is desirable to provide
an improved polarizing beam splitter that can withstand an intense
photon flux for thousands of hours without showing signs of
deterioration. In addition, it is desirable to provide a polarizing
beam splitter that is amenable to economical, large scale
fabrication.
[0020] The need to meet these, and other, criteria has resulted in
only a few types of polarizers finding actual use in a projection
system. Many attempts have been made to incorporate both wide
angular aperture and high fidelity polarization into the same beam
splitting device. The relative success of these efforts is
described below. Thin film interference filters are the type of
polarizer cited most frequently in efforts to make a polarizing
beam splitter that is also used as an analyzer. MacNeille was the
first to describe such a polarizer that was effective over a wide
spectral range (U.S. Pat. No. 2,403,731). It is composed of
thin-film multi-layers set diagonally to the incident light,
typically within a glass cube, so it is bulky and heavy compared to
a sheet polarizer. What is more, it must be designed for a single
angle of incidence, usually 45.degree., and its performance is poor
if light is incident at angles different from this by even as
little as 2.degree.. Others have improved on the design (e.g. J.
Mouchart, J. Begel, and E. Duda, Applied Optics 28, 2847-2853,
1989; and L. Li and J. A. Dobrowolski, Applied Optics 13,
2221-2225, 1996). All of them found it necessary to seriously
reduce the wavelength range if the angular aperture is to be
increased. This can be done in certain designs (U.S. Pat. Nos.
5,658,060 and 5,798,819) in which the optical design divides the
light into appropriate color bands before it arrives at the
polarizing beam splitter. In this way, it is possible to reduce the
spectral bandwidth demands on the beam splitter and expand its
angular aperture, but the additional components and complexity add
significant cost, bulk, and weight to the system.
[0021] Even so, these improved beam splitter cubes are appearing on
the market, and are currently available from well known vendors
such as Balzers and OCLI. They typically offer an F/# of
f/2.5-f/2.8, which is a significant improvement over what was
available 2 years ago, but is still far from the range of
F/1.2-F/2.0 which is certainly within reach of the other key
components in optical projection systems. Reaching these f-numbers
has the potential to improve system efficiency by as much as a
factor of 4. They would also enable the projection display engineer
to make previously impossible design trade-offs to achieve other
goals, such as reduced physical size and weight, lower cost,
etc.
[0022] In a technology far from visible optics, namely radar, wire
grids have been used successfully to polarize long wavelength radar
waves. These wire grid polarizers have also been used as
reflectors. They are also well known as optical components in the
infrared (IR), where they are used principally as transmissive
polarizer elements.
[0023] Although it has not been demonstrated, some have postulated
possible use of a wire grid polarizer in display applications in
the visible portion of the spectrum. For example, Grinberg (U.S.
Pat. No. 4,688,897) suggested that a wire grid polarizer serve as
both a reflector and an electrode (but not simultaneously as an
analyzer) for a liquid crystal display.
[0024] Others have posed the possible use of a wire grid polarizer
in place of a dichroic polarizer to improve the efficiency of
virtual image displays (see U.S. Pat. No. 5,383,053). The need for
contrast or extinction in the grid polarizer, however, is
explicitly dismissed, and the grid is basically used as a
polarization sensitive beam steering device. It does not serve the
purpose of either an analyzer, or a polarizer, in the U.S. Pat. No.
5,383,053 patent. It is also clear from the text that a broadband
polarizing cube beam splitter would have served the purpose as
well, if one had been available. This technology, however, is
dismissed as being too restricted in acceptance angle to even be
functional, as well as prohibitively expensive.
[0025] Another patent (U.S. Pat. No. 4,679,910) describes the use
of a grid polarizer in an imaging system designed for the testing
of IR cameras and other IR instruments. In this case, the
application requires a beam splitter for the long wavelength
infra-red, in which case a grid polarizer is the only practical
solution. The patent does not suggest utility for the visible range
or even mention the need for a large angular aperture. Neither does
it address the need for efficient conversion of light into a
viewable image, nor the need for broadband performance.
[0026] Other patents also exist for wire-grid polarizers in the
infrared portion of the spectrum (U.S. Pat. Nos. 4,514,479,
4,743,093; and 5,177,635, for example). Except for the exceptions
just cited, the emphasis is solely on the transmission performance
of the polarizer in the IR spectrum.
[0027] These references demonstrate that it has been known for many
years that wire-grid arrays can function generally as polarizers.
Nevertheless, they apparently have not been proposed and developed
for image projection systems. One possible reason that wire grid
polarizers have not been applied in the visible spectrum is the
difficulty of manufacture. U.S. Pat. No. 4,514,479 teaches a method
of holographic exposure of photoresist and subsequent etching in an
ion mill to make a wire grid polarizer for the near infrared
region; in U.S. Pat. No. 5,122,907, small, elongated ellipsoids of
metal are embedded in a transparent matrix that is subsequently
stretched to align their long axes of the metal ellipsoids to some
degree. Although the transmitted beam is polarized, the device does
not reflect well. Furthermore, the ellipsoid particles have not
been made small enough to be useful in the visible part of the
electromagnetic spectrum. Accordingly, practical applications have
been generally limited to the longer wavelengths of the IR
spectrum.
[0028] Another prior art polarizer achieves much finer lines by
grazing angle evaporative deposition (U.S. Pat. No. 4,456,515).
Unfortunately, the lines have such small cross sections that the
interaction with the visible light is weak, and so the optical
efficiency is too poor for use in the production of images. As in
several of these prior art efforts, this device has wires with
shapes and spacings that are largely random. Such randomness
degrades performance because regions of closely spaced elements do
not transmit well, and regions of widely spaced elements have poor
reflectance. The resulting degree of polarization (extinction) is
less than maximal if either or both of these effects occur, as they
surely must if placement has some randomness to it.
[0029] For perfect (and near perfect) regularity, the mathematics
developed for gratings apply well. Conversely, for random wires
(even if they all have the same orientation) the theory of
scattering provides the best description. Scattering from a single
cylindrical wire has been described (H. C. Van de Hulst, Light
Scattering by Small Particles, Dover, 1981). The current
random-wire grids have wires embedded throughout the substrate. Not
only are the positions of the wires somewhat random, but the
diameters are as well. It is clear that the phases of the scattered
rays will be random, so the reflection will not be strictly
specular and the transmission will not retain high spacial or image
fidelity. Such degradation of the light beam would prevent its use
for transfer of well resolved, high-information density images.
[0030] Nothing in the prior art indicates or suggests that an
ordered array of wires can or should be made to operate over the
entire visible range as a spathic PBS, at least at the angles
required when it serves both as a polarizer and analyzer. Indeed,
the difficulty of making the narrow, tall, evenly spaced wires that
are required for such operation has been generously noted (see
Zeitner, et. al. Applied Optics, 38, 11 pp. 2177-2181 (1999), and
Schnabel, et. al., Optical Engineering 38,2 pp. 220-226 (1999)).
Therefore, it is not surprising that the prior art for image
projection similarly makes no suggestion for use of a spathic PBS
as part of a display device.
[0031] Tamada and Matsumoto (U.S. Pat. No. 5,748,368) disclose a
wire grid polarizer that operates in both the infrared and a
portion of the visible spectrum; however, it is based on the
concept that large, widely spaced wires will create resonance and
polarization at an unexpectedly short wavelength in the visible.
Unfortunately, this device works well only over a narrow band of
visible wavelengths, and not over the entire visible spectrum. It
is therefore not suitable for use in producing images in full
color. Accordingly, such a device is not practical for image
display because a polarizer must be substantially achromatic for an
image projection system.
[0032] Another reason wire grid polarizers have been overlooked is
the common and long standing belief that the performance of a
typical wire grid polarizer becomes degraded as the light beam's
angle of incidence becomes large (G.R. Bird and M. Parrish, Jr.,
"The Wire Grid as a Near-Infrared Polarizer," J. Opt. Soc. Am., 50,
pp. 886-891, (1960); the Handbook of Optics, Michael Bass, Volume
II, p. 3-34, McGraw-Hill (1995)). There are no reports of designs
that operate well for angles beyond 35.degree. incidence in the
visible portion of the spectrum. Nor has anyone identified the
important design factors that cause this limitation of incidence
angle. This perceived design limitation becomes even greater when
one realizes that a successful beam splitter will require suitable
performance in both transmission and reflection simultaneously.
[0033] This important point deserves emphasis. The extant
literature and patent history for wire grid polarizers in the IR
and the visible spectra has almost entirely focused on their use as
transmission polarizers, and not on reflective properties. Wire
grid polarizers have been attempted and reported in the technical
literature for decades, and have become increasingly common since
the 1960s. Despite the extensive work done in this field, there is
very little, if any, detailed discussion of the production and use
of wire grid polarizers as reflective polarizers, and nothing in
the literature concerning their use as both transmissive and
reflective polarizers simultaneously, as would be necessary in a
spathic polarizing beam splitter for use in imaging devices. From
the lack of discussion in the literature, a reasonable investigator
would conclude that any potential use of wire grid polarizers as
broadband visible beam splitters is not apparent, or that it was
commonly understood by the technical community that their use in
such an application was not practical.
[0034] Because the conventional polarizers described above were the
only ones available, it was impossible for Takanashi (U.S. Pat. No.
5,239,322) to reduce his projection device to practice with
anything but the most meager results. No polarizer was available
which supplied the performance required for the Takanashi
invention, namely, achromaticity over the visible part of the
spectrum, wide angular acceptance, low losses in transmission and
reflection of the desired light polarizations, and good extinction
ratio.
[0035] There are several important features of an image display
system which require specialized performance of transmission and
reflection properties. For a projector, the product of
p-polarization transmission and s-polarization reflection
(R.sub.ST.sub.P) must be large if source light is to be efficiently
placed on the screen. On the other hand, for the resolution and
contrast needed to achieve high information density on the screen,
it is important that the converse product (R.sub.PT.sub.S) be very
small (i.e. the transmission of s-polarized light multiplied by the
reflection of p-polarized light must be small).
[0036] Another important feature is a wide acceptance angle. The
acceptance angle must be large if light gathering from the source,
and hence the conversion efficiency, is maximized. It is desirable
that cones of light (either diverging or converging) with
half-angles greater than 20.degree. be accepted.
[0037] An important consequence of the ability to accept larger
light cones and work well at large angles is that the optical
design of the imaging system is no longer restricted. Conventional
light sources can be then be used, bringing their advantages of low
cost, cool operation, small size, and low weight. A wide range of
angles makes it possible for the designer to position the other
optical elements in favorable positions to improve the size and
operation of the display.
[0038] Another important feature is size and weight. The
conventional technology requires the use of a glass cube. This cube
imposes certain requirements and penalties on the system. The
requirements imposed include the need to deal with thermal loading
of this large piece of glass and the need for high quality
materials without stress birefringence, etc., which impose
additional cost. In addition, the extra weight and bulk of the cube
itself poses difficulties. Thus, it is desirable that the beam
splitter not occupy much volume and does not weigh very much.
[0039] Another important feature is robustness. Modern light
sources generate very high thermal gradients in the polarizer
immediately after the light is switched on. At best, this can
induce thermal birefringence which causes cross talk between
polarizations. What is more, the long duration of exposure to
intense light causes some materials to change properties (typically
yellowing from photo-oxidation). Thus, it is desirable for the beam
splitter to withstand high temperatures as well as long periods of
intense radiation from light sources.
[0040] Still another important feature is uniform extinction (or
contrast) performance of the beam splitter over the incident cone
of light. A McNeille-type thin film stack polarizer produces
polarized light due to the difference in reflectivity of
S-polarized light as opposed to P-polarized light. Since the
definition of S and P polarization depends on the plane of
incidence of the light ray, which changes orientation within the
cone of light incident on the polarizer, a McNeille-type polarizer
does not work equally well over the entire cone. This weakness in
McNeille-type polarizers is well known. It must be addressed in
projection system design by restricting the angular size of the
cone of light, and by compensation elsewhere in the optical system
through the use of additional optical components. This fundamental
weakness of McNeille prisms raises the costs and complexities of
current projection systems, and limits system performance through
restrictions on the f-number or optical efficiency of the beam
splitter.
[0041] Other important features include ease of alignment.
Production costs and maintenance are both directly affected by
assembly criteria. These costs can be significantly reduced with
components which do not require low tolerance alignments.
[0042] The prior patent (U.S. Pat. No. 6,234,634) advantageously
teaches the use of a wire grid polarizer as the PBS for both
polarizing and analyzing in an image projection system. However,
the wire grid polarizer itself presents various challenges. For
example, the wire grid can be fragile or susceptible to damage in
environments with high humidity, significant air pollution, or
other conditions. Thus, it is desirable to protect the wire grid.
Because wire grid polarizers are wavelength sensitive optical
devices, imbedding the polarizer in a material or medium with an
index of refraction greater than one will always change the
performance of the polarizer over that available in air for the
same structure. Typically, this change renders the polarizer less
suitable for the intended application. Imbedding the polarizer,
however, provides other optical advantages. For example, imbedding
the polarizer may provide other beneficial optical properties, and
may protect the polarizer, although the performance of the
polarizer itself, or polarization, may be detrimentally effected.
Therefore, it is desirable to obtain the optimum performance of
such an imbedded wire-grid polarizer.
[0043] Wire grids are typically disposed on an outer surface of a
substrate, such as glass. Some wire grids have been totally encased
in the substrate material, or glass. For example, U.S. Pat. No.
2,224,214, issued Dec. 10, 1940, to Brown, discloses forming a
polarizer by melting a powdered glass packed around wires, and then
stretching the glass and wires. Similarly, U.S. Pat. No. 4,289,381,
issued Sep. 15, 1981, to Garvin et al., discloses forming a
polarizer by depositing a layer of metallization on a substrate to
form the grid, and then depositing substrate material over the
grid. In either case, the wires or grid are surrounded by the same
material as the substrate. As stated above, such encasement of the
wires or grids detrimentally effects the optical performance of the
grid.
[0044] U.S. Pat. No. 5,748,368, issued May 5, 1998, to Tamada et
al., discloses a narrow bandwidth polarizer with a grid disposed on
a substrate, and a wedge glass plate disposed over the grid. A
matching oil is also applied over the elements which is matched to
have the same refractive index as the substrate. Thus, the grid is
essentially encased in the substrate or glass because the matching
oil has the same refractive index. Again, such encasement of the
grid detrimentally effects the optical performance of the gird.
[0045] The key factor that determines the performance of a wire
grid polarizer is the relationship between the center-to-center
spacing, or period, of the parallel grid elements and the
wavelength of the incident radiation. If the grid spacing or period
is long compared to the wavelength, the grid functions as a
diffraction grating, rather than as a polarizer, and diffracts both
polarizations (not necessarily with equal efficiency) according to
well-known principles. When the grid spacing or period is much
shorter than the wavelength, the grid functions as a polarizer that
reflects electromagnetic radiation polarized parallel to the grid
elements, and transmits radiation of the orthogonal
polarization.
[0046] The transition region, where the grid period is in the range
of roughly one-half of the wavelength to twice the wavelength, is
characterized by abrupt changes in the transmission and reflection
characteristics of the grid. In particular, an abrupt increase in
reflectivity, and corresponding decrease in transmission, for light
polarized orthogonal to the grid elements will occur at one or more
specific wavelengths at any given angle of incidence. These effects
were first reported by Wood in 1902 (Philosophical Magazine,
September 1902), and are often referred to as "Wood's Anomalies".
Subsequently, Rayleigh analyzed Wood's data and had the insight
that the anomalies occur at combinations of wavelength and angle
where a higher diffraction order emerges (Philosophical Magazine,
vol. 14(79), pp. 60-65, July 1907). Rayleigh developed an equation
to predict the location of the anomalies (which are also commonly
referred to in the literature as "Rayleigh Resonances").
[0047] The effect of the angular dependence is to shift the
transmission region to larger wavelengths as the angle increases.
This is important when the polarizer is intended for use as a
polarizing beam splitter or polarizing turning mirror because such
uses require high angles of incidence.
[0048] A wire grid polarizer is comprised of a multiplicity of
parallel conductive electrodes supported by a substrate. Such a
device is characterized by the pitch or period of the conductors;
the width of the individual conductors; and the thickness of the
conductors. A beam of light produced by a light source is incident
on the polarizer at an angle .THETA. from normal, with the plane of
incidence orthogonal to the conductive elements. The wire grid
polarizer divides this beam into a specularly reflected component,
and a non-diffracted, transmitted component. For wavelengths
shorter than the longest resonance wavelength, there will also be
at least one higher-order diffracted component. Using the normal
definitions for S and P polarization, the light with S polarization
has the polarization vector orthogonal to the plane of incidence,
and thus parallel to the conductive elements. Conversely, light
with P polarization has the polarization vector parallel to the
plane of incidence and thus orthogonal to the conductive
elements.
[0049] In general, a wire grid polarizer will reflect light with
its electric field vector parallel to the wires of the grid, and
transmit light with its electric field vector perpendicular to the
wires of the grid, but the plane of incidence may or may not be
perpendicular to the wires of the grid as discussed here.
[0050] Ideally, the wire grid polarizer will function as a perfect
mirror for one polarization of light, such as the S polarized
light, and will be perfectly transparent for the other
polarization, such as the P polarized light. In practice, however,
even the most reflective metals used as mirrors absorb some
fraction of the incident light and reflect only 90% to 95%, and
plain glass does not transmit 100% of the incident light due to
surface reflections.
[0051] Applicants' prior patent (U.S. Pate. No. 6,122,103) shows
transmission and reflection of a wire grid polarizer with two
resonances which only affect significantly the polarizer
characteristics for P polarization. For incident light polarized in
the S direction, the reflectivity of the polarizer approaches the
ideal. The reflection efficiency for S polarization is greater than
90% over the visible spectrum from 0.4 .mu.m to 0.7 .mu.m. Over
this wavelength band, less than 2.5% of the S polarized light is
transmitted, with the balance being absorbed. Except for the small
transmitted component, the characteristics of the wire grid
polarizer for S polarization are very similar to those of a
continuous aluminum mirror.
[0052] For P polarization, and high angle of incidence, the
transmission and reflection efficiencies of the wire grid are
affected by the resonance effect at wavelengths below about 0.5
.mu.m. At wavelengths longer than 0.5 .mu.m, the wire grid
structure acts as a lossy dielectric layer for P polarized light.
The losses in this layer and the reflections from the surfaces
combine to limit the transmission for P polarized light.
[0053] Applicants' prior patent (U.S. Pat. No. 6,122,103) also
shows the calculated performance of a different type of prior-art
wire gird polarizer, as described by Tamada in U.S. Pat. No.
5,748,368. As stated above, an index matching fluid has been used
between two substrates such that the grid is surrounded by a medium
of constant refractive index. This wire grid structure exhibits a
single resonance at a wavelength about 0.52 .mu.m. There is a
narrow wavelength region, from about 0.58 to 0.62 .mu.m, where the
reflectivity for P polarization is very nearly zero. U.S. Pat. No.
5,748,368 describes a wire grid polarizer that takes advantage of
this effect to implement a narrow bandwidth wire gird polarizer
with high extinction ratio. The examples given in the Tamada patent
specification used a grid period of 550 nm, and produced a
resonance wavelength from 800 to 950 nm depending on the grid
thickness, conductor width and shape, and the angle of incidence.
The resonance effect that Tamada exploits is different from the
resonance whose position is described above. While the two
resonances may be coincident, they do not have to be. Tamada
exploits this second resonance. Furthermore, there are thin film
interference effects which may come into play. The bandwidth of the
polarizer, where the reflectivity for the orthogonal-polarized
light is less than a few percent, is typically 5% of the center
wavelength. While this type of narrow band polarizer may have some
applications, many visible-light systems, such as liquid crystal
displays, require polarizing optical elements with uniform
characteristics over the visible-spectrum wavelengths from 400 nm
to 700 nm.
[0054] A necessary requirement for a wide band polarizer is that
the longest wavelength resonance point must either be suppressed or
shifted to a wavelength shorter than the intended spectrum of use.
The wavelength of the longest-wavelength resonance point can be
reduced in three ways. First, the grid period can be reduced.
However, reducing the grid period increases the difficulty of
fabricating the grid structure, particularly since the thickness of
the grid elements must be maintained to ensure adequate
reflectivity of the reflected polarization. Second, the incidence
angle can be constrained to near-normal incidence. However,
constraining the incidence angle would greatly reduce the utility
of the polarizer device, and preclude its use in applications such
as projection liquid crystal displays where a wide angular
bandwidth centered on 45 degrees is desired. Third, the refractive
index of the substrate could be lowered. However, the only
cost-effective substrates available for volume manufacture of a
polarizer device are several varieties of thin sheet glass, such as
Corning type 1737F or Schott type AF45, all of which have a
refractive index which varies between 1.5 and 1.53 over the visible
spectrum.
[0055] As stated above, the wire grid polarizer can include a
multiplicity of parallel conductive electrodes supported by a
substrate. The substrate itself, however, can have certain optical
consequences that can limit the utility of a wire grid polarizer
used in such an image display described above. For example, the
substrate can cause aberrations of astigmatism and coma if a
non-collimated beam of light passes through the substrate tilted at
an angle. One reason cube polarizing beam splitters are sometimes
used is because light enters such cube polarizers with the optic
axis normal to the cube surface, thus minimizing these
aberrations.
[0056] Light striking the substrate at other than normal incidence
can suffer from a lateral shift in position along the sloping
direction of the substrate. Consequently, a diverging light cone
striking the substrate suffers astigmatic aberration and coma
causing the otherwise round area of the beam to become elongated in
one direction. This, combined with chromatic aberration (color
separation) as polychromatic light disperses through the tilted
substrate, causes unacceptable distortion in high quality imaging
optical systems. These aberrations occur regardless of the flatness
of the substrate. Therefore, flat plate transmissive optics cannot
be used in imaging applications unless the aberrations are
corrected or rendered negligible.
SUMMARY OF THE INVENTION
[0057] It has been recognized that it would be advantageous to
develop an image projection system capable of providing bright
images and good image contrast, and which is inexpensive. It also
has been recognized that it would be advantageous to develop an
image projection system with a polarizing beam splitter that
reduces aberrations of astigmatism and coma, and/or that produces a
transmitted or reflected beam with reduced geometric distortions.
It also has been recognized that it would be advantageous to
develop an image projection system with a polarizing beam splitter
which is protected against environmental degradation and other
sources of damage while reducing detrimental effects of the
protection on the performance of the beam splitter.
[0058] It also has been recognized that it would be advantageous to
develop an image projection system with a polarizing beam splitter
capable of utilizing divergent light (or having a smaller F/#),
capable of efficient use of light energy or with high conversion
efficiency, and which is durable. It also has been recognized that
it would be advantageous to develop an image projection system with
a polarizing beam splitter having a high extinction ratio, uniform
response over the visible spectrum, good color fidelity, that is
spathic, robust and capable of resisting thermal gradients.
[0059] It also has been recognized that it would be advantageous to
develop an image projection system with a polarizing beam splitter
capable of being positioned at substantially any incidence angle so
that significant design constraints are not imposed on the image
projection system, but substantial design flexibility is permitted.
It also has been recognized that it would be advantageous to
develop an image projection system with a polarizing beam splitter
which efficiently transmits p-polarized light and reflects
s-polarized light across all angles in the entire cone of incident
light. It also has been recognized that it would be advantageous to
develop an image projection system with a polarizing beam splitter
which is light-weight and compact. It also has been recognized that
it would be advantageous to develop an image projection system with
a polarizing beam splitter which is easy to align.
[0060] The invention provides an image projection system with a
polarizing beam splitter which advantageously is a wire grid
polarizer. The wire grid polarizing beam splitter has a generally
parallel arrangement of thin, elongated elements. The arrangement
is configured, and the elements are sized, to interact with
electromagnetic waves of the source light beam to generally
transmit one polarization of light through the elements, and
reflect the other polarization from the elements. Light having a
polarization oriented perpendicular to a plane that includes at
least one of the elements and the direction of the incident light
beam is transmitted, and defines a transmitted beam. The opposite
polarization, or light having a polarization oriented parallel with
the plane that includes at least one of the elements and the
direction of the incident light beam, is reflected, and defines a
reflected beam.
[0061] The system includes a light source for producing a visible
light beam. The polarizing beam splitter is located proximal to the
light source in the light beam. The system also includes a
reflective liquid crystal array. The array may be located proximal
to the polarizing beam splitter in either the reflected or
transmitted beam. The array modulates the polarization of the beam,
and creates a modulated beam. The array is oriented to direct the
modulated beam back to the beam splitter. The arrangement of
elements of the beam splitter interacts with electromagnetic waves
of the modulated beam to again generally transmit one polarization
and reflect the other polarization. Thus, the reflected portion of
the modulated beam defines a second reflected beam, while the
transmitted portion defines a second transmitted beam. The array
alters the polarization of the beam to encode image information on
the modulated beam. The beam splitter separates the modulated
polarization from the unmodulated beam, thus making the image
visible on a screen.
[0062] A screen is disposed in either the second reflected or
second transmitted beam. If the array is disposed in the reflected
beam, then the screen is disposed in the second transmitted beam.
If the array is disposed in the transmitted beam, then the screen
is disposed in the second reflected beam.
[0063] Unlike the bulky, heavy beam splitters of the prior art, the
beam splitter of the present invention is a generally planar sheet.
The beam splitter is also efficient, thus providing greater
luminous efficacy of the system.
[0064] In accordance with one aspect of the present invention, the
beam splitter advantageously includes an embedded wire grid
polarizer with an array of parallel, elongated, spaced-apart
elements sandwiched between first and second layers. The elements
form a plurality of gaps between the elements, and the gaps
advantageously provide a refractive index less than the refractive
index of the first or second layer. Preferably, the gaps include
air or have a vacuum.
[0065] In accordance with another aspect of the present invention,
the elements of the wire grid polarizer can be disposed on a
substrate. Preferably, the substrate is very thin, or has a
thickness less than approximately 5 millimeters, to reduce
astigmatism, coma, and/or chromatic aberrations. In addition, the
wire grid polarizer and substrate preferably transmits a
transmitted beam with reduced geometric distortions, preferably
less than approximately 3 standard wavelengths per inch.
[0066] In accordance with another aspect of the present invention,
the substrate preferably has a surface with a flatness less than
approximately 3 standard wavelengths deviation per inch to reduce
distortions in the reflected beam.
[0067] In accordance with another aspect of the present invention,
the beam splitter is capable of being oriented with respect to the
light beam and the modulated beam at incidence angles between
approximately 0 to 80 degrees.
[0068] In accordance with another aspect of the present invention,
the light beam has a useful divergent cone with a half angle
between approximately 10 and 25.degree.. The beam splitter is used
at a small F-number, preferably between approximately 1.2 and
2.5.
[0069] In accordance with another aspect of the present invention,
the beam splitter has a conversion efficiency of at least 50%
defined by the product of the s-polarization reflected light and
the p-polarization transmitted light (R.sub.ST.sub.P). In addition,
the s-polarization transmitted light and the p-polarization
reflected light are both less than 5%. Furthermore, the percentage
of reflected light and the percentage of the transmitted light of
the modulated beam is greater than approximately 67%.
[0070] In accordance with another aspect of the present invention,
the system may include a pre-polarizer disposed between the light
source and the beam splitter, and/or a post-polarizer disposed
between the beam splitter and the screen.
[0071] Additional features and advantages of the invention will be
apparent from the detailed description which follows, taken in
conjunction with the accompanying drawings, which together
illustrate, by way of example, features of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0072] FIG. 1a is a schematic view of the general operation of a
preferred embodiment of an image projection system of the present
invention using a wire grid polarizing beam splitter of the present
invention.
[0073] FIGS. 1b and 1c are schematic views of the image projection
system of the present invention in different configurations.
[0074] FIG. 2a is a graphical plot showing the relationship between
wavelength and transmittance for both S and P polarizations of a
preferred embodiment of the wire grid polarizing beam splitter of
the present invention.
[0075] FIG. 2b is a graphical plot showing the relationship between
wavelength and reflectance for both S and P polarizations of a
preferred embodiment of the wire grid polarizing beam splitter of
the present invention.
[0076] FIG. 2c is a graphical plot showing the relationship between
wavelength, efficiency and transmission extinction of a preferred
embodiment of the wire grid polarizing beam splitter of the present
invention.
[0077] FIG. 3 is a graphical plot showing the performance of the
preferred embodiment of the wire grid polarizing beam splitter of
the present invention as a function of the incident angle.
[0078] FIG. 4a is a graphical plot showing the theoretical
throughput performance of an alternative embodiment of the wire
grid polarizing beam splitter of the present invention.
[0079] FIG. 4b is a graphical plot showing the theoretical
extinction performance of an alternative embodiment of the wire
grid polarizing beam splitter of the present invention.
[0080] FIG. 4c is a graphical plot showing the theoretical
extinction performance of an alternative embodiment of the wire
grid polarizing beam splitter of the present invention.
[0081] FIG. 5a is a schematic view of the general operation of an
alternative embodiment of an image projection system of the present
invention.
[0082] FIGS. 5b and 5c are schematic views of the image projection
system of the present invention in different configurations.
[0083] FIG. 6 is a schematic view of the general operation of an
alternative embodiment of an image projection system of the present
invention.
[0084] FIG. 7 is a perspective view of the wire grid polarizing
beam splitter of the present invention.
[0085] FIG. 8 is a cross sectional side view of the wire grid
polarizing beam splitter of the present invention.
[0086] FIG. 9 is a cross-sectional view of an embedded wire grid
polarizer of the present invention.
DETAILED DESCRIPTION
[0087] For the purposes of promoting an understanding of the
principles of the invention, reference will now be made to the
exemplary embodiments illustrated in the drawings, and specific
language will be used to describe the same. It will nevertheless be
understood that no limitation of the scope of the invention is
thereby intended. Any alterations and further modifications of the
inventive features illustrated herein, and any additional
applications of the principles of the invention as illustrated
herein, which would occur to one skilled in the relevant art and
having possession of this disclosure, are to be considered within
the scope of the invention.
[0088] As illustrated in FIG. la, a display optical train of an
image projection system of the present invention, indicated
generally at 10, is shown. The image projection system 10
advantageously has a wire grid polarizer as the beam splitter,
indicated generally at 14. The wire grid polarizing beam splitter
14 (WGP-PBS) efficiently reflects light of one polarization from a
source 20 to a reflective liquid crystal array 26, and then
efficiently transmits reflected light of the opposite polarization
to a display screen 25.
[0089] For adequate optical efficiency, the WGP-PBS 14 must have
high reflectivity (R.sub.S) of the desired polarization from the
light source 20, and it must have high transmissivity (T.sub.P) of
the opposite polarization from the liquid crystals array 26. The
conversion efficiency is proportional to the product of these two,
R.sub.ST.sub.P, so deficiency in one factor can be compensated to
some extent by improvement in the other.
[0090] Examples of wire grid polarizing beam splitters 14 of the
present invention advantageously show the following characteristics
which demonstrate the advantage of using a WGP-PBS 14 of the
present invention as both the polarizer and analyzer in display
devices for the visible portion of the spectrum. Theoretical
calculations of further improvements indicate that even better
polarizing beam splitters will be available.
[0091] Referring to FIGS. 2a and 2b, the measured transmissivity
and reflectivity, respectively, for both S and P polarizations of a
WGP-PBS are shown. In FIG. 2c, the efficiency of the WGP-PBS is
shown as the product of the transmissivity and reflectivity. In
addition, the extinction is also shown in FIG. 2c. In FIGS. 2a-2c,
the WGP-PBS is oriented to reflect the s-polarization and transmit
the p-polarization at incident angles of 30.degree., 45.degree. and
60.degree.. For an image projection system, such as a projector,
the product of the reflected s-polarization and transmitted
p-polarization (R.sub.ST.sub.P) must be large if source light is to
be efficiently placed on the screen. On the other hand, for the
resolution needed to achieve high information density on the
screen, it is important that the converse product (R.sub.PT.sub.S)
be very small (i.e. the transmission of s-polarized light
multiplied by the reflection of p-polarized light must be small).
It is clear from the figures that the wire grid polarizing beam
splitter of the present invention meets these standards over the
entire spectrum without degradation by Rayleigh resonance or other
phenomena.
[0092] Another important feature is a wide acceptance angle. This
must be large if light gathering from the source, and hence the
conversion efficiency, is maximized. Referring to FIG. 3, the
performance of the wire grid polarizing beam splitter of the
present invention is shown for various portions of the light cone
centered around the optical axis which is inclined at 45.degree..
In FIG. 3, the first referenced angle is the angle in the plane of
incidence while the second referenced angle is the angle in the
plane perpendicular to the plane of incidence. It is clear that the
WGP-PBS of the present invention is able to accept cones of light
(either diverging or converging) with half-angles between
approximately 10 and 25.degree..
[0093] Referring to FIGS. 4a-4c, theoretical calculations for an
alternative embodiment of a wire grid polarizing beam splitter
indicate that significantly larger light cones and/or other
enhancements will be possible. FIGS. 4a and 4b show the theoretical
throughput and extinction, respectively, of a wire grid polarizing
beam splitter with a period p reduced to 130 nm. In addition, the
grid height or thickness is 130 nm; the line-spacing ratio is 0.48;
the substrate groove depth is 50 nm; and the substrate is BK7
glass. It should be noted in FIG. 4a that the throughput is grouped
much more closely than the throughput shown in FIG. 2c. Therefore,
performance can be improved by reducing the period p. It should be
noted in FIG. 4b that the extinction is significantly increased in
comparison to FIG. 2c.
[0094] FIG. 4c shows the theoretical extinction of another
alternative embodiment of the wire grid polarizing beam splitter
with the period p further reduced. The wavelength is 420 nm and the
incidence angle is 30.degree.. It should be noted that the
extinction increases markedly as the period p is reduced.
[0095] As indicated above, an important consequence of the ability
to accept larger light cones with a WGP-PBS that will work well at
large angles is that the PBS no longer restricts the optical design
of the imaging system. Thus, conventional light sources can be
used, with the advantage of their low cost, cooler operation, small
size, and low weight. The wide range of angles over which the
WGP-PBS works well makes it possible for the designer to position
the other optical elements in favorable positions to improve the
size and operation of the display. Referring to FIGS. 1b and 1c,
the design flexibility provided by the wide range of angles of the
PBS of the present invention is demonstrated. As shown in FIG. 1b,
the light source 20 and array 26 may be positioned closer together,
with both having a relatively small incident angle with respect to
the PBS 14. Such a configuration is advantageous for a compact
design of the components of the system 10. Alternatively, as shown
in FIG. 1c, the light source 20 and array 26 may be positioned
farther apart, with both having a relatively large incident angle.
In should be noted that in either case, the incidence angles vary
greatly from the 45 degree angle typically required by traditional
beam splitters.
[0096] Yet other features of wire grids provide advantages for
display units. The conventional technology requires the use of a
glass cube. This cube imposes certain requirements and penalties on
the system. The requirements imposed include the need to deal with
thermal loading of this large piece of glass, the need for high
quality materials without stress birefringence, etc., which impose
additional cost, and the extra weight and bulk of the cube itself.
The WGP-PBS of the present invention advantageously is a divided or
patterned thin film that does not occupy much volume and does not
weigh very much. It can even be integrated with or incorporated
into other optical elements such as color filters, to further
reduce part count, weight, and volume of the projection system.
[0097] The WGP-PBS of the present invention is also very robust.
Modern light sources generate very high thermal gradients in the
polarizer immediately after the light is switched on. At best, this
can induce thermal and stress birefringence which causes cross talk
between polarizations. At worst, it can delaminate multilayer
polarizers or cause the cemented interface in a cube beam splitter
to separate. What is more, the long duration of exposure to intense
light causes some materials to change properties (typically
yellowing from photo-oxidation). However, wire grid polarizing beam
splitters are made of chemically inert metal that is well attached
to glass or other substrate materials. They have been shown to
withstand high temperatures as well as long periods of intense
radiation from light sources.
[0098] The WGP-PBS of the present invention also is easy to align.
It is a single part that needs to be adjusted to direct the source
beam onto the liquid crystal array. This is the same simple
procedure that would be used for a flat mirror. There is another
adjustment parameter, namely, the angular rotation about the normal
to the WGP surface. This determines the orientation of polarization
in the light beam. This adjustment is not critical because the WGP
functions as its own analyzer and cannot be out of alignment in
this sense. If there are other polarizing elements in the optical
train, the WGP-PBS should be oriented with respect to their
polarization, but slight misalignment is not critical because:
according to Malus' law, angular variation makes very little
difference in the intensity transmitted by polarizers if their
polarization axes are close to being parallel (or
perpendicular).
[0099] In order to be competitive with conventional polarizers, the
product R.sub.ST.sub.P must be above approximately 50%. This
represents a lower estimate which would only be practical if the
WGP-PBS was able to gather significantly more light from the light
source than the conventional polarizing beam splitters. The
estimate of 50% comes from an assumption that the best conventional
beam splitter, a modern MacNeille cube beam splitter, can deliver
an f/4 of about f/2.5 at best. An optical system which was twice as
fast, or capable of gathering twice as much light, would then have
an f/# of 1/{square root}2 of this value, or about f/1.8, which is
certainly a reasonable f/# in optical image projection systems. A
system which is twice as fast, and therefore capable of gathering
twice the light from the source, would approximately compensate for
the factor of 2 decrease in the R.sub.ST.sub.P product over the
conventional cube beam splitter, resulting in an equivalent
projection system performance. In fact, since a WGP-PBS can
potentially be used down below f/1.2 (a factor of four increase)
this seemingly low limit can still produce very bright images. Of
course, an R.sub.ST.sub.P product which is over this minimum value
will provide even better performance.
[0100] Another important performance factor is contrast in the
image, as defined by the ratio of intensities of light to dark
pixels. One of the significant advantages of the WGP-PBS is the
improved contrast over compound incident angles in comparison to
the prior art cube beam splitter such as a McNeille prism. The
physics of the McNeille prism polarizes light by taking advantage
of the difference in reflectivity of S vs. P polarization at
certain angles. Because S and P polarization are defined with
respect to the plane of incidence, the effective S and P
polarization for a particular ray in a cone of light rotates with
respect to the ray along the optical axis as various rays within
the cone of light are considered. The consequence of this behavior
is the well-known compound angle problem in which the extinction of
the polarizer is significantly reduced for certain ranges of angles
within the cone of light passing through the polarizing beam
splitter, significantly reducing the average contrast over the
cone.
[0101] The WGP-PBS, on the other hand, employs a different physical
mechanism to accomplish the polarization of light which largely
avoids this problem. This difference in behavior is due to the fact
that the polarization is caused by the wire grids in the beam
splitter which have the same orientation in space regardless of the
plane of incidence for any particular ray in the cone of light.
Therefore, even though the plane of incidence for any particular
ray is the same when incident on a McNeille prism or a WGP, the
polarization effect is only dependent on the plane of incidence in
the case of the McNeille prism, meaning the compound angle
performance of the WGP is much improved over that provided by the
cube beam splitter.
[0102] The fact that the function of the WGP-PBS is independent of
the plane of incidence means that the WGP-PBS can actually be used
with the wires or elements oriented in any direction. The preferred
embodiment of the invention has the elements oriented parallel to
the axis around which the polarizer is tilted so that the light
strikes the WGP-PBS at an angle. This particular orientation is
preferred because it causes the polarization effects of the surface
reflections from the substrate to be additive to the polarization
effects from the grid. It is possible, however, to produce a
WGP-PBS which functions to reflect the P-polarization and transmit
the S-polarization (which is exactly opposite what has been
generally described herein) over certain ranges of incident angles
by rotating the grid elements so they are perpendicular to the tilt
axis of the WGP-PBS. Similarly, the grid elements can be placed at
an arbitrary angle to the tilt axis to obtain a WGP-PBS which
functions to transmit and reflect light with polarizations aligned
with the projection of this arbitrary angle onto the wavefront in
the light beam. It is therefore clear that WGP-PBS which reflect
the P-polarization and transmit the S-polarization, or which
reflect and transmit light with polarization oriented at arbitrary
angles are included within this invention.
[0103] The compound angle performance advantage of the WGP-PBS
provides an inherently more uniform contrast over the entire light
cone, and is one of the reasons the WGP is suitable for very small
f-numbers. But, of course, it is not the only factor affecting the
image contrast. The image contrast is governed to a large extent by
low leakage of the undesired polarization, but in this case the
product T.sub.SR.sub.P is not the important parameter, because the
image generating array which lies in sequence after the first
encounter with the beam splitter but before the second also takes
part in the production of the image contrast. Therefore, the final
system contrast will depend on the light valve performance as well
as the polarizer extinction. However, lower bounds on the required
beam splitter performance can be determined with the assumption
that the light valve performance is sufficient enough that it can
be assumed to have an essentially infinite contrast. In this case,
the system contrast will depend entirely on the beam splitter
performance.
[0104] Referring to FIG. la, there are two different functions
fulfilled by the beam splitter 14. The first is the preparation of
the polarized light before it strikes the liquid crystal array 26
or other suitable image generation device. The requirement here is
that the light be sufficiently well polarized that any variations
in the polarization of the light beam created by the light valve
can be adequately detected or analyzed such that the final image
will meet the desired level of performance. Similarly, the beam
splitter 14 must have sufficient performance to analyze light which
is directed by the light valve back to the beam splitter so that
the desired system contrast performance is achieved.
[0105] These lower bounds can be determined fairly easily. For
reasons of utility and image quality, it is doubtful that an image
with a contrast of less than 10:1 (bright pixel to adjacent dark
pixel) would have much utility. Such a display would not be useful
for dense text, for example. If a minimum display system contrast
of 10:1 is assumed, then an incident beam of light is required
which has at least 10 times the light of the desired polarization
state over that of the undesired polarization state. In terms of
polarizer performance, this would be described as having an
extinction of 10:1 or of simply 10.
[0106] The second encounter with the beam splitter 14 which is
going to analyze the image, must be able to pass the light of the
right polarization state, while eliminating most of the light of
the undesired state. Again, assuming from above a light beam with
an image encoded in the polarization state, and that this light
beam has the 10:1 ratio assumed, then a beam splitter is desired
which preserves this 10:1 ratio to meet the goal of a system
contrast of 10:1. In other words, it is desired to reduce the light
of the undesired polarization by a factor of 10 over that of the
right polarization. This again leads to a minimum extinction
performance of 10:1 for the analysis function of the beam
splitter.
[0107] Clearly, higher system contrast will occur if either or both
of the polarizer and analyzer functions of the beam splitter have a
higher extinction performance. It is also clear that it is not
required that the performance in both the analyzer function and the
polarizer function of the beam splitter be matched for a image
projection system to perform adequately. An upper bound on the
polarizer and analyzer performance of the beam splitter is more
difficult to determine, but it is clear that extinctions in excess
of approximately 20,000 are not needed in this application. A good
quality movie projection system as found in a quality theater does
not typically have an image contrast over about 1000, and it is
doubtful that the human eye can reliably discriminate between an
image with a contrast in the range of several thousand and one with
a contrast over 10,000. Given a need to produce an image with a
contrast of several thousand, and assuming that the light valves
capable of this feat exist, an upper bound on the beam splitter
extinction in the range of 10,000-20,000 would be sufficient.
[0108] The above delineation of the minimum and maximum bounds on
the wire grid beam splitter is instructive, but as is clear from
the demonstrated and theoretical performance of a wire grid beam
splitter as shown above, much better than this can be achieved. In
accordance with this information, the preferred embodiment has
R.sub.ST.sub.P.gtoreq.65%, and R.sub.P or T.sub.S or both are
.gtoreq.67%, as shown in FIGS. 2a-2c. The preferred embodiment
would also employ the wire grid polarizing beam splitter in the
mode where the reflected beam is directed to the image generating
array, with the array directing the light back to the beam splitter
such that it passes through, or is transmitted through, the beam
splitter. This preferred embodiment is shown in FIG. 1a.
[0109] Alternatively, as shown in the image display system 60 of
FIG. 5a, the wire grid polarizing beam splitter 14 may efficiently
transmit light of one polarization from the source 20 to the
reflective liquid crystal array 26, and then efficiently reflect
the reflected light of the opposite polarization to the display
screen 25. The second embodiment of the image projection system 60
is similar to that of the preferred embodiment shown in FIG. 1a,
with the exception that the beam splitter 14 would be employed in a
manner in which the source beam of light is transmitted or passed
through the beam splitter 14 and directed at the image generating
array 26, then is reflected back to the beam splitter 14 where it
is reflected by the beam splitter and analyzed before being
displayed on the screen 25.
[0110] Again, referring to FIGS. 5b and 5c, the design flexibility
provided by the wide range of angles of the PBS of the present
invention is demonstrated. As shown in FIG. 5b, the array 26 and
screen 25 may be positioned closer together, with both having a
relatively small incident angle with respect to the PBS 14.
Alternatively, as shown in FIG. 5c, the array 26 and screen 25 may
be positioned farther apart, with both having a relatively large
incident angle.
[0111] As shown in FIG. 6, a third embodiment of image projection
system 80 provides an alternative system design which may assist in
achieving a desired level of system performance. This third
embodiment would include one or more additional transmissive or
reflective polarizers which work in series with the wire grid
polarizing beam splitter to increase the extinction of either or
both of the polarizing and analyzing functions to achieve the
necessary system contrast performance. Another reason for
additional polarizers would be the implementation of a polarization
recovery scheme to increase the system efficiency. A pre-polarizer
82 is disposed in the source light beam between the light source 20
and the WGP-PBS 14. A post-polarizer or clean-up polarizer 84 is
disposed in the modulated beam, or the beam reflected from the
array 26, between the array 26 and the screen 25, or between the
WGP-PBS 14 and the screen 25. The third embodiment would still
realize the advantages of the wire grid beam splitter's larger
light cone, durability, and the other advantages discussed
above.
[0112] As shown in the figures, the image display system may also
utilize light gathering optics 90 and projection optics 92.
[0113] Referring to FIGS. 7 and 8, the wire grid polarizing beam
splitter 14 of the present invention is shown in greater detail.
The polarizing beam splitter is further discussed in greater detail
in co-pending U.S. application Ser. No. 09/390,833, filed Sep. 7,
1999, entitled "Polarizing Beam Splitter", which is herein
incorporated by reference.
[0114] As described in the co-pending application, the polarizing
beam splitter 14 has a grid 30, or an array of parallel, conductive
elements, disposed on a substrate 40. The source light beam 130
produced by the light source 20 is incident on the polarizing beam
splitter 14 with the optical axis at an angle .THETA. from normal,
with the plane of incidence preferably orthogonal to the conductive
elements. An alternative embodiment would place the plane of
incidence at an angle 73 to the plane of conductive elements, with
.THETA. approximately 45.degree.. Still another alternative
embodiment would place the plane of incidence parallel to the
conductive elements. The polarizing beam splitter 14 divides this
beam 130 into a specularly reflected component 140, and a
transmitted component 150. Using the standard definitions for S and
P polarization, the light with S polarization has the polarization
vector orthogonal to the plane of incidence, and thus parallel to
the conductive elements. Conversely, light with P polarization has
the polarization vector parallel to the plane of incidence and thus
orthogonal to the conductive elements.
[0115] Ideally, the polarizing beam splitter 14 will function as a
perfect mirror for the S polarized light, and will be perfectly
transparent for the P polarized light. In practice, however, even
the most reflective metals used as mirrors absorb some fraction of
the incident light, and thus the WGP will reflect only 90% to 95%,
and plain glass does not transmit 100% of the incident light due to
surface reflections.
[0116] The key physical parameters of the wire grid beam splitter
14 which must be optimized as a group in order to achieve the level
of performance required include: the period p of the wire grid 30,
the height or thickness t of the grid elements 30, the width w of
the grids elements 30, and the slope of the grid elements sides. It
will be noted in examining FIG. 8 that the general cross-section of
the grid elements 30 is trapezoidal or rectangular in nature. This
general shape is also a necessary feature of the polarizing beam
splitter 14 of the preferred embodiment, but allowance is made for
the natural small variations due to manufacturing processes, such
as the rounding of corners 50, and fillets 54, at the base of the
grid elements 30.
[0117] It should also be noted that the period p of the wire grid
30 must be regular in order to achieve the specular reflection
performance required to meet the imaging fidelity requirements of
the beam splitter 14. While it is obviously better to have the grid
30 completely regular and uniform, some applications may have
relaxed requirements in which this is not as critical. However, it
is believed that a variation in period p of less than 10% across a
meaningful dimension in the image (such as the size of a single
character in a textual display, or a few pixels in an image) is
required to achieve the necessary performance.
[0118] Similarly, reasonable variations across the beam splitter 14
in the other parameters described, such as the width w of the grid
elements 30, the grid element height t, the slopes of the sides, or
even the corner rounding 50, and the fillets 54, are also possible
without materially affecting the display performance, especially if
the beam splitter 14 is not at an image plane in the optical
system, as will often be the case. These variations may even be
visible in the finished beam splitter 14 as fringes, variations in
transmission efficiency, reflection efficiency, color uniformity,
etc. and still provide a useful part for specific applications in
the projection imaging system.
[0119] The design goal which must be met by the optimization of
these parameters is to produce the best efficiency or throughput
possible, while meeting the contrast requirements of the
application. As stated above, the minimum practical extinction
required of the polarizing beam splitter 14 is on the order of 10.
It has been found that the minimum required throughput
(R.sub.ST.sub.P) of the beam splitter 14 in order to have a
valuable product is approximately 50%, which means either or both
of R.sub.P and T.sub.S must be above about 67%. Of course, higher
performance in both the throughput and the extinction of the beam
splitter will be of value and provide a better product. In order to
understand how these parameters affect the performance of the wire
grid beam splitter, it is necessary to examine the variation in
performance produced by each parameter for an incident angle of
45.degree., and probably other angles of interest.
[0120] The performance of the wire grid beam splitter 14 is a
function of the period p. The period p of the wire grid elements 30
must fall under approximately 0.21 .mu.m to produce a beam splitter
14 which has reasonable performance throughout the visible
spectrum, though it would be obvious to those skilled in the art
that a larger period beam splitter would be useful in systems which
are expected to display less than the full visible spectrum, such
as just red, red and green, etc.
[0121] The performance of the wire grid beam splitter 14 is a
function of the element height or thickness t. The wire-grid height
t must be between about 0.04 and 0.5 .mu.m in order to provide the
required performance.
[0122] The performance of the wire grid beam splitter 14 is a
function of the width to period ratio (w/p) of the elements 30. The
width w of the grid element 30 with respect to the period p must
fall within the ranges of approximately 0.3 to 0.76 in order to
provide the required performance.
[0123] The performance of the wire grid beam splitter 14 is a
function of the slopes of the sides of the elements 30. The slopes
of the sides of the grid elements 30 preferably are greater than 68
degrees from horizontal in order to provide the required
performance.
[0124] As indicated above, other factors can effect the performance
and/or durability of the WG-PBS. For example, WG-PBS may be
subjected to strenuous optical environments, such as high flux
illumination and other physically harsh conditions, for long
periods of time, which can effect the durability of the WG-PBS.
Thus, it is desirable to protect the WG-PBS. As stated above,
however, embedding the polarizer in a material or medium with an
index of refraction greater than one will always change the
performance of the polarizer over that available in air in the same
structure. Therefore, it is desirable to protect the polarizer,
while optimizing its performance.
[0125] As illustrated in FIG. 9, an embedded wire grid polarizer of
the present invention is shown, indicated generally at 200. The
polarizer 200 includes a first optical medium, material, layer or
substrate 201; a second optical medium, material or layer 203; and
a plurality of intervening elongated elements 205 sandwiched
between the first and second layers 201 and 203. As indicated
above, although certain advantages are obtained by encasing or
imbedding the elements, the polarization or performance of the
elements is detrimentally effected. Thus, the polarizer 10 of the
present invention is designed to optimize the performance when
imbedded, as discussed below.
[0126] The first and second layers 201 and 203 have respective
first and second surfaces 202 and 204 which face one another and
the elements 205. The layers 201 and 203, or material of the
layers, also have respective first and second refractive indices.
The first and second optical mediums 201 and 203 each have a
thickness t.sub.L1 and t.sub.L2, and are considered to be thick in
an optical sense. They may be, for example, sheets of glass or
polymer, an optical quality oil or other fluid, or other similar
optical materials. The thickness t.sub.L1 or t.sub.L2 may be
anywhere from a few microns to essentially infinite in extent.
Preferably, the thickness t.sub.L1 and t.sub.L2 of the layers 201
and 203 is greater than 1 micron. The optical media 201 and 203 may
be the same materials, such as two sheets of glass, or they may be
chosen to be different materials, such as an oil for material 203
and glass for material 201. The elements 205 may be supported by
the first layer or the substrate 201.
[0127] The intervening array of elements 205 includes a plurality
of parallel, elongated, spaced-apart, conductive elements 205. The
elements 205 have first and second opposite surfaces 205a and 205b,
with the first surfaces 205a facing towards the first surface 202
or first layer 201, and second surfaces 205b facing towards the
second surface 204 or second layer 203. The first surfaces 205a of
the elements 205 may contact and be coupled to the first surface
202 of the first layer 201, while the second surfaces 205b may
contact and be coupled to the second surface 204 of the second
layer 203, as shown in FIG. 9. The array of elements 205 is
configured to interact with electromagnetic waves of light in the
visible spectrum to generally reflect most of the light of a first
polarization, and transmit most of the light of a second
polarization.
[0128] The dimensions of the elements 205, and the dimensions of
the arrangement of elements 205, are determined by the wavelength
used, and are tailored for broad or full spectrum visible light.
The elements 205 are relatively long and thin. Preferably, each
element 205 has a length that is generally larger than the
wavelength of visible light. Thus, the elements 205 have a length
of at least approximately 0.7 .mu.m (micrometers or microns). The
typical length, however, may be much larger. In addition, the
elements 205 are located in generally parallel arrangement with the
spacing, pitch, or period P of the elements smaller than the
wavelength of light. Thus, the pitch will be less than 0.4 .mu.m
(micrometers or microns).
[0129] The period of the elements 205, and the choices of the
materials for the optical mediums 201 and 203 are all made to
obtain and enhance the desired interactions with the light rays
209, 211 and 213. The ray of light 209 is typically an unpolarized
beam of light containing roughly equal amounts of the two
polarizations known in the field as S polarization and P
polarization. However, the ray of light 209 may be altered in
specific applications to be partially or mostly of either
polarization as well. The period P of the elements 205 is chosen
such that the wire grid will specularly reflect most of the light
of the S polarized light 211, and transmit most of the P polarized
light 213.
[0130] The optical materials also are chosen to aid in this
process. For example, it is possible to choose optical material 201
to be equally transmissive to both S and P polarizations, while
choosing an optical material 203 that would absorb S polarized
light or otherwise aid in the transmission of P polarized light and
the reflection of S polarized light. In the preferred embodiment,
the optical material composing the layers 201 and 203 is glass.
Other materials are also suitable depending on the particular
application. For example, the second layer 203 can be a sheet of
glass or plastic. In addition the second layer can be a layer of
vacuum deposited film, or optical thin film, such as silicon
dioxide, silicon nitride, magnesium fluoride, titanium oxide, etc.
The second layer 203 also can be formed by chemically treating the
surface of the elements and first layer to leave a thin film, such
as hexamethyl disilazane. Such a layer can be one or several atomic
monolayers that is less sensitive to the environmental conditions.
Alternatively, this chemical treatment may be chosen to actively
impede the physical mechanisms which cause the damage to the wire
grid structure in the harsh environment. The second layer also can
include a plurality of films of material mentioned herein.
[0131] The intervening elongated elements 205 are not very large.
They will typically be arranged in a regular, ordered array having
a period P on the order of 0.3 .mu.m or less, with the width
w.sub.R of the ribs 205 and the width w.sub.S of the spaces or gaps
207 separating the elements on the order of 0.15 .mu.m or less. The
width of the elements 205 and the spaces 207 can be varied to
achieve desired optical performance effects, which will be further
described below. The height or thickness t.sub.R of these elements
205 will typically be between that required for the elements to be
optically opaque (approximately 40 nm in the case of aluminum) up
to a height of perhaps 1 .mu.m. The upper bound is fixed by
considerations of manufacturing practicality as well as optical
performance. In the preferred embodiment, the elements 205 are
typically composed of materials such as aluminum or silver if the
polarizer is to be used across the entire visible spectrum.
However, if it is only required in a particular case to provide a
polarizer which performs in a portion of the spectrum, such as in
red light, then other materials such as copper or gold could be
used.
[0132] An important factor to obtaining the optimum performance of
the imbedded wire grid polarizer 200 is the material disposed
within the spaces or gaps 207. The gaps 207, formed between the
elements 205, advantageously provide a refractive index less than
the refractive index of at least one of the layers 201 and 203,
such as the first layer 201. Applicants have found that, when the
gaps 207 provide a lower refractive index, the performance of the
polarizer 200 is improved over wire grids totally encapsulated in a
material with a constant refractive index. In the preferred
embodiment, this material will be air or vacuum, but for reasons of
practicality or performance in certain applications, other
materials may be used.
[0133] It is desirable that this material have the lowest
refractive index n possible while meeting other necessary design
constraints such as manufacturability. These other constraints may
require that the material filling the spaces 207 between the
elongated elements 205 be the same material as that composing
either or both of the optical materials 201 and 203. Or, the
material filling the spaces 207 between the elongated elements 205
may be chosen to be a material different from the optical materials
201 and 203.
[0134] As mentioned, in the preferred embodiment, the material in
the spaces 207 will be air or vacuum. Other materials which may be
used include water (index 1.33), magnesium fluoride (index 1.38) or
other common optical thin film materials which can be deposited
using evaporation, sputtering, or various chemical vapor deposition
processes, optical oils, liquid hydrocarbons such as naptha,
toluene, etc. or other materials with suitably low indices. The
material in the gaps 207 also can include plastics, or fluorinated
hydrocarbons (Teflon).
[0135] In addition, the substrate 201 (FIG. 9) or 40 (FIG. 7) of
the WG-PBS can affect the performance of the WG-PBS. As stated
above, orienting the substrate of the WG-PBS at an angle with
respect to the light can result in aberrations of astigmatism and
coma when non-collimated light passes therethrough. These
aberrations occur regardless of the flatness of the substrate.
Therefore, flat plate transmissive optics cannot be used in imaging
applications unless the aberrations are corrected or rendered
negligible.
[0136] This problem can be avoided in an imaging application if the
beam of light containing the image is reflected from the front
surface of the plate rather than passed through it, because it is
the transit through the tilted substrate that causes the optical
aberrations, such as a lateral shift in position along the sloping
direction of the substrate. Such a configuration requires a flat
substrate in order to avoid distortions in the beam that result in
distortions in the final image. Depending on the application,
flatness less than approximately 3 standard wavelengths deviation
per inch is preferable; less than approximately 1 standard
wavelength deviation per inch is more preferable; and less than
{fraction (1/10)}.sup.th standard wavelengths deviation per inch is
most preferable.
[0137] For the transmitted beam, it is desirable to reduce
astigmatism and chromatic aberration. Thus, the substrate
preferably is very thin, or has a thickness less than approximately
5 millimeters.
[0138] Another important consideration for the image system of the
present invention is to have a transmitted wave from the WG-PBS be
sufficiently free of geometric distortions. For the transmitted
beam, the distortions are typically caused by deviations from
parallel between the two surfaces of the substrate. It is
preferable that the transmitted beam have a geometric distortion
less than approximately 3 standard wavelengths deviation per inch;
more preferably less than approximately 1/2 standard wavelength
deviation per inch; and most preferably less than approximately
{fraction (1/10)}.sup.th standard wavelength deviation per
inch.
[0139] It is to be understood that the above-described arrangements
are only illustrative of the application of the principles of the
present invention. Numerous modifications and alternative
arrangements may be devised by those skilled in the art without
departing from the spirit and scope of the present invention and
the appended claims are intended to cover such modifications and
arrangements. Thus, while the present invention has been shown in
the drawings and fully described above with particularity and
detail in connection with what is presently deemed to be the most
practical and preferred embodiment(s) of the invention, it will be
apparent to those of ordinary skill in the art that numerous
modifications, including, but not limited to, variations in size,
materials, shape, form, function and manner of operation, assembly
and use may be made, without departing from the principles and
concepts of the invention as set forth in the claims.
* * * * *