WO1997024637A1

WO1997024637A1 - Fresnel rhomb polarization converter

Info

Publication number: WO1997024637A1
Application number: PCT/US1997/000066
Authority: WO
Inventors: Rodney S. Harris
Original assignee: Hughes-Jvc Technology Corporation
Priority date: 1996-01-03
Filing date: 1997-01-03
Publication date: 1997-07-10

Abstract

A compact optical polarization converter (10) for a liquid crystal light valve projector includes a beam splitting prism (12) with a first coated surface (26) made of alternating layers of a first and second material. An input beam of light (43) is incident upon the first coated surface, at Brewster's angle for the first and second materials, which transmits a first beam having a first polarization along a first path (44) and reflects a second beam having a second polarization along a second path (45), towards a second surface (24). The second surface reflects the second beam such that it is displaced from, and approximately parallel to, the first path. A beam steering prism (32) refracts the first beam. A retarding device (34) rotates the first beam from the first polarization state to the second polarization state. A displacing prism (38) translates the beams closer together.

Description

FRESNEL RHOMB POLARIZATION CONVERTER

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to polarization converters and, more particularly, to polarization converters for use in video projection systems requiring a highly polarized beam of light to be generated from an unpolarized broadband light source.

2. Discussion Liquid crystal light valve (LCLV) projectors have been actively pursued to realize high resolution, high quality and full-color large screen displays. One problem still facing these conventional LCLV projectors is their unsatisfactory total light output and homogeneity due to low luminous efficiency. One reason for the low luminous efficiency is that a polarizer typically employed in conventional LCLV projectors has low efficiency. These conventional polarizers separate the unpolarized input beam of light into a first beam of light having a first polarization state and a second beam of light having a second polarization state. Because light of only one polarization is used, either the first or second beam of light is discarded. Therefore, at most, these polarizers are 50% efficient assuming no loss due to reflection and absorption in the polarizer. However, such losses do occur and the maximum efficiency falls below 50%.

To increase the efficiency of the optical system, polarization converting optics were developed. For example, see, Imai, Shiratori, Tashiro, Sakamoto, and Kubota, "High-Brightness Liquid Crystal Light Valve Projector Using a New Polarization Converter", SPIE Vol. 1255 Large-Screen Projection Displays II (1990) , hereby incorporated by reference. The polarization converter disclosed therein consists of a polarizing beam splitter (PBS) , a polarization direction rotator and a synthesizer. The PBS separates the unpolarized light from the light source into a first beam of light having a first polarization state and a second beam of light having a second polarization state. The polarization direction rotator changes the polarization of the first beam of light to the second polarization state. Subsequently, the two beams of light are synthesized. The conversion efficiency of this polarization converter is 100% minus losses due to reflection and absorption in the polarization converter components. The polarization converter disclosed therein utilizes a prism having a 45° angle of incidence which includes three dielectric films which are made from costly and exotic materials such as CaF₂/ZnS. The losses of this polarization converter due to reflection and absorption in the polarization converter are significant. Furthermore, the use of a 45* prism results in reflectivity and phase distortion.

It is therefore desirable to increase the efficiency of polarization converters in extracting highly polarized light from an unpolarized broadband light source. By improving the efficiency, the contrast of the projector system is increased through the presentation of less divergent light to system polarizer elements as well as increasing light throughput efficiency. Furthermore, it is desirable to decrease the reflection and absorption loss and increase reflectivity and phase preservation of polarization converters.

SUMMARY OF THE INVENTION A compact optical polarization converter, according to the invention, for increasing the extraction of polarized light from an input beam of unpolarized light includes a beam splitting means having a first coated surface for splitting said input beam of light into a first beam of light of a first polarization state and a second beam of light of a second polarization state. The first coated surface transmits said first beam of light along a first path and reflects said second beam of light towards a second surface. The second surface reflects said second beam of light on a second path displaced from and approximately parallel to said first path. The first coated surface includes a stack of alternating layers of a first material having a first index of refraction and a second layer having a second index of refraction. The angle of incidence of said input beam of light with respect to said first coated surface is approximately equal to Brewster's angle for said first and second materials. According to other features of the invention, the first material is titanium dioxide, the second material is silicon dioxide and said beam splitting means has a rhomboid shaped cross-section. According to still other features of the invention, the polarization converter further includes beam steering means, located adjacent said beam splitting means, for refracting said first beam of light. A retarding means, positioned adjacent said beam steering means, rotates said first beam of light from said first polarization state to said second polarization state. A beam displacing means, located in said first and second paths, translates said first and second beams of light closer together.

According to another aspect of the invention, a compact optical polarization converter extracts polarized light from an input beam of unpolarized light for input to a light valve projection system. A beam splitting means splits said input beam of light into a first beam of light of a first polarization state and a second beam of light of a second polarization state and transmits said first beam of light along a first path and said second beam of light on a second path displaced from and approximately parallel to said first path. A beam steering means, located adjacent said beam splitting means, refracts said first beam of light. A retarding means, positioned adjacent said beam steering means, rotates said first beam of light from said first polarization state to said second polarization state. A beam displacing means translates said first and second beams of light closer together.

According to another feature of the invention, the beam splitting means includes a first coated surface for splitting said input beam of light from said light source means into said first beam of light of said first polarization state and said second beam of light of said second polarization state.

In another feature of the invention, the first coated surface transmits said first beam of light along said first path and reflects said second beam of light towards a second surface which reflects said second beam of light on said second path. The first coated surface includes a stack of alternating layers of a first material having a first index of refraction and a second layer having a second index of refraction. An angle of incidence of said input beam of light with respect to said first coated surface is approximately equal to Brewster's angle for said first and second materials. The beam splitting means has a rhomboid cross-section and the first surface and said second surface are located on opposite parallel sides of said beam splitting means.

According to still another aspect of the invention, a compact optical polarization converter extracts polarized light from first and second input beams of unpolarized light for input to a video projection system. A first beam splitting means splits said first input beam of light into a first beam of light of said first polarization state and a second beam of light of said second polarization state. The first beam splitting means transmits said first beam of light along a first path and said second beam of light on a second path displaced from and approximately parallel to said first path. A first beam steering means, located adjacent said first beam splitting means, refracts said first beam of light. A second beam splitting means splits said second input beam of light into a third beam of light of said first polarization state and a fourth beam of light of said second polarization state. The second beam splitting means transmits said third beam of light along a third path and said fourth beam of light on a fourth path displaced from and approximately parallel to said third path. A second beam steering means, located adjacent said second beam splitting means, refracts said third beam of light. A first retarding means, located adjacent said first beam steering means, rotates said first beam of light from said first polarization state to said second polarization state. A second retarding means, located adjacent said second beam steering means, rotates said third beam of light from said first polarization state to said second polarization state. A beam displacing means, located in said first, second, third and fourth paths, translates said first, second, third and fourth beams of light closer together.

Other objects, features and advantages will be readily apparent. BRIEF DESCRIPTION OF THE DRAWINGS

The various advantages of the present invention will become apparent to those skilled in the art after studying the following specification and by reference to the drawings in which:

Figure IA is an end view of an unpolarized input light bea ;

Figure IB is a side view of a first polarization converter according to a first embodiment of the present invention for receiving the input light beam from Figure IA;

Figure IC is an end view of light output by the polarization converter of Figure IB;

Figure 2 is a side view of a displacement prism illustrating parameters for calculating the displacement provided thereby;

Figure 3 is a side view of a polarization converter according to a second embodiment of the present invention;

Figure 4 is a side view of a polarization converter according to a third embodiment of the present invention;

Figure 5 is a side view of a polarization converter employing multiple light sources according to a fourth embodiment of the present invention; and

Figure 6 is an end view of light output by the polarization converter of Figure 3.

DETAILED DESCRIPTION Referring to Figures IA, IB and IC, a polarization converter 10, according to the present invention, is illustrated and includes a polarization beam splitter (PBS) 12 which receives light from an unpolarized broadband light source 16. PBS 12 includes a first set of parallel sides 18 and 20 and a second set of parallel sides 22 and 24. Side 22 preferably includes a first polarizing multi-layer coating 26 which forms a first total internal reflecting ("TIR") surface. Preferably coating 26 transmits and reflects wavelengths in the visible light region. More specifically, coating 26 preferably transmits and reflects wavelengths from 425-675 nanometers (nm) and have a center wavelength of 545 nm.

Polarization converter 10 further includes a beam steering prism 32 having a first side 33 located adjacent side 22 of PBS 12. Beam steering prism 32 refracts light passing through coating 26 as will be described below. A polarization rotation device 34 is preferably located adjacent a second side 35 of beam steering prism 32. Polarization rotation device 34 rotates light from a first polarization state to a second polarization state or from the second polarization state to the first polarization state. Preferably, polarization rotation device 34 is an achromatic polymer half-wave retarder. A beam displacement prism 38 includes a first section

39, a second section 40, and a center section 41. Beam displacement device 38 is symmetrical about a dotted center line 42 illustrated in Figure 1. First and second sections 39 and 40 translate beams incident thereupon in a direction towards center line 42. An output of beam displacement device 38 is input to an LCLV or LCD type video projection system. Polarization beam splitter 12 preferably has a rhomboid-shaped cross-section. In a preferred embodiment, polarization beam splitter is made from glass. In a highly preferred embodiment, polarization beam splitter is made from BK7 or SF57 glass. Preferably, beam steering prism 32 is bonded to the rhomboid-shaped polarization beam splitter using index matching adhesive.

In use, an input beam of light 43, output by light source 16, is directed towards side 22 of polarization beam splitter 12 at an angle x (relative a line perpendicular to side 22) . Angle x is preferably approximately equal to Brewster's angle, the angle at which the reflectance of P- polarized light is minimized for the first and second materials selected for coating 26.

First coating 26 transmits a first beam of light 44 having a first polarization state, for example P-polarized light, along a first path. First beam of light 44 is then refracted by beam steering prism 32. First coating 26 reflects a second beam of light 45 of a second polarization state, for example S-polarized light. Second beam of light 45 then travels towards a second surface defined by side 24.

Second surface or side 24 reflects second beam of light 45 in a direction perpendicular to side 20, along a second path parallel to the first path, and towards second section 40 of beam displacement device 38. Second surface or side 24 in Figure IB functions as a TIR surface without polarizing coatings. First beam of light 44 is refracted by beam steering prism 32 and is rotated from the first polarization state to the second polarization state by polarization rotation device 34.

For example, if first beam of light 44 is P-polarized light, first beam of light 45 would be rotated to S- polarized light by polarization rotation device 34. Alternately, if first beam of light 44 is S-polarized light, first beam of light 44 would be rotated to P- polarized light by polarization rotation device 34.

After polarization rotation, first beam of light 44 has the second polarization state and is designated as 44" in Figure 1. First beam of light 44' and second beam of light 45 are translated towards each other by beam displacement device 38. As can be appreciated, polarization converter 10 provides two beams of light 44 ' and 45 in Figure 3 having a Gaussian distribution and the same polarization state. Polarization rotation device 34 also provides optical path length compensation, for example utilizing the thickness of the retarder substrates, to correct differing path lengths of first beam of light 44 and 44' and second beam of light 45.

Leakage of P-polarization light from the first polarizing surface 26 is reflected along with the S- polarization light, reduces polarization purity and contrast. Undesirable differences in the indices of refraction of the external medium (preferably a glass rhomboid prism) and the outer-most layer of coating 26 (preferably silicon dioxide) contribute to the P- polarization light not achieving the theoretical 100% transmission at Brewsters' angle. P-polarization reflection leakage due to index mismatch is small compared to the effects of light ray incidence angles that are not exactly equal to the Brewsters' angle. Any deviation from the Brewsters' angle value as dictated by the materials used for the polarizing coatings will result in reductions in contrast (dynamic range) of a projectors' performance. Broadband, incoherent light sources with relatively large arc diameters, such a xenon and metal-halide lamps, inherently produce beams that can only be "collimated" with divergences of 3.0 degrees, half-angle.

A "pre-polarizer" reduces P-polarization leakage regardless of the cause. Implementation of polarizing coatings on the second TIR surface or side 24 and a beam steering prism will reduce P-polarization leakage whether it arises from index or angular mismatching. The trade¬ offs are the cost of coating the second surface and the additional prism weighed against the benefits of eliminating the P-polarization leakage. In systems where high contrast is critical, the cost may be justifiable.

Therefore, for the polarization converter depicted in Figure IB, the second reflecting surface 24 functions as a TIR surface with no polarization coatings. Surface 24 should be kept as clean and free of contaminants as possible. Surface 24 reflects both polarization orientations (S-polarization and P-polarization leakage) as the second beam of light 45. Surface 24 provides a highly reflective and broadband surface to the S-polarization light.

Figure 2 illustrates parameters relevant to the calculation of the displacement "d" provided by beam displacement prism 38. In Figure 2, the variable "t" denotes the thickness of the beam displacement prism along a line perpendicular to leading and trailing edges 50 and 52. The angle "i" represents the angle that an incident beam of light defines relative to a dotted line 53, which is perpendicular to leading and trailing edges 50 and 52, before impact. The angle "i"' denotes the angle defined by the path of the incident beam inside of beam displacement prism 38 relative to a line perpendicular to leading and trailing edges 50 and 52. As can be appreciated by skilled artisans, angle i' is less than angle i due to refraction caused by beam steering prism 38.

The variable "d" represents the displacement of a light beam, incident upon the beam displacement prism, toward center line 42. The displacement d can be calculated as follows:

. _ t cos (90 - i + i') cosi'

In a highly preferred embodiment, first coating 26 includes a plurality of alternating layers of a first - 13 - material having a first index of refraction and a second material having a second index of refraction lower than the first index of refraction. For example, the coatings can include ten layers of the first material interspersed with nine layers of the second material. Preferably, coating 26 has a quarter wavelength thickness (at a center frequency) . In a highly preferred embodiment, the first material is titanium dioxide (Ti0₂) and the second material is silicon dioxide (Si0₂) , both of which are readily available, stable materials.

Brewster's angle can be calculated as follows:

θ_B = arctan

(* )

where the n_H equals the index of refraction of the first material and n_L equals the index of refraction of the second material. In a highly preferred embodiment, the index of refraction of Ti0₂ is 2.32 and the index of refraction of

SiO, is 1.46. In such a case, Brewster's angle is approximately 54.6" and coating 26 transmits 90% of the P- polarized light (T_p=90%) and reflects 99.5% of the S- polarized light (R_s=99.5%).

Other suitable materials for the first and second materials are set forth in Table I below:

FRESNEL RHOMB POLARIZATION CONVERTER THIN-FILM MATERIALS

Incidence Glass/Oil LOW High Index Bandwidth Angle Index Index (calculated) (nanometers)

45 1.52 1.25 2.11 326 47 1.46 1.25 2.05 306

47 1.52 1.25 2.43 442

49 1.52 1.35 2.18 300

49 1.55 1.35 2.34 356

49 1.46 1.25 2.33 410

51 1.52 1.38 2.29 324

51 1.55 1.38 2.47 383

51 1.52 1.35 2.44 391

53 1.55 1.46 2.34 302

53 1.46 1.35 2.31 349

(54.6) 55 1.52 1.46 2.39 319

55 1.46 1.38 2.4 365

55 1.55 1.46 2.57 377

55 1.46 1.35 2.58 441

58 1.46 1.46 2.34 307

Suitable high index materials include A-Silicon dioxide (Si02) n_H=1.46; B=Titanium dioxide (Ti02) n_H=2.2- 2.7; C=Zinc sulphide (ZnS) n_H=2.35; and D=Lead chloride (PbF2) n_H=1.75-2.3. Suitable low index materials include 1-Magnesium fluoride (MgF2) n_L=1.35-1.38; 2=Calcium fluoride (CaF2) n_L=1.22-1.26; 3= Cryolite (Na3AlF6) n_L=1.35; and 4=Sodium fluoride (NaF) n_L=1.34. Suitable external mediums include i=Glass (Bk7) n_L=1.518; ii=Glass (fused silica) n_L=1.46; iii=Glass(crown) n_L=1.55; iv=Index fluid n_L=1.5187; and v=Index fluid n_L=1.46.

As can be appreciated, polarization converters having an incident angle (Brewster's angle) between 45° and 58* are suitable. Advantageously, displacement prism 38 reduces the distance imparted to the first and second beams without appreciably increasing the cone angle. Convergence of the two beams at the light valve can easily be achieved with a reasonably long focal length condenser which also keeps the cone angle low. A low cone angle value improves the contrast of the projector system as well as increases the light output by coupling more light through the projection lens input pupil. Polarization converter 10 increases the efficiency of the extraction of polarized light from an unpolarized light source. Furthermore, polarization converter 10 decreases reflection and absorption loss over prior polarization converters while increasing phase preservation. Referring to Figure 3, as alternate polarization converter 80 is illustrated. For purposes of clarity, reference numbers from Figure IB will be used where appropriate. If a second broadband polarizing coating 82 is applied to second surface 24 to remove P-polarization leakage from light reflected by the first coating 26, a second beam steering prism 84 is placed in contact with the polarizing layer 82. Prism 84 gives the P-polarization leakage a refractive path 86 and 86' out of the optical system. Without the beam steering prism 84 in contact with the last layer of coating 82, a glass-air interface at an angle greater than the critical angle of approximately 33* would cause the P-polarization light to be reflected with the S-polarization light, thereby defeating the purpose of second coating 82.

As can be appreciated, the polarizing coating of

Figures IB and 3 can be applied to the hypotenuse surfaces of the two beam steering prisms as an alternative to the surfaces of PBS. Applications of the coatings to the PBS is preferred.

The devices described in conjunction with Figures IB and 3 are preferably exposed to light generated by xenon arc lamps which provide less than 750 watts. Above this lever of power, the optical adhesive between the coating and the beam steering prism hypotenuse are subjected to heat densities that cause delamination and/or expansion. Expansion causes separation of the PBS at coating and the beam steering prism interfaces. A gap between the two prisms causes total reflection of both polarizations at surface 24 (Figures IB and 3) and coating 82 (Figure 3) surfaces. These devices are therefore less effective with the arc lamps providing more than 750 watts. Optical adhesive is preferably Norland #65.

With reference to Figure 4, an alternate polarization converter 90 is illustrated. For purposes of clarity, reference numbers from Figures IB and 3 are used where appropriate. The polarization converter of Figure 4 is suitable for high power arc lamps providing more than 750 watts and P-polarization leakage reduction applications if the PBS is replaced with an optical index matching fluid. Broadband, Brewsters' angle, polarizing coatings are applied to the hypotenuse surfaces of the two beam steering prisms. When assembled into the configuration of Figure 4, index matching fluid 92 can be placed into a sealed cavity formed by the two beam steering prisms 32 and 84 and aperture windows 95 and 96. As can be seen, the rhomboid- shaped configuration is maintained by the optical index fluid 92 held within the boundaries formed by the beam steering prisms 32 and 84 and aperture windows 95 and 96. This configuration eliminates the need for optical adhesives at the coating interfaces thus eliminating the problem of prism separation at the coating interface at high incident light powers. The phenomena of stress induced birefringence, causing non-uniform polarization variations across the light beam field, is also reduced. This effect is inherent in PBS which are solid glass which is being subjected to non-uniform thermal stimulation. Fluid seals are formed by use of 598 Ultra Black silicon gasket adhesive sold by Loctite*. Some expansion relief such as a small silicon rubber or metal bellows (not shown) may be used for the fluid chamber.

Referring to Figure 5, a polarization converter 100 for multiple light sources is illustrated. For purposes of clarity, reference numerals from Figure IB are used where appropriate. Polarization converter 100 includes first and second polarization converters 102 and 104 which operate in a manner similar to polarization converter 10 described above. First polarization converter 102 also has a configuration similar to polarization converter 10 described in Figure IB. Second polarization converter 104, however, has a configuration which is flipped with respect to an imaginary horizontal axis. Second polarization converter 104 lies adjacent (and inward with respect to a plane defined by the page containing Figure 4) to first polarization converter 102.

In addition to first and second polarization converters 102 and 104, polarization converter 100 additionally includes a second beam displacement prism 110 for providing horizontal beam displacement in a manner analogous to the vertical beam displacement provided by beam displacement prism 38.

A multiple beam light source 120 includes first and second light sources 122 and 124 which are separated by one or more ceramic insulators 125. First light source 122 includes a xenon arc lamp 126 with an elliptical reflector 128. First and second light sources 122 and 124 further include heat exchangers 130 and 132 which are, in turn, coupled to a plurality of spaced cooling fins 140 which are preferably made of copper.

In a highly preferred embodiment, a suitable light source 120, available from ILC Technology, Co. in Sunyvale, California, consists of two 1.0" diameter, 500 watt, elliptical reflectors, xenon arc lamps integrated into a convoluted/open-celled metallic foam heat exchanger. At two/lumens/watt/channel, light source 120 provides 2000/lumens (500 lumens per channel for each of the four channels illustrated in Figure 6) from a relatively small package. The lamps are electrically isolated by an injection molded ceramic insulator. Cooling fans (not shown) are of the push-pull type.

In use, dual polarization converter 100 operates in a manner analogous to polarization converter 10 in most relevant respects. Referring to Figure 6, dual polarization converter 100 outputs first, second, third and fourth beams of light 144A' , 145A, 144B' and 145B, respectively. First light source 122 generates an input beam of light 143A which is incident upon first coating 26 of first polarization converter 102. First coating 26 splits the input beam of light 143A, transmits the first beam of light 144A to beam steering prism 32 and reflects the second beam of light 145A towards second surface or side 24. Polarization rotation device 34 rotates the polarization of first beam of light 144A (designated as 144A') from the first polarization state to the second polarization state. Second beam of light 145A is reflected by second surface or side 24 and is transmitted to vertical beam displacement prism 38. Note that polarization rotation device 34*' is located inwardly (in a direction into the page containing Figure 5) and is associated with polarization converter 104. Therefore, second beam of light 145A does not pass through polarization rotation device 34'.

Second light source 124 generates an input beam of light 143B which is incident upon coating 26' of second polarization converter 104. Coating 26' splits the input - 20 - beam of light 143B, transmits the third beam of light 144B to beams steering prism 32' and reflects the fourth beam of light 145B towards second surface or side 24'. The polarization of third beam of light 144B is rotated (and designated as 144B') from the first polarization state to the second polarization state by polarization rotation device 34'. Fourth beam of light 145B is reflected by second surface or side 24' and is transmitted to vertical beam displacement prism 38. Note that polarization rotation device 34 is located outwardly (in a direction out of the page containing Figure 5) and is associated with polarization converter 102. Therefore, fourth beam of light 145B does not pass through polarization rotation device 34. Vertical beam displacement prism 38 translates first beam of light 144A' and second beam light 145A in a direction towards center line 42. Likewise, horizontal beam displacement prism 110 translates first and second beam 144A' and 145A in a horizontal direction towards center line 42.

Advantageously, vertical and horizontal displacement prisms 38 and 110 reduce the distance imparted to the first and second beams without appreciably increasing the cone angle. Convergence of the four beams of light at the light valve can easily be achieved with a reasonably long focal length condenser which also keeps the cone angle low. A low cone angle value improves the contrast of the projection system as well as increases the light output by coupling more light through the projection lens input pupil.

In view of the foregoing, it can be appreciated that the present invention increases the efficiency of polarized light which can be obtained from an unpolarized broadband light source. While this invention has been described in connection with a particular example thereof, no limitations intended thereby except as defined by the following claims. This is because the skilled practioner will realize that other modifications can be made without departing from the spirit of this invention after studying the specification, the drawings and the foregoing claims.

Claims

CLAIMS What is Claimed is:

1. A compact optical polarization converter for increasing the extraction of polarized light from an input beam of unpolarized light, comprising: a polarization beam splitter including a first coated surface for splitting said input beam of light into a first beam of light of a first polarization state and a second beam of light of a second polarization state, said first coated surface transmitting said first beam of light along a first path and reflecting said second beam of light towards a second surface which reflects said second beam of light on a second path displaced from and approximately parallel to said first path, said first coated surface including a stack of alternating layers of a first material having a first index of refraction and a second layer having a second index of refraction, wherein the angle of incidence of said input beam of light with respect to said first surface is approximately equal to Brewster's angle for said first and second materials.

2. The compact optical polarization converter of Claim 1 wherein said first material is titanium dioxide and said second material is silicon dioxide.

3. The compact optical polarization converter of Claim 2 wherein said angle is 54.6 degrees.

4. The compact optical polarization converter of Claim 1 wherein said polarization beam splitter means has a rhomboid shaped cross-section.

5. The compact optical polarization converter of Claim 1 wherein said polarization beam splitter means has a rhomboid shaped cross-section and said first coated surface and said second surface are located on opposite parallel sides thereof.

6. The compact optical polarization converter of Claim 5 wherein said second surface of said polarization beam splitter is a second coated surface.

7. The compact optical polarization converter of Claim 6 further comprising: first beam steering means, located adjacent said second coated surface, for eliminating leakage of light in said second beam having a first polarization state.

8. The compact optical polarization converter of Claim 1 further comprising: second beam steering means, located adjacent said first coated surface of said polarization beam splitter, for refracting said first beam of light.

9. The compact optical polarization converter of Claim 8 further comprising: retarding means, positioned adjacent said beam steering means, for rotating said first beam of light from said first polarization state to said second polarization state.

10. The compact optical polarization converter of Claim 7 wherein the retarding means also compensates for differing path lengths of said first and second paths.

11. The compact optical polarization converter of Claim 9 further comprising: beam displacing means, located in said first and second paths, for translating said first and second beams of light closer together.

12. The compact optical polarization converter of Claim 1 wherein said beam splitting means and said beam displacement means comprise glass.

13. The compact optical polarization converter of Claim 5 wherein said polarization beam splitter comprises a sealed cavity including first and second aperture windows and optical index fluid.

14. A compact optical polarization converter for extracting polarized light from an input beam of unpolarized light for input to a light valve projection system, comprising: beam splitting means for splitting said input beam of light into a first beam of light of a first polarization state and a second beam of light of a second polarization state and for transmitting said first beam of light along a first path and said second beam of light along a second path displaced from and approximately parallel to said first path; beam steering means, located adjacent said beam splitting means, for refracting said first beam of light; retarding means, positioned adjacent said beam steering means, for rotating said first beam of light from said first polarization state to said second polarization state; and beam displacing means for translating said first and second beams of light closer together.

15. The compact optical polarization converter of Claim 14 wherein said beam splitting means includes a first coated surface for splitting said input beam of light into said first beam of light of said first polarization state and said second beam of light of said second polarization state.

- 27 - 16. The compact optical polarization converter of Claim 15 wherein said first coated surface transmits said first beam of light along said first path and reflects said second beam of light towards a second surface which reflects said second beam of light on said second path.

17. The compact optical polarization converter of Claim 16 wherein said first coated surface includes a stack of alternating layers of a first material having a first index of refraction and a second material having a second index of refraction, and wherein an angle of incidence of said input beam of light with respect to said first coated surface is approximately equal to Brewster's angle for said first and second materials.

18. The compact optical polarization converter of Claim 15 wherein said beam splitting means has a rhomboid cross-section and wherein said first coated surface and said second surface are located on opposite parallel sides of said beam splitting means.

19. A compact optical polarization converter for extracting polarized light from first and second beams of unpolarized light for input to a projection system, comprising: first beam splitting means for splitting said first input beam of light into a first beam of light of a first polarization state and a second beam of light of a second polarization state, said first beam splitting means transmitting said first beam of light along a first path and said second beam of light along a second path displaced from and approximately parallel to said first path; first beam steering means, located adjacent said first beam splitting means, for refracting said first beam of light; second beam splitting means, located adjacent said light source means, for splitting said second input beam of light from said light source means into a third beam of light of said first polarization state and a fourth beam of light of said second polarization state, said second beam splitting means transmitting said third beam of light along a third path and said fourth beam of light along a fourth path displaced from and approximately parallel to said third path; second beam steering means, located adjacent said second beam splitting means, for refracting said third beam of light; first retarding means, located adjacent said first beam steering means, for rotating said first beam of light from said first polarization state to said second polarization state; second retarding means, located adjacent said second beam steering means, for rotating said third beam of light from said first polarization state to said second polarization state; and beam displacing means, located in said first, second, third and fourth paths, for translating said first, second, third and fourth beams of light closer together.

20. The compact optical polarization converter of Claim 19 wherein said first beam splitting means includes a first coated surface for splitting said first input beam of light into said first beam of light of said first polarization state and said second beam of light of said second polarization state, and wherein said second beam splitting means includes a third coated surface for splitting said second input beam of light from said light source means into said third beam of light of said first polarization state and said fourth beam of light of said second polarization state.

21. The compact optical polarization converter of Claim 20 wherein said first coated surface transmits said first beam of light along said first path and reflects said second beam of light towards a second surface which reflects said second beam of light on said second path, wherein said third coated surface transmits said third beam of light along said third path and reflects said fourth beam of light towards a fourth surface which reflects said fourth beam of light on said fourth path.

22. The compact optical polarization converter of

Claim 21 wherein said first and third coated surfaces each include a stack of alternating layers of a first material having a first index of refraction and a second layer having a second index of refraction, and wherein an angle of incidence of said first and second input beams of light with respect to said first and third surfaces is approximately equal to Brewster's angle for said first and second materials.

23. The compact optical polarization converter of Claim 19 wherein said first and second beam splitting means have a rhomboid cross-section.