CA2247435A1 - Polarized illumination system for lcd projector - Google Patents

Abstract

An illumination system (10) is described that efficiently produces linear polarized light for use in LCD projection. A polarizing beam splitter (16) and half-wave retarder plate (20) produce two adjacent collimated beams of light having a common polarization direction. These adjacent beams are spatially integrated (22) into a single collimated polarized beam whose aspect ratio is subsequently converted to match that of the LCD format.

Description

CA 0224743~ 1998-08-26 POLA~IZED ILLUMINATIC~N SYSTEM FOR LCD PROJECTOR

Background of the Invention s 1. Field of the Invention The present invention generally relates to optical ilh-min~tion systems, and more particularly to a polarized ill~lmin~tion system having a novel spatialintegrator, incl~l~ling variations for controlling aspect ratio, the system being suited for use with electronic projection displays, particularly of the liquid crystal display type.
0 2. Description of the Prior Art With the increasing use of liquid crystal display (LCD) devices in projection systems, there is a re~uirement for an efficient source of polarized light.
Polarized light has been tr~litit~n~lly produced by an absorbing polarizer or a polarizing beam splitter (PLS) cube. For polarizing beam splitters, half of the light is 15 r~flected and this reflected polarized light is either thrown away, or converted to the same polarization as the tr~n.emittecl beam. When converted, the r~o-flected beam is then redirected to the LCD, along with the ~l~n~ led beam, to achieve a polarized light source that is brighter than one using a simple absorbing polarizer.
An early application of converting polarized light in illl~min~tion 20 systems was for automotive he7.~ mps, as described by 7~ n~ler in "~e~ ht~ for Motor-Vehicles with Polarized Light," Lichttechnik, pp. 100-103 (1973). In this early design, no attempt was made to achieve spatial recolllbinalion of the separate beams, or to control the beam cross-section. With the advent of LCD projection systems, this technology achieved renewed interest, for example as described by Imai et al., "A
25 Novel Polarization Converter for High-Brightness Li~uid Crystal Light Valve Projector," SPIEProceedings, vol. 1225, pp. 52-58 (1990~, and Shinsuke et al., "A
Polarization-Transforming Optics for a High Tllmin~nce LCD Projector," Proceedings of ~he SID, vol. 32/4, pp. 301-304 (1991).
Japanese Patent Application (Kokai) No. 61-122626 describes a polarizing illllmin~tion device, that spatially integrates the separated beams at the LCD
plane by means of a pair of wedge prisms. As shown in Figure 1, the device uses a CA 0224743~ 1998-08-26 W O 97137266 pcTrus97l~4565 light source 1, collimator 2, PBS cube 3, right angle prism 4, half-wave retarder 5, wedge prisms 6 and 7, LCD panel 8, and projection lens 9. There must be a considerable rli.~t~nce between the wedge prisms and the LCD, since the beams are converging, and the incidence angle on the LCD must be kept small, which unduly s restricts use of the device.
U.S. Patent Nos. 4,913,529 and 4,969,730 describe converting polarized light projection illtlmin~tion systems using polarizing plates or a PBS cube, collll)inillg the sepa-~led beams at the ~CD with steering prisms U.S. Patent No.
5,381,278 uses collvt~l~,ing and diverging lenses to redirect the separated beams to the 10 LCD panel. Japanese Patent Application (Kokai) No. 71-99185 uses dual polarizing beam splitters, achieving a beam of oblong cross-section, but with no spatial recombination of the separate beams. European Patent Application No. 615,148 achieves polarization conversion and spatial reco,..billaLion by recycling lightbackwards to the light source reflector, as does European Patent Application No.573,905, ~ign~1 to Minnesota Mining and ~mlf~etl7ring Co. (3M--assignee ofthe present invention). U.S. Patent No. 5,181,054 achieves spatial reco".bi,.ation ofthe separated ~eams by sending the beams through the LCD in opposite directions, as does U.S. Patent No. 5,428,469 (also ~ nfd to 3M). U.S. Patent No. 5,446,510 achievesa common co11imation angle for the separate beams, but without any spatial integration.
U.S. Patent Nos. 5,042,921 and 5,124,841 describe polarization converters with spatial integration achieved by refracting microprisms. The polarization converter described in European Patent Application No. 463,500 preserves the aspect ratio ofthe original beam, but requires the use oftwo LCD's, and European Patent Application No. 456,427 m~t-h~s the beam size to the LCD panel by the use of back reflections from a lamp reflector having a rect~n~-l~r exit aperture.
The efficiency of these systems is limited by the complexity of the optics, quality of the ~,, reflectance coatings, degree of spatial reco~ illalion and beam shaping, and high cl~o"~alic dispersion of the refracting elements.
Spatial integration of the separate beams is illlpOI la~ll since the separated beams usually differ in intensity and color temperature. Collimation of the CA 0224743~ 1998-08-26 W O 97/37266 PCTnUS97/04565 beam incident on the LCD is important, since most displays of this type work best with a common and low incidence angle ofthe illn~ g light. Also, collim~ted light can be more efflciently focused by a field lens to the projection lens. Beam shaping is hllpol lan~ to ll ~ "nil m~ciml Im light through the rect~n~-l~r aperture of the LCD.
5 Lastly, compactn~c~ is desirable to reduce the size of the projector unit. None of the rOI egoin~, systems provides optimum performance in all of these areas.

Summaly of the Invention The present invention provides a polarized illllmin~tion system 10 particularly suited for an LCD projector, the system generally comprising a light source, means for collim~tin~ light from the source into an ~ L~nce beam, means for sepa~ g the entrance beam into a first beam having a first polarization state, and a second beam having a second polarization state, retarder means for converting said first beam from said first polarization state to said second polarization state, means for 15 spatially illlegr~ling said first and second beams to create a collim~tecl, exit beam having an aspect ratio, and means for rh~nging the aspect ratio of said exit beam. The i.lleg,~Li.lg means preferably comprises a spatial integrator cell having a first microprismatic element with a planar surface and a surface of p~ lic grooves opposite said planar surface, and a second mic~ ,;s,ll~lic element with another planar 20 surface and another surface of pli~ lic grooves opposite the other planar surface and facing said surface of prismatic grooves on said first microprismatic element, there further being first and second, generally opposed reflecting surfaces a~jacent first and second sides, respectively, of the second microprismatic element.
The means for sepa,~li,lg the light may comprise a polarizing beam 2~ splitter (PBS~ which creates the first and second beams, and the PBS may be adapted to direct the first and second beams to first and second reflectorized wedge prisms, respectively. In several embodiments, the spatial integrator cell has an exit element which is combined with a first beam coll,plession element, and a second beam co,..p.es~ion element is positioned arljac~nt the first beam com,ol-ession elP!m~nt to 30 achieve the changed aspect ratio. The first beam cc"..ples~ion element may be a positive cylinder Fresnel lens, and the second beam co.-.p. es~ion element a negative CA 0224743~ 1998-08-26 W O 97/37266 P~TAJS97/04~65 cylinder lens Alternatively, the first beam compression element may be a first mic"op..s,..dLic element, and the second beam conlp,ession element a second microprismatic element positioned at an oblique angle with respect to the first microprismatic ele-ment The means for ch~nginp. the aspect ratio of the exit beam may 5 further be designed to additionally cause the exit beam to deviate by approxim~tPly 9oo~
The current invention avoids many of the deficiencies of the prior art, by producing a converted secondary beam of collim~ted polarized light, and spatially reco",binillg this beam with the primary beam using total internally reflective linear lo mic~op.isms, preserving the polarization and collimation with minim~l cluolllaLic dispersion The aspect ratio of the spatially integrated beam is then efficientlyconverted to match the format of an LCD panel There is no requi.t;,.~..~ for anyback-reflection or recycling of the beam in the direction of the light source, and the single collim~ted beam passing through the LCD panel can be more efficiently focused to the projection lens Brief Description of the Drawin~s The invention will best be understood by reference to the acco--ipa"ying drawings, wherein Figure 1 is a diagram of a prior art polarizing ilhlmin~tion device using wedge prism beam integrators;
Figure 2 is a diagram of one embodiment of the polarized illllmin~tion system of the current invention;
Figure 3 is a diagram of one configuration of the spatial integrator cell used in the present invention;
Figure 3a is an enlal~,c;...enL of the linear microprismatic ~lem~nt structure on the spatial integrator of Figure 3;
Figure 4 is a diagram of one type of prior art aspect ratio converter using positive and negative cylinder lenses;
Figure S is a diagram showing application of positive/negative cylinder Fresnel lenses to the c~rrent invention;

CA 0224743~ 1998-08-26 W O 97/37266 PCT~US97/04565 Figure 6 shows prior art anamorphic beam compression using identical prism pair, with minim~l clll omalic dispersion;
Figure 7 is a diagram of a spatial integrator of the present invention with anamorphic beam col,.~.ession using identical Fresnel prisms;
s Figure 8 is a diagram of anamorphic beam compression using a prismatic pair with 90~ beam deviation and l~ lAI chromatic dispersion;
Figure 9 shows a reflective wedge prism as an anall-oll.hic beam co...~, t;ssor;
Figure 10 shows a combined spatial integrator/anamorphic beam o colpl essor;
Figure 11 shows an LCD projector using an embodiment of the current invention; and Figure 12 shows another embodiment of the current invention which elimin~tes the need for a right angle prism in conjunction with the PBS cube.

Desçription of the Pl ~I - ed Embodiment With reference now to the figures, and in particular with reference to Figure 2, there is depicted one embodiment 10 of the polarized illllmin~tion system of the present invention. System 10 is particularly adapted for use with a liquid crystal display (LCD) projector, and the cor"binalion generally comprises a light source or lamp 12, a collimator or parabolic reflector 14, means 16 for sepa-~li"g p- and s-polarized light, taking the form of a polarizing beam splitter (PBS), mean 18 for redi. e~;li.lg one of the polarized beams (in this case, the reflected beam) parallel with the other polarized beam (the ll ~ ed beam), taking the form of a right angle prism 2s reflector, a half-wave retarder plate 20, a spatial integrator cell 22, an analyzer 24, an l,CD panel 26, and a projection lens 28. Embodiment 10 is a single-panel, tr~n~mi~ive LCD projector, but those skilled in the art will appleciate that the general principle can be extended to color and pseudo-color tran~mi~sive LCD projectors using multiplepanels, as well as reflective LCD light valve projectors.
Randomly polarized light from light source 12 is collim~ted by parabolic reflector 14 (or some other means) and enters PBS cube 16. The p-polarized CA 0224743~ 1998-08-26 W O 97137266 PCT~US97/04565 light is trAn~mitted and the reflected s-polarized light is turned 90~ by right angle reflecting prism 18, where it is then converted to p-polarized light by half-wave (~/2~
retarder plate 20. The ~dj~cçnt and spatially separated collimAted light beams enter the microprismatic spatial integrator cell 22 where they are spatially integrated and exit as s a single collim~ted polarized beam. Components 12-18 are individually known in the prior art and nearly any co.,ven~ional components will suffice. Therefore, in embodiment 10, the primary novelty resides in spatial integrator 22.
Figure 3 shows spatial integrator cell 22. Each ~ c~nt entrance beam is equally deviated by the first microprismatic element 32, which consists of a series of 0 60~ equilateral linear microprisms 34 (see Figure 3 a~. Half of the light passes directly to a second, oppositely disposed microprismatic element 36. The other half of the light is reflected offside mirrors 38 and 4~, where it then passes to the opposite mic. op. i~ ic ~I~m~nt 36. By controlling the length of cell 22, both beams exit the cell as a spatially h~e~ ed and collim~ted beam. The spatial integrator cell length 5 L = A tan(~/6), where A is the half-height of the cell (see Figure 2), and light rays entering the center of cel1 22 exit at the edge of the cell. Figure 3a shows an enlarged section of first linear mil;,opli~-na~ic element 32, where all the prism angles a = 60~, and the deviation angle ~ = 60~. There is no refraction, hence no cluo~..alic dispersion, at either miclupli~ aLic elemP.nt since all ray deviations occur by total internal 20 reflection (TIR), i.e., the angle of incitl~n~e at the interface between air and the rnicroprismatic element is zero. For slight deviations from perfect collimation, there is sub~ lly no chromatic dispersion for rays which are refracted at either surface, and also reflected from the TIR facets, since the microprisms are 60~ equilateral triangles.
Since PBS cube 16 forms ~ r~nt square beams, each having an aspect 25 ratio AR = 1:1, the polarization converted beam entering and exiting the spatial integrator cell has an Al~ = 2:1. Since most LCD panels have an AR = 4:3 = 1.33, the AR should be adjusted for efficient illtlmin~tion ofthe LCl~ panel. One standardmethod of aspect ratio conversion that preserves the direction of the light beam is by the use of positive and negative cylinder lenses 42 and 44, respectively, as shown in 30 Figure 4. A novel variation of this method is used in the current invention by forming a positive cylinder Fresnel lens 46 in the exit surface of the spatial integrator cell 48, as CA 0224743~ 1998-08-26 W O 97/37266 PCTrUS97/04565 shown if Figure 5. The negative cylinder lens 50 can be continuous or of the Fresnel type.
Another standard method of converting aspect ratio, or anamorphic beam co,,,pl ession, uses a pair of identical prisms as shown in Figure 6. The chromatic dispersion of the first prism ~2 is c~nc~led by the opposite dispersion of the second prism 54, and the direction of the collim~ted light is preserved. A similar effect may be achieved in the current invention by forming the first element as a series of linear microprisms on the exit element 56 of a spatial integrator cell 58. A second linear microprismatic element 60 is then set at an oblique angle o equal to the ray deviation of 0 the first elçment, as shown in Figure 7. For example, for acrylic plastic (nd =1.492) with prism angles a = 40.52~, and ~ = 35.25~, the aspect ratio of the beam is converted from 2/1 to 4/3, with no chromatic dispersion.
It is also possible to perform anamorphic beam col~.plession in conjunction with the present invention with a 90~ beam deviation, by using two prisms as shown in Figure 8. Here the first refracting prism 62 is followed by a second prism 64 that deviates the beam by refraction and total internal reflection. By specifying the vertex angles and tilt angles as shown, the chromatic dispersion of the first prism is canceled out by the dispersion of the second prism. For example, using optical crown glass ~nd = 1.523) for both prisms, and ~1 = 30~, ~1~2= 8~, al = 18.6~, a2 = 68~, a3 =
38.2~, then the beam is converted from an AR - 2:1 to an AR = 4:3 with negligblecl"~3-"alic dispersion.
If a beam deviation of 90~ is introduced between the spatial integrator and the LCD panel, there are several other methods of achieving the desired aspect ratio conversion. One method is shown in Figure 9, which utilizes a reflective wedge prism 66. The incident beam, having an AR = 2:1 is converted to an output beam with an AR = 4:3 by specifying the tilt angle ~ and the prism wedge angle a. Here atan(A'/A), and a = (~ - ~')/2, where: A' = width of compressed exit beam, A = width of entrance beam, W O 97/37266 PCTrUS97/04565 = angle of incidence of exiting rPflected ray at refracting surface, and ~' = angle of refraction of entrance ray at refracting surface.
The ~ value shown is an applo~ll,ation. In practice, it is necess~ry to account for the Ir varying prism thickness and its effect on the coml)les~ion of the exit beam. Thus ~ and 5 a are iteratively adjllcted until the exit beam is col.lp.t;~sed to achieve the desired aspect ratio. For a reflecting wedge prism 66 of optical crown glass (nd = 1.523), 17.0~, and a ~ 13.9~, a beam compression ratio A'/A = 0.375 can be obtained.
Figure lQ shows a spatial integrator 70 where anamorphic beam COn~n ession and spatial integration of dual incoming beams has been performed using l~ reflecting wedge prisms 72 and 74, and auxiliary planar side mirrors 76 and 78. The prism deviation angle o is 60~, and the entrance angle for the 60~ linear microprismatic e~eln~ntc 80 and 82 is 60~. For the figure shown:
A' = (4/3) A, atan ~cos(7~/6) - C cos(~/3~) / (C cos(~/3) + cos (~/6))), and a= (~ ')/2, where: C = anamorphic colllplt;ssion factor of wedge prism, = angle of incidence of exiting reflected ray at refracting surface of wedge prism, ~' = angle of refraction of e~ ce ray at refracting surface of wedge prism, and ~ = tilt angle of the wedge prisms.
For C = 2/3, two ~dj~c~nt collim~ted entrance beams of the same polarization, each of cross-section A x A, are combined into an integrated, collim~ted and polarized exit beam of cross-section 4A~3 x A, having an aspect ratio AR = 4:3, Each reflecting2s wedge prism has re~active index nd = 1.523, ~ = 26.0~, and a = 15.2~. In the system of ~igure 10, the reflecting surfaces of the wedge prisms are generally opposed, but not parallel. Figure 11 shows a compact LCD illllmin~tion and projection system 84 using this combined spatial integrator and anamorphic beam colllpl essor.
Figure 12 shows a polarization converter 86 that uses two reflecting 30 prisms 88 and 90 and a single 60~ microprismatic element 92 to spatially integrate the two beams and convert the aspect ratio. Collim~ted light enters a polarization beam CA 0224743~ 1998-08-26 W O 97/37266 PCTrUS971~4565 splitter 94 where the p-polarized light is tr~n~mitted and s-polarized light is reflected and converted to p-polarized light by the half-wave retarder 96. The two beams enter reflecting prisms at a 45~ entrance angle, are co.llpressed by a factor C, and exit the prisms at an angle of 60~. There is a 75~ deviation angle o between the input and exit beams ofthe reflecting prism. The co,..plessed beams are then spatially integrated into a single coll;~ ted polarized beam b~ the 60~ miclup~ n~lic element. An incomingbeam having an aspect ratio AR = 1 can be converted to an outgoing beam having an aspect ratio AR = 4:3 by the use of r~flçctin~ prisms having a refractive index nd=
1.523, a vertex angle oc = 10.3~, a tilt angle ~ = 26~, and an anamorphic compression lo factor C = 2/3.

- ~xample A single panel LCD projection system was constructed as an optical breadboard to evaluate components of the current invention. A 24 volt, 250 watt 5 tlln~ctcn-halogen lamp ( EH.T type ) having a spherical back reflector, produced a beam of collim~ted light of ~ 50 mm diameter by means of a pair of glass con~len~ing lenses.
The beam was masked down to a 32 mm by 32 mrn square aperture, and a piece of heat reflecting glass was positioned near this square aperture.
A broadband 450-680 nm PBS cube (Melles Griot #03 PBB 007) 20 produced a tr~n~mitted p-polarized beam and a r.-flecte<l s-polarized beam. A 45~
uncoated prism (Fflmllntl Scientific #32531) deviated the reflçcted beam 90~ by total internal reflection to be adjac~nt to the tr~n.cmitted beam. The s-polarized beam was converted to a p-polarized beam by a half-wave retarder sheet (Polaroid #605208). A
spatial integrator cell was constructed using two linear 60~ microprism elements, each 25 element 32 mm wide by 64 mm high, of 2 mm thick acrylic, with each microprismwidth = 0.25 mm. The separation ofthe microprism elements was L = 18.5 mm.
Spatial integration and common polarization of the two beams was verified by e,~...;..i,~g the output from the spatial integrator cell.
An anamorphic beam-colllpl essing reflecting wedge prism was 30 constructed from acrylic plastic (n~= 1.492), having a length of 100 mm, a width of 38 mm, and a wedge angle = 14.3~. The reflecting wedge prism changed the beam CA 0224743~ 1998-08-26 W O 97137266 PCTrUS97/04~65 dimension exiting the spatial integrator from 64 mm by 32 mm (AR = 2:1) to 32 mmby 24 mm (AR = 4:3).
A VGA co~pa~ ie 1.3" diagonal monochrome LCD module (Seiko Epson #P13VM115/125), with an analyzer on the ex~t side, was illllmin~ted by s the collim~tec~ and polarized light beam exiting the wedge prism. A plano-convex field 1ens focused the light from the LCD module into a 3" focal length, f/2.5 coated anastigmat projection lens (JML Optical Industries). Using a Spectra-Physics brightness spot meter, measured brightnP~s~ increase of the projected screen image was # 70 %, when the output of the converted polarized light was added to the primary 10 beam.
Although the invention has been described with reference to specific embodim~n~, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embo~lim~nt~ as well as alternative embo~iim~nt~ of the invention, will become appa,t;"l to persons skilled in the art upon reference to the 15 description of the invention. It is therefore contemplated that such modifications can be made without departing from the spirit or scope of the present invention as defined in the appended claims.

Claims (20)

CLAIMS:

1. An article for spatially integrating two collimated light beams, comprising:
a first reflecting surface;
a second reflecting surface generally opposed to said first reflecting surface;
means for deviating light from the light beams between and onto said first and second reflecting surfaces; and means for collecting light so deviated from the light beams and reflected off of said first and second reflecting surfaces, and collimating the collected light into an exit beam.

2. The article of Claim 1 wherein said first and second reflecting surfaces are parallel.

3. The article of Claim 1 wherein said first and second reflecting surfaces are formed on first and second reflectorized wedge prisms, respectively.

4. The article of Claim 1 wherein said means for deviating light includes a polarizing beam splitter.

5. The article of Claim 1 wherein said means for deviating light comprises a microprismatic element having a planar surface facing the two light beams and a surface of prismatic grooves opposite said planar surface.

6. The article of Claim 1 wherein said means for collecting light comprises a microprismatic element having a surface of prismatic grooves facing the two light beams, and a planar surface opposite said groove surface.

7. The article of Claim 1 wherein said means for deviating light and said means for collecting light cause substantially no chromatic dispersion

8. The article of Claim 1 wherein the exit beam has an aspect ratio, and said collecting means includes a first beam compression element, and further comprising a second beam compression element positioned adjacent said first beamcompression element to change the aspect ratio of the exit beam.

9. The article of Claim 3 wherein:

said means for deviating light comprises a polarizing beam splitter which creates the two collimated beams from a single collimated beam, a first of the two collimated beams having a first polarization state, and a second of the two collimated beams having a second polarization state;
said polarizing beam splitter directs the first collimated beam to said first reflectorized wedge prism, and directs the second collimated beam to said second reflectorized wedge prism; and a half-wave retarder is interposed between said polarizing beam splitter and said first reflectorized wedge prism.

10. The article of Claim 5 wherein said means for deviating light comprises a first microprismatic element, and said means for collecting light comprises a second microprismatic element having a surface of prismatic grooves facing the two light beams, and a planar surface opposite said groove surface.

11. The article of Claim 10 wherein prisms formed on said prismatic grooves of both said first and second microprismatic element have approximately 60°
angles.

12. A polarized illumination system comprising:
a light source;
means for collimating light from said source into an entrance beam;
means for separating said entrance beam into a first beam having a first polarization state, and a second beam having a second polarization state;
means for converting said first beam from said first polarization state to said second polarization state;
means for spatially integrating said first and second beams to create a collimated, exit beam having an aspect ratio; and means for changing the aspect ratio of said exit beam.

13. The polarized illumination system of Claim 12 wherein said integrating means includes:
a first microprismatic element having a planar surface and a surface of prismatic grooves opposite said planar surface;
a second microprismatic element having another planar surface and another surface of prismatic grooves opposite said other planar surface and facing said surface of prismatic grooves on said first microprismatic element;
a first reflecting surface adjacent a first side of said second microprismatic element; and a second reflecting surface adjacent a second side of said second microprismatic element, said second reflecting surface being generally opposed to said first reflecting surface.

14. The polarized illumination system of Claim 12 wherein:
said means for separating light comprises a polarizing beam splitter which creates the first and second beams;
said means for spatially integrating said first and second beams includes first and second reflectorized wedge prisms; and said polarizing beam splitter directs the first beam to said first reflectorized wedge prism, and directs the second beam to said second reflectorized wedge prism.

15. The polarized illumination system of Claim 12 wherein:

said means for spatially integrating said first and second beams includes a spatial integrator cell having an exit element; and said means for changing the aspect ratio of said exit beam includes a first beam compression element attached to said exit element, and a second beam compression element positioned adjacent said first beam compression element.

16. The polarized illumination system of Claim 12 wherein said means for changing the aspect ratio of said exit beam also causes said exit beam to deviate by approximately 90°.

17. The polarized illumination system of Claim 13 wherein said first and second reflecting surfaces are respectively comprised of first and second auxiliary planar mirrors, and further comprising:
a first reflectorized wedge prism located between said first and second microprismatic elements, proximate said first auxiliary planar mirror; and a second reflectorized wedge prism located between said first and second microprismatic elements, proximate said second auxiliary planar mirror and generally opposed to said first reflectorized wedge prism.

18. The polarized illumination system of Claim 15 wherein:
said first beam compression element is a positive cylinder Fresnel lens;
and said second beam compression element is a negative cylinder lens.

19. The polarized illumination system of Claim 15 wherein:
said first beam compression element is a first microprismatic element;
and said second beam compression element is a second microprismatic element positioned at an oblique angle with respect to said first microprismatic element.

20. An electronic projector comprising:
a light source;
means for collimating light from said source into an entrance beam;
means for separating said entrance beam into a first beam having a first polarization state, and a second beam having a second polarization state;
a half-wave retarder adjacent said separating means for converting said first beam from said first polarization state to said second polarization state;a spatial integrator cell positioned to receive the first and second beams, said cell including a first reflecting surface, a second reflecting surface generally opposed to said first reflecting surface, means for deviating light from the first and second beams between and onto said first and second reflecting surfaces, and an exit element for collecting light so deviated from the light beams and reflected off of said first and second reflecting surfaces, and for collimating the collected light into an exit beam having an aspect ratio;
means for changing the aspect ratio of said exit beam, including a first beam compression element attached to said exit element, and a second beam compression element positioned adjacent said first beam compression element;
display means positioned to receive the exit beam having the changed aspect ratio;
a projection lens; and a field lens positioned to direct light from said display means to said projection lens.