Abstract:
A housing (100) for mounting a wire grid polarizing beamsplitter (122) and a spatial light modulator (30) in alignment with an output optical path comprises a front plate having an opening for admitting incident illumination provided along an illumination axis. A modulator mounting plate ( 110 ) is spaced apart from and parallel to the front plate, for mounting the spatial light modulator in the optical output path of the illumination axis. First and second polarizer support plates are spaced apart from each other and extend between the front plate and the modulator mounting plate. The respective facing inner surfaces of the first and second support plates provide coplanar support features for supporting the wire grid polarizing beamsplitter between the inner surfaces. The wire grid polarizing beamsplitter extends between the facing inner surfaces. The surface of the wire grid polarizing beamsplitter is a fixed angle with respect to the surface of the spatial light modulator on the modulator mounting plate. The fixed angle defining an output optical axis along the output optical path.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly-assigned copending U.S. patent application Ser. No. 09/813,207, filed Mar. 20, 2001, entitled A DIGITAL CINEMA PROJECTOR, by Kurtz et al.; U.S. patent application Ser. No. 10/040,663, filed Jan. 7, 2002, entitled DISPLAY APPARATUS USING A WIRE GRID POLARIZING BEAMSPLITTER WITH COMPENSATOR, by Mi et al.; and U.S. patent application Ser. No. 10/050,309, filed Jan. 16, 2002, entitled PROJECTION APPARATUS USING SPATIAL LIGHT MODULATOR, by Joshua M. Cobb, the disclosures of which are incorporated herein. 
    
    
     FIELD OF THE INVENTION 
     This invention generally relates to digital imaging apparatus and more particularly relates to a frame for and method for mounting polarization components and a reflective LCD spatial light modulator. 
     BACKGROUND OF THE INVENTION 
     Initially introduced as small-scale imaging devices for business presentation markets, digital color projectors have steadily improved in overall imaging capability and light output capacity. In order for digital motion picture projectors to compete with conventional motion picture film projectors such as those used in theaters, however, a number of significant technical hurdles remain. Unlike conventional motion picture projectors, high-quality digital projection systems provide separate color modulation paths for red, green, and blue (RGB) color image data. The design of digital color projection apparatus requires that monochromatic light beams carrying images formed on each of the individual color channels be combined, with proper intensity and registration, in order to project a full color image. 
     Referring to FIG. 1, there is shown a simplified schematic for a digital motion picture projection apparatus  10  as described in U.S. patent application Ser. No. 10/050,309, incorporated herein by reference. Each color channel (r=Red, g=Green, b=Blue) uses similar components for forming a modulated light beam. Individual components within each path are labeled with an appended r, g, or b, appropriately. For the description that follows, however, distinctions between color channels are specified only when necessary. A light source  20  provides unmodulated light, which is conditioned by uniformizing optics  22  to provide a uniform illumination, directed through an illumination relay lens  80  to a dichroic separator  27 . Dichroic separator  27  splits the white light into red, green, and blue color channels. Following any of the three color channels, light goes to a light modulation assembly  38  in which a relay lens  82  directs light through a prepolarizer  70  to a polarizing beamsplitter  24 . Light having the desired polarization state is transmitted through polarizing beamsplitter  24  and is then modulated by a spatial light modulator  30 , which selectively modulates the polarization state of the incident light over an array of pixel sites. The action of spatial light modulator  30  forms an image. The modulated light from this image, reflected from polarizing beamsplitter  24 , is transmitted along an optical axis O r /O g /O b  through an analyzer  72  and is directed by a magnifying relay lens  28 , through an optional folding mirror  31 , to a dichroic combiner  26 , typically an X-cube, Philips prism, or combination of dichroic surfaces in conventional systems. An optional color-selective polarization filter  60  may also be provided in the modulated light path. Dichroic combiner  26  combines the red, green, and blue modulated images from separate optical axes O r /O g /O b  to form a combined, multicolor image for a projection lens  32  along a common optical axis O for projection onto a display surface  40 , such as a projection screen. 
     The reflective liquid crystal device (LCD) of FIG. 1 is a type of spatial light modulator that is widely used in digital projector design. This device accepts polarized light and modulates the polarization of the incident light to provide colored light beam as output. For obtaining polarized light, a polarizing beamsplitter prism, such as a McNcille prism, is typically employed along with the support of one or more polarizing elements, configured as polarizers and analyzers. 
     Because modulated light must be combined from each of three color channels in order to synthesize a color image, correct registration of the modulated light is important. When the modulated light is reflected from the surface of spatial light modulator  30 , angular errors in the relative alignment of each LCD surface can cause significant shifts in resolution, yielding unsatisfactory image quality. Further image quality problems, such as loss of contrast, can be the result of imperfect alignment of polarization support components, particularly for polarizing beamsplitter  24 . Moreover, thermal expansion effects can cause further drift in registration and degrade polarization components performance. Thermal expansion becomes a particular concern with high-end projection apparatus, since high brightness is required in these applications. At the same time, compact optical packaging is desirable, with minimized optical path length between image-forming components and the projection lens. These conflicting requirements complicate the design of high-brightness projection apparatus. 
     The negative impact of thermal expansion on image registration is well known in the art. In response to this problem, U.S. Pat. No. 6,345,895 (Maki et al.) discourages use of a mounting base for supporting reflective spatial light modulators, polarizing beamsplitters, and related polarization support components. Significantly, the U.S. Pat. No. 6,345,895 disclosure even teaches away from the use of a mounting base formed from metals or composite materials having low coefficients of expansion. Instead, the approach proposed U.S. Pat. No. 6,345,895 mounts spatial light modulator components directly to glass prism components used for beamsplitting or color combining, so that components in the optical path remain in alignment with thermal expansion. This same overall type of approach is also taught in U.S. Pat. No. 6,375,330 (Mihalakis); U.S. Pat. No. 6,053,616 (Fujimori et al.); and U.S. Pat. No. 6,056,407 (linuma et al.). 
     One recognized problem with attachment to prism components is in achieving the initial alignment itself. As one example, U.S. Pat. No. 6,406,151 (Fujimori) describes methods for adhesively affixing LCD components to a prism with alignment. While attachment directly to a glass or plastic prism surface may have advantages for minimizing thermal expansion effects, there appear to be a number of drawbacks with solutions that use adhesives, compounding thermal dissipation concerns for the LCD itself and making component replacement a costly and time-consuming procedure. 
     Recently, as is disclosed in U.S. Pat. No. 6,122,103 (Perkins et al.), high quality wire grid polarizers have been developed for use in the visible spectrum. While existing wire grid polarizers may not exhibit all of the necessary performance characteristics needed for obtaining the high contrast required for digital cinema projection, these devices have a number of advantages. Chief among these advantages are the following: 
     (i) Good thermal performance. Wire grid polarizers do not exhibit the thermal stress birefringence that is characteristic of glass-based polarization devices, as was noted above. 
     (ii) Robustness. Wire grid polarizers have been shown to be able to withstand anticipated light intensity, temperature, vibration, and other ambient conditions needed for digital cinema projection. 
     (iii) Good angular response. These devices effectively provide a higher numerical aperture than is available using conventional glass polarization beamsplitters, which allows relatively higher levels of light throughput when compared against conventional devices. 
     (iv) Good color response. These devices perform well under conditions of different color channels. It must be noted, however, that response within the blue light channel may require additional compensation. 
     U.S. Patent Nos. 6,234,634 and 6,447,120 (both to Hansen et al.) and U.S. Pat. No. 6,585,378 (Kurtz et al.) disclose image projection apparatus using wire grid polarizing beamsplitters. The wire grid polarizing beamsplitter offers advantages over conventional prism-based polarizing beamsplitters, particularly due to its small size and weight. It can be appreciated that there could be advantages for light modulation in a combination using wire grid polarizer and analyzer components. However, as with the more conventional beamsplitter and polarizers employed in prior art projection apparatus, wire grid components are themselves subject to thermal expansion effects and must be properly aligned with respect to the spatial light modulator within each color channel, with thermal effects taken into account. 
     An article in the SID 02 Digest entitled “The Mechanical-Optical Properties of Wire-Grid Type Polarizer in Projection Display System” by G. H. Ho et al., presents some of the key design considerations for deploying wire grid polarizer components in imaging apparatus using reflective LCD spatial light modulators. Noting problems caused by mechanical constraint and thermal stress in a comparatively low-power projection apparatus, the Ho et al. article highlights the overall negative impact of conventional mounting techniques for wire grid polarizing beamsplitters. Notably, the Ho et al. disclosure is directed to an imaging system that uses a reflective LCD spatial light modulator that transmits modulated light thru a wire grid polarizing beamsplitter. Inherent problems in that type of system include astigmatism, which can be corrected using techniques described in the Ho et al. article. Among other problems noted in the Ho et al. article are surface deformation caused by thermal effects on the wire grid polarizing beamsplitter. It can be appreciated that problems for low-to intermediate-power projection apparatus, as highlighted in the Ho et al. article, would be even more pronounced for higher energy projection equipment. 
     Among key design considerations for mounting a wire grid polarizing beamsplitter is maintaining the surface of this component at an accurate. 45 degree orientation relative to both the surface of the spatial light modulator and the surface of an analyzer. A related problem that must be resolved in electronic projection apparatus design is alignment of the spatial light modulator itself relative both to the wire grid polarizing beamsplitter and to the projection optical path. Maintaining precision alignment without the negative effects of thermal drift is a key design goal for high-end electronic projection apparatus. 
     Unlike the imaging application of the Ho et al. configuration, projection apparatus  10  of FIG. 1 (of which the present invention is part) uses reflective LCD spatial light modulator&#39;s  30   r,    30   g,    30   b  that direct modulated light back to the corresponding polarizing beamsplitter&#39;s  24   r,    24   g,    24   b,  which in turn reflect light towards the imaging lens. In order to substitute wire grid polarizing beamsplitter&#39;s for conventional prism based polarizing beamsplitter components, the thermal effects highlighted by Ho et al. must be considered. However, because the position of the polarizing beamsplitter is as a reflective surface in the path of modulated light, the inherent thermal impact on imaging problems is even more pronounced than for the system described in the Ho et al. article. That is, with wire grid polarizing beamsplitter components used in place of polarizing beamsplitter&#39;s  24   r,    24   g,    24   b,  convergence, contrast, and general wave front aberrations are serious concerns for the optical designer. These optical effects are due to surface deformation, lateral shifts, or tilt and/or rotations, and all of which can be induced by thermal stress. Ho et al. not only does not consider the problems encountered with high intensity illumination, but these specific problems incurred in a reflective structure, and the solutions thereof, are also not considered by Ho et al. 
     As another recent reference, U.S. Patent Application Publication 2003/0117708 (Kane) discloses a sealed enclosure comprising of a wire grid polarizing beamsplitter, a spatial light modulator and a projection lens having the interior space filled with a inert gas or vacuum. Among the goals stated in U.S. 2003/0117708 are protection of the wire grid component from corrosion and handling and modular packaging of the optics assembly. While this approach may be useful in some small-scale projection environments employing only a single spatial light modulator, the apparatus and method of U.S. 2003/0117708 would not be suitable for the high-heat environment of a full-color projection apparatus designed for commercial use, such as for use in motion picture theaters. Moreover, high-quality digital projection requires the use of a separate spatial light modulator for each color channel, with high-quality projection optics. In order to provide suitable contrast, additional support components for the polarizing beamsplitter are needed to provide further polarization selectivity. The relative alignment of these supporting polarization components with the polarizing beamsplitter and with the overall imaging path is significant. No provision is made for deploying or adding these supporting components in U.S. 2003/0117708. In addition, the U.S. 2003/0117708 methods do not anticipate nor provide solutions due to thermal distortion and stress birefringence that would be induced in a high-heat environment, as a result of over constraint and heat containment within the sealed enclosure. 
     Thus it can be seen that, while wire grid polarizers and polarizing beamsplitters offer some advantages for digital projection apparatus, problems of alignment and complexities presented by thermal expansion effects must be resolved in order to obtain suitable performance from these components. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an apparatus and technique for mounting spatial light modulator and supporting polarization components that is mechanically robust, that allows thermal expansion without degrading image quality, and that allows straightforward alignment of components in the light modulation path. With this object in mind, the present invention provides a housing for mounting a wire grid polarizing beamsplitter and a spatial light modulator in alignment with an output optical path, comprising: 
     (a) a front plate having an opening for admitting incident illumination provided along an illumination axis; 
     (b) a modulator mounting plate, spaced apart from and parallel to the front plate, for mounting the spatial light modulator in the path of the illumination axis; 
     (c) first and second polarizer support plates, spaced apart from each other and extending between the front plate and the modulator mounting plate; the respective facing inner surfaces of the first and second support plates providing coplanar support features for supporting the wire grid polarizing beamsplitter between the inner surfaces; and 
     the wire grid polarizing beamsplitter being extended between and normal to the facing inner surfaces, the surface of the wire grid polarizing beamsplitter at a fixed angle with respect to the surface of the spatial light modulator on the modulator mounting plate, the fixed angle defining an output optical axis along the output optical path. 
     It is a feature of the present invention that it provides a modular housing for a spatial light modulator and supporting polarization components for a single color channel. 
     It is an advantage of the present invention that it provides a mounting method for accurately aligning a wire grid polarizing beamsplitter relative to the optical path for modulated light. Using the apparatus and method of the present invention, no adjustment to polarizing beamsplitter position is necessary once the housing is mounted in place. Only slight adjustment for spatial light modulator positioning is necessary in any color channel. 
     It is a further advantage of the apparatus and method of the present invention it allows conventional optical fabrication tolerances to be used in manufacture of a precision alignment housing. 
     It is a further advantage of the present invention that it allows replacement of the spatial light modulator for a single color channel without necessitating re-adjustment of supporting polarization components. The complete set of modulation and polarization components for a single color channel are packaged as a unit, allowing ease of removal for serviceability. 
     It is yet a further advantage of the present invention that it provides a mounting arrangement for polarization components that is robust and allows for thermal expansion effects. 
     These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the present invention, it is believed that the invention will be better understood from the following description when taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a schematic block diagram showing the overall arrangement of components for a projection apparatus using a reflective LCD spatial light modulator; 
     FIG. 2 is a perspective view showing components of the housing of the present invention, in relation to other components in the optical path; 
     FIG. 3 is a perspective view showing the components of the housing of the present invention enlarged, without mounting details shown in FIG. 2; 
     FIG. 4 is a perspective view showing internal components of the housing of the present invention, with the analyzer removed; 
     FIG. 5 is a perspective view showing internal components of the housing of the present invention for modulation and polarization, with the top cover plate removed; 
     FIG. 6 is a perspective view showing internal components of the housing of the present invention, with the top cover plate removed and with representative light cones shown for illumination and modulated light; and 
     FIG. 7 is a perspective, exploded view showing the key support structure separated from the mounting plate used for attachment of the spatial light modulator. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present description is directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. 
     Referring to FIGS. 2 and 3, there are shown perspective views of a housing  100  for mounting spatial light modulator  30  and its supporting polarization components for a single color modulation channel, the blue channel in a preferred embodiment, to a chassis wall  104  within projection apparatus  10 . Monochrome illumination I is directed to turning mirror  31  which reflects illumination I through magnifying relay lens  82  and through a ring bore  102  into housing  100 . Output modulated light along axis O b  is then directed through reducing relay lens  28  to combining and projection optics, as was described with reference to FIG. 1. A modulator mount  106  is attached as part of housing  100 . 
     Referring to FIG. 4, there is shown a perspective front view with analyzer  72  removed to show internal components and overall structure of housing  100 . Modulator mount  106  attaches to a modulator mounting plate  110 . Fitted into modulator mounting plate  110  are a top plate  112  and a base plate  120 , separating modulator mounting plate  110  and ring bore  102 . Within housing  100 , a wire grid polarizing beamsplitter  122  is disposed at a fixed diagonal angle relative to the surface of spatial light modulator  30 . A registration recess  118  is provided along the edge of base plate  120 , providing a seat for the bottom edge of analyzer  72 . Prepolarizer  70  is mounted within a recess  108  provided by ring bore  102  and is lightly fixed in position using a flexible, compliant adhesive, such as an RTV type adhesive. 
     Referring to FIG. 7, there is shown a perspective view of housing  100  with modulator mounting plate  110  removed. The portion of housing  100  consisting of top plate  112 , base plate  120 , and ring bore  102  can be fabricated as a single unit, such as by casting. In whatever manner housing  100  is fabricated, corresponding support features on facing surfaces of base plate  120  and top plate  112  must be mutually aligned in order to register wire grid polarizing beamsplitter  122  and analyzer  72  between these surfaces with minimal constraint. Wire grid polarizing beamsplitter  122  is fitted against coplanar registration surfaces  124  and  124 ′ on base plate  120  and top plate  112 , respectively. The bottom edge of wire grid polarizing beamsplitter  122  seats atop a beamsplitter seating base  128 . In one embodiment, coplanar registration surfaces  124  and  124 ′ are aligned to be coplanar by machining, following assembly of top plate  112  and base plate  120  to ring bore  102 . Edge guides  126  and  126 ′ are likewise machined in the same operation to be colinear with the edge of polarizing beamsplitter  122  when in housing  100 . 
     Similarly, for supporting analyzer  72 , a registration recess  118  on base plate  120  is aligned so that its rear surface is coplanar with a side surface  116  of top plate  112 . Slots  114  are provided in top and base plates  112  and  120 , maximizing air flow  109 , ambient or dedicated forced air, across one or both surfaces of the polarizing beamsplitter, also providing additional cooling to adjacent polarization and modulation components. Furthermore, cooling the polarization beamsplitter can have the added benefit of preventing a differential thermal expansion of the polarizing beamsplitter and/or its mount, that could cause the polarizing beamsplitter to rotate from its normal position and thus induce a convergence (screen position) error. 
     Base plate  120  and top plate  112  can be fitted into modulator mounting plate  110  and ring bore  102  using conventional mating methods for machined or cast metal components. Pins and detents may be used for alignment of these components to form the outer shell of housing  100  as shown in FIG.  4 . The components are then screwed together to provide housing  100  as a single, modular component. For uniform thermal expansion, similar materials are used for fabrication of base plate  120 , top plate  112 , modulator mounting plate  110 , and ring bore  102 . In a preferred embodiment, base plate  120 , top plate  112 , modulator mounting plate  110 , and ring bore  102  are made of aluminum. Alternately, some other material having a low coefficient of thermal expansion could be used, such as Invar or some types of stainless steel for example. 
     Precision alignment with the illumination system (axis I as shown in FIGS. 2 and 3) is not critical; there is some tolerance allowable for alignment in the path of unmodulated light. Advantageously, housing  100  provides self-centering to illumination axis I, within allowable tolerance, so that further manual alignment is unnecessary. Referring back to FIG. 3, the barrel of relay lens  82  provides this self-centering by fitting into ring bore  102 , which is itself fastened to chassis wall  104 . 
     Alignment of Polarizing Components 
     Referring to FIG. 5, there is shown a perspective view of polarization and modulation components, with top plate  112  and ring bore  102  removed and with analyzer  72  shown in place. FIG. 5 shows details of the configuration of modulator mounting plate  110  and of base plate  120 . As was noted with respect to FIG. 7, base plate  120  has coplanar registration surface  124 , or an equivalent type of mechanical feature that acts as a datum for seating wire grid polarizing beamsplitter  122  at the needed fixed angle with respect to spatial light modulator  30 . In a preferred embodiment, this fixed angle is at 45 degrees. Beamsplitter seating base  128 , shown most clearly in FIG. 7, then provides a vertical datum for alignment of wire grid polarizing beamsplitter  122  in the y direction as indicated in FIG.  5 . Coplanar registration surface  124  provides a datum for alignment of wire grid. polarizing beamsplitter  122  in the z-direction. An edge guide  126 . in base plate  120  serves as a datum point for horizontal alignment of wire grid polarizing beamsplitter  122  along coplanar registration surface  124 , that is, in the x direction as indicated in FIG.  5 . As is shown in FIG. 7, a corresponding edge guide  126 ′ in top plate  112  is aligned with edge guide  126  in base plate  120  to provide a pair of datum points for horizontal (x-direction) alignment of one edge of polarizing beamsplitter  122  that extends between base plate  120  and top plate  112 . 
     In the design of housing  100 , thermal expansion of polarizing components is permitted in controlled directions, opposite datum points or surfaces. The use of edge guide  126  and coplanar registration surface  124  allows thermal expansion of wire grid polarizing beamsplitter  122  outward from the corner point of contact near edge guide  126 . A surface of wire grid polarizing beamsplitter  122  near its bottom edge is seated against coplanar registration surface  124  on base plate  120 ; the top edge of wire grid polarizing beamsplitter  122  lies against the surface of coplanar registration surface  124 ′ on top plate  112 , with allowance provided for thermal expansion along this top edge. A small amount of flexible, compliant adhesive, such as an RTV type adhesive, can be used to stabilize the bottom edge of wire grid polarizing beamsplitter  122  against seating base  128  and to stabilize the top edge of wire grid polarizing beamsplitter  122  to the surface of coplanar registration surface  124 ′ on top plate  112 . Similarly, analyzer  72 , seated against registration recess  118  as is shown in FIG. 7, can expand at its top edge, which is flexibly adhered to side surface  116 . By allowing some tolerance for thermal expansion and allowing expansion only in predictable directions (x and y as shown in FIG.  5 ), the design of housing  100  thereby minimizes bending or other distortion of wire grid polarizing beamsplitter  122  and of analyzer  72  due to heat effects. 
     It can be observed that the fabrication of housing  100  as shown in FIGS. 4,  5 , and  7  allows an initial, approximate positioning of polarization and modulation components relative to projection optics for a color channel, that is, providing initial alignment of the three polarization components (prepolarizer  70 , wire grid polarizing beamsplitter  122 , and analyzer  72 ), and of spatial light modulator  30 . There remains, of course, some small tolerance related to alignment of the edges of wire grid polarizing components with the precise polarization axis of these components, accurate to within about 0.5 degrees using current fabrication techniques. 
     Conventional optical tolerances and machining practices can be employed in fabrication of housing  100 . Advantageously, housing  100  enables the three polarization components to be assembled with needed precision, not requiring further adjustment once these components are set in place. Housing  100  can then be mounted against chassis wall  104 . Precision alignment to the output optical path (for example, to O b  in FIGS. 3 or  5 ) is then obtained by adjusting the relative position of spatial light modulator  30  on modulator mounting plate  110 . This final precision alignment is a minor adjustment, typically on the order of a few microns, and can be made once projection apparatus  10  assembly is complete. 
     For providing image registration with the needed accuracy, the following alignments are of particular importance: 
     (i) alignment of wire grid polarizing beamsplitter  122  to the output optical axis, O b  as shown in FIG. 5; 
     (ii) alignment of wire grid polarizing beamsplitter  122  with respect to spatial light modulator  30 ; and 
     (iii) alignment of analyzer  72  to wire grid polarizing beamsplitter  122  and to the output optical axis, O b . 
     Thus, with the apparatus and method of the present invention, alignments (i) and (iii) above are accomplished by assembling components within housing  100  and mounting housing  100  to chassis wall  104 , as was shown in FIG.  2 . Alignment (ii) above requires that spatial light modulator  30  be positioned against modulator mounting plate  10  and adjusted in place. With this arrangement, then, only one in situ adjustment, that of spatial light modulator  30 , is needed for optical alignment of light modulation assembly  38  components within each color channel. 
     FIG. 6 shows a perspective view showing representative light cones transmitted through and reflected from wire grid polarizing beamsplitter  122 . 
     The alignment of prepolarizer  70  to illumination path I, provided by its mounting within ring bore  102 , is sufficiently within tolerance when housing  100  is fully assembled. 
     Alignment of Spatial Light Modulator  30   
     Referring back to FIG. 1, the problem of alignment for spatial light modulator  30  can be more readily appreciated. Each color channel O r /O g /O b  must be aligned with respect to dichroic combiner  26  in order for precise alignment to output optical axis O. Using housing  100 , the position of each spatial light modulator  30  when initially mounted onto modulator mounting plate  110  will already be within some reasonable alignment tolerance, typically within a few pixels, for example. Slight adjustment of each spatial light modulator  30  position, using a projected image target, such as would be familiar to those skilled in the optical alignment arts, then allows final alignment within projection apparatus  10 . When this alignment is achieved, each spatial light modulator  30  can be potted in place, using adhesives and techniques well known in the opto-mechanical arts. 
     A secondary design consideration with the implementation of housing  100  relates to minimizing light leakage that could reduce image contrast. Referring to FIG. 5, some stray light S from the illumination path I can be reflected from the surface of wire grid polarizing beamsplitter  122  rather than being fully transmitted to spatial light modulator  30 . Any type of reflective surface in the path of this unwanted, reflected stray light S could reflect some portion of this light through wire grid polarizing beamsplitter  122  in the direction of output axis O b , thereby reducing contrast. Thus, the use of non-reflective materials within the path of possible stray, reflected light S is recommended. In one embodiment, light-absorbing materials are provided in the path of stray light S. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention as described above, and as noted in the appended claims, by a person of ordinary skill in the art without departing from the scope of the invention. For example, the designations “top” and “bottom” refer to the layout of housing  100  and its components in one embodiment; the angular orientation of housing  100  could be varied within the scope of the present invention. Coplanar datum surfaces for alignment could be provided by an arrangement of suitably placed mounting points provided on top and base plates  112  and  120 , such as using pins or other locating features. Analyzer  72  and prepolarizer  70  components could themselves be wire grid polarizing components or could be other types of conventional planar polarization devices. Analyzer  72  could be a polymer-based polarizer for example. 
     Unlike conventional mounting approaches in electronic image projection systems that mount polarizer components to glass prism components in order to compensate for thermal expansion, housing  100  of the present invention provides a separate structure that maintains these components in the needed positional relationship with respect to each other. Where U.S. Pat. No. 6,345,895 discourages supporting modulation and polarization components on a metal base, the present invention provides housing  100  employing base plate  120  as a primary supporting structure for these components. Unlike prior art solutions that require numerous settings and adjustments for obtaining the needed alignment of polarization components with each other and with the spatial light modulator, housing  100  of the present invention maintains the position of these components so that only minor adjustment of spatial light modulator  30  is needed to align modulation and polarization components of a color channel with color combining optics. At the same time, the design of housing  100  provides this precise alignment using fabrication and machining techniques that employ merely standard optical tolerances. Unlike apparatus that attach components to a combining prism, housing  100  of the present invention allows each color channel to be independently assembled, adjusted, and serviced, minimizing the impact of adjustments in a single color channel on projection apparatus  10  as a whole. Unlike prior art solutions that comprise multiple sheet metal components, housing  100  of the present invention provides a single, sturdy frame for mounting polarization and modulation components, suitable for a high-energy projection system. 
     Thus, what is provided is an apparatus and method for mounting polatization components and a reflective LCD spatial light modulator in a configuration that is thermally robust and allows straightforward alignment techniques. 
     PARTS LIST 
       10  Projection apparatus 
       20  Light source 
       22  Uniformizing optics 
       24  Polarizing beamsplitter 
       24   r  Polarizing beamsplitter, red 
       24   g  Polarizing beamsplitter, green 
       24   b  Polarizing beamsplitter, blue 
       26  Dichroic combiner 
       27  Dichroic separator 
       28  Magnifying relay lens 
       28   r  Magnifying relay lens, red 
       28   g  Magnifying relay lens, green 
       28   b  Magnifying relay lens, blue 
       30  Spatial light modulator 
       30   r  Spatial light modulator, red 
       30   g  Spatial light modulator, green 
       30   b  Spatial light modulator, blue 
       31  Folding mirror 
       32  Projection lens 
       38  Light modulation assembly 
       38   r  Light modulation assembly, red 
       38   g  Light modulation assembly, green 
       38   b  Light modulation assembly, blue 
       40  Display surface 
       60  Color-sensitive polarization filter 
       60   r  Color-selective polarization filter, red 
       60   g  Color-selective polarization filter, green 
       60   b  Color-selective polarization filter, blue 
       70  Prepolarizer 
       72  Analyzer 
       80  Illumination relay lens 
       82  Relay lens 
       100  Housing 
       102  Ring bore 
       104  Chassis wall 
       106  Modulator mount 
       108  Recess 
       109  Air flow 
       110  Modulator mounting plate 
       112  Top plate 
       114  Slots 
       116  Side surface 
       118  Registration recess 
       120  Base plate 
       122  Wire grid polarizing beamsplitter 
       124  Coplanar registration surfaces 
       124 ′ Coplanar registration surfaces 
       126  Edge guides 
       126 ′ Edge guides 
       128  Beamsplitter seating base