Patent Publication Number: US-2023152590-A1

Title: Compact polarized illuminators using reflective polarizers

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation filing of U.S. application Ser. No. 17/248,963, filed Feb. 16, 2021, now allowed, which is a divisional filing of U.S. application Ser. No. 15/536,703, filed Jun. 16, 2017, issued as U.S. Pat. No. 10,955,666, which is a national stage filing under 35 C.F.R. 371 of PCT/US15/66563, filed Dec. 18, 2015, which claims the benefit of U.S. Provisional Application No. 62/099,138, filed Dec. 31, 2014, the disclosures of which are incorporated by reference in their entireties herein. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to optical projection systems, with particular application to such systems that are small in size, e.g., small enough so that all of the optical components of the projection system can fit within the palm of one&#39;s hand, or so that the projection system can be incorporated into a device or apparatus that can be worn by a user, such as a helmet, eyewear, or other headgear. The invention also relates to components relating to such projection systems, such as compact polarized illuminators, compact polarizing beamsplitters, and related articles, systems, and methods. 
     BACKGROUND 
     Projection systems and components such as illuminators and beamsplitters, including polarized illuminators and polarizing beamsplitters, are known. However, many such systems and components are not designed to provide high efficiency or high brightness in a small physical size suitable for use in compact applications, such as wearable or pocket-sized electro-optical devices. 
     BRIEF SUMMARY 
     We have developed new families of projection systems, and related components such as polarized illuminators and polarizing beamsplitters, that are highly suited for use in compact applications. In some cases, miniaturization is enabled or promoted by the use of a folded optical path, as provided in a reflective cavity or a beamsplitter. In some cases, miniaturization is promoted by the use of an illumination beam that converges at the location where it impinges upon a spatial light modulator. In some cases, miniaturization is promoted by the use of a beamsplitter that comprises opposed prisms of substantially different sizes. In some cases, miniaturization is promoted by beamsplitters whose obliquely disposed partial reflector defines a first rectangular reference space, and where at least a portion of a light source or at least a portion of a projector lens is disposed within such first rectangular reference space. In some cases, miniaturization is exemplified by systems in which a ratio of areas of the first rectangular reference space and a second rectangular reference space is within a range from 30% to 70%, where the second rectangular reference space is just large enough to encompass the optical components of the projector. In some cases, miniaturization is promoted by the use of projector lens that is small compared to the spatial light modulator, e.g., a lateral dimension of the projector lens, and/or one or more of its individual lenses, may be no more than 30% or 50% or 70% of the corresponding lateral dimension of the spatial light modulator. 
     We disclose for example compact projectors that include a beamsplitter, a light source, a spatial light modulator, and a projector lens. The beamsplitter includes a reflective polarizer, the reflective polarizer obliquely disposed to define a diagonal of a first rectangular reference space. The light source is disposed proximate the reflective polarizer and is configured to emit an input light beam towards the reflective polarizer. The spatial light modulator is disposed to receive an output illumination beam derived from the input light beam, the spatial light modulator adapted to selectively reflect the output illumination beam to provide a patterned light beam, which the projector lens is adapted to receive. The beamsplitter, the light source, the spatial light modulator, and the projector lens are encompassed by a second rectangular reference space. The first rectangular reference space has an area A1 and the second rectangular reference space has an area A2, and 30%&lt;A1/A2&lt;70%. 
     We disclose projectors that include a reflective polarizer, a light source, a spatial light modulator, and a projector lens. The reflective polarizer is obliquely disposed to define a diagonal of a first rectangular reference space. The light source is disposed proximate the reflective polarizer and is configured to emit an input light beam towards the reflective polarizer. The spatial light modulator is disposed to receive an output illumination beam derived from the input light beam, and adapted to selectively reflect the output illumination beam to provide a patterned light beam. The projector lens is adapted to receive the patterned light beam. At least a portion of the light source, or at least a portion of the projector lens, is disposed within the first rectangular reference space. 
     We disclose beamsplitters that include a first prismatic body comprising a first prism, a second prismatic body comprising a second prism, and a reflective polarizer sandwiched between the first and second prismatic bodies. The first prism is substantially smaller than the second prism. 
     We disclose illuminators that include a reflector and a reflective polarizer disposed to form a reflective cavity with the reflector. The illuminators also include a retarder film disposed within the reflective cavity, and a light source disposed to emit a polarized input light beam into the reflective cavity through an aperture in the reflector. The reflector, the reflective polarizer, and the retarder film are configured to produce an output illumination beam from the input light beam, and the output illumination beam is polarized. 
     We disclose illuminators that include a reflector, a reflective polarizer disposed obliquely relative to the reflector, and a retarder film disposed between the reflector and the reflective polarizer, and a light source disposed to emit an input light beam of a first polarization state through the reflective polarizer towards the reflector. The reflector, the reflective polarizer, and the retarder film are configured to produce an output illumination beam from the input light beam, and the output illumination beam has a second polarization state orthogonal to the first polarization state. 
     We also disclose projectors that include the foregoing illuminators. 
     Related methods, systems, and articles are also discussed. 
     These and other aspects of the present application will be apparent from the detailed description below. In no event, however, should the above summaries be construed as limitations on the claimed subject matter, which subject matter is defined solely by the attached claims, as may be amended during prosecution. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The inventions will now be described with reference to the following figures, in which: 
         FIG.  1    is a schematic top view of a compact projection system; 
         FIG.  2    is a schematic front view of a reflector used in the projection system of  FIG.  1   ; 
         FIG.  3    is a schematic front view of a spatial light modulator used in the projection system of  FIG.  1   ; 
         FIG.  4    is a schematic side view of a portion of a compact projection system, wherein the compact illuminator comprises a “solid cavity” type reflective cavity; 
         FIG.  5    is a schematic side view of a portion of a compact projection system, wherein the compact illuminator comprises a “hollow cavity” type reflective cavity; 
         FIG.  6 A  is a schematic perspective view of an optical body for use as a reflective cavity of a compact illuminator, and  FIGS.  6 B,  6 C, and  6 D  are schematic rear, side, and top views of the optical body; 
         FIG.  7    is a schematic side view of another compact projection system; 
         FIG.  8    is a schematic side view of an optical body for use as a reflective cavity of a compact illuminator, where one surface is roughened to promote light scattering and make the output illumination beam more spatially uniform; 
         FIG.  9 A  is a graph of irradiance versus position at the output plane of the illumination beam, where the surface roughness provides a Gaussian scattering angle of 0.01 degrees; 
         FIG.  9 B  is a graph similar to that of  FIG.  9 A , but where the surface roughness provides a Gaussian scattering angle of 0.1 degrees; 
         FIG.  9 C  is a graph similar to that of  FIG.  9 A , but where the surface roughness provides a Gaussian scattering angle of 1 degree; 
         FIG.  9 D  is a graph similar to that of  FIG.  9 A , but where the surface roughness provides a Gaussian scattering angle of 2 degrees; 
         FIG.  9 E  is a graph similar to that of  FIG.  9 A , but where the surface roughness provides a Gaussian scattering angle of 4 degrees; 
         FIG.  10 A  is a duplicate of  FIG.  9 C ; 
         FIG.  10 B  is a graph similar to that of  FIG.  9 A , but where the reflective cavity utilizes a hollow cavity, and the surface roughness provides a Gaussian scattering angle of 1 degree; 
         FIG.  11    is a schematic top or side view of another compact projection system, this projector having an illuminator that uses a polarizing beamsplitter of substantially different prism sizes; 
         FIG.  11 A  is the same schematic view as  FIG.  11   , but with two rectangular reference spaces superimposed on the projector; 
         FIG.  11 B  is the same schematic view as  FIG.  11   , but with on-axis and off-axis image rays of a pass polarization state drawn to show their propagation through the system; 
         FIG.  11 C  is the same schematic view as  FIG.  11   , but with on-axis and off-axis image rays of a block polarization state drawn to show their propagation through the system; 
         FIG.  12    is a schematic top or side view of another compact projection system, this projector also having an illuminator that uses a polarizing beamsplitter of substantially different prism sizes; 
         FIG.  13    is a schematic top or side view of another compact projection system, this projector also having an illuminator that uses a polarizing beamsplitter of substantially different prism sizes; 
         FIG.  14    is a schematic top or side view of another compact projection system, this projector having an illuminator that uses both a reflective cavity and a polarizing beamsplitter of substantially different prism sizes; 
         FIG.  14 A  is a magnified schematic view of the reflecting layer and the retarding layer on the outer surface of the optical body in  FIG.  14   ; 
         FIG.  14 B  is the same schematic view as  FIG.  14   , but with two rectangular reference spaces superimposed on the projector; 
         FIG.  15    is a schematic top or side view of another compact projection system, this projector having an illuminator that uses both a reflective cavity and a polarizing beamsplitter, the projection system also including a detector device; 
         FIG.  15 A  is the same schematic view as  FIG.  15   , but with two rectangular reference spaces superimposed on the projector; and 
         FIG.  16    is a schematic side view of another projection system. 
     
    
    
     In the figures, like reference numerals designate like elements. 
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     As mentioned above, we have developed new families of projection systems and components thereof that are well suited to miniaturization. These systems and components may use one or more of the following features: a folded optical path, as in a reflective cavity or a (e.g. polarizing) beamsplitter; an illumination beam that is converging at the place where it impinges upon the spatial light modulator; a beamsplitter that uses opposed prisms of substantially different sizes; a beamsplitter whose obliquely disposed partial reflector defines a first rectangular reference space, and where at least a portion of the light source or at least a portion of the projector lens is disposed within such first rectangular reference space; a system in which a ratio of areas of the first rectangular reference space and a second rectangular reference space is within a range from 30% to 70% or 40% to 70%, where the second rectangular reference space is just large enough to encompass the optical components of the projector such as a beamsplitter, the light source, the spatial light modulator, and the projector lens; a system in which the projector lens is small compared to the active area of the spatial light modulator, e.g., a lateral dimension of the projector lens, and/or one or more of its individual lenses, may be no more than 30% or 50% or 70% of the corresponding lateral dimension of the spatial light modulator; a system in which the collection efficiency of the projector lens is substantially uniform over the area of the spatial light modulator. 
     Turning then to  FIG.  1   , we see there a schematic top view of a compact projector  110 . For reference purposes, the projector  110  is drawn in the context of a Cartesian x-y-z coordinate system, where it is assumed that the x-z plane defines a horizontal plane and the y-axis is a vertical axis, but other conventions may also be used. The projector  110  includes an illuminator  140 , a spatial light modulator  150 , and a projector lens  160 . The z-axis of the coordinate system is assumed to be parallel to an optical axis  142  of the projector  110  and its component parts. The illuminator  140  produces an output illumination beam  172  that impinges upon the spatial light modulator  150 . In exemplary embodiments, the output illumination beam  172  is substantially spatially uniform over the entire active area of such modulator  150 , so that the brightness of the projected image is also substantially uniform. A conventional electronic controller (not shown) couples to the spatial light modulator  150  and controls the states of the individual elements (pixel elements)  152  in an image-wise fashion. The pixel elements  152  are usually arranged in a grid of rows and columns to provide a rectangular active area. A given pixel element  152  may have two states—“on” or “off”, as in the case of a monochrome display—or it may have red, green, and blue sub-elements to provide a full color image. Other conventional configurations of the spatial light modulator  150  are also contemplated. In the embodiment of  FIG.  1   , the spatial light modulator  150  is a transmissive-type modulator. The spatial light modulator  150  thus converts the output illumination beam  172  into a transmitted patterned light beam  174  which contains the image-wise or spatially patterned information from the electronic controller. The modulator  150  may be a non-polarized transmissive device, such as a microelectromechanical system (MEMS), or it may be a liquid crystal based modulator. In the latter case, the modulator  150  selectively rotates the polarization of the light exiting the array of pixels, and further in that case a polarizer (not shown) is inserted into the projector  110  after the spatial light modulator to filter out “on” pixels from “off” pixels. 
     The patterned light beam  174  is then intercepted by the projector lens  160  to produce a projected output beam  176 . The output beam  176  may produce a real image, e.g. an image that can be displayed on a physical surface or substrate remotely disposed relative to the projector  110 , or it may produce a virtual image, e.g. one that may be viewed directly by the eye of a user. The projector lens  160  typically, but not in all cases, is a module that includes a plurality of individual lenses arranged in series. In the embodiment of  FIG.  1   , the projector lens  160  includes individual lenses  161 ,  162 ,  163 ,  164 , and  165 . These lenses are drawn schematically, but the reader will understand that the individual lenses have curved surfaces, suitable thicknesses, and are composed of suitable optical glasses or plastics, to provide high quality optical performance. In one exemplary embodiment, the projector lens is a five-element module containing five individual lenses as set forth in the table provided further below. In an alternative embodiment, a four-element projector lens can be obtained by omitting the fifth lens (lens  5 ) from referenced five-element projector lens. The scale of  FIG.  1    is accurate in the sense that the projector lens  160 , and its individual lenses, each have a lateral dimension (e.g. parallel to the x-axis, or parallel to the y-axis, or along a diagonal of the spatial light modulator  150 ) that is substantially smaller than a corresponding lateral dimension of the spatial light modulator  150 . This is made possible by the fact that the output illumination beam  172  is a converging beam. For example, the lateral dimension of the projector lens  160 , and/or one or more of its individual lenses, may be no more than 30% or 50% or 70% of the corresponding lateral dimension of the spatial light modulator  150 . In an exemplary embodiment based on the projector lens table provided further below, the lateral dimension of the (five-element) projector lens is 2.88 mm and the length of the diagonal of the 5:4 spatial light modulator is 6 mm, for a percentage of 48%. The lateral dimension of the individual lens closest to the spatial light modulator is 2.8 mm, for a percentage of 47%. 
     The purpose of the illuminator  140  is to illuminate the active area of the spatial light modulator  150  so that an optical image or pattern can be produced. In many, but not all, cases, it is desirable for the output illumination beam  172  to be a converging light beam at the place where it impinges upon the spatial light modulator  150 , as suggested by light ray  144 . It is also often desirable for the output illumination beam  172  to be relatively uniform in brightness over the active area of the spatial light modulator  150 . Furthermore, it is often desirable to accomplish this illumination in a package that is physically small, and that uses high efficiency, high brightness light sources in order to keep heat generation low and device size small. A logical option for high efficiency, high brightness sources is one or more discrete, solid state light sources such as light emitting diodes (LEDs). However, other suitable light sources can also be used. 
     In this regard, “light emitting diode” or “LED” refers to a diode that emits light, whether visible, ultraviolet, or infrared, although in most practical embodiments the emitted light will have a peak wavelength in the visible spectrum, e.g. from about 400 to 700 nm. The term LED includes incoherent encased or encapsulated semiconductor devices marketed as “LEDs”, whether of the conventional or super radiant variety, as well as coherent semiconductor devices such as laser diodes, including but not limited to vertical cavity surface emitting lasers (VCSELs). An “LED die” is an LED in its most basic form, i.e., in the form of an individual component or chip made by semiconductor processing procedures. For example, the LED die may be formed from a combination of one or more Group III elements and of one or more Group V elements (III-V semiconductor). The component or chip can include electrical contacts suitable for application of power to energize the device. Examples include wire bonding, tape automated bonding (TAB), or flip-chip bonding. The individual layers and other functional elements of the component or chip are typically formed on the wafer scale, and the finished wafer can then be diced into individual piece parts to yield a multiplicity of LED dies. The LED die may be configured for surface mount, chip-on-board, or other known mounting configurations. Some packaged LEDs are made by forming a polymer encapsulant over an LED die and an associated reflector cup. Some packaged LEDs also include one or more phosphor materials that are excited by an ultraviolet or short wavelength visible LED die, and fluoresce at one or more wavelengths in the visible spectrum. An “LED” for purposes of this application should also be considered to include organic light emitting diodes, commonly referred to as OLEDs. 
     At the heart of the illuminator  140  is a light source  120 , which may include one or more LEDs, including in some cases one or more laser diodes. Several such LEDs can be combined to produce a desired spectral distribution of light. For example, the outputs of red-, green-, and blue-emitting LEDs may be combined to provide nominally white light, or white-emitting LEDs may be used instead or additionally. Alternatively, one or more LEDs of a specific non-white color may be used to produce colored (non-white) illumination, e.g., red, or green, or blue illumination, in which case the projected image will be monochrome rather than full color. In the schematic drawing of  FIG.  1   , the light source  120  is drawn as a single LED die  122  disposed behind a polarizer  124 . The light source  120  is assumed to emit white light, as in the case where the LED die emits blue or UV light and is covered with a thin coating (not shown) of a yellow- or white-emitting phosphor. The polarizer  124 , which may be any suitable polarizer including an absorbing linear polarizer, a multilayer polymeric reflective polarizer, or a laminate of a reflective polarizer and an absorbing polarizer, predominantly transmits only one polarization state of light, causing the light emitted by the source  120  to be polarized. In alternative embodiments, a light guide, lens, or color combiner may be inserted between the LED die  122  and the polarizer  124  to allow the LED die (or other active light source, if desired) to be mounted remotely from the polarizer. In other alternative embodiments, the polarizer  124  may be omitted, such that the light emitted by the source  120  is unpolarized. In still other cases, the polarizer  124  may be retained and a retarder film, such as a quarter wave retarder, may be added atop the polarizer  124  such that the source  120  emits rotationally polarized (circularly or elliptically polarized) light. However, in the particular embodiment shown in  FIG.  1   , the light source  120  emits polarized light because the polarizer  124  is included in front of the LED die  122 . The arrow  170  schematically represents an input light beam emitted by the light source  120  into a reflective cavity, which is also part of the illuminator  140  and is discussed further below. The input light beam  170  is assumed to comprise broad band (and polarized) white light, and is assumed to cover a distribution of propagation directions, e.g. in a Gaussian distribution of angles centered about the optical axis  142 . For illustrative purposes, a representative light ray  144  is shown originating from the light source  120  and propagating through the projector  110 . A first ray portion  144   a  is part of the input light beam  170 , and it is shown to have a polarization state P1 as a result of the polarizer  124 . 
     The input light beam  170  is emitted into a reflective cavity  130  formed by a reflective polarizer  134  and a reflector  132 . The reflector  132  has a high specular reflectivity (for example, in some cases at least 70%, or at least 80%, or at least 90%, or at least 95%) for light within the spectral range of the input light beam  170 , and for all polarization states, and in some cases it also substantially preserves the degree of the polarization of an incident light ray in the reflected ray. Any known structure or material that can supply these characteristics can be used. Metal coatings, optically enhanced metal coatings, multilayer interference structures or films, whether stacks of alternating inorganic materials, or stacks of coextruded polymers appropriately oriented or processed to provide a high reflectivity over a range of angles and over the spectral range of interest and for all polarizations, may be used for this purpose. See for example U.S. Pat. No. 5,882,774 (Jonza et al.). Alternatively, a simple metal coating such as a layer of aluminum or a layer of silver may in some cases be used for the reflector  132 . The reflective polarizer  134  provides a high reflectivity (for example, in some cases at least 70%, or at least 80%, or at least 90%) for one polarization state of light within the spectral range of interest, while also providing a low reflectivity (for example, in some cases less than 30%, or less than 20%, or less than 10%) and corresponding high transmission for light of an orthogonal polarization state in such spectral range. Any known structure or material that can supply these characteristics can be used. A wire grid polarizer, cholesteric reflective polarizer, or multilayer polymeric reflective polarizing film (comprising an interference stack of coextruded polymers appropriately oriented or processed to provide a high reflectivity for one polarization state and low reflectivity for an orthogonal polarization state) may be used for the reflective polarizer  134 . As a specific example, Advanced Polarizer Film (APF) available from 3M Company, St. Paul, Minn. may be used for the reflective polarizer  134 . Given an appropriate reflector  132  and reflective polarizer  134 , light of one polarization state can reflect back and forth within the reflective cavity  130  as illustrated by the light ray  144 . In the embodiment illustrated, the reflective polarizer  134  is oriented to provide high reflectivity for light of the first polarization state P1, and thus it will provide low reflectivity and high transmission for light of an orthogonal second polarization state P2. 
     In order for the polarized light emitted by the light source  120  to emerge from the illuminator  140 , the polarization state should be rotated from the first polarization state P1 (which is highly reflected by the reflective polarizer  134 ) to the orthogonal second polarization state P2 (which is highly transmitted by the reflective polarizer  134 ). To accomplish this rotation, we include a retarder film  136  in the reflective cavity  130 . The retarder film  136  may be located near, and may be substantially coextensive with, the reflector  132  as shown in the figure, but the retarder film  136  may alternatively be located elsewhere in the reflective cavity  130 . The amount of retardation provided by the retarder film  136  is selected to provide the desired rotation of the polarization state based on the number of passes of a representative light ray through the retarder film. In the depicted embodiment, light passes through the retarder film two times, which would lead one to select a (single-pass) retardance of a quarter-wave of light for the retarder film  136 . Thus, in reference to the light ray  144 , upon reflection at the reflective polarizer  134 , the first polarization state P1 is maintained in the reflected ray portion  144   b . This ray portion  144   b  passes through the retarder film  136 , and is reflected at the reflector  132 . The resulting reflected ray portion  144   c  passes again through the retarder film  136 , after which it acquires the second polarization state P2 (orthogonal to the state P1) as a result of the two passes through the retarder film. This ray portion  144   c  is thus highly transmitted by the reflective polarizer  134  and emerges from the illuminator  140  as ray portion  144   d , still having the second polarization state P2. The ray portion  144   c  is one of the many light rays that make up the output illumination beam  172  discussed above. 
     The reflector  132  and the reflective polarizer  134  may have the same or similar lateral dimensions, e.g., parallel to the x-axis, or parallel to the y-axis, or along a diagonal of the spatial light modulator  150 . Furthermore, the reflector  132  and reflective polarizer  134  may be shaped to have convex or concave curvatures as appropriate to focus the light from the light source to produce an output illumination beam  172  that is converging at the place where such beam impinges on the spatial light modulator  150 . One example of such curvatures is illustrated in  FIG.  1   . The curvatures may be simple spherical curvatures, or they may be aspherical. The curvatures may also in some cases be anamorphic, i.e., more highly or strongly curved in one plane (e.g. the y-z plane) than in an orthogonal plane (e.g. the x-z plane). In other cases the curvatures may be rotationally symmetric about the optical axis  142 . 
     In order for light from the light source  120  to enter the reflective cavity  130 , an aperture  138  is provided in the reflector  132 . Note, however, that a light ray within the input light beam  170  that travels parallel to the optical axis  142  (as well as light rays within a narrow angular cone centered about the optical axis  142 ) will be reflected by the reflective polarizer  134  but then will not be reflected by the reflector  132  due to the absence of the reflector  132  in the region of the aperture  138 , and such light therefore also will not become part of the output illumination beam  172 . This may give rise to a darkened area in the vicinity of the optical axis  142  in the output illumination beam  172 , at the position of the spatial light modulator  150 , such darkened area roughly analogous to a shadow of the aperture  138 . To reduce this shadowing effect, it is helpful to make the aperture  138 , as well as the light source  120 , as small as possible. 
     One or more scattering elements may also be included in the illuminator  140  to help further reduce the shadowing effect and make the output illumination beam  172  more spatially uniform. One example of such a scattering element is a grooved, textured, or otherwise roughened surface included as part of the reflective cavity  130 . To the extent the reflective cavity  130  makes uses of lenses or other optical bodies to which the various reflectors or films are applied, such optical bodies may be made by single point diamond turning, where the tooling pattern creates a series of grooves that scatter a fraction of the incident light toward the optical axis  142 . Another example of a scattering element is a layer of scattering material that is disposed within the reflective cavity  130 . Such a layer may for example be or comprise a microparticle filled adhesive layer of film, and/or a microstructured surface coated with a layer of a different refractive index. Regardless of which type of scattering element is used, it or they should substantially preserve the polarization state of light so as not to detract from the operation of the illuminator as already described. In some cases, the scattering element(s) may provide a spatially uniform scattering, i.e., a scattering that is the same as a function of the radial distance from the optical axis  142 . In other cases, the scattering element(s) may be designed to provide a spatially non-uniform scattering, e.g., a maximum amount of scattering at or near the optical axis  142 , and reduced scattering at increased radial distances from the optical axis  142 . 
     By inspection of  FIG.  1   , one can see that it is important to preserve the polarization state of light propagating within the reflective cavity  130 —except, of course, for the deliberate change in polarization caused by the retarder film  136 . If a lens or other optical body substantially filled the space between the reflector  132  and the reflective polarizer  134 , and if such optical body was made of a material that had a residual optical birefringence, such residual birefringence could change the polarization state of a given light ray as it traversed the reflective cavity  130 , such that, for example, the ray portion  144   a  was not substantially reflected at the reflective polarizer  134 , or the ray portion  144   c  was not substantially transmitted at the reflective polarizer  134 . For this reason, it is desirable to substantially fill the volume of the reflective cavity  130  with a material or medium having little or no birefringence so that proper operation of the illuminator  140  can be maintained. In one class of examples, referred to herein as “solid cavity”, a majority of the cavity volume, and in some cases substantially the entire cavity volume, comprises one or more solid light-transmissive materials having very low birefringence, such as PMMA, cyclic polyolefins, inorganic glass, or silicones. In another class of examples, referred to herein as “hollow cavity”, a majority of the cavity volume, and in some cases substantially the entire cavity volume, comprises air or vacuum. Some advantages of the hollow cavity approach include no measurable birefringence, reduced weight, and improved masking of the shadowing effect, as discussed further below. 
     In the embodiment of  FIG.  1   , the aperture  138  is provided not only in the reflector  132  but also in the retarder film  136 . Because of this, as the light ray  144  travels from the light source  120  to the spatial light modulator  150 , it passes through the retarder film  136  exactly two times. In an alternative embodiment, the aperture in the retarder film  136  could be omitted, such that the retarder film  136  was intact and continuous, with no central hole. In that case, the light ray  144  would pass through the retarder film  136  once in the ray portion  144   a , and once again in the ray portion  144   b  and again in the ray portion  144   c , for a total of three passes through the retarder film  136 . The performance of such an embodiment may not be optimal but may be adequate for some applications. The single pass retardation of the retarder film  136  in such embodiments would be selected to provide the desired rotation of the polarization state based on three passes of a representative light ray through the retarder film, which would give a result somewhat less than a quarter-wave of light. 
       FIG.  2    is taken from a different perspective to illustrate possible outer boundaries or shapes of the reflective cavity  130  and its constituent parts. In particular,  FIG.  2    is a front view of a reflector  232  which may be the same as or similar to reflector  132 , the reflector  232  being a part of a projector that may be the same as or similar to the projector  110  already described. An aperture  238  is provided in the reflector  232 , the aperture  238  likewise being the same as or similar to the aperture  138  in  FIG.  1   . From the perspective of  FIG.  2   , the entire aperture can clearly be seen. The aperture  238  is typically but not necessarily centered on an optical axis  242  of the reflective cavity. In some cases, the reflector  232  may have a round (circular) outer boundary or edge  232   a  as would be typical for a conventional round lens. In other cases, the projector size and weight may be decreased by truncating the reflector  232  to have a reduced boundary or edge  232   b . The boundary  232   b  drawn in  FIG.  2    is a rectangle whose aspect ratio is about 5:4. This 5:4 aspect ratio is intended to substantially match the aspect ratio of the active area of the spatial light modulator  150  for efficient matching of the illumination optics to the spatial light modulator, although the actual length and width dimensions of the reflector  232  may be somewhat larger than those of the spatial light modulator. Note that although only the reflector  232  is depicted in  FIG.  2   , the other chief components of the reflective cavity  130 , such as the reflective polarizer  134  and the retarder film  136 , may be provided with boundaries or edges that match or substantially match that of the reflector  232 , e.g., edge  232   a  or edge  232   b.    
       FIG.  3    schematically illustrates a front view of a spatial light modulator  350 , which may be the same as or similar to the spatial light modulator  150  in  FIG.  1   . Alternatively, the spatial light modulator  350  may represent a reflective-type spatial light modulator, as discussed in some of the embodiments below. The active area of the modulator  350  is filled with rows and columns of individual pixel elements  352 , only some of the rows and columns being shown in  FIG.  3    for simplicity. The length and width of the active area is typically substantially rectangular, and the rectangle often has a length-to-width aspect ratio of 5:4. Of course, other aspect ratios may be used, but it is sometimes desirable to match the relative shape (e.g. as characterized by an aspect ratio) of the illumination optics to that of the spatial light modulator. An electronic controller couples to the spatial light modulator  350  and controls the states of all of the individual pixels, as explained above. The spatial light modulator  350  may be configured such that the difference between “on” pixels and “off” pixels is given by a rotation in the polarization of the outgoing light ray, and/or an angular deflection of the outgoing light ray. 
       FIG.  4    illustrates a portion of a compact projection system including a compact illuminator, the illuminator comprising a “solid cavity” type reflective cavity as explained above. Thus, in  FIG.  4   , an illuminator  440  includes a light source  420  and a reflective cavity  430 . The light source  420  includes an LED die  422  and a polarizer  424 , but the light source  420  may alternatively be or comprise any of the variations discussed above with regard to light source  120 . The illuminator  440  and light source  420 , as well as a spatial light modulator  450  (having pixel elements  452 ), are arranged along an optical axis  442 . The light source injects an input light beam  470  into the reflective cavity  430 , which is defined by a reflective polarizer  434  and a reflector  432 . Disposed within the reflective cavity is a retarder film  436 . The retarder film  436  is located adjacent the reflector  432 , and an aperture  438  is provided in both the reflector and the retarder film. Light from the polarized light source  420  exits the reflective cavity  430  as output illumination beam  472 , which illuminates the active area of the spatial light modulator  450 . The spatial light modulator  450  converts the beam  472  into a transmitted patterned light beam  474 , the beam  474  containing image-wise or spatially patterned information. All of the foregoing elements of the embodiment of  FIG.  4    may be the same as or similar to the corresponding elements discussed above in connection with  FIG.  1   , and the embodiment of  FIG.  4    may be used in a projector the same as or similar to that of  FIG.  1   . Similarly, the manner in which polarized light from the light source  420  reflects back and forth within the cavity  430 , and its polarization rotated to allow it to emerge as output illumination beam  472 , is the same as or similar to the operation of the illuminator  140 , and need not be repeated here. 
     However, the particular construction of the reflective cavity  430  is worthy of some additional observations. The space between the reflector  432  and the reflective polarizer  434  defines a cavity volume, and substantially all of that cavity volume is occupied by one or more solid light-transmissive materials, in particular, a first optical lens or body  431  and a second optical lens or body  433 . The bodies  431 ,  433  are cemented together along an interface  435  with a suitable optical adhesive or other optical bonding material. For ease of manufacture the interface  435  may be planar. The bodies  431 ,  433  are composed of suitable very low birefringence optical materials as discussed in more detail above, and they may be composed of different such optical materials or the same optical material. In an alternative embodiment the two optical bodies  431 ,  433  may be replaced with a single unitary optical body without any interface  435  therein but having the same outer surfaces. The optical body  431  has an outer curved surface  431   a  to which the retarder film  436  is applied directly or by a suitable optical bonding material. The reflector  432  is applied atop the retarder film so that the retarder film is properly positioned between the reflector  432  and the reflective polarizer  434 . The reflective polarizer  434 , in turn, is applied to an outer curved surface  433   a  of the optical body  433 . Central portions of the reflector  432  and the retarder film  436  are etched, cut, or otherwise omitted to define the aperture  438 , which is appropriately sized to the light source  420 . At the aperture  438 , the outer surface  431   a  of the optical body  431  may be exposed. 
       FIG.  5    illustrates a portion of a compact projection system similar to  FIG.  4   , but for the opposite case in which the illuminator comprises a hollow cavity type reflective cavity rather than a solid cavity. Thus, in  FIG.  5   , an illuminator  540  includes a light source  520  and a reflective cavity  530 . The light source  520  includes an LED die  522  and a polarizer  524 , but the light source  520  may alternatively be or comprise any of the variations discussed above with regard to light source  120 . The illuminator  540  and light source  520 , as well as a spatial light modulator  550  (having pixel elements  552 ), are arranged along an optical axis  542 . The light source injects an input light beam  570  into the reflective cavity  530 , which is defined by a reflective polarizer  534  and a reflector  532 . Disposed within the reflective cavity is a retarder film  536 . The retarder film  536  is located adjacent the reflector  532 , and an aperture  538  is provided in both the reflector and the retarder film. Light from the polarized light source  520  exits the reflective cavity  530  as output illumination beam, which illuminates the active area of the spatial light modulator  550 . The spatial light modulator  550  converts the illumination beam into a transmitted patterned light beam  574 , the beam  574  containing image-wise or spatially patterned information. All of the foregoing elements of the embodiment of  FIG.  5    may be the same as or similar to the corresponding elements discussed above in connection with  FIG.  1   , and the embodiment of  FIG.  5    may be used in a projector the same as or similar to that of  FIG.  1   . Similarly, the manner in which polarized light from the light source  520  reflects back and forth within the cavity  530 , and its polarization rotated to allow it to emerge as an output illumination beam, is the same as or similar to the operation of the illuminator  140 , and need not be repeated here. 
     However, the particular construction of the reflective cavity  530  is worthy of some additional observations. The space between the reflector  532  and the reflective polarizer  534  defines a cavity volume, and substantially all of that cavity volume is occupied by air, or vacuum, rather than any solid light-transmissive materials. This is made possible by supporting the opposed reflectors and retarder film on the inward-facing major surfaces of two optical bodies that are spaced apart from each other. In particular, a first optical lens or body  531  has a curved major surface  531   a  that faces a second optical lens or body  533 , and the second body  533  has a curved major surface  533   a  that faces the first body  531 . The bodies  531 ,  533  may be held firmly and stably in their relative positions by a suitable substrate or framework attached to the outer edges of the bodies. The bodies  531 ,  533  may be composed of suitably transparent optical materials to allow light from the light source  520  to pass through the body  531 , and light exiting the reflective cavity  530  to pass through the body  533 . The bodies  531 ,  533  may also be composed of relatively low birefringence optical materials so that the polarization state of light passing through the body  531 , as well as the polarization state of light passing through the body  533 , is not significantly rotated. But the amount of birefringence that can be tolerated in the bodies  531 ,  533  is substantially greater than that of bodies  431 ,  433 , due to the substantially shorter optical path lengths for light rays passing through the bodies  531 ,  533  than for light rays passing through bodies  431 ,  433 . This is a result of being able to design the bodies  531 ,  533  to have thicknesses (the dimension measured along the z-axis) that are substantially less than the on-axis thickness of the reflective cavity  530 . The optical body  531  has an inner curved surface  531   a  to which the reflector  532  is applied directly or by a suitable optical bonding material. The retarder film  536  is applied atop the retarder film so that the retarder film is properly positioned between the reflector  532  and the reflective polarizer  534  when the optical bodies  531 ,  533  are properly mounted. The reflective polarizer  534 , in turn, is applied to an inner curved surface  533   a  of the optical body  533 . Central portions of the reflector  532  and the retarder film  536  are etched, cut, or otherwise omitted to define the aperture  538 , which is appropriately sized to the light source  520 . At the aperture  538 , the inner surface  531   a  of the optical body  531  may be exposed. 
     Some advantages of hollow cavity-type illuminators may include one or more of: reduced weight; no measurable birefringence in the reflective cavity, including no birefringence resulting from thermally induced stress, as may occur in a solid optical body; the curved surfaces  531   a ,  533   a  can be formed in thin substrates by microreplication, reducing stray birefringence concerns and reducing any absorptive losses; and improved masking of the shadowing effect, i.e., improved spatial uniformity of the output illumination beam, as demonstrated below in connection with  FIGS.  10 A and  10 B . 
       FIGS.  6 A through  6 D  illustrate various views of a particular optical body  637  that has been found to be useful in at least some of the disclosed compact illuminators and projectors.  FIG.  6 A  is a perspective view,  FIG.  6 B  is a rear view,  FIG.  6 C  is a side view, and  FIG.  6 D  is a top view of the optical body  637 . The  FIGS.  6 A- 6 D  are at least approximately to scale with regard to relative lengths, widths, and thicknesses of the various illustrated features. The optical body  637  is composed of a first optical body  631  attached to a second optical body  633  along a planar interface  635 . The bodies  631 ,  633  are analogous to the optical bodies  432 ,  433  of  FIG.  4   , and those optical bodies of  FIG.  4    can be designed precisely as set forth here in connection with the optical body  637 . 
     The first and second optical bodies  631 ,  633  are assumed to be made of the same very low birefringence optical material, in particular, annealed PMMA, which has a refractive index of about 1.49 at a visible wavelength of 550 nm. The first optical body  631  has an outer major convex surface  631   a  and the second optical body  633  has an outer major concave surface  633   a , and both of these surface curvatures are oriented along the same optical axis  642  of the optical body  637 . These two surface curvatures are each also aspherical, but are rotationally symmetric about the optical axis  642  (ignoring the rectangular outer boundary or edge of the bodies  637 ,  631 ,  633 ). 
     The surface  631   a  has a radius of curvature of 11.156 mm, a conic constant of 0.11055, and the following polynomial aspheric coefficients: 
     4 th  order aspheric coefficient: 0.00012286; 
     6 th  order aspheric coefficient: −1.3845E-06; and 
     8 th  order aspheric coefficient: 5.2850E-08, 
     where exponential notation is used for numbers of very small magnitude. The aspheric coefficients and other information relating to the curvature of surfaces  631   a  (and  633   a ) is provided herein using the nomenclature of LightTools™ illumination design software. 
     The surface  633   a  has a paraxial radius of curvature of 58.562 mm, a conic constant of 29.052, and the following polynomial aspheric coefficients: 
     4 th  order aspheric coefficient: 2.2997E-05; 
     6 th  order aspheric coefficient: 1.2025E-05; and 
     8 th  order aspheric coefficient: 7.0933E-08. 
     The overall length (dimension along the x-axis) and width (dimension along the y-axis) of the optical body  637  and its component bodies  631 ,  633  is 9.2 millimeters and 7.4 millimeters, respectively. The 9.2 mm length and 7.4 mm width are substantially in the proportion of 5:4. The axial thickness of the optical body  637 , i.e., the physical thickness (dimension along the z-axis) of the optical body  637  measured at the optical axis  642 , is 4.38 millimeters. 
     The performance of a compact projection system utilizing the optical body  637 , in an embodiment similar to that of  FIG.  4   , was modeled with commercial optical modeling software. Such modeling is discussed below in connection with  FIGS.  8  and  9 A through  9 E . 
       FIG.  7    is a schematic side view of another compact projection system, this one using an illuminator having a hollow cavity type of reflective cavity similar to  FIG.  5   . In  FIG.  7   , a projector includes a compact illuminator  740 , a spatial light modulator  750  (having pixel elements  752 ), and a projector lens  760  (a lens module having individual lenses  761 ,  762 ,  763 ,  764 , and  765 ). The illuminator  740  includes a light source  720  and a reflective cavity  730 . The light source  720  includes an LED die  722  and a polarizer  724 , but the light source  720  may alternatively be or comprise any of the variations discussed above with regard to light source  120 . The illuminator  740  and light source  720 , as well as the spatial light modulator  750  and the projector lens  760 , are arranged along an optical axis  742 . The light source injects an input light beam  770  into the reflective cavity  730 , which is defined by a reflective polarizer  734  and a reflector  732 . Disposed within the reflective cavity is a retarder film  736 . The retarder film  736  is located adjacent the reflector  732 , and an aperture  738  is provided in both the reflector and the retarder film. Light from the polarized light source  720  exits the reflective cavity  730  as output illumination beam, which illuminates the active area of the spatial light modulator  750 . The spatial light modulator  750  converts the illumination beam into a transmitted patterned light beam  774 , the beam  774  containing image-wise or spatially patterned information. All of the foregoing elements of the embodiment of  FIG.  7    may be the same as or similar to the corresponding elements discussed above in connection with  FIGS.  1  and  5   . Similarly, the manner in which polarized light from the light source  720  reflects back and forth within the cavity  730 , and its polarization rotated to allow it to emerge as an output illumination beam, is the same as or similar to the operation of the illuminator  140 . In brief, referring to representative light ray  744 : a first ray portion  744   a , which is part of the input light beam  770 , has a polarization state P1; a second ray portion  744   b  is generated by reflection from reflective polarizer  734 ; a third ray portion  744   c  is generated by reflection from reflector  732 , and has a rotated second polarization state P2 due to two passes of the light ray through the retarder film  736 ; a fourth ray portion  744   d , which is part of the output illumination beam, also has the second polarization state P2 and impinges upon the spatial light modulator  750 ; a fifth ray portion  744   e , which is spatially modulated by the modulator  750  as a function of whether the particular pixel  752  through which the beam passes is in an “on” or “off” state. If we assume the ray portion  744   e  is passed by the modulator  750 , it then goes on to be captured by the projector lens  760  and emitted to a remote surface or user. 
     Similar to the embodiment of  FIG.  5   , the projector and illuminator of  FIG.  7    utilize a hollow cavity type reflective cavity  730 . A first optical lens or body  731  has a curved major surface  731   a  that faces a second optical lens or body  733 , and the second body  733  has a curved major surface  733   a  that faces the first body  731 . These bodies  731 ,  733  are analogous to the optical bodies  531 ,  533  described above, except the curvatures of the respective major surfaces have been changed. Relative to the  FIG.  5    embodiment, the curvature of the surface  731   a  has been reduced (greater radius of curvature), and the curvature of the surface  733   a  has been flipped from convex to concave. In this regard, there is substantial design flexibility in selecting the curvatures of the reflectors that form the reflective cavity in the disclosed embodiments (both hollow cavity types and solid cavity types), and even non-curved or planar shapes can be used. But in many cases it is nevertheless desirable to select a set of curvatures that will produce an illumination beam that is converging at the place where it impinges upon the spatial light modulator. 
     As mentioned above, the performance of a compact projection system utilizing the optical body  637  was modeled with commercial optical modeling software. One added feature that was investigated as part of the modeling was the addition of surface roughness on one of the curved outer surfaces of the optical body, as illustrated generally in  FIG.  8   , to introduce controlled amounts of light scattering. In that figure, an optical body  837  is composed of a first optical body  831  and a second optical body  833  joined together along a planar interface  835 , and aligned along an optical axis  842 . The first optical body  831  has an outer curved surface  831   a , and the second optical body  833  has an outer curved surface  833   a . For ease of illustration, a reflector, a reflective polarizer, and a retarder film are omitted from  FIG.  8   , but the reader will understand that such optical elements are applied to the surfaces  831   a ,  833   a  in the same fashion as they are applied to the respective curved surfaces  431   a ,  433   a  as described above, to form a reflective cavity using the optical body  837 . Specifically, the retarder film and reflector (with suitable apertures) are applied to the curved surface  831   a , and the reflective polarizer is applied to the curved surface  833   a . A light source  820 , having an LED die  822  and a polarizer  824 , injects an input light beam  870  into the reflective cavity, and an output illumination beam  872  emerges from the other side of the reflective cavity to illuminate the spatial light modulator, represented schematically in  FIG.  8    by reference number  854 . Propagation of light back and forth within the reflective cavity is substantially as described in  FIG.  1   . 
     The optical modeling was able to create a controlled amount of surface roughness along the curved surface  831   a , while otherwise maintaining the original nominal curvature of that surface. The same controlled surface roughness was also imputed to the reflector (see e.g. reflector  432  in  FIG.  4   ), which was assumed to be applied to such surface. The modeling assumed the curvatures, thicknesses, and material properties discussed above for the optical body  637  of  FIG.  6   . The optical modeling also assumed:
         the reflector (refer again to reflector  432  in  FIG.  4   ) had a reflectivity of 100% for all polarizations of light;   the reflective polarizer (refer e.g. to reflective polarizer  434  in  FIG.  4   ) had a 100% reflectivity for a first polarization state and 0% reflectivity (100% transmission) for an orthogonal second polarization state;   the size of the aperture (refer e.g. to aperture  438  in  FIG.  4   ) was 0.3 by 0.5 millimeters;   a spatial light modulator having a 5:4 length-to-width aspect ratio and diagonal measuring 6.0 millimeters;   a projector lens module that included five individual lenses arranged in series as shown in the following table, where lens  1  refers to the individual lens that is farthest from the spatial light modulator and lens  5  refers to the individual lens that is closest to the spatial light modulator, where a negative radius (R1, R2) denotes a concave curvature, a positive radius denotes a convex curvature, CT denotes center thickness, RICH denotes R1 center height, and DIA denotes the lens diameter:       

                            5-element Projector Lens Details                                          Lens    Material   R1  (mm)   R2  (mm)   CT  (mm)   R1CH  (mm)   DIA  (mm)                                                 1   LAFN21   2.5732   −8.0998   0.6   7.8738   2.8        2   SF5   −9.5469   −2.454   0.55   7.0738   2.55       3   SF2   7.6121   −2.8515   0.25   6.2238   2.11       4   NBFD3   2.8515   5.393   0.75   5.9738   2.06       5   SF5   −26.823   15.894   0.5   4.355    2.88                    
Note from the table that the overall lateral dimension (diameter) of the projector lens is only 2.88 millimeters, and the diameter of the individual lens that is nearest the spatial light modulator (lens  1 ) is only 2.8 mm.
 
     The optical modeling simulated the distribution of light produced by the output illumination beam as imaged by the projector lens onto a far-field detector plane, and calculated the brightness or intensity (irradiance) of that far field image as a function of position. The modeling was repeated for different amounts of surface roughness on the curved surface  831   a , and the results are shown in  FIGS.  9 A  through  9 E. In  FIGS.  9 A,  9 B,  9 C,  9 D, and  9 E , surface roughnesses corresponding to Gaussian diffusion angles of 0.01 degrees, 0.1 degrees, 1 degree, 2 degrees, and 4 degrees respectively were assumed. In the figures, the solid curves  902   a ,  902   b ,  902   c ,  902   d , and  902   e  represent the calculated irradiance-versus-position curves along the horizontal or x-axis in the far-field plane, and the dashed curves  904   a ,  904   b ,  904   c ,  904   d , and  904   e  represent the calculated irradiance-versus-position curves along the vertical or y-axis in the far-field plane. Inspection and comparison of  FIGS.  9 A through  9 E  confirms that increased amounts of surface roughness and light scattering yield a more uniform irradiance distribution and less pronounced dark spot or shadow in the center of the image. 
     The optical modeling software was then used again to establish a comparison between a solid cavity type illuminator and a hollow cavity type illuminator. The results of  FIGS.  9 A through  9 E  all assume a solid cavity type illuminator. We simulated a hollow cavity type illuminator by programming the modeling software to replace all of the annealed PMMA material within the reflective cavity with air (refractive index=1), but keeping everything else including the curvatures, distances, reflectivities, and so forth the same. This comparison was done for the case of a surface roughness corresponding to a Gaussian diffusion angle of 1 degree. The results for the solid cavity type system were shown in  FIG.  9 C , and are reproduced in  FIG.  10 A  for ease of comparison. The results for the corresponding hollow cavity type system are shown in  FIG.  10 B , where solid curve  1002   b  represents the calculated irradiance-versus-position curves along the horizontal or x-axis in the far-field plane, and the dashed curve  1004   b  represents the calculated irradiance-versus-position curves along the vertical or y-axis in the far-field plane. Comparison of  FIGS.  10 A and  10 B  confirm the conclusion that a hollow cavity design for the compact illuminator provides improved masking of the shadowing effect and improved spatial uniformity of the output illumination beam. 
     Turning now to  FIG.  11   , we see there a different type of compact illuminator and projector  1110 . Rather than using a reflective cavity configuration as depicted in  FIG.  1   , the projector  1110  uses a polarizing beamsplitter  1180 , and the beamsplitter uses opposed prisms of substantially different sizes. The projector also comprises a light source  1120 , which includes an LED die  1122  and polarizer  1124 , a spatial light modulator  1150  (see  FIG.  11 A ), which includes pixel elements  1152 , and projector lens  1160 , which includes individual lenses  1161 ,  1162 ,  1163 ,  1164 . The light source, spatial light modulator, and projector lens may be the same as or similar to the various light sources, spatial light modulators, and projectors discussed in the preceding embodiments, except that the spatial light modulator in the projector  1110  is designed to operate in reflection rather than in transmission. As a result, the spatial light modulator  1150  may for example be or comprise a Liquid Crystal on Silicon (LCoS) device. Such devices may be flat (as illustrated schematically), or they may include a curved reflector array that is curved in one or more axes. 
     The light source  1120  emits a polarized input light beam  1170  into the beamsplitter  1180  towards an obliquely disposed reflective polarizer  1184 . The reflective polarizer  1184  may be the same as or similar to the reflective polarizers discussed in the preceding embodiments, except that the reflective polarizer  1184  may if desired by optimized for oblique angle performance and in an immersed configuration (surrounded by solid light-transmissive optical materials such as glass or plastic). The reflective polarizer is sandwiched between two opposed prismatic bodies to form the beamsplitter  1180 . A first such prismatic body  1190  contains two facets  1192   a ,  1192   b  which define a relatively small prism  1192 . The input light beam  1170  enters the beamsplitter  1180  through the facet  1192   a , and spatially patterned light from the spatial light modulator (as explained below) exits the beamsplitter through the facet  1192   b . For size comparison purposes, the boundaries of the small prism  1192  can be ascertained by extending the facets  1192   a ,  1192   b  to the reflective polarizer  1184  to define the labeled points c and b, respectively. A third point, a, lies at the intersection of the facets  1192   a ,  1192   b , at the apex of the small prism  1192 . The set of points a, b, c may thus be considered to define the corners of the small prism  1192 . 
     The other prismatic body, opposite the first prismatic body  1190 , is a relatively larger prism  1195 . The corners of this prism  1195  are clearly seen as labeled points d, e, and f. Attached to the side surfaces of the prism  1195  are lenses or other optical bodies having exterior curved surfaces. In alternative embodiments, one or both of these optical bodies may be combined with the prism  1195  in a unitary prismatic body. 
     A reflector  1182  is applied to one of these exterior curved surfaces as shown in  FIG.  11   , and a retarder film  1186  is disposed between the reflector  1182  and the reflective polarizer  1184 . The reflector  1182  may be the same as or similar to the reflectors  132 ,  232 ,  432 ,  532 ,  732  discussed above, e.g., the reflector  1182  may have a high specular reflectivity (for example, in some cases at least 70%, or at least 80%, or at least 90%, or at least 95%) for light within the spectral range of the input light beam  1170 , and for all polarization states, and in some cases it also substantially preserves the polarization state of an incident light ray in the reflected ray. The retarder film  1186  may be the same as or similar to the retarding films discussed in the preceding embodiments. 
     The light source  1120  in combination with the beamsplitter  1180  serves as an illuminator. As will be described in relation to a representative light ray  1144 , light from the light source  1120  propagates within the beamsplitter  1180  until it emerges as an output illumination beam  1172 . The output illumination beam  1172  illuminates the active area of the spatial light modulator  1150  (see  FIG.  11 A ) so that an optical image or pattern can be produced. In many, but not all, cases, it is desirable for the output illumination beam  1172  to be a converging light beam at the place where it impinges on the spatial light modulator  1150 . It is also often desirable for the output illumination beam  1172  to be relatively uniform in brightness over the active area of the spatial light modulator  1150 . 
     In reference to the light ray  1144 , a first ray portion  1144   a  is part of the input light beam  1170 , which enters the beamsplitter  1180  through the facet  1192   a . The ray portion  1144   a  is caused by the polarizer  1124  to have a second polarization state P2. The reflective polarizer  1184  is configured and oriented to highly transmit light of the second polarization state P2, and highly reflect light of the orthogonal first polarization state P1. Therefore, upon encountering the reflective polarizer  1184 , the ray portion  1144   a  is highly transmitted through the reflective polarizer  1184  into the prism  1195  to become ray portion  1144   b , still having the second polarization state P2. The ray then passes through the retarder film  1186  to become ray portion  1144   c , is reflected by the reflector  1182  to become ray portion  1144   d , and passes through the retarder film  1186  again to become ray portion  1144   e . As a result of the two passes through the retarder film  1186 , the ray portion  1144   e  has the first polarization state P1, orthogonal to the original second polarization state P2. Upon encountering the reflective polarizer  1184 , the ray portion  1144   e  is highly reflected to become ray portion  1144   f , and finally emerging from the lower curved surface of the illuminator as ray portion  1144   g , which can be considered part of the output illumination beam  1172 . 
     The spatial light modulator  1150  (see  FIG.  11 A ) is placed in the output illumination beam  1172  at the output of the illuminator, and it selectively reflects incident light in an image-wise fashion. For example, pixel elements  1152  in an “on” state may rotate the polarization state of the reflected light by 90 degrees, whereas pixel elements  1152  in an “off” state may produce no such polarization rotation, or vice versa. In either case, the spatial light modulator  1150  converts the output illumination beam  1172  into a reflected patterned light beam  174  which is transmitted through the same beamsplitter  1180 , through the facet  1192   b  to the projector lens  1160 , as explained below. The projector lens  1160  then transforms the reflected patterned light beam into a projected output beam  1176 , analogous to the projected output beams discussed in the preceding embodiments. 
     One can see that, in connection with the operation of the beamsplitter  1180 , it is again important to preserve the polarization state of light propagating within the beamsplitter, except for the deliberate change in polarization caused by the retarder film  1186 . It is therefore desirable for the solid light-transmissive optical materials that make up the prisms  1192 ,  1195  and other optical bodies connected thereto to be made of very low birefringence materials as explained above. 
     One design feature of the projector  1110  that facilitates miniaturization is the substantial size disparity of the prisms  1192 ,  1195 . By making the prism  1192  substantially smaller than the other prism, the light source  1120 , and/or the projector lens  1160 , can be brought closer to the reflective polarizer  1184 , thus reducing the overall projector size. Note also however that the small prism  1192  need not be microscopic in size relative to the larger prism; instead, the small prism  1192  is large enough so that light that passes through the first prismatic body  1190  is predominantly, or mostly, also transmitted through the small prism  1192 . The relative sizes of the opposed prisms can be characterized in terms of their hypotenuse lengths and/or their prism heights. The hypotenuse length of the small prism  1192 , which we may call HL1, is the distance between the points b and c, and the hypotenuse length of the other prism  1195 , which we may call HL2, is the distance between the points d and f. The prism heights of the prisms  1192 ,  1195 , which we may call H1 and H2, respectively, are illustrated in  FIG.  11    as being measured relative to the reflective polarizer  1184 . To characterize the size disparities of the opposed prisms, we may specify that the ratio HL1/HL2 is in a range from 40% to 70%, and/or that the ratio H1/H2 is similarly in a range from 40% to 70%. 
       FIG.  11 A  is a substantial duplicate of  FIG.  11   , where like reference numbers designate like elements with no further need for discussion, except that the reflective spatial light modulator  1150 , already described above, is now shown in its proper position to intercept the output illumination beam  1172 . Superimposed on the figure are two rectangular reference spaces  1103 ,  1105 . The first reference space  1103  is a rectangular space defined by the obliquely oriented reflective polarizer  1184 . More specifically, the reference space  1103  is a rectangle whose diagonal line is the reflective polarizer  1184 . This roughly corresponds to the rectangular space occupied by a conventional beamsplitter. On the other hand, the second reference space  1105  may be described as the smallest rectangular space that encompasses the optical components (exclusive of any mechanical mounting hardware or the like) of the projector. For the projector  1110 , those optical components are the beamsplitter  1180 , the light source  1120 , the spatial light modulator  1150 , and the projector lens  1160 . 
     As a result of our miniaturization efforts, several new relationships can be satisfied by the projector  1110  and/or other disclosed projectors. 
     For example, at least a portion of the light source  1120 , or at least a portion of the projector lens  1160 , are disposed within the first rectangular reference space  1103 . In some cases at least a portion of the light source  1120  and at least a portion of the projector lens  1160  are both disposed within the first rectangular reference space  1103 . In some cases, at least a portion of the light source, or at least a portion of the projector lens, but not both, are disposed within the first rectangular reference space  1103 . The first rectangular reference space  1103  may be split into two portions, along the diagonal of the reflective polarizer  1184 . When so split, the at least a portion of the light source  1120 , or the at least a portion of the projector lens  1160 , is disposed within one such portion of the first rectangular reference space  1103 . Furthermore, the at least a portion of the light source and the at least a portion of the projector lens may both be disposed within such portion of the first rectangular reference space  1103 . If the light source  1120  comprises an LED die  1122 , the LED die  1122  may be entirely disposed within the first rectangular reference space  1103 . If the projector lens  1160  comprises a plurality of individual lenses  1161 ,  1162 ,  1163 ,  1164 , at least one of the individual lenses may be entirely disposed within the first rectangular reference space  1103 . 
     Also, the projector can be miniaturized to such an extent that the optical components of the projector (exclusive of any mechanical mounting hardware or the like) can fit within a space that is only slightly larger than the space occupied by a conventional beamsplitter for that projector. Stated differently, if A1 is the area of the first rectangular reference space  1103 , and A2 is the area of the second rectangular reference space  1105 , then the ratio of A1/A2 may be in a range from 30% to 70%, or from 40% to 70%. 
     The foregoing relationships relating to reference spaces, locations of elements within or outside of such spaces, comparisons of areas of such spaces, and so forth can be ascertained with reasonable accuracy by reference to  FIGS.  11  and  11 A  despite the fact that these figures are schematic in nature. Although schematic, the relative sizes and positions of pertinent system components in these figures are believed to be reasonably representative of how such components would appear in an actual scale drawing. This can also be said for at least  FIGS.  14 ,  14 B,  15 , and  15 A  below. Based on  FIG.  11 A , the ratio of the area of reference space  1103  to the area of reference space  1105  (A1/A2) is at least 60%. 
       FIGS.  11 B and  11 C  are provided to further illustrate the passage of light through the projector  1110  after the output illumination beam  1172  is reflected by the spatial light modulator  1150 .  FIG.  11 B  covers the case of light reflected by “on” pixels of the modulator  1150 , whereas  FIG.  11 C  covers the case of light reflected by “off” pixels of the modulator. Otherwise, in these figures, like reference numbers to those in  FIGS.  11  and  11 A  refer to like elements, and need no further explanation. 
     In  FIG.  11 B , light from the output illumination beam  1172  is reflected by an on-axis pixel  1152   a  and by an off-axis pixel  1152   b , both of these pixels assumed to be in an “on” state such that the reflected light has a rotated polarization, i.e., the reflected light has the second polarization state P2 rather than state P1. Light of this polarization state P2 is highly transmitted by the reflective polarizer  1184 . Thus, the light reflected by the “on” pixels is transmitted through the reflective polarizer  1184  and exits the beamsplitter  1180  through the facet  1192   b  of the prism  1192 , and is collected by the projector lens  1160  to ultimately form the projected output beam  1176 . 
     On the other hand, in  FIG.  11 C , light from the output illumination beam  1172  is again reflected by the on-axis pixel  1152   a  and by the off-axis pixel  1152   b , but now these pixels are assumed to be in an “off” state such that the reflected light does not have a rotated polarization, i.e., the reflected light has the first polarization state P1. Light of this polarization state P1 is highly reflected by the reflective polarizer  1184 . Thus, the light reflected by the “off” pixels is reflected by the reflective polarizer  1184  and thereafter reflected by the reflector  1182  towards the left side of the beamsplitter  1180 , where such rays may be absorbed or otherwise lost. Light reflected by the “off” pixels thus do not get collected by the projector lens  1160 , and do not substantially contribute to the projected output beam  1176 . 
       FIGS.  12  and  13    represent modifications to the projector  1110 , where the first prismatic body  1190  is replaced by alternative prismatic bodies. These replacements do not substantially change the manner in which light propagates between the light source, spatial light modulator, and projection lens, and thus that description will not be repeated for these figures. 
     In  FIG.  12   , a projector  1210  uses a polarizing beamsplitter  1280  having opposed prisms of substantially different sizes. The projector  1210  also comprises a light source  1220 , a spatial light modulator  1250  which includes pixel elements  1252 , and a projector lens  1260  which includes individual lenses, and all of these elements may be the same as or similar to corresponding elements in the projector  1110 . 
     The beamsplitter  1280  includes an obliquely disposed reflective polarizer  1284  sandwiched between two opposed prismatic bodies: a first such prismatic body  1290  contains two facets  1292   a ,  1292   b  which define a relatively small prism  1292 , and the other prismatic body is a relatively larger prism  1295 . Polarized light from the light source  1220  enters the beamsplitter  1280  through the facet  1292   a , and spatially patterned light from the spatial light modulator  1250  exits the beamsplitter through the facet  1292   b . The boundaries of the small prism  1292  can be ascertained by extending the facets  1292   a ,  1292   b  to the reflective polarizer  1284  to define the labeled points c and b, respectively, and a third point, a, lies at the intersection of the facets  1292   a ,  1292   b . The corners of the larger prism  1295  are clearly seen as labeled points d, e, and f. Attached to the side surfaces of the prism  1295  are lenses or other optical bodies having exterior curved surfaces. A reflector  1282  is applied to one of these exterior curved surfaces, and a retarder film  1286  is disposed between the reflector  1282  and the reflective polarizer  1284 . The reflector  1282  and retarder film  1286  may be the same as or similar to corresponding elements of the projector  1110 . The light source  1220  in combination with the beamsplitter  1280  serves as an illuminator. Light that is reflected by “on” pixels of the spatial light modulator is transmitted through the reflective polarizer  1284  and exits the beamsplitter  1280  through the facet  1292   b  of the prism  1292 , and is collected by the projector lens  1260  to ultimately form the projected output beam  1276 . 
     In this embodiment, the first prismatic body  1290  covers only a portion of the reflective polarizer  1284 , and the first prismatic body  1290  comprises no prisms other than the prism  1292 . The locations of the points a, b, c, d, e, f may be substantially the same as corresponding points in the projector  1110 , hence, the prism dimensions H1, H2, HL1, and HL2, and their various ratios may satisfy the same conditions as discussed above. 
     In  FIG.  13   , a projector  1310  uses a polarizing beamsplitter  1380  having opposed prisms of substantially different sizes. The projector  1310  also comprises a light source  1320 , a spatial light modulator  1350  which includes pixel elements  1352 , and a projector lens  1360  which includes individual lenses, and all of these elements may be the same as or similar to corresponding elements in the projector  1110 . 
     The beamsplitter  1380  includes an obliquely disposed reflective polarizer  1384  sandwiched between two opposed prismatic bodies: a first such prismatic body  1390  contains two facets  1392   a ,  1392   b  which define a relatively small prism  1392 , and the other prismatic body is a relatively larger prism  1395 . Polarized light from the light source  1320  enters the beamsplitter  1380  through the facet  1392   a , and spatially patterned light from the spatial light modulator  1350  exits the beamsplitter through the facet  1392   b . The boundaries of the small prism  1392  can be ascertained by extending the facets  1392   a ,  1392   b  to the reflective polarizer  1384  to define the labeled points c and b, respectively, and a third point, a, lies at the intersection of the facets  1392   a ,  1392   b . The corners of the larger prism  1395  are clearly seen as labeled points d, e, and f. Attached to the side surfaces of the prism  1395  are lenses or other optical bodies having exterior curved surfaces. A reflector  1382  is applied to one of these exterior curved surfaces, and a retarder film  1386  is disposed between the reflector  1382  and the reflective polarizer  1384 . The reflector  1382  and retarder film  1386  may be the same as or similar to corresponding elements of the projector  1110 . The light source  1320  in combination with the beamsplitter  1380  serves as an illuminator. 
     In this embodiment, the first prismatic body  1390  covers substantially all of the reflective polarizer  1384 , and the first prismatic body  1390  comprises two prisms—prism  390  and prism  396 —in addition to the prism  1392 . The locations of the points a, b, c, d, e, f may be substantially the same as corresponding points in the projector  1110 , hence, the prism dimensions H1, H2, HL1, and HL2, and their various ratios may satisfy the same conditions as discussed above. 
       FIG.  14    illustrates still another compact projector  1410  and illuminator. This projector combines the reflective cavity configuration depicted in  FIG.  1    with the use of a polarizing beamsplitter having opposed prisms of substantially different sizes. 
     In this case, a light source  1420  injects an input light beam into a reflective cavity  1430  formed by a reflector  1432  and a reflective polarizer  1434 . A retarder film  1436  is disposed in the cavity, and an aperture  1438  is provided in the reflector  1432  and the retarder film  1436 . The reflector  1432 , reflective polarizer  1434 , and retarder film  1436  may be applied to the outer surfaces of an optical lens or body  1431  as shown, and may otherwise be the same as or similar to corresponding components of the reflective cavity  430  in  FIG.  4   . Light that exits the reflective cavity  1430  enters a polarizing beamsplitter  1480 . 
     The polarizing beamsplitter  1480  includes an obliquely disposed reflective polarizer  1484  sandwiched between two opposed prismatic bodies: a first such prismatic body  1490  contains two facets  1492   a ,  1492   b  which define a relatively small prism  1492 , and the other prismatic body is a relatively larger prism  1495 . Polarized light exiting the reflective cavity  1430  enters the beamsplitter  1480  through a facet of the prism  1495 , and is reflected by the reflective polarizer  1484  so that it exits the beamsplitter  1480  and impinges upon the spatial light modulator  1450 . Light whose polarization is rotated by “on” pixels of the spatial light modulator  1450  re-enter the beamsplitter  1480  and now pass through the reflective polarizer  1484 , traverse the small prism  1492 , and exit the beamsplitter  1480  by the facet  1492   b . The boundaries of the small prism  1492  can be ascertained by extending the facets  1492   a ,  1492   b  to the reflective polarizer  1484  to define the labeled points c and b, respectively, and a third point, a, lies at the intersection of the facets  1492   a ,  1492   b . The corners of the larger prism  1495  are clearly seen as labeled points d, e, and f. Attached to the lower side surface of the prism  1495  is a lens or other optical body having an exterior curved surface for optional focusing or convergence of the exiting illumination beam. In this embodiment, the beamsplitter  1480  need not contain any retarder film or reflector other than the reflective polarizer  1484 . 
     The prisms  1492 ,  1495  have respective prism heights H1 and H2 as shown, and these parameters, as well as the respective hypotenuse lengths HL1 and HL2, and their various ratios, may satisfy the same conditions discussed above. 
     With reference to representative light ray  1444 , a first ray portion  1444   a  has a first polarization state P1 which is substantially reflected by the reflective polarizer  1434 , such that the reflected ray portion  1444   b  passes through the retarder film  1436  and is reflected by the reflector  1432 . This reflection results in the ray portion  1444   c  which passes again through the retarder film  1436 , thereupon acquiring the second polarization state P2 which is highly transmitted by the reflective polarizer  1434 . The ray portion  1444   c  thus passes through the reflective polarizer  1434  to provide ray portion  1444   d , which enters the beamsplitter  1480  and is reflected by the reflective polarizer  1484  to produce reflected ray portion  1444   e . This ray exits the beamsplitter  1480  as ray portion  1444   f , reflects from an “on” pixel of the spatial light modulator  1450  (thus rotation the polarization state from P2 back to P1) as ray portion  1444   g , re-enters the beamsplitter  1480  as ray portion  1444   h  (still of polarization state P1), now passes through the reflective polarizer  1484 , traverses the prism  1492  as ray portion  1444   i , and exits the beamsplitter  1480  at facet  1492   b  on its way to the projector lens  1460 . Such light collected by the projector lens  1460  ultimately forms the projected output beam  1476  of the projector  1410 . 
       FIG.  14 A  is a magnified view of the reflector  1432  and retarder film  1436  on the outer surface  1431   a  of the optical body  1431 . 
       FIG.  14 B  is a substantial duplicate of  FIG.  14   , where like reference numbers designate like elements with no further need for discussion, and where two rectangular reference spaces  1403 ,  1405  are superimposed on the figure. A first reference space  1403  is a rectangular space defined by the obliquely oriented reflective polarizer  1484 . More specifically, the reference space  1403  is a rectangle whose diagonal line is the reflective polarizer  1484 . A second reference space  1405  is the smallest rectangular space that encompasses the optical components of the projector, i.e., the beamsplitter  1480 , the light source  1420 , the spatial light modulator  1450 , and the projector lens  1460 . The relationships discussed above with respect to corresponding rectangular reference spaces, in connection with  FIG.  11   , can be seen to apply at least in part to the reference spaces  1403 ,  1405  of the projector  1410  also. In particular, the ratio of the area of reference space  1403  to the area of reference space  1405  (A1/A2) is at least 45%. 
     The projector  1510  of  FIG.  15    is similar to that of  FIG.  14    insofar as it also combines the reflective cavity configuration depicted in  FIG.  1    with the use of a polarizing beamsplitter. However, the projector  1510  also makes use of an unused side of the beamsplitter by adding a detector device, such that the projector  1510  is also capable of functioning as a camera. 
     A light source  1520  injects an input light beam into a reflective cavity  1530  formed by a reflector  1532  and a reflective polarizer  1534 . A retarder film  1536  is disposed in the cavity, and an aperture  1538  is provided in the reflector  1532  and the retarder film  1536 . The reflector  1532 , reflective polarizer  1534 , and retarder film  1536  may be applied to the outer surfaces of an optical lens or body  1531  as shown, and may otherwise be the same as or similar to corresponding components of the reflective cavity  430  in  FIG.  4   . Light that exits the reflective cavity  1530  enters a polarizing beamsplitter  1580 . 
     The polarizing beamsplitter  1580  includes an obliquely disposed reflective polarizer  1584  sandwiched between two opposed prismatic bodies: a first prismatic body  1590  and a second prismatic body or prism  1595 . Polarized light exiting the reflective cavity  1530  enters the beamsplitter  1580  through a facet of the prism  1595 , and is reflected by the reflective polarizer  1584  so that it exits the beamsplitter  1580  and impinges upon the spatial light modulator  1550 . Light whose polarization is rotated by “on” pixels of the spatial light modulator  1550  re-enter the beamsplitter  1480  and now pass through the reflective polarizer  1584 , traverse the prismatic body  1590 , and exit the beamsplitter  1580  in a well or hole sized to fit at least a part of the projector lens  1560 . Attached to the lower side surface of the prism  1595  is a lens or other optical body having an exterior curved surface for optional focusing or convergence of the exiting illumination beam. In this embodiment, the beamsplitter  1580  need not contain any retarder film or reflector other than the reflective polarizer  1584 . 
     With reference to representative light ray  1544 , a first ray portion  1544   a  has a first polarization state P1 which is substantially reflected by the reflective polarizer  1534 , such that the reflected ray portion  1544   b  passes through the retarder film  1536  and is reflected by the reflector  1532 . This reflection results in the ray portion  1544   c  which passes again through the retarder film  1536 , thereupon acquiring the second polarization state P2 which is highly transmitted by the reflective polarizer  1534 . The ray portion  1544   c  thus passes through the reflective polarizer  1534  to provide ray portion  1544   d , which enters the beamsplitter  1580  and is reflected by the reflective polarizer  1584  to produce reflected ray portion  1544   e . This ray exits the beamsplitter  1580  as ray portion  1544   f , reflects from an “on” pixel of the spatial light modulator  1550  (thus rotation the polarization state from P2 back to P1) as ray portion  1544   g , re-enters the beamsplitter  1580  as ray portion  1544   h  (still of polarization state P1), now passes through the reflective polarizer  1584 , traverses the prismatic body  1590  as ray portion  1544   i , and exits the beamsplitter  1580  on its way to the projector lens  1560 . Such light collected by the projector lens  1560  ultimately forms a projected output beam of the projector  1510 . 
     Incoming light ray  1545  represents light originating from a body outside of the projector, but passing through the projector lens  1545 . A first ray portion  1545   a  enters the projector lens  1560 , exits the lens  1560  as ray portion  1545   b , and enters the beamsplitter  1580  as ray portion  1545   c . Assuming the ray portion  1545   a  was originally unpolarized, the ray portion  1545   c  will also be unpolarized, and will contain both components of the first polarization state P1 and of the second polarization state P2 as shown. When the ray portion  1545   c  encounters the reflective polarizer  1584 , the P1 polarization state will be transmitted into a ray portion  1545   d , and the P2 polarization state will be reflected into a ray portion  1545   e , which then exits the beamsplitter  1580  at an unused facet thereof, where a detector device  1596  has been placed. The detector device  1596  is shown as having an array of detector elements, such as in a charge-coupled device (CCD) detector array, but other known detector arrays or even an individual detector element may also be used. By including the detector device  1596  in the projector  1510 , the same projector lens  1560  that is used to project an image to a remote location can also be used to collect light from the remote location and capture that light as a camera image or the like. Such a detector device can also be incorporated into the projector  1410  of  FIG.  14    by placing the same or similar array at the unused facet  1492   a , or at a spaced distance from such facet  1492   a  with one or more lenses or other optical elements disposed therebetween. 
       FIG.  15 A  is a substantial duplicate of  FIG.  15   , where like reference numbers designate like elements with no further need for discussion, and where two rectangular reference spaces  1503 ,  1505  are superimposed on the figure. A first reference space  1503  is a rectangular space defined by the obliquely oriented reflective polarizer  1584 . More specifically, the reference space  1503  is a rectangle whose diagonal line is the reflective polarizer  1584 . A second reference space  1505  is the smallest rectangular space that encompasses the optical components of the projector, namely, the beamsplitter  1580 , the light source  1520 , the spatial light modulator  1550 , the projector lens  1560 , and the detector device  1596 . The relationships discussed above with respect to corresponding rectangular reference spaces, in connection with  FIG.  11   , can be seen to apply at least in part to the reference spaces  1503 ,  1505  of the projector  1510  also. In particular, the ratio of the area of reference space  1503  to the area of reference space  1505  (A1/A2) is at least 40%. 
       FIG.  16    schematically illustrates still another projector and illumination system. The projector  1610  requires no reflective cavity or beamsplitter. Rather, light from a light source  1620 , as represented by ray portion  1644   a  of a representative light ray  1644 , is simply collected by a pair of lenses L1, L2, although more or fewer than two lenses can be used. The lenses focus the light to produce a converging output illumination beam, represented by ray portion  1644   b . This beam impinges upon a transmissive spatial light modulator  1650 , producing a spatially patterned beam as represented by ray portion  1644   c . The spatially patterned beam is then collected and projected to a remote location by a projector lens  1660 . As a result of the converging illumination beam, the projector lens  1660 , or at least one of its individual component lenses, may have a maximum lateral dimension LD1 (e.g. as measured along the x-axis, or along the y-axis, or along a diagonal of the spatial light modulator  1650 ) that is less than the corresponding lateral dimension LD2 of the spatial light modulator  1650 , or that alternatively satisfies the relationship 30%&lt;LD1/LD2&lt;70% 
     Unless otherwise indicated, all numbers expressing quantities, measurement of properties, and so forth used in the specification and claims are to be understood as being modified by the term “about”. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the specification and claims are approximations that can vary depending on the desired properties sought to be obtained by those skilled in the art utilizing the teachings of the present application. Not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, to the extent any numerical values are set forth in specific examples described herein, they are reported as precisely as reasonably possible. Any numerical value, however, may well contain errors associated with testing or measurement limitations. 
     The following are embodiments of the present invention. 
     Embodiment 1 is a compact projector, comprising: a beamsplitter comprising a reflective polarizer, the reflective polarizer obliquely disposed to define a diagonal of a first rectangular reference space; a light source disposed proximate the reflective polarizer, the light source configured to emit an input light beam towards the reflective polarizer; a spatial light modulator disposed to receive an output illumination beam derived from the input light beam, the spatial light modulator adapted to selectively reflect the output illumination beam to provide a patterned light beam; and a projector lens adapted to receive the patterned light beam; wherein the beamsplitter, the light source, the spatial light modulator, and the projector lens are encompassed by a second rectangular reference space; and wherein the first rectangular reference space has an area A1 and the second rectangular reference space has an area A2, and wherein 30%&lt;A1/A2&lt;70%. 
     Embodiment 2 is the projector of embodiment 1, wherein 40%&lt;A1/A2&lt;70%. 
     Embodiment 3 is the projector of embodiment 1, wherein the beamsplitter comprises a first prism and a second prism disposed on opposite sides of the reflective polarizer. 
     Embodiment 4 is the projector of embodiment 3, wherein the first and second prisms have respective first and second hypotenuse lengths HL1 and HL2, and wherein 40%&lt;HL1/HL2&lt;70%. 
     Embodiment 5 is the projector of embodiment 3, wherein the first and second prisms have respective first and second prism heights H1 and H2, and wherein 40%&lt;H1/H2&lt;70%. 
     Embodiment 6 is the projector of embodiment 1, wherein the projector lens has a maximum lateral dimension LD1 and the spatial light modulator has a maximum lateral dimension LD2, and wherein LD1 is less than LD2. 
     Embodiment 7 is the projector of embodiment 6, wherein 30%&lt;LD1/LD2&lt;70%. 
     Embodiment 8 is the projector of embodiment 1, wherein the light source comprises an LED die and a polarizer, and wherein the input light beam is polarized. 
     Embodiment 9 is the projector of embodiment 1, wherein the projector further comprises a detector to receive light originating from a body outside of the projector through the projector lens. 
     Embodiment 10 is a compact projector, comprising: a reflective polarizer obliquely disposed to define a diagonal of a first rectangular reference space; a light source disposed proximate the reflective polarizer, the light source configured to emit an input light beam towards the reflective polarizer; a spatial light modulator disposed to receive an output illumination beam derived from the input light beam, the spatial light modulator adapted to selectively reflect the output illumination beam to provide a patterned light beam; a projector lens adapted to receive the patterned light beam; wherein at least a portion of the light source, or at least a portion of the projector lens, is disposed within the first rectangular reference space. 
     Embodiment 11 is the projector of embodiment 10, wherein the at least a portion of the light source and the at least a portion of the projector lens are both disposed within the first rectangular reference space. 
     Embodiment 12 is the projector of embodiment 10, wherein only one of the at least a portion of the light source and the at least a portion of the projector lens is disposed within the first rectangular reference space. 
     Embodiment 13 is the projector of embodiment 11, wherein the reflective polarizer divides the first rectangular reference space into a first portion and a second portion, and wherein the at least a portion of the light source, or the at least a portion of the projector lens, is disposed within the first portion of the first rectangular reference space. 
     Embodiment 14 is the projector of embodiment 13, wherein the at least a portion of the light source and the at least a portion of the projector lens are both disposed within the first portion of the first rectangular reference space. 
     Embodiment 15 is the projector of embodiment 10, wherein the light source comprises an LED die, and wherein the LED die is entirely disposed within the first rectangular reference space. 
     Embodiment 16 is the projector of embodiment 10, wherein the projector lens comprises a plurality of individual lenses in series, and wherein at least one of the individual lenses is entirely disposed within the first rectangular reference space. 
     Embodiment 17 is the projector of embodiment 10, wherein the reflective polarizer is part of a beamsplitter that also includes a first and second prism disposed on opposite sides of the reflective polarizer. 
     Embodiment 18 is the projector of embodiment 17, wherein the first and second prisms have respective first and second hypotenuse lengths HL1 and HL2, and wherein 40%&lt;HL1/HL2&lt;70%. 
     Embodiment 19 is the projector of embodiment 17, wherein the first and second prisms have respective first and second prism heights H1 and H2, and wherein 40%&lt;H1/H2&lt;70%. 
     Embodiment 20 is the projector of embodiment 10, wherein the projector lens has a maximum lateral dimension LD1 and the spatial light modulator has a maximum lateral dimension LD2, and wherein LD1 is less than LD2. 
     Embodiment 21 is the projector of embodiment 20, wherein 30%&lt;LD1/LD2&lt;70%. 
     Embodiment 22 is the projector of embodiment 10, wherein the projector further comprises a detector to receive light originating from a body outside of the projector through the projector lens. 
     Embodiment 23 is a polarizing beamsplitter, comprising: a first prismatic body comprising a first prism; a second prismatic body comprising a second prism; and a reflective polarizer sandwiched between the first and second prismatic bodies; wherein the first prism is substantially smaller than the second prism. 
     Embodiment 24 is the beamsplitter of embodiment 23, wherein the first and second prisms have respective first and second hypotenuse lengths HL1 and HL2, and wherein 40%&lt;HL1/HL2&lt;70%. Embodiment 25 is the beamsplitter of embodiment 23, wherein the first and second prisms have respective first and second prism heights H1 and H2, and wherein 40%&lt;H1/H2&lt;70%. 
     Embodiment 26 is the beamsplitter of embodiment 23, wherein the beamsplitter is configured such that light that passes through the first prismatic body passes predominantly through the first prism. 
     Embodiment 27 is the beamsplitter of embodiment 26, wherein the reflective polarizer comprises a first major surface facing the first prismatic body, and wherein the first prismatic body covers most of the first major surface. 
     Embodiment 28 is the beamsplitter of embodiment 27, wherein the first prismatic body covers substantially all of the first major surface. 
     Embodiment 29 is the beamsplitter of embodiment 23, wherein the first prismatic body comprises at least one prism other than the first prism. 
     Embodiment 30 is the beamsplitter of embodiment 23, wherein the first prismatic body comprises no prisms other than the first prism. 
     Embodiment 31 is a compact polarized illuminator, comprising: a reflector; a reflective polarizer disposed to form a reflective cavity with the reflector; a retarder film disposed within the reflective cavity; and a light source disposed to emit a polarized input light beam into the reflective cavity through an aperture in the reflector; wherein the reflector, the reflective polarizer, and the retarder film are configured to produce an output illumination beam from the input light beam, and wherein the output illumination beam is polarized. 
     Embodiment 32 is the illuminator of embodiment 31, wherein at least one of the reflector and the reflective polarizer is curved, and wherein the output illumination beam is converging. 
     Embodiment 33 is the illuminator of embodiment 31, wherein light follows a light path from the light source to the output illumination beam that includes passing through the aperture, reflecting from the reflective polarizer, reflecting from the reflector, passing through the reflective polarizer, and passing at least two times through the retarder film. 
     Embodiment 34 is the illuminator of embodiment 31, wherein the reflective cavity defines a cavity volume, and a majority of the cavity volume comprises at least one solid light-transmissive material. 
     Embodiment 35 is the illuminator of embodiment 31, wherein the reflective cavity defines a cavity volume, and a majority of the cavity volume comprises air or vacuum. 
     Embodiment 36 is the illuminator of embodiment 31, wherein the retarder film is proximate the reflector, and the aperture is also in the retarder film. 
     Embodiment 37 is the illuminator of embodiment 31, wherein the illuminator further comprises a scattering element to make the output illumination beam more spatially uniform. 
     Embodiment 38 is the illuminator of embodiment 37, wherein the scattering element comprises a roughened surface, and the roughened surface is part of the reflective cavity. 
     Embodiment 39 is the illuminator of embodiment 37, wherein the scattering element comprises a layer of scattering material within the reflective cavity. 
     Embodiment 40 is the illuminator of embodiment 31, wherein the light source comprises an LED and a polarizer. 
     Embodiment 41 is a compact polarized illuminator, comprising: a reflector; a reflective polarizer disposed obliquely relative to the reflector; a retarder film disposed between the reflector and the reflective polarizer; and a light source disposed to emit an input light beam of a first polarization state through the reflective polarizer towards the reflector; wherein the reflector, the reflective polarizer, and the retarder film are configured to produce an output illumination beam from the input light beam, and wherein the output illumination beam has a second polarization state orthogonal to the first polarization state. 
     Embodiment 42 is the illuminator of embodiment 41, wherein the reflective polarizer is part of a beamsplitter that also includes a first and second prism disposed on opposite sides of the reflective polarizer. 
     Embodiment 43 is the illuminator of embodiment 42, wherein the first and second prisms have respective first and second hypotenuse lengths HL1 and HL2, and wherein 40%&lt;HL1/HL2&lt;70%. 
     Embodiment 44 is the illuminator of embodiment 42, wherein the first and second prisms have respective first and second prism heights H1 and H2, and wherein 40%&lt;H1/H2&lt;70%. 
     Embodiment 45 is a projector, comprising: the polarized illuminator of embodiment 31; a spatial light modulator disposed to intercept the output illumination beam so as to produce a spatially patterned beam; and a projector lens to receive the spatially patterned beam. 
     Embodiment 46 is the projector of embodiment 45, wherein the spatial light modulator is a transmissive spatial light modulator. 
     Embodiment 47 is the projector of embodiment 45, wherein the spatial light modulator is a reflective spatial light modulator. 
     Embodiment 48 is the projector of embodiment 45, wherein the projector lens has a maximum lateral dimension LD1 and the spatial light modulator has a maximum lateral dimension LD2, and wherein LD1 is less than LD2. 
     Embodiment 49 is the projector of embodiment 45, wherein the projector further comprises a detector to receive light originating from a body outside of the projector through the projector lens. 
     Embodiment 50 is a projector, comprising: the polarized illuminator of embodiment 41; a spatial light modulator disposed to intercept the output illumination beam so as to produce a spatially patterned beam; and a projector lens to receive the spatially patterned beam. 
     Embodiment 51 is the projector of embodiment 50, wherein the spatial light modulator is a reflective spatial light modulator. 
     Embodiment 52 is the projector of embodiment 50, wherein the projector lens has a maximum lateral dimension LD1 and the spatial light modulator has a maximum lateral dimension LD2, and wherein LD1 is less than LD2. 
     Embodiment 53 is the projector of embodiment 50, wherein the projector further comprises a detector to receive light originating from a body outside of the projector through the projector lens. 
     Various modifications and alterations of this invention will be apparent to those skilled in the art without departing from the spirit and scope of this invention, and it should be understood that this invention is not limited to the illustrative embodiments set forth herein. The reader should assume that features of one disclosed embodiment can also be applied to all other disclosed embodiments unless otherwise indicated. It should also be understood that all U.S. patents, patent application publications, and other patent and non-patent documents referred to herein are incorporated by reference, to the extent they do not contradict the foregoing disclosure.