Abstract:
An optical system and method that utilize multiple low-etendue lasers to illuminate multiple spots one or more spatial light modulators. Stereoscopic systems may be formed by using different wavelengths or different polarizations for each spot. Light from each spot is guided to each eye of the viewer by wearing 3D glasses.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Effective light sources for projectors may be constructed from lamps with high brightness such as high intensity discharge (HID) lamps, light emitting diodes (LEDs), or lasers. A light source with low etendue can be efficiently coupled through a projector when the etendue of the light source is less than or equal to the etendue of the following optical system in the projector. 
         [0002]    Most stereoscopic projection systems can be characterized as one of two basic types: (1) time-sequential projection that uses one spatial light modulator (SLM) per color and alternately shows left eye images and right eye images in rapid sequence, and (2) simultaneous projection that uses two SLMs per color, one for the left eye images and one for the right eye images. In a third type, split image projection, there is only one SLM per color, and the left and right eye images are formed simultaneously on separate parts or pixels of the single SLM. 
         [0003]    Stereoscopic left and right images may be formed by using spectral selection, for example as described in U.S. Pat. No. 6,283,597, the complete disclosure of which is incorporated herein by reference. In the spectral selection method, first wavelength bands of red, green, and blue are passed to the left eye, and second wavelength bands of red, green, and blue are passed to the right eye. The first bands and second bands are distinct so that there is little or no overlap between the first and second bands. 
         [0004]    The optical designs of most digital image projectors use SLMs to switch each pixel on and off in order to create a visual image. The SLMs may be reflective, such as liquid crystal on silicon (LCOS) devices and digital micromirror devices (DMDs), or may be transmissive such as liquid crystal display (LCD) devices. 
       SUMMARY OF THE INVENTION 
       [0005]    In general, in one aspect, an optical system that includes two light sources and a spatial light modulator. The first light source has a first optical output that is processed by the first part of the spatial light modulator and the second light source has a second optical output that is processed by the second part of the spatial light modulator. 
         [0006]    Implementations may include one or more of the following features. The first light source may have an etendue lower than 0.1 mm 2  sr and may include a laser. The first part of the spatial light modulator may be used to form an image for the left eye, and the second part of the spatial light modulator may be used to form an image for the right eye. There may be a beam combiner which forms a combined image by combining the image for the left eye with the image for the right eye. There may be an anamorphic lens which expands or compresses the image for the left eye of the viewer such that a single axis is expanded or compressed relative to an orthogonal axis. The combined image may have a checkerboard pattern of pixels, wherein the checkerboard pattern alternates pixels for the left eye and pixels for the right eye. All of the pixels of the spatial light modulator may be utilized by the first part of the spatial light modulator and the second part of the spatial light modulator. The first optical output may include a first wavelength band and the second optical output may include a second wavelength band that may be different than the first wavelength band. The second optical output may include a second wavelength band of red light, a second wavelength band of green light, and a second wavelength band of blue light. The second wavelength band of red light may be different than the first wavelength band of red light. The second wavelength band of green light may be different than the first wavelength band of green light. The second wavelength band of blue light may be different than the first wavelength band of blue light. The first wavelength band may be visible light and the second wavelength band may be infrared light. The processing of the spatial light modulator may spatially form the first optical output. The spatial light modulator may include a reflective liquid-crystal light valve, a liquid-crystal-on-silicon light valve, a digital-micromirror-device light valve, or a transmissive liquid-crystal light valve. There may also be a mixing rod, and the first optical output may enter the mixing rod. The first optical output may have a first polarization state and the second optical output may have a second polarization state, and the first polarization state may be different than the second polarization state. The first polarization state may be orthogonal to the second polarization state. 
         [0007]    In general, in one aspect, a stereoscopic display system that includes two laser light sources, two mixing rods, three spatial light modulators, and two beam combiners. The first laser light source illuminates the first mixing rod. The second laser light source illuminates the second mixing rod. The first and second mixing rods illuminate the first beam combiner. The first beam combiner illuminates the first spatial light modulator, the second spatial light modulator, and the third spatial light modulator. The first spatial light modulator, the second spatial light modulator, and the third spatial light modulator illuminate the second beam combiner. A beam of light from the first laser light source is processed by a first part of the first spatial light modulator, a first part of the second spatial light modulator, and a first part of the third spatial light modulator. A beam of light from the second laser light source is processed by a second part of the first spatial light modulator, a second part of the second spatial light modulator, and a second part of the third spatial light modulator. 
         [0008]    In general, in one aspect, a method of illumination that includes generating a first beam of light, generating a second beam of light, processing the first beam of light with a first part of a spatial light modulator to form a third beam of light, and processing the second beam of light with a second part of the spatial light modulator to form a fourth beam of light. 
         [0009]    Implementations may include one or more of the following features. The third beam of light may be combined with the fourth beam of light. The first beam of light may have an etendue lower than 0.1 mm 2  sr. The first part of the spatial light modulator may be used to form an image for the left eye and the second part of the spatial light modulator may be used to form an image for the right eye. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0010]      FIG. 1  is a top view of a projector optical design with dual illumination using LCOS SLMs; 
           [0011]      FIG. 2  is a top view of a projector optical design with dual illumination using DMD SLMs; 
           [0012]      FIG. 3  is a top view of a projector optical design with dual illumination using transmissive LCD SLMs; 
           [0013]      FIG. 4  is a front view of a portrait-oriented SLM with two images located one above the other; 
           [0014]      FIG. 5  is a front view of a landscape-oriented SLM with two images located one above the other; 
           [0015]      FIG. 6  is a front view of a portrait-oriented SLM with two images far apart and located one above the other; 
           [0016]      FIG. 7  is a front view of a landscape-oriented SLM with two images located on the left and right of each other; 
           [0017]      FIG. 8  is a front view of a landscape-oriented SLM with two images located one diagonal to the other; 
           [0018]      FIG. 9  is a front view of a landscape-oriented SLM with an anamorphic pattern of pixels; 
           [0019]      FIG. 10  is a front view of a landscape-oriented SLM with a checkerboard pattern of pixels; 
           [0020]      FIG. 11  is a top view of low etendue illumination compared to high etendue illumination; and 
           [0021]      FIG. 12  is a flow chart of a method of dual illumination. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    Split image projection has the advantage of using fewer SLMs and other optical components compared to simultaneous projection. Split image projection also has the advantage of not requiring active glasses such as those used in time sequential projection. Technological progress leads towards ever higher and higher pixel counts per SLM which also tends to favor using more than one image per SLM while still allowing sufficient pixels in each image to achieve high resolutions such as 1920×1080 pixels (full high definition) which is also known as 2K. Very high resolution 4K SLMs, (which may be 4096×2160 resolution) are available for cinema applications. Two 2K images may be processed on two parts of one 4K SLM. In the case of stereoscopic projection, one of the 2K images may be viewed by the left eye, and the other 2K image may be viewed by the right eye. Dual illumination allows one low-etendue light source to illuminate one part of the SLM, and a second low-etendue light source to illuminate a second part of the same SLM. Other advantages of low-etendue light sources will also be seen in the following examples. 
         [0023]      FIG. 1  shows a projector optical design with dual illumination using LCOS SLMs. First light source  100  produces first beam segment  102  which is spread by first lens system  104  to make second beam segment  106 . Second beam segment  106  is homogenized by first mixing rod  108  to produce third beam segment  110 . Third beam segment  110  is collimated by second lens system  112  to form fourth beam segment  114 . Fourth beam segment  114  partially reflects from first dichroic beamsplitter (DBS)  132  to form fifth beam segment  172  and partially transmits to form sixth beam segment  136 . Fifth beam segment  172  reflects from first minor  175  to form seventh beam segment  176 . Seventh beam segment  176  enters first polarizing beamsplitter (PBS)  180  and is reflected to form eighth beam segment  184 . Eighth beam segment  184  is processed by first SLM  185  which rotates the polarization of each pixel depending on the desired brightness of the pixel and reflects eighth beam segment  184  back along its input path to reenter first PBS  180 . On a pixel-by-pixel basis, if the polarization is not changed relative to the input beam, the light reflects inside first PBS  180  to go back towards first light source  100 . If the polarization has been changed relative to the input beam, some or all of the light (depending on how much the polarization has been changed) passes through first PBS  180  to form ninth beam segment  186 . 
         [0024]    Sixth beam segment  136  partially reflects from second DBS  138  to form tenth beam segment  158  and partially transmits to form eleventh beam segment  142 . Tenth beam segment  158  enters second PBS  160  and is reflected to form twelfth beam segment  164 . Twelfth beam segment  164  is processed by second SLM  166  which rotates the polarization of each pixel depending on the desired brightness of the pixel and reflects twelfth beam segment  164  back along its input path to reenter second PBS  160 . On a pixel-by-pixel basis, if the polarization is not changed relative to the input beam, the light reflects inside second PBS  160  to go back towards first light source  100 . If the polarization has been changed relative to the input beam, some or all of the light (depending on how much the polarization has been changed) passes through second PBS  160  to form thirteenth beam segment  168 . 
         [0025]    Eleventh beam segment  142  enters third PBS  144  and is reflected to form fourteenth beam segment  146 . Fourteenth beam segment  146  is processed by third SLM  150  which rotates the polarization of each pixel depending on the desired brightness of the pixel and reflects fourteenth beam segment  146  back along its input path to reenter third PBS  144 . On a pixel-by-pixel basis, if the polarization is not changed relative to the input beam, the light reflects inside third PBS  144  to go back towards first light source  100 . If the polarization has been changed relative to the input beam, some or all of the light (depending on how much the polarization has been changed) passes through third PBS  144  to form fifteenth beam segment  154 . 
         [0026]    First beam combiner  188  combines ninth beam segment  186 , thirteenth beam segment  168 , and fifteenth beam segment  154  to form sixteenth beam segment  190 . Sixteenth beam segment  190  reflects from second mirror  191  to form seventeenth beam segment  193 . Seventeenth beam segment  193  reflects from third DBS  192  to form beam segment  194 . 
         [0027]    Second light source  116  produces eighteenth beam segment  118  which is spread by third lens system  120  to make nineteenth beam segment  122 . Nineteenth beam segment  122  is homogenized by second mixing rod  124  to produce twentieth beam segment  126 . Twentieth beam segment  126  is collimated by fourth lens system  128  to form twenty-first beam segment  130 . Twenty-first beam segment  130  partially reflects from first DBS  132  to form twenty-second beam segment  174  and partially transmits to form twenty-third beam segment  134 . Twenty-second beam segment  174  reflects from first minor  175  to form twenty-fourth beam segment  178 . Twenty-fourth beam segment  178  enters first polarizing beamsplitter (PBS)  180  and is reflected to form twenty-fifth beam segment  182 . Twenty-fifth beam segment  182  is processed by first SLM  185 , which rotates the polarization of each pixel depending on the desired brightness of the pixel and reflects twenty-fifth beam segment  182  back along its input path to reenter first PBS  180 . On a pixel-by-pixel basis, if the polarization is not changed relative to the input beam, the light reflects inside first PBS  180  to go back towards first light source  100 . If the polarization has been changed relative to the input beam, some or all of the light (depending on how much the polarization has been changed) passes through first PBS  180  to form twenty-sixth beam segment  187 . 
         [0028]    Twenty-third beam segment  134  partially reflects from second DBS  138  to form twenty-seventh beam segment  156  and partially transmits to form twenty-eighth beam segment  140 . Twenty-seventh segment  156  enters second PBS  160  and is reflected to form twenty-ninth beam segment  162 . Twenty-ninth beam segment  162  is processed by second SLM  166 , which rotates the polarization of each pixel depending on the desired brightness of the pixel and reflects twenty-ninth beam segment  162  back along its input path to reenter second PBS  160 . On a pixel-by-pixel basis, if the polarization is not changed relative to the input beam, the light reflects inside second PBS  160  to go back towards first light source  100 . If the polarization has been changed relative to the input beam, some or all of the light (depending on how much the polarization has been changed) passes through second PBS  160  to form thirtieth beam segment  170 . 
         [0029]    Twenty-eighth beam segment  140  enters third PBS  144  and is reflected to form thirty-first beam segment  148 . Thirty-first beam segment  148  is processed by third SLM  150 , which rotates the polarization of each pixel depending on the desired brightness of the pixel and reflects thirty-first beam segment  148  back along its input path to reenter third PBS  144 . On a pixel-by-pixel basis, if the polarization is not changed relative to the input beam, the light reflects inside third PBS  144  to go back towards first light source  100 . If the polarization has been changed relative to the input beam, some or all of the light (depending on how much the polarization has been changed) passes through third PBS  144  to form thirty-second beam segment  152 . 
         [0030]    First beam combiner  188  combines twenty-sixth beam segment  187 , thirtieth beam segment  170 , and thirty-second beam segment  152  to form thirty-third beam segment  189 . Thirty-third beam segment  189  passes through third DBS  192  to combine with seventeenth beam segment  193  in forming thirty-fourth beam segment  194 . Thirty-fourth beam segment  194  passes through fifth lens system  195  to form thirty-fifth beam segment  196  which passes outside of the projector to make a viewable image on a projection screen (not shown). 
         [0031]    First beam combiner  188  may be an X-prism. Second minor  191  and third DBS  192  form second beam combiner  197 . First lens system  104 , second lens system  112 , third lens system  120 , fourth lens system  128 , and fifth lens system  195  may be formed from a single lens or any number of lenses that guide the light beams into the desired positions. The sizes of components and distances between components are not shown to scale in  FIG. 1 . Some optical components may be positioned against other optical components so that there is no gap between the components. Auxiliary optical components such as polarizers, relay lenses, skew ray plates, polarization rotation plates, and trim filters are not shown in  FIG. 1 . The three SLMs shown in  FIG. 1  may be each assigned to a primary color so that one is red, one is green, and one is blue. First light source  100  may output sub-bands red 1, green 1, and blue 1 whereas second light source  116  may output sub-bands red 2, green 2, and blue 2. First DBS  132  may reflect blue while passing green and red. Second DBS  138  may reflect green while passing red. Third DBS  192  may reflect sub-bands red 1, green 1, and blue 1 while passing sub-bands red 2, green 2, and blue 2. First light source  100  and second light source  116  may output polarized light. 
         [0032]      FIG. 2  shows a projector optical design with dual illumination using DMD SLMs. First light source  202  produces first beam segment  204  which is spread by first lens system  206  to make second beam segment  208 . Second beam segment  208  is homogenized by first mixing rod  210  to produce third beam segment  212 . Third beam segment  212  is collimated by second lens system  214  to form fourth beam segment  216 . Fourth beam segment  216  enters first subprism  234  and reflects from the interface of first subprism  234  and second subprism  268  to form fifth beam segment  238 . Fifth beam segment  238  partially reflects from the interface between third subprism  240  and fourth subprism  256  and then from the entrance face of third subprism  240  to form sixth beam segment  244 . Fifth beam segment  238  also partially transmits from the interface between third subprism  240  and fourth subprism  256  and partially transmits from the interface between fourth subprism  256  and fifth subprism  248  to form seventh beam segment  252 . Fifth beam segment  238  also partially reflects from the interface between fourth subprism  256  and fifth subprism  248  and then reflects from the interface between fourth subprism  256  and third subprism  240  to form eighth beam segment  260 . Sixth beam segment  244  is processed by first SLM  246  which flips micromirrors for each pixel depending on the desired brightness of the pixel. For darker pixels, more light is directed back through the prism systems until the light is absorbed in a beam dump (not shown) and for brighter pixels, more light is directed to the output path which reflects from the entrance face of third subprism  240  and then from the interface between third subprism  240  and fourth subprism  256  to form ninth beam segment  266 . 
         [0033]    Seventh beam segment  252  is processed by second SLM  254  which flips micromirrors for each pixel depending on the desired brightness of the pixel. For darker pixels, more light is directed back through the prism systems until the light is absorbed in a beam dump (not shown) and for brighter pixels, more light is directed to the output path which passes through the interface between fifth subprism  248  and fourth subprism  256 , then passes through the interface between fourth subprism  256  and third subprism  240  to join sixth beam segment  244  in forming ninth beam segment  266 . 
         [0034]    Eighth beam segment  260  is processed by third SLM  262  which flips micromirrors for each pixel depending on the desired brightness of the pixel. For darker pixels, more light is directed back through the prism systems until the light is absorbed in a beam dump (not shown) and for brighter pixels, more light is directed to the output path which reflects from the interface between fourth subprism  256  and third subprism  240 , then reflects from the interface between fourth subprism  256  and fifth subprism  248 , then passes through the interface between fourth subprism  256  and third subprism  240  to join sixth beam segment  244  and seventh beam segment  252  in forming ninth beam segment  266 . 
         [0035]    Ninth beam segment  266  passes through first subprism  234  and second subprism  268  to form tenth beam segment  272 . Tenth beam segment  272  passes through first DBS  278  to form eleventh beam segment  280 . 
         [0036]    Second light source  218  produces twelfth beam segment  220  which is spread by third lens system  222  to make thirteenth beam segment  224 . Thirteenth beam segment  224  is homogenized by second mixing rod  226  to produce fourteenth beam segment  228 . Fourteenth beam segment  228  is collimated by fourth lens system  230  to form fifteenth beam segment  232 . Fifteenth beam segment  232  enters first subprism  234  and reflects from the interface of first subprism  234  and second subprism  268  to form sixteenth beam segment  236 . Sixteenth beam segment  236  partially reflects from the interface between third subprism  240  and fourth subprism  256  and then from the entrance face of third subprism  240  to form seventeenth beam segment  242 . Sixteenth beam segment  236  also partially transmits from the interface between third subprism  240  and fourth subprism  256  and partially transmits from the interface between fourth subprism  256  and fifth subprism  248  to form eighteenth beam segment  250 . Sixteenth beam segment  236  also partially reflects from the interface between fourth subprism  256  and fifth subprism  248  and then reflects from the interface between fourth subprism  256  and third subprism  240  to form nineteenth beam segment  258 . Seventeenth beam segment  242  is processed by first SLM  246  which flips micromirrors for each pixel depending on the desired brightness of the pixel. For darker pixels, more light is directed back through the prism systems until the light is absorbed in a beam dump (not shown) and for brighter pixels, more light is directed to the output path which reflects from the entrance face of third subprism  240  and then from the interface between third subprism  240  and fourth subprism  256  to form twentieth beam segment  264 . 
         [0037]    Eighteenth beam segment  250  is processed by second SLM  254  which flips micromirrors for each pixel depending on the desired brightness of the pixel. For darker pixels, more light is directed back through the prism systems until the light is absorbed in a beam dump (not shown) and for brighter pixels, more light is directed to the output path which passes through the interface between fifth subprism  248  and fourth subprism  256 , then passes through the interface between fourth subprism  256  and third subprism  240  to join seventeenth beam segment  242  in forming twentieth beam segment  264 . 
         [0038]    Nineteenth beam segment  258  is processed by third SLM  262  which flips micromirrors for each pixel depending on the desired brightness of the pixel. For darker pixels, more light is directed back through the prism systems until the light is absorbed in a beam dump (not shown) and for brighter pixels, more light is directed to the output path which reflects from the interface between fourth subprism  256  and third subprism  240 , then reflects from the interface between fourth subprism  256  and fifth subprism  248 , then passes through the interface between fourth subprism  256  and third subprism  240  to join seventeenth beam segment  242  and eighteenth beam segment  250  in forming twentieth beam segment  264 . 
         [0039]    Twentieth beam segment  264  passes through first subprism  234  and second subprism  268  to form twenty-first beam segment  270 . Twenty-first beam segment  270  reflects from first mirror  274  to form twenty-second beam segment  276  and then reflects from first DBS  278  to join tenth beam segment  272  in forming eleventh beam segment  280 . Eleventh beam segment  280  passes through fifth lens system  282  to form twenty-third beam segment  284  which passes outside of the projector to make a viewable image on a projection screen (not shown). 
         [0040]    First subprism  234  and second subprism  268  form total internal reflection (TIR) prism  288 . Third subprism  240 , fourth subprism  256 , and fifth subprism  248  form Philips prism  286 . Minor  274  and DBS  278  form beam combiner  290 . First lens system  206 , second lens system  214 , third lens system  222 , fourth lens system  230 , and fifth lens system  282  may be formed from a single lens or any number of lenses that guide the light beams into the desired positions. The sizes of components and distances between components are not shown to scale in  FIG. 2 . Some optical components may be positioned against other optical components so that there is no gap between the components. Auxiliary optical components such as polarizers, relay lenses, skew ray plates, polarization rotation plates, and trim filters are not shown in  FIG. 2 . The three SLMs shown in  FIG. 2  may be each assigned to a primary color so that one is red, one is green, and one is blue. First light source  202  may output sub-bands red 1, green 1, and blue 1 whereas second light source  218  may output sub-bands red 2, green 2, and blue 2. The interface between third subprism  240  and fourth subprism  256  may reflect blue while passing green and red. The interface between fourth subprism  256  and fifth subprism  248  may transmit green while reflecting red. First DBS  278  may transmit sub-bands red 1, green 1, and blue 1 while reflecting sub-bands red 2, green 2, and blue 2. 
         [0041]      FIG. 3  shows a projector optical design with dual illumination using transmissive LCD SLMs. First light source  300  produces first beam segment  302  which is spread by first lens system  304  to make second beam segment  306 . Second beam segment  306  is homogenized by first mixing rod  308  to produce third beam segment  310 . Third beam segment  310  is collimated by second lens system  312  to form fourth beam segment  314 . Fourth beam segment  314  partially reflects from first DBS  332  to form fifth beam segment  364  and partially transmits to form sixth beam segment  336 . Fifth beam segment  364  reflects from first mirror  368  to form seventh beam segment  372 . Seventh beam segment  372  is processed by first SLM  374  which rotates the polarization of each pixel depending on the desired brightness of the pixel. On a pixel-by-pixel basis, depending on the amount of polarization rotation, the light is transmitted or absorbed to a varying degree by a polarizer (not shown) to form eighth beam segment  376 . 
         [0042]    Sixth beam segment  336  partially reflects from second DBS  338  to form ninth beam segment  381  and partially transmits to form tenth beam segment  340 . Ninth beam segment  381  is processed by second SLM  382  which rotates the polarization of each pixel depending on the desired brightness of the pixel. On a pixel-by-pixel basis, depending on the amount of polarization rotation, the light is transmitted or absorbed to a varying degree by a polarizer (not shown) to form eleventh beam segment  384 . 
         [0043]    Tenth beam segment  340  reflects from second minor  344  to form twelfth beam segment  348 . Twelfth beam segment  348  reflects from third mirror  350  to form thirteenth beam segment  354 . Thirteenth beam segment  354  is processed by third SLM  356  which rotates the polarization of each pixel depending on the desired brightness of the pixel. On a pixel-by-pixel basis, depending on the amount of polarization rotation, the light is transmitted or absorbed to a varying degree by a polarizer (not shown) to form fourteenth beam segment  360 . 
         [0044]    First beam combiner  362  combines eighth beam segment  376 , eleventh beam segment  384 , and fourteenth beam segment  360  to form fifteenth beam segment  385 . Fifteenth beam segment  385  reflects from fourth minor  387  to form sixteenth beam segment  388 . Sixteenth beam segment  388  reflects from third DBS  390  to form seventeenth beam segment  391 . 
         [0045]    Second light source  316  produces eighteenth beam segment  318  which is spread by third lens system  320  to make nineteenth beam segment  322 . Nineteenth beam segment  322  is homogenized by second mixing rod  324  to produce twentieth beam segment  326 . Twentieth beam segment  326  is collimated by fourth lens system  328  to form twenty-first beam segment  330 . Twenty-first beam segment  330  partially reflects from first DBS  332  to form twenty-second beam segment  366  and partially transmits to form twenty-third beam segment  334 . Twenty-second beam segment  366  reflects from first minor  368  to form twenty-fourth beam segment  370 . Twenty-fourth beam segment  370  is processed by first SLM  374  which rotates the polarization of each pixel depending on the desired brightness of the pixel. On a pixel-by-pixel basis, depending on the amount of polarization rotation, the light is transmitted or absorbed to a varying degree by a polarizer (not shown) to form twenty-fifth beam segment  378 . 
         [0046]    Twenty-third beam segment  334  partially reflects from second DBS  338  to form twenty-sixth beam segment  380  and partially transmits to form twenty-seventh beam segment  342 . Twenty-sixth beam segment  380  is processed by second SLM  382  which rotates the polarization of each pixel depending on the desired brightness of the pixel. On a pixel-by-pixel basis, depending on the amount of polarization rotation, the light is transmitted or absorbed to a varying degree by a polarizer (not shown) to form twenty-eighth beam segment  383 . 
         [0047]    Twenty-seventh beam segment  342  reflects from second minor  344  to form twenty-ninth beam segment  346 . Twenty-ninth beam segment  346  reflects from third minor  350  to form thirtieth beam segment  352 . Thirtieth beam segment  352  is processed by third SLM  356  which rotates the polarization of each pixel depending on the desired brightness of the pixel. On a pixel-by-pixel basis, depending on the amount of polarization rotation, the light is transmitted or absorbed to a varying degree by a polarizer (not shown) to form thirty-first beam segment  358 . 
         [0048]    First beam combiner  362  combines twenty-fifth beam segment  378 , twenty-eighth beam segment  383 , and thirty-first beam segment  358  to form thirty-second beam segment  386 . Thirty-second beam segment  386  passes through third DBS  390  to combine with sixteenth beam segment  388  in forming seventeenth beam segment  391 . Seventeenth beam segment  391  passes through fifth lens system  392  to form thirty-third beam segment  393  which passes outside of the projector to make a viewable image on a projection screen (not shown). 
         [0049]    First beam combiner  362  may be an X-prism. Second minor  387  and third DBS  390  form second beam combiner  394 . First lens system  304 , second lens system  312 , third lens system  320 , fourth lens system  328 , and fifth lens system  392  may be formed from a single lens or any number of lenses that guide the light beams into the desired positions. The sizes of components and distances between components are not shown to scale in  FIG. 3 . Some optical components may be positioned against other optical components so that there is no gap between the components. Auxiliary optical components such as polarizers, relay lenses, skew ray plates, polarization rotation plates, and trim filters are not shown in  FIG. 3 . The three SLMs shown in  FIG. 3  may be each assigned to a primary color so that one is red, one is green, and one is blue. First light source  300  may output sub-bands red 1, green 1, and blue 1 whereas second light source  316  may output sub-bands red 2, green 2, and blue 2. First DBS  332  may reflect blue while passing green and red. Second DBS  338  may reflect green while passing red. Third DBS  390  may reflect sub-bands red 1, green 1, and blue 1 while passing sub-bands red 2, green 2, and blue 2. First light source  300  and second light source  316  may output polarized light. 
         [0050]      FIG. 4  shows a portrait-oriented SLM with two images located one above the other. First image  402  is formed in one part of SLM  400  and second image  404  is formed in another, distinct part of SLM  400 . First image  402  and second image  404  are located such that most of the un-used pixels are above and below each image. In the case of stereoscopic systems, first image  402  may be the left eye image and second image  404  may be the right eye image. 
         [0051]      FIG. 5  shows a landscape-oriented SLM with two images located one above the other. First image  502  is formed in one part of SLM  500  and second image  504  is formed in another, distinct part of SLM  500 . First image  502  and second image  504  are located such that most of the un-used pixels are on the left and right of each image. In the case of stereoscopic systems, first image  502  may be the left eye image and second image  504  may be the right eye image. 
         [0052]      FIG. 6  shows a portrait-oriented SLM with two images far apart and located one above the other. First image  602  is formed in one part of SLM  600  and second image  604  is formed in another, distinct part of SLM  600 . First image  602  and second image  604  are located such that most of the un-used pixels are between the two images. In the case of stereoscopic systems, first image  602  may be the left eye image and second image  604  may be the right eye image. By placing first image  602  far from second image  604  an increased guard band is formed between the two images that may reduce the amount of cross-talk or light spillage between the two images. 
         [0053]      FIG. 7  shows a landscape-oriented SLM with two images located on the left and right of each other so that they form a central band across the horizontal center of the SLM. First image  702  is formed in one part of SLM  700  and second image  704  is formed in another, distinct part of SLM  700 . First image  702  and second image  704  are located such that most of the un-used pixels are formed into one band above the images and one band below the images. In the case of stereoscopic systems, first image  702  may be the left eye image and second image  704  may be the right eye image. 
         [0054]      FIG. 8  shows a landscape-oriented SLM with two images located one diagonal to the other. First image  802  is formed in one part of SLM  800  and second image  804  is formed in another, distinct part of SLM  800 . First image  802  and second image  804  are located such that most of the un-used pixels are above and below the two images in diagonally opposite corners. In the case of stereoscopic systems, first image  802  may be the left eye image and second image  804  may be the right eye image. 
         [0055]      FIG. 9  shows a landscape-oriented SLM with an anamorphic pattern of pixels with one image located above the other. First image  902  is formed in one part of SLM  900  and second image  904  is formed in another, distinct part of SLM  900 . First image  902  and second image  904  use substantially all of the pixels in SLM  900 . An anamorphic lens may be used to compress the horizontal axis (relative to the vertical axis) or expand the vertical axis (relative to the horizontal axis) such that the final viewable images are formed with the desired aspect ratio. In the case of stereoscopic systems, first image  902  may be the left eye image and second image  904  may be the right eye image. 
         [0056]      FIG. 10  shows a landscape-oriented SLM with a checkerboard pattern of pixels. Pixels of the first image on SLM  1000  are shown cross-hatched such as pixel  1002  and pixels of the second image on SLM  1000  are shown not cross-hatched such as pixel  1004 . A 31×15 array of pixels is shown for clarity, but SLMs typically have many more pixels to form high resolution images. The checkerboard pattern uses substantially all the pixels of the SLM. In the case of stereoscopic systems, the first image may be the left eye image and second image may be the right eye image. 
         [0057]      FIG. 11  shows low etendue illumination of an SLM and an optical component compared to high etendue illumination of the same SLM and optical component. First beam segment  1100  passes through optical component  1102  to form second beam segment  1104 . Second beam segment  1104  passes through SLM  1106  to form third beam segment  1108 . Alternatively, fourth beam segment  1110  passes through optical component  1102  to form fifth beam segment  1112 . Fifth beam segment  1112  passes through SLM  1106  to form sixth beam segment  1114 . First beam segment  1100 , second beam segment  1104 , and third beam segment  1108  have high etendue. Fourth beam segment  1110 , fifth beam segment  1112 , and sixth beam segment  1114  have low etendue. First beam segment  1100 , second beam segment  1104 , and third beam segment  1108  can be seen to have higher angles of incidence for rays near the edges of the beam segments. Fourth beam segment  1110 , fifth beam segment  1112 , and sixth beam segment  1114  can be seen to have lower angles of incidence for rays near the edges of the beam segments. Optical component  1102  may be any component that processes light such as a polarizer, skew ray plate, polarization rotation plate, interference filter, beamsplitter, minor, or lens assembly. Skew ray plates are used to compensate the polarization state of rays at high angle of incidence. Polarization rotation plates make a controlled change in polarization such as changing linear polarization to circular polarization. SLM  1106  may be any sort of SLM such as DMD, LCD, or LCOS. Optical component  1102  and SLM  1106  are shown operating in transmission, but may alternatively operate in reflection. Optical component  1102  is shown to in the light path before SLM  1106 , but alternatively, optical component  1102  may be after SLM  1106 . The included angles of beam segments shown in  FIG. 11  are for illustrative purposes only. The actual beam angles may be larger or smaller depending on the design of the actual optical system. 
         [0058]      FIG. 12  shows a flow chart of a method of dual illumination. In this method, an SLM is illuminated by two light sources. In step  1200 , a first beam of light is generated. In step  1202 , the first beam of light is processed by the first part of an SLM. In step  1204 , a second beam of light generated. In step  1206 , the second beam of light is processed by the second part of the same SLM. In optional step  1208 , the first beam of light after processing is combined with the second beam of light after processing. 
         [0059]    When considering a light source, etendue is an optical property that characterizes how spread out the light beam is in both area and angle. In simple terms, the approximate etendue of a light source may be computed by multiplying the emitting area of the source by the solid angle that the light beam subtends. Lasers have low etendue whereas arc lamps, filament lamps, and LEDs have high etendue. If the light source has sufficiently low etendue, it is possible to focus light through a subsequent optical system with high efficiency. Laser light sources enable the independent illumination of more than one part of an SLM with high brightness. As an example, the beam from a semiconductor laser may have a cross-sectional area of 1 mm 2  and a beam divergence of 10 milliradians which makes an etendue of approximately 0.01 mm 2  sr. Most lasers have etendues less than 0.1 mm 2  sr, which allows effective illumination of multiple parts of an SLM. An example of a high etendue light source is an arc lamp which may have an emitting area of 3 mm 2  and a beam divergence of 12.6 radians which makes an etendue of approximately 38 mm 2  sr. 
         [0060]    When considering an optical system which accepts light from a light source, etendue is the optical property that characterizes how much light the optical system can accept in both aperture area and angle. In simple terms, the approximate etendue of an optical system may be computed by multiplying the area of the entrance pupil by the solid angle of the light path as seen from the entrance pupil. For an optical system of a fixed etendue such as a projector SLM, associated lens systems, and auxiliary optical components, the etendue of the light source should be lower than or equal to the etendue of the optical system in order to efficiently illuminate the optical system without vignetting. Additional advantages may be gained by using an even lower source etendue. Low source etendue means that the angle of incidence is smaller, especially for rays that are near the edge of the beam. A low angle of incidence means that certain optical components may be simplified or may operate more effectively. For example, polarization uniformity may be improved in LCD and LCOS SLMs, skew ray plates may not be necessary, PBSs and polarization filters may have higher extinction ratios, multilayer interference filters may have less angle shift, and lens assemblies may be less subject to optical aberrations. 
         [0061]    Laser light sources may include optical parametric oscillators (OPOs). The detailed operation of OPOs is described in U.S. Pat. No. 5,740,190, the complete disclosure of which is incorporated herein by reference. By suitably designing the OPO and controlling operation parameters such as temperature, when the input light is green, the outputs may be blue and red, thus making all three colors required for a full-color projection display. By mixing red, green, and blue light, other colors may be generated in the projector. 
         [0062]    Prisms and beamsplitters are used in projectors and other optical systems to control the path of light beams. DBSs split or combine wavelength bands of light that form various colors and are usually constructed from interference coatings on flat substrates or prism surfaces. PBSs split or combine different polarizations of light and may be constructed from interference coatings, prisms, or by other techniques such as wire grids. Philips prisms consist of three subprisms with DBSs on two of the internal faces. TIR prisms have an air gap inside that makes total internal reflection when the incidence angle of the beam is greater than the critical angle. X-prisms consist of 4 subprisms assembled into a cube such that the internal surfaces have DBSs along both diagonal faces. Depending on their roles in the light path, prisms and beamsplitters may act as beam separators, beam combiners, or both at the same time. 
         [0063]    SLMs may be one, two, or three-dimensional. In each case, an SLM processes an incoming beam of light to produce an outgoing beam of light which has pixels formed in a two-dimensional array. A one-dimensional SLM has a single pixel which is scanned in two directions to form a two-dimensional image. A one-dimensional SLM has pixels arranged in a one-dimensional line segment which is scanned in one direction to form a two-dimensional image. A two-dimensional SLM has pixels arranged in a two-dimensional shape such as a rectangle. 
         [0064]    A mixing rod is used to make a light beam more spatially uniform and to form the beam into a specific cross-section, such as rectangular, so that the beams can better match the shape of an SLM. A mixing rod may be constructed from a solid rectangular parallelepiped where total internal reflection guides the rays of light inside to make multiple bounces within the mixing rod. In the case of dual illumination, there are two mixing rods, and a thin air gap may be used to keep the light within each rod while keeping the rods as close as possible. If the light sources are linearly polarized, orthogonal orientation of the mixing rods relative to the polarization state of the light will maintain the linear polarization state of the light sources. If circular polarization is desired at the output of the projectors, a quarter-wave rotation plate may be used to convert linear polarization to circular polarization. Alternatively, instead of mixing rods, other types of beam homogenizers may be used such as fly&#39;s eye lenses or diffusers. 
         [0065]    Anamorphic lenses expand or compress one axis relative to the other, orthogonal axis. For example, an anamorphic lens may be used to compress the horizontal axis relative to the vertical axis, so that the 4:1 aspect ratios of the images in  FIG. 9  become 2:1 aspect ratios. The use of an anamorphic lens allows substantially all of the pixels of SLM  900  to be used for imaging so that there are few or no un-used pixels. A small number of un-used pixels may surround the images as guard bands if necessary to allow for alignment tolerances. 
         [0066]    Projection lens systems such as fifth lens system  195  in  FIG. 1 , fifth lens system  282  in  FIG. 2 , and fifth lens system  392  in  FIG. 3  may consist of many individual lens elements that are combined into one lens system designed to project a large image onto a screen that is located many meters away from the projector. Functions such as image shifting, zooming, focusing, and other image control features may be built in the projection lens system.  FIGS. 1 through 3  show a second beam combiner and one projection lens system, but alternatively, two projection lens systems may be used, one for each image. A second beam combiner is not necessary if two projection lens systems are used, but a beam separator may be required to increase the spacing between the two beams so that the beams can pass through the two projection lens systems. 
         [0067]    Dual illumination of a projector is advantageous because light output may be increased relative to designs that use only one light source. This is particularly important for 3D projection systems that are often operated below desired brightness levels. Also, the light is efficiently used in a dual illumination system because the light is directed only to the pixels that form the images, and does not illuminate un-used pixels. In the configurations of  FIGS. 9 and 10 , a double benefit is that all the light is used and also all the pixels are used. 
         [0068]    In one example of dual illumination, a wider gamut can be obtained by using more than three primary colors where the colors come from more than one light source. Red, green, and blue, may be generated by one light source whereas yellow (or yellow and cyan) may be generated by another light source. The two light sources may illuminate separate parts of an SLM or may overlap to illuminate the same part of the SLM. 
         [0069]    In another example of dual illumination, an SLM may be illuminated with different wavelengths of the same primary color in order to reduce speckle. The checkerboard pattern of  FIG. 10  may be illuminated such that the pixels that are cross-hatched process one wavelength of light, and the pixels that are not cross-hatched process another wavelength of light. The two wavelengths of light may be generated by two separate light sources, or may be generated by one light source with two output wavelengths. In the case where most of the speckle results from the green band, only the green band need be broken into two sub-bands to significantly reduce visible speckle. 
         [0070]    Other optical systems include those with more than two light sources which may be utilized to illuminate two or more parts of each SLM, optical systems that are not imaging such as laser-beam spatial-shaping systems, optical systems that include non-visible light such as ultraviolet or infrared radiation, optical systems that use infrared radiation to simulate night-vision scenes, and optical systems that use inexpensive SLMs with resolution of 2K or less that are subdivided into more than one part. 
         [0071]    Other implementations are also within the scope of the following claims.