Abstract:
Light combiners and light splitters, and methods of using light combiners and light splitters are described. In particular, the description relates to light combiners and splitters that combine and split, respectively, light of different wavelength spectrums using polarizing beam splitters. The light combiners include arrangements of four polarizing beam splitters, such that three different wavelength spectrums of light can be directed into three of the polarizing beam splitters, and a combined light can be received from the fourth polarizing beam splitter. The light splitters can be the same configuration as the light combiners, but the direction of light travel is reversed to split, rather than combine, light. Polychromatic light can be directed into one of the polarizing beam splitters, and light having three different wavelength spectrums can be received from the other three polarizing beam splitters. The three different wavelength spectrums of light, the combined light, and the polychromatic light can be unpolarized light. The light combiners can be useful as unpolarized white light sources, such as in digital micro-mirror display projection systems.

Description:
FIELD OF TECHNOLOGY 
       [0001]    This description generally relates to light combiners and light splitters, and methods of using light combiners and light splitters. In particular, the description relates to light combiners and splitters that combine and split, respectively, light of different wavelength spectrums using polarizing beam splitters. 
       BACKGROUND 
       [0002]    Projection systems used for projecting an image on a screen can use multiple wavelength spectrum light sources, such as light emitting diodes (LEDs), with different wavelength spectrums to generate the illumination light. Several optical elements are disposed between the LEDs and the image display unit to combine and transfer the light from the LEDs to the image display unit. The image display unit can use various methods to impose an image on the light. For example, the image display unit may use polarization, as with transmissive or reflective liquid crystal displays (LCDs). 
         [0003]    Still other projection systems used for projecting an image on a screen can use white light configured to imagewise reflect from a digital micro-mirror array, such as the array used in Texas Instruments&#39; Digital Light Processor (DLP®) displays. In the DLP® display, individual mirrors within the digital micro-mirror array represent individual pixels of the projected image. A display pixel is illuminated when the corresponding mirror is tilted so that incident light is directed into the projected optical path. A rotating color wheel placed within the optical path is timed to the reflection of light from the digital micro-mirror array, so that the reflected white light is filtered to project the color corresponding to the pixel. The digital micro-mirror array is then switched to the next desired pixel color, and the process is continued at such a rapid rate that the entire projected display appears to be continuously illuminated. The digital micro-mirror projection system requires fewer pixelated array components, which can result in a smaller size projector. 
       SUMMARY 
       [0004]    Image brightness is an important parameter of a projection system. The brightness of color light sources and the efficiencies of collecting, combining, homogenizing and delivering the light to the image display unit all effect brightness. As the size of modern projector systems decreases, there is a need to maintain an adequate level of output brightness while at the same time keeping heat produced by the light sources at a low level that can be dissipated in a small projector system. There is a need for a light combining system that combines multiple color lights with increased efficiency to provide a light output with an adequate level of brightness without excessive power consumption by light sources. 
         [0005]    Generally, the present description relates to light combiners comprising polarizing beam splitters, and methods of using light combiners. The present description also relates to light splitters comprising polarizing beams splitters, and methods of using light splitters. 
         [0006]    In one aspect, a light combiner includes an arrangement of four polarizing beam splitters, each of which include two prisms each having two prism faces and two end faces, and a reflective polarizer disposed between the two prisms. The prism faces and ends can be polished so that total internal reflection can occur within each prism. Each of the faces and ends of each polarizing beam splitter can be in contact with an optically transmissive material having a refractive index lower than the refractive index of the prisms. The optically transmissive material can be air. The optically transmissive material can be an optical adhesive that bonds components of the light combiner together. The reflective polarizer can be a Cartesian reflective polarizer aligned to a first polarization direction, such as a polymeric multilayer optical film. The light combiner also includes four filters disposed between each pair of adjacent polarizing beam splitters. Each of the filters can change the polarization direction of at least one wavelength spectrum of light, while allowing other wavelength spectrums of light to remain unchanged. A reflector that changes the polarization direction and propagation direction of polarized light can be positioned adjacent one face of each of the four polarizing beam splitters. The polarization rotating reflector can be a quarter-wave retarder and a reflector, and the quarter-wave retarder can be aligned at 45° to the first polarization direction. 
         [0007]    In another aspect, a method of combining light using the light combiner is described. A first, second and third wavelength spectrum of light is directed toward the first, second and third polarizing beam splitter respectively, and combined light is received from the fourth polarizing beam splitter. In one embodiment, each of the first, second and third wavelength spectrums of light are unpolarized, and the combined light is also unpolarized. 
         [0008]    In yet another aspect, a method of splitting light using the light combiner is described. Polychromatic light is directed toward the fourth polarizing beam splitter, and a first, second and third wavelength spectrum of light is received from the first, second and third polarizing beam splitter, respectively. In one embodiment, the polychromatic light is unpolarized, and each of the first, second and third wavelength spectrums of light are also unpolarized. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Throughout the specification reference is made to the appended drawings, where like reference numerals designate like elements, and wherein: 
           [0010]      FIG. 1  is a perspective view of a polarizing beam splitter. 
           [0011]      FIG. 2  is a perspective view of a polarizing beam splitter with a quarter-wave retarder. 
           [0012]      FIG. 3  is a top schematic view showing a polarizing beam splitter with polished faces. 
           [0013]      FIGS. 4A-4D  are top schematic views of a light combiner. 
           [0014]      FIGS. 5A-5D  are top schematic views of a light combiner. 
           [0015]      FIGS. 6A-6D  are top schematic views of a light combiner. 
           [0016]      FIGS. 7A-7D  are top schematic views of a light combiner. 
           [0017]      FIGS. 8A-8D  are top schematic views of a light combiner. 
       
    
    
       [0018]    The figures are not necessarily to scale. Like numbers used in the figures refer to like components. However, it will be understood that the use of a number to refer to a component in a given figure is not intended to limit the component in another figure labeled with the same number. 
       DETAILED DESCRIPTION 
       [0019]    The light combiners described herein receive different wavelength spectrum lights and produce a combined light output that includes the different wavelength spectrum lights. In some embodiments, the combined light has the same etendue as each of the received lights. The combined light can be a polychromatic combined light that comprises more than one wavelength spectrum of light. In one aspect, each of the different wavelength spectrums of light correspond to a different color light (e.g. red, green and blue), and the combined light output is white light. For purposes of the description provided herein, “color light” and “wavelength spectrum light” are both intended to mean light having a wavelength spectrum range which may be correlated to a specific color if visible to the human eye. The more general term “wavelength spectrum light” refers to both visible and other wavelength spectrums of light including, for example, infrared light. 
         [0020]    When two or more unpolarized color lights are directed to the color combiner, each are split according to polarization by a reflective polarizer in a polarizing beam splitter (PBS). The light can be collimated, convergent, or divergent when it enters the PBS. Convergent or divergent light entering the PBS can be lost through one of the faces or ends of the PBS. To avoid such losses, all of the exterior faces of the PBS can be polished to enable total internal reflection (TIR) within the PBS. Enabling TIR improves the utilization of light entering the PBS, so that substantially all of the light entering the PBS within a range of angles is redirected to exit the PBS through the desired face. 
         [0021]    At least one polarization component of each color light entering the light combiner passes through to a polarization rotating reflector. The polarization rotating reflector reverses the propagation direction of the light and alters the magnitude of the polarization components, depending of the type and orientation of a retarder disposed in the polarization rotating reflector. The polarization rotating reflector can include a mirror and a retarder. The retarder can provide any desired retardation, such as an eighth-wave retarder, a quarter-wave retarder, and the like. In embodiments described herein, there is an advantage to using a quarter-wave retarder and an associated reflector. Linearly polarized light is changed to circularly polarized light as it passes through a quarter-wave retarder aligned at an angle of 45° to the axis of light polarization. Subsequent reflections from the reflective polarizer and quarter-wave retarder/reflectors in the color combiner result in efficient combined light output from the light combiner. In contrast, linearly polarized light is changed to a polarization state partway between s-polarization and p-polarization (either elliptical or linear) as it passes through other retarders and orientations, and can result in a lower efficiency of the combiner. 
         [0022]    The components of a light combiner including prisms, reflective polarizers, quarter-wave retarders, mirrors and filters can be bonded together by a suitable optical adhesive. The optical adhesive used to bond the components together can have a lower index of refraction than the index of refraction of the prisms used in the light combiner. A light combiner that is fully bonded together offers advantages including alignment stability during assembly, handling and use. 
         [0023]    The embodiments described above can be more readily understood by reference to the Figures and their accompanying description, which follows. 
         [0024]      FIG. 1  is a perspective view of a PBS. PBS  100  includes a reflective polarizer  190  disposed between the diagonal faces of prisms  110  and  120 . Prism  110  includes two end faces  175 ,  185 , and a first and second prism face  130 ,  140  having a 90° angle between them. Prism  120  includes two end faces  170 ,  180 , and a third and fourth prism face  150 ,  160  having a 90° angle between them. The first prism face  130  is parallel to the third prism face  150 , and the second prism face  140  is parallel to the fourth prism face  160 . The identification of the four prism faces shown in  FIG. 1  with a “first”, “second”, “third” and “fourth” serves to clarify the description of PBS  100  in the discussion that follows. Reflective polarizer  190  can be a Cartesian reflective polarizer or a non-Cartesian reflective polarizer. A non-Cartesian reflective polarizer can include multilayer inorganic films such as those produced by sequential deposition of inorganic dielectrics, such as a MacNeille polarizer. A Cartesian reflective polarizer has a polarization axis direction, and includes both wire-grid polarizers and polymeric multilayer optical films such as can be produced by extrusion and subsequent stretching of a multilayer polymeric laminate. In one embodiment, reflective polarizer  190  is aligned so that one polarization axis is parallel to a first polarization direction  195 , and perpendicular to a second polarization direction  196 . In one embodiment, the first polarization direction  195  can be the s-polarization direction, and the second polarization direction  196  can be the p-polarization direction. As shown in  FIG. 1 , the first polarization direction  195  is perpendicular to each of the end faces  170 ,  175 ,  180 ,  185 . 
         [0025]    A Cartesian reflective polarizer film provides the polarizing beam splitter with an ability to pass input light rays that are not fully collimated, and that are divergent or skewed from a central light beam axis. The Cartesian reflective polarizer film can comprise a polymeric multilayer optical film that comprises multiple layers of dielectric or polymeric material. Use of dielectric films can have the advantage of low attenuation of light and high efficiency in passing light. The multilayer optical film can comprise polymeric multilayer optical films such as those described in U.S. Pat. No. 5,962,114 (Jonza et al.) or U.S. Pat. No. 6,721,096 (Bruzzone et al.). 
         [0026]      FIG. 2  is a perspective view of the alignment of a quarter-wave retarder to a PBS, as used in some embodiments. Quarter-wave retarders can be used to change the polarization state of incident light. PBS retarder system  200  includes PBS  100  having first and second prisms  110  and  120 . A quarter-wave retarder  220  is disposed adjacent the first prism face  130 . Reflective polarizer  190  is a Cartesian reflective polarizer film aligned to first polarization direction  195 . Quarter-wave retarder  220  includes a quarter-wave polarization direction  295  that can be aligned at 45° to first polarization direction  195 . Although  FIG. 2  shows polarization direction  295  aligned at 45° to first polarization direction  195  in a clockwise direction, polarization direction  295  can instead be aligned at 45° to first polarization direction  195  in a counterclockwise direction. In some embodiments, quarter-wave polarization direction  295  can be aligned at any degree orientation to first polarization direction  195 , for example from 90° in a counter-clockwise direction to 90° in a clockwise direction. It can be advantageous to orient the retarder at approximately +/−45° as described, since circularly polarized light results when linearly polarized light passes through a quarter-wave retarder so aligned to the polarization direction. Other orientations of quarter-wave retarders can result in s-polarized light not being fully transformed to p-polarized light, and p-polarized light not being fully transformed to s-polarized light upon reflection from the mirrors, resulting in reduced efficiency of the light combiners described elsewhere in this description. 
         [0027]      FIG. 3  shows a top view of a path of light rays within a polished PBS  300 . According to one embodiment, the first, second, third and fourth prism faces  130 ,  140 ,  150 ,  160  of prisms  110  and  120  are polished external surfaces that are in contact with a material having an index of refraction “n 1 ” that is less than the index of refraction “n 2 ” of prisms  110  and  120 . According to another embodiment, all of the external faces of the PBS  300  (including end faces, not shown) are polished faces that provide TIR of oblique light rays within PBS  300 . The polished external surfaces are in contact with a material having an index of refraction “n 1 ” that is less than the index of refraction “n 2 ” of prisms  110  and  120 . TIR improves light utilization in PBS  300 , particularly when the light directed into PBS is not collimated along a central axis, i.e. the incoming light is either convergent or divergent. At least some light is trapped in PBS  300  by total internal reflections until it leaves through third prism face  150 . In some cases, substantially all of the light is trapped in PBS  300  by total internal reflections until it leaves through third prism face  150 . 
         [0028]    As shown in  FIG. 3 , light rays L 0  enter first prism face  130  within a range of angles θ 1 . Light rays L 1  within PBS  300  propagate within a range of angles θ 2  such that Snell&#39;s law is satisfied at prism faces  140 ,  160  and the end faces (not shown). Light rays “AB”, “AC” and “AD” represent three of the many paths of light through PBS  300 , that intersect reflective polarizer  190  at different angles of incidence before exiting through third prism face  150 . Light rays “AB” and “AD” also both undergo TIR at prism faces  140  and  160 , respectively, before exiting. It is to be understood that ranges of angles θ 1  and θ 2  can be a cone of angles so that reflections can also occur at the end faces of PBS  300 . In one embodiment, reflective polarizer  190  is selected to efficiently split light of different polarizations over a wide range of angles of incidence. A polymeric multilayer optical film is particularly well suited for splitting light over a wide range of angles of incidence. Other reflective polarizers including MacNeille polarizers and wire-grid polarizers can be used, but are less efficient at splitting the polarized light. A MacNeille polarizer does not efficiently transmit light at high angles of incidence. Efficient splitting of polarized light using a MacNeille polarizer can be limited to incidence angles below about 6 or 7 degrees from the normal, since significant reflection of both polarization states occur at larger angles. Efficient splitting of polarized light using a wire-grid polarizer typically requires an air gap adjacent one side of the wires, and efficiency drops when a wire-grid polarizer is immersed in a higher index medium. 
         [0029]    In one aspect,  FIG. 4A  is a top view schematic representation of a light combiner  400  that includes a first, second, third and fourth PBS  420 ,  440 ,  460 ,  480 , respectively. A first, second, third and fourth filter,  431 ,  432 ,  433  and  434 , respectively, is disposed between each pair of adjacent PBSs ( 420  and  480 ,  420  and  440 ,  440  and  460 ,  460  and  480 ), respectively. The first, second, third and fourth filters  431 ,  432 ,  433  and  434 , can be color-selective stacked retardation polarization (CSSRP) filters. In the present description, reference is made to CSSRP filters throughout; however, any filter capable of effecting the wavelength selective rotation of polarization as described, can be used. Rotation of polarization in each of the CSSRP filters,  431 ,  432 ,  433  and  434 , is dependent on the color of light passing through each of the filters. According to one aspect, each of the filters comprises a ColorSelect™ filter available from ColorLink Incorporated, Boulder, Colo. A polarization rotating reflector comprising retarder  425  and mirror  430  is disposed facing a fourth prism face  424 ,  444 ,  464  of each of the first, second and third PBS  420 ,  440 ,  460 , respectively. In one embodiment, retarder  425  is a quarter-wave retarder orientated at 45° to a first polarization direction  195 . 
         [0030]    First PBS  420  includes a first prism  405  having a first and second prism face  421 ,  422  having a 90° angle between them, and a second prism  406  having a third and fourth prism face  423 ,  424  having a 90° angle between them. A reflective polarizer  190  is disposed between first and second prisms  405 ,  406  such that first prism face  421  is opposite third prism face  423 . Reflective polarizer  190  can be a Cartesian reflective polarizer aligned to the first polarization direction  195  (in this view, perpendicular to the page). Reflective polarizer  190  can instead be a non-Cartesian polarizer. 
         [0031]    Second PBS  440  includes a first prism  445  having a first and second prism face  441 ,  442  having a 90° angle between them, and a second prism  446  having a third and fourth prism face  443 ,  444  having a 90° angle between them. The reflective polarizer  190  is disposed between first and second prisms  445 ,  446  such that first prism face  441  is opposite third prism face  443 . 
         [0032]    Third PBS  460  includes a first prism  465  having a first and second prism face  461 ,  462  having a 90° angle between them, and a second prism  466  having a third and fourth prism face  463 ,  464  having a 90° angle between them. The reflective polarizer  190  is disposed between first and second prisms  465 ,  466  such that first prism face  461  is opposite third prism face  463 . 
         [0033]    Fourth PBS  480  includes a first prism  485  having a first and second prism face  481 ,  482  having a 90° angle between them, and a second prism  486  having a third and fourth prism face  483 ,  484  having a 90° angle between them. The reflective polarizer  190  is disposed between first and second prisms  485 ,  486  such that first prism face  481  is opposite third prism face  483 . 
         [0034]    An optically transmissive material  435  is disposed adjacent each of the prism faces. The optically transmissive material  435  can be any material that has an index of refraction lower than the index of refraction of prisms  405 ,  406 ,  445 ,  446 ,  465 ,  466 ,  485 ,  486 . In one embodiment, the optically transmissive material  435  is air. In another embodiment, the optically transmissive material  435  is an optical adhesive which bonds the retarders  425  and the CSSRP filters  431 ,  432 ,  433 ,  434 , to their respective prism faces. 
         [0035]    In one aspect, a method of combining light using the light combiner  400  is shown in  FIG. 4A . A first wavelength spectrum light  450  is directed toward first prism face  421  of first PBS  420 , a second wavelength spectrum light  470  is directed toward first prism face  441  of second PBS  440 , a third wavelength spectrum light  490  is directed toward first prism face  461  of third PBS  460 , and a combined light  401  is received from first prism face  481  of fourth PBS  480 . In one embodiment, at least two of the first, second or third wavelength spectrum light  450 ,  470 ,  490  is directed toward the respective prism faces  421 ,  441 ,  461 , and combined light  401  is received from first prism face  461  of fourth PBS  480 . 
         [0036]    In one embodiment, first, second and third wavelength spectrum light  450 ,  470 ,  490  are unpolarized light, and the combined light  401  is also unpolarized. Each of the first, second, and third lights  450 ,  470 ,  490  can comprise light from a light emitting diode (LED) source. Various light sources can be used such as lasers, laser diodes, organic LED&#39;s (OLED&#39;s), and non solid-state light sources such as ultra high pressure (UHP), halogen or xenon lamps with appropriate collectors or reflectors. An LED light source can have advantages over other light sources, including economy of operation, long lifetime, robustness, efficient light generation and improved spectral output. 
         [0037]    In one embodiment, first and third CSSRP filters  431 ,  433  are selected to change the polarization direction of the first wavelength spectrum light  450 , and the second and fourth CSSRP filters  432 ,  434  are selected to change the polarization direction of the third wavelength spectrum light  490 . In a further embodiment shown in  FIGS. 4A-4D , the first, a second and the third wavelength spectrum light  450 ,  470 ,  490  are green, red and blue unpolarized light, respectively, the first and third CSSRP filters  431 ,  433  are green CSSRP filters, the second and fourth CSSRP filters  432 ,  434  are blue CSSRP filters, and the combined light  401  is white unpolarized light. 
         [0038]    Turning now to  FIG. 4B , the optical path of unpolarized green light  450  through light combiner  400  is described. In this embodiment, unpolarized green light  450  enters first PBS  420  through first prism face  421  and exits fourth PBS  480  through first prism face  481  as unpolarized green light comprising green light  458  having the first polarization direction, and green light  453  having the second polarization direction. 
         [0039]    Green light  450  enters first PBS  420  through first prism face  421 , intercepts reflective polarizer  190 , and is split into green light  451  having the first polarization direction and green light  452  having the second polarization direction. 
         [0040]    Green light  451  having the first polarization direction exits first PBS  420  through third prism face  423 , changes polarization direction as it passes through first CSSRP filter  431 , and enters fourth PBS  480  through second prism face  482  as green light  453  having the second polarization direction. Green light  453  having the second polarization direction reflects from reflective polarizer  190 , and exits fourth PBS  480  through first prism face  481  as green light  453  having the second polarization direction. 
         [0041]    Green light  452  having the second polarization direction exits first PBS  420  through second prism face  422 , passes through second CSSRP filter  432  without change of polarization, enters second PBS  440  through third prism face  443 , reflects from reflective polarizer  190 , exits second PBS  440  through fourth prism face  444 , and changes to green circularly polarized light  499 G as it passes through quarter-wave retarder  425 . Green circularly polarized light  499 G reflects from mirror  430 , changes direction of circular polarization, and changes to green light  454  having the first polarization direction as it passes through quarter-wave retarder  425 . Green light  454  having the first polarization direction enters second PBS  440  through fourth prism face  444 , passes through reflective polarizer  190 , exits second PBS  440  through second prism face  442 , and changes polarization direction as it passes through third CSSRP filter  433 , to become green light  456  having the second polarization direction. Green light  456  having the second polarization direction enters third PBS  460  through third prism face  463 , reflects from reflective polarizer  190 , exits third PBS  460  through fourth prism face  464 , changes to green circularly polarized light  499 G as it passes through quarter-wave retarder  425 , changes direction of circular polarization as it reflects from mirror  430 , and becomes green light  458  having the first polarization direction as it again passes through quarter-wave retarder  425 . Green light  458  having the first polarization direction enters third PBS  460  through fourth prism face  464 , passes through reflective polarizer  190 , exits third PBS  460  through second prism face  462 , passes through fourth second CSSRP filter  434  without change of polarization, enters fourth PBS  480  through fourth prism face  484 , passes through reflective polarizer  190 , and exits fourth PBS through first prism face  481  as green light  458  having the first polarization direction. 
         [0042]      FIG. 4C  shows the optical path of unpolarized red light  470  through light combiner  400 . In this embodiment, unpolarized red light  470  enters second PBS  440  through first prism face  441  and exits fourth PBS  480  through first prism face  481  as unpolarized red light comprising red light  474  having the first polarization direction, and red light  473  having the second polarization direction. 
         [0043]    Red light  470  enters second PBS  440  through first prism face  441 , intercepts reflective polarizer  190 , and is split into red light  471  having the first polarization direction and red light  472  having the second polarization direction. 
         [0044]    Red light  471  having the first polarization direction exits second PBS  440  through third prism face  443 , passes unchanged through second CSSRP filter  432 , enters first PBS  420  through second prism face  422 , passes through reflective polarizer  190 , exits first PBS  420  through fourth prism face  424 , and changes to red circularly polarized light  499 R as it passes through quarter-wave retarder  425 . Red circularly polarized light  499 R changes the direction of circular polarization as it reflects from mirror  430 , changes to red light  473  having the second polarization direction as it passes through quarter-wave retarder  425 , and re-enters first PBS  420  through fourth prism face  424 . Red light  473  having the second polarization direction reflects from reflective polarizer  190 , exits first PBS  420  through third prism face  423 , passes unchanged through first CSSRP filter  431 , enters fourth PBS  480  through second prism face  482 , reflects from reflective polarizer  190 , and exits fourth PBS  480  through first prism face  481  as red light  473  having the second polarization direction. 
         [0045]    Red light  472  having the second polarization direction exits second PBS  440  through second prism face  442 , passes through third CSSRP filter  433  without change of polarization, enters third PBS  460  through third prism face  463 , reflects from reflective polarizer  190 , exits third PBS  460  through fourth prism face  464 , and changes to red circularly polarized light  499 R as it passes through quarter-wave retarder  425 . Red circularly polarized light  499 R reflects from mirror  430 , changes direction of circular polarization, and changes to red light  474  having the first polarization direction as it passes through quarter-wave retarder  425 . Red light  474  having the first polarization direction enters third PBS  460  through fourth prism face  464 , passes through reflective polarizer  190 , exits third PBS  460  through second prism face  462 , passes unchanged through fourth CSSRP filter  434 , enters fourth PBS  480  through third prism face  483 , passes through reflective polarizer  190 , and exits fourth PBS  480  through first prism face  481  as red light  474  having the first polarization direction. 
         [0046]      FIG. 4D  shows the optical path of unpolarized blue light  490  through light combiner  400 . In this embodiment, unpolarized blue light  490  enters third PBS  460  through first prism face  461  and exits fourth PBS  480  through first prism face  481  as unpolarized blue light comprising blue light  494  having the first polarization direction, and blue light  497  having the second polarization direction. 
         [0047]    Blue light  490  enters third PBS  460  through first prism face  441 , intercepts reflective polarizer  190 , and is split into blue light  491  having the first polarization direction and blue light  492  having the second polarization direction. 
         [0048]    Blue light  491  having the first polarization direction exits third PBS  460  through third prism face  463 , passes unchanged through third CSSRP filter  433 , enters second PBS  440  through second prism face  442 , passes through reflective polarizer  190 , exits second PBS  440  through fourth prism face  444 , and changes to blue circularly polarized light  499 B as it passes through quarter-wave retarder  425 . Blue circularly polarized light  499 B changes the direction of circular polarization as it reflects from mirror  430 , changes to blue light  493  having the second polarization direction as it passes through quarter-wave retarder  425 , and re-enters second PBS  440  through fourth prism face  444 . Blue light  493  having the second polarization direction reflects from reflective polarizer  190 , exits second PBS  440  through third prism face  443 , and changes polarization direction as it passes through second CSSRP filter  432 , to become blue light  495  having the first polarization direction. Blue light  495  having the first polarization direction enters first PBS  420  through second prism face  422 , passes through reflective polarizer  190 , exits first PBS  420  through fourth prism face  481 , and changes to blue circularly polarized light  499 B as it passes through quarter-wave retarder  425 . Blue circularly polarized light  499 B changes direction of circular polarization as it reflects from mirror  430 , changes to blue light  497  having the second direction of polarization as it passes through quarter-wave retarder  425 , enters first PBS  420  through fourth prism face  424 , reflects from reflective polarizer  190 , and exits first PBS  420  through third prism face  423 . Blue light  497  having the second polarization direction passes through first CSSRP filter  431  without change of polarization, enters fourth PBS  480  through second prism face  482 , reflects from reflective polarizer  190 , and exits fourth PBS  480  through first prism face  481  as blue light  497  having the second polarization direction. 
         [0049]    Blue light  492  having the second polarization direction exits third PBS  490  through second prism face  462 , changes polarization as it passes through fourth CSSRP filter  434  to become blue light  494  having the first polarization direction. Blue light  494  having the first polarization direction enters fourth PBS  480  through third prism face  483 , passes through reflective polarizer  190 , and exits fourth PBS  480  through first prism face  481  as blue light  494  having the first polarization direction. 
         [0050]    In a further aspect, a method of splitting light using the light combiner  400  includes changing the propagation direction of the first, second, third, and combined light,  450 ,  470 ,  490 ,  401 , respectively, shown in  FIG. 4A-4D . Combined light  401  is directed toward first prism face  481  of fourth PBS  480 , and at least one of the first, second and third wavelength spectrum light is received from first prism face  421 ,  441 ,  461  of first, second and third PBS  420 ,  440 ,  460 , respectively. 
         [0051]      FIG. 5A  describes one embodiment of a light combiner  500 , where the first, second, third and fourth CSSRP filters,  431 ,  432 ,  433  and  434  of light combiner  400  are replaced by a first, second, third and fourth CSSRP filters,  531 ,  532 ,  533  and  534 , respectively. 
         [0052]    In one aspect, a method of combining light using the light combiner  500  is shown in  FIG. 5A . A first wavelength spectrum light  550  is directed toward first prism face  421  of first PBS  420 , a second wavelength spectrum light  570  is directed toward first prism face  441  of second PBS  440 , a third wavelength spectrum light  590  is directed toward first prism face  461  of third PBS  460 , and a combined light  501  is received from first prism face  481  of fourth PBS  480 . In one embodiment, at least two of the first, second or third wavelength spectrum light  550 ,  570 ,  590  are directed toward the respective prism faces  421 ,  441 ,  461 , and combined light  501  is received from first prism face  461  of fourth PBS  480 . In one embodiment, first, second and third wavelength spectrum light  550 ,  570 ,  590  are unpolarized light, and the combined light  501  is also unpolarized. Each of the first, second, and third lights  550 ,  570 ,  590  can comprise light from a light emitting diode (LED) source. Various light sources can be used such as lasers, laser diodes, organic LED&#39;s (OLED&#39;s), and non solid-state light sources such as ultra high pressure (UHP), halogen or xenon lamps with appropriate collectors or reflectors. An LED light source can have advantages over other light sources, including economy of operation, long lifetime, robustness, efficient light generation and improved spectral output. 
         [0053]    In one embodiment, first and third CSSRP filters  531 ,  533  are selected to change the polarization direction of the first wavelength spectrum light  550 , and the second and fourth CSSRP filters  532 ,  534  are selected to change the polarization direction of the third wavelength spectrum light  590 . In a further embodiment shown in  FIGS. 5A-5D , the first, a second and the third wavelength spectrum light  550 ,  570 ,  590  are red, green and blue respectively, the first and third CSSRP filters  531 ,  533  are red/cyan CSSRP filters, and the second and fourth CSSRP filters  532 , 534  are blue/yellow CSSRP filters. 
         [0054]    Turning now to  FIG. 5B , the optical path of unpolarized red light  550  through light combiner  500  is described. In this embodiment, unpolarized red light  550  enters first PBS  420  through first prism face  421  and exits fourth PBS  480  through first prism face  481  as unpolarized red light comprising red light  558  having the first polarization direction, and red light  553  having the second polarization direction. 
         [0055]    Red light  550  enters first PBS  420  through first prism face  421 , intercepts reflective polarizer  190 , and is split into red light  551  having the first polarization direction and red light  552  having the second polarization direction. 
         [0056]    Red light  551  having the first polarization direction exits first PBS  420  through third prism face  423 , changes polarization direction as it passes through first CSSRP filter  531 , and enters fourth PBS  480  through second prism face  482  as red light  553  having the second polarization direction. Red light  553  having the second polarization direction reflects from reflective polarizer  190 , and exits fourth PBS  480  through first prism face  481  as red light  553  having the second polarization direction. 
         [0057]    Red light  552  having the second polarization direction exits first PBS  420  through second prism face  422 , passes through second CSSRP filter  532  without change of polarization, enters second PBS  440  through third prism face  443 , reflects from reflective polarizer  190 , exits second PBS  440  through fourth prism face  444 , and changes to red circularly polarized light  599 R as it passes through quarter-wave retarder  425 . Red circularly polarized light  599 R reflects from mirror  430 , changes direction of circular polarization, and changes to red light  554  having the first polarization direction as it passes through quarter-wave retarder  425 . Red light  554  having the first polarization direction enters second PBS  440  through fourth prism face  444 , passes through reflective polarizer  190 , exits second PBS  440  through second prism face  442 , and changes polarization direction as it passes through third CSSRP filter  533 , to become red light  556  having the second polarization direction. Red light  556  having the second polarization direction enters third PBS  460  through third prism face  463 , reflects from reflective polarizer  190 , exits third PBS  460  through fourth prism face  464 , changes to red circularly polarized light  599 R as it passes through quarter-wave retarder  425 , changes direction of circular polarization as it reflects from mirror  430 , and becomes red light  558  having the first polarization direction as it again passes through quarter-wave retarder  425 . Red light  558  having the first polarization direction enters third PBS  460  through fourth prism face  464 , passes through reflective polarizer  190 , exits third PBS  460  through second prism face  462 , passes through fourth second CSSRP filter  534  without change of polarization, enters fourth PBS  480  through fourth prism face  484 , passes through reflective polarizer  190 , and exits fourth PBS through first prism face  481  as red light  558  having the first polarization direction. 
         [0058]      FIG. 5C  shows the optical path of unpolarized green light  570  through light combiner  500 . In this embodiment, unpolarized green light  570  enters second PBS  440  through first prism face  441  and exits fourth PBS  480  through first prism face  481  as unpolarized green light comprising green light  574  having the first polarization direction, and green light  573  having the second polarization direction. 
         [0059]    Green light  570  enters second PBS  440  through first prism face  441 , intercepts reflective polarizer  190 , and is split into green light  571  having the first polarization direction and green light  572  having the second polarization direction. 
         [0060]    Green light  571  having the first polarization direction exits second PBS  440  through third prism face  443 , passes unchanged through second CSSRP filter  532 , enters first PBS  420  through second prism face  422 , passes through reflective polarizer  190 , exits first PBS  420  through fourth prism face  424 , and changes to green circularly polarized light  599 G as it passes through quarter-wave retarder  425 . Green circularly polarized light  599 G changes the direction of circular polarization as it reflects from mirror  430 , changes to green light  573  having the second polarization direction as it passes through quarter-wave retarder  425 , and re-enters first PBS  420  through fourth prism face  424 . Green light  573  having the second polarization direction reflects from reflective polarizer  190 , exits first PBS  420  through third prism face  423 , passes unchanged through first CSSRP filter  531 , enters fourth PBS  480  through second prism face  482 , reflects from reflective polarizer  190 , and exits fourth PBS  480  through first prism face  481  as green light  573  having the second polarization direction. 
         [0061]    Green light  572  having the second polarization direction exits second PBS  440  through second prism face  442 , passes through third CSSRP filter  533  without change of polarization, enters third PBS  460  through third prism face  463 , reflects from reflective polarizer  190 , exits third PBS  460  through fourth prism face  464 , and changes to green circularly polarized light  599 G as it passes through quarter-wave retarder  425 . Green circularly polarized light  599 G reflects from mirror  430 , changes direction of circular polarization, and changes to green light  574  having the first polarization direction as it passes through quarter-wave retarder  425 . Green light  574  having the first polarization direction enters third PBS  460  through fourth prism face  464 , passes through reflective polarizer  190 , exits third PBS  460  through second prism face  462 , passes unchanged through fourth CSSRP filter  534 , enters fourth PBS  480  through third prism face  483 , passes through reflective polarizer  190 , and exits fourth PBS  480  through first prism face  481  as green light  574  having the first polarization direction. 
         [0062]      FIG. 5D  shows the optical path of unpolarized blue light  590  through light combiner  500 . In this embodiment, unpolarized blue light  590  enters third PBS  460  through first prism face  461  and exits fourth PBS  480  through first prism face  481  as unpolarized blue light comprising blue light  594  having the first polarization direction, and blue light  597  having the second polarization direction. 
         [0063]    Blue light  590  enters third PBS  460  through first prism face  441 , intercepts reflective polarizer  190 , and is split into blue light  591  having the first polarization direction and blue light  592  having the second polarization direction. 
         [0064]    Blue light  591  having the first polarization direction exits third PBS  460  through third prism face  463 , passes unchanged through third CSSRP filter  533 , enters second PBS  440  through second prism face  442 , passes through reflective polarizer  190 , exits second PBS  440  through fourth prism face  444 , and changes to blue circularly polarized light  599 B as it passes through quarter-wave retarder  425 . Blue circularly polarized light  599 B changes the direction of circular polarization as it reflects from mirror  430 , changes to blue light  593  having the second polarization direction as it passes through quarter-wave retarder  425 , and re-enters second PBS  440  through fourth prism face  444 . Blue light  593  having the second polarization direction reflects from reflective polarizer  190 , exits second PBS  440  through third prism face  443 , and changes polarization direction as it passes through second CSSRP filter  532 , to become blue light  595  having the first polarization direction. Blue light  595  having the first polarization direction enters first PBS  420  through second prism face  422 , passes through reflective polarizer  190 , exits first PBS  420  through fourth prism face  481 , and changes to blue circularly polarized light  599 B as it passes through quarter-wave retarder  425 . Blue circularly polarized light  599 B changes direction of circular polarization as it reflects from mirror  430 , changes to blue light  597  having the second direction of polarization as it passes through quarter-wave retarder  425 , enters first PBS  420  through fourth prism face  424 , reflects from reflective polarizer  190 , and exits first PBS  420  through third prism face  423 . Blue light  597  having the second polarization direction passes through first CSSRP filter  531  without change of polarization, enters fourth PBS  480  through second prism face  482 , reflects from reflective polarizer  190 , and exits fourth PBS  480  through first prism face  481  as blue light  597  having the second polarization direction. 
         [0065]    Blue light  592  having the second polarization direction exits third PBS  490  through second prism face  462 , changes polarization as it passes through fourth CSSRP filter  534  to become blue light  594  having the first polarization direction. Blue light  594  having the first polarization direction enters fourth PBS  480  through third prism face  483 , passes through reflective polarizer  190 , and exits fourth PBS  480  through first prism face  481  as blue light  594  having the first polarization direction. 
         [0066]    In a further aspect, a method of splitting light using the light combiner  500  includes changing the propagation direction of the first, second, third, and combined light,  550 ,  570 ,  590 ,  501 , respectively, shown in  FIG. 5A-5D . Combined light  501  is directed toward first prism face  481  of fourth PBS  580 , and at least one of the first, second and third wavelength spectrum light is received from first prism face  421 ,  441 ,  461  of first, second and third PBS  520 ,  540 ,  560 , respectively. 
         [0067]    In one aspect,  FIG. 6A  is a top view schematic representation of a light combiner  600  that includes a first, second, third and fourth PBS  620 ,  640 ,  660 ,  680 , respectively. A first, second, third and fourth CSSRP filter,  631 ,  632 ,  633 , and  634 , respectively, is disposed between each pair of adjacent PBSs ( 620  and  680 ,  620  and  640 ,  640  and  660 ,  660  and  680 ), respectively. Rotation of polarization in each of the CSSRP filters,  631 ,  632 ,  633 , and  634 , is dependent on the color of light passing through each of the individual filters. Each individual CSSRP filter is adapted to allow light of at least one color to pass through the filter unchanged, while altering the polarization direction of at least one other color. According to one aspect, each of the filters comprise a ColorSelect™ filter available from ColorLink Incorporated, Boulder, Colo. A polarization rotating reflector comprising retarder  425  and mirror  430  is disposed facing a fourth prism face  424 ,  444 ,  464 ,  484  of each of the first, second, third and fourth PBS  620 ,  640 ,  660 ,  680 , respectively. In one embodiment, retarder  425  is a quarter-wave retarder orientated at 45° to a first polarization direction  195 . 
         [0068]    First PBS  620  includes a first prism  405  having a first and fourth prism face  421 ,  424  having a 90° angle between them, and a second prism  406  having a second and third prism face  422 ,  423  having a 90° angle between them. A reflective polarizer  190  is disposed between first and second prisms  405 ,  406  such that first prism face  421  is opposite third prism face  423 . Reflective polarizer  190  can be a Cartesian reflective polarizer aligned to the first polarization direction  195  (in this view, perpendicular to the page). Reflective polarizer  190  can instead be a non-Cartesian polarizer. 
         [0069]    Second PBS  640  includes a first prism  445  having a first and fourth prism face  441 ,  444  having a 90° angle between them, and a second prism  446  having a second and third prism face  442 ,  443  having a 90° angle between them. The reflective polarizer  190  is disposed between first and second prisms  445 ,  446  such that first prism face  441  is opposite third prism face  443 . 
         [0070]    Third PBS  660  includes a first prism  465  having a first and fourth prism face  461 ,  464  having a 90° angle between them, and a second prism  466  having a second and third prism face  462 ,  463  having a 90° angle between them. The reflective polarizer  190  is disposed between first and second prisms  465 ,  466  such that first prism face  461  is opposite third prism face  463 . 
         [0071]    Fourth PBS  680  includes a first prism  485  having a first and fourth prism face  481 ,  484  having a 90° angle between them, and a second prism  486  having a second and third prism face  482 ,  483  having a 90° angle between them. The reflective polarizer  190  is disposed between first and second prisms  485 ,  486  such that first prism face  481  is opposite third prism face  483 . 
         [0072]    An optically transmissive material  435  is disposed adjacent each of the prism faces. The optically transmissive material  435  can be any material that has an index of refraction lower than the index of refraction of prisms  405 ,  406 ,  445 ,  446 ,  465 ,  466 ,  485 ,  486 . In one embodiment, the optically transmissive material  435  is air. In another embodiment, the optically transmissive material  435  is an optical adhesive which bonds the retarders  425  and the CSSRP filters  631 ,  632 ,  633 ,  634 , to their respective prism faces. 
         [0073]    In one aspect, a method of combining light using the light combiner  600  is shown in  FIG. 6A . A first wavelength spectrum light  650  is directed toward first prism face  421  of first PBS  620 , a second wavelength spectrum light  670  is directed toward first prism face  441  of second PBS  640 , a third wavelength spectrum light  690  is directed toward first prism face  461  of third PBS  660 , and a combined light  601  is received from first prism face  481  of fourth PBS  680 . In one embodiment, at least two of the first, second or third wavelength spectrum light  650 ,  670 ,  690  is directed toward the respective prism faces  421 ,  441 ,  461 , and combined light  601  is received from first prism face  461  of fourth PBS  680 . 
         [0074]    In one embodiment, first, second and third wavelength spectrum light  650 ,  670 ,  690  are unpolarized light, and the combined light  601  is also unpolarized. Each of the first, second, and third lights  650 ,  670 ,  690  can comprise light from a light emitting diode (LED) source. Various light sources can be used such as lasers, laser diodes, organic LED&#39;s (OLED&#39;s), and non solid-state light sources such as ultra high pressure (UHP), halogen or xenon lamps with appropriate collectors or reflectors. An LED light source can have advantages over other light sources, including economy of operation, long lifetime, robustness, efficient light generation and improved spectral output. 
         [0075]    In one embodiment, first and third CSSRP filters  631 ,  633  are selected to change the polarization direction of the second and third wavelength spectrum light  670 ,  690 , and the second and fourth CSSRP filters  632 ,  634  are selected to change the polarization direction of the first and second wavelength spectrum light  650 ,  670 . In a further embodiment shown in  FIGS. 6A-6D , the first, second and third wavelength spectrum light  650 ,  670 ,  690  are green, red and blue unpolarized light, respectively, the first and third CSSRP filters  631 ,  633  are green/magenta CSSRP filters that rotate the polarization direction of red and blue light while preserving the polarization direction of green light; the second and fourth CSSRP filters  632 ,  634  are yellow/blue CSSRP filters that rotate the polarization direction of red and green light while preserving the polarization direction of blue light; and the combined light  601  is white unpolarized light. 
         [0076]    Turning now to  FIG. 6B , the optical path of unpolarized green light  650  through light combiner  600  is described. In this embodiment, unpolarized green light  650  enters first PBS  620  through first prism face  421  and exits fourth PBS  680  through first prism face  481  as unpolarized green light comprising green light  658  having the first polarization direction, and green light  653  having the second polarization direction. 
         [0077]    Green light  650  enters first PBS  620  through first prism face  421 , intercepts reflective polarizer  190 , and is split into green light  651  having the first polarization direction and green light  652  having the second polarization direction. 
         [0078]    Green light  651  having the first polarization direction exits first PBS  620  through third prism face  423 , passes unchanged through first CSSRP filter  631 , enters fourth PBS  680  through second prism face  482 , passes through reflective polarizer  190 , exits fourth PBS  680  through fourth prism face  484 , and changes to green circularly polarized light  699 G as it passes through quarter-wave retarder  425 . Green circularly polarized light  699 G changes direction of circular polarization as it reflects from mirror  430 , changes to green light  653  having the second polarization direction as it passes through quarter-wave retarder  425 , enters fourth PBS  680  through fourth prism face  484 , reflects from reflective polarizer  190 , and exits fourth PBS  680  through first prism face  481  as green light  653  having the second polarization direction. 
         [0079]    Green light  652  having the second polarization direction exits first PBS  620  through fourth prism face  424 , and changes to green circularly polarized light  699 G as it passes through quarter-wave retarder  425 . Green circularly polarized light  699 G changes direction of circular polarization as it reflects from mirror  430 , changes to green light  654  having the first polarization direction as it passes through quarter-wave retarder  425 , re-enters first PBS  620  through fourth prism face  424 , passes through reflective polarizer  190  and exits first PBS through second prism face  422 . Green light  654  having the first polarization direction changes to green light  656  having the second polarization direction as it passes through second CSSRP filter  632 , enters second PBS  640  through third prism face  443 , reflects from reflective polarizer  190 , exits second PBS  640  through second prism face  442 , passes through third CSSRP filter  633  without change of polarization, and enters third PBS  660  through third prism face  463 . Green light  656  having the second polarization direction reflects from reflective polarizer  190 , exits third PBS  660  through second prism face  462 , changes to green light  658  having the first polarization direction as it passes through fourth CSSRP filter  634 , enters fourth PBS  680  through third prism face  483 , passes through reflective polarizer  190  and exits fourth PBS  680  through first prism face  481  as green light  658  having the first polarization direction. 
         [0080]      FIG. 6C  shows the optical path of unpolarized red light  670  through light combiner  600 . In this embodiment, unpolarized red light  670  enters second PBS  640  through first prism face  441  and exits fourth PBS  680  through first prism face  481  as unpolarized red light comprising red light  678  having the first polarization direction, and red light  677  having the second polarization direction. 
         [0081]    Red light  670  enters second PBS  640  through first prism face  441  and intercepts reflective polarizer  190  where it is split into red light  671  having the first polarization direction and red light  672  having the second polarization direction. 
         [0082]    Red light  671  having the first polarization direction, exits second PBS  640  through third prism face  443  and changes to red light  673  having the second polarization direction as it passes through second CSSRP filter  632 . Red light  673  having the second polarization direction enters first PBS  620  through second prism face  422 , reflects from reflective polarizer  190 , exits first PBS  620  through third prism face  423 , and changes to red light  675  having the first polarization direction as it passes through first CSSRP filter  631 . Red light  675  having the first polarization direction enters fourth PBS  680  through second prism face  482 , passes through reflective polarizer  190 , exits fourth PBS  680  through fourth prism face  484  and changes to red circularly polarized light  699 R as it passes through quarter-wave retarder  425 . Red circularly polarized light  699 R changes direction of circular polarization as it reflects from mirror  430 , changes to red light  677  having the second polarization direction as it passes through quarter-wave retarder  425 , enters fourth PBS  680  through fourth prism face  484 , reflects from reflective polarizer  190 , and exits fourth PBS  680  through first prism face  481  as red light  677  having the second polarization direction. 
         [0083]    Red light  672  having the second polarization direction reflects from reflective polarizer  190 , exits second PBS  640  through fourth prism face  444 , changes to red circularly polarized light  699 R as it passes through quarter-wave retarder  425 , changes direction of circular polarization as it reflects from mirror  430 , and changes to red light  674  having the first polarization direction as it again passes through quarter-wave retarder  425 . Red light  674  having the first polarization direction enters second PBS  640  through fourth prism face  444 , passes through reflective polarizer  190 , exits second PBS  640  through second prism face  442 , and changes to red light  676  having the second polarization direction as it passes through third CSSRP filter  633 . Red light  676  having the second polarization direction enters third PBS  660  through third prism face  463 , reflects from reflective polarizer  190 , exits third PBS  660  through second prism face  462 , and changes to red light  678  having the first polarization direction as it passes through fourth CSSRP filter  634 . Red light  678  having the first polarization direction enters fourth PBS  680  through third prism face  483 , passes through reflective polarizer  190 , and exits fourth PBS  680  through first prism face  481  as red light  678  having the first polarization direction. 
         [0084]      FIG. 6D  shows the optical path of unpolarized blue light  690  through light combiner  600 . In this embodiment, unpolarized blue light  690  enters third PBS  660  through first prism face  461  and exits fourth PBS  680  through first prism face  481  as unpolarized blue light comprising blue light  694  having the first polarization direction, and blue light  697  having the second polarization direction. 
         [0085]    Blue light  690  enters third PBS  660  through first prism face  461  and intercepts reflective polarizer  190  where it is split into blue light  691  having the first polarization direction and blue light  692  having the second polarization direction. 
         [0086]    Blue light  691  having the first polarization direction exits third PBS  660  through third prism face  463 , and changes to blue light  693  having the second polarization direction as it passes through third CSSRP filter  633 . Blue light  693  having the second polarization direction enters second PBS  640  through second prism face  442 , reflects from reflective polarizer  190 , exits second PBS  640  through third prism face  443 , and passes unchanged through second CSSRP filter  632 . Blue light  693  having the second polarization direction enters first PBS  620  through second prism face  422 , reflects from reflective polarizer  190 , exits first PBS  620  through third prism face  423 , changes to blue light  695  having the first polarization direction as it passes through first CSSRP filter  631 , and enters fourth PBS  680  through second prism face  482 . Blue light  695  having the first polarization direction, passes through reflective polarizer  190 , exits fourth PBS  680  through fourth prism face  484 , and changes to blue circularly polarized light  699 B as it passes through quarter-wave retarder  425 . Blue circularly polarized light  699 B changes direction of circular polarization as it reflects from mirror  430 , changes to blue light  697  having the second polarization direction as it passes through quarter-wave retarder  425 , enters fourth PBS  680  through fourth prism face  484 , reflects from reflective polarizer  190 , and exits fourth PBS  680  through first prism face  481  as blue light  697  having the second polarization direction. 
         [0087]    Blue light  692  having the second polarization direction reflects from reflective polarizer  190 , exits third PBS  660  through fourth prism face  464 , changes to blue circularly polarized light  699 B as it passes through quarter-wave retarder  425 , changes direction of circular polarization as it reflects from mirror  430 , and changes to blue light  694  having the first polarization direction as it again passes through quarter-wave retarder  425 . Blue light  694  having the first polarization direction enters third PBS  660  through fourth prism face  464 , passes through reflective polarizer  190 , exits third PBS  660  through second prism face  462 , and passes unchanged through fourth CSSRP filter  634 . Blue light  694  having the first polarization direction enters fourth PBS  680  through third prism face  483 , passes through reflective polarizer  190 , and exits fourth PBS  680  through first prism face  481  as blue light  694  having the first polarization direction. 
         [0088]    In a further aspect, a method of splitting light using the light combiner  600  includes changing the propagation direction of the first, second, third, and combined light,  650 ,  670 ,  690 ,  601 , respectively, shown in  FIG. 6A-6D . Combined light  601  is directed toward first prism face  481  of fourth PBS  680 , and at least one of the first, second and third wavelength spectrum light is received from first prism face  421 ,  441 ,  461  of first, second and third PBS  620 ,  640 ,  660 , respectively. 
         [0089]    In one aspect,  FIG. 7A  is a top view schematic representation of a light combiner  700  that includes a first, second, third and fourth PBS  720 ,  740 ,  760 ,  780 , respectively. A first, second, third and fourth CSSRP filter,  731 ,  732 ,  733  and  734 , respectively, is disposed between each pair of adjacent PBSs ( 720  and  780 ,  720  and  740 ,  740  and  760 ,  760  and  780 ), respectively. Rotation of polarization in each of the CSSRP filters,  731 ,  732 ,  733  and  734 , is dependent on the color of light passing through each of the filters. According to one aspect, each of the filters comprises a ColorSelect™ filter available from ColorLink Incorporated, Boulder, Colo. A polarization rotating reflector comprising retarder  425  and mirror  430  is disposed facing a fourth prism face  424 ,  444 ,  464  of each of the first, second and third PBS  720 ,  740 ,  760 , respectively. In one embodiment, retarder  425  is a quarter-wave retarder orientated at 45° to a first polarization direction  195 . 
         [0090]    First PBS  720  includes a first prism  405  having a first and second prism face  421 ,  422  having a 90° angle between them, and a second prism  406  having a third and fourth prism face  423 ,  424  having a 90° angle between them. A reflective polarizer  190  is disposed between first and second prisms  405 ,  406  such that first prism face  421  is opposite third prism face  423 . Reflective polarizer  190  can be a Cartesian reflective polarizer aligned to the first polarization direction  195  (in this view, perpendicular to the page). Reflective polarizer  190  can instead be a non-Cartesian polarizer. 
         [0091]    Second PBS  740  includes a first prism  445  having a first and fourth prism face  441 ,  444  having a 90° angle between them, and a second prism  446  having a second and third prism face  442 ,  443  having a 90° angle between them. The reflective polarizer  190  is disposed between first and second prisms  445 ,  446  such that first prism face  441  is opposite third prism face  443 . 
         [0092]    Third PBS  760  includes a first prism  465  having a first and fourth prism face  461 ,  464  having a 90° angle between them, and a second prism  466  having a second and third prism face  462 ,  463  having a 90° angle between them. The reflective polarizer  190  is disposed between first and second prisms  465 ,  466  such that first prism face  461  is opposite third prism face  463 . 
         [0093]    Fourth PBS  780  includes a first prism  485  having a first and second prism face  481 ,  482  having a 90° angle between them, and a second prism  486  having a third and fourth prism face  483 ,  484  having a 90° angle between them. The reflective polarizer  190  is disposed between first and second prisms  485 ,  486  such that first prism face  481  is opposite third prism face  483 . 
         [0094]    An optically transmissive material  435  is disposed adjacent each of the prism faces. The optically transmissive material  435  can be any material that has an index of refraction lower than the index of refraction of prisms  405 ,  406 ,  445 ,  446 ,  465 ,  466 ,  485 ,  486 . In one embodiment, the optically transmissive material  435  is air. In another embodiment, the optically transmissive material  435  is an optical adhesive which bonds the retarders  425  and the CSSRP filters  731 ,  732 ,  733 ,  734 , to their respective prism faces. 
         [0095]    In one aspect, a method of combining light using the light combiner  700  is shown in  FIG. 7A . A first wavelength spectrum light  750  is directed toward first prism face  421  of first PBS  720 , a second wavelength spectrum light  770  is directed toward first prism face  441  of second PBS  740 , a third wavelength spectrum light  790  is directed toward first prism face  461  of third PBS  760 , and a combined light  701  is received from first prism face  481  of fourth PBS  780 . In one embodiment, at least two of the first, second or third wavelength spectrum light  750 ,  770 ,  790  is directed toward the respective prism faces  421 ,  441 ,  461 , and combined light  701  is received from first prism face  461  of fourth PBS  780 . In one embodiment, first, second and third wavelength spectrum light  750 ,  770 ,  790  are unpolarized light, and the combined light  701  is also unpolarized. Each of the first, second, and third lights  750 ,  770 ,  790  can comprise light from a light emitting diode (LED) source. Various light sources can be used such as lasers, laser diodes, organic LED&#39;s (OLED&#39;s), and non solid-state light sources such as ultra high pressure (UHP), halogen or xenon lamps with appropriate collectors or reflectors. An LED light source can have advantages over other light sources, including economy of operation, long lifetime, robustness, efficient light generation and improved spectral output. 
         [0096]    In one embodiment, first CSSRP filter  731  is selected to change the polarization direction of the first wavelength spectrum light  750 , second CSSRP filter  732  is selected to change the polarization direction of the third wavelength spectrum light  790 , third CSSRP filter  733  is selected to change the polarization direction of the second and third wavelength spectrums light  770  and  790 , and the fourth CSSRP filter  734  is selected to change the polarization direction of the first and second wavelength spectrums light  750  and  770 . In a further embodiment shown in  FIGS. 7A-7D , the first, second and the third wavelength spectrum light  750 ,  770 ,  790  are green, red and blue unpolarized light, respectively, the first CSSRP filter  731  is a green/magenta CSSRP filter, the second CSSRP filter  432  is a blue/yellow CSSRP filter, the third CSSRP filter  733  is a magenta/green CSSRP filter, the fourth CSSRP filter  734  is a cyan/red CSSRP filter, and the combined light  701  is white unpolarized light. 
         [0097]    Turning now to  FIG. 7B , the optical path of unpolarized green light  750  through light combiner  700  is described. In this embodiment, unpolarized green light  750  enters first PBS  720  through first prism face  421  and exits fourth PBS  780  through first prism face  481  as unpolarized green light comprising green light  754  having the first polarization direction, and green light  753  having the second polarization direction. 
         [0098]    Green light  750  enters first PBS  720  through first prism face  421 , intercepts reflective polarizer  190 , and is split into green light  751  having the first polarization direction and green light  752  having the second polarization direction. 
         [0099]    Green light  751  having the first polarization direction exits first PBS  720  through third prism face  423 , changes polarization direction as it passes through first CSSRP filter  731 , and enters fourth PBS  780  through second prism face  482  as green light  753  having the second polarization direction. Green light  753  having the second polarization direction reflects from reflective polarizer  190 , and exits fourth PBS  780  through first prism face  481  as green light  753  having the second polarization direction. 
         [0100]    Green light  752  having the second polarization direction exits first PBS  720  through second prism face  422 , passes through second CSSRP filter  732  without change of polarization, enters second PBS  740  through third prism face  443 , reflects from reflective polarizer  190 , exits second PBS  740  through second prism face  442 , passes through third CSSRP filter  733  without change of polarization, enters third PBS  760  through third prism face  463 , reflects from reflective polarizer  190 , exits third PBS  760  through second prism face  462 , and changes to green light  754  having the first polarization direction as it passes through fourth CSSRP filter  734 . Green light  754  having the first polarization direction enters fourth PBS  780  through third prism face  483 , passes through reflective polarizer, and exits fourth PBS  780  through first prism face  481  as green light  754  having the first polarization direction. 
         [0101]      FIG. 7C  shows the optical path of unpolarized red light  770  through light combiner  700 . In this embodiment, unpolarized red light  770  enters second PBS  740  through first prism face  441  and exits fourth PBS  780  through first prism face  481  as unpolarized red light comprising red light  778  having the first polarization direction, and red light  773  having the second polarization direction. 
         [0102]    Red light  770  enters second PBS  740  through first prism face  441 , intercepts reflective polarizer  190 , and is split into red light  771  having the first polarization direction and red light  772  having the second polarization direction. 
         [0103]    Red light  771  having the first polarization direction exits second PBS  740  through third prism face  443 , passes unchanged through second CSSRP filter  732 , enters first PBS  720  through second prism face  422 , passes through reflective polarizer  190 , exits first PBS  720  through fourth prism face  424 , and changes to red circularly polarized light  799 R as it passes through quarter-wave retarder  425 . Red circularly polarized light  799 R changes the direction of circular polarization as it reflects from mirror  430 , changes to red light  773  having the second polarization direction as it passes through quarter-wave retarder  425 , and re-enters first PBS  720  through fourth prism face  424 . Red light  773  having the second polarization direction reflects from reflective polarizer  190 , exits first PBS  720  through third prism face  423 , passes unchanged through first CSSRP filter  731 , enters fourth PBS  780  through second prism face  482 , reflects from reflective polarizer  190 , and exits fourth PBS  780  through first prism face  481  as red light  773  having the second polarization direction. 
         [0104]    Red light  772  having the second polarization direction exits second PBS  740  through fourth prism face  444 , and changes to red circularly polarized light  799 R as it passes through quarter-wave retarder  425 . Red circularly polarized light  799 R changes the direction of circular polarization as it reflects from mirror  430 , changes to red light  774  having the first polarization direction as it passes through quarter-wave retarder  425 , enters second PBS  740  through fourth prism face  444 , passes through reflective polarizer  190 , exits second PBS  740  through second prism face  442 , and changes to red light  776  having the second polarization direction as it passes through third CSSRP filter  733 . Red light  776  having the second polarization direction enters third PBS  760  through third prism face  463 , reflects from reflective polarizer  190 , exits third PBS  760  through second prism face  462 , and changes to red light  778  having the first polarization direction as it passes through fourth CSSRP filter  734 . Red light  778  having the first polarization direction enters fourth PBS  780  through third prism face  483 , passes through reflective polarizer  190 , and exits fourth PBS  780  through first prism face  481  as red light  778  having the first polarization direction. 
         [0105]      FIG. 7D  shows the optical path of unpolarized blue light  790  through light combiner  700 . In this embodiment, unpolarized blue light  790  enters third PBS  760  through first prism face  461  and exits fourth PBS  780  through first prism face  481  as unpolarized blue light comprising blue light  796  having the first polarization direction, and blue light  795  having the second polarization direction. 
         [0106]    Blue light  790  enters third PBS  760  through first prism face  461 , intercepts reflective polarizer  190 , and is split into blue light  791  having the first polarization direction and blue light  792  having the second polarization direction. 
         [0107]    Blue light  791  having the first polarization direction exits third PBS  760  through third prism face  463 , and changes to blue light  793  having the second polarization direction as it passes through third CSSRP filter  733 , enters second PBS  740  through second prism face  442 , reflects from reflective polarizer  190 , exits second PBS  740  through third prism face  443 , and changes to blue light  794  having the first polarization direction as it passes through second CSSRP filter  732 . Blue light  794  having the first polarization direction enters first PBS  720  through second prism face  422 , passes through reflective polarizer  190 , exits first PBS  720  through fourth prism face  424 , and changes to blue circularly polarized light  799 B as it passes through quarter-wave retarder  425 . Blue circularly polarized light  799 B changes direction of circular polarization as it reflects from mirror  430 , changes to blue light  795  having the second polarization direction as it passes through quarter-wave retarder  425 , enters first PBS  720  through fourth prism face  424 , reflects from reflective polarizer  190 , and exits first PBS  720  through third prism face  423 . 
         [0108]    Blue light  795  having the second polarization direction passes unchanged through first CSSRP filter  731 , enters fourth PBS  780  through second prism face  482 , reflects from reflective polarizer  190 , and exits fourth PBS  780  through first prism face  481  as blue light  795  having the second polarization direction. 
         [0109]    Blue light  792  having the second polarization direction exits third PBS  790  through fourth prism face  464 , changes to blue circularly polarized light  799 B as it passes through quarter-wave retarder  425 , changes direction of circular polarization as it reflects from mirror  430 , and changes to blue light  796  having the first polarization direction as it passes through quarter-wave retarder  425 . Blue light  796  having the first polarization direction enters third PBS  760  through fourth prism face  464 , passes through reflective polarizer  190 , exits third PBS  760  through second prism face  462 , passes unchanged through fourth CSSRP filter  734 , enters fourth PBS  780  through third prism face  483 , passes through reflective polarizer  190 , and exits fourth PBS  780  through first prism face  481  as blue light  796  having the first polarization direction. 
         [0110]    In a further aspect, a method of splitting light using the light combiner  700  includes changing the propagation direction of the first, second, third, and combined light,  750 ,  770 ,  790 ,  701 , respectively, shown in  FIG. 7A-7D . Combined light  701  is directed toward first prism face  481  of fourth PBS  780 , and at least one of the first, second and third wavelength spectrum light is received from first prism face  421 ,  441 ,  461  of first, second and third PBS  720 ,  740 ,  760 , respectively. 
         [0111]    In one aspect,  FIG. 8A  is a top view schematic representation of a light combiner  800  that includes a first, second, third and fourth PBS  820 ,  840 ,  860 ,  880 , respectively. A first, second, third and fourth CSSRP filter,  831 ,  832 ,  833  and  834 , respectively, is disposed between each pair of adjacent PBSs ( 820  and  880 ,  820  and  840 ,  840  and  860 ,  860  and  880 ), respectively. Rotation of polarization in each of the CSSRP filters,  831 ,  832 ,  833  and  834 , is dependent on the color of light passing through each of the filters. According to one aspect, each of the filters comprises a ColorSelect™ filter available from ColorLink Incorporated, Boulder, Colo. A polarization rotating reflector comprising retarder  425  and mirror  430  is disposed facing a fourth prism face  424 ,  444 ,  464  of each of the first, second and third PBS  820 ,  840 ,  860 , respectively. In one embodiment, retarder  425  is a quarter-wave retarder orientated at 45° to a first polarization direction  195 . 
         [0112]    First PBS  820  includes a first prism  405  having a first and fourth prism face  421 ,  424  having a 90° angle between them, and a second prism  406  having a second and third prism face  422 ,  423  having a 90° angle between them. A reflective polarizer  190  is disposed between first and second prisms  405 ,  406  such that first prism face  421  is opposite third prism face  423 . Reflective polarizer  190  can be a Cartesian reflective polarizer aligned to the first polarization direction  195  (in this view, perpendicular to the page). Reflective polarizer  190  can instead be a non-Cartesian polarizer. 
         [0113]    Second PBS  840  includes a first prism  445  having a first and second prism face  441 ,  442  having a 90° angle between them, and a second prism  446  having a third and fourth prism face  443 ,  444  having a 90° angle between them. The reflective polarizer  190  is disposed between first and second prisms  445 ,  446  such that first prism face  441  is opposite third prism face  443 . 
         [0114]    Third PBS  860  includes a first prism  465  having a first and fourth prism face  461 ,  464  having a 90° angle between them, and a second prism  466  having a second and third prism face  462 ,  463  having a 90° angle between them. The reflective polarizer  190  is disposed between first and second prisms  465 ,  466  such that first prism face  461  is opposite third prism face  463 . 
         [0115]    Fourth PBS  880  includes a first prism  485  having a first and second prism face  481 ,  482  having a 90° angle between them, and a second prism  486  having a third and fourth prism face  483 ,  484  having a 90° angle between them. The reflective polarizer  190  is disposed between first and second prisms  485 ,  486  such that first prism face  481  is opposite third prism face  483 . 
         [0116]    An optically transmissive material  435  is disposed adjacent each of the prism faces. The optically transmissive material  435  can be any material that has an index of refraction lower than the index of refraction of prisms  405 ,  406 ,  445 ,  446 ,  465 ,  466 ,  485 ,  486 . In one embodiment, the optically transmissive material  435  is air. In another embodiment, the optically transmissive material  435  is an optical adhesive which bonds the retarders  425  and the CSSRP filters  831 ,  832 ,  833 ,  834 , to their respective prism faces. 
         [0117]    In one aspect, a method of combining light using the light combiner  800  is shown in  FIG. 8A . A first wavelength spectrum light  850  is directed toward first prism face  421  of first PBS  820 , a second wavelength spectrum light  870  is directed toward first prism face  441  of second PBS  840 , a third wavelength spectrum light  890  is directed toward first prism face  461  of third PBS  860 , and a combined light  801  is received from first prism face  481  of fourth PBS  880 . In one embodiment, at least two of the first, second or third wavelength spectrum light  850 ,  870 ,  890  is directed toward the respective prism faces  421 ,  441 ,  461 , and combined light  801  is received from first prism face  461  of fourth PBS  880 . In one embodiment, first, second and third wavelength spectrum light  850 ,  870 ,  890  are unpolarized light, and the combined light  801  is also unpolarized. Each of the first, second, and third lights  850 ,  870 ,  890  can comprise light from a light emitting diode (LED) source. Various light sources can be used such as lasers, laser diodes, organic LED&#39;s (OLED&#39;s), and non solid-state light sources such as ultra high pressure (UHP), halogen or xenon lamps with appropriate collectors or reflectors. An LED light source can have advantages over other light sources, including economy of operation, long lifetime, robustness, efficient light generation and improved spectral output. 
         [0118]    In one embodiment, first and third CSSRP filters  831 ,  833  are selected to change the polarization direction of the first wavelength spectrum light  850 , and the second and fourth CSSRP filters  832 ,  834  are selected to change the polarization direction of the first and second wavelength spectrums light  850  and  870 . In a further embodiment shown in  FIGS. 8A-8D , the first, a second and the third wavelength spectrum light  850 ,  870 ,  890  are red, green and blue unpolarized light, respectively, the first and third CSSRP filters  831 ,  833  are red/cyan CSSRP filters, the second and fourth CSSRP filters  832 ,  834  are yellow/blue CSSRP filters, and the combined light  801  is white unpolarized light. 
         [0119]    Turning now to  FIG. 8B , the optical path of unpolarized red light  850  through light combiner  800  is described. In this embodiment, unpolarized red light  850  enters first PBS  820  through first prism face  421  and exits fourth PBS  880  through first prism face  481  as unpolarized red light comprising red light  858  having the first polarization direction, and red light  853  having the second polarization direction. 
         [0120]    Red light  850  enters first PBS  820  through first prism face  421 , intercepts reflective polarizer  190 , and is split into red light  851  having the first polarization direction and red light  852  having the second polarization direction. 
         [0121]    Red light  851  having the first polarization direction exits first PBS  820  through third prism face  423 , changes polarization direction as it passes through first CSSRP filter  831 , and enters fourth PBS  880  through second prism face  482  as red light  853  having the second polarization direction. Red light  853  having the second polarization direction reflects from reflective polarizer  190 , and exits fourth PBS  880  through first prism face  481  as red light  853  having the second polarization direction. 
         [0122]    Red light  852  having the second polarization direction exits first PBS  820  through fourth prism face  424 , and changes to red circularly polarized light  899 R as it passes through quarter-wave retarder  425 . Red circularly polarized light  899 R reflects from mirror  430 , changes direction of circular polarization, and changes to red light  854  having the first polarization direction as it passes through quarter-wave retarder  425 . Red light  854  having the first polarization direction enters first PBS  820  through fourth prism face  424 , passes through reflective polarizer  190 , exits first PBS  820  through second prism face  422 , and changes polarization direction as it passes through first CSSRP filter  831 , to become red light  855  having the second polarization direction. Red light  855  having the second polarization direction enters second PBS  840  through third prism face  443 , reflects from reflective polarizer  190 , exits second PBS  840  through fourth prism face  444 , changes to red circularly polarized light  899 R as it passes through quarter-wave retarder  425 , changes direction of circular polarization as it reflects from mirror  430 , and becomes red light  856  having the first polarization direction as it again passes through quarter-wave retarder  425 . Red light  856  having the first polarization direction enters second PBS  840  through fourth prism face  444 , passes through reflective polarizer  190 , exits second PBS  840  through second prism face  442 , changes to red light  857  having the second polarization direction as it passes through third CSSRP filter  433 . Red light  857  having the second polarization direction enters third PBS  860  through third prism face  463 , reflects from reflective polarizer  190 , exits third PBS  860  through second prism face  462 , and changes to red light  858  having the first polarization direction as it passes through fourth CSSRP filter  434 . Red light  858  having the first polarization direction enters fourth PBS  880  through third prism face  483 , passes through reflective polarizer  190 , and exits fourth PBS  880  through first prism face  481  as red light  858  having the first polarization direction. 
         [0123]      FIG. 8C  shows the optical path of unpolarized green light  870  through light combiner  800 . In this embodiment, unpolarized green light  870  enters second PBS  840  through first prism face  441  and exits fourth PBS  880  through first prism face  481  as unpolarized green light comprising green light  874  having the first polarization direction, and green light  873  having the second polarization direction. 
         [0124]    Green light  870  enters second PBS  840  through first prism face  441 , intercepts reflective polarizer  190 , and is split into green light  871  having the first polarization direction and green light  872  having the second polarization direction. 
         [0125]    Green light  871  having the first polarization direction exits second PBS  840  through third prism face  443 , and changes to green light  873  as it passes through second CSSRP filter  832 . Green light  873  having the second polarization direction enters first PBS  820  through second prism face  422 , reflects from reflective polarizer  190 , exits first PBS  820  through third prism face  423 , passes unchanged through first CSSRP filter  831 , enters fourth PBS  880  through second prism face  482 , reflects from reflective polarizer  190  and exits fourth PBS  880  through first prism face  481  as green light  873  having the second polarization direction. 
         [0126]    Green light  872  having the second polarization direction exits second PBS  840  through second prism face  442 , passes through third CSSRP filter  433  without change of polarization, enters third PBS  860  through third prism face  463 , reflects from reflective polarizer  190 , exits third PBS  860  through second prism face  462 , and changes to green light  874  having the first polarization direction as it passes through fourth CSSRP filter  834 . Green light  874  having the first polarization direction enters fourth PBS  880  through third prism face  483 , passes through reflective polarizer  190 , and exits fourth PBS  880  through first prism face  461  as green light  874  having the first polarization direction. 
         [0127]      FIG. 8D  shows the optical path of unpolarized blue light  890  through light combiner  800 . In this embodiment, unpolarized blue light  890  enters third PBS  860  through first prism face  461  and exits fourth PBS  880  through first prism face  481  as unpolarized blue light comprising blue light  894  having the first polarization direction, and blue light  893  having the second polarization direction. 
         [0128]    Blue light  890  enters third PBS  860  through first prism face  441 , intercepts reflective polarizer  190 , and is split into blue light  891  having the first polarization direction and blue light  892  having the second polarization direction. 
         [0129]    Blue light  891  having the first polarization direction exits third PBS  860  through third prism face  463 , passes unchanged through third CSSRP filter  833 , enters second PBS  840  through second prism face  442 , passes through reflective polarizer  190 , exits second PBS  840  through fourth prism face  444 , and changes to blue circularly polarized light  899 B as it passes through quarter-wave retarder  425 . Blue circularly polarized light  899 B changes the direction of circular polarization as it reflects from mirror  430 , changes to blue light  893  having the second polarization direction as it passes through quarter-wave retarder  425 , and re-enters second PBS  840  through fourth prism face  444 . Blue light  893  having the second polarization direction reflects from reflective polarizer  190 , exits second PBS  840  through third prism face  443 , passes unchanged through second CSSRP filter  832 , and enters first PBS  820  through second prism face  422 . Blue light  893  having the second polarization direction reflects from reflective polarizer  190 , exits first PBS  820  through third prism face  483 , passes unchanged through first CSSRP filter  831 , enters fourth PBS  880  through second prism face  482 , reflects from reflective polarizer  190 , and exits fourth PBS  880  through first prism face  481  as blue light  893  having the second polarization direction. 
         [0130]    Blue light  892  having the second polarization direction exits third PBS  860  through fourth prism face  464 , changes to blue circularly polarized light  899 B as it passes through quarter-wave retarder  425 , changes the direction of circular polarization as it reflects from mirror  430 , and changes to blue light  894  having the first polarization direction as it passes through quarter-wave retarder  425 . Blue light  894  having the first polarization direction enters third PBS  860  through fourth prism face  464 , passes through reflective polarizer  190 , exits third PBS  860  through second prism face  462 , passes unchanged through fourth CSSRP filter  834 , enters fourth PBS  880  through third prism face  483 , passes through reflective polarizer  190  and exits fourth PBS  880  through first prism face  481  as blue light  894  having the first polarization direction. 
         [0131]    In a further aspect, a method of splitting light using the light combiner  800  includes changing the propagation direction of the first, second, third, and combined light,  850 ,  870 ,  890 ,  801 , respectively, shown in  FIG. 8A-8D . Combined light  801  is directed toward first prism face  481  of fourth PBS  880 , and at least one of the first, second and third wavelength spectrum light is received from first prism face  421 ,  441 ,  461  of first, second and third PBS  820 ,  840 ,  860 , respectively. 
         [0132]    Unless otherwise indicated, all numbers expressing feature sizes, amounts, and physical properties 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 foregoing specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings disclosed herein. 
         [0133]    Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations can be substituted for the specific embodiments shown and described without departing from the scope of the present disclosure. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this disclosure be limited only by the claims and the equivalents thereof.