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 polarizing beam splitters include a reflective polarizer to efficiently split incident light into transmitted and reflected beams having different polarization directions. Reflectors and quarter-wave retarders are positioned facing selected prism faces of the polarizing beam splitters, to affect the polarization state of light passing through the prism faces. The reflectors can be dichroic filters adapted to reflect light that is outside a selected wavelength range, so that light of different wavelength spectrums can be affected at different prism faces. The surfaces of each polarizing beam splitter can be polished so that the light utilization efficiency is increased due to total internal reflection within the polarizing beam splitter. The light combiners can combine up to five unpolarized different color lights to produce an unpolarized polychromatic light output, which may be white light useful for a projection display. The light splitters can split unpolarized polychromatic light to produce up to five unpolarized different color light outputs.

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 a polarizing beam splitter that includes two prisms having four prism faces and two ends, and a reflective polarizer that is disposed between the diagonal faces of the two prisms. The prism faces and ends can be polished so that total internal reflection can occur within the prism. The reflective polarizer can be a Cartesian reflective polarizer aligned to a first polarization direction. The reflective polarizer can be a polymeric multilayer optical film. The light combiner includes quarter-wave retarders disposed facing three of the four external prism faces. The quarter-wave retarders can be aligned to the first polarization direction. A reflector is disposed facing each of the quarter-wave retarders. 
         [0007]    In another aspect, a light combiner used for combining two lights having different wavelength spectrums includes two reflectors that are dichroic filters that transmit a first and second wavelength of light respectively, and reflect other wavelengths of light. The light combiner includes a third reflector that is a mirror. In a further aspect, a light combiner used for combining three lights having different wavelength spectrums includes three reflectors that are dichroic filters that transmit a first, second and third wavelength of light, respectively, and reflect other wavelengths of light. In some embodiments, at least some of the prisms, reflective polarizer, quarter-wave retarders, reflectors and dichroic filters are bonded together with an optical adhesive. 
         [0008]    In yet a further aspect, a method of combining light of two or three wavelength spectrums includes providing a light combiner having a polarizing beam splitter including a first, second and third dichroic filter that transmits light having a first, second and third wavelength spectrum, respectively and reflect other wavelengths of light, facing three of the four prism faces; directing light having the first, second and third wavelength spectrum toward the dichroic filters; and receiving combined light from the fourth prism face. The first and second lights can be unpolarized, and the combined light can also be unpolarized. 
         [0009]    In another aspect, a method of splitting polychromatic light includes providing a light combiner including first, second and third dichroic filters that transmit light having a first, second and third wavelength spectrum, facing three of the four prism faces, directing polychromatic combined light toward the fourth prism face, and receiving light having the first, second and third wavelength spectrum from the first, second and third dichroic filters. The polychromatic light can be unpolarized, and the received lights can also be unpolarized. In some embodiments, the third dichroic filter is replaced by a mirror, and first and second wavelength spectrum light is received from the remaining two dichroic filters. 
         [0010]    In one aspect, a light combiner includes two polarizing beam splitters that each includes two prisms having four prism faces and two ends, and a reflective polarizer disposed between the diagonal faces of each of the two prisms. The two polarizing beam splitters are positioned so that two of the prism faces are facing each other. The prism faces and ends can be polished so that total internal reflection can occur within each polarizing beam splitter. The reflective polarizers can be Cartesian reflective polarizers aligned to a first polarization direction. The reflective polarizers can be polymeric multilayer optical films. The light combiner includes quarter-wave retarders disposed facing five of the six external prism faces. The quarter-wave retarders are aligned to the first polarization direction. A reflector is disposed facing each of the quarter-wave retarders. 
         [0011]    In still a further aspect, a light combiner used for combining two lights having different wavelength spectrums includes two reflectors that are dichroic filters that transmit a first and second wavelength of light respectively and reflect other wavelengths of light; and third, fourth and fifth reflectors that are mirrors. 
         [0012]    In another aspect, a light combiner used for combining three lights having different wavelength spectrums includes three reflectors that are dichroic filters that transmit a first, second and third wavelength of light respectively, and reflect other wavelengths of light; and fourth and fifth reflectors that are mirrors. 
         [0013]    In a further aspect, a light combiner used for combining four lights having different wavelength spectrums includes four reflectors that are dichroic filters that transmit a first, second, third and fourth wavelength of light respectively, and reflect other wavelengths of light; and a fifth reflector that is a mirror. 
         [0014]    In yet another aspect, a light combiner used for combining five lights having different wavelength spectrums includes five reflectors that are dichroic filters that transmit a first, second, third, fourth and fifth wavelength of light respectively, and reflect other wavelengths of light. 
         [0015]    In one aspect, a sixth dichroic filter and an additional quarter-wave retarder are disposed between the two prisms to improve the performance of the light combiner. In some embodiments, at least some of the prisms, reflective polarizer, quarter-wave retarders, reflectors and dichroic filters are bonded together with an optical adhesive. 
         [0016]    In another aspect, a method of combining light of from two to five wavelength spectrums includes providing a light combiner having two polarizing beam splitters, disposing a first through fifth dichroic filter that transmit light having a first through fifth wavelength spectrum respectively, and reflect other wavelengths of light, on five of the six external prism faces; directing light having the first through fifth wavelength spectrum toward the dichroic filters; and receiving combined light from the sixth external prism face. The first through fifth lights can be unpolarized, and the combined light can also be unpolarized. 
         [0017]    In a further aspect, a method of splitting polychromatic light includes the steps of providing a light combiner including first through fifth dichroic filters that transmit light having a first through fifth wavelength spectrum respectively, and reflect other wavelengths of light, on five of the six external prism faces; directing polychromatic light toward the sixth prism face; and receiving light having the first through fifth wavelength spectrum from the first through fifth dichroic filters. The polychromatic light can be unpolarized, and the received lights can also be unpolarized. Up to three dichroic filters can be replaced by mirrors, and light can be received from the remaining two dichroic filters. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    Throughout the specification reference is made to the appended drawings, where like reference numerals designate like elements, and wherein: 
           [0019]      FIG. 1  is a perspective view of a polarizing beam splitter. 
           [0020]      FIG. 2  is a perspective view of a polarizing beams splitter with quarter-wave retarders. 
           [0021]      FIGS. 3A-3D  are top schematic views of a light combiner. 
           [0022]      FIG. 4  is a top schematic view showing a polarizing beam splitter. 
           [0023]      FIG. 5  is a top schematic view of a light splitter. 
           [0024]      FIGS. 6A-6B  are top schematic views of a light combiner. 
           [0025]      FIGS. 7A-7B  are top schematic views of a light combiner. 
           [0026]      FIG. 8  is a top schematic view of a light splitter. 
           [0027]      FIGS. 9A-9C  are top schematic views of a light combiner. 
       
    
    
       [0028]    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 
       [0029]    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. 
         [0030]    Also for the purposes of the description provided herein, the term “facing” refers to one element disposed so that a perpendicular line from the surface of the element follows an optical path that is also perpendicular to the other element. One element facing another element can include the elements disposed adjacent each other. One element facing another element further includes the elements separated by optics so that a light ray perpendicular to one element is also perpendicular to the other element. 
         [0031]    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. 
         [0032]    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 components and their orientation in the polarization rotating reflector. The polarization rotating reflector includes a reflector and a retarder. In one embodiment, the reflector can be a mirror that reflects the transmission of light by reflection. In one embodiment, the reflector can be a dichroic filter that transmits one wavelength spectrum of light and reflects other wavelengths of light. The dichroic filter can reflect other wavelengths of light by reflecting the light. 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 can be 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. 
         [0033]    According to one aspect, a light combiner comprises two PBSs with associated quarter-wave retarders and reflectors arranged in cascade, to produce combined light. Light from up to five different sources can be directed into five of the six exterior prism faces of the two cascaded PBSs, and combined light is received from the sixth exterior prism face. 
         [0034]    The components of a light combiner including prisms, reflective polarizers, quarter-wave retarders, mirrors and dichroic 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. 
         [0035]    The embodiments described above can be more readily understood by reference to the Figures and their accompanying description, which follows. 
         [0036]      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 . 
         [0037]    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.). 
         [0038]      FIG. 2  is a perspective view of the alignment of quarter-wave retarders 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 facing each of the first and second prism faces,  130  and  140 . Reflective polarizer  190  is a Cartesian reflective polarizer film aligned to first polarization direction  195 . Quarter-wave retarders  220  include a quarter-wave polarization direction  295  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. 
         [0039]      FIG. 3A  is a top view of a light combiner. In  FIG. 3A , a light combiner  300  includes PBS  100  having reflective polarizer  190  disposed between the diagonal faces of prisms  110  and  120 . Prism  110  includes first and second prism faces  130 ,  140  having a 90° angle between them. Prism  120  includes third and fourth prism face  150 ,  160  having a 90° angle between them. 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. 
         [0040]    Light combiner  300  includes quarter-wave retarders  220  disposed facing the first, second and third prism faces  130 ,  140 ,  150 . Quarter-wave retarders  220  are aligned at a 45° angle to the first polarization direction  195 . An optically transmissive material  340  is disposed between each quarter-wave retarder  220  and their respective prism faces. The optically transmissive material  340  can be any material that has an index of refraction lower than the index of refraction of prisms  110 ,  120 . In one embodiment, the optically transmissive material  340  is air. In another embodiment, the optically transmissive material  340  is an optical adhesive which bonds quarter-wave retarders  220  to their respective prism faces. 
         [0041]    Light combiner  300  includes a first, second and third reflector  310 ,  320 ,  330  disposed facing quarter-wave retarders  220  as shown. Each of the reflectors  310 ,  320 ,  330  can be separate from the adjacent quarter-wave retarder  220  as shown in  FIG. 3A . Further, each of the reflectors  310 ,  320 ,  330  can be in direct contact with the adjacent quarter-wave retarder  220 . Alternatively, each of the reflectors  310 ,  320 ,  330  can be adhered to the adjacent quarter-wave retarder  220  with an optical adhesive. The optical adhesive can be a curable adhesive. The optical adhesive can also be a pressure-sensitive adhesive. 
         [0042]    Light combiner  300  can be a two color combiner. In this embodiment, two of the reflectors  310 ,  320 ,  330  are a first and a second dichroic filter selected to transmit a first and a second color light respectively, and reflect other colors of light. The third reflector is a mirror. By mirror is meant a specular reflector selected to reflect substantially all colors of light. The first and second color light can have minimum overlap in the spectral range, however there can be substantial overlap if desired. 
         [0043]    In one embodiment shown in  FIG. 3A , light combiner  300  is a three color combiner. In this embodiment, reflectors  310 ,  320 ,  330  are first, second and a third dichroic filter selected to transmit the first, second, and a third color light respectively, and reflect other colors of light. In one aspect, the first, second and third color light have minimum overlap in the spectral range, however there can be substantial overlap, if desired. A method of using light combiner  300  of this embodiment includes directing a first light  350  having the first color toward first dichroic filter  310 , directing a second light  360  having the second color toward second dichroic filter  320 , directing a third light  370  having the third color toward third dichroic filter  330 , and receiving combined light  380  from the fourth face of PBS  100 . The path of each of the first, second and third light  350 ,  360 ,  370  are further described with reference to  FIGS. 3B-3D . 
         [0044]    In one embodiment, each of the first, second and third light  350 ,  360 ,  370  can be unpolarized light and the combined light  380  is unpolarized. In a further embodiment, each of the first, second and third lights  350 ,  360 ,  370  can be red, green and blue unpolarized light, and the combined light  380  can be unpolarized white light. Each of the first, second, and third lights  350 ,  360 ,  370  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. 
         [0045]    Turning now to  FIG. 3B , the optical path of first light  350  through light combiner  300  is described for the embodiment where first light  350  is unpolarized. In this embodiment, unpolarized light comprising light ray  351  having the second polarization direction, and light ray  355  having the first polarization direction, exit PBS  100  through fourth prism face  160 . 
         [0046]    First light  350  is directed through first dichroic filter  310 , quarter-wave retarder  220 , and enters PBS  100  through third prism face  150 . First light  350  intercepts reflective polarizer  190  and is split into light ray  352  having the first polarization direction and light ray  351  having the second polarization direction. Light ray  351  having the second polarization direction is reflected from reflective polarizer  190  and exits PBS  100  through fourth prism face  160 . 
         [0047]    Light ray  352  having the first polarization direction passes through reflective polarizer  190 , exits PBS  100  through first prism face  130 , and changes to circularly polarized light  390  as it passes through quarter-wave retarder  220 . Circularly polarized light  390  reflects from third dichroic filter, changing the direction of circular polarization, and passes again through quarter-wave retarder  220 , entering PBS  100  through first prism face  130  as light ray  354  having the second polarization direction. Light ray  354  reflects from reflective polarizer  190 , exits PBS  100  through second prism face  140 , and changes to circularly polarized light  390  as it passes through quarter-wave retarder  220 . Circularly polarized light  390  reflects from second dichroic filter  320 , changing the direction of circular polarization, and passes again through quarter-wave retarder  220 , entering PBS  100  through second prism face  140  as light ray  355  having the first polarization direction. Light ray  355  having the first polarization direction passes through reflective polarizer  190  and exits PBS  100  through fourth prism face  160 . 
         [0048]    Turning now to  FIG. 3C , the optical path of second light  360  through light combiner  300  is described for the embodiment where second light  360  is unpolarized. In this embodiment, unpolarized light comprising light ray  365  having the second polarization direction, and light ray  362  having the first polarization direction, exit PBS  100  through fourth prism face  160 . 
         [0049]    Second light  360  is directed through second dichroic filter  320 , quarter-wave retarder  220 , and enters PBS  100  through second prism face  140 . Second light  360  intercepts reflective polarizer  190  and is split into light ray  362  having the first polarization direction and light ray  361  having the second polarization direction. Light ray  362  having the first polarization direction passes through reflective polarizer  190  and exits PBS  100  through fourth prism face  160 . 
         [0050]    Light ray  361  having the second polarization direction, is reflected from reflective polarizer  190 , exits the first prism face  130  of PBS  100 , and changes to circularly polarized light  390  as it passes through quarter-wave retarder  220 . Circularly polarized light  390  reflects from third dichroic filter  330 , changing the direction of circular polarization, and passes again through quarter-wave retarder  220 , entering PBS  100  through first prism face  130  as light ray  363  having the first polarization direction. Light ray  363  passes through reflective polarizer  190 , exits PBS  100  through third prism face  150 , and changes to circularly polarized light  390  as it passes through quarter-wave retarder  220 . Circularly polarized light  390  reflects from first dichroic filter  310 , changing the direction of circular polarization, and passes again through quarter-wave retarder  220 , entering PBS  100  through third prism face  150  as light ray  365  having the second polarization direction. Light ray  365  having the second polarization state, reflects from reflective polarizer  190  and exits PBS  100  through fourth prism face  160 . 
         [0051]    Turning now to  FIG. 3D , the optical path of third light  370  through light combiner  300  is described for the embodiment where third light  370  is unpolarized. In this embodiment, unpolarized light comprising light ray  375  having the second polarization direction, and light ray  373  having the first polarization direction, exits PBS  100  through fourth prism face  160 . 
         [0052]    Third light  370  is directed through third dichroic filter  330 , quarter-wave retarder  220 , and enters PBS  100  through first prism face  130 . Third light  370  intercepts reflective polarizer  190  and is split into light ray  372  having the first polarization direction and light ray  371  having the second polarization direction. Light ray  372  having the first polarization direction passes through reflective polarizer  190 , exits the third prism face  150 , and changes to circularly polarized light  390  as it passes through quarter-wave retarder  220 . Circularly polarized light  390  reflects from first dichroic filter  310 , changing the direction of circular polarization, and passes again through quarter-wave retarder  220 , entering PBS  100  through third prism face  150  as light ray  374  having the second polarization state. Light ray  374  having the second polarization direction reflects from reflective polarizer  190  and exits PBS  100  through fourth prism face  160 . 
         [0053]    Light ray  371  having the second polarization direction, reflects from reflective polarizer  190 , exits PBS  100  through the second prism face  140  and changes to circularly polarized light  390  as it passes through quarter-wave retarder  220 . Circularly polarized light  390  reflects from second dichroic filter  320 , changing the direction of circular polarization, passes again through quarter-wave retarder  220  and enters PBS  100  through second prism face  140  as light ray  373  having the second polarization direction. Light ray  373  having the first polarization direction, passes through reflective polarizer  190  and exits PBS  100  through fourth prism face  160 . 
         [0054]      FIG. 4  shows a path of light rays within a polished PBS  400 . 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  400  (including end faces, not shown) are polished faces that provide TIR of oblique light rays within PBS  400 . 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  400 , 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  400  by total internal reflections until it leaves through third prism face  150 . In some cases, substantially all of the light is trapped in PBS  400  by total internal reflections until it leaves through third prism face  150 . 
         [0055]    As shown in  FIG. 4 , light rays L 0  enter first prism face  130  within a range of angles θ 1 . Light rays L 1  within PBS  400  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  400 , 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  400 . 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. 
         [0056]      FIG. 5  is a top view schematic representation of a light splitter  500  according to one aspect of the invention. Light splitter  500  uses the same components as the light combiner shown in  FIGS. 3A-3D , but functions in reverse, i.e. combined light  580  is directed toward fourth prism face  160 , and split into a first, second and third received light  550 ,  560 ,  570  having first, second and third color, respectively. In  FIG. 5 , light splitter  500  includes PBS  100  having reflective polarizer  190  disposed between the diagonal faces of prisms  110 ,  120 . Prism  110  includes first and second prism faces  130 ,  140  having a 90° angle between them. Prism  120  includes third and fourth prism faces  150 ,  160  having a 90° angle between them. Reflective polarizer  190  can be a Cartesian reflective polarizer aligned to the first polarization direction  195  (in this view, perpendicular to the page), or a non-Cartesian polarizer, but a Cartesian reflective polarizer is preferred. 
         [0057]    Light splitter  500  also includes quarter-wave retarders  220  disposed facing the first, second and third prism faces  130 ,  140 ,  150 . The quarter-wave retarders  220  are aligned at a 45° angle to the first polarization direction  195 , as described elsewhere. An optically transmissive material  340  is disposed between each of the quarter-wave retarders  220  and their respective prism faces. Optically transmissive material  340  can be any material that has an index of refraction lower than the index of refraction of prisms  110 , 120 . In one aspect, optically transmissive material  340  can be air. In one aspect, the optically transmissive material  340  can be an optical adhesive which bonds quarter-wave retarders  220  to their respective prism faces. 
         [0058]    Light splitter  500  includes first, second and third reflector  310 ,  320 ,  330  disposed facing quarter-wave retarders  220  as shown. In one aspect, reflectors  310 ,  320 ,  330  can be separated from the adjacent quarter-wave retarder  220  as shown in  FIG. 3A . In one aspect, reflectors  310 ,  320 ,  330  can be in direct contact with the adjacent quarter-wave retarder  220 . In one aspect, reflectors  310 ,  320 ,  330  can be adhered to the adjacent quarter-wave retarder  220  with an optical adhesive. 
         [0059]    In one embodiment, light splitter  500  is a two color splitter. In this embodiment, two of the reflectors  310 ,  320 ,  330  are first and second dichroic filter selected to transmit first and second color light, respectively, and reflect other colors of light. The third reflector is a mirror. By mirror is meant a specular reflector selected to reflect substantially all colors of light. In one aspect, the first and second color light have minimum overlap in the spectral range, however there can be substantial overlap, if desired. 
         [0060]    In one embodiment, light splitter  500  is a three color splitter. In this embodiment, reflectors  310 ,  320 ,  330  are first, second and third dichroic filter selected to transmit first, second, and third color lights, respectively, and reflect other colors of light. In one aspect, first, second and third color lights have minimum overlap in the spectral range, however there can be substantial overlap, if desired. A method of using light splitter  500  of this embodiment includes the steps of directing combined light  580  toward fourth prism face  160  of PBS  100 , receiving first light  550  having the first color from dichroic filter  310 , receiving second light  560  having the second color from second dichroic filter  320 , and receiving third light  570  having the third color from third dichroic filter  330 . The optical path of each of the combined, first, second and third received lights  580 ,  550 ,  560 ,  570  follow the description in  FIGS. 3B-3D , however, the direction of all of the light rays is reversed. 
         [0061]    In one embodiment, combined light  580  can be unpolarized light, and each of the first, second and third lights  550 ,  560 ,  570  are unpolarized lights. In one embodiment, combined light  580  can be unpolarized white light, and each of the first, second and third lights  550 ,  560 ,  570  are red, green and blue unpolarized lights. According to one aspect, combined light  580  comprises 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. 
         [0062]      FIG. 6A  is a top view of a light combiner  600  comprising PBS  100  and a second PBS  100 ′ according to one embodiment. PBS  100  comprises reflective polarizer  190  disposed between the diagonal faces of prisms  110 , 120 . Prism  110  includes first and second prism faces  130 ,  140  having a 90° angle between them. Prism  120  includes third and fourth prism faces  150 ,  160  having a 90° angle between them. Second PBS  100 ′ comprises reflective polarizer  190 ′ disposed between the diagonal faces of prisms  110 ′,  120 ′. Prism  110 ′ includes fifth and sixth prism faces  140 ′,  130 ′ having a 90° angle between them. Prism  120 ′ includes seventh and eighth prism faces  160 ′,  150 ′ having a 90° angle between them. Reflective polarizers  190 ,  190 ′ can be Cartesian reflective polarizers aligned to the first polarization direction  195  (in this view, perpendicular to the page). Reflective polarizers  190 ,  190 ′ can be non-Cartesian polarizers, but Cartesian reflective polarizers are preferred. Second PBS  100 ′ is disposed adjacent to PBS  100  so that fourth prism face  160  is facing fifth prism face  140 ′. Fourth prism face  160  and fifth prism face  140 ′ can be separated by a gap, or adhered to each other using an optical adhesive. An optical adhesive, if used, should satisfy the refractive index relationship provided elsewhere to enable TIR at the prism faces. 
         [0063]    Light combiner  600  includes quarter-wave retarders  220  disposed facing the first, second, third, sixth and seventh prism faces  130 ,  140 ,  150 ,  130 ′,  160 ′. Quarter-wave retarders  220  are aligned at a 45° angle to the first polarization direction  195 , as described elsewhere. An optically transmissive material  340  is disposed between each quarter-wave retarder  220  and their respective prism faces. Optically transmissive material  340  can be any material that has an index of refraction lower than the index of refraction of prisms  110 ,  120 ,  110 ′,  120 ′. In one aspect, optically transmissive material  340  can be air. In another aspect, optically transmissive material  340  can be an optical adhesive which bonds quarter-wave retarders  220  to their respective prism faces. 
         [0064]    Light combiner  600  includes a first, second, third, fourth and fifth reflector  610 ,  620 ,  630 ,  640 ,  660  disposed facing quarter-wave retarders  220  as shown. In one embodiment, reflectors  610 ,  620 ,  630 ,  640 ,  660  can be separated from the adjacent quarter-wave retarder  220  as shown in  FIG. 6A . In another embodiment, reflectors  610 ,  620 ,  630 ,  640 ,  660  can be in direct contact with the adjacent quarter-wave retarder  220 . In one embodiment, reflectors  610 ,  620 ,  630 ,  640 ,  650  can be adhered to the adjacent quarter-wave retarder  220  with an optical adhesive. 
         [0065]    In one embodiment, light combiner  600  is a two color combiner. In this embodiment, two of reflectors  610 ,  620 ,  630 ,  640 ,  660  are first and second dichroic filter selected to transmit first and second color light respectively, and reflect other colors of light. The remaining three reflectors are mirrors. In one aspect, the first and second colors of light have minimum overlap in the spectral range, however there can be substantial overlap, if desired. 
         [0066]    In one embodiment, light combiner  600  is a three color combiner. In this embodiment, three of reflectors  610 ,  620 ,  630 ,  640 ,  660  are first, second, and third dichroic filters selected to transmit first, second, and third color light, respectively, and reflect other colors of light. The remaining two reflectors are mirrors. In one aspect, the first, second, and third colors of light have minimum overlap in the spectral range, however there can be substantial overlap, if desired. 
         [0067]    In one embodiment, light combiner  600  is a four color combiner. In this embodiment, four of reflectors  610 ,  620 ,  630 ,  640 ,  660  are first, second, third and a fourth dichroic filters selected to transmit first, second, third and a fourth color light respectively, and reflect other colors of light. The remaining reflector is a mirror. In one aspect, the first, second, third and fourth colors of light have minimum overlap in the spectral range, however there can be substantial overlap, if desired. 
         [0068]    In one embodiment shown in  FIG. 6A , light combiner  600  is a five color combiner. In this embodiment, reflectors  610 ,  620 ,  630 ,  640 ,  660  are first, second, third, fourth and a fifth dichroic filters selected to transmit first, second, third, fourth and a fifth color light respectively, and reflect other colors of light. In one aspect, the first, second, third, fourth and fifth colors of light have minimum overlap in the spectral range; however there can be substantial overlap, if desired. A method of using light combiner  600  of this embodiment includes the steps of directing a first light  670  having the first color toward first dichroic filter  610 , directing a second light  692  having the second color toward second dichroic filter  620 , directing a third light  694  having the third color toward third dichroic filter  630 , directing a fourth light  696  having the fourth color toward fourth dichroic filter  640 , directing a fifth light  698  having the fifth color toward fifth dichroic filter  660 , and receiving combined light  680  from the seventh face of second PBS  100 ′. The optical path of the first light  670  is described with reference to  FIG. 6B . For brevity, the optical paths of the second, third, fourth and fifth lights  692 ,  694 ,  696 ,  698  are not included, but can be determined by following the procedure described for  FIG. 6B . 
         [0069]    In one embodiment, each of the first, second, third, fourth and fifth lights  670 ,  692 ,  694 ,  696 ,  698  can be unpolarized light and the combined light  680  is unpolarized. In one embodiment, each of the first, second, third, fourth and fifth lights  670 ,  692 ,  694 ,  696 ,  698  can be red, green, blue, yellow and cyan unpolarized light, and the combined light  680  is unpolarized white light. According to one aspect, each of the first, second, third, fourth and fifth lights  670 ,  692 ,  694 ,  696 ,  698  comprises 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. 
         [0070]    Turning now to  FIG. 6B , the optical path of first light  670  through light combiner  600  is described for the embodiment where first light  670  is unpolarized. In this embodiment, unpolarized light comprising light ray  676  having the second polarization direction, and light ray  678  having the first polarization direction, exits second PBS  100 ′ through eighth prism face  150 ′. 
         [0071]    First light  670  is directed through first dichroic filter  610 , quarter-wave retarder  220 , and enters PBS  100  through third prism face  150 . First light  670  intercepts reflective polarizer  190  and is split into light ray  672  having the first polarization direction and light ray  671  having the second polarization direction. 
         [0072]    Light ray  671  having the second polarization direction, is reflected from reflective polarizer  190 , exits PBS  100  through fourth prism face  160 , and enters fifth prism face  140 ′ of second PBS  100 ′. Light ray  671  reflects from reflective polarizer  190 ′ as light ray  677  having the second polarization direction, exits second PBS  100 ′ through sixth prism face  130 ′, and changes to circularly polarized light  690  as it passes through quarter-wave retarder  220 . Circularly polarized light  690  reflects from fourth dichroic filter  640 , changing the direction of circular polarization, passes through quarter-wave retarder  220 , and enters second PBS  100 ′ through sixth prism face  130 ′ as light ray  678  having the first polarization state. Light ray  678  having the first polarization direction passes through reflective polarizer  190 ′ and exits second PBS  100 ′ through eighth prism face  150 ′. 
         [0073]    Light ray  672  having the first polarization direction exits PBS  100  through first prism face  130 , and changes to circularly polarized light  690  as it passes through quarter-wave retarder  220 . Circularly polarized light  390  reflects from third dichroic filter  630 , changing the direction of circular polarization, and passes through quarter-wave retarder  220 , entering PBS  100  through first prism face  130  as light ray  673  having the second polarization state. Light ray  673  reflects from reflective polarizer  190 , exits PBS  100  through second prism face  140 , and changes to circularly polarized light  690  as it passes through quarter-wave retarder  220 . Circularly polarized light  690  reflects from second dichroic filter  620 , changing the direction of circular polarization, and passes through quarter-wave retarder  220 , entering PBS  100  through second prism face  140  as light ray  674  having the first polarization state. Light ray  674  having the first polarization direction, passes through reflective polarizer  190 , exits PBS  100  through fourth prism face  160 , and enters second PBS  100 ′ through fifth prism face  140 ′. Light ray  674  passes through reflective polarizer  190 ′, exits second PBS  100 ′ through seventh prism face  160 ′, and changes to circularly polarized light  690  as it passes through quarter-wave retarder  220 . Circularly polarized light  690  reflects from fifth dichroic filter  660 , changing the direction of circular polarization, passes through quarter-wave retarder  220 , entering second PBS  100 ′ through seventh prism face  160 ′ as light ray  675  having the second polarization state. Light ray  675  reflects from reflective polarizer  190 ′ and exits second PBS  100 ′ through eighth prism face  150 ′ as light ray  676  having the second polarization direction. 
         [0074]    In one embodiment, the operation of light combiner  600  shown in  FIGS. 6A and 6B  can be improved by modifying the optical path of light rays entering second PBS  100 ′ through fourth and fifth reflectors  640  and  660 . A sixth dichroic filter and an additional quarter-wave retarder can be positioned between PBS  100  and second PBS  100 ′ to modify the optical path. This embodiment is further described below, with reference to  FIGS. 7A and 7B . 
         [0075]      FIG. 7A  is a top schematic view of the optical path of second light  692  through light combiner  700  according to one embodiment of the invention. Light combiner  700  comprises the light combiner  600  of  FIGS. 6A and 6B  with an additional sixth dichroic filter  770  and an additional quarter-wave retarder  220  disposed between fourth prism face  160  and fifth prism face  140 ′. Sixth dichroic filter  770  is disposed facing fourth prism face  160  and additional quarter-wave retarder  220  is disposed facing fifth prism face  140 ′. Optically transmissive material  340  is disposed between the sixth dichroic filter  770 , additional quarter-wave retarder  220 , and the fourth and fifth prism faces  160 ,  140 ′, respectively. Sixth dichroic filter  770  is selected to reflect at least one of the fourth and fifth colors of light, and transmit other colors of light. 
         [0076]    Second light  692  passes through second dichroic filter  620 , quarter-wave retarder  220 , enters PBS  100  through second prism face  140 , intercepts reflective polarizer  190 , and is split into light ray  710  having the first polarization direction and light ray  730  having the second polarization direction. Light ray  710  passes through reflective polarizer  190  and exits PBS  100  through fourth prism face  160 . 
         [0077]    Light ray  730  reflects from reflective polarizer  190 , exits PBS  100  through first prism face  130 , and changes to circularly polarized light  690  as it passes through quarter-wave retarder  220 . Circularly polarized light  690  reflects from third dichroic filter  630 , changing the direction of circular polarization, and passes through quarter-wave retarder  220 , entering PBS  100  through first prism face  130  as light ray  732  having the first polarization state. Light ray  732  passes through reflective polarizer  190 , exits PBS  100  through third prism face  150 , and changes to circularly polarized light  690  as it passes through quarter wave retarder  220 . Circularly polarized light  690  reflects from first dichroic filter  610 , changing the direction of circular polarization, passes through quarter-wave retarder  220 , entering PBS  100  through third prism face  150  as light ray  734  having the second polarization state. Light ray  734  reflects from reflective polarizer  190  and leaves PBS  100  through fourth prism face  160  as light ray  736  having the second polarization direction. 
         [0078]    It is to be understood that first and third lights  670  and  694  (shown in  FIG. 6A ) have optical paths through PBS  100  of  FIG. 7A  that are readily traced using the same method and with the same result as described for second light  692 , but are omitted here for brevity. First and third lights  670  and  694  also leave PBS  100  through fourth prism face  160  in both first and second polarization directions. 
         [0079]    After leaving PBS  100  through fourth prism face  160 , both light rays  710  and  736  pass through sixth dichroic filter  770  and change to circularly polarized light rays  712  and  738  as they pass through quarter-wave retarder  220 . Circularly polarized light rays  712  and  738  intercept reflective polarizer  190 ′ and are split into light rays  716  and  740  having the first polarization direction, and light rays  714  and  742  having the second polarization direction. 
         [0080]    Light rays  716  and  740  exit second PBS  100 ′ through seventh prism face  160 ′ and change to circularly polarized light  690  as they pass through quarter-wave retarder  220 . Circularly polarized light  690  reflects from fifth dichroic filter  660 , changing the direction of circular polarization, passes through quarter-wave retarder  220 , and enters second PBS  100 ′ through seventh prism face  160 ′ as light rays  722  and  748  having the second polarization state. Light rays  722  and  748  reflect from reflective polarizer  190 ′ and exit second PBS  100 ′ through eighth prism face  150 ′ as light rays  724  and  750 , both having the second polarization state. 
         [0081]    Light rays  714  and  742  are reflected from reflective polarizer  190 ′, exit second PBS  100 ′ through sixth prism face  130 ′, and change to circularly polarized light  690  as they pass through quarter-wave retarder  220 . Circularly polarized light  690  reflects from fourth dichroic filter  640 , changing the direction of circular polarization, passes through quarter-wave retarder  220 , and enter second PBS  100 ′ through sixth prism face  130 ′ as light rays  718  and  744  having the first polarization state. Light rays  718  and  744  reflect pass through reflective polarizer  190 ′ and exit second PBS  100 ′ through eighth prism face  150 ′ as light rays  720  and  746 , both having the first polarization state. 
         [0082]      FIG. 7B  shows the optical path of fifth and sixth light rays  696  and  698  through the light combiner  700  shown in  FIG. 7A . Fifth and sixth light rays  696  and  698  enter second PBS  100 ′ and are prevented from entering PBS  100  by reflection from the sixth dichroic filter  770 . A small amount of light is lost when light passes through or reflects from the reflective polarizers  190  and  190 ′. Sixth dichroic filter  770  can reduce these losses for fifth and sixth light rays  696  and  698  by preventing them from entering PBS  100 , thereby improving the operation of light combiner  700 . 
         [0083]    Fourth light  696  passes through fourth dichroic filter  640 , quarter-wave retarder  220 , enters second PBS  100 ′ through sixth prism face  130 ′, intercepts reflective polarizer  190 ′ and is split into light ray  752  having the first polarization direction and light ray  754  having the second polarization direction. Light ray  752  having the first polarization passes through reflective polarizer  190 ′ and exits second PBS  100 ′ through eighth prism face  150 ′. 
         [0084]    Light ray  754  reflects from reflective polarizer  190 ′, exits second PBS  100 ′ through fifth prism face  140 ′, and changes to circularly polarized light  690  as it passes through quarter-wave retarder  220 . Circularly polarized light  690  reflects from sixth dichroic filter  770 , changing the direction of circular polarization, passes through quarter-wave retarder  220  and enters second PBS  100 ′ through fifth prism face  140 ′ as light ray  755  having the first polarization state. Light ray  755  passes through reflective polarizer  190 ′, exits second PBS  100 ′ through seventh prism face  160 ′, and changes to circularly polarized light  690  as it passes through quarter-wave retarder  220 . Circularly polarized light  690  reflects from fifth dichroic filter  660 , changing the direction of circular polarization, passes through quarter-wave retarder  220  and enters second PBS  100 ′ through seventh prism face  160 ′ as light ray  756  having the second polarization state. Light ray  756  reflects from reflective polarizer  190 ′ and exits second PBS  100 ′ through eighth prism face  150 ′ as light ray  757  having the second polarization state. 
         [0085]    Fifth light  698  passes through fifth dichroic filter  660 , quarter-wave retarder  220 , enters second PBS  100 ′ through seventh prism face  160 ′, intercepts reflective polarizer  190 ′ and is split into light ray  758  having the first polarization direction and light ray  762  having the second polarization direction. Light ray  762  having the second polarization direction reflects from reflective polarizer  190 ′ and exits second PBS  100 ′ through eighth prism face  150 ′. 
         [0086]    Light ray  758  passes through reflective polarizer  190 ′, exits second PBS  100 ′ through fifth prism face  140 ′, and changes to circularly polarized light  690  as it passes through quarter-wave retarder  220 . Circularly polarized light  690  reflects from sixth dichroic filter  770 , changing the direction of circular polarization, passes through quarter-wave retarder  220  and enters second PBS  100 ′ through fifth prism face  140 ′ as light ray  759  having the second polarization state. Light ray  759  reflects from reflective polarizer  190 ′ as light ray  760 , exits second PBS  100 ′ through sixth prism face  130 ′, and changes to circularly polarized light  690  as it passes through quarter-wave retarder  220 . Circularly polarized light  690  reflects from fourth dichroic filter  640 , changing the direction of circular polarization, passes through quarter-wave retarder  220  and enters second PBS  100 ′ through sixth prism face  130 ′ as light ray  761  having the first polarization state. Light ray  761  passes through reflective polarizer  190 ′ and exits second PBS  100 ′ through eighth prism face  150 ′ as light ray  761  having the first polarization state. 
         [0087]      FIG. 8  is a top view schematic representation of a light splitter  800  according to one aspect of the invention. In one embodiment, light splitter  800  can use the same components as light combiner  600  shown in  FIGS. 6A and 6B . In one embodiment, light splitter  800  can use the same components as light combiner  600  shown in  FIGS. 7A and 7B . Light splitter  800  functions in reverse of light combiner  600 , i.e. polychromatic combined light  810  is directed toward eighth prism face  150 ′, and split into first, second, third, fourth and fifth received light  820 ,  830 ,  840 ,  850 ,  860  having first, second, third, fourth and fifth color. In  FIG. 8 , light splitter  800  comprises the components of light combiner  600  described with reference to  FIGS. 6A and 6B   
         [0088]    In one embodiment, light splitter  800  is a two color splitter. In this embodiment, two of the reflectors  610 ,  620 ,  630 ,  640 ,  660  are first and second dichroic filters selected to transmit first and second color light respectively, and reflect other colors of light. The remaining three reflectors are mirrors. In one aspect, first and second color lights have minimum overlap in the spectral range, however there can be substantial overlap, if desired. 
         [0089]    In one embodiment, light splitter  800  is a three color splitter. In this embodiment, three of the reflectors  610 ,  620 ,  630 ,  640 ,  660  are first, second and third dichroic filters selected to transmit first, second, and a third color lights respectively, and reflect other colors of light. The remaining two reflectors are mirrors. In one aspect, the first, second and third colors of light have minimum overlap in the spectral range, however there can be substantial overlap, if desired. 
         [0090]    In one embodiment, light splitter  800  is a four color splitter. In this embodiment, four of the reflectors  610 ,  620 ,  630 ,  640 ,  660  are first, second, third and fourth dichroic filters selected to transmit first, second, third and fourth color lights respectively, and reflect other colors of light. The remaining reflector is a mirror. In one aspect, the first, second, third and fourth colors of light have minimum overlap in the spectral range, however there can be substantial overlap, if desired. 
         [0091]    In one embodiment, light splitter  800  is a five color splitter. In this embodiment, reflectors  610 ,  620 ,  630 ,  640 ,  660  are first, second, third, fourth and fifth dichroic filters selected to transmit first, second, third, fourth and fifth color lights respectively, and reflect other colors of light. In one aspect, the first, second, third and fourth colors of light have minimum overlap in the spectral range, however there can be substantial overlap, if desired. A method of using light splitter  800  of this embodiment includes the steps of directing a combined light  810  toward eighth prism face  150 ′ of second PBS  100 ′, and receiving a first light  860  having the first color from first dichroic filter  610 , receiving a second light  850  having the second color from second dichroic filter  620 , receiving a third light  840  having the third color from third dichroic filter  630 , receiving a fourth light  830  having the fourth color from fourth dichroic filter  640 , and receiving a fifth light  820  having the fifth color from fifth dichroic filter  660 . The optical path of each of the combined, first, second, third, fourth and fifth received lights  860 ,  850 ,  840 ,  830 ,  820  follow the description provided referring to  FIG. 6B , however, the direction of all of the light rays is reversed. 
         [0092]    In one embodiment, combined light  810  can be unpolarized light, and each of the first, second, third, fourth and fifth received lights  860 ,  850 ,  840 ,  830 ,  820  are unpolarized lights. In one embodiment, combined light  810  can be unpolarized white light, and each of the first, second, third, fourth and fifth received lights  860 ,  850 ,  840 ,  830 ,  820  are red, green, blue, yellow and cyan unpolarized lights. According to one aspect, combined light  810  comprises 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. 
         [0093]      FIGS. 9A-9C  are top views of a light combiner according to another aspect of the description. In  FIGS. 9A-9C , paths of first through third light rays  950 ,  960 ,  970  are described through unfolded light combiner  900 . Unfolded light combiner  900  can be one embodiment of light combiner  300  described with reference to  FIGS. 3A-3D . In this embodiment, the first through third light sources  940 ,  942 ,  944  are disposed on the same plane  930 . In one embodiment, plane  930  can be a heat exchanger common to the three light sources. Unfolded light combiner  900  includes third prism  910  and fourth prism  920  disposed facing first prism face  130  and third prism face  150 , respectively, of PBS  100 , described elsewhere. Third prism  910  and fourth prism  920  are each a “turning prism”. First and third light  950 ,  970  emanating from first and third light sources  940 ,  944  on plane  930  are turned by third and fourth prisms  910 ,  920  to enter PBS  100  in a direction perpendicular to first and second prism faces  120 ,  130 , respectively. 
         [0094]    Unfolded light combiner  900  includes quarter-wave retarders  220  disposed facing the first, second and third prism faces  130 ,  140 ,  150 . Quarter-wave retarders  220  are aligned at a 45° angle to the first polarization direction  195 . An optically transmissive material  340  is disposed between each quarter-wave retarder  220  and their respective prism faces. The optically transmissive material  340  can be any material that has an index of refraction lower than the index of refraction of prisms  110 ,  120 . In one embodiment, the optically transmissive material  340  is air. In another embodiment, the optically transmissive material  340  is an optical adhesive which bonds quarter-wave retarders  220  to their respective prism faces. 
         [0095]    Unfolded light combiner  900  includes third and fourth prisms  910 ,  920 . Third prism  910  includes fifth and sixth prism faces  912 ,  914  and diagonal prism face  916  between them. Fifth and sixth prism faces  912 ,  914  are “turning prism faces”. Fifth prism face  912  is positioned to receive light from third light source  944  and direct light to first prism face  130 . Fourth prism  920  includes seventh and eight prism faces  922 ,  924  and diagonal prism face  926  between them. Seventh and eighth prism faces  922 ,  924  also are “turning prism faces”. Seventh prism face  922  is positioned to receive light from first light source  940  and direct light to third prism face  150 . 
         [0096]    Fifth, sixth, seventh and eighth prism faces  912 ,  914 ,  922 ,  924 , and diagonal prism faces  916 ,  926  can be polished for preservation of TIR, as described elsewhere. Diagonal prism faces  916 ,  926  of third and fourth prisms  910 ,  920  can also include a metal coating; a dielectric coating; an organic or inorganic interference stack; or a combination to enhance reflection. 
         [0097]    Unfolded light combiner  900  further includes a first, second and third reflector  310 ,  320 ,  330  disposed to receive light from first, second and third light sources  940 ,  942 ,  944 . In one embodiment shown in  FIGS. 9A-9C , first reflector  310  and the associated retarder  220  are disposed facing seventh and eighth prism faces  922 ,  924 , respectively, and are also facing third prism face  150  of PBS  100 . In one embodiment, third reflector  330  and the associated retarder  220  are disposed facing fifth and sixth prism faces  912 ,  914 , respectively, and are also facing first prism face  130  of PBS  100 . In another embodiment (not shown), first reflector  310  and associated retarder  220  are positioned facing one another in a manner similar to the positioning of second reflector  320  and the associated retarder  220  (e.g. adjacent each other). In this case first reflector  310  and retarder  220  can either be placed adjacent to prism face  922 , or adjacent to prism face  150 . In principle, unfolded light combiner  900  can function regardless of the separation between reflectors and associated retarders, provided the orientation of each relative to the path of the light rays is unchanged, i.e. each is substantially perpendicular to the path of the light ray. However, depending on the nature of the reflection from diagonal prism faces  926  and  916 , there may be more or less polarization mixing introduced by the reflection from those faces. This polarization mixing may result in lost light efficiency, and can be minimized by placing the reflectors  310  and  330  adjacent to prism faces  120  and  130 . 
         [0098]    Each of the reflectors  310 ,  320 ,  330  can be separate from the associated quarter-wave retarder  220  as shown in  FIG. 9A-9C . Further, each of the reflectors  310 ,  320 ,  330  can be in direct contact with the adjacent quarter-wave retarder  220 . Alternatively, each of the reflectors  310 ,  320 ,  330  can be adhered to the adjacent quarter-wave retarder  220  with an optical adhesive. The optical adhesive can be a curable adhesive. The optical adhesive can also be a pressure-sensitive adhesive. 
         [0099]    Unfolded light combiner  900  can be a two color combiner. In this embodiment, two of the reflectors  310 ,  320 ,  330  are a first and a second dichroic filter selected to transmit a first and a second color light respectively, and reflect other colors of light. The third reflector is a mirror. By mirror is meant a specular reflector selected to reflect substantially all colors of light. The first and second color light can have minimum overlap in the spectral range; however there can be substantial overlap if desired. 
         [0100]    In one embodiment shown in  FIGS. 9A-9C , unfolded light combiner  900  is a three color combiner. In this embodiment, reflectors  310 ,  320 ,  330  are first, second and a third dichroic filter selected to transmit the first, second, and a third color light respectively, and reflect other colors of light. In one aspect, the first, second and third color light have minimum overlap in the spectral range, however there can be substantial overlap, if desired. A method of using unfolded light combiner  900  of this embodiment includes directing a first light  950  having the first color toward first dichroic filter  310 , directing a second light  960  having the second color toward second dichroic filter  320 , directing a third light  970  having the third color toward third dichroic filter  330 , and receiving combined light from the fourth face  160  of PBS  100 . The path of each of the first, second and third light  950 ,  960 ,  970  are further described with reference to  FIGS. 9A-9C . 
         [0101]    In one embodiment, each of the first, second and third light  950 ,  960 ,  970  can be unpolarized light and the combined light is unpolarized. In a further embodiment, each of the first, second and third lights  950 ,  960 ,  970  can be red, green and blue unpolarized light, and the combined light can be unpolarized white light. Each of the first, second, and third lights  950 ,  960 ,  970  can comprise light as described elsewhere with reference to  FIGS. 3A-3D . 
         [0102]    In one aspect, unfolded light combiner  900  can include an optional light tunnel  935  disposed between each of the first, second and third light source  940 ,  942 ,  944  and the respective fifth, second and seventh prism faces  912 ,  140 ,  922 . A single optional light tunnel  935  is shown in  FIGS. 9A-9C  to indicate placement relative to third light source  944 ; however, it is to be understood that optional light tunnel  935  can be placed adjacent to any combination of first, second and third light source  940 ,  942 ,  944  and the respective prism faces  922 ,  140 ,  912 . The light tunnels  935  can be useful to partially collimate light originating from the light source, and decrease the angle that the light enters PBS  100 . Light tunnels  935  are an optional component for the unfolded color combiner  900 , and can also be optional components for any of the color combiners and splitters described herein. The light tunnels could have straight or curved sides, or they could be replaced by a lens system. Different approaches may be preferred depending on specific details of each application, and those with skill in the art will face no difficulty in selecting the optimal approach for a specific application. 
         [0103]    Turning now to  FIG. 9A , the optical path of first light  950  through unfolded light combiner  900  is described for the embodiment where first light  950  is unpolarized. In this embodiment, unpolarized light comprising light ray  951  having the second polarization direction, and light ray  956  having the first polarization direction, exit PBS  100  through fourth prism face  160 . 
         [0104]    First light  950  is directed through first dichroic filter  310 , enters fourth prism  920  through seventh prism face  922 , reflects from diagonal  926 , exits fourth prism  920  through eighth prism face  924 , passes through quarter-wave retarder  220 , and enters PBS  100  through third prism face  150 . First light  950  intercepts reflective polarizer  190  and is split into light ray  952  having the first polarization direction and light ray  951  having the second polarization direction. Light ray  951  having the second polarization direction is reflected from reflective polarizer  190  and exits PBS  100  through fourth prism face  160 . 
         [0105]    Light ray  952  having the first polarization direction passes through reflective polarizer  190 , exits PBS  100  through first prism face  130 , and changes to circularly polarized light  953  as it passes through quarter-wave retarder  220 . Circularly polarized light  953  enters third prism  910  through sixth prism face  914 , reflects from diagonal  916  changing direction of circular polarization, exits third prism  910  through fifth prism face  912 , and reflects from third dichroic filter  330 , again changing the direction of circular polarization and becoming circularly polarized light  954 . Circularly polarized light  954  enters third prism  910  though fifth prism face  912 , reflects from diagonal  916  changing the direction of circular polarization, exits third prism  910  through sixth prism face  914  and becomes light ray  955  having the second polarization state as it passes through quarter-wave retarder  220 . Light ray  955  having the second polarization state enters PBS  100  through first prism face  130 , reflects from reflective polarizer  190 , exits PBS  100  through second prism face  140 , changes to circularly polarized light  390  as it passes through quarter-wave retarder  220 , reflects from second dichroic filter  320 , changing the direction of circular polarization, and becomes first light  956  having the first polarization direction as it passes through quarter-wave retarder  220 . First light  956  having the first polarization direction enters PBS  100  through second prism face  140  passes through reflective polarizer  190 , and exits PBS  100  through fourth prism face  160  as first light  956  having the first polarization direction. 
         [0106]    Turning now to  FIG. 9B , the optical path of second light  960  through unfolded light combiner  900  is described for the embodiment where second light  960  is unpolarized. In this embodiment, unpolarized light comprising light ray  968  having the second polarization direction, and light ray  961  having the first polarization direction, exit PBS  100  through fourth prism face  160 . 
         [0107]    Second light  960  is directed through second dichroic filter  320 , quarter-wave retarder  220 , and enters PBS  100  through second prism face  140 . Second light  960  intercepts reflective polarizer  190  and is split into light ray  961  having the first polarization direction and light ray  962  having the second polarization direction. Light ray  961  having the first polarization direction passes through reflective polarizer  190  and exits PBS  100  through fourth prism face  160 . 
         [0108]    Light ray  962  having the second polarization direction, is reflected from reflective polarizer  190 , exits the first prism face  130  of PBS  100 , and changes to circularly polarized light  963  as it passes through quarter-wave retarder  220 . Circularly polarized light  963  enters third prism  910  through sixth prism face  914 , reflects from diagonal  916  changing the direction of circular polarization, exits third prism  910  through fifth prism face  912 , reflects from third dichroic filter  330  again changing the direction of circular polarization, and enters third prism  910  through fifth prism face  912 , as circularly polarized light  964 . Circularly polarized light  964  reflects from diagonal  916  changing direction of circular polarization, exits third prism  910  through sixth prism face  914  and changes to second light  965  having the first polarization direction as it passes through retarder  220 . Second light  965  having the first polarization direction enters PBS  100  through first prism face  130 , passes unchanged through reflective polarizer  190 , exits PBS  100  through third prism face  150 , changes to circularly polarized light  966  as it passes through quarter-wave retarder  220 , and enters fourth prism  920  through eighth prism face  924 . Circularly polarized light  966  reflects from diagonal  992 , changes direction of circular polarization, exits fourth prism  920  through seventh prism face  922 , reflects from first dichroic filter  310  changing the direction of circular polarization and enters fourth prism  920  through seventh prism face  922  as circularly polarized light  967 . Circularly polarized light  967  reflects from diagonal  926 , changes direction of circular polarization, exits fourth prism  920  through eighth prism face  924 , changes to second light  968  having the second polarization direction as it passes through retarder  220 , enters PBS  100  through third prism face  150 , reflects from reflective polarizer  190 , and exits PBS  100  through fourth prism face  160  as second light  968  having the second polarization direction. 
         [0109]    Turning now to  FIG. 9C , the optical path of third light  970  through unfolded light combiner  900  is described for the embodiment where third light  970  is unpolarized. In this embodiment, unpolarized light comprising light ray  976  having the second polarization direction, and light ray  972  having the first polarization direction, exits PBS  100  through fourth prism face  160 . 
         [0110]    Third light  970  is directed through third dichroic filter  330 , enters third prism  910  through fifth prism face  912 , reflects from diagonal  916 , exits third prism  910  through sixth prism face  914 , passes through quarter-wave retarder  220 , and enters PBS  100  through first prism face  130 . Third light  970  intercepts reflective polarizer  190  and is split into light ray  973  having the first polarization direction and light ray  971  having the second polarization direction. Light ray  973  having the first polarization direction passes through reflective polarizer  190 , exits the third prism face  150 , changes to circularly polarized light  974  as it passes through quarter-wave retarder  220  and enters fourth prism  920  through eighth prism face  924 . Circularly polarized light  974  reflects from diagonal  926  changing the direction of circular polarization, exits fourth prism  920  through seventh prism face  922 , reflects from first dichroic filter  310  changing the direction of circular polarization, enters fourth prism  920  through seventh prism face  922 , and becomes circularly polarized light  975  as it reflects from diagonal  926  again changing the direction of circular polarization. Circularly polarized light  975  exits fourth prism  920  through eighth prism face  923 , changes to third light ray  976  having the second polarization direction as it passes through quarter-wave retarder  220 , enters PBS  100  through third prism face  150 , reflects from reflective polarizer  190 , and exits PBS  100  through fourth prism face  160  as third light  976  having the second polarization direction. 
         [0111]    Light ray  971  having the second polarization direction, reflects from reflective polarizer  190 , exits PBS  100  through the second prism face  140  and changes to circularly polarized light  390  as it passes through quarter-wave retarder  220 . Circularly polarized light  390  reflects from second dichroic filter  320 , changing the direction of circular polarization, passes again through quarter-wave retarder  220  and enters PBS  100  through second prism face  140  as light ray  972  having the first polarization direction. Light ray  972  having the first polarization direction, passes through reflective polarizer  190  and exits PBS  100  through fourth prism face  160 . 
         [0112]    In one aspect, any of the  2 ,  3 ,  4 , and  5  color light combiners and splitters described herein can be unfolded in a manner similar to that described with reference to  FIGS. 3A-3D  and  FIGS. 9A-9C . Prisms can be added to direct the light from a common plane to one of the input faces of the PBS (combiners), or from the PBS to a common plane (splitters). The unfolded light combiners can benefit from positioning of the input light sources along a common plane, for example, so that a common heat exchanger can be used to remove heat generated by the light sources. The unfolded light splitters can likewise benefit from having the split colors of light emitted from the same plane. 
         [0113]    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. 
         [0114]    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.