Abstract:
A compact polarization conversion system (PCS) for use in optical display systems is capable of emitting substantially polarized output light in response to unpolarized input light. The PCS includes a polarizer and one or more substantially planar optical element arrays in optical communication with the polarizer. The polarizer converts the input light having plural polarization states into output light having a substantially single polarization state. Each optical element array comprises a plurality of optical elements formed and positioned in a specific two-dimensional arrangement for altering at least one optical characteristic of the input light to produce desired characteristics in the output light. The optical elements can include any suitable combination of micro-waveguides, micro-tunnels, micro-lenses, micro-prisms.

Description:
RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of U.S. patent application Ser. No. 10/458,390 filed on Jun. 10, 2003, titled “Light Guide Array, Fabrication Methods, and Optical System Employing Same”. This application also claims the benefit of U.S. Provisional Application Nos. 60/548,814, 60/548,293 and 60/548,619, all filed on Feb. 27, 2004. It is also related to U.S. patent application Ser. No. ______, titled “Compact Projection System Including A Light Guide Array” Attorney Docket No. 00024.0006.NPUS00, filed on Feb. 25, 2005 and U.S. patent application Ser. No. ______, titled “Light Recycler And Color Display System Including Same”, Attorney Docket No. 00024.0007.NPUS00, filed on Feb. 25, 2005. The subject matter of the aforementioned applications is hereby incorporated by reference as though set forth in full. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention relates generally to optics, and in particular, to a polarization conversion system that converts input light with mixed polarization states to output light with a substantially single polarization state.  
       BACKGROUND  
       [0003]     Many direct view and projection display systems are based on liquid crystal display (LCD) technology that require light of a single polarization state. Since most light sources produce light with mixed polarization states, such display systems typically use half of the provided light and discard the other half. In order to enhance the brightness of a display system, many polarization conversion systems have been developed to convert the polarization state of the discarded light to a polarization state usable by the display system.  
         [0004]     Known polarization conversion systems typically split a light beam into two sub-beams according to their polarization states, change the polarization state of one sub-beam to a usable polarization state using a wave plate, and then recombine both sub-beams, sending them through the display system.  
         [0005]     The more advanced systems use an array of polarization beam splitters (PBSs) coupled either with a fly&#39;s eye lens system or an integrating rod. Recent polarization conversion systems use either a limited number of PBSs or a single reflective polarizer coupled to an integrating rod, thus, providing more compactness and lower cost than the ones that use an array of PBSs. Examples of such polarization conversion systems are shown in  FIGS. 1A-1E .  
         [0006]      FIG. 1A  shows a perspective view of a prior art polarization conversion system  25  consisting of an apertured reflective plate  21 , a light rod or tunnel  22 , a quarter wave plate  23  and a reflective polarizer  24 . Input light  19  is focused into the aperture  20  of the reflective plate  21  and travels toward the reflective polarizer  24 , which reflects light with one polarization state (e.g., s state) and passes light with an orthogonal polarization state (e.g., p state). The reflected light (e.g., s state) passes through the quarter wave plate  23  and continues toward the apertured reflective plate  21 . Some of this light passes through aperture  20  toward the light source and the rest is reflected toward the reflective polarizer  24  by the reflective plate  21 . Since the polarization state of this light is converted into the orthogonal state (e.g., p state) after passing through the quarter wave plate  23  for the second time, this light passes through the reflective polarizer  24  when it reaches it the second time. This effectively converts unpolarized input light into polarized output light without discarding a large portion of the input light energy, and thus, improves the intensity of the polarized output light.  
         [0007]      FIGS. 1B and 1C  show two prior art polarization conversion systems  35  and  45  similar to that of  FIG. 1A , except for the replacement of the reflective polarizer  24  by two polarization beam splitters  30  and  31  ( FIG. 1B ) and a mirror  40  with a single polarization beam splitter  41  ( FIG. 1C ). Polarization conversion systems of  FIGS. 1A-1C  have been described in Published European Patent Application No. 1,315,022 A1, to Drazic, Hall and O&#39;Donnell, which is hereby incorporated by reference.  
         [0008]      FIGS. 1D-1F  use polarization beam splitters (PBSs) and mirrors as a replacement for the apertured reflective plate  21  of  FIGS. 1A-1C , thus, providing a higher efficiency.  
         [0009]      FIG. 1D  shows a perspective view of a prior art polarization conversion system  65 , which consists of two polarization beam splitters  60   a  and  60   b , a rhomb  62 , a half wave plate  63  and a light pipe  64 . Input light  61  is focused into the first PBS cube  60   a  as shown in FIG. ID. Light with one polarization state (e.g., p state) is transmitted to the light pipe  64  and light with orthogonal polarization state (e.g., s state) is reflected toward the second PBS cube  60   b . At the surface of the second PBS cube  60   b , light with an orthogonal polarization state (e.g., s state) is reflected toward the half wave plate  63  where its polarization state is converted into the orthogonal state (e.g., p state) and enters the light pipe  64 . Such a system  65  has been commercialized by OCLI, Inc., A JDS Uniphase Company of Santa Rosa, Calif.  
         [0010]      FIG. 1E  shows a perspective view of a prior art polarization conversion system  80 , which consists of a polarization beam splitter cube  73 , a prism reflector  71 , a half wave plate  74 , a spacer  75  and a light pipe  76 . Input light  72  is coupled into the PBS cube  73  either directly as shown in  FIG. 1E  or through other arrangements such as a tapered light pipe. Light with one polarization state (e.g., p state) is transmitted to the light pipe  76  through the spacer  75  and light with the orthogonal polarization state (e.g., s state) is reflected toward a prism reflector  71 . At the surface of the prism reflector  71 , light with the orthogonal polarization state (e.g., s state) is reflected toward the half wave plate  74 , where its polarization state is converted into the other state (e.g., p state) and enters the light pipe  76 .  
         [0011]      FIG. 1F  shows a perspective view of a prior art polarization conversion system  100 , which consists of a polarization beam splitter cube  93 , a prism reflector  91 , a quarter wave plate with a reflector  92  and a light pipe  94 . Input light  95  is coupled into the PBS cube  93  as shown in  FIG. 1F  or delivered via a tapered light pipe (not shown). Light with one polarization state (e.g., p state) is transmitted to the prism reflector  91 , which in turn reflects it toward the light pipe  94 . Light with the orthogonal polarization state (e.g., s state) is reflected toward the quarter wave plate  92  where it enters and exits the quarter wave plate  92  toward the light pipe  94  with the opposite polarization state (e.g., p state). The systems 80,100 are further described in U.S. Pat. No. 6,587,269 B2, to Kenneth K. Li, which is hereby incorporated by reference.  
         [0012]     It is important that polarization conversion systems operate with minimal light loss, are physically compact, and relatively inexpensive. Although known polarization converters are useful in some applications, there is a need for improved polarization conversion systems that are more compact, light weight, efficient and cost-effective.  
       SUMMARY  
       [0013]     The present invention provides a compact, light weight, efficient and cost-effective polarization conversion system (PCS) for use in optical displays.  
         [0014]     According to one advantageous aspect of the present invention, various embodiments of the polarization conversion system provide a pre-selected spatial distribution of output light in terms of intensity and cone angle. This allows the PCS to be readily adapted to efficiently couple light from sources having wide variety of sizes and shapes into light valves (e.g., LCDs) of various shapes and sizes.  
         [0015]     In accordance with an exemplary embodiment of the present invention, a PCS is capable of emitting substantially polarized output light in response to unpolarized input light. The PCS includes a polarizer and one or more substantially planar optical element arrays in optical communication with the polarizer. The polarizer converts the input light having plural polarization states into output light having a substantially single polarization state. Each optical element array comprises a plurality of optical elements (e.g., micro-elements) formed and positioned in a specific two-dimensional arrangement for altering at least one optical characteristic of the input light to produce desired characteristics in the output light. The optical elements can include any suitable combination of micro-guides, micro-tunnels, micro-lenses, micro-prisms.  
         [0016]     The phrase “optical communication” means that the optical components of the PCS are arranged so that at least some of the input light received by the PCS passes through both the polarizer and the optical element arrays at some point before being emitted as output light. The phrase does not specifically limit the relative order in which the polarizer and optical element arrays receive incident light. For example, in some embodiments, the polarizer receives the input light first, and then passes it to the optical element arrays. In other embodiments, the order is reversed and the optical element arrays receive the input light first and then pass it to the polarizer.  
         [0017]     Other embodiments, features, aspects, advantages, systems and methods of the invention will be or will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional embodiments, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     It is to be understood that the drawings are solely for purposes of illustration and not as a definition of the limits of the invention. Furthermore, it is to be understood that the drawings are not necessarily drawn to scale and that, unless otherwise stated, they are merely intended to conceptually illustrate the systems, structures and methods described herein. In the figures, like reference numerals designate corresponding parts throughout the different views.  
         [0019]      FIGS. 1A-1F  show perspective views of prior art polarization conversion systems.  
         [0020]      FIGS. 2A-2D  show perspective views of four polarization conversion systems utilizing a homogenizer in accordance with four exemplary embodiments of the present invention.  
         [0021]      FIG. 2E  shows perspective views of a first type of light homogenizer usable in PCSs of FIGS.  2 A-D.  
         [0022]      FIG. 2F  shows a plan view of a two-dimensional circulation optical element array included in the homogenizer of  FIG. 2E .  
         [0023]      FIG. 2G  shows a cross sectional view of the optical element array of  FIG. 2F .  
         [0024]      FIG. 2H  shows plan view of an extraction optical element array included in the homogenizer of  FIG. 2E   
         [0025]      FIG. 2I  shows a cross sectional view of the extraction optical element array of  FIG. 2H .  
         [0026]      FIG. 2J  shows a plan view of a collimating optical element array included in the homogenizer of  FIG. 2E .  
         [0027]      FIG. 2K  shows a cross sectional view of the collimating optical element array of  FIG. 2J .  
         [0028]      FIG. 2L  shows a cross sectional view of the homogenizer of  FIG. 2E .  
         [0029]      FIG. 2M  shows a perspective view of a second type of light homogenizer usable in PCSs of FIGS.  2 A-D.  
         [0030]      FIG. 2N  shows a front plan view of an extraction optical element array included in the homogenizer of  FIG. 2M .  
         [0031]      FIG. 2O  shows a cross sectional view of the optical element array of  FIG. 2N .  
         [0032]      FIG. 2P  shows a cross sectional view of the second type of homogenizer shown in  FIG. 2M   
         [0033]      FIGS. 3A-3C  show perspective views of three polarization conversion systems in accordance with three additional exemplary embodiments of the present invention.  
         [0034]      FIG. 3D  shows a perspective view of a homogenizer usable in the PCSs shown in  FIGS. 3A-3C .  
         [0035]      FIG. 3E  shows a front plan view of a circulation tunnel optical element array of the homogenizer of  FIG. 3D .  
         [0036]      FIG. 3F  shows a cross-sectional view of the optical element array shown in  FIG. 3E .  
         [0037]      FIGS. 4A-4D  show perspective views of four polarization conversion systems in accordance with four more exemplary embodiments of the present invention.  
         [0038]      FIG. 4E  shows a front plan view of a reflective plate usable in the homogenizer of the PCSs of FIGS.  4 A-D.  
         [0039]      FIG. 4F  shows a cross sectional view of the reflective plate shown in  FIG. 4E .  
         [0040]      FIG. 4G  shows a front plan view of an optical element array used in the homogenizer of the PCSs of FIGS.  4 A-D.  
         [0041]      FIG. 4H  shows a cross sectional view of the optical element array of  FIG. 4G .  
         [0042]      FIG. 4I  shows a perspective view of an alternative homogenizer usable in the PCSs of  FIGS. 4A-4B , which is implemented with a collimating optical element array.  
         [0043]      FIGS. 4J-4K  show cross sectional views of two versions of the homogenizer of  FIG. 4I .  
         [0044]      FIG. 4L  shows a perspective view of a further alternative homogenizer structure, which is implemented without a collimating optical element array.  
         [0045]      FIG. 4M  shows a cross sectional view of the homogenizer of  FIG. 4L .  
         [0046]      FIGS. 5A-5C  show perspective views of three compact polarization conversion systems in accordance with three further exemplary embodiments of the present invention.  
         [0047]      FIG. 5D  shows a front plan view of a single-plate homogenizer usable in the PCSs shown in  FIGS. 5A-5C .  
         [0048]      FIG. 5E  shows a cross sectional view of the single-plate homogenizer of  FIG. 5D .  
         [0049]      FIGS. 6A-6B  show perspective views of two additional homogenizers usable in the PCSs disclosed herein.  
         [0050]      FIGS. 6C-6D  show front and back plan views, respectively, of an optical element array included in the homogenizers of  FIGS. 6A-6B .  
         [0051]      FIG. 6E  shows a cross sectional view of the waveguide shown in  FIGS. 6C-6D .  
         [0052]      FIG. 6F  shows a front plan view of a collimating optical element array included in the homogenizers of  FIGS. 6A-6B .  
         [0053]      FIG. 6G  shows a cross sectional view of the collimating optical element array of  FIG. 6F .  
         [0054]      FIG. 6H-6I  shows cross views, respectively, of the homogenizers shown in  FIG. 6A-6B . 
     
    
     DETAILED DESCRIPTION  
       [0055]     A feature of the present system is the use of optical element arrays, solid light pipes or tunnels, wave plates, polarization beam splitters and reflective polarizers to form polarization conversion systems (PCSs).  
         [0056]     A first type of polarization conversion system (depicted in FIGS.  2 A-D) uses circulation, extraction and collimating arrays, polarization beam splitters and a wave plate. This polarization conversion system provides high efficiency and compactness when compared to other polarization conversion system of this disclosure.  
         [0057]     A second type of polarization conversion system (depicted in FIGS.  3 A-C) uses a reflective plate with a distributed aperture for light recycling, a wave plate as well as circulation, extraction and collimating arrays coupled with a reflective polarizer or polarization beam splitters.  
         [0058]     A third type of polarization conversion system (depicted in FIGS.  4 A-D) uses a reflective plate with a single aperture that has an optical element array fabricated on its surface, a wave plate, light pipe or tunnel, collimating array coupled with a reflective polarizer or polarization beam splitters.  
         [0059]     A fourth type of polarization conversion system (depicted in FIGS.  5 A-C) uses a single optical element array, a wave plate in addition to a reflective polarizer or polarization beam splitters, thus, providing the most compact polarization conversion system of this disclosure.  
         [0060]     As used throughout the figures, the z-axis designates the primary optical axis of the PCSs.  
         [0061]     Turning now to the drawings, and in particular to FIGS.  2 A-D, there are illustrated, respectively, perspective views of four PCSs  210 ,  230 ,  250  and  270  utilizing a homogenizer  204  of  FIG. 2E , in accordance with four respective embodiments of the invention. In addition, homogenizers  304 ,  950  and  970  of  FIG. 2M  and  FIGS. 6A-6B  can be used in such systems instead of homogenizer  204  to provide a selected spatial light distribution to the next stage of an optical display system.  
         [0062]     In accordance with a first embodiment of the invention,  FIG. 2A  shows a polarization conversion system  210  consisting of two polarization beam splitters  200   a  and  200   b , a rhomb  202 , a half wave plate  203  and the homogenizer  204 . Input light  201  is focused into the first polarization beam splitter cube  200   a  as shown in  FIG. 2A . Light with one polarization state (e.g., p state) is transmitted to the homogenizer  204 ,  304 ,  950  and  970  and light with orthogonal polarization state (e.g., s state) is reflected toward the second polarization beam splitter cube  200   b.    
         [0063]     At the surface of the second polarization beam splitter cube  200   b , light with orthogonal polarization state (e.g., s state) is reflected toward the half wave plate  203  where its polarization state is converted into the orthogonal state (e.g., p state) and enters the light homogenizer  204  (or alternatively, homogenizer  304 ,  950  or  970 ). The substantially polarized output light  205  exits from the homogenizer  204 .  
         [0064]     The structure and operation of homogenizers  204 ,  304 ,  950  and  970  are described below in connection with  FIGS. 2E-2P  and  FIGS. 6C-6I .  
         [0065]      FIG. 2B  shows a second embodiment of a polarization conversion system  230 , which consists of a polarization beam splitter cube  223 , a prism reflector  221 , a half wave plate  224 , spacer  225  and a light homogenizer  204 .  
         [0066]     Input light  222  is coupled into the polarization beam splitter cube  223  as shown in  FIG. 2B . Light with one polarization state (e.g., p state) is transmitted to the light homogenizer  204 ,  304 ,  950  and  970  through a spacer  225  and light with orthogonal polarization state (e.g., s state) is reflected toward a prism reflector  221 .  
         [0067]     At the surface of the prism reflector  221 , light with orthogonal polarization state (e.g., s state) is reflected toward the half wave plate  224  where its polarization state is converted into the orthogonal state (e.g., p state) and enters the light homogenizer  204 , (or alternatively, homogenizer  304 ,  950  or  970 ). The substantially polarized output light  227  exits from the homogenizer  204 .  
         [0068]      FIG. 2C  shows a third embodiment of polarization conversion system  250  which consists of a polarization beam splitter cube  243 , a prism reflector  241 , a quarter wave plate with a reflector  242  and a light homogenizer  204  (or alternatively, homogenizer  304 ,  950  or  970 ).  
         [0069]     Input light  245  is coupled into the polarization beam splitter cube  243  as shown in  FIG. 2C . Light with one polarization state (e.g., p state) is transmitted to the prism reflector  241 , which in turn reflects it toward the light homogenizer  204 ,  304 ,  950  or  970  Light with orthogonal polarization state (e.g., s state) is reflected toward the quarter wave plate  242  where it enters and exits the quarter wave plate  242  toward the light homogenizer  204 ,  304 ,  950  or  970  with a converted polarization state (e.g., p state). The substantially polarized output light  227  exits from the homogenizer  204 .  
         [0070]     In a fourth embodiment of the invention, the quarter wave plate with a reflector  242  is placed as shown  FIG. 2D .  
         [0071]      FIGS. 2E and 2M  show perspective views of two light homogenizers  204  and  304 .  FIG. 2E  shows a perspective view of light homogenizer  204 , which consists of three optical element arrays  204   a,    204   b,  and  204   c.    
         [0072]      FIG. 2F  shows a plan front view of a two-dimensional optical element array  204   a,  which consists of circulation micro-elements  1202  arranged in two dimensions (x and y).  
         [0073]      FIG. 2G  shows a cross-sectional view of optical element array  204   a  along line B of  FIG. 2F  with an exploded three-dimensional view of micro-element  1202 . Each micro-element has four sidewalls  1204  as well as entrance  1203  and exit  1205  apertures. Neighboring micro-elements  1202  are separated by air or material with lower index of refraction than that of the micro-element  1202  itself. Reflective layer  1200  is bonded to or deposited on the four sidewalls of array  204   a  and sidewalls of circulation micro-elements  1202  are coated with a reflective layer  1201 .  
         [0074]      FIGS. 2H and 2J  show front plan views of two-dimensional optical element arrays  204   b  and  204   c,  which consist of extraction micro-elements  1212  and collimating micro-elements (i.e., micro-prisms)  1222 , respectively, arranged in two dimensions (x and y).  
         [0075]      FIGS. 2I and 2K  show cross-sectional views of optical element arrays  204   b  and  204   c  along line C of  FIGS. 2H and 2J . Exploded three-dimensional views of micro-element  1212  and  1222  are shown with their corresponding sidewalls  1214  and  1223  as well as entrance  1213  and  1223  and exit  1215  and  1224  apertures. Reflective layers  1210  and  1221  are bonded to or deposited on the four sidewalls of array  204   b  and  204   c.  In addition, reflective layer  1211  is deposited on areas between extraction micro-elements  1212 .  
         [0076]     As shown in  FIG. 2K , each micro-prism  1202  has four sidewalls  1223  (two sidewalls are shown in the perspective view of the exploded micro-prism) as well as entrance  1223  and exit  1224  apertures. Sidewalls of micro-elements  1202 ,  1212 , and  1222  of arrays  204   a,    204   b  and  204   c  are aligned with the axes of polarization beam splitter cubes  200 ,  223  and  243  so that the polarization state of light entering the homogenizer  204  is maintained.  
         [0077]      FIG. 2L  shows a cross sectional-view of homogenizer  204  along plane A of  FIG. 2E . Homogenizer  204  consists of circulation optical element array  204   a,  extraction optical element array  204   b  and micro-prism array  204   c,  which are attached, glued, or bonded together as shown in  FIGS. 2E and 2L .  
         [0078]      FIGS. 2M and 2P  show a perspective view and corresponding cross-sectional view, respectively, of homogenizer  304  along line A of  FIG. 2M . Homogenizer  304  consists of circulation optical element array  204   a,  plain glass plate  304   b  with reflective layer on its four sidewalls and extraction optical element array  304   c,  all which are attached, glued, or bonded together as shown in  FIGS. 2M and 2P .  
         [0079]      FIGS. 2N and 2O  show a front plan view and corresponding cross-sectional view, respectively, of extraction optical element array  304   c  along line C of  FIG. 2N .  
         [0080]     The operation of homogenizers  204  and  304  is based on circulating the input light within the body of an optical element array  204   b  or glass plate  304   b  using circulating optical element array  204   a.  The circulated light is uniformly extracted out of the body of the micro-guide  204   b  or glass plate  304   b  using extraction micro-elements  1212  and  1302  of optical element arrays  204   b  and  304   c . Light is received by optical element array  204   a  and impinges on the circulation micro-elements  1202  within the circulation array  204   a  which increases the cone angle of preferably all received light so that it is guided within the body of optical element array  204   b  and  304   b  via total internal reflection (TIR) and reflection unless it is extracted by micro-elements  1212  and  1302 . In other words, the function of circulation array  204   a  is to deliver light to array  204   b  and plate  304   b  with an incidence angle θ larger than the critical angle θ c  of the array  204   b  and plate  304   b.    
         [0081]     Polarization beam splitters  200   a ,  200   b ,  223 , and  243  ( FIGS. 2A-2D ) split the light they receive into two sub-beams with two hot spots at the center of each sub-beam. In order to get a uniform spatial distribution of light energy over the PCS output cross section, the density of extraction micro-elements  1212  and  1302  within arrays  204   b  and  304   c  is designed to be inversely proportional to the intensity of light within the body of the optical element arrays  204   b  and  304   b.  Extraction micro-elements  1212  and  1302  can be distributed non-uniformly or randomly within arrays  204   b  and  304   c  and can be distributed to get a selected distribution of light in terms of intensity and cone angle.  
         [0082]      FIGS. 2H and 2N  show that the density of extraction micro-elements  1212  and  1302  is low where input light is high and increase toward array edges and center. As a result, the light delivered by extraction array  204   b  and  304   c  has a highly uniform cross-sectional distribution of intensity and angle. Light extracted by micro-elements  1212  enters the micro-prism array  204   c  with an angle β in  and exits with an angle β out , thus, a collimated and uniform light beam is delivered by homogenizer  204 .  
         [0083]     In homogenizer  304  ( FIG. 2P ), the collimation of extracted light is achieved by the collimating nature of the extraction micro-elements  1302 . By changing the distribution of extraction micro-elements  1212  and  1302  and their design parameters such as size and taper, it is possible to deliver light with a certain spatial distribution in terms of angle and intensity. For example, higher spatial intensity near homogenizer edges can be used to compensate for the usual lower light intensity near screen edges in projection display systems.  
         [0084]     The polarization conversion systems (PCSs)  210 ,  230 ,  250  and  270  disclosed herein have five key advantages over known polarization conversion systems (FIGS.  1 A-F). First, the polarization conversion systems disclosed herein can use larger input apertures (i.e., larger cross-sectional input area of the PCS) while maintaining the etendue of the input light or that of a lamp/reflector. This leads to increasing the efficiency of the polarization conversion system and displays utilizing such PCSs. Second, the PCSs disclosed herein provide more control over the spatial light distribution and uniformity in terms of intensity and exit divergence angle when compared to that of known PCSs. The capability of designing and distributing individual micro-elements within an extraction optical element array provides control over the spatial distribution of light intensity and cone angle over the entire cross section of the exit aperture of a PCS. For example, PCSs can provide more light at higher angles, thus, overcoming the typical angle dependent loss in a conventional display system and leading to more uniform light intensity at the screen. Third, higher coupling efficiency between the light source and the display panels (i.e., modulator) in a display system can be provided by the use of collimating elements within the inventive PCS, which results in a more efficient use of light by the light valve, thus, reducing the required number of light sources and/or their power. In this case, collimating optical element arrays do not increase the etendue of light beam delivered to the light valve, thus enhancing coupling efficiency and increasing display brightness.  
         [0085]     Fourth, the PCSs disclosed herein provide a superior level of compactness and light-weight. The length of the inventive PCSs can be lower than the lengths of known PCSs by one or more orders of magnitude resulting in very compact light-weight display and illumination systems. In addition, the high PCS efficiency enables the use of small size display panels (≦0.5″) which results in using smaller optical components such as the projection lens, thus, leading to very compact projection systems.  
         [0086]     Fifth, lower display system cost is achieved by using the inventive PCSs disclosed herein due to the reduced size of the optical components used within the projection display system. As the size of optical components is reduced, their cost is reduced and the cost of the overall system is reduced.  
         [0087]      FIGS. 3A-3C  show perspective views, respectively, of three polarization conversion systems  410 ,  490  and  510  utilizing a homogenizer  404 , which provides the desired spatial light distribution to the next stage in an optical display system, in accordance with further embodiments of the present invention.  
         [0088]     Homogenizers  204  and  304  of  FIGS. 2E and 2M  can be used alternatively to provide the function of homogenizer  404  of  FIGS. 3A-3C  as long as the distributions of extraction micro-elements within arrays  204   b  and  304   c  are modified to account for the spatial intensity of input light  400 , which is related to the spatial intensity of light within the body of arrays  204   b  and  304   b.    
         [0089]     The input light beam  400  in  FIGS. 3A-3C  is not divided into two sub-beams as it is the case of input light beam of  FIGS. 2A-2D . As shown in  FIG. 3D , homogenizer  404  of  FIGS. 3A-3C  has a structure similar to either that of homogenizers  204  or  304  and uses a circulation micro-tunnel array  1404   a  of  FIGS. 3E-3F , which performs the same function as that of circulation optical element array  204   a . Either arrays  204   b  and  204   c  or arrays  304   b  and  304   c  can be used to perform the functions of arrays  404   b  and  404   c  of  FIG. 3D .  
         [0090]      FIG. 3E  shows a front plan view of array  1404   a  and  FIG. 3F  shows a cross-sectional view of array  1404   a  along line B of  FIG. 3E . Micro-tunnels  1402  are hollow with a reflective coating  1401   a  on their sidewalls  1401  and have entrance  1403  and exit  1405  apertures as shown in  FIG. 3F . The array  1404   a  is coated with a reflective layer  1400  on its four edges. The reflective layers described herein can be deposited aluminum or any other suitable reflective material.  
         [0091]     Circulation arrays  204   a  and  1404   a  of homogenizers  204 ,  304  and  404  accepts the input light from a light source such as an arc lamp and delivers it to the next stage for circulation. Since these arrays  204   a  and  1404   a  are coated with reflective layers  1201  and  1401   b  on the sidewalls of its micro-elements  1202  and the front surface of its micro-tunnels  1402 , a substantial amount of the light traveling in the opposite direction (i.e. in the negative z direction) is reflected back toward the circulation arrays  204   b ,  304   b  and  404   b . Thus, array  204   a  and  1404   a  acts as a one directional aperture that passes a substantial amount of light entering from one side and reflects a substantial amount of light entering from the opposite side. This kind of unidirectional aperture provides more efficient polarization conversion systems  410 ,  490  and  510  than known polarization conversion systems  25 ,  35  and  45  of  FIGS. 1A-1C .  
         [0092]     According to one embodiment,  FIG. 3A  shows a polarization conversion system  410  consisting of a homogenizer  404 , a quarter wave plate  405  and a reflective polarizer  406  such as a Proflux brand from Moxtek company. Input light  400  is focused into the homogenizer  404  as shown in  FIG. 3A  and travels toward the reflective polarizer  406 . Light with one polarization state (e.g., p state) is transmitted through reflective polarizer  406  to the next stage and light with orthogonal polarization state (e.g., s state) is reflected toward the homogenizer  404  where it passes through the quarter wave plate  405  and impinges on homogenizer  404 . This light is reflected or refracted back toward the quarter wave plate  405  by the reflective layers and refractive micro-elements of homogenizer  404  where its polarization state is converted into the orthogonal state (e.g., p state) and passes through the reflective polarizer  406 , and is emitted as substantial polarized output light  407 .  
         [0093]      FIGS. 3B and 3C  show two polarization conversion systems  490  and  510  similar to that of  FIG. 3A  except for the replacement of the reflective polarizer  406  by an assembly of two polarization beam splitters  485  and  486  each disposed at an angle θ of 45° to the axis of the light path ( FIG. 3B ) and an assembly of a mirror  505  with a single polarization beam splitter  506  disposed at an angle β of 45° to the axis of the light path ( FIG. 3C ). The light path in  FIGS. 3A-3C  is parallel to the z-axis. When compared to polarization conversion systems (PCSs) of  FIG. 2 , PCSs  410 ,  490  and  510  of  FIG. 3  provide more compactness and collect more light due to doubling the size of the input aperture of the PCSs of  FIG. 3 . In addition, polarization conversion systems  410 ,  490  and  510  have the same key advantages as these of PCSs of  FIG. 2 .  
         [0094]      FIGS. 4A-4D  show perspective views, respectively, of four polarization conversion systems  610 ,  650 ,  690  and  710 , which utilize a compact homogenizer  608  to provide the required spatial light uniformity, in accordance with further embodiments of the present invention. Homogenizer  608  consists of three elements, a reflective plate  602 , light guide  603  and optional collimating optical element array  604  as shown in  FIGS. 4I-4M . The three elements  602 ,  603  and  604  of homogenizer  608  can be arranged within the PCSs  610 ,  650 ,  690  and  710  as a block followed by the quarter wave plate  605  ( FIGS. 4A and 4C ) and can be distributed within the PCSs  610 ,  650 ,  690  and  710  in various ways such as shown in  FIGS. 4B and 4D . In general, the quarter wave plate  605  can be placed either between reflective plate  602  and light guide  603 , between light guide  603  and optical element array  604 , or after optical element array  604 .  
         [0095]      FIG. 4A-4B  show polarization conversion systems  610  and  650  consisting of a homogenizer  608 , a quarter wave plate  605  and a reflective polarizer  606 . Both polarization conversion systems  610 , 650  are similar except for the placement of quarter wave plate  605 .  
         [0096]      FIGS. 4C and 4D  show two polarization conversion systems  690  and  710  that do not use a reflective polarizer but rather use an assembly of two polarization beam splitters  685  and  686  each disposed at an angle θ of 45° to the axis of the light path ( FIG. 4C ) and an assembly of a mirror  705  with a single polarization beam splitter  706  disposed at an angle β of 45° to the axis of the light path ( FIG. 4D ).  
         [0097]      FIGS. 4E and 4F  show a front plan view and a cross-sectional view, respectively, of reflective plate  602  along line A of  FIG. 4E . In  FIG. 4E , reflective plate  602  has reflective layers  602   b  and  602   c  coated on its surface and edges and has a two dimensional array  602 A of micro-elements  602   a  fabricated on both sides of its optically transmissive aperture  601  which has an area of d 1 ×d 2 . Aperture  601  can have any suitable shape such as circular, oval, rectangular, square and irregular. Micro-elements  602   a  can be arranged in a one or two dimensional array  602 A and their distribution can be random, uniform, or non-uniform. Each micro-elements  602   a  is a tapered solid micro-guide with entrance  602   d  and exit  602   f  apertures and four sidewalls  602   e  (only two are shown in  FIG. 4F ).  
         [0098]     Reflective layers  602   b  and  602   c  can be dielectric mirrors that do not rotate the polarization state of reflected light. Other types, tapers, sizes and shapes of micro-elements  602   a  are possible and they are not required to preserve the polarization state of input light. Light guide  603  can be solid light guide made of optically transmissive material such as glass with polished surfaces or hollow light guide with reflective sidewalls and can also be straight or tapered with an exit aperture of cross section aspect ratio as the display panel used in the projection system.  
         [0099]      FIGS. 4G and 4H  show a front plan and a cross-sectional view, respectively, of optical element array  604  along line A of  FIG. 4G . Collimating micro-elements  604   a  are fabricated in a two dimensional array  604  on both sides of an optically transmissive substrate and are aligned in a way that do not rotate the polarization of light. The edges of the substrate are coated with a reflective layer  604   c.  The cross-section aspect ratio of optical element array  604  is preferably equal to that of the display panel used in the projection system.  
         [0100]      FIGS. 4I-4M  show two additional homogenizers  608  and  609 .  FIGS. 4I and 4L  show perspective views of homogenizers  608  and  609 , respectively, and  FIGS. 4J-4K  and  4 M show cross-sectional views along plane B of  FIGS. 4I and 4L , respectively.  
         [0101]     In homogenizer  608 , array  602  is flipped so that it diverges rather than collimates the input light, which results in achieving required light uniformity with a short light pipe/tunnel  603 . If one uses straight (i.e. no taper) or collimating micro-pipes within array  602  ( FIG. 4M ), a longer light pipe/tunnel  603  will be required to achieve the required light uniformity assuming that the entrance and exit apertures of light pipe/tunnel  603  remain equal in all cases. As shown in  FIG. 4M , homogenizer  609  is implemented without a collimating array  604  at its exit aperture but uses array  602  to collimate input light. The efficiency of homogenizer  608  can be increased by coating the sidewalls of micro-elements  602   a  of reflective plate  602  by a reflective coating as shown in  FIG. 4K . Polarization conversion systems (PCSs)  610 ,  650 ,  690  and  710  of  FIG. 4  have the same key advantages as these of PCSs of  FIG. 2 .  
         [0102]      FIGS. 5A-5C  show perspective views, respectively, of three compact polarization conversion systems  810 ,  850  and  890  utilizing a single-plate homogenizer  801 , in accordance with three further embodiments of the present invention. Homogenizer  801  provides the required spatial distribution of light and acts as a unidirectional reflective plate.  FIGS. 5D and 5E  show a top view and a cross sectional view of homogenizer  801  along line B of  FIG. 5D .  
         [0103]     As shown in  FIG. 5E , circulation array  1807   a  and extraction array  1808   a  are fabricated on the back side of substrate  1802 . Extraction array  1808   a  consists of extraction micro-elements  1803   a  and  1803   b  which overlap with circulation micro-elements  1804   b . On the front side of substrate  1802 , there are circulation array  1808   b  and an optional collimating array  1807   b . Collimating array  1807   b  can be eliminated or replaced by an optical element array of another type.  
         [0104]     Circulation array  1808   b  consists of one dimensional micro-elements, which extend in the y-direction and are coated with a reflective layer  1804   c.  The function of array  1808   b  is to collimate light impinging on it so that it exits the surface of array  801  perpendicularly (i.e., substantially parallel to the Z-axis). Extraction micro-elements within array  1808   b  are preferably made of micro-elements that collimate light in two directions rather than one. Such micro-elements may be micro-prisms or micro-lenses that are arranged in a two dimensional array. Micro-elements within extraction arrays  1808   a  and  1808   b  are distributed over the surface of the substrate  1802  so that light is extracted uniformly from the body of the substrate  1802 . It is possible to have a homogenizer  801  with a single extraction array either  1808   a  or  1808   b.  For simplicity of illustration, the circulation array  1807   a  is shown to have one circulating micro-element  1801  as shown in  FIG. 5D . The number, size and shape of circulating micro-element  1801  are some of the design parameters of circulation array  1807   a.    
         [0105]     A reflective layer  1804   a  is bonded or deposited on the four edges of substrate  1802 . Reflective and refractive micro-elements  1803   a,    1803   b,    1801  and  1804   c  of homogenizer  801  are aligned so that they do not rotate the light polarization. However, micro-elements of array  1807   b  may not have to follow this restriction. The operation of collimating  1807   b,  circulation  1807   a  and extraction  1808   a  and  1808   b  optical element arrays is substantially the same as the operation of the already discussed collimating, circulation and extraction arrays. Thus, homogenizer  801  and polarization conversion systems  810 ,  850  and  890  operate in a similar manner to those  410 ,  490  and  510  of  FIGS. 3A-3C .  
         [0106]     Polarization conversion systems  810 ,  850  and  890  have same key advantages as these PCSs of  FIG. 2 . In addition, they provide higher compactness in comparison with PCSs of  FIG. 2  but at a lower efficiency due to the small size of their input aperture  1807   b.    
         [0107]      FIGS. 6A-6B  show perspective views of two homogenizers  950  and  970 , which can be used in the implementation of polarization conversion systems (PCSs)  210 ,  230 ,  250  and  270  of  FIGS. 2A-2D . Homogenizer  950  uses two optical element arrays  910  and  925  in its structure, whereas, homogenizer  970  uses in addition to that a light pipe/tunnel  935 .  
         [0108]      FIGS. 6C and 6D  show front and back side views of optical element array  910  and  FIG. 6E  shows a cross-sectional view of  FIGS. 6B-6C  along line A. Two collimating optical element arrays  900 A are shown on the front surface of optical element array  910 , which correspond to the location of the divided hot spot as delivered by polarization beam splitter cubes of  FIG. 2A-2D  to the homogenizers  950  and  970 . On the back side of array  910 , there are extraction micro-guides  900   b  arranged in an array in the xy-plane.  
         [0109]     Distribution of these extraction micro-guides  900   b  can be uniform ( FIG. 6D ), non-uniform or random. Exploded three dimensional views of collimating micro-guides  900   a  and extraction micro-guides  900   b  are shown in  FIG. 6E .  FIGS. 6F and 6G  show a perspective view and cross sectional view of collimating optical element array  925  along line C of  FIG. 6F . As shown in  FIG. 6F-6G , micro-prisms  920  are distributed over the surface of array  925  in areas that do not correspond to the divided hot spot (i.e., collimating array  900 A). A three dimensional view of micro-prisms  920  is shown in  FIG. 6G . Cross sectional views of homogenizers  950  and  970  are shown in  FIGS. 6H-6I  along plane B of  FIGS. 6A-6B .  
         [0110]     The operation of homogenizers  950  and  970  is based on collimating part of the light in the hot spot, which is made of substantially high angles, that passes through the entrance apertures of the collimating micro-guides  900   a  of array  900 A. The hot-spot light that passes through the sidewalls of micro-guides  900   a  is diverged (i.e., cone angle is increased) and gets spatially separated from the collimated light as it reaches the extraction micro-guides  900   b . For simplicity of illustration, rays A 1  and A 2  represent the hot-spot light that goes through the entrance apertures of the collimating micro-guides  900   a  and rays B 1  and B 2  represent the hot-spot light that goes through their sidewalls as shown in  FIG. 6H . Light extracted from the body of array  910  is collimated by micro-prism array  925  and light collimated by array  900 A travels through plates  910  and  925  without encountering any micro-elements. Outside the hot-spot area, light intensity is lower and light is made of substantially low angles. This light passes through the extraction micro-guides  900   b  and exits with a larger cone angle, thus, becoming a candidate for collimation by the micro-prism array  925 . Light that enters micro-prism array  925  with low angles (i.e., already collimated) exits micro-prisms  920  with a high angle and gets recycled back toward array  910  via TIR and reflections off of coated plate edges  902  and  922 . Such case is represented by rays C 1  and C 2  as shown in  FIG. 6H . Light exiting plate  925  enters light pipe/tunnel  935  for further homogenization then to next stage ( FIG. 6I ) or is directly delivered to the next stage ( FIG. 6H ).  
         [0111]     In general, the polarization conversion systems disclosed herein preferably have the same cross section aspect ratio as the display panel used in the display system. Since a wave plate is used to rotate polarization by 90 degrees, micro-elements and optical element arrays used to implement polarization conversion systems can be aligned with other components in the polarization conversion system so that polarization is preserved when light is recycled through these micro-elements and optical element arrays. This kind of alignment enhances the efficiency of the polarization conversion system. In polarization conversion system where no wave plate is used to convert the polarization of recycled light, micro-elements and optical element arrays can be designed and aligned to achieve the function of the wave plate (i.e., rotate polarization randomly or non-randomly).  
         [0112]     Micro-elements used within optical element arrays disclosed herein may include micro-guides, micro-tunnels, micro-lenses, micro-prisms and combinations of different types in a single optical element array. Such micro-elements are typically separated from adjacent micro-elements by either air or material with lower index of refraction than that of the micro-elements themselves. Design parameters of each micro-element within an array include shape and size of entrance and exit apertures, depth, sidewalls shape and taper, and orientation. Micro-elements within an array can have uniform, non-uniform, random or non-random distributions and range from thousands to millions with each micro-element being distinct in its design parameters.  
         [0113]     The size of the entrance/exit aperture of each circulation micro-element is preferably ≧5 μm in case of visible light in order to avoid light diffraction phenomenon. However, it is possible to design micro-elements with sizes of entrance/exit aperture being &lt;5 μm. In such case, the design should consider the diffraction phenomenon and behavior of light at such scales to provide homogeneous light distributions in terms of intensity, viewing angle and color over a certain area. Micro-elements can be arranged as a one-dimensional array, two-dimensional array, or circular array and can be aligned or oriented individually.  
         [0114]     The specific shapes, sizes and arrangements of the optical element arrays described herein are only a small subset of the possible optical element arrays that can be used within the scope and spirit of the invention. Some of the other array types that are usable with the PCSs disclosed herein are described in the U.S. Patent Applications identified in the immediately following paragraph.  
         [0115]     Techniques for manufacturing the optical element arrays and PCSs disclosed herein are described in U.S. patent application Ser. No. 10/458,390, titled “Light Guide Array, Fabrication Methods and Optical System Employing Same” and U.S. patent application Ser. No. ______, titled “Compact Projection System Including A Light Guide Array”, Attorney Docket No. 00024.0006.NPUS00, filed on Feb. 25, 2005, both of which are incorporated herein by reference.  
         [0116]     While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that more embodiments and implementations, other than those specifically described above, are possible that are within the scope of this invention. Further, the foregoing summary, detailed description and drawings are considered as illustrative only of the principles of the invention and are not intended to limit the scope of the invention. Since other modifications and changes may be or become apparent to those skilled in the art, the invention is thus not limited the exact embodiments shown and described above, and accordingly, all suitable modifications and equivalents are deemed to fall within the scope of the invention, as it is defined by the claims below.