Abstract:
A polarization beam splitter includes at least six prisms assembled together to form a single solid components. At least one diagonal interface is formed by a combination of two or more prism surfaces. The solid polarization beam splitter component has at least four light entrance/exit surfaces with at least one of the light entrance/exit surfaces including a step. At least one of the prisms has a non-triangular cross-sectional shape. At least one surface of a prism that forms a portion of the diagonal interface has a polarization beam splitting material disposed thereon resulting in a diagonal interface that includes a polarization beam splitting material. The polarization beam splitter can be incorporated into various image projection apparatus including 2D, multiple image, and 3D projection apparatuses.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Provisional Patent Application 61/473,165 filed 8 Apr. 2011, and U.S. patent application Ser. No. 13/233,036 filed 15 Sep., 2011 the disclosures of which are incorporated by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to polarization beam splitters in general and, more particularly, to polarization beam splitters for use in projection apparatus that can produce multiple images and/or three-dimensional images. 
       BACKGROUND 
       [0003]    There are many applications in which multiple images must be displayed either sequentially or simultaneously. Current apparatus which can project multiple images are bulky, usually being complete duplicates of single-image projection apparatus. Typically, multiple light sources are required. 
         [0004]    However, there is a need in the art for compact projection apparatus which can project multiple images, in which the multiple images each optionally display different image information. Such a projector could be used for wide-screen projection, three-dimensional image creation, and interactive imaging applications. 
         [0005]    Polarization beam splitters for such projection apparatus must also be compact. In addition to being compact, the PBS structures must be easy to fabricate since machining of small optical components greatly increases their expense. Thus there is a need in the art for improved polarization beam splitters which are compact and easy to fabricate. Such polarization beam splitters can be used in compact image projection apparatus. 
         [0006]      FIGS. 2A-2C  depict polarization beam splitters with two LCoS spatial light modulators. The polarization beam splitter needs to have the same vertical and horizontal lengths. However, light has a certain cone angle; the longer the length, the bigger the cone will be. As seen in  FIG. 2B , there is significant light leakage from the polarization beam splitter. This leakage will cause an undesirable ghost image due to total internal reflection. Thus it is necessary, for this design, to either enlarge the size of the PBS or control the light solid angle. Further, as seen in  FIG. 2C , there are large regions (indicated in part by the oval shapes) of the PBS that are unused by the light emitted from the spatial light modulator, making the device inefficient. 
         [0007]    As seen in  FIG. 3 , a more efficient polarization beam splitter with less light leakage and greater PBS volume utilization can be created using stepped optical surfaces adjacent to the spatial light modulators (as well as stepped surfaces on the top and bottom PBS surfaces). While such a design advantageously reduces size, improves brightness, and improves contrast, fabrication is difficult, particularly for the precision machining of the stepped optical surfaces. 
         [0008]    Thus, there remains a need in the art for improved polarization beam splitters that are compact, have high brightness and contrast, and are simple and cost-effective to fabricate. 
       SUMMARY OF THE INVENTION 
       [0009]    In one aspect, the invention comprises a polarization beam splitter that includes at least six prisms assembled together to form a single solid component. At least one diagonal interface is formed by a combination of two or more prism surfaces. The solid polarization beam splitter component has at least four light entrance/exit surfaces with at least two of the light entrance/exit surfaces forming a step. At least one of the prisms has a non-triangular cross-sectional shape. At least one surface of a prism that forms a portion of the diagonal interface has a polarization beam splitting material disposed thereon resulting in a diagonal interface that includes a polarization beam splitting material. 
         [0010]    In one embodiment, the present invention is directed to a projection apparatus that has at least a first light source for providing light that includes the polarization beam splitter described above. Image forming light modulators, such as LCoS spatial light modulators, are provided adjacent the multi-prism beam splitter. Projectors that form a single image or two images can be created using the polarization beam splitters of the present invention. 
         [0011]    In another projector embodiment for forming two images, first and second LCoS spatial light modulators and a first light source are configured such that the first LCoS spatial light modulator and the first light source are positioned adjacent one side of the polarization beam splitter and the second LCoS spatial light modulator and the second projection optics system share an opposite-facing side of the polarization beam splitter. In this configuration, light from the first light source is formed into first and second polarized beams that are directed in orthogonal directions by the polarization beam splitter such that the first polarized beam is directed into the first LCoS spatial light modulator and the second polarized beam is directed into the second LCoS spatial light modulator. 
         [0012]    A first image source modulates the first LCoS spatial light modulator and a second image source for producing images which may be the same or different from images produced using the first image source, modulates the second LCoS spatial light modulator. The first and second LCoS spatial light modulators and the first and second projection optics systems are configured such that a first modulated reflected output image from the first LCoS spatial light modulator is output to the first projection optics system and a second modulated reflected output image from the second LCoS spatial light modulator output to the second projection optics system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIGS. 1A-1C  depict polarization beam splitter configurations of the present invention. 
           [0014]      FIGS. 2A-2C  depict light passing through polarization beam splitters with two spatial light modulators. 
           [0015]      FIG. 3  depicts light passing through a modified/stepped polarization beam splitter with two spatial light modulators. 
           [0016]      FIGS. 4A and 4B  depict formation of a polarization beam splitter with either an internal PPL coating on surfaces to be adhered to one another or an internal antireflection coating on the surface of the prism if the prisms are not adhered to one another. 
           [0017]      FIGS. 5A and 5B  depict an image projection system using the polarization beam splitters of the present invention to form a single 2-D or 3-D image. 
           [0018]      FIG. 6  depicts an image projection apparatus using the polarization beam splitters of the present invention to form two images. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    Turning to the drawings in detail,  FIGS. 1A-1C  depict polarization beam splitter structures of the present invention. As seen in  FIG. 1A , each polarization beam splitter includes six prisms  101 ,  102 ,  103 ,  104 ,  105 , and  106 . The six prisms are cemented together to form an integrated polarization beam splitter  100 . A surface coating  112  or other beamsplitting element is formed on one or more facets of each prism  101 ,  102 ,  103 ,  104 ,  105 , and  106  that faces another prism facet; the elements are selected such that when the six prisms are assembled, the surfaces form a pair of orthogonal surfaces  113  and  114  of prism  100 . The surface  113  is formed by facets of  101 ,  102 ,  103 ,  104 ,  105  and  106 , the surface  114  is formed by facets of  102 ,  103 ,  105  and  106 . 
         [0020]    In one embodiment, both orthogonal surfaces  113  and  114  include a polarization beam splitting material or coating; alternatively, the polarization beam splitting coating can be formed on portions of the orthogonal surfaces with one or more portions including a mirror coating (for certain imaging applications to be discussed below). The polarization beam splitting material includes coatings, gratings, etc. such that two orthogonal diagonal beam splitter surfaces are created. Optionally, a grating micro structure or MOF film can also be attached between the two orthogonal surfaces  113  and  114  to be used as the polarization beam splitter element rather than a PBS coating material. For a portion of an orthogonal surface having a mirror surface rather than a polarization beam splitter element, the mirror surface reflects incident light of any polarization in a direction 90° from the mirror surface. The mirror coating can be selected form thin film coatings such as aluminum, silver, gold, or other metal or reflective coatings. The material of the prisms is selected from glass, plastic, crystal or other optical materials. 
         [0021]    As seen in  FIG. 1A , elements  102  and  105  include a step formed in the outer surface; this surface is a non-optical surface, that is, light is not expected to enter or exit through the exterior-facing surface of prism elements  102  and  105 . Further, it is noted that prisms  102  and  105  do not have triangular cross sections. Similarly, prism elements  103  and  106  also do not have triangular cross sections. For any exterior surface through which light enters or exits, an optional antireflection coating can be provided. 
         [0022]    To avoid having to machine a step in the optical surfaces formed by the combination of prisms  103  and  104  and the combination of prisms  101  and  106 , the prism sizes are selected such that, when assembled, a step is formed at the interface between the respective prisms. Further, a polarizer coating or material can be placed at the interface between prisms  103  and  104  and the interface between prisms  101  and  106  to improve the contrast ratio to be discussed below. As seen in  FIG. 1A , two prisms have triangular cross sections (prisms  101  and  103 ) and one angle of the triangular cross section is 90 degrees while the other two acute angles are different from one another. Further, the assembled polarization beam splitter of  FIG. 1A  has four stepped external surfaces. 
         [0023]    In the embodiment of  FIG. 1B , the prisms  101 ′,  102 ′,  103 ′,  104 ′,  105 ′ and  106 ′ correspond, functionally, to prisms  101 - 106  of  FIG. 1A . However, the geometry is different. To eliminate the step from the top and bottom non-optical surfaces, the step is replaced by a sloped diagonal in prisms  102 ′ and  105 ′. Thus only prisms  103 ′ and  106 ′ have non-triangular cross-sectional shapes. As a result, only two of the external beam splitter surfaces are stepped surfaces. Similarly, in the embodiment of  FIG. 1C , prisms  101 ″,  102 ″,  103 ″,  104 ″,  105 ″, and  106 ″ are configured such that a step is not used on the non-optical exterior surfaces of prisms  102 ″ and  105 ″ and only two exterior surfaces are stepped. However, these prisms are selected to have a non-triangular cross-section such that the resultant polarization beam splitter  100 ″ has an overall geometry that is approximately cubic. That is the non-stepped surfaces of the polarization beam splitter are substantially planar and intersect at approximately right angles with the stepped surfaces of the polarization beam splitter. 
         [0024]    As seen in  FIGS. 4A and 4B , a polarizer coating or material  210 ,  220  can be interposed between prism subcomponents  101  and  106  and between prism subcomponents  103  and  104  to enhance the contrast ratio. Alternatively, as seen in  FIGS. 4A and 4B , an antireflection coating can be interposed on each surface in between prism subcomponents  101  and  106  and between prism subcomponents  103  and  104 . Advantageously, the antireflection coating reduces undesirable light reflections at these interfaces if they are not cemented to one another. 
         [0025]      FIGS. 5A and 5B  depict projection apparatus  500  and projection apparatus  500 ′ respectively using polarization beam splitters  100  and  101  in connection with light source  400  to form a single 2-D or 3-D image. Light source  400  is unpolarized and is incident to polarization beam splitter  100  ( FIG. 5A ) or  100 ′ ( FIG. 5B ). The light source may be a white light source or combined or sequential colored light sources (e.g., red, blue, green LED light sources as depicted in  FIGS. 5A and 5B ). On the same side of polarization beam splitter  100  as light source  400  is a first reflective spatial light modulator  300 . In an exemplary embodiment, reflective spatial light modulator  300  is a liquid crystal on silicon (LCoS) modulator; however, any low profile reflective spatial light modulator that can be positioned adjacent the polarization beam splitters of the present invention can be used in the projection apparatus  500 ,  500 ′ of the present invention. Exemplary spatial light modulators (for all of the embodiments of the present invention) include, but are not limited to the liquid crystal on silicon spatial light modulators, digital micromirror device spatial light modulators, digital light processor spatial light modulators, MEMS spatial light modulators, liquid crystal spatial light modulators, mirror-based spatial light modulators, or any other low profile spatial light modulator that can process image information for projection. Note that the spatial light modulators in any projector may be the same kind of spatial light modulator or two or more kinds of spatial light modulators, depending upon the projector application. 
         [0026]    On an opposite polarization beam splitter surface, in line with incident light source  400 , is a second reflective spatial light modulator  302 ; again, in this embodiment a LCoS spatial light modulator is depicted as light modulator  302  but other reflective spatial light modulators can be selected. Incident light from source  400  passes through incident light source optics  410  and enters the polarization beam splitter  100 . When the light reaches the polarization beam splitter coated diagonal interface  113 , light of one polarization passes through polarization beam splitter  100  in a straight line and is incident on second reflective spatial light modulator  302 . Light of the opposite polarization is reflected by the polarization beam splitter surface  113  and is again reflected by polarization beam splitter surface  114  into first spatial light modulator  300 . In the embodiment of  FIG. 5A , a straight line indicates P-polarized light while a dashed line indicates S-polarized light. However, the opposite configuration can also be used (with P and S polarization light reversed) depending upon the selected beamsplitter element. 
         [0027]    Modulated light of the opposite polarization is reflected by each reflective spatial light modulator  300 ,  302 . Light that exits reflective spatial light modulator  300  passes through polarization beam splitter  100  towards projection optics  502 . Light that exits reflective spatial light modulator  302  is reflected by the polarization beam splitter surface  114  and is again reflected by polarization beam splitter surface  113  towards projection optics  502 . In this manner, two modulated images are combined to form a single 2D image of enhanced brightness or a 3D image, depending upon the selected modulation performed by modulators  300  and  302 . 
         [0028]    In  FIG. 5B , incident light from source  400  is processed in a substantially similar manner by polarization beam splitter  100 ′ and spatial light modulators  300 ′ and  302 ′ to form a selected 2D or 3D image output through projection optics  502 ′. 
         [0029]    In  FIG. 6 , an image projection apparatus  700  is formed which can project two modulated images. As in the embodiment of  FIG. 5A , a light source  400  sends incident light through light source optics  410  into a polarization beam splitter  100 ′. Spatial light modulator  300  is positioned on the same side as the incident light surface while second spatial light modulator  302  is positioned on the opposite surface of the polarization beam splitter  100 ′; again, in this embodiment a LCoS spatial light modulator is depicted as light modulator  302  but other reflective spatial light modulators can be selected. Incident light from source  400  passes through incident light source optics  410  and enters polarization beam splitter  100 ′. When the light reaches the polarization beam splitter coated diagonal interface  113 ′, light of one polarization passes through the beam splitter in a straight line and is incident on second reflective spatial light modulator  302 . Light of the opposite polarization is reflected by the polarization beam splitter surface  113  and is again reflected by polarization beam splitter surface  114 ′ into first modulator  300 . As in the embodiment of  FIG. 5A , a straight line indicates P-polarized light while a dashed line indicates S-polarized light. However, the opposite configuration can also be used (with P and S polarization light reversed) depending upon the selected beam splitter element. 
         [0030]    Modulated light of the opposite polarization is reflected by each reflective spatial light modulator  300 ,  302 . Light that exits reflective spatial light modulator  300  is incident on mirror surface  118  and is reflected towards a second set of projection optics  504 . Similarly, light that exits reflective spatial light modulator  302  is also reflected by mirror surface  118  towards the first set of projection optics  502 . In this manner, two modulated images are formed. 
         [0031]    Depending upon the application, the modulated images can be the same or different. For the application of  FIG. 6 , it may be desirable to display the same image in two different locations and, optionally, two different sizes. For example, the main image display can be selected to be SVGA/XGA/720P/WSVGA resolution with a 0.3˜0.4 inch active area; the smaller image projected from projection optics  404  can be of WVGA resolution with around a 0.2 inch active area. Alternatively, it may be desirable to display two different images on a single screen of the same image size and resolution that are “stitched together” to form a single wide screen image. Note that the projection screen can be a flat screen, a wall, a metal screen or any surface which can be projected upon. 
         [0032]    While the foregoing invention has been described with respect to various embodiments, such embodiments are not limiting. Numerous variations and modifications would be understood by those of ordinary skill in the art. For example, the polarization beam splitters of the present invention can be employed in the image projection apparatus disclosed in U.S. patent application Ser. No. 13/233,036 incorporated by reference above. Such variations and modifications are considered to be included within the scope of the following claims.