Abstract:
A polarized light source, is provided, comprising a lamp providing randomly polarized light, an integrator for directing light from the lamp generally along an axis of the integrator, and a wire-grid polarizer disposed at one end of the integrator. This polarized light source recovers a portion of the light produced by the lamp having an undesired polarization. A Liquid Crystal on Silicon (LCOS) light engine including a polarized light source with polarization recovery is also provided.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention is related to an apparatus and method for producing more polarized light of the desired polarization in a Liquid Crystal on Silicon (LCOS) light engine.  
       BACKGROUND OF THE INVENTION  
       [0002]     Liquid Crystal On Silicon (LCOS) imagers are gaining popularity in projection system applications such as rear-projection televisions. LCOS projection systems require polarized light, which can be modulated by the LCOS imager. Typically, a polarizing means, such as a polarizing beam splitter (PBS) is used to separate light by polarization, allowing light of the desired polarization to pass to the LCOS imager, and deflecting light of the undesired polarization away from the projection path. One of the problems with systems using polarized light in a projection system is the inherent waste in such systems by “throwing away” half (or more) of the light being produced by the lamp (i.e., the undesired polarization light) in the process of producing light of the desired polarization.  
         [0003]     One approach to provide polarized light and recapture a portion of the alternately polarized light uses an integrator having small input and output apertures surrounded by reflective surfaces. A polarization means is disposed within the integrator to pass only light of a desired polarization. A quarter-wave plate (QWP) surrounds the output aperture, such that polarization of the light continues to be rotated and the light is reflected back into the integrator until it is both incident on the output aperture and has the desired polarization. This approach, however, is complicated and expensive to produce and maintain. Also, heat generated by the QWP and polarization means is difficult to dissipate, due to their location within the integrator. Additionally, placement of the QWP and polarization means within the integrator requires significant redesign of the entire illumination system.  
       SUMMARY OF THE INVENTION  
       [0004]     A polarized light source, is provided, comprising a lamp providing randomly polarized light, an integrator for directing light from the lamp generally along an axis of the integrator, and a wire-grid polarizer disposed at one end of the integrator. This polarized light source recovers a portion of the light produced by the lamp having an undesired polarization. A Liquid Crystal on Silicon (LCOS) light engine including a polarized light source with polarization recovery is also provided. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]     An exemplary embodiment of the present invention is described below with reference to the drawings in which:  
         [0006]      FIG. 1  shows a polarized light source according to an exemplary embodiment of the invention; and  
         [0007]      FIG. 2  shows a Liquid Crystal on Silicon (LCOS) light engine according to an exemplary embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0008]     In an exemplary embodiment of the present invention, an apparatus provides polarized light and recovers a portion of such light having an alternate polarization. A lamp  10  produces light  2  with random polarization. The lamp  10  may be, for example, a mercury arc lamp having an elliptical or a parabolic reflector (not shown) for directing light in a desired direction, such as a UHP™ lamp from Philips Lighting of Eindhoven, The Netherlands, or another UHP-type lamp.  
         [0009]     The random polarization light  2  enters an integrator  20 , which directs the random polarization light along an axis of the integrator  20 . As is known in the art, the integrator  20  may comprise an elongate hollow structure having reflective inside surfaces that reflect the randomly polarized light  2  to channel it through the integrator  20  from an input aperture  21  disposed at an input end  22  of the integrator  20  to an output aperture  23  disposed at an output end  24  of the integrator  20 . Alternatively, the integrator  20  may comprise a solid transparent elongate structure with a reflective coating, such as silver plating applied to its surfaces to reflect the randomly polarized light  2  to channel it through the integrator  20  from the input aperture  21  disposed at the input end  22  to the output aperture  23  disposed at the output end  24 . For use in a projection system, the integrator  20  preferably has a rectangular output aperture  23  having the same aspect ratio as the desired image to be projected (e.g., 9×16).  
         [0010]     A primary polarizer  30  is disposed at an end  22 ,  24  of the integrator  20 . In the embodiment shown in  FIG. 1 , the polarizer  30  is a reflective wire-grid polarizer operating in the visible light spectrum and disposed at the output end  24  of the integrator  20 , proximate to and aligned with the output aperture  23 . Moreover, in the exemplary polarized light source illustrated in  FIG. 1 , only a clean-up polarization means is required in addition to the polarizer  30 . This is an improvement over existing systems using multiple polarizers. As will be appreciated by those skilled in the art, light of a desired polarization  2 ′ passes through polarizer  30  while light of an alternate or undesired polarization is reflected.  
         [0011]     The reflected light  4 , having an undesired polarization, is channeled back through the integrator  20  to the lamp  10 , where it is reflected back toward the integrator  20 . The inventors have determined that the lamp rotates the polarization of a portion of the reflected light  4  from an undesired polarization to the desired polarization. Thus, recycled light  6 , the portion of light reflected by the lamp  10  and having the desired polarization is channeled through the integrator  20  and passes through the polarizer  30 .  
         [0012]     A portion of the recycled light  6  is again rejected, and again recycled. The total recycled light  6 ′ is equal to the cumulative light of the desired polarization that passes through the polarizer  30  throughout this recycling process. The total polarized light output provided by the exemplary polarized light source is equal to the initial desired polarization light  2 ′ plus the total recycled light  6 ′.  
         [0013]      FIG. 2  shows an exemplary embodiment of the present invention, in which an LCOS light engine is provided using a polarized light source with polarization recovery. A lamp  10  produces random polarization light  2 , which is directed toward the integrator  20  by a parabolic or elliptical reflector (not shown) in the lamp  10 . The random polarization light  2  is channeled by the integrator  20  from an input aperture  21  at an input end  22  to an output aperture  23  at an output end  24 . A polarizer  30  is disposed proximate the output end  24 , aligned with the output aperture  23 . The polarizer  30  is preferably a reflective wire grid polarizer operating in the visible light spectrum. As described above, light having a preferred polarization, as defined by the orientation of the wire-grid of the polarizer  30 , passes through the polarizer  30 , while light having undesired polarization is reflected by the polarizer  30 . Recycled light  6 ′ having the desired polarization also passes through the polarizer. Also, because the wire-grid polarizer  30  is located proximate the end of the integrator  20 , the light from the integrator  20  does not have an opportunity to diverge so that the wire-grid polarizer  30  used in the exemplary embodiment of the present only needs to be slightly larger than the output aperture  23  of the integrator  20 .  
         [0014]     After passing through the polarizer  30 , the polarized light having the desired polarization  2 ′,  6 ′ is focused by a relay lens set  40  into an imaging assembly. The imaging assembly comprises a polarizing beam splitter (PBS)  50  and an LCOS imager  60 . The PBS  50  passes light of one orientation through and deflects light of an opposite polarization. In the embodiment illustrated in  FIG. 2 , light having the desired polarization enters a first face  51  of the PBS  50  and is deflected through a second face  52  onto the LCOS imager  60 . The LCOS imager modulates the light on a pixel-by-pixel basis to form a matrix of modulated pixels of light  90 , rotates the polarization of the modulated light, and reflects the modulated light back through second face  52 . Because the matrix of modulated pixels of light  90  has a polarization opposite to the polarization of the polarized light  2 ′,  6 ′, it passes through the PBS  50  and out of third face  53 . A projection lens set (not shown) then projects the matrix of modulated light  90  onto a screen (not shown) to form a viewable image.  
         [0015]     In addition to directing the polarized light  2 ′,  6 ′ to the LCOS imager  60  and directing the matrix of modulated pixels of light  90  to the projection lens set, the PBS  50  also performs clean-up polarization (i.e., it prevents leakage of light having an undesired polarization from passing to the LCOS imager  60 ). Although the PBS  50  is used for clean-up polarization in the illustrated embodiment, because it is needed for creating the LCOS projection path, other polarization means may be used for clean-up polarization, such as a linear polarizer.  
         [0016]     One advantage of the present invention is that no change is required to existing light engine (or imager) architecture. Thus, a significant cost reduction may be realized as compared to a system that requires a redesign of the light engine architecture. Also, since the wire-grid polarizer  30  used in the exemplary embodiment of the present invention only needs to be slightly larger than the output aperture of the integrator, the surface area of the wire-grid polarizer (and thus its cost) is reduced by at least a factor of four, as compared to a polarizer disposed proximate the imager assembly (i.e., after the relay lens set). Due to its proximity to an end  22 ,  24  of the integrator  20 , the polarizer  30  need only be about the size of the aperture  21 ,  23  to which it is proximate, plus a guard band to prevent edge leakage. Thus, the polarizer  30  can be smaller, and therefore less expensive, than a polarization means disposed proximate an imaging assembly.  
         [0017]     With an integrator having an oversize light pipe (16×9×80 mm), lumen throughput may be increased by about 19%. With a more typically sized light pipe (8×4.5×40 mm) lumen throughput may be increased by about 14%.  
         [0018]     The foregoing illustrates some of the possibilities for practicing the invention. Many other embodiments are possible within the scope and spirit of the invention. It is, therefore, intended that the foregoing description be regarded as illustrative rather than limiting, and that the scope of the invention is given by the appended claims together with their full range of equivalents.