Abstract:
An apparatus and method is discloses for providing a substantially uniform, homogenous, polarized light. A beam of substantially un-polarized light is provided by a conventional light source. The un-polarized light is converted to at least four real images by using a light pipe and appropriate lensing. Each of the real images have a light region and one more dark regions at a first image plane. A portion of light from a light region in each real image is directed to one or more dark regions in each of the real images in a polarization dependent manner and the altered image is into a single image of substantially polarized light.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present invention claims priority from U.S. Provisional Patent Application No. 60/562,370 filed Apr. 15, 2004, entitled “Illumination System Utilizing Polarization Recovery”, and U.S. Provisional Patent Application No. 60/569,746 filed May 10, 2004, entitled “Illumination System Utilizing Polarization Recovery” which are incorporated herein by reference for all purposes. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to an illuminating apparatus such as is commonly used in a projection display system and more particularly to an illumination system that provides an enhanced polarized output.  
       BACKGROUND OF THE INVENTION  
       [0003]     In projection display systems the function of the illumination system is to provide uniform illumination to a spatial light modulator (SLM). Typically the illumination system in a projection display consists of a light source such an arc lamp or light emitting diodes (LEDs), a homogenizer of some type for providing homogenized light at the SLM, and a relay lens for providing a beam having a desired size, shape and telecenetricity.  
         [0004]     In some projection display systems, liquid crystal spatial light modulators are used, and it is desirable to illuminate such modulators with polarized light. A polarizer can be added to the illumination system to filter the light, so as to provide a polarized beam to the SLM, however this method has an associated loss of 50% of the light, as one polarization state is absorbed or otherwise lost. Illumination systems utilizing polarization recovery techniques seek to take this lost light and covert it to the desired polarization state so it can be recovered and utilized, thus increasing the efficiency of the system and hence the brightness of projection. Prior art methods of performing this polarization recovery and conversion include the use of lenslet arrays and a polarization conversion array (PCA) or the use of a polarization converting light pipe (PCLP).  
         [0005]     Projection displays incorporating transmissive liquid crystal SLMs typically use a lenslet array and a PCA; some reflective liquid crystals SLMs also known as liquid crystal on silicon (LCoS) also use a lenslet array and PCA. Prior art  FIG. 1  of the accompanying drawings is a schematic view of the essential portions of another liquid crystal projector disclosed in U.S. Pat. No. 6,139,157. In  FIG. 1 , light emitted by a lamp  201   a  is reflected toward an image display element  207  by a reflector  203 , and enters a first lens array  201   b  comprising a plurality of lenses arranged into the form of a grating. The aforementioned light is condensed near the lenses of a second lens array  202  similar in construction to the first lens array  201  and comprising lenses having the same degree of focal length as the interval between the first lens array  201  and the second lens array  202  and arranged into the form of a grating by the lenses of the first lens array  201 , and is caused to be transmitted through the lenses of the second lens array  202 , whereafter it enters a polarization converting element  204 .  
         [0006]     The light beam which has entered the polarization converting element  204  is separated into different polarized components (S component and P component) by a polarization separating surface  204   a , and S-polarized light reflected by the polarization separating surface  204   a  is reflected by a reflecting mirror  204   b  and is transmitted through a half wavelength plate  205 , whereby it is converted into the same polarized state as the P-polarized light transmitted through the polarization separating surface  204   a.    
         [0007]     P-polarized light beams having the same directions of polarization which have emerged from the polarization converting element  204  illuminate the image display element  207  provided near the focus position (the surface to be irradiated) of a condensing lens  206 , through the condensing lens  206 . An image displayed by the image display element  207  is projected onto a predetermined surface by a projection lens.  
         [0008]     On the other hand, the liquid crystal projector shown in  FIG. 1  forms a plurality of secondary light source images by a the fly-eye type optical integrator and superposes the light beams from the plurality of secondary light source images one upon another on the surface to be irradiated to uniformly illuminate the surface  207  to be irradiated.  
         [0009]     The liquid crystal projector of  FIG. 1 , however, requires first and second lens arrays of the same degree of size as an opening in the reflecting mirror  203  and therefore, the entire apparatus tends to be bulky.  
         [0010]     There have heretofore been proposed various liquid crystal projectors for illuminating a liquid crystal panel by a light beam from a light source, and enlarging and projecting image light such as transmitted light or reflected light from the liquid crystal panel onto a screen or a wall by a projection lens.  
         [0011]     Usually the liquid crystal panel utilizes the polarizing characteristic of liquid crystal. Therefore, usually, polarizing filters such as a polarizer and an analyzer are provided before and behind the liquid crystal panel. The polarizing filter has the characteristic of transmitting therethrough polarized light polarized in a particular direction of polarization of incident light, and intercepting polarized light of which the direction of polarization is orthogonal thereto. Therefore, the light from the light source utilized in the liquid crystal projector has had at least a half thereof intercepted by the polarizer which is a polarizing filter and thus, the brightness of the image projected onto the screen or the wall has not been sufficient.  
         [0012]     Projection systems being manufactured incorporating LCos SLMs currently employ PCLPs in their illumination system. The most common form of PCLP includes a reflector in the form of a light pipe having two PBSs at the input end of the light pipe. Light from the light source is incident upon one PBS. The PBS spits the light into S and P polarization states. P-polarized light is transmitted and the S-polarized light is reflected. The P-polarized light is then incident upon a rotator in the form of a half-wave retarder which is oriented such that the emerging light has its polarization axis rotated by 90 degrees such that it has been converted to S-polarized light. The S-polarized light split off by the PBS is then incident upon the second PBS where it is reflected and re-directed into the light pipe. S-polarized light thus emerges from both PBSs and is then homogenized by the light pipe. The PCLP tends to be a lower cost system than the aforementioned lenslet array PCA, but suffers from lower contrast, approximately 6:1 emerging from the PCLP as compared with the 20 to 30:1 from the lenslet array PCA, and lower efficiency; about 72% compared with approximately 80%.  
         [0013]     Prior art U.S. Pat. No. 6,139,157 incorporated herein by reference is directed to an illuminating apparatus which appears to have several elements in common with the instant invention. A light source, light pipe, lenses, a reflective polarization beam splitter, a polarization conversion element and a target plane are all shown in this patent. Notwithstanding, the invention shown in  FIG. 13  of this patent, shown herein as  FIG. 2 , is absent an obstruction or opaque region at an input end of the light pipe. The applicant believes that by not providing a light pipe with masked region at the input end, a uniform polarized beam will not result. By providing the obstruction or masked region at an input end of the light pipe, and ensuring that the polarization translation and separation occurs at predetermined locations in dependence upon the configuration of the masked region, a desired uniform polarized beam will result. The description of  FIG. 13  is as follows:  
         [0014]     “The polarization converting element  74  shown in  FIG. 13  uses a transparent plate provided with a polarization separating surface  701  on the surface thereof as reflecting means for bending the optical path by 90.degree. and a reflecting mirror  702  on the back thereof, and the optical paths of S-polarized light reflected by the surface and P-polarized light reflected by the back are deviated in parallelism to each other, and a half wavelength plate  703  is disposed on the optical path of the S-polarized light reflected by the surface (or the optical path of the P-polarized light reflected by the back) to thereby uniformize the directions of polarization of the two lights.” 
         [0015]     Also, in the system of prior art  FIG. 13 , half wavelength plates  703  are periodically provided on the flat surface of the plano-convex integrated lens  5  to thereby convert the direction of polarization of the incident S-polarized light into the same direction as the P-polarized light.” 
         [0016]     It is unclear whether the structure shown in  FIG. 13  functions as it is intended to. Since there is no structure taught which blocks light from the input end, it would appear as if all light of one linear polarization incident upon surface  701  is reflected upon the entire receiving surface of 5. It would also appear as if all light reflected from surface  702  is reflected upon the same region of 5. Therefore it is not clear how this embodiment works to selectively rotate one linear polarization and not the other.  
         [0017]     It is an object of this invention to provide a system and method for uniformly illuminating a spatial light modulator which incorporates a novel, relatively low cost method of polarization recovery.  
         [0018]     It is an object of this invention to provide a simple, low cost system for providing substantially uniform polarized light for use in a projection display system.  
       SUMMARY OF THE INVENTION  
       [0019]     In accordance with an aspect of this invention an illuminating system is provided comprising: 
        a) a light pipe having an input end for receiving substantially unpolarized light from a light source and an output end for outputting reflected light, the input end having a masked portion and an unmasked portion, the masked portion for substantially preventing light from entering the light pipe and the unmasked portion for allowing light to enter into the light pipe, wherein the light pipe is for reflecting light and for producing a plurality of virtual images, each virtual image having a dark non-illuminated region corresponding to the masked region and an illuminated region corresponding to the unmasked region adjacent to the dark region;     b) a lens optically coupled to the output end of the light pipe for providing a plurality of real images at or about an image plane, wherein each real image corresponds to one of the virtual images such that each real image has a dark non-illuminated region corresponding to the masked portion and an illuminated region corresponding to the unmasked portion, wherein the plurality of real images together form an array of dark regions and illuminated regions; and,     c) a polarization converting system optically coupled to receive light from the array of dark and light regions and for filling the dark regions with some light from the illuminated regions in a polarization dependent manner, such that some light is spatially translated from a illuminated region to a dark region, said polarization converting system for converting light so that substantially all of the light passing therethough is uniformly polarized.        
 
         [0023]     At least 80% of the light that passes through the polarization converting system is uniformly polarized and preferably 90% or greater.  
         [0024]     In accordance with the invention there is provided an illuminating system comprising: 
        a) a light source;     b) a light pipe having an end thereof masked in a predetermined manner so as to produce virtual images each having a dark region corresponding to the masked region and having an unmasked illuminated region adjacent the masked region;     c) a lens coupled to the light pipe for producing real images corresponding to the virtual images wherein each real image has an illuminated region and an un-illuminated region;     d) a polarization conversion system for receiving light form the lens and for filling the un-illuminated regions with light from adjacent illuminated regions in a polarization dependent manner so as to produce a polarized light beam formed from the real images.        
 
         [0029]     In accordance with another aspect of the invention, there is provided, in a projection display system, a method of providing a substantially uniform, homogenous, polarized light source comprising the steps of: 
        providing a beam of substantially un-polarized light;     converting the un-polarized light to at least four real images each having a light region and one more dark regions at a first image plane;     directing a portion of light from a light region in each real image to one or more dark regions in each of the real images in a polarization dependent manner; and,     homogenizing the altered image into a single image of substantially polarized light.       
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]     Exemplary embodiments of the invention will now be described in conjunction with the figures in which:  
         [0035]      FIG. 1  is a side view of a prior art system for polarization conversion using a fly&#39;s eye array and a PCA.  
         [0036]      FIG. 2  is a side view of a polarization conversion system as shown in U.S. Pat. No. 6,139,157.  
         [0037]      FIG. 3  is a side view of a preferred embodiment of the illumination system in accordance with this invention.  
         [0038]      FIG. 4   a  is a diagram illustrating an input end face of a light pipe of  FIG. 3 .  
         [0039]      FIG. 4   b  is a diagram of multiple images of the light pipe input face, as would be seen at the polarization converter plane.  
         [0040]      FIG. 5  is a diagram showing the S and P-polarized light stitched together side-by-side on the polarization converter plate.  
         [0041]      FIG. 6  is an alternative embodiment of the illumination system shown in  FIG. 3 .  
         [0042]      FIG. 7  is a diagram of the polarization separator and translator of the system of  FIG. 6 . 
     
    
     DETAILED DESCRIPTION  
       [0043]     Referring now to  FIG. 3  an illumination system in accordance with a preferred embodiment of this invention is shown. A light source  30  having a reflector  32  is optically coupled to a light pipe  34 . The light pipe is conventional in all respects except one. The light pipe  34  has a masked region at its input end, not seen in  FIG. 3  but clearly illustrated in  FIG. 4   a . The light pipe  34  may be a light tunnel or other light reflecting structure having an input end that can be masked and an output end for outputting light. A polarization separator and translator  36  is disposed to receive substantially collimated light from the light pipe via lenses  35   a  and  35   b  in the form of a plurality of real images. A similar element  74  is shown in detail in  FIG. 2 . Each real image is a reduced image of the input end of the light pipe. A more detailed description of this follows. The polarization separator and translator  36  is disposed to separate the S and P-polarized light and then spatially translate the light in a polarization dependent manner such that the S-polarized light and P-polarized light is incident upon the polarization converter (PC) plate  38  in a predetermined spatial pattern. The polarization converter  38  is a plate having striped regions  52  with a half wave retarding material present in alternating regions. Light of one linear polarization falls on the striped regions  52  while light of the other linear polarization passes through glass regions  50  unaffected as is shown in  FIG. 5 . Relay elements  39   a  and  39   b  ensure that the light emerging from the PC  38  is uniformly distributed over the SLM plane  31 .  
         [0044]     In operation light from the light source  30  is focused by the reflector  32  on the input end  33   a  of the light pipe or light tunnel  34 . Light traverses the light pipe from the input end face to the output face  33   b , experiencing multiple reflections from the walls of the light pipe. Total internal reflection (TIR) from the light pipe walls prevents light from escaping the light pipe. After multiple reflections within the light pipe  34  the light becomes homogenized, with the output face  33   b  of the light pipe representing an essentially uniformly emitting surface. A hollow tunnel made up of mirrors can also be used, and is considered to be an equivalent. The light pattern at the output end face  33   b  is then imaged by the illumination relays  37   a ,  37   b ,  39   a  and  39   b  on to the SLM  31 . In this embodiment the polarization separator translator is disposed after the first two relay elements  37   a  and  37   b . This region is essentially collimated space where the light exiting the relay element  37   b  is collimated. The function of the polarization separator translator  36  is to separate the orthogonal linear polarization states and then shift or translate one laterally sideways with respect to the other. As is shown in  FIG. 3 , unpolarized light is incident upon a PBS  36   a , for example a Moxtek Proflux™ wire grid polarizer with S-polarization being reflected and Polarization being transmitted. The P-polarized light is then reflected off a mirror  36   b  at the back of the translator  36  through the PBS. The separation of the mirror  36   b  from the PBS of translator  36  causes the P-polarized light to be shifted sideways relative to the S-polarized light. This separation needs to be controlled precisely as will be shown later.  
         [0045]     As is shown in  FIG. 4   a  the input end face of the light pipe has two masked opaque regions  42   a  and  42   b  and has a light transmissive window  42   c , therebetween. Preferably, the width of the windowed region  42   c  is d/2 and the width of the masked regions are each d/4. The purpose of the masked regions is to yield an array of illuminated or light regions  45   a  and non-illuminated or dark  45   b  regions on the image plane. The light pipe receives the real image shown in  FIG. 4   a , and as light bounces within the light pipe a plurality of smaller virtual images are constructed, each a smaller version of the real image shown in  FIG. 4   a , at a virtual image plane about the input end of the light pipe. These virtual images would be captured as real images in the image plane of the PC  38 . Since the light impinging upon the separator and translator  36  is substantially collimated, the same real images corresponding to and being conjugates of the virtual images would be present upon the separator and translator  36 . Hence, the pattern in  FIG. 4   b  would be imaged onto the elements  36  and  38 .  
         [0046]     However, the polarization separator and translator  36  alters the multiple real images in a predetermined manner, such that approximately half the light within illuminated regions  45   a  is directed to non-illuminated or dark regions  45   b  by way of spatial translation, so as to stitch together side-by-side S-polarized light and P-polarized light. Conveniently, the PC  38  at a plane which is conjugate to the input face  33   b  of the light pipe as formed by the intervening relay optics has its retarding striped regions  52  coincident with only the S or P polarized light but not both. In summary the real image shown in  FIG. 4   b  is altered by the polarization separator and translator  36  so that light is evenly distributed in a polarization dependent manner such that the dark regions are illuminated with light of an orthogonal polarization state to the illuminated regions after polarization dependent translation and conversion occurs. If designed correctly, both the S-polarized light and the P-polarized light will appear as alternating bars of bright and dark regions. The P-polarized light will be shifted sideways relative to the S-polarized stripes such that the bright P-regions fall on top of the S-polarized dark regions with little or minimal overlap and vice versa.  
         [0047]     In this exemplary embodiment the PC  38  is designed to transform the P-polarized light into S-polarized light. One way to perform this is to laminate strips of half wave retarder material such as plastic retarder film, quartz, mica or other birefringent materials to a transparent substrate such as glass in a pattern which matches the pattern of the of the bright regions of the P-polarized light. The retarder is designed to rotate or retard the polarization axis by 90 degrees. Light emerging from the PC  38  will then be S-polarized light. Of course S-polarized light can be transformed into P polarized light in a similar manner if desired.  
         [0048]     Although the embodiments described heretofore have been described with respect to linearly polarized light, embodiments may be envisaged wherein circularly polarized light is used, so that all of the output light is uniformly circularly polarized. The element  36  would have to be replaced by a plate that will discriminate and translate with regard to right and left handed circularly polarized light, rather than S or P-polarized light.  
         [0049]     Turning now to  FIG. 6  an alternative embodiment of the invention is shown, wherein the polarization separator and translator  36  and polarization converter  38  of  FIG. 3  are replaced with an optional mirror  62  for folding light incident thereon, and for redirecting the light to a polarization separator, translator, and converter  64  shown in more detail in  FIG. 7 . Unpolarized light having S and P linear polarization components is incident upon an array of elements as shown. The absence of light between the arrows indicating beams of S and P-polarized light is due to the dark regions as a function of the input of the light pipe  32  or reflector being masked. The light shown by way of example, as three arrows, representing three beams is incident upon polarization beam splitters  75   a ,  75   b , and  75   c . S-polarized light is reflected while P-polarized light passes therethrough. The reflected S-polarized light reflected from  75   a ,  75   b  and  75   c  is then directed in a same direction as the P-polarized light and becomes P-polarized light as it passes through a retarder. Retarders  77   a ,  77   b , and  77   c  provide the required retardation so that all of the output light becomes P-polarized. This embodiment of the invention also takes advantage of the plural real images which represent the masked input to provide illuminated and non-illuminated regions. In this embodiment individual beams are split and stitched, one beside the other, to fill in the dark regions; this also provides an opportunity to rotate the S-polarized light while not affecting the P-polarized light after filling in the dark regions.  
         [0050]     In the embodiments shown in  FIG. 3  and  FIG. 7  the light pipe or reflector has an obstruction preventing light from entering. This appears in the plural real images directed into the collimated space as images having dark and light regions. These dark regions can subsequently be filled in with about 50% of the light present in the light regions, in a polarization dependent manner, such that the remaining light is of one linear polarization and the translated shifted light is of an orthogonal polarization. One of the two linear polarizations can conveniently be retarded or rotated to be the same as the other, thereby providing a substantially uniform beam having a same polarization state.  
         [0051]     Of course numerous other embodiments may be envisaged without departing from the spirit and scope of the invention.