Abstract:
Disclosed is a technology that corrects astigmatism introduced by crossing beam splitting/combining plates. This technology may be used in the context of video projection systems, whereby the plates may be used for combining and/or splitting beams between two or more light modulating panels. In one embodiment, a system is provided for projecting an optical image, where the system includes a lens configured to receive and focus light rays to project the image. The system also includes a first beam splitting/combining plate that refracts light rays passing therethrough, where the refracting creates pluralities of light rays having corresponding optical paths causing each of the pluralities to be focused on corresponding focal planes by the lens. In addition, the system also includes a second beam splitting/combining plate configured to receive the pluralities of rays from the first beam splitting/combining plate, where the second beam splitting/combining plate is oriented with respect to the first beam splitting/combining plate so as to compensate the corresponding optical paths of the pluralities of rays such that each of the pluralities are focused on substantially the same focal plane by the lens.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This Application claims the benefit of U.S. Provisional Application Ser. No. 60/470,158, filed on May 13, 2003, and entitled “Astigmatic Correction in LCoS Video Projection Color Management Architectures Using Shared Plate Beam Splitters,” which is commonly assigned with the present application and incorporated herein by reference for all purposes. 
     
    
     
       TECHNICAL FIELD  
         [0002]    Disclosed embodiments herein relate generally to multiple panel color management architectures for Liquid Crystal on Silicon (LCoS) video projection systems. Specifically it relates to the efficient use of two plate-beamsplitters tilted about orthogonal axes relative to each other to correct for astigmatic aberrations.  
         BACKGROUND  
         [0003]    ColorLink, Inc., the assignee of the present disclosure, and others have developed and even patented several architectures that use a single polarizing beam splitter (PBS) for color separation and recombination between two reflective liquid crystal on silicon (LCoS) microdisplay panels. For example, see U.S. Pat. No. 6,183,091, U.S. Published Patent Application No. US 2001/0000971, U.S. patent application Ser. No. 10/000,227, and U.S. patent application Ser. No. 10/713,548, all commonly owned with the present disclosure and incorporated herein by reference in their entirety for all purposes. In addition, other patents exist in the field to well known companies, such as Sanyo Electric Co., Ltd., Advanced Digital Optics, Inc., Sony and JVC. In addition, as evidenced in such prior references, the use of plate wire grid PBSs for combining and separating light with isolated panels is also known. However, when employing such PBSs in these systems, the refraction imparted by the thickness and composition of the PBS plate typically results in an “astigmatism” in the final projected image where portions of the light rays are focus on one focal plane while other light rays are focus on one or more other focal panels. Accordingly, what is needed in the art is an architecture for optical display systems that does not suffer from such astigmatism in the projected image.  
         BRIEF SUMMARY  
         [0004]    Disclosed is a technology that corrects astigmatism introduced by crossing plates, including crossing plates in a two-panel mode. This technology may be used in the context of video projection systems, and in certain embodiments the second plate may be used for combining and splitting beams. In one embodiment, a system is provided for projecting an optical image, where the system includes a lens configured to receive and focus light rays to project the image. The system also includes a first beam splitting/combining plate that refracts light rays passing therethrough, where the refracting creates pluralities of light rays having corresponding optical paths causing each of the pluralities to be focused on corresponding focal planes by the lens. In addition, the system also includes a second beam splitting/combining plate configured to receive the pluralities of rays from the first beam splitting/combining plate, where the second beam splitting/combining plate is oriented with respect to the first beam splitting/combining plate so as to compensate the corresponding optical paths of the pluralities of rays such that each of the pluralities are focused on substantially the same focal plane by the lens. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    For a more complete understanding of this disclosure, and the advantages of the systems and methods herein, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:  
         [0006]    [0006]FIG. 1 illustrates a conventional projection system suffering from astigmatism of the projected image;  
         [0007]    [0007]FIG. 2 illustrates one embodiment of an anastigmatic compensation scheme according to the principles disclosed herein that employs double plates tilted about orthogonal axes, both axes of which are orthogonal to the optic propagation axis of the system; FIG. 3 illustrates one embodiment of a system having an architecture in which the double plate compensation is part of the optical combining architecture; and  
         [0008]    [0008]FIG. 4 illustrates another embodiment of a projection system incorporating the disclosed principles of astigmatism correction. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0009]    Referring initially to FIG. 1, illustrated is a conventional projection system  100  suffering from astigmatism of the projected image. The illustrated system employs a single color panel  110  and a single tilted plate  120 . In such a system, when light rays  130  pass through the plate  120 , various rays are refracted so as to travel is different directions. As a result, when the light rays  130  pass through a focusing lens  140 , some lights rays  130  are focused on one image focal plane  150   a,  while other rays  130  are focused on another focal plane  150   b.  Such an astigmatism results because rays  130  in the vertical and horizontal dimensions are imaged in the different planes  150   a,    150   b . Thus, in any one plane the image is either uniformly blurred or sharp in only one of the orthogonal dimensions.  
         [0010]    Turning now to FIG. 2, illustrated is one embodiment of a projection system  200  employing a double plate solution to the astigmatism problem. As with the system  100  in FIG. 1, this system  200  also includes a color panel  110  and a first plate  120  oriented in a predetermined manner with respect to the color panel  110 . Also illustrated are light rays  130  projecting through the plate  120  and through the lens  140 .  
         [0011]    In accordance with the principles of the present disclosure, the system  200  further includes compensation plate  210  positioned between the plate  120  and the projection lens  140 . The additional second plate  210  refracts the light rays  130  in the vertical plane to an optical path similar to the optical path of the light rays in the horizontal plane. Thus, an uncorrected lens  140  is allowed to converge the two sets of rays in a single focal plane. In other words, sharp imaging is possible without the astigmatism.  
         [0012]    Among other things, the technology described in this disclosure provides for using a first order astigmatism correction technique of crossing two tilted polarizing beamsplitter (PBS) plates in a reflective microdisplay video projection system. In doing this, the advantages of using plate PBSs in such projection systems are had without penalty with regard to astigmatic correction of any projection optics. This technique and architecture may be used, for example, in systems wherein a single plate PBS is used to split and combine light from two panels. Such systems are attractive from physically compact and cost standpoints. In certain aspects, the correction is adequate for certain color channels, although it may be inadequate or not required for others. Due to the color variation of human eye&#39;s imaging performance, certain inadequacies of performance in the optical system become tolerable or acceptable.  
         [0013]    Looking now at FIG. 3, illustrated one embodiment of a system  300  having an architecture in which the double plate compensation is part of the optical combining architecture. More specifically, in this embodiment of the system  300  three panels  310 ,  320 ,  330 . Of course, in other embodiments, the disclosed techniques may be used with any pair of the panels, of course any single panel is also compensated in the illustrated embodiment. Note also the plate  340  that is shared between panels  320  and  310  has its reflecting surface buried. This allows correction for both panels  310 ,  320  by virtue of the reflected rays from panel  320  passing through the tilted plate between it and the buried reflecting layer. In certain embodiments, this may or may not be necessary since the panel  320  that has light reflected off this compound reflector may not require very good imaging.  
         [0014]    In short, in this approach any pair of panels  310 ,  320  are compensated for astigmatism by the plate beam splitter  340 . Note that the reflecting surface  350   a  of the plate  350  is facing the projection lens  360  and the lower panel  330 . For this reason the lower panel  330  imaging path does not need astigmatic compensation. However, as light rays project from either or both panels  310  and  320  and pass through plates  340 , the astigmatism in the light rays in introduced. The plane of the compensation plate  350  is rotated about an axis that is orthogonal to that of the plates  340 . Moreover, to assist in accurately compensating the astigmatism introduced by the plates  340 , the overall thickness of the compensation plate  350  may beneficially be selected to match the thickness of the plates  340 . Once compensated, the light rays travel into the projection lens  360 , which may now focus all the light rays on the same focal plane.  
         [0015]    Referring now to FIG. 4, illustrated in another embodiment of a projection system  400  incorporating the disclosed principles of astigmatism correction. The illustrated embodiment is a low cost three-panel architecture having red  410 , blue  415 , and green  420  panels. This embodiment takes in white light  425 , which is passes through a pre-polarizer  430  before being selectively polarized with a ColorSelect® green/magenta filter  435 . S-polarized magenta light then reflects off a plate PBS  440  (denoted as a wire grid PBS (WGPBS) in this embodiment) before being split with a polarization preserving dichroic mirror  445 . The dichroic mirror  445  is a red/blue mirror for use with the red  410  and blue  415  panels. Reflected light is either of the same polarization, i.e. in their OFF-state, or orthogonal, the ON-state. Similar polarizations get recombined by the dichroic plate, but are then reflected back toward the source by the WGPBS. Orthogonal polarizations are also recombined at the dichroic mirror  445 , but pass through the WGPBS  440  and are imaged onto an image plane via a projection lens  450 . An output green/magenta ColorSelect® filter  455  and p-polarizer  460  allow transmission of the magenta light.  
         [0016]    Green light is left p-polarized after the input G/M filter  435  and is transmitted through the WGPBS  440  toward the green panel  420 . In the OFF-state, the panel  420  does not change the green light&#39;s polarization state by reflection, which causes most of the light to be transmitted by the WGPBS  440  toward the source. The light that is reflected by the WGPBS  440  gets rotated to s-polarized light by the output G/M filter  455  to be absorbed by the output polarizer  460 . In its ON-state the green light becomes s-polarized by reflection by the panel  420  and is then reflected off the WGPBS  440  toward the projection lens  450 . The output G/M filter  455  rotates it to p-polarization allowing transmission through the output polarizer  460  where it hits the projection lens  450 .  
         [0017]    This embodiment is a very low cost embodiment, and potentially high performance, architecture employing the principles disclosed herein. The crossed plates (plate  440  and  445 ) act to correct the astigmatism as per the concept outlined above, allowing use of WGPBSs  440  that do not need quarter wave or similar geometric compensation. As before, the astigmatism is removed by having the WGPBS  440  rotated orthogonally to the plate  445 . The high angle performance of the WGPBS  440  would also allow small panels or larger lamps to be used with the related throughput improvements over conventional PBSs. One downside of the architecture is an increase in optical path length and the ray angles due to the low index of air in the optical path compared this the high index of alternative lead glass prisms. This in turn puts demand on the projection lens design and increases optical component sizes in telecentric systems. One solution, which takes advantage of the angular tolerance of the WGPBS  440  and ColorSelect® filters  435 ,  460 , is to go off-telecentric and use field lenses at the panels  410 ,  415 ,  420 . In addition, the dichroic coating in this embodiment may be graded in this or other embodiments to avoid or mitigate non-uniformity issues.  
         [0018]    Turning finally to FIG. 5, illustrated is an embodiment of a projection system  500  employing two PBS plates (again denoted as WGPBSs), and which would not demand polarization preservation by the dichroic plate(s). As with the system  400  in FIG. 4, this system  500  includes red  510 , blue  515 , and green  520  panels for processing incoming white light  525 . However, this embodiment now employs two WGPBSs  530 ,  535  to correct any astigmatism problems present in the propagated light rays.  
         [0019]    Generally, when using PBSs adjacent all color panels  510 ,  515 ,  520  there is a requirement that light cannot enter and exit through the same PBS port. Because of this, the illustrated embodiment provides a little more complicated (but nevertheless low cost) 3D arrangement to illuminate the panels  510 ,  515 ,  520 . Also, to ensure the same polarization of the red and blue light entering and leaving the shared WGPBS  530 , RB ColorSelect® filters  550 ,  555  are used to sandwich the R/B WGPBS  530 , as shown.  
         [0020]    This embodiment of the disclosed technique operates first by splitting green from the magenta light via the input dichroic plate  540 . A mirrors  560   a  then direct the green light through a WGPBS  535  toward the green panel  520 , while other mirrors  560   b  direct the magenta light toward a pre-polarizer  565  and RB ColorSelect® filter  555 . The RB filter  555  then rotates the polarization of red light relative to the blue allowing separation and recombination with a second, shared WGPBS plate  530 . After reflecting off the Red and Blue panels  510 ,  515 , the magenta light is recombined to be either transmitted toward the first WGPBS  535  if the panels  510 ,  515  have altered the polarization states, or reflected back to the source in the case where the panels  510 ,  515  are OFF and do not alter polarization. The transmitted light is recombined to be uniform p-polarized by a second RB filter  550  and passes though the first WGPBS  535 , an output G/M filter  570 , and an output clean-up polarizer  575  so that it may be imaged onto the screen by the projection lens  580 . The green light is either projected or not, as discussed with respect to the embodiment shown in FIG. 4.  
         [0021]    Also, as mentioned above, the thicknesses of the WGPBSs  530 ,  535  employed to correct astigmatism will typically be made near equivalent. That is the combined thickness of the PBS substrate and its covering glass plate  530  with the near equivalent to the thickness of the substrate of the PBS plate  535 . As for the embodiments in FIGS. 3 and 4 the reflecting surface is sandwiched between its substrate and this cover glass plate to compensate astigmatism for both transmitted and reflected light. In addition, as shown, there are internal lenses  585  that allow off-telecentric illumination, which minimizes component size for any given illumination f/#. This could be extended further to field lenses directly adjacent the panels as described for the first embodiment. Also, for the embodiments in FIGS. 4 and 5, and as stated earlier, the blue port may not require astigmatic compensation. In this case, the reflecting surface of the WGPBS  530  would be facing the blue panel  515 . Removing the covering glass plate would act to minimize ghost images on screen from both the red and blue channels.  
         [0022]    While various embodiments of projection techniques in accordance with the principles disclosed herein, have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the invention(s) should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with any claims and their equivalents issuing from this disclosure. Furthermore, the above advantages and features are provided in described embodiments, but shall not limit the application of such issued claims to processes and structures accomplishing any or all of the above advantages.  
         [0023]    Additionally, the section headings herein are provided for consistency with the suggestions under 37 CFR 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the invention(s) set out in any claims that may issue from this disclosure. Specifically and by way of example, although the headings refer to a “Technical Field,” such claims should not be limited by the language chosen under this heading to describe the so-called technical field. Further, a description of a technology in the “Background” is not to be construed as an admission that technology is prior art to any invention(s) in this disclosure. Neither is the “Brief Summary” to be considered as a characterization of the invention(s) set forth in issued claims. Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is only a single point of novelty in this disclosure. Multiple inventions may be set forth according to the limitations of the multiple claims issuing from this disclosure, and such claims accordingly define the invention(s), and their equivalents, that are protected thereby. In all instances, the scope of such claims shall be considered on their own merits in light of this disclosure, but should not be constrained by the headings set forth herein.