Abstract:
A method and apparatus for enhancing performance of a projection system by blocking incident angle light rays without increasing the F-number of the system includes a skew filter having a shaped aperture. The skew filter blocks a substantial portion of the skew light rays while allowing other light rays to pass through the projection system. It is emphasized that this abstract is provided to comply with the rules requiring an abstract which will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or the meaning of the claims.

Description:
[0001]     This is a non-provisional application claiming priority of Provisional Patent Application No. 60/533,163 filed on Dec. 29, 2003, which is hereby incorporated by reference in its entirety as if fully set forth herein. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention generally relates to performance enhancement in optical, image projection and communications systems. Specifically, the present invention relates to projection displays that incorporate polarized light sources and modulate the throughput of incident light.  
       BACKGROUND OF THE INVENTION  
       [0003]     In existing optical systems and other applications involving processing light rays, contrast and corner color uniformity limitations exist due to light rays that are incident to a polarizing beam splitter (PBS) at skew angles. Contrast performance of polarizing beam splitter (PBS)-based projection systems is limited by the angular performance of the PBS components. A typical PBS includes right-angle prisms that have multi-layer stacks coated on the surfaces corresponding to the hypotenuse of the right-angles of the adjoining prisms. The combination of right angle prisms and multi-layer stacks are designed so that at a 45° incidence angle to the adjoining surface, the incident beam will satisfy the Brewster&#39;s angle condition for the p-polarization component of the incident beam such that most of the p-polarization component is transmitted while the s-polarization component of the incident beam is rejected. This occurs because the spectral width of rejection bands for a multi-layer stack is different for s- and p-components of an incident beam. However, for a converging or diverging beam, the problem of depolarization, or the transmission and rejection of unwanted light, occurs due to the fact that even if the incident beam is purely polarized with respect to the incident plane of the PBS layer, the non-collimated nature of the incident light results in components which have propagation vectors that are not orthogonal to either of the p- or s-planes of the PBS layer. The result is a rotationally asymmetric transmission contrast ratio.  
         [0004]     These skew rays therefore degrade the contrast ratio by depolarizing the incident beam, which then leaks through other polarizing and analyzing elements in the projection system. Degradation in contrast limits the ability to display colors in a resulting image.  
         [0005]     One existing method of addressing these contrast and color corner uniformity limitations includes increasing an F-number of the optical system. Increasing the F-number blocks the incident-angle rays, resulting in increased contrast but also reduced light throughput. As the amount of light entering the optical system is reduced, the contrast is reduced, making the resulting image less and less bright.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention provides a method and apparatus for enhancing performance of a projection system by blocking incident angle light rays without increasing the F-number of the system. In one embodiment of the present invention, a method of reducing leakage of unwanted polarization in a projection apparatus comprises introducing a light source to the projection apparatus for producing a plurality of light rays, the plurality of light rays including orthogonally-polarized light rays and skew light rays having multiple polarization components, and preventing the transmission of a substantial portion of the skew light rays to a polarization apparatus by applying a skew filter at a filter position in the projection apparatus, the skew filter including an aperture with a shape configured to allow the orthogonally-polarized rays to pass into the polarization apparatus and to block the skew light rays from entering the polarization apparatus by following a constant contrast curve of a polarizing beam splitter for a cone of light incident on to the polarizing beam splitter. In another embodiment of the present invention, a method of increasing contrast in an image processing apparatus without increasing the F-number comprises rejecting a substantial portion of a plurality of skew rays introduced by a light source by applying a skew filter at a filter position in the image processing apparatus, the light source introducing a plurality of orthogonally-polarized rays and a plurality of skew rays having multiple polarization components, and processing the plurality of orthogonal rays in a polarization apparatus, the polarization apparatus including a plurality of polarizing beam filters for transmitting a plurality of orthogonal rays, wherein the skew filter includes an aperture having a shape configured to follow a constant contrast curve of at least one polarizing beam splitter in the plurality of polarizing beam splitters for a cone of light incident thereto.  
         [0007]     In another embodiment of the present invention, an image projection apparatus comprises a light source, the light source producing a plurality of light rays including skew light rays and orthogonally polarized light rays, a polarization apparatus including at least one polarizing beam splitter, a plurality of lenses through which the plurality of light rays passes to the polarization apparatus, and a skew filter positioned at a filter position and having a shaped aperture for blocking the passage of a substantial portion of the skew light rays to the polarization apparatus while allowing a substantial portion of the orthogonally polarized rays to pass through to the polarization apparatus, wherein the shaped aperture of the skew filter has a shape which follows a constant contrast curve of the at least one polarizing beam splitter for a cone of light incident to the at least one polarizing beam splitter. Another embodiment of the present invention includes a contrast enhancement apparatus in an image projection system comprising an angular light rejection plate configured to block a substantial portion of angular light from entering a polarization apparatus and to allow orthogonally polarized light to enter the polarization apparatus, the polarization apparatus having at least one polarizing beam splitter, the at least one polarization beam splitter having a right angle prism having multi-layer filter stacks, the polarization apparatus configured to process the orthogonally polarized light to produce an image having enhanced contrast.  
         [0008]     In yet another embodiment of the present invention, a method of increasing contrast in an image processing apparatus without increasing the F-number comprises means for introducing a plurality of light rays to a polarization apparatus, the plurality of light rays including skew light rays which enter the polarization apparatus at incident angles and orthogonally polarized light rays which enter the polarization apparatus at orthogonal angles, means for rejecting a substantial portion of the skew light rays without reducing the F-number of the image processing apparatus, and means for processing a substantial portion of the orthogonally polarized light rays in the polarization apparatus, the polarization apparatus including a plurality of polarizing beam splitters for transmitting the orthogonally polarized light rays.  
         [0009]     The foregoing and other aspects of the present invention will be apparent from the following detailed description of the embodiments, which makes reference to the several figures of the drawings as listed below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a top view depicting a projection apparatus according to the present invention;  
         [0011]      FIG. 2 ( a ) is a constant contrast curve of a cone of light rays emerging from a single PBS;  
         [0012]     FIGS.  2 ( b ) and  2 ( c ) are side views of a skew filter according to one embodiment of the present invention;  
         [0013]      FIG. 3 ( a ) is a constant contrast curve of a cone of light rays emerging from two PBSs in sequence with planes of the PBS layer orthogonal to each other;  
         [0014]     FIGS.  3 ( b ) and  3 ( c ) are side views of a skew filter according to another embodiment of the present invention;  
         [0015]      FIG. 4  is a side view of a skew filter according to another embodiment of the present invention;  
         [0016]      FIG. 5  is a close-up view of a color management system according to one embodiment of the present invention;  
         [0017]      FIG. 6  is a frequency representation of s and p polarization components of light rays;  
         [0018]      FIG. 7  is a top view of a projection apparatus according to the present invention;  
         [0019]      FIG. 8 ( a ) is a perspective view depicting a cone of rays incident on a PBS in a projection apparatus that leads to depolarization according to the present invention; and  
         [0020]      FIG. 8 ( b ) is a side view depicting a cone of rays emerging from a PBS. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0021]     In the following description of the present invention reference is made to the accompanying drawings which form a part thereof, and in which is shown, by way of illustration, exemplary embodiments illustrating the principles of the present invention and how it may be practiced. It is to be understood that other embodiments may be utilized to practice the present invention and structural and functional changes may be made thereto without departing from the scope of the present invention.  
         [0022]      FIG. 1  is a top view of a projection apparatus  10  according to one embodiment of the present invention. The projection apparatus  10  includes a light source  12 , a first fly&#39;s eye integrator lens  14 , a second fly&#39;s eye integrator lens  16 , a UV/IR filter  18 , and a skew filter  20 . The projection apparatus  10  also includes a first relay lens  44  and a second relay lens  46 .  
         [0023]     The projection apparatus  10  also includes a polarization apparatus  48 . The polarization apparatus  48  modulates light from the light source  12  and includes a color management system  22  that includes a polarizing beam splitter  24 . The polarizing beam splitter  24  includes a right angle prism  26 , the right angle prism  26  having a plurality of multi-layer filter stacks  28 . A right angle prism  26  may be substantially composed of glass, and a multi-layer filter stack  28  may have a coating on at least one surface. In one embodiment of the present invention, the color management system  22  includes a plurality of polarizing beam splitters  24 , each including a right angle prism  26  and a plurality of multi-layer stacks  28 . In one embodiment of the present invention, the UV/IR filter  18  is located between the first fly&#39;s eye integrator lens  14  and the light source  12 , and the second relay lens  46  is located between the first relay lens  44  and a first polarizing beam splitter  24  in the plurality of polarizing beam splitters  24 .  
         [0024]     The light source  12  of the projection apparatus  10  produces light rays  32 . The light rays  32  include orthogonally-polarized light rays  34  and skew light rays  36 . The orthogonally-polarized light rays  34  and the skew light rays  36  each include s-polarization components and p-polarization components. The s-polarization and p-polarization components of the skew light rays  36  are at least partially incident to an optical plane. The orthogonally-polarized light rays  34  enter the polarizing beam splitter  24  at orthogonal angles, such that the p- and s-polarization components are properly modulated by the polarizing beam splitter  24  as explained herein. The skew light rays  36 , however, enter the polarizing beam splitter  24  at angles incident to the internal components of the polarizing beam splitter  24 , such that the p and s polarization components are not properly modulated, resulting in leakage of the p- and s-polarization components, and color degradation in a resulting image. Therefore, the presence of the skew light rays  36  results in leakage of unwanted polarization in the projection apparatus  10 .  
         [0025]     The projection apparatus  10  of the present invention may be utilized in any type of communications system or image processing system. For example, optical systems for use in applications including, but not limited to, high-definition television, may include a projection apparatus  10  as described herein. The present invention can also be used to improve other limitations in optical systems that suffer from effects of skew rays. For example, the performance of optical systems utilizing dichroic mirrors can also be improved by performing analysis of off-axis skew ray response to arrive at optimal aperture sizes and optimal filter position location in the illumination optics electronics. Additionally, any image projection or processing apparatus or system may utilize a projection apparatus  10  having the features and characteristics described herein. Where the projection apparatus  10  is employed in an image processing apparatus or system, preventing the transmission of the skew light rays  36  therefore increases a contrast of a resulting image in the image processing system. Further applications include satellite communications systems, and other communications systems in which incident waves or signals must be filtered or blocked to improve signal transmission quality and to improve the resulting output quality.  
         [0026]     A typical contrast performance of polarizing beam splitter-based projection systems is limited by the angular performance of the right-angle prisms  26  that have multi-layer stacks  28 . The combination of a glass prism  26  and multi-layer filter stacks  28  are typically designed so that at a 45° incidence angle to the adjoining surface, the incident light ray will satisfy the Brewster&#39;s angle condition for the p-polarization component, such that most of the p-polarization component of the incident light ray is passed while the s-polarization component of the incident light ray is rejected. Therefore, the polarizing beam splitter  24  transmits, or allows to pass, p-polarization components, and reflects, or rejects, s-polarization components. This occurs because the spectral width of rejection bands for a multi-layer stack  28  is different for s and p components of an incident beam. However, for a converging or diverging beam, the problem of de-polarization, or the transmission and rejection of unwanted light, occurs due to the fact that even if the incident beam is purely polarized with respect to the incident plane of the polarizing beam splitter layer, the non-collimated nature of the incident light results in components which have propagation vectors that are not orthogonal to either of the p- or s-planes of the polarizing beam splitter layer.  
         [0027]     The projection apparatus  10  also includes a plurality of micro-displays  30 . The plurality of micro-displays  30  are commonly known in the existing art for use in optical and image projection systems for transferring light into images on a screen or other apparatus.  
         [0028]     The skew filter  20  includes a shaped aperture  38 . The shaped aperture  38  of the skew filter  20  has a shape which follows a constant contrast curve  50  of the polarizing beam splitter  24  for a cone of light incident on to the polarizing beam splitter  24 . The shaped aperture  38  is a hole  40  in the skew filter  20  shaped to allow a substantial portion of the orthogonally-polarized light rays  34  to pass through the projection apparatus  10  while at the same time rejecting a substantial portion of the skew light rays  36  from entering the projection apparatus  10 . A variety of shapes may be utilized with the present invention to follow a constant contrast curve  50  of the polarizing beam splitter  24  for a cone of light incident on to the polarizing beam splitter  24 , as described in detail herein, and depending on the configuration of the color management system  22 . In image processing applications including color management systems, the shape of the shaped aperture  38  may be modified to further optimize the overall performance due to different primary colors as well as other polarization control components in the color management system  22 . The shaped aperture  38  is generally configured to lie in the middle of the skew filter  20 , but as will be seen below, any number of shapes, sizes and locations of the shaped aperture  38  can be utilized with the present invention.  
         [0029]      FIG. 2  includes side views (b) and (c) of two possible shaped aperture  38  shapes of a skew filter  20  according to one embodiment of the present invention in which the color management system  22  includes one polarizing beam splitter  24 .  FIG. 2 ( a ) shows a constant contrast curve  50  for light rays modulated by a color management system  22  having one polarizing beam splitter  24 . In  FIG. 2 , the shaped aperture  38  of the skew filter  20  may be shaped as shown in (b) or (c). The shapes shown in (b) and (c) are optimally configured to allow a substantial portion of orthogonally-polarized light rays  34  to enter the color management system  22  having one polarizing beam splitter  24  while blocking the skew light rays  36 . The solid lines inside (a) represent the orthogonally-polarized light rays  34  while the dotted lines represent the skew light rays  36 .  
         [0030]      FIG. 3  includes side views (b) and (c) of two possible shaped aperture  38  shapes of a skew filter  20  according to another embodiment of the present invention in which the color management system  22  includes two polarizing beam splitters  24 .  FIG. 3  (a) shows a constant contrast curve  50  for light rays modulated by a color management system  22  having two polarizing beam splitters  24 . In  FIG. 3 , the shaped aperture  38  of the skew filter  20  may be shaped as shown in (b) or (c). The shapes shown in (b) and (c) are optimally configured to allow a substantial portion of orthogonally-polarized light rays  34  to enter the color management system  22  having two polarizing beam splitters  24  while blocking the skew light rays  36 . The solid lines inside (a) represent the orthogonally-polarized light rays  34  while the dotted lines represent the skew light rays  36 .  
         [0031]      FIG. 4  is a side view of a skew filter  20  having a shaped aperture  38  according to another embodiment of the present invention to maximize overall performance in a color management system  22  having four or more polarizing beam splitters  24 . In this embodiment, the shaped aperture  38  is substantially cross-shaped to block a substantial portion of the skew light rays while allowing passage of the orthogonally-polarized light rays  34 .  
         [0032]     In  FIGS. 1-4 , the skew filter  20  is a device made of a material sufficient to block light rays  32  from passing through the device, such as a metal. The device itself may be a plate or other solid instrument that can be inserted, fixably positioned, and removed from a projection apparatus  10 . In one embodiment, the skew filter  20  may be substantially square in shape. It is to be understood, however, that the skew filter  20  may comprise any shape and material suitable for performing the present invention. The shaped aperture  38  of the skew filter  20  may be a hole  40  through which a desired amount of light may pass. In all embodiments, the shaped aperture  38  is optimally configured to allow passage of a desired amount of light. Use of a shaped aperture  38  as described in  FIGS. 1-4  allows a projection apparatus  10  to utilize a greater amount of overall light rays  32  than by increasing an F-number of the optical system in the projection apparatus  10 . Increasing an F-number restricts the passage of light rays  32  in a uniform fashion without regard to the type of light ray  32  blocked, thereby reducing the color clarity in a resulting image. By utilizing a shaped aperture  38  as in  FIGS. 1-4 , specific light rays  32  can be blocked to maximize the color and clarity of a resulting image by minimizing the passage of unwanted, or incident light rays  32 . Therefore, the present invention provides a method and apparatus for increasing and enhancing contrast in an image processing apparatus without increasing the F-number.  
         [0033]     The skew filter  20  of the present invention may be placed at a filter position  42 . The filter position  42  is any position in the projection apparatus  10  at which a substantial portion of the skew light rays  36  are spatially located. The filter position  42  may also be any position where a substantial portion of the skew light rays  36  are blocked and a substantial portion of the orthogonally-polarized light rays  34  are allowed to pass. In one embodiment, the filter position  42  is located between the first relay lens  44  and the second relay lens  46 . In another embodiment, the filter position  42  is located between the second relay lens  46  and the polarizing beam splitter  24 . In embodiments with a plurality of polarizing beam splitters  24 , the filter position  42  is located between the second relay lens  46  and first polarizing beam splitter  24  in the plurality of polarizing beam splitters  24 .  
         [0034]     Another embodiment of the present invention may be a “soft” aperture that has a predetermined transmission profile and shape that is optimized for a desired illumination profile at the object and to give the target contrast enhancement. Such an aperture  38  can be made, for example, by spatially varying the thickness of thin film absorbing material following the well-known relationship, 
 
 T ( x, y )=exp[α· d ( x, y )], 
        where T(x, y) is the spatial transmission profile of the aperture, α is the absorption coefficient and d(x, y) is the film thickness profile.        
 
         [0036]      FIG. 5  is a close-up view of a color management system according to one embodiment of the present invention.  FIG. 5  shows a color management system  22  including four polarizing beam splitters  22  and a plurality of micro-displays  30 . The light rays  32  enter the color management system  22  and are modulated by the polarizing beam splitters  24  and then transmitted through the micro-displays  30 .  FIG. 6  is a frequency representation of s and p polarization components of light rays. As shown in  FIG. 6 , s-polarization components of light rays  32  have a broader frequency range than p-polarization components. A substantial portion of the s-polarization components are reflected, or rejected, by the polarizing beam splitters  24  of the color management system  22 , while a substantial portion of the p-polarization components are transmitted by the polarizing beam splitters  24  of the color management system  22 .  
         [0037]      FIG. 7  is a design drawing of another top view of a projection apparatus  10  according to the present invention.  FIG. 7  shows the projection apparatus  10  and the positioning of the skew filter  20  therein in one embodiment of the present invention.  
         [0038]      FIG. 8 ( a ) is a perspective view depicting a cone of light rays  32  incident on a polarizing beam splitter  24  in a projection apparatus  10  that leads to depolarization according to the present invention. In  FIG. 8 ( a ), a cone of light rays  32  is shown projecting into a color management system  22 . The color management system  22  includes a right angle prism  26  of a polarizing beam splitter  24  layer which processes the cone of light rays  32  and produces the light rays  32  exiting the polarizing beam splitter  24 .  FIG. 8 ( b ) is cross-section of the constant contrast curves  50  produced by the polarizing beam splitter  24 .  FIG. 8 ( b ) shows the cone of light rays  32  emerging from the polarizing beam splitter  24 .  FIG. 8 ( b ) also depicts higher constant contour curves  50  located in the center of the cone of lights rays  32  emerging from the polarizing beam splitter  24 .  
         [0039]     It is to be understood that other embodiments may be utilized and structural and functional changes may be made without departing from the scope of the present invention. The foregoing descriptions of embodiments of the invention have been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Accordingly, many modifications and variations are possible in light of the above teachings. For example, the skew filter  20  may have any size or shape, and include a shaped aperture  38  of any size and shape, which is capable of application to a projection apparatus  10  as described in this specification and which is capable of minimizing the amount of leakage of unwanted polarization. Additionally, the skew filter  20  may be placed at any position in the projection apparatus  10 , including between the light source  12  and the first fly&#39;s eye integrator lens  14 , and between the first fly&#39;s eye integrator lens  14  and the second fly&#39;s eye integrator lens  16 . In yet another embodiment, the shape of the skew filter  20  is controlled by an algorithm that continually measures the constant contrast curve  50  of the light rays  32  emanating from the polarization apparatus  48 , determines the optimal shape of the aperture  38 , and adjusts the shape of the aperture  38  by manipulating the skew filter  20  accordingly. It is therefore intended that the scope of the invention be limited not by this detailed description.