Abstract:
A projection system is disclosed that utilizes a modulator, preferably in the form of an LCD matrix, to modulate light in accordance with a video image to be displayed. The light is supplied to the LCD matrix, reflected back off the matrix through a lens to a mirror, and then imaged through another lens. The mirror is placed in the path between the light source and the LCD matrix.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to LCD projectors, and more particularly, to a method and apparatus for providing an LCD projector that eliminates inefficiencies present in prior systems by maximizing the useful area for projection.  
         BACKGROUND OF THE INVENTION  
         [0002]    Generic LCD projectors are becoming more widely used in business applications. These types of projectors are typically used for business presentations, educational sessions, etc.  
           [0003]    [0003]FIG. 1 shows a typical reflective liquid crystal device (LCD) projector. The arrangement of FIG. 1 represents a monochrome type of projector, but extension to color systems is known to those of skill in the art. For example, color systems may be implemented using an X-cube and a plurality of different color specific modulators. This technique is well known in the art, and will not be described in detail.  
           [0004]    In the arrangement of FIG. 1, LCD  101  is a matrix of reflective LCD elements. Each element may rotate the polarization of incident light by up to 90 degrees. A polarizing beam splitter  102  passes light polarized in a first direction but reflects light polarized in a second and orthogonal direction.  
           [0005]    In operation, light from lamp  104  is prepolarized by prepolarizer  105  and transmitted horizontally through polarized beam splitter (PBS)  102 . The light exiting lamp  104  is collimated into substantially parallel columns. The polarized light passes plane  107  and is incident upon LCD  101 .  
           [0006]    The elements of R-LCD  101  are arranged to change the polarization of the incident light in accordance with a video signal driving R-LCD  101 . This technique results in the light being reflected back from R-LCD  101  in a variety of different states. More specifically, some of the light is reflected back after having its polarization rotated, and other light remains polarized in the same direction as when it was initially incident upon R-LCD  101 . Additionally, the light may have its polarization only partially rotated, providing for shades of gray. Each of the numerous elements in the LCD matrix may independently rotate the polarization of incident light by a different amount.  
           [0007]    Upon being reflected back, the light which has not had its polarization changed passes back through plane  107  and is absorbed. Light which was incident upon R-LCD elements and which did have its polarization changed will not pass through plane  107 , but will instead be reflected up through the post-polarizer  109  for projection as an image through projection lens  110 . Light which has had its polarization changed by some degree will partially pass and result in gray shades rather than black and white. In short, the polarization may be rotated by any amount between zero and 90 degrees, with angles between these two extremes representing shades of gray.  
           [0008]    Several problems exist with R-LCD projectors of the type shown in FIG. 1. One problem is that the rays are not strictly S-polarized or P-polarized as they hit the plane  107 . This results in decreased contrast. For a more complete description of this problem and a proposed solution, see U.S. Pat. No. 5,453,859, issued Sep. 26, 1995.  
           [0009]    Another problem associated with the systems of the type shown in FIG. 1 is stressed induced birefringence in the PBS  102 . This phenomenon occurs because the PBS is warmed non-uniformly by the optical beam passing through it. The differential warming of the glass induces stress in the glass, which in turn induces birefringence in the glass. Prior attempts at solving the problem have been less than optimum.  
           [0010]    A second prior art LCD projector design utilizes an off-axis LCD projector of the type shown in FIG. 2. A lamp  104  and prepolarizer  105  transmit polarized light to a reflective LCD  101 . The polarization of the light is then either changed or not, or changed to some degree, by the state of the various LCD elements. As was the case for FIG. 1, the LCD elements are driven by a video signal, and thus, the polarization of the reflected light beam  203  has been modulated in accordance with the video signal. That reflected light beam is then transmitted through a post-polarizer  109  for projection via lens  110 .  
           [0011]    The basic problem with the arrangement of FIG. 2 is the inefficient use of the pupil of the projection lens, which is located approximately at plane “B.” More specifically, a large portion of the optical beam that would otherwise be captured by the projection lens is blocked by the path of the light from lamp  104 . Thus, the usable pupil of the system is approximately one quarter of the full pupil of the projection lens, significantly reducing system efficiency. FIG. 3 depicts the pupil utilization in the prior art off-axis projector such as that shown in FIG. 2. It can be seen that about three quarters of the pupil area is wasted.  
           [0012]    In view of the above there exists a need in the art for an improved reflective LCD projector which can efficiently utilize a larger pupil area and which eliminates the foregoing problems.  
         SUMMARY OF THE INVENTION  
         [0013]    The above and other problems with the prior art are overcome in accordance with the present invention. A light source provides light to a modulator, which reflects back a modulated light signal that has been modulated in accordance with a video signal to be imaged. A mirror is interposed between the light source and the modulator. A portion of the light from the light source is blocked from reaching the modulator by the back of the mirror, but the modulated reflected light is focused entirely on the reflecting surface of the mirror and reflected through a projection lens by the mirror.  
           [0014]    The mirror is positioned such that it only blocks a small portion of the incident light from the light source, thereby increasing efficiency. It may be placed directly in the path of incident light. In an additional embodiment, the mirror may be curved.  
           [0015]    In a preferred embodiment, the modulator is a matrix of R-LCD elements.  
           [0016]    Color may be obtained by utilizing an X-cube or similar device. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 in the prior art LCD projector;  
         [0018]    [0018]FIG. 2 is a different type of prior art LCD projector;  
         [0019]    [0019]FIG. 3 a  shows a depiction of an inefficient use of the pupil in the prior art arrangement of FIG. 2;  
         [0020]    [0020]FIG. 3 b  shows a depiction of the utilization of the pupil in the illumination path of the current invention and  
         [0021]    [0021]FIG. 4 shows an exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]    [0022]FIG. 4 shows an exemplary embodiment of the present invention including a light source  401 , lenses  405  and  406 , a mirror  407  and polarizer  429 , and an LCD  411 . Modulators  410 - 412  and X-cube  415  provide color in a conventional manner. It is noted that polarizers  430 - 432  may be used instead of, not in addition to, polarizer  429 . For explanation we assume polarizer  429  is utilized.  
         [0023]    In operation, light from source  401  leaves reflector  402  and is transmitted substantially parallel towards lens  406 . Mirror  407  blocks the light from passing left to right over a substantially circular cross-sectional area. Since the mirror  407  is tilted as shown, the mirror is elliptical in shape such that the cross-section, viewing the tilted mirror from point  420 , is circular. In a prototype, a mirror from Edmund Scientific, model number 30837, was found to give satisfactory performance. The circular cross section of the mirror may be substantially concentric with a cross section of the light emitted by light source  401 . Plane “A” is located approximately at the pupil plane of the illumination path.  
         [0024]    The mirror may also be curved slightly in order to add power to the projection lens. More specifically, if the mirror is curved, power may be added to the projection lens even though elements  405  and  406  of the projection lens remain at the same power. Given the design and performance parameters of the mirror, those of skill in the art can readily calculate the required curvature, and software packages for performing such calculations are available commercially.  
         [0025]    After passing the mirror, the light then passes through lens element  406  but contains a substantially circular dim portion, which represents the cross-section that has been blocked by mirror  407 . After being polarized, the light is transmitted through X-cube  415  and is modulated by the R-LCDs  410 ,  411  and  412 .  
         [0026]    After reflecting off of the LCD matrix  411 , and being modulated thereby, the light is then transmitted back through X-cube  415  and polarizer  429 . The reflected light is then focused upon the reflecting surface of mirror  407  for projection through projection lens elements  405 - 407  to projection screen  433 . Note that  406  and  405  must act together as two elements of the projection lens, along with mirror  407 . Although we refer to lenses  405  and  406 , these lenses actually act together as lens elements to form a projection lens with mirror  407 .  
         [0027]    The operation of the X-cube  415  will not be described in specific detail since such an X-cube is known to those of skill in this art. Suffice it to say however, that white light enters the cube at surface  421  and the red, green and blue components are directed toward modulators  410 ,  411  and  412  respectively. The polarization of the red, green and blue components of the white light is modulated by modulators  410 ,  411  and  412  respectively. The red, green and blue components then are recombined into white light and the white light then exits the X-cube at surface  421 . After passing through polarizer  429 , the components with the undesired polarization are removed and the white light beam has been modulated in accordance with the video signal driving the LCDs  410 ,  411  and  412 . This technique is conventional to those of skill in the art and will not be described in detail herein.  
         [0028]    The distance between lens element  406 , R-LCD matrix  411  and mirror  407  is such that the light incident upon lens element  406  is focused upon mirror  407 . It is noted that the polarizer  429  may be relocated in positions  430 ,  431  and  432  as indicated in FIG. 4. These additional positions are preferred as they result in no optical elements being placed between the polarizer and the LCD which gives maximum contrast. This location for the polarizer also results in the reduced power loading on the polarizers due to the fact that there are separate polarizers in the red, green and blue channels. This results in increased lifetime. Note that due to the presence of mirror  407  in the pupil plane “A”, the system cannot be telecentric, unlike most other reflective LCD projection systems.  
         [0029]    The system as described represents a full color projector. A monochrome projector could be built in accordance with the invention by omitting the X-prism  415 , light modulators  410  and  412  and polarizers  430  and  432  if polarizers  430 - 432  are used rather than polarizer  429 .  
         [0030]    While the above describes the preferred embodiment of the invention, various modifications will be apparent to those of skill in the art. Such modifications are intended to be covered by the following claims.