Patent Document

BACKGROUND 
   Digital projectors often include micro-displays that include arrays of pixels (e.g., 1280×1024, etc.) Each pixel usually includes a micro-electromechanical system (MEMS) device, such as a micro-mirror, liquid crystal on silicon (LcoS) device, interference-based modulator, etc. A micro-display is used with a light source and projection lens of the digital projector. The micro-display receives light from the light source. When the pixels of the micro-display are ON, the pixels direct the light to the projection lens. The projection lens images and magnifies the micro-display. When the pixels are OFF, they direct the light from the light source away from the projection lens. However, some light may still be directed to the projection lens when the pixels are OFF, e.g., due to reflections from packaging of the micro-display, etc. This degrades the “Black/White Contrast ratio” that is often defined as the ratio of the light imaged by the projection lens when all of the pixels in the micro-display are ON to the light imaged by the projection lens when all of the pixels are OFF and is a measure of the blackness of the projector&#39;s black state. 
   SUMMARY 
   One embodiment of the invention provides a method of operating a projector that includes receiving light at a modulator of the projector, reflecting the light from the modulator, returning the reflected light to the modulator, and re-reflecting the reflected light from the modulator. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a portion of a projector, according to an embodiment of the invention. 
       FIGS. 2A–2D  illustrate a portion of a projector in operation, according to another embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   In the following detailed description of the present embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that process, electrical or mechanical changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims and equivalents thereof. 
     FIG. 1  illustrates a portion of a projector, e.g., for use in a front or rear projection system, according to an embodiment of the invention. The projector includes a light modulator  110 . For one embodiment, light modulator  110  is a multi-color modulator, e.g., red, blue, and green. For another embodiment, light modulator  110  is a single color, such as red, and another modulator  120 , shown by dashed lines, is a two-color modulator, such as blue and green. Modulators  110  and  120  may include pixilated MEMS devices, such as micro-mirrors, or interference-based modulators, LCD devices, etc. An optical system  130  is included for receiving light from a light source  135  and delivering the light to modulator  110  or modulators  110  and  120 . Optical system  130  also delivers modulated light from modulator  110  or modulators  110  and  120  to an outlet  140  of the projector. 
   Optical system  130  includes a polarizer (or polarizing beam splitter)  150 , such as a polarizing beam spitting cube or plate. Polarizer  150  is located between modulator  110  and a lens  155 . Polarizer  150  is also located between a mirror  160  and lens  155 . Lens  155  is located between polarizer  150  and a mirror  165 . Polarizer  150  passes light having one polarization and reflects light having another polarization. For embodiments having modulators  110  and  120 , a dichroic beam splitter  170 , such as a dichroic beam spitting cube or plate, is located between polarizer  150  and modulator  120  and between modulator  110  and lens  155 . Dichroic beam splitter  170  separates light according to its color, e.g., dichroic beam splitter  170  may pass red light to modulator  110  and reflect blue and green light to modulator  120 . 
   A quarter wavelength plate  175  is located between polarizer  150  and modulator  110 . For one embodiment, quarter wavelength plate  175  is butted against modulator  110 . For another embodiment, a face  176  of quarter wavelength plate  175  is substantially co-planer with mirror  160 . For another embodiment, a quarter wavelength plate  180  is located between modulator  120  and dichroic beam splitter  170 . 
   For one embodiment, optical system  130  may be formed as a transparent solid, such as glass, so that the various components or optical system are integral with the solid. That is, the material of the transparent solid physically interconnects the elements. For another embodiment, the components of optical system are physically discrete, i.e., are not physically connected. For example, gas filled spaces, e.g., air, or evacuated spaces may separate the components of optical system  130 . 
     FIGS. 2A–2D  illustrate optical system  130  in operation, according to another embodiment of the invention. Optical system  130  receives linearly (or plane) polarized light at a first polarization from light source  135 . The light enters optical system  130  along an illumination path  205 , as shown in  FIG. 2A . For one embodiment, illumination path  205  is contained within an angle  210  with respect to an axis  215  that is substantially parallel to face  176  of quarter wavelength plate  175  ( FIG. 2A ). 
   As shown in  FIG. 2A , the incoming light rays, represented by a ray  220   1 , are reflected off polarizer  150  to lens  155 . The subscript “1” of “220” is used to denote the first polarization. Ray  220   1  is refracted upon exiting lens  155  and is subsequently reflected by mirror  165  to lens  155 . Ray  220   1  is refracted upon exiting lens  155  and enters quarter wavelength plate  175 . Note that the polarization of ray  220   1  changes, e.g., becomes circularly polarized, when it passes through quarter wavelength plate  175 . However, the subscript “1” will be retained for purposes of discussion. 
   When modulator  110  is on it reflects substantially all of the light of ray  220   1 , back through quarter wavelength plate  175 . Passing ray  220   1  through quarter wavelength plate  175 , reflecting it back through quarter wavelength plate  175  using modulator  110  rotates the polarization of ray  220   1  to a second linear (or plane) polarization upon exiting quarter wavelength plate  175  that is orthogonal to the first polarization. Specifically, the circularly polarized light changes to linearly (or plane) polarized light at the second polarization when it passes back through quarter wavelength plate  175 . The exiting ray is referred to as ray  222   2 , where the subscript “2” denotes the second polarization, as shown in  FIG. 2A . 
   When modulator  110  is off, it absorbs and/or transmits most of the light of ray  220   1  and/or reflects the light of ray  220   1  away from optical system  130 . The remaining fraction is reflected from modulator  110  back into optical system  130 . This reflected light passes back through quarter wavelength plate  175  so that the polarization of ray  220   1  is rotated to the second polarization upon exiting quarter wavelength plate  175 , as just described, as the ray  222   2 . Therefore, ray  222   2  contains substantially all of the light that ray  220   1  does when modulator  110  is on and a fraction of the light that ray  220   1  does when modulator  110  is off. 
   Note that for embodiments that include modulators  110  and  120  and dichroic beam splitter  170 , dichroic beam splitter  170  reflects one or more color components of ray  220   1  corresponding to modulator  120  and passes one or more color components of ray  220   1  corresponding to modulator  110 . 
   As shown in  FIG. 2A  ray  222   2  enters lens  155 .  FIG. 2B  shows ray  222   2  passing through lens  155  and being refracted onto mirror  165  upon exiting lens  155 . Mirror  165  reflects ray  222   2  back to lens  155 . Ray  222   2  is refracted upon entering lens  155  and subsequently passes therethrough to polarizer  150 . Polarizer  150  passes ray  222   2  therethrough and onto mirror  160 . Mirror  160  reflects ray  222   2  to lens  155 . 
     FIG. 2C  shows ray  222   2  passing through lens  155  and being refracted onto mirror  165  upon exiting lens  155 . Mirror  165  reflects ray  222   2  back to lens  155 . Ray  222   2  is refracted upon entering lens  155  and subsequently passes therethrough to quarter wavelength plate  175 . Ray  222   2  passes through quarter wavelength plate  175  and reflects off modulator  110  back through quarter wavelength plate  175 . As described above this rotates the polarization of ray  222   2  orthogonally back to the first polarization so that a ray  224   1  exits quarter wavelength plate  175  and enters lens  155 . 
   Note that for the reasons described above, ray  224   1  contains substantially all of the light that ray  222   2  does when modulator  110  is on and a fraction of the light that ray  222   2  does when modulator  110  is off. This means that when modulator  110  is off ray  224   1  is blacker than ray  222   2  and thus the contrast is improved compared to passing the light into modulator  110  only once while modulator  110  is in the off state. Moreover, when modulator  110  is off and since ray  222   2  contains a fraction of the light of the ray  220   1  when ray  220   1  enters quarter wavelength plate  175  in  FIG. 2A , ray  224   1  contains a fraction of a fraction, e.g., about a square of the fraction, of the light of the ray  220   1 . 
     FIG. 2D  shows ray  224   1  passing through lens  155  and being refracted onto mirror  165  upon exiting lens  155 . Mirror  165  reflects ray  224   1  back to lens  155 . Ray  224   1  is refracted upon entering lens  155  and subsequently passes therethrough to polarizer  150 . Polarizer  150  reflects ray  224   1  to a projection path  230  and to outlet  140  of the projector, as shown in  FIG. 2D . For one embodiment, projection path  230  is contained within an angle  240  with respect to axis  215 . Comparing the projection path  230  of  FIG. 2D  with the illumination path  205  of  FIG. 2A  shows that the angle  210  containing illumination path  205  and the angle  240  containing projection path  230  are on opposite sides of axis  215 . This separates illumination path  205  from projection path  230 . 
   Note that due to the polarization state of the light at polarizer  150 , polarizer  150  does not allow light to exit optical system  130  until it passes into modulator  110  twice. When modulator  110  is off, this reduces the light exiting system  130 , thereby producing blacker blacks and a higher contrast. 
   CONCLUSION 
   Although specific embodiments have been illustrated and described herein it is manifestly intended that this invention be limited only by the following claims and equivalents thereof.

Technology Category: 3