Abstract:
A lens array is placed over a cover glass of a pixel based LCoS display device. The lens array is situated so that an individual lens in the array corresponds to an individual pixel in the display device. Each lens collects light and/or reflections from a high contrast portion of its corresponding pixel and forwards it to a viewing area. The lens array reduces effects of low contrast portions of the pixels (particularly, for example, perimeters of adjacent pixels having relatively large differences in electrical field strengths).

Description:
CLAIM OF PRIORITY 
   This invention claims priority to U.S. provisional patent application Ser. No. 60/447,681 filed Feb. 14, 2003, which is incorporated herein by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The invention disclosed in this document is a means to increase the contrast ratio of a LCoS microdisplay based video projector. It is accomplished by utilizing a micro lens array to focus light into the central, higher contrast portion of each pixel. 
   2. Discussion of the Background 
   SUMMARY OF THE INVENTION 
   The present inventor has realized the need to increase contrast in display devices. In one embodiment, the present invention eliminates or reduces use of the lower contrast portions of array type display devices such as LCDs. 
   Roughly described, the invention comprises an array of lenses wherein each lens is positioned to be in line with a corresponding pixel or set of pixels. Preferably, each lens has a one-to-one correspondence with one of the pixels. The pixels are, for example, pixels of an LCoS microdisplay. The lenses are constructed to focus light onto and from a selected portion of each pixel. The selected portion is, for example, a predefined area of a pixel (e.g., a central area of the pixel, or, an area of the pixel having a highest/best resolution representing the intended darkness or color of the pixel). 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a top view of a conventional LCoS microdisplay; 
       FIG. 1B  is a side view of the convention LCoS microdisplay of  FIG. 1 ; 
       FIG. 1C  is a top view of an LCoS microdisplay row; 
       FIG. 1D  is a side view of an LCoS microdisplay row without lateral field effects; 
       FIG. 1E  is a side view of an LCoS microdisplay row with lateral field effects; 
       FIG. 2  is a close up of the side view of  FIG. 1B ; 
       FIG. 3  is a drawing of a pixelated lens array and a pixelated lens array mounted on a light modulator each according to several embodiments of the present invention; and 
       FIG. 4  is an example of a projection device with pixelated lens array microdisplay packages according to an embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   As illustrated in  FIG. 1A , the pixels in a LCoS microdisplay  100  are defined by an array of reflective electrodes on the silicon substrate. The pixels are rectangular and arranged in a rectilinear grid. The counter electrode is a single electrode on the cover glass (e.g., see  FIG. 1B ). 
   The electric field applied to the layer of liquid crystal associated with each pixel can be independently controlled. This is illustrated in  FIG. 1C  and  FIG. 1D . In these figures it is assumed that one pixel is in the dark state and all surrounding pixels are in a bright state. The electric field is established by applying a voltage to the individual pixel on the silicon and a reference voltage to the single electrode on the cover glass. Regardless of whether the drive scheme of the silicon substrate is analog or digital, the layer of liquid crystal responds to the RMS of the applied voltage. 
   The ideal case is illustrated in the central portion of  FIG. 1D . When a pixel&#39;s voltage is equal to the reference voltage, the electric field across the associated layer of liquid crystal layer is uniformly 0 and the pixel will be in a uniform bright state. When a pixel&#39;s voltage is at V high, the voltage across the associated liquid crystal layer is maximized and the pixel will be in a uniform dark state. When the pixel voltage is between the reference voltage and V high, that pixel will be in a reflective state between full bright and full dark. 
   A more realistic case is illustrated in the lower portion of  FIG. 1E . As before, we see that an electric field is established between the dark pixels to which a V high has been applied and the counter electrode which is at the reference voltage. Also as before, the electric field is absent between the bright pixel to which the reference voltage is applied and the counter electrode. Note, however, that a lateral electric field exists between the dark pixel and the bright pixels adjacent to it. A lateral field does not exist between adjacent pixels when both are in the bright state. Stated more generally, a lateral field will exist whenever adjacent pixels have voltage values that differ. The greater the voltage difference, the larger the lateral field. The effect of the lateral field is discussed with reference to  FIG. 2 . 
     FIG. 2  is a closer look at the side views of pixels from the bottom portion of  FIG. 1E  with the addition of graphics to indicate the orientation of molecules in the liquid crystal layer. The orientation chosen for this illustration is representative of one of the many possible electro-optic configurations used in LCoS microdisplays (details of other possible examples would be different but the point about to be made on the effect of a lateral field will still apply). A Brightness scale  250  indicates relative brightness for each part of the illustrated pixels. For example, a bright pixel  252  is graphically illustrated as having a bright portion at the center and left of the pixel, and a dark portion on the right (near the dark pixel  253 ). The illustrated dark and bright areas also correspond to the corresponding molecular orientation shown directly above. 
   The lateral field penetrates from a perimeter in the liquid crystal layer (e.g., above a 1st pixel electrode  200 ) towards the centers of adjacent pixels (e.g., above pixel electrodes  205  and  215 ) which have voltage values different in their corresponding pixel electrodes. The larger the lateral field, the deeper the penetration. The lateral field adds vectorally to whatever electric field exists between the pixel and the counter electrode. The orientation of liquid crystal molecules in the region effected by of the lateral field will be different from that in the balance of the pixel. This disruption in orientation affects the electro optic effect in such a way as to render non-uniform the shade of gray intended for the pixel. More specifically, in the case of a pixel intended to be fully dark, the perimeter of the pixel is rendered less dark. The adjacent pixels, intended to be fully bright are darkened around a portion, or portions of their perimeters. From the macroscopic standpoint, this produces dark pixels that are less black and bright pixels that are less bright thus reducing the overall display contrast ratio. 
   The invention disclosed in the document is a means to deal with lateral field effects. By doing so, the contrast ratio of each pixel and, hence, the overall display is maximized. 
   The invention is explained with reference to  FIG. 3 . As shown, a lens array  310  has been added to a microdisplay  300 . One lens is associated and aligned with each pixel (e.g., lens  320  is associated with the pixel produced by pixel electrode  325 ). The function of the lens is to converge the light used to illuminate the microdisplay into the central portion of its associated pixel. In this way, the high contrast portion of each pixel is used to modulate the light or illumination  350  that eliminates from the microdisplay (e.g., reduces or eliminates the use of the perimeter portion of the pixel, which, if adjacent to a pixel of a different voltage, would likely have a darkening or lightening effect). 
   The lens array illustrated in  FIG. 3  is depicted as if were made of plastic and the lenses focusing light through the use of diffractive/digital optics. Other mechanisms by which the lens array could be formed would include the use of glass or other substrate materials and the lenses formed from conventional refractive lenses. 
   The pixilated lens array may be constructed, for example on a sheet and glued to the microdisplay (e.g., using optical adhesive). As shown in  FIG. 3 , each lens comprises several stair-stepped layers of transparent optical material (e.g., glass, plastic, etc.). 
     FIG. 4  is a diagram of a video projection system  400  having at least one pixelated lens array microdisplay package according to at least one of the embodiments of the present invention. As shown in  FIG. 4 , white light  410  is generated by a light source  405 . The light is collected, homogenized and formed into a proper shape by a condenser  415 . UV and IR components are eliminated by filters (e.g., hot/cold mirrors  416 / 417 ). The white light  410  then enters a prism assembly  450  (e.g., comprising a set of beamsplitters) at an input face  472 . The white light  410  is separated into red, green and blue light beams by the prism assembly. The individual beams are separated based on polarization, color separation, or other techniques (e.g., cholesterics), according to the properties of beam splitting layer(s) in the beam splitters. Polarization and/or other management of the red, green, and blue light beams is performed, at least in part by the beamsplitters and other optical elements (e.g., optical elements  482 / 484  which may take the form of waveplates, linear polarizers, etc.). 
   A set of pixelated lens array microdisplay packages  452 A,  452 B, and  452 C (each comprising, for example, a Liquid Crystal on Silicon (LCoS) microdisplay and a pixelated lens array) are provided and positioned on processing faces of the prism assembly such that each package corresponds to one of the light beams (e.g., red, green, and blue) (the prism assembly  450  with the attached microdisplays is called a kernel  480 ). The light beams follow different paths (light channels) within the prism assembly  450  such that each beam is directed to a pixelated lens array microdisplay package that modulates and reflects the light beam. The microdisplay that modulates and reflects the green beam “displays” the green content of a full color video image. The reflected green beam then contains the green content of the full color video image. Similarly, blue and red content of the full color video image is imparted into the blue and red light beams by the “blue” and “red” microdisplays. On a pixel by pixel basis, the microdisplays modulate and reflect (“display”) the colored light beams. 
   The lens array portion of the pixelated lens array microdisplay package directs the light to be modulated toward high contrast regions of the microdisplay. The high contrast portions may be, for example, non-perimeter portions of pixels or non-perimeter portions of groups of pixels of the microdisplay. The microdisplay modulates and reflects a light beam that is now modulated. 
   The prism assembly  450  then recombines the modulated beams into a modulated white light beam  460  that contains the full color video image. The resultant modulated white light beam  460  exits the prism assembly  450  and enters a projection lens  465 . Finally, the image-containing beam (white light beam  460  that has been modulated and now contains the full color image) is projected onto a screen  470 . 
   The above described video projection system may be utilized in television sets, HDTV televisions, monitors, computer monitors, display systems, home entertainment systems, presentation projectors, and the like. 
   Kernels and prism assemblies are commercially available in many varying configurations. The present invention includes any such configurations utilizing a pixelated lens array or other optical component according to the present invention. Example additional configurations include, but are not limited to, the various quad style configurations described in Berman et al., U.S. patent application Ser. No. 10/342,219, filed Jan. 13, 2003, and entitled “Design of Prism Assemblies and Kernel Configurations for Use in Projection Systems,” the contents of which are incorporated herein by reference in their entirety. In yet other embodiments, the prism assembly may include any of a fourth color channel, channels of different primary colors (e.g., Yellow, Magenta, Cyan), or be designed with single or multiple light channels and configured to utilize a set of sequential primary colors. 
   In describing preferred embodiments of the present invention illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the present invention is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents which operate in a similar manner. For example, a lens array may be described as individual lenses placed at appropriate locations, a transparent material cut or ground to a shape acting as a lens array, or any other device having an equivalent function or capability, whether or not listed herein, may be substituted therewith. Furthermore, the inventors recognize that newly developed technologies not now known may also be substituted for the described parts and still not depart from the scope of the present invention. All described items, including, but not limited to lens arrays, light modulators (e.g., microdisplay), substrates, materials, etc should also be consider in light of any and all available equivalents. 
   Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.