Patent Publication Number: US-2009219735-A1

Title: Structure for light emitting device array

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority of U.S. Provisional Patent Application No. 61/032,981, filed on Mar. 2, 2008. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the polarization of light source. More specifically, the present invention provides a structure and method to produce polarized light applicable for display devices requiring polarized light source, such as liquid crystal display. 
     2. Description of the Prior Art 
     Light source currently used in a flat panel display is first distributed through a light guide to illuminate the entire area of the display. The light then passes through a polarizing film of the same size as the display area. An example of such device is provided in  FIG. 1  wherein the light source  101  and the reflector  102  direct the light to a light guide  103 . The structures  104 , arranged on one face of the light guide  103  and a reflecting surface placed behind this face, direct the light traversing the light guide toward the opposite face of the light guide where the light exits the light guide and illuminates a display screen. The light exiting the light guide is not polarized as shown to comprise polarization components  111  and  112  in  FIG. 1 . A sheet polarizer of the same size as the image display screen is placed between the light guide and the image display screen to produce linearly polarized light to illuminate the image display screen. Such display structure requires a large area polarizing film, the same size as the display. Furthermore, a polarizing film removes 50% of the light from the incoming non-polarized light. An ordinary polarizer allows light of one polarization  111  to transmit and removes the orthogonal polarization  112  from the light. The removed light is typically absorbed or dispersed and mostly lost as heat. In order to improve the efficiency of light utilization, special material and structure have to be processed into the polarizing film to reflect, rather than absorb, the light of orthogonal polarization. The reflected light is then re-processed and re-directed back into the system. An example of such film is DBEF. A typical structure using such polarizer is shown in the right part of  FIG. 1  where  105  is a DBEF that allows the polarization  113  to transmit and reflects the orthogonal polarization light  114 . The special material and structure of such film increases the cost. This is particularly unfavorable when such cost is scaled with the size of the display. 
     Furthermore, as re-processing the reflected light involves multiple passes of the light back and forth between the reflector  106  and DBEF, the light passes through dispersing and absorbing elements such as structure  104  multiple times. The efficiency of light utilization is still very limited even with such conventional re-processing structures. 
     The present invention provides a structure and method to improve on both factors. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and structure to produce light polarization before the light entering the large area light guide typical used in a flat panel display. In this invention, the polarization is generated in a fairly confined area before distributing to illuminate a large area. Two orthogonal polarizations are created and directed to different paths. In one preferred embodiment, one component of the polarized light is re-directed to an optically active device to have its polarization modified to be the same as the first before merging into the optical path of the first component. High utilization of light and small area processing thus provide improved efficiency in both energy and material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  is a schematic of a prior art light source. 
         FIG. 2  is a schematic illustration of the present invention. 
         FIG. 3  is a schematic illustration of the present invention. 
         FIG. 4  is a drawing of a preferred embodiment of the present invention. 
         FIG. 5  is a schematic drawing of the present invention. 
         FIG. 6  is a schematic illustration of light distribution structure in the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     The present invention is herein described in detail with reference to the drawings. 
       FIG. 2  illustrates the schematic of a preferred embodiment of the present invention, wherein an apparatus comprises a light source  201 , a light guide  203 , and a polarizer arranged between the light source and the light guide. The light is polarized via a polarizer  202  in a small area before entering the small face of the light guide, thereby reducing the size of the polarizer. 
     The light guide  203  comprises light re-directing structures on one of its large faces  2032 A. Such structures preserve the polarization, and direct the traversing light toward and to exist the opposite large face  2032 B of the light guide. 
     The light source may further comprise structures such as reflector  2010  or collimator to direct light toward the light guide. 
       FIG. 3  provides further detail of a preferred embodiment of the present invention, where  301  is a light source,  303  is a light guide, and  302  is an assembly of polarizer comprising multiple elements of polarizer shown as  3021 ,  3022  and so on. The polarizer is arranged between the light source  301  and the light guide  303 . The light from the light source  301  is directed to the polarizer  302  and become polarized before entering the light guide. 
     In a preferred embodiment, the polarizer assembly  302  may further comprise an optical active element  3023  positioned between the polarizer element  3022  and the optical guide  303 . 
     In a preferred embodiment, said light guide  303  comprises a face  3031  of small area, and a face  3032  of large area. The apparatus is arranged so that the light emerging form the polarizer is directed into the light guide via the face  3031  of small area, and exiting the light guide via the face  3032  of large area. 
     In a preferred embodiment the polarizer assembly  302  further comprises an enclosure element  3024 . Such enclosure element provides mechanical support to the elements of polarizer  3021  and  3022 . One preferred embodiment of the enclosure element  3024  is a mechanical frame supporting the polarizer  302  and  3022 . The mechanical frame is made to fix the polarizer in positions. Another preferred embodiment of the enclosure  3024  is a bonding chemical that bonds the polarizers into one unit. 
     In a preferred embodiment, the polarizer elements  3021  and  3022  are integrated as facets within the host  3024 , and the integrated polarizer assembly  302  is constructed as a single slab or film. One preferred method of making such assembly is to immerse or embed polarizer  3021  and  3022  in transparent epoxy resin or polymer which is the host material. The host material is cured or solidified and molded into the shape of the assembly slab. Another preferred method of producing such embodiment is to immerse the polarizer  3021  and  3022  in polycarbonate plastic. 
       FIG. 4  provides further detail of a preferred embodiment and a preferred operation of the present invention. The light source comprises at least one light element  4011 . The polarizer assembly  402  comprises multiple elements of polarizer  4021  and  4022 , and an optically active element  4123 . The light output  407  from the light element  401  is directed to a polarizing element  4021 . Polarizer  4021  splits light beam  407  into two orthogonal polarization light beams  408  and  409 , where light  408  is transmitting with one polarization and  409  is reflected by  4021  and polarized orthogonally to the polarization of  408 . The polarizer  4021  is arranged to direct the reflected light  409  to a second polarizer  4022 , where polarizer  4022  is oriented to reflect the polarized light  409 . The reflection by polarizer  4022  re-directs light  409  into a direction of  410  which is in the same direction of light  408 . Light  410  passes through an optically active element  4023  before merging into the direction of light  408 . The optically active element  4023  operates to re-orient the polarization of  410  to the same polarization orientation as of light  408 . The emerge lights of  408  and  410  thus comprise the same polarization. 
     In a preferred embodiment, the optically active element comprises material that rotates a polarization by an angle. Examples of such optically active materials comprise quartz, calcite, and certain organic materials comprising polyamide, polyester and polyimide. The device is so prepared that rotates the polarization of light  410  by an amount equal to the angular difference between the two orthogonal polarizations so that the light  410  emerging from the device  4023  has its polarization aligned in the same orientation as the transmissive light  408 . 
     In a preferred embodiment, the polarizing element comprises a plurality of alternating layers of different indices of refraction, thereby allowing one polarization to pass and reflecting the orthogonal polarization. 
     In another preferred embodiment, said polarizing elements comprise a plurality of repeating stacks; wherein each stack comprises at least two layers; wherein one of the two layers is optically anisotropic; wherein said two layers have substantially similar indices of refraction in one direction of polarization, and different indices of refraction in the other direction of polarization. 
     In another preferred embodiment, said polarizing elements comprise grating with parallel metal wires. In a preferred embodiment such metal wires stretch perpendicular to the light path, where the spacing between the metal wires and the thickness of the metal wires are in the same order of magnitude of the wavelength of the visible light. 
     In a preferred embodiment, the light source may be further structured with a plurality of light elements shown as  4011  and  4011 A in  FIG. 4 , where each light element directs its light output to an element of polarizer in the polarizer assembly, as illustrated in  FIG. 4 . The light elements are separated by a structure  4012 , such as collimating device or isolation reflectors, to direct the light  407  to the corresponding polarizing element  4021 . 
       FIG. 5  illustrates a preferred embodiment of the arrangement of the polarizer and the light guide. The light guide  503  comprises a face  5031  of small area and a face  5032  of large area. The light emerging from the polarizer assembly is directed into the small face  5031 . The light traverses inside the light guide and is directed to exit the light guide via the large face  5032 . 
     A preferred embodiment of the light guide is shown in  FIG. 6 , where the light guide comprises a face  6031  of small area and a large face  6032 . Structures  6033  are arranged along at least one of the large faces, re-directing the passing light toward the opposite face of large area where the light exits the light guide. Examples of light re-directing structures comprise v-shaped groves, curved surfaces such as spherical or cylindrical bumps or indents. Such structures re-direct or reflect lights at their interfaces between materials of different refractive indices, and preserve the polarization of the light. 
     Various structures may be used to achieve the function of a polarizing apparatus of the present invention. Specific embodiments of the polarizing elements were provided in this description to illustrate the operation of the principles of this invention. The application of the principles of the present invention however is not limited by such examples. It is conceivable that various types of materials and structures may be used to construct such polarizing elements, and all such variations are embraced by the present invention. 
     Although various embodiments utilizing the principles of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other variances, modifications, and extensions that still incorporate the principles disclosed in the present invention. The scope of the present invention embraces all such variances, and shall not be construed as limited by the number of elements, number of layers, or specific direction and angles.