Abstract:
An optical display panel which provides improved viewing contrast for front projection applications, and a method of producing a stacked optical waveguide panel for front projection applications, are disclosed. The optical panel includes a plurality of stacked optical waveguides, wherein each waveguide has a back face and an outlet face at opposing ends of each waveguide, and wherein each waveguide is formed of a core between an opposing pair of cladding layers, and at least one reflector connected to the back face of at least one waveguide, wherein the at least one reflector receives image light incident through at least one waveguide from the outlet face, and wherein the at least one reflector redirects the image light back through the at least one waveguide out of the outlet face. In the preferred embodiment, the outlet face is rendered black by inclusion of black within or between cladding layers. The method includes stacking a plurality of clear strips of plastic, placing a double sided, dark colored adhesive between each strip of plastic, pressing the stack, forming, at two opposite ends of the stack, a back face and an outlet face, and connecting at least one reflector to the back face.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
         [0001]    Not applicable.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention is directed generally to a planar optical display, and, more particularly, to a high contrast front projection display panel and a method of making a high contrast front projection display panel.  
           [0004]    2. Description of the Background  
           [0005]    Video display screens typically use cathode ray tubes (CRTs) for projecting an image onto the outlet face of the screen. A typical screen of this type has a width to height ratio of 4:3 with 525 vertical lines of resolution. An electron beam must be scanned both horizontally and vertically on the screen to form a number of pixels, which collectively form the image. Conventional cathode ray tubes have a practical limit in size and are relatively deep to accommodate the required electron gun. Larger screen televisions are available which typically include various forms of image projection for increasing the screen image size. However, such screens may experience limited viewing angle, limited resolution, decreased brightness, and decreased contrast, particularly in display screens using front projections. This is, in part, due to the use of white screens to allow the screen to reflect the front projection back to the user. Thus, because the screen is white, the darkest black level that can be displayed is “screen white”, the color of the screen when the projection is off, due to the fact that black light cannot be projected. Consequently, the projection must be either on, or off, to produce white, or black, respectively. Thus, where black is viewed on a front screen projection system, the viewer is actually seeing the white of the background, i.e the absence of projected light, which the human eye sees as black in the context of the white light projected elsewhere on the background, meaning that the presence of the optical spectrum projected onto the white background forms a “whiter than white” color, which the eye sees as white. This is the reason that a room must be darkened in order for a viewer to see black on a front projection screen.  
           [0006]    Optical panels can be created using a plurality of stacked waveguides, and may be rendered black using at least one black cladding layer between transparent cores of the waveguides. The cladding layers disclosed therein have a lower index of refraction than the waveguide cores for effectuating substantial internal reflection of the image light channeled through the cores, and thereby improve contrast, i.e. thereby improve the appearance of black images on a screen. Such optical panel displays have typically been operated in a rear projection mode.  
           [0007]    Therefore, the need exists for a display panel that allows for front projection, while also providing the appearance of a black screen to improve viewing contrast and to eliminate the need to dim lights in order to allow a viewer to see black images.  
         BRIEF SUMMARY OF THE INVENTION  
         [0008]    The present invention is directed to an optical display panel which provides improved viewing contrast for front projection applications. The optical panel includes a plurality of stacked optical waveguides, wherein each waveguide has a back face and an outlet face at opposing ends of each waveguide, and wherein each waveguide is formed of a core between an opposing pair of cladding layers, and at least one reflector connected to the back face of at least one waveguide, wherein the at least one reflector receives image light incident through at least one waveguide from the outlet face, and wherein the at least one reflector redirects the image light back through the at least one waveguide out of the outlet face. In the preferred embodiment, the outlet face is rendered black by inclusion of black within or between cladding layers.  
           [0009]    The present invention is also directed to a method of producing a stacked optical waveguide panel for front projection applications. In one preferred embodiment of the present invention, clear strips of plastic, which are preferably approximately {fraction (3/4)}″ by 40″, and approximately {fraction (20/1000)}″ thick, are stacked, with a thin double sided black adhesive strip between each plastic strip. The stack may include 2000-3000 of the strips. The strip stack is then pressed under high pressure to eliminate air bubbles and improve adhesion. Another method includes coating a plurality of glass sheets on each of two faces with a first substance having an index of refraction lower than that of the glass sheets, placing a first coated glass sheet into a trough sized slightly larger than the first coated glass sheet, filling the trough with a thermally curing black epoxy, stacking the plurality of coated glass sheets within the filled trough, curing the epoxy, forming, at two opposite ends of the stack, a back face and an outlet face, and connecting at least one reflector to the back face.  
           [0010]    The optical display panel for front projection applications solves problems experienced in the prior art by providing a display panel that allows for front projection, while also providing the appearance of a black screen to improve viewing contrast and to eliminate the need to dim lights in order to allow a viewer to see black images. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0011]    For the present invention to be clearly understood and readily practiced, the present invention will be described in conjunction with the following figures, wherein:  
         [0012]    [0012]FIG. 1 is an isometric view illustrating a cross section of a high contrast front projection display panel;  
         [0013]    [0013]FIG. 2 illustrates the use of a high contrast front projection display panel for movie projection;  
         [0014]    [0014]FIG. 3A is a cross sectional view of a high contrast front projection display panel having a planar diffusor and planar reflective portion;  
         [0015]    [0015]FIG. 3B is a cross sectional view of a high contrast front projection display panel having a planar diffusor and an angled reflective portion;  
         [0016]    [0016]FIG. 3C illustrates the reflection of light in a high contrast front projection display panel;  
         [0017]    [0017]FIG. 3D is a cross sectional view of a high contrast front projection display panel having a diffusive reflector;  
         [0018]    [0018]FIG. 3E is a cross sectional view of a high contrast front projection display panel having an embossed diffusive reflector; and  
         [0019]    [0019]FIG. 4 is an isometric view illustrating a plurality of stacked waveguides.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for purposes of clarity, many other elements found in a typical optical display panel. Those of ordinary skill in the art will recognize that other elements are desirable and/or required in order to implement the present invention. However, because such elements are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements is not provided herein.  
         [0021]    [0021]FIG. 1 is an isometric view schematic illustrating a display panel  10 . The display panel  10  may include a plurality of stacked optical waveguides  16   a , an outlet face  16  at one end of a body  18  formed by the plurality of stacked waveguides  16   a , a back face  12  at a second end of the body  18 , at least one reflector  19  that reflects light within the body  18  at the back face  12 , and a light generator  21 .  
         [0022]    The body  18  is preferably solid and receives light  14  along the surface of the outlet face  16 . The light  14  is passed through the body  18  after entering the outlet face  16 , and is reflected back through the body  18  from the at least one reflector  19  to the outlet face  16 . In a preferred embodiment of the present invention, the body  18  is formed of the length, height, and width of the plurality of stacked waveguides  16   a.    
         [0023]    The plurality of stacked waveguides  16   a  forms the body  18  of the panel  10 , forms at one end of the stack  16   a  the back face  12 , and at a second end the outlet face  16 . The waveguides  16   a  may be formed of any material known in the art to be suitable for passing electromagnetic waves therethrough, such as, but not limited to, plastics, or glass. The preferred embodiment of the present invention is implemented using individual glass or plastic or polymer sheets, which are typically approximately 0.010-0.020″ thick, and which may be of a manageable length and width. The polymer used may be a suitable plastic laminate, such as Lexan®, which is commercially available from the General Electric Company®, or any polymers or acrylics, such as Plexiglass.  
         [0024]    The waveguides  16   a  are in the form of sheets or ribbons extending the full width of the outlet face  16  and are stacked to collectively form at their upper ends the height of the outlet face  16 . The waveguides  16   a  are disposed along their longitudinal light transmitting axes. The number of waveguides  16   a  may be selected for providing a corresponding vertical resolution of the outlet face  16 . For example,  525  of the waveguides  16   a  may be stacked to produce  525  lines of vertical resolution in the outlet face  16 . Since the waveguides  16   a  extend the full width of the outlet face  16 , horizontal resolution may be controlled by horizontal modulation of the image light  14 .  
         [0025]    Each of the plurality of waveguides includes a central core  26  for channeling the image light  14  through the waveguides, and each core  26  is disposed between cladding layers  28 . In a preferred embodiment of the present invention, the cladding layers  28  extend completely from the back face  12  to the outlet face  16  along the entire width of the outlet face  16 . A black layer  30  may be disposed within or between adjoining cladding layers  28  for absorbing ambient light  32  at the outlet face  16 , and may form multi-layer cladding layers  28 . The term black is used herein to encompass not only pure black color, but additionally, any functionally comparable dark color suitable for use in the present invention, such as dark blue. The black layer  30  is only necessary within the viewable region of the outlet face, but, in a preferred embodiment of the present invention, the black layer  30  extends completely from the back face  12  to the outlet face  16  along the entire width of the outlet face  16 . Additionally, the cladding layers  28  may be formed of gradients.  
         [0026]    Each central core  26  has a first index of refraction. The cladding layers  28  have a second index of refraction, lower than that of the central core  26 , for ensuring total internal reflection of the image light  14  as it travels from the outlet face  16  to the back face  12 , and back to the outlet face  16 . The core is thus bi-directional. In a preferred embodiment of the present invention, the cladding layers  28  are transparent in order to effectuate total internal reflection of the image light  14 , and thereby maximize the brightness of the light  14  at the outlet face  16 . The black layers  30 , if separate from the cladding layers, may have any index of refraction.  
         [0027]    The back face  12  and outlet face  16  are formed by the plurality of waveguides  16   a , wherein one end of each waveguide  16   a  forms a back face for that waveguide, and wherein the opposite end of each waveguide  16   a  forms an outlet for that waveguide  16   a . Each waveguide  16   a  extends horizontally, and the plurality of stacked waveguides  16   a  extends vertically. The light  14  may be displayed on the outlet face in a form such as, but not limited to, a video image  14   a . Consequently, in a preferred embodiment the plurality of waveguides  16   a  are stacked approximately parallel to the horizontal, thus placing the outlet face  16  and the back face  12  in the same plane from the horizontal and approximately equidistant from the horizontal.  
         [0028]    The outlet face  16  is formed by the plurality of stacked optical waveguides  16   a . The outlet face  16  is at one end of the body  18 , and receives light  14  from the light generator  21 . In the preferred embodiment, this light  14  is incident to the outlet face  16  at the critical angle or lower of the waveguide  16   a , thus allowing for total internal reflection of the light within the waveguide  16   a , thereby allowing for approximately all light projected from the light generator  21  to reach the back face  12 . The outlet face  16  is defined as the front of the body  18 . Additionally, the panel  10  has a height from the top to the bottom of the outlet face  16 , and a width from the left to the right of the outlet face  16 . The width and height may be selected to produce width to height aspect ratios of 4:3 or 16:9, for example, for use in a typical television application.  
         [0029]    The light generator  21  generates light  14  and passes the light to outlet face  16 . The light generator  21  may be a white light projector, such as an overhead projector, or may include a light source, and/or a light modulator, and/or imaging optics, such as a video or movie projector. The light  14  may be initially generated, for example, by the light source. The light source may be, for example, a bright incandescent bulb, a laser, an arc lamp, an LED, an RF excited gas discharge lamp, any solid state light source, or any phosphorescent, luminescent, or incandescent light source. The light  14  from the source may then be modulated by the modulator for defining individual picture elements, known in the art as pixels. Alternatively, the light may define a simple lighted item, such as an on/off switch. The imaging optics may include light folding mirrors or lenses. The imaging optics may be optically aligned between the outlet face  16  and the light modulator for compressing or expanding and focusing the light  14  as required to fit the outlet face  16 . The light  14 , after entry into the outlet face  16 , travels through the panel body  18  to the back face  12 , and reaches the at least one reflector  19 . The light  14  is projected at the waveguide critical angle or lower over the outlet face  16 , and is thus directed generally horizontally upon reflection from the at least one reflector  19  for projection outwardly from the outlet face  16 .  
         [0030]    The at least one reflector  19  is connected to at least one of the back faces  12 , or is embossed into at least one of the back faces  12 , in order to redirect the light  14 , which is incident in a direction generally horizontally inward through the body  18  from the outlet face  16 , back to a direction generally horizontally outward from the outlet face  16 . The at least one reflector may be within, pressed into, or without, the body  18  at the back face  12 . The at least one reflector may be connected to the back face  12  by an optical connection, being placed directly adjacent to the back face, or being glued to the back face, with or without air gaps, for example. The reflective portion of the reflector  19  may be, but is not limited to, a mirrored surface, such as a retro-reflector, a total internal reflection (TIR) retro-reflector, a reflective serration, a reflective coating, such as a reflective tape, a lens or series of lenses, a micro-lens or series of micro-lenses, a plane mirror, or a prism. Only light entering each waveguide  16   a  at the critical angle or lower reaches the back face reflector  19 , as most ambient and other light will enter the waveguide  16   a  at an angle greater than the critical angle, and will consequently be absorbed by the cladding between the waveguides  16   a , rather than being reflected from the outlet face  16  to the back face  19 . Therefore, ambient and other light not entering the waveguide at the critical angle or lower will not be reflected by the at least one reflector  19  back to the outlet face  16 , and light entering at the critical angle or lower will be so reflected. The at least one reflector may be a reflector  19  placed at the back face  12  of each waveguide  16   a , when covered with the at least one reflector  19 , causes reflection to occur back through the waveguide  16   a  and out the outlet face  16 , or the at least one reflector  19  may cover several or all waveguide back faces  12  which constitute the body  18 .  
         [0031]    Additionally, in a preferred embodiment, the at least one reflector includes a diffuser or disperser to reflect incoming light out of the outlet face  16  at, for example, 30 degrees from the vertical axis and 120 degrees from the horizontal axis. This dispersion allows for viewing by a much larger number of viewers, as those viewers can be off angle and, through the dispersion of the image light, still view the image. For example, as shown in FIG. 2, a movie projector may project a movie onto the outlet face  16 , which movie is then reflected back out the outlet face  16 , at a dispersed angle, to a wide viewing audience.  
         [0032]    The diffuser  19   a  may be attached to the reflective portion  19   b  of the reflector  19 , between the reflective portion  19   b  and the at least one back face  12 , as shown in FIG. 3A. The diffuser  19   a  may be planar in nature, as may be the reflective portion  19   b , as shown in FIG. 3A, or the reflective portion  19   b  may be angled, and may be a retroreflector, such as a TIR or mirrored surface, with a planar diffuser  19   a  between that angled reflective portion  19   b  and the at least one back end, as shown in FIG. 3B. In the embodiments of FIGS. 3A and 3B, horizontal spreading is preferably completely dependent on the diffuser  19   a , while vertical spreading is dependent on the diffuser  19   a  and the waveguide absorption angle, as shown in FIG. 3C. The vertical and horizontal dispersion angles should thus be tailored to the audience location, and the diffuser angle of diffusion should be chosen accordingly.  
         [0033]    In an additional preferred embodiment shown in FIG. 3D, the reflector  19  is a diffusive mirror, which combines the reflective portion  19   b  and the diffusor  19   a  into a single element. The diffusive mirror may be a glass mirror or a plastic mirror, and includes the reflective portion  19   b  on the diffusive mirror at a plane farthest from the at least one back face  12 . A diffusive microstructure is preferably present on the glass or plastic under the reflective portion  19   a  of the reflector  19 . FIG. 3E illustrates the reflector  19  as an embossed reflective and/or diffusive microstructure, which is embossed directly onto the at least one back face  12 .  
         [0034]    The plurality of stacked waveguides  16   a , including the at least one reflector, may be formed by several methods. The plurality of stacked waveguides is shown in FIG. 4. A plurality of glass sheets may be used as the central cores  26 , and may be individually coated with, or dipped within, a clear, or black, substance having an index of refraction lower than that of the glass, such as, but not limited to, polyurethane, clear coat containing dyes, silicones, cyanoacreylates, low index refraction epoxys, plastics, and polymers, thereby forming a coated glass sheet. This clear or black substance is the opposed cladding layers  28 . Where a clear cladding layer is placed, a first coated glass sheet may then be placed in a trough sized slightly larger than the first coated glass sheet. The trough may then be filled with a thermally curing black epoxy. The black epoxy need not possess the properties of a suitable cladding layer.  
         [0035]    After filling of the trough with either clear sheets in a black epoxy, or black coated sheets in any epoxy, the coated glass sheets are repeatedly stacked, and a layer of epoxy forms between each coated glass sheet. The stacking is preferably repeated until between approximately 500 and 800 sheets have been stacked. Uniform pressure may then be applied to the stack, thereby causing the epoxy to flow to a generally uniform level between coated glass sheets. The stack may then be baked to cure at 80 degrees Celsius for such time as is necessary to cure the epoxy, and the stack is then allowed to cool slowly in order to prevent cracking of the glass.  
         [0036]    The back face  12  and the outlet face  16  may be cut as planar or curved as desired, and the back face  12  may be specially shaped to form a desired shaped surface to allow for proper operation of the at least one reflector  19 . The cut portions of the panel  10  may then be polished with a diamond polisher to remove any saw marks. The at least one reflector  19  is then added to the back face, either in the form of a coating placed on the back face or faces  12 , a mirror, lens, or prism glued to the back face or faces  12 , or a reflective attachment, such as a reflective tape, being fastened to the back face or faces  12 .  
         [0037]    In an additional preferred embodiment, clear strips of plastic, which are preferably approximately {fraction (3/4)}″ by 40″, and approximately {fraction (20/1000)}″ thick, are stacked, with a thin double sided black adhesive strip between each plastic strip. The stack may include 2000-3000 of the strips. The strip stack is then pressed under high pressure to remove air bubbles and increase adhesion. In one embodiment, the adhesive is Research AR8350, {fraction (1/1000)}″ to {fraction (2/1000)}″ thick black double sided adhesive. The adhesive may be shades other than black, such as dark blue, and preferably rolls out like a form of tape, in a plastic/adhesive/plastic/adhesive format. The pressure applied to the completed stack is preferably in excess of 1,000 pounds.  
         [0038]    In a second embodiment of the present invention, the coated glass sheets or plastic strips may be coated with a black substance, such as spray paint, before being stacked with an adhesive, which need not be a dark shade in this embodiment, between the strips, or before being placed into the epoxy trough. In another embodiment of the present invention, the coated blackened glass sheets may be individually fastened using glue or epoxy. In another embodiment of the present invention, both the clear substance and the black layer could be formed of a suitable substance and placed, in turn, on the glass core using sputtering techniques known in the art, or deposition techniques known in the art.  
         [0039]    Those of ordinary skill in the art will recognize that many modifications and variations of the present invention may be implemented. The foregoing description and the following claims are intended to cover all such modifications and variations.