Patent Document

RELATED U.S. PATENT DOCUMENTS 
     Ser. No. 12/004,115 Dec. 19, 2007; Ser. No. 12/583,156 Aug. 13, 2009 which are included here as reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to displays. More particularly the invention concerns displays comprising an optical waveguide, a light source and a plurality of electromechanical picture elements. 
     2. Discussion of the Prior Art 
     Currently liquid crystal displays dominate the flat panel display market. The overall light efficiency of a typical prior art liquid crystal display (LCD) is below 10% mainly due to the fact that light from the backlight assembly has to pass several layers of polarizers, color and neutral density filters. A further problem with LCDs is the slow response time of the liquid crystal resulting in objectionable visible motion artifacts when displaying motion images. 
     Recently, micro-mechanical flat panel displays based on an optical waveguide were proposed as a viable alternate to LCDs. These displays typically consist of a planar waveguide with parallel surfaces on which a matrix of electrically driven micro-mechanical picture elements is constructed. Light from a light source is introduced to the waveguide from one or more sides of the waveguide and is confined within the waveguide by total internal reflections. Light is extracted from the planar surface of the waveguide by coupling to evanescent waves or by deforming the surface of the planar waveguide to produce an image. There is an inherent optical crosstalk problem when picture elements are simultaneously activated to display an image. The state of one picture element changes the brightness of other picture elements. 
     Another common problem concerns the use of mirror surfaces to redirect light to the viewer. The same mirror surface reflects the ambient light back to the viewer thereby significantly reducing the contrast at high levels of ambient light. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a display that effectively overcomes the optical crosstalk problem typically found in prior art optical waveguide-based displays. In one form of the invention this object is achieved by providing a display that comprises a light source and an optical waveguide. The optical waveguide distributes light to a plurality of light exits. At each light exit a picture element modulates light by selectively directing the light to the viewer or to a light absorber. 
     Another object of the invention is to provide a high contrast display of the character that operates at high levels of ambient light. Embodiments of the invention achieve this object by providing a display wherein the majority of the viewing surface is coated with a light-absorbing coating. 
     Another object of the invention is to provide a display that can compete with LCD&#39;s in light efficiency, picture quality and cost. Increased light efficiency is achieved by providing a display in which light travels most of the light path by total internal reflections. Improved picture quality is achieved by providing fast and efficient light modulators. 
     The foregoing as well as other objects of the invention will be achieved by, the novel display illustrated in the accompanying drawings and described in the specification that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a generally perspective view of the display of the present invention. 
         FIG. 2  is a cross-sectional view taken along lines  2 - 2  of  FIG. 1 . 
         FIG. 3  is an enlarged view of the area designated as  3 - 3  in  FIG. 2 . 
         FIG. 4  is a generally cross-sectional view of an alternate form of the display of the invention. 
         FIG. 5  is an enlarged view of the area designated as  5 - 5  in  FIG. 4 . 
     
    
    
     DESCRIPTION OF THE INVENTION 
     Referring to the drawings and particularly to  FIGS. 1 and 2 , one form of the display of the invention is shown there and generally designated by the numeral  20 . As best seen in  FIG. 1 , display  20  here includes a generally rectangular shaped optical waveguide  21  that is substantially wedge-shaped cross section. Waveguide  21  is preferably constructed from acrylic or other optically transparent material, having a refractive index n 1  with a value between approximately 1.45 and approximately 1.6 and comprises parallel first and second end surfaces  26  and  27  that are joined by parallel side surfaces  28  and  29  (see  FIG. 1 ). Waveguide  21  also includes a major upper surface  30  and a lower surface  31  converging with upper surface  30 . The lower surface  31  as generally shown in  FIG. 1  is a flat surface and forming an angle  22  ( FIG. 2 ) with a value between approximately 0.1 degrees to approximately 2.0 degrees with the upper surface  30 . Also the lower surface  31  may be a curved surface forming varying angles with the upper surface  30  of the waveguide  21  or include a plurality of stepwise facets for controlling the display light uniformity. A plurality of substantially equally spaced-apart micro-prisms  32  are constructed at upper surface  30  and, as generally shown in  FIG. 1 , extend between side surfaces  28  and  29 . Micro-prisms  32  may be molded or constructed using lithography from a UV curing polymer having a refractive index n 2  with a value between approximately 1.45 and approximately 1.6. LED light sources  25  are installed proximate the wide edge  26  of the waveguide  21  and a plurality of tilting micro-shutters  33  are constructed between micro-prisms  32 . In  FIG. 2 , one column of the tilting micro-shutters is designated as  33   a ,  33   b , and  33   c .  FIG. 1  also illustrates a section of a cover assembly  34 . More detailed construction of the cover assembly  34  is illustrated in  FIG. 3 . 
     Now referring to  FIG. 3  of the drawings where more details of multi-layer optical coatings are shown. The first layer is a light reflecting layer  35  constructed from metal or multilayer dielectric materials on the upper surface  30  of the waveguide  21 . The light reflecting layer  35  is patterned to form plurality of light reflecting regions  36  and light transmitting regions  37 . The second optical layer is a light transmitting layer  39  formed from a fluoropolymer or other substantially transparent material having a refractive index n 3  with a value between approximately 1.3 and approximately 1.4. The light transmitting layer  39  is formed only in the light transmitting regions  37  as shown in  FIG. 3  on the upper surface  30  of the waveguide  21 . Also the light transmitting layer  39  may be a continuous layer formed between the light reflecting layer  35  and upper surface  30  of the waveguide  21 . The third optical layer is a light absorbing layer  40  formed from a dielectric material on light reflecting layer  35  and is patterned to partially cover light reflecting layer  35 . A black oxide layer may be formed on upper surface of light reflecting layer  35  instead of light absorbing layer  40 . Also the light absorbing layer  40  may be replaced with a light absorbing film and placed below the lower surface  31  of the waveguide  21 . 
     Further illustrated in  FIG. 3  are micro-prisms  32 . Each micro-prism  32  comprises a light input facet  41  which is optically coupled to the upper surface  30  of waveguide  21  via light transmitting layer  39  and a light exit facet  42  which is inclined with respect to the upper surface  30  of waveguide  21  and forms an angle  23  with a value between approximately 45 degrees to approximately 65 degrees. Micro-prisms  32  further include a facet  43  which is inclined opposite to the light exit facet  42  and an upper facet  47  which is generally parallel to the light input facet  41 . 
       FIG. 3  also illustrates one of the tilting micro-shutters  33   b  which typifies the construction of each of the micro-shutters of the present form of the invention. Micro-shutter  33   b  comprises a thin aluminum alloy elastic film that is affixed to the upper facet  47  of micro-prism  32   b  and it tilts in two directions at about axis  50  that is substantially parallel to the upper surface  30  of the waveguide  21 . For absorbing light, a black oxide layer or a black polymer film may be formed on surfaces of micro-shutters  33 . 
     Further illustrated in  FIG. 3  is a cover assembly  34  which is affixed to the upper surface  30  of waveguide  21  with spacers  58  (see  FIG. 2 ). Cover assembly  34  comprises a substrate  44  made of glass or other substantially transparent material. A light absorbing layer  51  constructed on the lower surface  46  of substrate  44  from conductive light absorbing film or a multilayer film that includes a conductor layer. The light absorbing layer  51  is patterned to form a plurality of display light exit regions  48  located directly above micro-shutters  33  and light absorbing regions  49 . The cover assembly  34  further includes a light shaping diffuser  52  formed on the upper surface  45  of substrate  44 . 
     In the present form of the invention, the tilting micro-shutters  33  operate by electrostatic attraction force. The conductive light reflecting layer  35  and conductive light absorbing layer  51  act as fixed electrodes for the tilting micro-shutters  33 . 
     When a suitable voltage is applied between the conductive light reflecting layer  35  and a micro-shutter  33 , the micro-shutter tilts down by electrostatic attraction force. When a suitable voltage is applied between the conductive light absorbing layer  51  and a micro-mirror  33 , the micro-shutter tilts up by electrostatic attraction force. 
     To prevent micro-shutter stiction, a small gap is required between the edge of the micro-shutters and the landing surfaces. This may be realized by constructing small spacers from a low friction material on landing surfaces or extending small portions of micro-shutters along the edge so the entire edge of the micro-shutters do not touch the landing surfaces. Additionally the black polymer coatings on the micro-shutters may be formed from a non stick material. 
     As best seen in  FIG. 2  of the drawings, light rays  55  entering from the wide edge  26  of the waveguide  21  reflect from the upper surface  30  and the lower surface  31  by total internal reflections and change angles towards normal with respect to the upper surface  30 . Light rays  55  exit the waveguide  21  from the light transmitting regions  37  ( FIG. 3 ) when the incident angle is less than the critical angle  38  defined by the refractive index n 1  of the waveguide  21  and refractive index n 3  of light transmitting layer  39 . Light rays passing through the light transmitting layer  39  enter the micro-prisms  32  from the light input facet  41  and change the angle defined by the refractive index n 2  of the micro-prisms. Light rays exit the micro-prisms from the light exit facets  42 . 
     Depending on the positions of the tilting micro-shutters, light rays are absorbed, or directed to the viewer. 
     When a tilting micro-shutter is in the up position, such as micro-shutter  33   b  ( FIG. 2 ), most light rays exiting from light exit facet  42  of micro-prisms  32  are absorbed in light absorber coatings of micro-shutters  33 . Any light reflected from the lower surface of the micro-shutters  33  will be absorbed in the light absorbing layer  40 . 
     When a micro-shutter is tilted down, such as micro-shutters  33   a  and  33   c , most light rays exiting from light exit facet  42  of micro-prisms  32  exit the display  20  from display light exit regions  48  and are directed to the viewer. 
     Referring now to  FIG. 4  of the drawings, a cross-sectional view of another embodiment of display of the present invention is there shown and generally designated by the numeral  70 . This latest embodiment is similar in some respect to the embodiment shown in  FIGS. 1 and 2  of the drawings and like numbers are used in  FIG. 4  to identify like components. 
     The display  70  is a full color display wherein each picture element comprises of Red, Green and Blue sub-pixels and includes dichoric filters for separating RGB colors from a white light source or from RGB light sources that are mixed in the waveguide  21 . 
     The display  70  includes optical waveguide  21  and LED light sources  25  that are installed proximate the wide edge  26  of the waveguide  21 . Display  70  also includes a substrate  72  constructed from a substantially transparent material such as glass having a refractive index n 4  with a value between approximately 1.45 and approximately 1.6. The lower surface  74  of substrate  72  is optically coupled to the upper surface  30  of waveguide  21  via an optical layer  71  formed from a substantially transparent material having a refractive index n 3  with a value between approximately 1.3 and approximately 1.4. 
     A plurality of equally spaced-apart micro-prisms  32  are constructed at upper surface  73  of substrate  72  and tilting micro-shutters  33  are constructed between micro-prisms  32 . The cover assembly  34  is affixed to the upper surface  73  of substrate  72  with spacers  58 . 
     Now referring to  FIG. 5  of the drawings where more details of multi-layer optical coatings are shown. The first optical layer is a dichroic filter  75  formed on the upper surface  73  of the substrate  72 . The second optical layer is a light reflecting layer  35  constructed from metal on the dichroic filter  75 . The light reflecting layer  35  is patterned to form plurality of light reflecting regions  36  and light transmitting regions  37 . The third optical layer is a light absorbing layer  40  formed on light reflecting layer  35  and is patterned to partially cover light reflecting layer  35 . 
     Also illustrated in  FIG. 5  are micro-prisms  32 . Each micro-prism  32  comprises a light input facet  41 , which is optically coupled to the upper surface  30  of waveguide  21  via dichroic filter  75 , substrate  72  and optical layer  71 . Each micro-prism  32  also includes a light exit facet  42 , a facet  43  which is inclined opposite to the light exit facet  42  and an upper facet  47  which is generally parallel to the light input facet  41 . 
       FIG. 5  also illustrates one of micro-shutters  33  and cover assembly  34  that was described before in  FIG. 3 . 
     As best seen in  FIG. 4  of the drawings, light rays  55  entering from the wide edge  26  of the waveguide  21  reflect from the upper surface  30  and the lower surface  31  by total internal reflections and change angles towards normal with respect to the upper surface  30 . Light rays  55  exit the waveguide  21  from the upper surface  30  and enter substrate  72  through the light transmitting layer  71  when the incident angle is less than critical angle  38  ( FIG. 5 ) defined by the refractive index n 1  of the waveguide  21  and refractive index n 3  of light transmitting layer  71 . Dichroic filters selectively pass RGB colors in the light transmitting regions  37  into the micro-prisms  32 . And light exits micro-prisms  32  from the light exit facets  42 . As before depending on the positions of the tilting micro-shutters  33 , light rays are absorbed, or directed to the viewer. 
     To increase the efficiency and reduce light scattering, various anti-reflection coatings may be applied to surfaces where light transitions between two different materials. Dichroic layers that comprise a low pass filter for the Blue color and a high pass filter for the Red color may be formed to overlap in the light reflecting regions  36 . 
     The above described displays will work with infrared, visible and ultraviolet light sources and combinations thereof. 
     Depending on the display size and resolution, each picture element of the display panel may include several tilting micro-shutters. Reducing the size of individual micro-shutters helps to reduce the required electrostatic actuation voltages. 
     Also, micro-shutters for each picture element may be grouped to modulate different levels of light when suitable voltage is applied between the fixed electrodes and a selected group of micro-shutters. This reduces the display addressing constraints. For example, each picture element may include 7 micro-shutters grouped in quantities of 1, 2 and 4 and selectively addressed to modulate 8 levels of light. Additionally, temporal artifacts inherent in pulse-width-modulation displays are reduced. 
     Having now described the invention in detail in accordance with the requirements of the patent statutes, those skilled in this art will have no difficulty in making changes and modifications in the individual parts or their relative assembly in order to meet specific requirements or conditions. Such changes and modification may be made without departing from the scope and spirit of the invention, as set forth in the following claims.

Technology Category: 3