Patent Publication Number: US-2009225531-A1

Title: Night vision imaging system (NVIS) compliant backlight

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention (Technical Field) 
     This present invention relates to night vision imaging systems (NVIS), and more particularly to backlights for transmissive displays required to meet NVIS radiance requirements. It also relates to general illumination assemblies, which are used with night vision goggles. 
     2. Background Art 
     The invention resulted from the desire to create a simple, low cost, scalable NVIS compliant backlight while maintaining optical efficiency and suitable heat sinking in an active-matrix liquid crystal display (AMLCD) system. Typically, optical filters are used to achieve NVIS compliance for full color displays and the filters are placed between the viewer and the light source. The optical filter may be over the entire backlight opening or just over the light source used for NVIS mode operation. 
     There are multiple examples of these types of backlight architectures. U.S. Pat. No. 5,661,578 discloses separate daytime and nighttime light sources. The NVIS compatible nighttime source is positioned to be out of sight of a user and produces an appropriately filtered, substantially collimated beam which diffusively reflects from the inside walls and base of the display housing before it illuminates the display surface. This device uses a NVIS filter located over an indirect nighttime lamp assembly used to achieve NVIS compliance. 
     U.S. Pat. No. 6,111,622 discloses a fluorescent lamp for daytime operation and a separate light source for nighttime operation which emits a low-intensity light. The nighttime light source is an electroluminescent panel which also acts as a daytime reflector of light. An integral image-splitting and collimating lens is provided to increase the uniformity of the light emitted and to provide wide angle viewability. U.S. Pat. No. 6,100,952 discloses a backlight that includes a ridged prismatic TIR with an embedded diffuser layer doped with an infrared (IR) absorbing dye, a ridged prismatic TIR without diffuser layer, an infrared rejecting filter layer, and either a holographic diffuser layer or a lenticular lens array layer, or a fresnel wedge layer. Both of these patents describe covering the entire AMLCD with an NVIS filter. 
     U.S. Pat. No. 6,842,204 teaches a LCD display which includes a normal mode light source, a liquid crystal display stack positioned so as to receive light from the normal mode light source, and a night vision imaging system (NVIS) mode light source. The NVIS mode light source includes a NVIS mode white light source, a NVIS mode red light source, a NVIS filter receiving light from both the NVIS mode white light source and the NVIS mode red light source, and a waveguide receiving light from the NVIS filter for reflection to the LCD stack. U.S. Pat. No. 5,211,463 discloses a duel system wherein the day lighting system uses conventional fluorescent or incandescent lamps for full color display during daytime use. The night lighting system uses a light source which is either filtered to remove infrared and near infrared wavelengths or which is chosen from a class of sources which does not emit such wavelengths. The day lighting system is disabled while the night system is in use. Both of the patents disclose devices that use a NVIS filter over the NVIS mode light sources with a light guide. 
     These existing prior art approaches for providing NVIS compliance for a full color display make use of primary and secondary lighting systems. Some systems use an optical filter covering the entire backlight opening and some systems use an optical filter covering just the light source used for NVIS mode operation. Filters that cover the entire display surface can be large and very expensive. The optical filters attenuate the visible spectrum significantly and can be a source of inefficiency in a high brightness display system. Other NVIS compliant backlight systems using indirect lighting techniques for the NVIS filtered light source are not easily scalable to other shapes and sizes or use indirect light sources that are difficult to construct and difficult to remove the generated heat. 
     SUMMARY OF THE INVENTION (DISCLOSURE OF THE INVENTION) 
     The present invention is a dual mode illumination system with a primary mode providing high efficiency high brightness illumination and a secondary mode providing NVIS compliant illumination. The primary light source is direct view, not NVIS filtered and used for non-NVIS applications. The secondary light source is filtered for NVIS compliance and is not in direct view. During NVIS applications the primary light source is turned off and light from the secondary source is directed toward the viewer through a light guide. Both light sources are mounted on the same plane. 
     A primary object of the present invention is to provide an illumination system which does not interfere with the use of night vision goggles. This system can be used for backlighting transmissive displays or as a general ambient illumination system. 
     A primary advantage of the invention it that it allows both the primary and NVIS light sources to be mounted on the same plane or orientation. If LEDs are used as the light sources, the NVIS mode and primary mode LEDs may be mounted on the same PWB. 
     Another advantage is the invention is easily scalable from small displays to large displays by scaling the size of the light guide and primary light source to the size of the AMLCD. 
     Another advantage is that light from the primary light source is unfiltered so the color and efficiency of the lighting system is not degraded by the NVIS filter during non-NVIS applications. 
     Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
       The accompanying drawings, which are incorporated into and form a part of the specification, illustrate several embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating a preferred embodiment of the invention and are not to be construed as limiting the invention. In the drawings: 
         FIG. 1  depicts a prior art light guide. 
         FIG. 2  is cross section of the preferred embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS (BEST MODES FOR CARRYING OUT THE INVENTION) 
     The present invention improves on the design and method for using a prior art light guide.  FIG. 1  depicts a typical prior art light guide used in LCD monitors. Light guide  14  is rectangular in shape. Light  12  from light source  22  enters ends of light guide  14  as shown. Reflectors  16  improve the efficiency of light source  22  by directing all emitted light  12  into light guide  14 . The light propagates the length of light guide  14  by total internal reflection. The prior art light guide  14  has a pattern of dots, grooves or other treatment (not shown) on the surface to scatter the propagating light toward diffuser  18  to provide diffuse light  20 . Reflector  24  improves efficiency be reflecting back-scattered light toward the diffuser. 
       FIG. 2  is a cross sectional view of the preferred embodiment of the invention. The preferred embodiment consists primarily of a primary light source  30 , secondary light source  32 , NVIS filter  34 , light guide  36 , light integrating cavity  38  and mounting plate  40 . Primary light source  30  is enclosed in a light-integrating cavity  38 . In primary mode or during non-NVIS applications the majority of light  42  produced by primary light source  30  passes through light guide  36 . Light guide  36  is made of a material highly transparent to visible light to maximize luminous efficiency. A diffuser  44  is placed in front of light guide  36  to give the illumination system a uniform light output. Because some light is reflected back into the light integrating cavity  38  from diffuser  44  and light guide  36 , the cavity  38  walls are be covered with a high reflectance material  46  to reflect light back into light integrating cavity  38  and eventually through light guide  36  and diffuser surface  44 . The primary light source  30  may be a bent fluorescent lamp or lamps, an array of straight fluorescent lamps or an array of LEDs. The reflective light-integrating cavity  38  should be of sufficient depth to provide diffuse light on the surface of diffuser  44 . 
     Secondary light source  32  may be a fluorescent lamp or LED. Primary light source  30  is disabled when secondary light source  32  is used for NVIS compliant operation. Light  48  from secondary light source  32  passes through an NVIS filter  34  and enters light guide  36  from the same surface  50  or plane as primary light source  30 . NVIS filter  34  limits the amount of infrared or near infrared spectral content in the light that interferes with the operation of NVIS goggles produced by secondary light source  32 . Secondary light source  32  is mounted in a sealed compartment  52  to ensure no light from secondary light source  32  enters light integration cavity  38  without passing through NVIS filter  34 . 
     Light guide  36 , which is commonly used in commercial LCD monitors as described above, is modified for this invention. The current invention uses the same principles as the common light guide except light enters from the rear instead of the ends. The rectangular shape of the common light guide is modified by beveling edges  54  of light guide  36 . Light from secondary light source  32  enters light guide  36  from the rear and reflects off of beveled edges  54  into the light guide  36 . Light propagating through light guide  36  reflects off of the light guide surface pattern (not shown) toward the diffuser  44  and provides a diffuse source of NVIS compliant light  56 . 
     Primary light source  30  and secondary light source  32  are attached to mounting plate  40  in the same orientation to emit light toward light guide  36  and diffuser  44 . Mounting plate  40  provides structural integrity to the illumination system and acts as a heat sink for the light sources. 
     Although the invention has been described in detail with particular reference to these preferred embodiments, other embodiments can achieve the same results. Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover in the appended claims all such modifications and equivalents. The entire disclosures of all references, applications, patents, and publications cited above, are hereby incorporated by reference.