Abstract:
An edge-lit backlighting system for liquid crystal displays employs a number of light strips in the form of thin, light weight sections of a thermally conductive substrate each of which mounts a plurality of light engines having an array of densely packed, primary color light emitting diodes, which, when collectively illuminated, produce a white light of desired reference white chromaticity.

Description:
FIELD OF THE INVENTION 
   This invention is directed to an edge-lit backlighting system for liquid crystal displays, and, more particularly, to a backlighting system employing a number of light strips in the form of thin, light weight sections of a thermally conductive substrate each of which mounts a plurality of light engines having an array of densely packed, primary color light emitting diodes (“LEDs”) which, when collectively illuminated, produce a white light of desired reference white chromaticity. 
   BACKGROUND OF THE INVENTION 
   Liquid Crystal Displays (“LCDs”) are the technology of choice for avionics displays and a number of other applications. Among the advantages of LCDs are: they conserve weight and space, have simple electrical interfaces, are capable of preserving contrast for sunlight viewing with appropriate enhancements, can be made sufficiently rugged to withstand difficult mechanical and environmental conditions, can be made to operate over extreme temperatures and can be made compatible with night vision imaging systems. The LCD is a transmissive device; as such, it requires a source of rear illumination (backlight) to render the intended image. Each of the subassemblies of the overall system, e.g. LCD and backlight, shoulder different responsibilities in the integrated system. Some performance attributes are shared, while others are unique to each subassembly. As such, design considerations for each subassembly must be friendly to the other to ensure the integrated system can accommodate the rigor of the intended application. 
   Currently, the most popular method of back-illuminating an LCD is to use a serpentine fluorescent lamp in a reflecting housing with a heavy diffuser overlying the lamp to provide balanced luminance. The serpentine fluorescent lamp is mounted in a frame or housing having a reflecting surface, and a diffuser panel or sheet is then placed between the lamp and LCD. An alternative arrangement is the so-called “edge-lit” backlight subassembly which employs one or more thin, elongated fluorescent lamps each mounted along a side edge of a light guide. A sheet or film of reflective material is located on one side of the light guide, and one or more enhancement films such as light diffusers or Brightness Enhancement Films (BEF) are positioned on the opposite side of the light guide in between it and the LCD. When the fluorescent lamp(s) are illuminated, the light guide transmits and directs the light toward the enhancement films to illuminate the LCD. 
   Backlight subassemblies of the type described above are considered the “weak link” which can compromise the optical performance, environmental performance, and life expectancy of LCDs systems in many applications. This is principally due to shortcomings of the fluorescent lamps. Although an effective means of producing visible light, fluorescent lamps have a number of deficiencies which present difficulties when employed in backlighting subassemblies. Fluorescent lamps have poor reliability and poor efficiency when used in a backlight system. It is estimated that as much as 75% of the theoretical maximum luminance is lost with fluorescent lamp backlights. Fluorescent lamp backlights are not mechanically robust, they are difficult to dim, they are difficult to start at cold temperature and have significantly reduced light output at both high and low temperatures. The color gamut of fluorescent backlights is reduced as compared to LEDs, for example, and reduced color gamut of the light which illuminates the LCD reduces image fidelity. Serpentine fluorescent lamps have a relatively deep profile which increases the overall weight and size of the LCD system, making it less than desirable in some applications. Further, fluorescent lamps of all types contain mercury which presents a disposal issue when the lamps are replaced. 
   A color display is an additive color system. It takes at least one red, green and blue sub-pixel to make a white light color group. The white light color group is commonly referred to as a pixel. In an LCD, there are discreet color filters, e.g., red, green and blue, resident at each sub-pixel. These filters subtract unwanted wavelengths of light from the aggregate white-light backlight to produce the desired color. With the relatively recent (circa 1997) development of blue LEDs, considering that red and green LEDs were already in existence, the LED has been suggested as a replacement for fluorescent lamps in backlight systems as discussed, for example, in U.S. Pat. Nos. 5,727,862 and 6,719,436. The LED produces light when electrons flow across a P—N junction doped with the proper light-emitting compound. Whereas phosphor chemistry employed in the manufacture of fluorescent lamps is a mature science, LED chemistry is still in its infancy and significant gains in efficiency compared to fluorescent lamps, perhaps on the order of 200%, are expected in the coming years. 
   There are three competing configurations for generating white light using LEDs. These include an assembly which mounts discrete red, green and blue LEDs immediately adjacent to one another so that when they are collectively illuminated white light is produced, an ultraviolet LED coated with a red, green and blue phosphor coating, and, a blue LED coated with a yellow phosphor coating. To enhance the images produced by an LCD, the backlight spectra should produce emission peaks for the red, green and blue emission bands which match the peak transmission of the color filters on the LCD. The latter two approaches for producing white light noted above perform relatively poorly in that respect. Although improved performance can be obtained with an assembly which combines red, green and blue LEDs, the question has been how clusters or assemblies of such primary color LEDs can be efficiently and effectively incorporated into a backlight system to obtain the desired physical and other performance parameters, e.g. size, weight, durability, luminance, intensity, color gamut etc. 
   SUMMARY OF THE INVENTION 
   This invention is directed to a an edge-lit backlighting subassembly for use with a LCD system which employs a multiplicity of primary color LEDs densely packed along a thin, elongated strip which can be broken or cut to a desired lengths and placed about the edges of the backlighting subassembly. The LEDs are chosen so that when they are collectively illuminated a white light of desired reference white chromaticity is produced with excellent color gamut. A heat transfer structure is provided to remove heat from the area of the LEDs. 
   In the presently preferred embodiment, a plurality of light engines each including an array of densely packed primary color LEDs are mounted at spaced intervals to a thermally conductive substrate such as a printed wiring board (PWB). The LEDs are preferably hosted on the light engine by a low temperature co-fired ceramic material having a metal heat spreader. In one embodiment, each light engine has two or more parallel legs consisting of several LEDs, and the LEDs in each leg are connected in series. The number of LEDs, and their respective colors, e.g., red, green or blue, are chosen to produce white light of desired reference white chromaticity. In an alternative embodiment, three arrays of LEDs are provided on each light engine, with the same color LEDs (red, green or blue) being grouped in respective arrays. A forward voltage and current is applied to the LEDs in each array which is separately controlled to permit independent variation of the intensity of the red, green or blue light produced by the different arrays. In either embodiment, when the LEDs are collectively illuminated, the resulting white light is transmitted to a light guide, which, in turn, transmits the light through one or more enhancement films to the LCD subassembly of the overall system. 
   In order to enhance the transmission of light between the light engines and the light guide, a reflective sheet is provided having an upturned, peripheral edge formed with a number of spaced apertures. Each aperture receives the cluster of LEDs on one of the light engines, and covers the spaces along the PWB between adjacent light engines. This prevents the white light collectively emitted from the LEDs from being absorbed by the PWB, or escaping in the area of the interface between the light engines and light guides, thus enhancing the overall efficiency of the backlight subassembly. 
   Heat transfer structure is provided to dissipate the heat produced by the LEDs on the light engines. As noted above, the LEDs are hosted on the light engines by a low temperature co-fired ceramic or similar material which mounts a metal heat spreader. Each light engine, in turn, is mounted by an interface material formed of silver and palladium, or a similar conductor, to the PWB. The PWB carries at least one conductive strip or plate, such as copper, which extends in a longitudinal direction along its length. Additionally, a number of via, also formed of copper or the like, extend in the perpendicular direction between the light engine on one side of the PWB and the opposite surface of the PWB. The PWB is then connected to a heat exchanger by a layer of flexible, heat conductive material which forms a seal at the interface of both the PWB and heat exchanger. The afore-mentioned structure collectively provides a path for the transfer of heat from the area of the LEDs to the heat exchanger, which, in turn, can be mounted to other heat conductive structure within which the system is housed to further dissipate heat. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The structure, operation and advantages of the presently preferred embodiment of this invention will become further apparent upon consideration of the following description, taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is an exploded, perspective view of a display system including an LCD subassembly and a backlighting subassembly; 
       FIG. 2  is a schematic view of a portion a light strip mounting a number of spaced light engines; 
       FIG. 3  is a side view of a light engine depicting one form of LED suitable for use in this invention; 
       FIG. 4  is a schematic view of one arrangement of an array of LEDs of a light engine employed herein; 
       FIG. 5  is a view similar to  FIG. 4 , except of an alternative LED array; 
       FIG. 6  is a schematic, disassembled view showing the connection between the light engines on the light strip and apertures formed in the upturned, peripheral edge of the light reflective sheet; 
       FIG. 7  is a side view of the heat transfer structure of this invention; and 
       FIG. 8  is an enlarged, cross sectional view of the printed wiring board which mounts the light engines. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring initially to  FIG. 1 , the light system  10  of this invention comprises an LCD subassembly  12  and a backlighting subassembly  14 . The backlighting subassembly  14  collectively includes a heat exchanger  16 , a light reflective sheet  18 , a number of light strips  20 , a light guide  22  and at least one light enhancement film  24 . The detailed construction of the LCD subassembly  12 , the heat exchanger  16 , the light guide  22  and the light enhancement film  24  forms no part of this invention and is therefore not discussed herein. The structure and operation of the light strips  20  is described first, followed by a discussion of their connection to the light reflective sheet  18  and how the entire light system  10  is assembled. 
   As best seen in  FIGS. 2 through 5 , each of the light strips  20  comprises a thermally conductive substrate such as a printed wiring board (PWB)  26  formed in an elongated strip which mounts a number of longitudinally spaced light engines  28 . Preferably, the PWB  26  has perforations  30  or other lines of weakness at intervals along its length where the PWB  26  may be broken or cut to form a strip of a desired length. It is contemplated that the PWB  26  with the light engines  28  will be formed in long rolls or the like for ease of transport and handling, and then separated along the perforations  30  to a desired length depending upon the physical size of a particular LCD subassembly  12 . 
   Each light engine  28  includes a clustered array  32  of primary color LEDs  34 , e.g. red, green and blue, as depicted by the corresponding letters in  FIGS. 4 and 5 . In the embodiment shown in  FIGS. 2 and 4 , each array  32  consists of two parallel legs of six LEDs  34  connected in series within each leg. To obtain a white light having a reference white chromaticity of 5400 Kelvins, for example, a combination of three green LEDs  34 , two red LEDs  34  and one blue LED  34  is employed in each parallel leg of an array  32 . It should be understood that the number of parallel legs and LEDs  34  in each array  32 , and the particular combination of primary color LEDs  34  selected, may be varied as desired to obtain a white light of desired reference white chromaticity when the LEDs are collectively illuminated. Further, the LEDs  34  can be connected in series with no parallel legs, if desired. The arrangement depicted in  FIGS. 2 and 4  is therefore provided for purposes of illustration only, and is not intended to limit the scope of this invention. 
   In the alternative embodiment illustrated in  FIG. 5 , an array  36  of LEDs  34  is shown which consists of three groups  38 ,  40  and  42  of four red, six green and two blue LEDs  34 , respectively. The red and green LEDs  34  in groups  38  and  40  are arranged in parallel legs with the LEDs  34  in each leg being connected in series. The two blue LEDs  34  in group  42  are also connected in series. As schematically depicted, a voltage is applied to each group  38 ,  40  and  42  of LEDs  34  which is separately controlled to permit independent variation of the intensity of red, green or blue light produced by the respective groups  38 ,  40  and  42 . Consequently, white light having a wide range of reference white chromaticity may be obtained from the array  36  illustrated in  FIG. 5  by simply adjusting the current applied to each group  38 ,  40  and  42  within an array  36 . This adds to the versatility of the backlighting subassembly  14  of this invention, making it readily adaptable for use with LCD subassemblies  12  produced by different manufacturers. 
   The detailed construction of a light engine  28  is best seen in  FIG. 3 . One type of LED  34  is shown in  FIG. 3  for purposes of illustration, but it is contemplated that others may be employed so long as they are available in red, green and blue. The LEDs  34  are hosted by a low temperature co-fired ceramic or similar layer  44  connected to a metal heat spreader  46 , preferably in the form of a plate made of copper or the like. A layer  48  of molybdenum is sandwiched between the heat spreader  46  and a lower copper plate  50 , which, in turn, is connected to a bottom layer  52  preferably formed of silver/palladium. As schematically shown in  FIG. 3 , adjacent LEDs  34  are electrically connected by an aluminum wire band  54 , although other suitable means of connection may be used. One end of the wire band  54  connects to a silver via  56  which mounts to a layer  58  of silver material or the like resting atop a dielectric layer  60 . A clear layer or dome  64  of low temperature glass ceramic or similar material encases the LEDs  34  on each light engine  28  on the PWB  26 , as shown. 
   As noted above, the backlighting subassembly  14  of this invention is an edge-lit system wherein white light from the light strips  20  is directed to the sides of the light guide  22  which then transmits and directs the light toward the LCD subassembly  12 . In order to increase overall efficiency of the system  10 , it is desirable to minimize the loss of light produced by the light engines  28  which can occur as a result of absorption or escape in directions other than toward the side edges of the light guide  22 . Referring to  FIGS. 1 and 6 , the light reflective sheet  18  is preferably formed with an upturned peripheral edge  66  extending along all four sides. The upturned peripheral edge  66  has a number of spaced apertures  68 , with a section  70  of the reflective sheet located in between adjacent apertures  68 . 
   Preferably, a total of four light strips  20  are employed, one for each side of the light guide  22 , although either less or more than four light strips  20  may be utilized. The light strips  20  are connected to the light reflective sheet  18  by inserting the dome  64  which encases the LEDs  34  on each light engine  28  into an aperture  68  in the upturned edge  66  of the light reflective sheet  18 . Each of the apertures  68  is sized to snugly receive the dome  64  of a light engine  28 , and the width of the section  70  of the reflective sheet in between the apertures  68  extends from an edge of one light engine  28  to the edge of the adjacent light engine  28 . This arrangement prevents light produced by the LEDs  34  from escaping along the top or bottom of the light strips  20  at the juncture with the light guide  22 . Further, the spaces along the PWB  26  in between adjacent light engines  28  is covered by the sections  70  of the reflective sheet so that virtually no light from the LEDs  34  is absorbed by the PWB  26 . The result is an efficient transfer of light produced by the light engines  28  to the side edges of the light guide  22 . 
   Another aspect of this invention relates to the structure provided to transfer heat away from the area of the LEDs  34  and light engines  28 . Referring now to  FIGS. 7 and 8 , as previously discussed the LEDs  34  on each light engine  28  are hosted by the low temperature co-fired ceramic or similar layer  44  which is connected to the metal heat spreader  46 . Heat is transferred from this location through the molybdenum layer  48  and copper plate  50  to the bottom layer  52  of the light engine  28  which is soldered or otherwise affixed to the PWB  26 . In the presently preferred embodiment, the PWB  26  has two, vertically spaced metal plates  72  and  74  which extend longitudinally along the length of the PWB  26 . A plurality of via  76 , preferably formed of copper, extend vertically through the PWB  26  from its top surface to the bottom beneath each one of the light engines  28 . For purposes of discussion, the terms “horizontal,” “vertical,” “top” and “bottom” refer to the corresponding directions in the orientation of the PWB  26  shown in  FIGS. 7 and 8 . A layer  78  of flexible thermal material is then sandwiched in between the bottom of the PWB  26  and heat exchanger  16 . The layer  78  has adhesive material on both sides to adhere to both the PWB  26  and heat exchanger  16 , and is sufficiently flexible to conform to the shape of those surfaces and form a seal. One type of thermal material which may be used for layer  78  is commercially available from the Parker Seals Company under the name “Therm-A-Gap.” As noted above, the detailed structure of the heat exchanger  16  forms no part of this invention and is therefore generically depicted as a section of heat conductive metal, such as aluminum, having fins  80 . See also  FIG. 1 . An efficient heat transfer path is therefore provided from each light engine  28  through the PWB  26  and thermal material layer  78  to the heat exchanger  16 . The heat exchanger  16 , in turn, may be mounted to other structure in which the system  10  is housed to provide for additional heat transfer, if desired. 
   In order to assemble the system  10 , four light strips  20  are mounted to the light reflective sheet  18  in the manner noted above, and they are received within a seat  82  formed in the heat exchanger  16 . The light guide  22  and light enhancement film(s)  24  are also received within the seat  82 , with the film(s)  24  overlying the light guide  22 , and a clip (not shown) or other attachment holds such elements in place within the heat exchanger  16  forming the completed backlighting subassembly  14 . The LCD subassembly  12  is then mounted to outside of the heat exchanger  16 , in a position overlying the backlighting subassembly  14 , to complete the system  10 . 
   While the invention has been described with reference to a preferred embodiment, it should be understood by those skilled in the art that various changes may be made and equivalents substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.