Abstract:
An omni-directional LED device is constructed such that light is prevented from exiting from the top of the LED device. In one embodiment, an opaque barrier is created and in some embodiments enhancement surfaces are created below the opaque barrier to increase lumen output from the device sides. In one embodiment, a reflecting structure is created to assist with horizontal light mixing. The horizontally mixed light is then redirected through a structure, such as an LED structure, to create a high lumen output, slender back-lighted display.

Description:
TECHNICAL FIELD 
     This invention relates to light emitting devices and more particularly to omni-directional light emitting diodes (LEDs) used to form a back-lighted array. 
     BACKGROUND OF THE INVENTION 
     Back-lighted display devices have become popular for a variety of display purposes. Some of these displays are large and some small. The actual light for such devices is provided, as the name implies, from behind (the side away from a viewer). Light emitting diodes (LEDs) are now increasingly being used to provide the light source in such back-lighted applications. 
     An example of an LED lighted application is a liquid crystal display (LCD) where a number of LEDs are spatially distributed behind the LCD so that their light transmits through the LCD. The light from each LED is filtered by the LCD to provide red, green and blue pixels. 
     LED devices are directional light sources where the light is typically directed towards the central axial direction of the LED. A common problem in using LEDs as back-lights is that the resultant LCD display does not display uniform luminance, i.e., the display surface brightness is not uniform. The region directly above the LED device typically is brighter than the rest. These “hot spots” do not have a pleasant look. 
     Another common problem is that the light coming from the LED devices are poorly mixed. In a back-lighted display, red, green and blue LED devices are typically used and the different colors must be uniformly mixed to ensure a uniform color, for example, white color, is obtained. If color mixing from the LCDs is poor, then different portions of the LCD show different colors. 
     One prior solution to prevent hot spots is to place a diffusing film above the back-lighted array. This solution helps minimize, but does not eliminate the problem. The diffusing film attenuates the amount of light transmitted through the LCD and thus the overall brightness of the back-light module drops. 
     A prior solution to ensure more uniform color mixing is to increase the distance between the LED devices and the LCD display so that a larger light mixing zone is created. This increases the chance of light mixing before reaching the LCD at the expense of increased device thickness. 
     BRIEF SUMMARY OF THE INVENTION 
     An omni-directional LED device is constructed such that light is prevented from exiting from the top of the LED device. In one embodiment, an opaque barrier is created and in some embodiments enhancement surfaces are created below the opaque barrier to increase lumen output from the device sides. 
     In one embodiment, a reflecting structure is created to assist with horizontal light mixing. The horizontally mixed light is then redirected through a structure, such as an LCD structure, to create a high lumen output, slender back-lighted display. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIGS. 1A ,  1 B,  1 C,  2 A,  2 B,  2 C,  3 A,  3 B, and  3 C illustrate one embodiment of the construction of an omni-directional LED; 
         FIG. 4  illustrates one embodiment of light being emitted out of the sides of the device; 
         FIG. 5  illustrates one embodiment of a LED device showing how light is mixed; 
         FIG. 6  illustrates one embodiment of LED devices used to form back-lighting in a display; and 
         FIGS. 7 and 8  illustrate prior art back-lighted displays. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Prior to beginning the detailed description it would be helpful to review prior art LED devices used for back-lighting purposes. 
       FIG. 7  shows device  70  having three LEDs ( 72 R,  72 G and  72 B) with light coming from their respective tops and passing through liquid crystal display (LCD)  71  forming hot spots  73 . These hot spots are due primarily to poor color mixing between the LEDs. 
       FIG. 8  shows the addition of diffusing film  82  to back-lighted device  80 . Device  80  has light mixing area  83  with a depth X. Note that the larger the value for X the more the light from the three LEDs ( 72 R,  72 G and  72 B) will mix. However, as X increases so does the dimensional profile of the device. This makes the device bulky and decreases the light passing through LCD  81 . The decreased light caused by the increased depth, coupled with the light decrease caused by diffusing film  82 , leaves room for improvement. 
       FIGS. 1A ,  1 B and  1 C illustrate one embodiment  10  of an LED device fabricated such that light is emitted omni-directionally, i.e., from all four sides, but not from the top  10 .  FIG. 1A  shows the plan view,  FIG. 1B  shows the front view and  FIG. 1C  shows the side view of device  10 . Since no, or very little, light is emitted from the top of the LED, hot-spots are eliminated as will be seen.  FIGS. 1A ,  1 B, and  1 C show top, front and side views of LED device  10  having substrate  12 . Device  10  has LED  15  with wire  16 . Area  13  is a clear encapsulate and area  14  is an opaque encapsulate. Note that while a device is shown having square sides, the device could be round or could have any number of sides surrounding the LED. 
     LED chip  15  has a pair of terminals (not shown) with the LED chip attached to a first terminal via wire  16 . A second electrical connection is made between the LED chip and a second terminal. The clear encapsulate has a low light attenuation characteristic. Second layer opaque encapsulate  14  covers the first layer on the top side. The second layer has a high light attenuation characteristic and does not cover the vertical sides of the optically transparent layer. As will be seen, this allows light emits from the LED device from all sides except for the top. 
       FIGS. 2A ,  2 B,  2 C and  3 A,  3 B,  3 C illustrate alternate embodiments. In  FIG. 2C  interfaces  21  between transparent encapsulate  13  and opaque encapsulate  14  of device  20  are shaped as reflectors so that light is reflected out sideways more efficiently.  FIG. 2B  shows the front view and  FIG. 2A  shows the plan view. 
     In  FIG. 3C , reflector  31  of device  30  has a different shape, but performs the same function, i.e., increasing the efficiency of sideways light output.  FIG. 3B  shows the front view and  FIG. 3A  shows the plan view. 
       FIG. 4  illustrates light  41  being reflected out sideways from device  30  ( 10 ,  20 ) in all directions, except for the top (and bottom). 
     Since the light is emitted sideways, mixing of light from each LED device is facilitated in the horizontal direction.  FIG. 5  shows LEDs  30  which can be implemented as a back-light using red, blue and green, if desired. LEDs  30  preferably are partially inserted into the light guide so that light emits into the light guide horizontally. In this embodiment, the reflecting features such as feature  53 , can be optionally provided so that the mixed light is directed toward LCD  51 . Hence, long vertical light-mixing portion (dimension X in  FIG. 8 ) can be reduced yielding a slimmer LCD inner portion  52  thereby allowing the entire device to be thinner. Light mixing region  52  can have a dimension Y that is less than 30 mm thick and can be a light guide or it can be air, if desired. The diffusion layer could, if desired, be eliminated from embodiment  50  thereby increasing the lumen output for a given power level. Note that while  FIG. 5  shows the LEDs lined up in a row, they can also be grouped together (for example, red, blue and green would be in groups of three) so that when the light that is mixed evenly from the group impacts the LCD it is essentially white and free of hot spots. Also note that while in the preferred embodiment all (or most) of the light is prevented from exiting the LED by the opaque layer, it could be possible to redirect whatever light comes from the top of the LED (or from the top areas of the sides of the LED) back down to features  53  to allow for mixing and/or scattering of the light prior to the light impacting top surface  51 . As is well known, by turning the power on or off to the different colored LEDs within each group, different colors are produced on the outer surface of the top surface. 
       FIG. 6  shows the top view of display  50  showing LEDs spaced apart beneath LCD  51 . In one embodiment, red, green and blue LEDs are arranged spatially so that horizontal light emitted from the LEDs are mixed and appears uniformly white at top surface  51 . Note that while  FIG. 6  shows LEDs populating fully the surface beneath LCD  51 , in an alternate embodiment, the LEDs can be placed near an edge and the light emitted from the LEDs is guided to distal edges. In yet another embodiment, LEDs can be placed around the perimeter and the light emitted from the LEDs guided towards the centre. In such arrangements, it would be helpful to use LEDs that have light coming from less than all four sides. One method for achieving this result would be to fabricate the second layer encapulate such that light is emitted in a preferred side. By adjusting the profile of any reflecting faces of the second layer encapsulate, light may emit from just one side. For example, if the second layer covers three sides (of a four sided device), then light is emitted from just one side. 
     Substrate  12  is preferably a printed circuit board (PCB) but other substrates such as ceramic, can be used. Circuit traces are fabricated on the surface of the PCB or ceramic substrate so as to form terminals for the placement of the LED and wire bond. Terminals can be formed on both surfaces of the substrate and connected using plated-through via holes. The terminals on the bottom side of the LED device are used for assembly to a mother PCB (not shown) if desired. In an alternate embodiment, a cavity can be fabricated on the substrate for the placement of the LED to enable construction of thinner LED device. 
     In an alternate embodiment, a plastic insert molded leadframe can be used. Leadframes made of metal are molded so that the terminals are encased in plastic material. In yet another alternate embodiment, a metal-core PCB is used as substrate  12  to facilitate heat dissipation. 
     The methods of fabrication of PCB (metal core and non-metal core) and ceramic substrates and plastic insert molded leadframe are all known in the art. 
     In the embodiment shown in  FIG. 1 , the first layer encapsulate (clear) is any organic polymer, inorganic material or air. In the preferred embodiment, the material is optically clear epoxy. Alternatively, silicone can also be used. In addition, inorganic material, such as glass or aluminum nitride, can be used. The first layer encapsulate can be deposited using any known process, such as transfer molding, insert molding, spraying, casting or capping. In an alternate embodiment, the first layer encapsulate can also optionally be made slightly diffused by adding diffusant. 
     The second layer encapsulate (opaque) can be constructed of organic or inorganic material. White opaque epoxy is preferred but, polycarbonate, PMMA, PVC, PBT can be used. The second layer encapsulate can be deposited using any known process such as transfer molding, insert molding, spraying, casting or capping. Alternatively, the second layer can be prefabricated as a separated component and then attached onto the substrate using an adhesive or any other known system of attachment. 
     If desired, a reflective mirror can be deposited on the surface of the first layer encapsulate prior to the placement of the second encapsulate. The mirror would act to further aid light extraction through the sides of the LED device. The reflective mirror can be constructed of metal or semiconductor material deposited using any known process such as sputtering or evaporation. 
     Although the second layer encapsulate is described as opaque, it should be noted that the opacity can be tuned to be different at different wavelengths of light. This can be achieved by fabricating the second layer encapsulate as a coating, for example, using multiple layers of coatings with varying refractive indices. 
     An illustrative manufacturing process is as follows: Attaching die, forming a wire bond, placing a first layer low light attenuation encapsulate around an LED chip, and second layer high light attenuation encapsulate on the top surface of the first layer encapsulate. 
     In an alternate embodiment, the first layer encapsulate can be impregnated with a wavelength converting material such as phosphor particles, nano-phosphors or luminescent dye. The wavelength converting material absorbs a first color light and then reemits a second color light as is well know. For example, by adding yellow phosphor particles in the first encapsulate and using a blue LED as light source, a composite emission of blue and yellow light is obtained. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.