Abstract:
A light source such as a semiconductor light emitting diode is positioned in a first opening in a transparent member, which may function as a waveguide in a display. The transparent member surrounds the light source. No light source is positioned in a second opening in the transparent member. In some embodiments, the first opening is shaped to direct light into the transparent member. In some embodiments, a reflector is positioned over the light source. The reflector includes a flat portion and a shaped portion. The shaped portion extends from the flat portion toward the light source.

Description:
FIELD OF INVENTION 
       [0001]    The present invention is directed to a lighting device including a light source disposed in an opening in a transparent member. Such a lighting device may be used, for example, as a backlight for a display. 
       BACKGROUND 
       [0002]    Semiconductor light emitting devices such as light emitting diodes (LEDs) are among the most efficient light sources currently available. Material systems currently of interest in the manufacture of high brightness LEDs capable of operation across the visible spectrum include group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials; and binary, ternary, and quaternary alloys of gallium, aluminum, indium, arsenic, and phosphorus. Often III-nitride devices are epitaxially grown on sapphire, silicon carbide, or III-nitride substrates and III-phosphide devices are epitaxially grown on gallium arsenide by metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. Often, an n-type region is deposited on the substrate, then a light emitting or active region is deposited on the n-type region, then a p-type region is deposited on the active region. The order of the layers may be reversed such that the p-type region is adjacent to the substrate. 
         [0003]    One promising use of semiconductor light emitting devices is for backlights in liquid crystal displays (LCDs). LCDs are commonly used in cellular phones, personal digital assistants (PDAs), portable music players, laptop computers, desktop monitors, and television applications. One embodiment of the present invention deals with a color or monochrome, transmissive LCD that requires backlighting, where the backlight may use one or more LEDs emitting white or colored light. Embodiments using blue LEDs and a remote phosphor layer for white light generation are also possible. The LEDs are distinguished from laser diodes in that the LEDs emit incoherent light. 
         [0004]    One backlight is described in US Published Application 2009-0045420, which is incorporated herein by reference, and illustrated in  FIG. 1 . “A side-emitting LED  10 , mounted on a mount  22 , is positioned in an opening in a section of solid, transparent waveguide material  36  . . . . Waveguide section  36  [is] positioned in slot  42  of a larger waveguide  40 .” 
       SUMMARY 
       [0005]    Embodiments of the invention are directed to lighting devices that can be used as backlights for displays. An object of the invention is to provide a transparent member with openings for actual and virtual light sources. A light source such as a semiconductor light emitting diode is positioned in a first opening of the transparent member, which may function as a waveguide in a display. The transparent member surrounds the light source. In embodiments of the lighting device, top-emitting LEDs, rather than side-emitting LEDs, may be used. No light source is positioned in a second opening in the transparent member. The second opening may act as a virtual light source. 
         [0006]    In some embodiments, a reflector is positioned over the light source. The reflector includes a flat portion and a shaped portion. The shaped portion extends from the flat portion toward the light source. Embodiments of the invention may reduce the number of light sources necessary for the lighting device to produce a desired amount of light with sufficient uniformity and mixing of the light. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a perspective view of a waveguide section positioned in a waveguide. 
           [0008]      FIG. 2  is a cross sectional view of a thin film flip chip LED with an overmolded dome lens. 
           [0009]      FIG. 3  is a top view of an LED disposed in an opening in a portion of a waveguide. 
           [0010]      FIG. 4  is a cross sectional view of an LED disposed in an opening in a waveguide. 
           [0011]      FIG. 5  is a top view of a portion of a waveguide with actual and virtual light sources. 
           [0012]      FIG. 6  is a top view of a portion of a waveguide including a feature to reflect light. 
           [0013]      FIG. 7  illustrates an LED with a shaped lens. 
           [0014]      FIG. 8  is a top view of a portion of a waveguide. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Backlights where the LEDs are positioned inside the waveguide, such as the backlight described in US 2009-0045420, are beneficial because the waveguide, and therefore the backlight, can be kept thin. It is also desirable to reduce the number of LEDs used in the backlight, to reduce the cost of the backlight. LEDs that emit a majority of light from the top surface of the LED may have slightly higher flux than the side-emitting LEDs described in US 2009-0045420. Embodiments of the invention are directed to devices such as backlights with top-emitting LEDs positioned inside a waveguide. 
         [0016]    One example of suitable top-emitting light is an LED with an overmolded dome lens. Examples of suitable LEDs with dome lenses are described in, for example, U.S. Pat. No. 7,344,902, which is incorporated herein by reference.  FIG. 2  is a simplified close-up view of an LED with a dome lens, described in U.S. Pat. No. 7,344,902, which is incorporated herein by reference. A single flip-chip LED die  10  is mounted on a mount  24  formed of any suitable material, such as a ceramic or silicon. The LED die  10  of  FIG. 2  has a p-metal contact  27 , p-type layers  28 , a light emitting active layer  30 , n-type layers  32 , and an n-metal contact  31  contacting the n-type layers  32 . Metal pads on mount  24  are directly metal-bonded to contacts  27  and  31 . Vias through mount  24  terminate in metal pads on the bottom surface of mount  24 , which are bonded to the metal leads  40  and  44  on a circuit board  45 . The metal leads  40  and  44  are connected to other LEDs or to a power supply. Circuit board  45  may be a metal plate (e.g., aluminum) with the metal leads  40  and  44  overlying an insulating layer. The molded lens  22  encapsulates the LED die  10 . 
         [0017]    The n-type layers  32 , light emitting layer  30 , and p-type layers  28  are grown on a growth substrate, then a portion of light emitting layer  30  and p-type layers  28  are etched away to reveal a portion of n-type layers  32 . N- and p-contacts  31  and  27  are formed on the exposed portions of the n- and p-type semiconductor layers. To reduce the thickness of LED  10  and to prevent light from being absorbed by the growth substrate, the growth substrate is removed by a method suitable to the substrate, such as etching, chemical-mechanical polishing, or laser melting, where a laser heats the interface of the III-nitride structure and growth substrate, melting a portion of the III-nitride structure and releasing the substrate from the semiconductor structure. The growth substrate may be removed after an array of LEDs are mounted on a submount wafer and prior to the LEDs/submounts being singulated (e.g., by sawing). 
         [0018]    After the growth substrate is removed, in some embodiments the remaining III-nitride structure is thinned and/or roughened or patterned, for example with a photonic crystal. 
         [0019]    A substantially planar phosphor layer, not shown in  FIG. 2 , may be positioned over the top of the LED for wavelength-converting at least some of the blue light emitted from the light emitting layer  30 . The phosphor layer may be preformed as a ceramic sheet and affixed to the LED layers, or the phosphor particles may be deposited, such as by electrophoresis. The light emitted by the phosphor layer, when mixed with blue light from the LED&#39;s active region, creates white light or another desired color. For example, a blue-emitting LED may be combined with a single yellow-emitting phosphor, or with a red-emitting phosphor and a green-emitting phosphor, to create white light. If a UV-emitting LED is used, a blue-emitting phosphor may be added. Phosphors emitting other colors of light may be added to the LED or somewhere else remotely in the system to achieve a desired color point of the mixed, white light. 
         [0020]    The dome lens  20  shown in  FIG. 2  is then molded over LED  10 . Embodiments of the invention may use thin film flip chip LEDs, as illustrated in  FIG. 2 , with or without dome lens  10 . Lens shapes other than the spherical dome lens shown in  FIG. 2  may be used to modify the radiation pattern of the device. The lens may be shaped to control ray angles and propagation in the waveguide such as slightly more collimated in the plane of the waveguide.  FIG. 7  illustrates a device with a shaped lens  76 . Lens  76  includes a top section  78  shaped to direct light into reflector  66  (described below in reference to  FIG. 4 ), and a bottom section shaped to direct light into waveguide  50 . 
         [0021]      FIG. 3  is a top view of a top-emitting LED positioned in a waveguide according to embodiments of the invention. Though an LED with a dome lens is illustrated, a device without a dome lens may be used. A domed LED  60 , mounted on a mount (not shown in  Fig. 3 ), is positioned in an opening  54  formed near the edge  52  of a solid, transparent waveguide  50 . Openings  54  may be large enough that the edge of the dome of LED  60  is between  100  micron and 2.5 mm from the edge of the opening, for easy placement of LED  60  in opening  54 . Waveguide  50  may be, for example, acrylic (e.g., PMMA), hard silicone, molded plastic, polycarbonate, or any other suitable material. A mirror film (not shown) may cover the bottom of waveguide  50 . The film may be, for example, enhanced specular reflector (ESR) film available from 3M corporation. 
         [0022]    Opening  54  is shaped to direct light into waveguide  50 . In the device shown in  FIG. 3 , a portion  58  of opening  54  near the edge  52  of waveguide  50  is V-shaped, while a portion  56  of opening  54  furthest from the edge is curved. Light  53  that is incident on the curved edge  56  is emitted into waveguide  50 , as illustrated. Light  51  that is incident on the V-shaped edge  58  is refracted toward waveguide edge  52 , where it may be reflected into waveguide  50 . Simple round or slightly elliptically-shaped openings  54  may also be used. In addition, the edges of the in-coupling edges  56  and  58  may be corrugated to spread the distribution of the light in the waveguide to achieve the desired uniformity while reducing the LED count. 
         [0023]      FIG. 4  is a cross section of a top-emitting LED positioned in a waveguide according to embodiments of the invention. Though an LED without a dome lens is illustrated, a device with a dome lens may be used. LED  60  includes a semiconductor LED structure  64  mounted on a support  62 . LED  60  may be positioned in the opening  54  in waveguide  50  such that support  62  is below the bottom  55  of waveguide  50  and semiconductor structure  64  is above the bottom  55  of waveguide  50 . A top surface  63  of support  62  may be reflective. 
         [0024]    Since LED  60  is a top-emitting LED, a reflector  66  is positioned over LED  60 , to redirect the emitted light into waveguide  50 . Reflector  66  may include a section  68  that protrudes below the top surface  57  of waveguide  50 , to direct light into waveguide  50 , as illustrated in  FIG. 4 . Protruding section  68  may be shaped to avoid reflecting light emitted normal to the top surface of semiconductor structure  64  directly back into semiconductor structure  64 , where it may be absorbed. One suitable shape for protruding section  68  is a cone, as illustrated in  FIG. 4 . The cone illustrated in  FIG. 4  may have dimensions between the diameter of the LED dome and the dimensions of opening  54 . Other shapes may be possible such as a curved or parabolic shape. The reflector angle can vary from, for example, 15° to 60°. Reflector  66  may be formed from any rigid material, such as plastic, and may be coated with a reflective material, such as a reflective metal, coating, paint, or dichroic stack. Alternatively, reflector  66  may be formed from a material that causes total internal reflection. Reflector  66  may have, for example, the same lateral extent as opening  54 , or slightly larger, so the edges of reflector  66  may rest on the top of waveguide  50 . 
         [0025]    In a large backlight, such as for a computer monitor or television, multiple LEDs are positioned along the edge  52  of waveguide  50 . The spacing of the LEDs and openings is a function of the number of LEDs used and the linear dimension of the backlight. For example, in a conventional  24  inch backlight,  60  LEDs may be used to achieve good color uniformity. Using the described embodiments the number of LEDs may be reduced to  15  to  30  LEDs for the same  24  inch backlight without sacrificing uniformity or increasing the bezel length. 
         [0026]      FIG. 5  is a top view of a portion of a waveguide. Two LEDs  60  are positioned in openings  54 , as illustrated above in  FIG. 3 . An opening  70  spaced between openings  54  does not contain an LED. Light emitted from LEDs  60  reflects off the edges of opening  70 , making opening  70  a “virtual” light source. In some embodiments, one or more edges of opening  70  are coated with a reflective material. The virtual sources may be placed between the LED openings and can extend in width from the LED width up to ¾ the space in between the LEDs. In some embodiments, opening  70  is filled with a material with a low index or refraction, such as air or low index of refraction silicone. Including openings  70  without LEDs positioned in the openings may improve the uniformity of the backlight, as compared with a backlight with the same number of LEDs without empty openings  70 , or for a given level of mixing and uniformity of the light in waveguide  50 , may permit LEDs  60  to be spaced further apart, as compared with a backlight without empty openings  70 . Improved mixing and uniformity may reduce the required backlight and monitor bezel height and can therefore be minimized or kept constant when reducing the number of LEDs. 
         [0027]      FIG. 6  is a top view of a waveguide with features formed on the edge of the waveguide between openings for LEDs. Instead of, or in addition to, the empty openings  70  illustrated in  FIG. 5 , in the device of  FIG. 6 , features  72  are formed on the edge  52  of waveguide  50 . Features are shaped to reflect light emitted by LEDs  60  toward adjacent LEDs into the body of waveguide  50 . One example of a suitable shape for feature  72  is a V-shaped notch in the edge  52  of waveguide  50 , as illustrated in  FIG. 6 . In some embodiments, feature  72  is curved. The edge  52  of waveguide  50 , including feature  72 , may be reflective, for example by coating the edge with a reflective film or attaching a film to the edge. As illustrated in  FIG. 6 , openings  54  in which LEDs  60  are positioned need not be shaped as illustrated in  FIG. 3 . In the device illustrated in  FIG. 6 , openings  54  are round. Other shapes are within the scope of the invention. The V-shaped features may start close to the LED and have a continuous curved shape. 
         [0028]    In some embodiments, the features illustrated in various figures may be combined. For example, as described above, top-emitting LEDs with or without dome lenses may be used. The top reflector illustrated in  FIG. 4  may be used over the openings in the devices illustrated in  FIGS. 3 ,  5 , and  6 . Openings with different shapes may be used in the various embodiments.  FIG. 8  illustrates some variations. In addition to or instead of features  72  shown in  FIG. 6 , the edge of waveguide  50  may include multiple features positioned adjacent to each other, such that the edge is continuously textured, as in the scalloped shape  80  illustrated in  FIG. 8 . Alternatively, multiple features on the edge may be positioned adjacent each other, then separated from the next group of multiple features on the edge by a section of untextured edge. Virtual sources  84  may be spaced closer to the edge of waveguide  50  than openings  86  in which actual light sources are positioned. Multiple virtual sources  84  may be positioned near a single actual source  86 . Other shapes may be used for openings for both virtual and actual light sources. Opening  82  illustrated in  FIG. 8  has a curved portion  82   a  that curves in toward the center of the opening, as opposed to curved portion  82   b  which curves out away from the center of the opening. 
         [0029]    The embodiments described above may be used as a backlight for an LCD display. In a finished display, a thin diffuser film may be affixed over the top surface of the waveguide  50  to diffuse the light. A brightness enhancement film (BEF) may be disposed over the diffuser film to redirect light to within a relatively small angle directly in front of the waveguide  50  to increase the brightness in the normal viewing direction. A conventional color or monochrome LCD is then disposed over waveguide  50 . The LCD can produce color images using pixel shutters (e.g., a liquid crystal layer in combination with a TFT array), polarizers, and RGB filters. Such LCDs are well known. Other applications of the embodiments described above include thin poster boxes. 
         [0030]    Having described the invention in detail, those skilled in the art will appreciate that, given the present disclosure, modifications may be made to the invention without departing from the spirit of the inventive concept described herein. Therefore, it is not intended that the scope of the invention be limited to the specific embodiments illustrated and described.