Abstract:
Some embodiments provide a luminance-enhanced light source. These embodiments include a thin-film LED mounted on a substrate and with a defined upper surface approximately hemispherically emitting light, with the upper surface being diffusely transmissive, a lower first layer of identically defined linear prismatic film separated from the upper surface by a non-evanescent air gap so as to cover the upper surface, a upper second layer of linear prismatic film, identical to but oriented orthogonally to the first layer, and a circumferential vertical reflective wall bordering on both of the first and second layers and extending in height from the substrate to the top of the second layer.

Description:
PRIORITY CLAIM 
       [0001]    This application is a Continuation of Internation Patent Application No. PCT/US07/75779 filed Aug. 13, 2007, entitled LED LIGHT RECYCLING FOR LUMINANCE ENHANCEMENT AND ANGULAR NARROWING, which claims the benefit of U.S. Provisional Application No. 60/822,075, filed Aug. 10, 2006, entitled LED LIGHT-RECYCLING FOR LUMINANCE-ENHANCEMENT AND ANGULAR-NARROWING, boht of which are incorporated herein by reference in their entireties. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to luminaries, and more particularly to luminaries in cooperation with light emitting diodes. 
       BACKGROUND 
       [0003]    The use of light emitting diodes (LED) has increased dramatically over the last few decades. Numerous applications for LEDs have been identified and continue to be identified. 
         [0004]    LEDs alone typically emitted relatively low light emissions as compared with many other types of light sources. Further, many LEDs often emit light in substantially a hemispheric emission pattern. As a result, the use of LEDs for some implementations has been limited. 
       SUMMARY OF THE EMBODIMENTS 
       [0005]    The present embodiments advantageously addresses the needs above as well as other needs through the provision of the methods and apparatuses for use in enhancing luminance of one or more LEDs. Some embodiments provide a luminance-enhanced light source. These embodiments include a thin-film LED mounted on a substrate and with a defined upper surface approximately hemispherically emitting light, said upper surface being diffusely transmissive, a lower first layer of identically defined linear prismatic film separated from said upper surface by a non-evanescent air gap so as to cover said upper surface, a upper second layer of linear prismatic film, identical to but oriented orthogonally to said first layer, and a circumferential vertical reflective wall bordering on both of said first and second layers and extending in height from said substrate to a top of said second layer. 
         [0006]    Other embodiments provide luminance-enhanced light sources. These sources include a thin-film LED with a defined upper surface hemispherically emitting light, a reflective upper layer in optical contact with said LED, said upper layer having an array of holes providing passage of luminance-enhanced light out of said LED, and an array of collimating means aligned in correspondence to said holes in order to receive said luminance-enhanced light and to expand a cross sectional exit area of the luminance-enhanced light to a majority of an area of said upper surface of said LED. 
         [0007]    Some embodiments provide luminance-enhanced light sources that include a line of a plurality of spaced LEDs and two linearly swept elliptical reflectors disposed symmetrically on opposing sides of the line of LEDs and defining an aperture above said line of LEDs, said reflectors with elliptical profiles each having a first focus on an opposite edge of said line of LEDs and a second focus on an opposite edge of said aperture. 
         [0008]    Further embodiments provide luminance-enhanced light sources that include an LED and a rotationally symmetric elliptical reflector, said reflector with elliptical profile having a circular focus defined at an opposite edge of the circular profile from the elliptical reflector where the circular focus has a radius substantially encompassing said LED. 
         [0009]    A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description of the invention and accompanying drawings which set forth an illustrative embodiment in which the principles of the invention are utilized. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0010]    The above and other aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein: 
           [0011]      FIG. 1  shows a cross-section of a thin-film LED. 
           [0012]      FIG. 2  shows same with a brightness enhancing film (BEF), positioned above it. 
           [0013]      FIG. 3  is a perspective view of same with a pair of crossed BEFs. 
           [0014]      FIG. 4A  is a perspective top view of an array of compound parabolic concentrators (CPCs). 
           [0015]      FIG. 4B  is a perspective bottom view of same. 
           [0016]      FIG. 5  shows a cross-section of a thin-film LED with an overlying array of 300 CPCs. 
           [0017]      FIG. 6  shows a cross-section of a thin-film LED with an overlying array of 200 CPCs. 
           [0018]      FIG. 7  shows luminance enhancement of a line of LEDs by the use of a cylindrical elliptical cavity. 
           [0019]      FIG. 8  shows luminance enhancement of an LED by use of a rotational symmetric elliptical cavity. 
           [0020]      FIG. 9  shows cross-section of luminance enhancement of LED by an air-filled elliptical cavity with a condenser lens at its exit aperture. 
       
    
    
       [0021]    Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. 
       DETAILED DESCRIPTION OF EMBODIMENTS 
       [0022]    Light emitting diode (LED) chips typically contain a thin volume of emitting semiconductor of relatively high refractive index (e.g., 2.5 to 3.5). This high index can cause a correspondingly high degree of light-trapping, which in many instances is deleterious for light extraction from the chip. Extraction is hindered by of internal absorption, which converts most of the trapped light into heat as path-length increasing due to repeated internal reflections. This repetition can be curtailed by asymmetric chip-shaping or by surface roughening. Whatever the extraction efficiency, however, the emitting surfaces of LEDs radiate typically into a nearly full hemisphere, which is to say, with low angular selectivity. 
         [0023]    Some luminaires are fashioned to transform such wide-angle radiation into intensity patterns that are, for example, usefully restricted to a beam. In the case of LEDs, such luminaires can be quite small (e.g., under an inch), but still typically much larger than the LED chips themselves. Additionally, these luminaires multiply emission area, but generally do not increase emission luminance since they typically are inherently passive devices. That is to say, the lit appearance of the luminaire will generally look no brighter than the source itself. Some present embodiments, however, provide methods of amplifying the chip&#39;s luminance itself, something heretofore generally seen only in lasers. 
         [0024]    Higher luminance is particularly valuable, for example, in image-projection applications, where the etendue of the spatial light modulator is a limiting factor on the flux that can be transferred through the system. Therefore, increasing that flux typically cannot be done by increasing the number of LEDs, but by increasing their luminance. Some embodiments increase luminance, for example by increasing the current. Additionally, the some present embodiments provide a higher luminance to the LED and apply a restriction in the emission angle, which can simplify for example the posterior condenser optics. 
         [0025]    Some present embodiments use Brightness Enhancement Films (BEF) atop the LED. These films are applied in other systems to backlights in order to increase their brightness (for example, by about 25% for one and about 50% for a crossed pair), but they typically employ highly reflective white coatings within the backlight. Some present embodiments, in contrast, use BEFs, in part, to enhance the LED luminance itself. 
         [0026]    Additionally or alternatively, some present embodiments relate generally to luminance enhancement of light emitting diodes (LED), most particularly of top-emitting LEDs. This enhancement is via light recycling, whereby a portion of the light extracted from an LED is returned into it. This is effective when an LED can reflect a relatively high percentage of any external light illuminating it. Although LEDs are not engineered with this external reflectivity being a specific goal, attaining high LED efficiency generally increases that reflectivity. 
         [0027]    Further, some embodiments provide luminance enhancement of LEDs over a restricted angular range with an etendue that is generally no larger than that of the LED chip itself. In some implementations these embodiments are evaluated based on how much they multiply chip-luminance and also by their output efficiency. In some applications, sufficiently high luminance-multiplication can outweigh low efficiency, as long as the increased heat load is dissipated effectively. 
         [0028]    Thin-film LEDs differ significantly from previous LEDs in their nearly zero lateral emission. They are typically made by peeling the thin top-layer off a conventional, thick (e.g., 0.5 mm) chip, then bonding it to a lower metallic electrode, typically the anode.  FIG. 1  shows a cross-section of thin-film LED  10 , comprising upper anode  1 , topmost semiconductor p-layer  2  (e.g., about 5 microns thick), emitting junction  3  (e.g., about 5 microns thick), and lower n-layer  4 , bonded to bottom electrode  5 , typically the cathode. Current source  7  provides power via upper feed-wire  6 , in electrical contact with anode  1 , and lower feed-wire  8 , which is in electrical contact with cathode  5 . With an aspect ratio over about 20:1, only a few percent of volume emission  9  of the active layer will escape out the sides, especially if the volume emission is not isotropic, but favored in the z direction (such as with quantum-well emitters). The category of thin-film LEDs encompasses thin configurations, generally regardless of the particularities of their fabrication. As such, substantially all emission is out the top. 
         [0029]    In this regard there is a distinction in the application of surface roughening of LEDs to extract trapped light. Some high-efficiency LED designs have a bottom diffusely reflecting layer, such as silver, to extract trapped light. When the bottom layer is specularly reflecting, the top surface can be roughened instead (or in addition). Some roughening methods can simulate a refractive-index gradient and thereby suppress Fresnel reflections by the top surface and correspondingly better transmit trapped light to the outside. Ironically, these gradient-index reductions of internal Fresnel reflections enhance external reflectivity and thus assist the recycling utilized by the present embodiments. 
         [0030]    Thicker LEDs, when placed inside a reflective cup but with a flat exit surface, are also top-emitting LEDs, and some present embodiments also apply to these LEDs. 
         [0031]      FIG. 2  shows thin-film LED  20 , identical to LED  10  of  FIG. 1  but also comprising a linear prismatic retro-reflecting film  23  on top. This film will reflect around half of the light received from the chip back into it. This retroreflection will cause the angular emission into air to be restricted to about 50° full angle in this plane. An air gap  22  is incorporated into the LED  20  in some embodiments to aid in the BEF functioning of film  23 , and in some instances allows the proper BEF functioning of film  23 . 
         [0032]    Further, in some embodiments the BEF pitch and thickness is small relative to the chip width, which at least in part aids in minimizing the light lost through the film&#39;s edges. This can be realized, for example, by using thin BEF&#39;s or by using large chips or even multiple chips with small spacing (preferably reflecting) in between. Additionally or alternatively, peripheral reflecting wall  24  can be used to surround both LED  20  and film  23 . This reflector acts to prevent light spilling out the side edges of the prismatic film. This reflecting wall  24  can be dispensed with if the vertical or nearly vertical plane of the BEF film  23  is essentially smooth, because most of the light within the BEF will remain trapped by total internal reflection off the edge. 
         [0033]      FIG. 3  is a perspective view showing thin-film LED  30  with first prismatic film  31  disposed just above the LED and with second prismatic film  32  above the first but oriented at 90° thereto. This embodiment emits a collimated output in an approximately circular cone of about 50° full angle. 
         [0034]      FIG. 4   a  is a perspective view of the top of a white block  40 , pierced by compound parabolic concentrator (CPC) shaped holes  41 , with exit apertures  42 . The CPC holes can be more closely spaced in some embodiments in attempts at least in part to limit or avoid non-emitting zones, and in some instances spaced such that their apertures overlap, resulting in hexagonal or squared-off exit apertures. Also, the CPCs can be made by crossing two linear profiles, so the input and exit apertures will be, in general, rectangular.  FIG. 4   b  is a further perspective view of block  40  of  FIG. 4   a,  from below, also showing entry apertures  43  and bottom surface  44 , which in some instances is diffusely reflecting (e.g., white). 
         [0035]      FIG. 5  shows LED  50  in cross-section, with unexaggerated vertical scale, comprising lower silver layer  51 , and semiconductor chip  52  internally layered as in  FIG. 1 . Atop LED  50  is metal CPC-hole array  53 , as in  FIG. 4   a.  In some instances it is made with air gaps  54  to promote total internal reflection (TIR) for recycling within the chip. TIR is generally more efficient than the reflection off the metal that would result if there were no air gap. Also shown is cathode contact  55 , incorporated into the metal of array  53  to deliver current to the top of LED  50 , and thereby not blocking exiting light, which is often an inescapable aspect of conventional LEDs. DC source  56  delivers the requisite direct current for operating the device. Unlike typical thin-film LEDs, there is no transparent cover. Instead, LED  50  emits directly into air. 
         [0036]      FIG. 6  depicts an LED  60  that is in correspondence with the LED  50  of  FIG. 5 , with the addition of transparent dielectric  67  filling the CPC array  63 . In order to output approximately the same 30° emission as the open-CPC array  53  of  FIG. 5 , those array  63  of  FIG. 6  are somewhat taller, and with smaller aperture width  68  than width  58  of  FIG. 5 . This gives the 50% greater concentration according to the refractive index (approximately 1.5) of dielectric  67 . The CPC shapes of  FIG. 6  have about a 20° output, which refracts to 30° as the light exits into air. Although the bottom-most part of transparent CPC  67  is too steep to operate by total internal reflection, an air gap between dielectric  67  and CPC array  63  will be beneficial over most of the CPC profile, which may introduce somewhat increased complexity. 
         [0037]    Some of the potential of these embodiments for luminance enhancement depends upon their overall luminous output being reduced by less than the reduction in area of the apertures immediately over the LED, such as  58  of  FIG. 5  or  68  of  FIG. 6 . The holes  42  of  FIG. 4  have an area that is about 75% that of the LED the array covers. The 30° CPC profiles of  FIG. 5  have about a 2:1 concentration in two dimensions, so that over the LED the hole fraction f H  is approximately 75%/4=19%, while the approximate 2.9:1 concentration of  FIG. 6  gives about f H =8.8%, little over half as much. In each case the total losses result in less flux reduction for there to be increased luminance. 
         [0038]    Some embodiments have more and smaller CPCs than the 4×4 arrays of  FIG. 5  and  FIG. 6 , so that trapped light, once it has been laterally diffused within the semiconductor, will not have as far to go to escape through the exit holes. This lateral light travel can be enhanced if a solid dielectric or reflective prism is placed between the LED and the CPCs. In some implementations, the CPC profiles, which may be difficult to make in small size, are approximated by a segmented linear profile and/or even by a straight profile. 
         [0039]    The white coating corresponding to reference numeral  52  of  FIG. 5  operates on the micro level through light-scattering by small transparent pieces of such high-index material, for example, as titanium dioxide (n˜2.5). The actual surface of such a white coating will exhibit about a portion of its 99% reflectivity, with deeper layers further scattering what is not scattered backwards to constitute reflection. A minimum thickness for total backscattering presumably is on the order of about tens to hundreds of microns (depending on the material being used), with a sacrifice of reflectivity for anything too thin, leaving some of the incident light allowed to be transmitted. Edmond Optics of New Jersey sells a highly reflective (diffuse) white coating called “Munsell White Reflective Coating”, which can be applied by a number of methods including spraying. The coating in its cured state is comprised primarily of a highly reflective Barium Sulfide binder. The coating yields a reflectance value of up to about 0.991 in the visible spectrum. The recommended minimum thickness of the coating to achieve the specified performance (above 98% reflectance in the visible spectrum) is about 0.64 mm, which is relatively large on the micro level scale. 
         [0040]    As a reflector profile, an ellipse will reflect a ray from a line between its foci to another point on the same line.  FIG. 7  shows line array  70  of closely spaced thin-film LEDs, that are aligned and mounted on planar substrate  71 . Slotted elliptical cylinder  72  has focal lines, depicted by dotted focal lines  77 , and is reflective on its inside walls, thus returning substantially all light from the LEDs back to their surface, or to the spaces  73  between the LEDs (thus are generally highly reflective for better efficiency). Similarly, end wall  74  is specularly reflective in some implementations. The emission out slot  75  is transversely restricted to angle  76 , although longitudinally it is as unrestricted as that of array  70  itself. Therefore, the device&#39;s etendue is reduced in the transversal plane as compared to that of the LEDs alone. This reduction may be useful, for example, for applications involving side injection into backlights, where the light collimation in this transversal plane is beneficial for efficient light extraction. Also with this application, multiple color chips can be used, with the recycling process providing some color mixing. 
         [0041]      FIG. 8  shows the application of a rotationally symmetric elliptical cavity  80 , according to some embodiments, with exit aperture  83 , shaped at least in part to restrict the angular emission of an LED or LED cluster  81 . The circle  82 , described by the ellipse&#39;s foci, can be selected, in some implementations, to be approximately equal to the LED area. 
         [0042]    When a rectangular LED or LED cluster is used, a non-rotational symmetric ellipsoid can be used, with its semi-axis in the plane of the LED, and showing a ratio similar to the aspect ratio of rectangular emitting area. 
         [0043]    In embodiments based on  FIG. 7  or  FIG. 8 , the elliptical profiles can be approximated by spherical ones for easier manufacturing. They can be either void or solid (with elliptical profile also along the exit aperture), the latter in some embodiments allowing the embodiment to act also as the primary optic dome encapsulating the LED. 
         [0044]    Since the exit aperture of the ellipsoid will act as an aperture stop, a condenser lens can be placed on the exit aperture for more optimum control and definition of the emitted ray bundle. Said lens by itself or in combination with others, could image the luminance-enhanced LED onto the entry aperture of, for example, a kaleidoscope prism (so the circular aperture of the ellipsoid will define the circular numerical aperture of the kaleidoscope). Alternatively, it could image the LED to infinity to illuminate a set of Kohler-integrating fly-eye lenses. In some other embodiments, the exit aperture is set as a rectangle with an aspect ratio, for example, of 4:3 or 16:9, typical for video and HD. Then the lens at the exit of the ellipsoid is the first element of a Kohler integrating system, while a second lens images the rectangular exit of the ellipsoid onto the spatial light modulator. 
         [0045]      FIG. 9  shows the cross section of an air-filled rotational symmetric elliptical reflector  90 , operable for increasing the luminance of LED or LED cluster  91 . While the device is made, according to some implementations, in one piece of transparent dielectric, it has interior specular reflective coating  92  surrounding central condenser lens  93 . Coating  92  is shown reflecting rays  95  back to the LED or LED cluster  91 . Condenser lens  93  refracts rays  95  from the LED or LED cluster  91 . 
         [0046]    For the embodiments of  FIG. 8  and  FIG. 9  the LED cluster can be comprised of LEDs of a variety of colors. In these embodiments the specular reflectivity of the interior walls provides color mixing, although in principle they typically cannot provide complete mixing because the color of each LED&#39;s own emission is unchanged in direction once it is emitted. Thus, for example, a mildly scattering (10°) holographic diffuser can be molded onto surface  94  of  FIG. 9 , to assist in color mixing. 
         [0047]    Some embodiments provide luminance enhancement. In some implementations, light is reflected by the one or more LEDs. The amount of light reflected by LEDs can be used as a method of light-recycling to increase LED luminance. Some embodiments are implemented with a single standard Brightness Enhancement Film or two-crossed BEFs. Additionally or alternatively, an array of CPCs positioned over the LED is utilized. Further, some embodiments use linear or rotational elliptical cavity with enhanced luminance and narrowed output angle. 
         [0048]    While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.