Abstract:
A LED light source is integrated with a heatsink and a collimator. Four isolated heatsinks form an optical taper in which a single color LED is mounted. The LEDs are arranged to form a reflective light recycling cavity. Up to four different colors can be combined inside the light recycling cavity to form a uniform and homogenous mixing of the colors at the exit aperture of the light recycling cavity and/or the exit aperture of the collimator/heatsink.

Description:
REFERENCE TO PRIOR APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/931,256, filed on May 22, 2007, which is herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    In U.S. Pat. Nos. 6,869,206, 6,960,872, 7,025,464, 7,040,774, and 7,048,385, a means of generating a high brightness light source uses light emitting diodes in a light recycling cavity. This present invention relates to an improved method and process for fabricating an integrated light recycling cavity, color combiner, optical collimator, and heatsink. In U.S. Pat. No. 7,040,774, a light recycling cavity contains different colored LEDs within the cavity. That light recycling cavity permits multiple LEDs of different colors to mix their outputs within the cavity. This multiple and mixed outputs can be very efficient in that the étendue of the light source is defined by the exit aperture of the light recycling cavity and not by the cumulative area of the LEDs within the cavity. 
         [0003]    In conventional light engines used for projection displays, a white light source, such as an arc lamp, must be broken up into multiple colors to generate a full color display. This white light source is very inefficient. For example, a color separation system using a color wheel of alternating red, green, and blue filters can maximally only be 33% efficient. For a light engine using separate distinct colored sources, the sources must be combined onto the same optical path using dichroic mirrors, which can also be inefficient. 
         [0004]    Light sources for microscopes, endoscopes, fiber optic illuminators, spotlights, and down lighting for general illumination all have a need for a tunable color temperature (e.g. being able to select the mix of red, green and blue or other color) projected. Space and volume is at a premium for optical systems that are very compact, such as pocket projectors. With conventional light sources, a relatively long optical train is required to collimate and integrate the colors into the projection engine. 
         [0005]    Adding to the overall size are the heatsinks required to dissipate the heat from the light source. These heatsinks add volume, which must be accommodated within a compact optical system. 
         [0006]    LEDs typically emit in a Lambertian full solid angle. To efficiently couple this light into an optical system requires collecting this wide angle light and collimating it. This collection and collimation is typically done with an optical tunnel or taper. The optical tunnel consists of four mirrored surfaces, which form a tunnel. Alternatively, a solid glass tapered rod may be used. The output aperture of the tunnel is larger than the input of the tunnel to collimate the light. The length of the tunnel must necessarily be made long to provide homogenization of the light that enters the tunnel. 40 mm to 80 mm is typical for the length of these tunnels. Thus, the tunnel not only collimates the light, it also aids in making the light more uniform. 
         [0007]    In the prior art, these optical tunnels are appended to the output of LEDs where the LEDs are mounted on their own heatsink, the heatsink protruding in the opposite direction of the tunnel. The combination of the heatsink and the optical tunnel add considerably to the overall size of the finished light source. 
         [0008]    Minimizing the manufacturing cost is paramount in consumer products, such as projectors, televisions, etc. Optical tunnels and heatsinks can add substantially to the cost of the finished product. 
         [0009]    There is a need for a light source for these various applications that is compact, inexpensive, and well collimated and can be tuned to different color temperatures or different colors depending on the application, mood, or environment. 
       SUMMARY OF THE INVENTION 
       [0010]    An integrated heatsink collimator forms a light emitting diode recycling light cavity. In one embodiment, four metal tapered heatsinks with mirrored interior faces form an optical taper light tunnel. One or more LEDs is mounted to each heatsink such that the LEDs are on opposite sides of a light recycling cavity. The bottom of the cavity can have an additional LED or a highly reflective specular or highly reflective diffused surface or a highly reflective pyramidal surface. 
         [0011]    Also, each heatsink can be electrically isolated from the other heatsinks, which form the structural elements of the recycling light cavity. Each heatsink has multiple heat dissipating fins. 
         [0012]    Light generated within the light recycling cavity exits the open end of the light recycling cavity and is guided and reflected along the highly polished and/or silver mirrored faces of the heatsink. The heatsinks are fitted together at a slight angle from one end to the other to form an optical tapered tunnel. Light emitted in a Lambertian (hemispherical) distribution will be collimated by the time it exits the end of the heatsink taper. 
         [0013]    Another embodiment of the invention is the unique LED submount and interconnect system which allows for precise alignment of the LEDs to the optical taper and a very efficient thermal coupling of the LED to the heatsink. Other aspects of the invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1A  is a perspective view of a finned heatsink with mirrored surface of the present invention. 
           [0015]      FIG. 1B  is a plan view of four heatsinks with mirrored surfaces assembled to form an optical tunnel of the present invention. 
           [0016]      FIG. 1C  is a plan view of the output side of the optical tunnel formed by the four mirrored heatsinks of the present invention. 
           [0017]      FIG. 1D  is a perspective view of the integrated heatsink optical tapered tunnel of the present invention. 
           [0018]      FIG. 2A  is perspective view of an LED mounted to an edge of a thin metal plate of the present invention. 
           [0019]      FIG. 2B  is a cross-sectional view of LED submount metal sandwich of the present invention. 
           [0020]      FIG. 2C  is a perspective view of LED submount sandwich with LED mounted and wirebonds of the present invention. 
           [0021]      FIG. 3A  is a perspective view of the LED sandwich assembly fastened to input face of mirrored heatsink of the present invention. 
           [0022]      FIG. 3B  is a plan view showing four LED sandwiches mounted to the input end face of the integrated heatsink tunnel of the present invention. 
           [0023]      FIG. 4A  is a plan view of reflective base plate mounted on bottom side of light recycling cavity of the present invention. 
           [0024]      FIG. 4B  is a perspective view of a bottom base plate with LED of the present invention. 
           [0025]      FIG. 4C  is a perspective view of the bottom base plate with reflecting pyramid of the present invention. 
           [0026]      FIG. 4D  is a cross-sectional view of the bottom base plate with reflecting pyramid inserted in the light recycling cavity of the present invention. 
           [0027]      FIG. 4E  is a cross-sectional view of a dichroic pyramid mounted on a bottom LED inserted in the light recycling cavity of the present invention. 
           [0028]      FIG. 4F  is a cross-sectional view of a flat dichroic plate mounted on a bottom mounted LED inside a light recycling cavity of the present invention. 
           [0029]      FIG. 5  is a perspective view of the completed integrated optical tunnel, heatsink and light recycling cavity of the present invention. 
           [0030]      FIG. 6  is a cross-sectional view of a solid optical taper inserted in the channel formed by the heatsinks and protruding down into the light recycling cavity of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0031]    The invention uniquely combines a finned heatsink and an optical tunnel (and/or taper) into a one piece integrated assembly. A further embodiment of the invention is coupling a light recycling cavity as described in U.S. Pat. Nos. 6,869,206 and 7,040,774 to provide a compact collimated multi-colored LED light source. A further embodiment of the invention is a novel LED mount and interconnect that provides both easy electrical connection and efficient heat spreading of the heat generated by the LED. The invention can be better understood by referring to the following drawings. 
         [0032]    Shown in  FIG. 1A  is a perspective view of a finned heatsink  100 . This heatsink can be made out of aluminum, copper, silver, etc. The heatsink, if made from aluminum, is easily fabricated at low cost by extrusion where the aluminum fins  102 ,  104  are formed by the extrusion process. Once extruded, the aluminum heatsinks are cut into smaller lengths appropriate to the optical tunnel to be fabricated. If a tapered optical tunnel is desired, the blocks are cut at an angle to match the taper angle. The blocks are then highly polished on one side  106  and/or coated with a dielectric mirror or protected silver mirror on that side  106 . Optionally, a thin glass mirror can be attached with an adhesive to form the mirrored surface on the side  106 . 
         [0033]    To form an optical taper in  FIG. 1B , the heatsinks  110  are assembled with their mirrored surfaces  112  facing to the inside of the channel created by the four heatsink block assembly. 
         [0034]    To form a collimating optical tunnel, the output area  130  in  FIG. 1C  of the mirrored channel tunnel is larger than the input area  132  of the channel. Étendue is preserved through this optical tunnel. Given a Lambertian light distribution on the input, the half angle of the output light bundle is equal to the arc sine of the square root of the ratio of the input area to the output area. Due to the novel arrangement of the mirrored optical block heatsinks, almost any output angle can be achieved by merely sliding the mirrored surfaces to open up a larger opening at the output end of the optical tunnel. 
         [0035]    For example, if it is desired to collimate light emitted by an LED or LED light recycling cavity to an included angle of 15° (half angle), the ratio of input area to output area must be approximately 20:1. A ±15° half angle corresponds to an f/2 optical system. Therefore, to efficiently couple an LED, LED array, or LED light recycling cavity with an output area of 1 mm 2  to a f/2 lens system, the output area of the optical tunnel needs to be approximately 20 mm. 
         [0036]    Most micro-valve display devices, such as digital mirror device, liquid crystal on silicon, etc., are not square, as they match the ratio of the display (4:3, 16:9, etc.). The optical tunnel formed by the mirrored heatsinks can easily be adjusted match any aspect ratio. For example, for a 4:3 display in the example just cited, an output aperture of dimensions 3.9 mm×5.2 mm would be selected. The output aperture  130  of the optical tunnel in  FIG. 1C  is formed by the four mirrored heatsinks  134 . Superimposed on this view is the input aperture  132  at the other end of the optical tunnel. 
         [0037]      FIG. 1D  is a perspective view of the integrated heatsink optical tapered tunnel  140 . The offset  143  of the heatsink blocks diverge outward to form the larger opening for the output of the optical taper. The length  142  of the optical tunnel is used to homogenize the light input into the tunnel. If there are four multi-colored LEDs placed at the input of the optical tunnel, the light will be mixed at the output side of the optical tunnel. 
         [0038]    For example, if red, green, blue, and yellow LEDs are placed with their emitting faces at the input side of the tunnel, a uniform white light will be produced at the output end of the tunnel if there is sufficient length in the tunnel for mixing. Further, the color temperature of the light emitted at the output may be adjusted by adjusting the drive currents to the LEDs. U.S. Pat. Nos. 6,869,206 and 7,040,774 have shown that a higher brightness LED light source may be achieved by forming an LED light recycling cavity. 
         [0039]    Another embodiment of this invention is a novel method to form such a cavity integrated to the optical tunnel/heatsinks. Each RGB LED is mounted to an edge of a thin metal plate to fabricate a multi-colored light recycling cavity. The metal plate may be silver, copper, gold, etc. This plate, which may be 10 mm by 10 mm, is the same thickness as the LED. 
         [0040]    The LED  210  in  FIG. 2A  has been eutectically soldered to a metal plate  214 . The metal plate may be silver, copper, gold, etc. If the LED  210  is 1 mm×1 mm, the thickness of the metal plate  212  is also preferably 1 mm. This plate  212  acts as both an electrical interconnect to the bottom of the LED and a heat spreader for the LED  210 . 
         [0041]    A metal sandwich  220  in  FIG. 2B  is fabricated to complete the electrical connection to the LED  210 . This sandwich  220  consists of the thin metal plate  214  with LED  210  attached, a thin insulating film or adhesive  222 , and another thin metal plate  224  as shown. The thin insulating film or adhesive  22  is between the first thin metal plate  214  and the second thing metal plate  224 . This sandwich  220  can be assembled by a lamination process and/or with high temperature adhesives. The thin outside metal  224 , is offset  226  so that its end face  228  is at the same level as the top of the LED  210 . 
         [0042]    As shown in  FIG. 2C , connection to the top contact  230  of the LED  210  is made via wirebonds  232  or ribbon wirebonds from the LED  210  across the thin insulating film  222  to the outside metal plate  224 . Preferable thickness of the thin insulating adhesive  222  is 15 μm to 25 μm. Preferable thickness of the outside thin metal plate  224  is 100 μm to 200 μm. 
         [0043]    As shown in  FIG. 3A , the LED  210  is mounted on the metal plate  214  and the sandwich  220  is fastened to the end face  310  of the heatsink  312  optical taper block such that the LED  210  emitting surface  314  lines up with the mirrored surface  316  of the heatsink block  312 . The LED  210  is fastened to the metal plate  214 . The metal plate  214  is intimately pressed against the end face  310  of the mirrored heatsink with the screw  318 . Thermal conductivity can be further enhanced between the LED submount metal plate  214  and heatsink  312  by using thermal grease or high conductivity thermal epoxy. 
         [0044]    Four LEDs  320 ,  322 ,  324 , and  326  in  FIG. 3B  are mounted on their sandwiches  328 ,  330 ,  332 ,  334 , which are in turn mounted to the input end faces  336 ,  338 ,  340 , and  342  of the mirrored optical tunnel heatsink. A reflective base plate  352  is placed over the opening  350 , as shown in  FIG. 4A , to complete the light recycling cavity. 
         [0045]    Optionally, a fifth LED  360  can be placed over the opening  350  (as seen in  FIG. 3B ) on the bottom of the light recycling cavity. As shown in  FIG. 4B , this LED  360  can be mounted to its own separate base plate heatsink  362 . The base plate has through holes  364  for screwing it to the sandwich base plate. 
         [0046]    Another embodiment of the invention is to insert, in place of a reflective surface or LED, a highly polished mirrored pyramid  430  on the center of the base plate  362 , as shown in  FIG. 4C . The pyramid  430  will protrude up through the bottom of the light recycling cavity. The pyramid&#39;s base will be the same dimensions as the input of the optical tunnel. The pyramid height can be from one third to one and a half times the height of the LED light recycling cavity. 
         [0047]    A cross-sectional view of the pyramid  430  mounted on the base plate  362  and inserted into the light recycling cavity is shown in  FIG. 4D . The pyramid enhances the light efficiency of the light recycling cavity by reflecting the light emitted by the side mounted LEDs  440 ,  442  and directing it out the output aperture of the light recycling cavity and into the input aperture  444  of the optical tunnel  446 . 
         [0048]    Another embodiment of this invention is to mount a pyramid with dichroic faces on top of the bottom or fifth LED in  FIG. 4E . For a multi-colored light recycling cavity using, for example red, green and blue, this can greatly enhance the efficiency of the cavity. Typically, red LEDs do not have high reflectivity to blue and green wavelength light. As shown in U.S. Pat. Nos. 6,869,206 and 7,040,774, the reflectivity of the LEDs determine the efficiency of the light recycling cavity. By placing the red LED  450  on the base plate  362  and placing a dichroic pyramid on top of the LED, the light emitted by the red LED  450  will pass through the two dichroic faces  454 ,  456  of the pyramid and up and out of the cavity. The dichroic coating on the faces  454 ,  456  are, however, highly reflective to blue and green wavelengths. Therefore, light emitted by the blue LED  458  and the green LED  460  is reflected off of the slope faces of the pyramid and out the light recycling cavity. Alternately, a flat dichroic plate can be used instead of a pyramid. 
         [0049]    Similarly, a flat dichroic on top of the red LED can perform a similar function to the pyramid as light emitted by the red LED will typically be collimated by the flat dichroic plate. The collimation is due to the light emitted at a large enough angle from the normal will be larger than the critical angle required for it to escape from the dielectric plate. The light ray  470  in  FIG. 4F  is emitted by the red LED  450  and passes through the dichroic  475  and out the light recycling cavity. Light ray  472 , however, emitted from LED  450  is greater than the critical angle required for it to escape and is reflected at point  474  back onto the LED  476 . The LED, being a partially diffused surface, can scatter light ray  472  in another direction, in this case making it more normal so that it can escape  474  the dichroic  475  and out the end of the cavity. 
         [0050]    The completed integrated optical taper  510  heatsinks  510 ,  512 ,  514 ,  516  and light recycling cavity with base plate  520  is shown in  FIG. 5 . 
         [0051]    This invention discloses a novel and unique way to form an integrated optical taper, integrated heatsink and light recycling cavity. Alternative ways of practicing this invention can be readily done by those skilled in the art. For example, a glass optical tunnel can be inserted in the channel formed by the four heatsinks. This optical tunnel can be hollow with mirrored inside surfaces or can be solid glass. 
         [0052]    Another embodiment of the invention can incorporate a solid glass taper that actually protrudes down inside the light recycling cavity as shown in  FIG. 6 . This glass taper  602  could have dichroic coatings on surfaces  604 ,  606 ,  608 . These dichroic coatings can be tuned to allow the light from each of the different colored LEDs to pass through them and into the solid glass taper where the three wavelengths would mix, reflecting  610  off the inside surfaces of the optical taper  602 , and out the end of the optical taper. 
         [0053]    A multitude of combinations of practicing this invention can be achieved by one skilled in the art. 
         [0054]    While the invention has been described in conjunction with specific embodiments and examples, it is evident to those skilled in the art that many alternatives, modifications and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all such alternatives, modifications and variations as fall within the spirit and scope of the appended claims.