Abstract:
An LED light engine system that incorporates light emitting diodes (LEDs) with one or more distinct colors, including broad band white light obtained from phosphors or a combination of LED die colors and LED die coated phosphors. The LED die or die arrays are mounted to a high thermal conductivity circuit board comprising COB technology which can include both the LED die and electronic drive components resulting in a compact and reliable design with improved thermal and optical performance. High efficiency non-imaging collection optics are coupled to the LEDs to efficiently capture substantially all of the light which they emit and reformat it as an output with substantially the same éntendue as that of the LED to provide high brightness sources. Feedback from the output back to a photosensor on the circuit board is provided to assure that the output of the collection optic remains constant.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority from U.S. Provisional Patent Application No. 61/222,499 entitled LIGHT EMITTING DIODE LIGHT ENGINE which was filed on Jul. 2, 2009 in the name of Thomas J. Brukilacchio, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention, in general, relates to the collection and monitoring of light emitted from high brightness light emitting diode (LED) die or die arrays coupled into dielectric non-imaging optics and directed toward a monitoring photodiode. Applications cover many markets including commercial, military and industrial illumination where high brightness illumination is required, coupling to optical fibers such as in endoscope, boroscope or microscope illumination, uniform illumination for machine vision, general illumination as in recessed lights, fluorescence imaging, and UV Curing. 
     BACKGROUND OF THE INVENTION 
     High brightness light emitting diode (LED) light sources are in high demand for challenging applications to replace conventional light sources that suffer from short life, poor efficiency and often contain toxic elements or compounds. We define an LED light engine as the combination of an LED board with LED die attached and a primary collection optic to, efficiently collect the light and substantially preserve the Etendue (solid angle, area, index squared product) and may include auxiliary electronics including temperature monitoring devices such as thermistors or thermocouples, photosenors for light monitoring, and drive electronics to control the LED drive current and voltage and an electrical connector. Prior art typically utilize tungsten or tungsten halogen, metal halide, and xenon arc lamps for related illumination applications. 
     Recently, LED based illumination systems have begun to appear in the market, but typically are based on prepackaged LED devices and suffer from relatively poor performance compared to the present invention. 
     Accordingly, it is a principle object of the present invention to provide a high brightness illumination source utilizing LEDs in combination with non-imaging collection optics. 
     It is yet another object of the present invention to provide an LED based system in which light is sampled from the output aperture of a collection optic and directed back to an on-board photosensor to allow for continuous monitoring or optical feedback control for applications that need to maintain constant light output or need to know how the light output changes with time and temperature. 
     Other objects of the invention will in part be obvious and will in part appear hereinafter when the following specification is read in connection with the appended drawings. 
     SUMMARY OF THE INVENTION 
     Prepackaged LEDs are defined as devices comprising an LED die or die array sitting on top of one or more thermally and electrically conductive materials each with associated thermal impedance with electrical leads and thermal backplane that are then intended to be attached to yet another board with additional thermal impedance. Examples of prepackaged devices include the Luxeon™ and Rebel™ product lines now sold by Philips, the Osram Dragon™ and Ostar™ product lines, and the CREE X-Lamp™ product line. 
     The present invention uses “Chip-on-Board” (COB) metal core printed circuit board (PCB) technology in conjunction with high brightness bare LED die attached to the board with solder, eutectic attachment, or conductive epoxy, and high efficiency compact non-imaging optics. This configuration provides a more compact, higher performance, longer life, and lower cost LED light engine relative to systems incorporating pre-packaged LED devices. The thermal impedance between the LED junction and the heat sink is significantly reduced for COB technology by placing the LED die directly on a metal core or on a thin, low thermal impedance dielectric and copper foil layer (or other high thermal conductivity material substrate), thereby increasing temperature dependant life and thermally dependant output power. Additionally, because there is no encapsulant or domed optic over the bare LED die, it is possible to get a much more compact and efficient substantially Etendue (area, solid angle, index squared product) preserving collection optics over the die. Cost is significantly reduced for COB configurations because there is not the additional expense of the components attached to the LED die for the case of pre-packaged LED devices. Additionally, much higher packing densities of LED die are possible, which significantly lowers current density and thereby increases efficiency and lowers total required heat dissipation. In particular for applications requiring a small diameter aperture such as fiber optic illumination, the present invention allows for a much more compact system with higher efficiency relative to one that can be constructed with prepackaged LED devices. 
     The invention herein is an LED light engine system which incorporates light emitting diodes (LEDs) with one or more distinct colors including broad band white light obtained from phosphors or a combination of LED die colors and LED die coated phosphors. The LED die or die arrays are mounted to a high thermal conductivity circuit board comprising COB technology which can include both the LED die and electronic drive components resulting in a more compact and reliable design with improved thermal and optical performance at lower cost relative to pre-packaged based LED systems and other non LED systems such as the industry standard tungsten halogen lamps, metal halide or Xenon arc lamps. In conjunction with high efficiency non-imaging collection optics, the resulting LED based light engines of the present invention are unmatched in brightness by other commercially available LED based illumination systems. 
     The light from the typically ultraviolet (UV), blue, green, amber, red, infrared or phosphor coated blue or UV LED die or die arrays is collected by a non-imaging concentrator which is substantially Etendue preserving. Thus, these light engines are ideally suited for applications such as surgical illumination for head lamps or endoscopes which are among the most challenging light applications that exist today. 
     The combination of COB technology and high efficiency non-imaging optics results in the preferred embodiment of the invention. A particular aspect of the present invention is the method in which light is sampled from the output aperture of the optic and directed back to an on-board photosensor to allow for continuous monitoring or optical feedback control for applications that need to maintain constant light output or need to know how the light output changes with time and temperature. Additionally, there is typically a temperature monitoring device such as a thermistor attached to the board to allow for continuous temperature monitoring and or control. 
     Another important aspect of a preferred embodiment of the present invention uses a reflective aperture to increase the brightness of the light engine which has particular application to Etendue limited applications such as fiber optic coupling. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The structure, operation, and methodology of the invention, together with other objects and advantages thereof, may best be understood by reading the following detailed description in connection with the drawings in which each part has an assigned numeral or label that identifies it wherever it appears in the various drawings and wherein: 
         FIG. 1  shows a preferred embodiment of an LED light engine comprisng a heat sink, LED board with LED die and a non-imaging collection optic; 
         FIG. 2  is a cross sectional view of the system of  FIG. 1 ; 
         FIG. 3  is a view of the system of  FIG. 1  with the heat sink, optic, and connector removed showing LED die, photosensor, thermistor and gain resistor; 
         FIG. 4  shows an enlarged view of the system of  FIG. 3  indicating the additional detail of wire bonds and alignment scribes for accurate LED die placement; 
         FIG. 5  shows the system of  FIG. 3  with the light blocking shroud which was shown in cross section in  FIG. 2 ; 
         FIG. 6  shows a cross section of the system of  FIG. 5 ; 
         FIG. 7  shows a cross section of the system of  FIG. 2  indicating the optical ray paths for the light reflected toward the monitoring photosensor and the blocking of the light by the shroud that would otherwise shunt light to the photosensor; 
         FIG. 8  shows greater detail of the light path for light reflected from the top of the collection optic down toward the photosensor; 
         FIG. 9  shows an isometric view of an alternative embodiment of the collection optic of  FIG. 1  which comprises a light homogenizing first section coupled into the compound parabolic concentrator (CPC) section; 
         FIG. 10  shows a bottom view of the optic of  FIG. 9 ; 
         FIG. 11  shows a cross sectional view of the optic of  FIG. 9 ; 
         FIG. 12  shows an alternative embodiment of the optic of  FIG. 1 ; 
         FIG. 13  shows a bottom view of the optic of  FIG. 12 ; 
         FIG. 14  shows an alternative embodiment of the optic of  FIG. 1 ; 
         FIG. 15  shows a bottom view of the optic of  FIG. 14 ; 
         FIG. 16  shows another embodiment of the optic of  FIG. 1  comprised of a square cross section taper; 
         FIG. 17  shows a bottom view of the optic of  FIG. 16 ; 
         FIG. 18  shows another embodiment of the system of  FIG. 1  with a mirrored aperture positioned at the output face of the collection optic for the purpose of increasing the brightness out of the central aperture; 
         FIG. 19  shows a cross section of the system of  FIG. 18 ; 
         FIG. 20  shows a bottom view of the optic shown in the system of  FIG. 18 ; 
         FIG. 21  shows a more detailed view of the optic of  FIG. 19  indicating the path of a ray reflected off the mirrored aperture which is scattered off the LED and back out the output aperture; 
         FIG. 22  shows a side view of the optical section of  FIG. 21 ; 
         FIG. 23  shows another embodiment of a collection optic and optic holder. 
         FIG. 24  shows a cross sectional view of the system of  FIG. 23 ; 
         FIG. 25  shows a second embodiment of the system of  FIG. 23  with a reduced mirrored aperture holder to increase brigthness of the output analogous to the system of  FIG. 18 ; 
         FIG. 26  shows a cross sectional view of the system of  FIG. 25 ; 
         FIG. 27  shows side on cross sectional view of the system of  FIG. 23 ; 
         FIG. 28  shows a side on cross sectional view of the system of  FIG. 25 ; 
         FIG. 29  shows a final embodiment of the system of  FIG. 18  which launches high brightness LED light into an optical fiber; 
         FIG. 30  shows a cross sectional view of the system of  FIG. 29 ; 
         FIG. 31  shows an isometric view of the cross sectional view of the system of  FIG. 29 ; 
         FIG. 32  shows a cross sectional view of the system of  FIG. 29  without the LED board showing detial of the mirrored aperture; 
         FIG. 33  shows the internal components of the system of  FIG. 29 ; 
         FIG. 34  shows a cross sectional view of the system of  FIG. 33 ; 
         FIGS. 35 ,  36 ,  37  and  38  show details of the optic of the system of  FIG. 29 ; 
         FIG. 39  shows an alternative embodiment using four collection optics in the place of one for the purpose of spreading out the thermal load of the LEDs; 
         FIG. 40  shows a cross sectional view of the system of  FIG. 29 ; 
         FIG. 41  shows the system of  FIG. 39  without the LED board; and 
         FIG. 42  shows a cross section of the system of  FIG. 41  without the LED die showing. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates to Light Emitting Diode (LED) illumination systems for which the Etendue (area, solid angle, index squared product) is substantially preserved and has application across many markets including general illumination, fiber optic coupling including microscopes, endoscopes and boroscopes, machine vision and inspection, ultra-violet (UV) curing, medical illumination, projection systems and fluorescence illumination. In particular the present invention offers higher performance in a more readily manufactured and reliable package in comparison to prior art. An important aspect of the invention is the way the associated collection optic provides a sampling of the light from the LED die, die array or phosphor emitted at the entrance aperture of the optic and passing out the upper portion of the said optic and is then reflected by total internal reflection (TIR) back down the outer wall of the optic toward a photosensor attached to the same LED board. In this way the sampled light is a good representation of the mixed light from the entire entrance aperture and also takes into account any changes in transmission of the optic due to light induced or age related changes in optic absorption. 
       FIG. 1  shows an isometric view  100  of a preferred embodiment of an LED light engine. A high thermal conductivity LED board  104  upon which LED are mounted, as will become apparent shortly, is shown attached to a heat sink  108 . A collection optic  102  is attached to the LED board from which light exits at surface  110 . A surface mounted multi-pin connector  106  is soldered to the LED board  104 . To maintain low LED junction temperature for improved output and lifetime, a high thermal conductivity substrate material such as, but not limited to, copper, aluminum, aluminum nitride, aluminum oxide, beryllium oxide, planar heat pipes, chemical vapor deposited (CVD) diamond, graphite, aluminum and copper composite materials, etc. is used to spread the heat in the plane of the board so as to reduce the heat flux through the back of the board to the heat sink. A thermally conductive material is typically placed between the back of the LED board  104  and the heat sink  108  so as to reduce the temperature rise across that interface. Suitable materials include the silicone/aluminum oxide materials sold under the name Gap Pad or Sil Pad from companies such as The Bergquist Company of Chanhassen, Minn. 55317 USA, standard thermal greases such as those based on aluminum oxide, silver or diamond powders, or Pyrolytic Graphite Sheet (PGS) such as is available from the Panosonic Corporation which is made from a highly oriented graphite polymer film. A multi-pin connector  106  is shown surface mounted to the LED board  104  and is capable of transmitting high currents on the order of 10&#39;s of Amps to the LEDs. 
       FIG. 2  shows a vertical cross sectional view of the system of  FIG. 1  indicating the detail of the non-imaging collection optic  102  having an input aperture  204 , a light blocking shroud  208 , and a photosensor  210 . The LED collection optic  102  is in a preferred embodiment of the form of a compound parabolic concentrator (CPC) such as described by Winston and Welford in a book entitled “High Collection Non-imaging Optics” published by Academic Press and is made of a tilted and shifted parabolic section according to the edge ray principle. Typical dielectric materials used to mold the optic  102  include, but are not limited to, highly transparent optical grade thermal plastics such as acrylic, polycarbonate, cyclic olefins (such as is available from Zeon Chemicals), or other transparent materials such as glass or silicone. A new class of polycarbonate manufactured by Bayer and include products such as LED2045 or LED2245 are particularly well suited due to their resistance to yellowing from exposure to short wavelength blue light or UV wavelengths. Additionally, the Bayer materials have a high glass transition temperature on the order of 147 Centigrade and have robust mechanical properties which yield rugged and reliable devices. The light emitted from the LED die, die array, and/or phosphors attached to the LED board  104 , directly under the entrance aperture  204  of the optic  102 , first pass through an index of refraction matching gel, typically made out of silicone, such as is available from the Nusil Corporation. The index matching gel increases the extraction efficiency of the light created within the LED die itself due to a reduction of the light totally internally reflected and thus trapped within the LED die. The light reflects off the side walls of the CPC and is directed toward the aperture  110  of  FIG. 1  where some of the light around the top outside edge of the optic is directed via a folded optical path by the process of total internal reflection (TIR) back down through an outer wall of the optic  206  toward the photosensor  210 . The purpose of the shroud  208  is to block light that would otherwise reach the photosenor by a more direct path. The advantage to sampling the light in this manner is that it represents a good average sampling from all the LED die over the entire input aperture  204  of the optic  102 . Additionally, as the optic ages, if it is made out of a polymer material, the increase in the absorption and thus loss of light would affect the light reaching the photosensor  210  and would be a better indicator of the light exiting aperture  110  than if light were to reach the photosensor directly from the LEDs. 
       FIG. 3  shows a detailed perspective view of the LED board  104  indicating an LED die array  306 , photosensor  210  and an associated gain resistor  302 , and a thermistor  304 . 
       FIG. 4  shows an enlarged close up view of the semiconductor components on board  104  of  FIG. 3 . The LED die  306  are attached directly to the gold coated copper substrate using standard attachment materials including solder, direct eutectic attachment and electrically and thermally conductive epoxy to achieve the best thermal performance. Laser scribed lines  404  are shown ablated into the metal substrate to act as alignment references for the die and are themselves aligned to the datums comprising the holes in the board through which kinematic pins in the optic  102  are positioned upon assembly. This assures that the input aperture  204  lines up with the LED die or die array  306 . Wire bonds  402  are shown attaching the top side of the LED die to the surrounding contact which is in turn routed to the connector  106 . 
       FIG. 5  shows a perspective view of the LED board  104  with the light blocking shroud  208  attached. The shroud  208  is designed to be held in place by the optic  102 . The light enters the shroud  208  at a hole  504  that is centered over the active area of the photosensor  210 . A vertical cross sectional view of the system of  FIG. 5  is shown in  FIG. 6 . The portion of the shroud  208  surrounding the photosensor  210  can be seen which acts to block light from the LED die  306  from directly reaching the photosensor. 
       FIG. 7  shows an enlarged cross sectional view of the system of  FIG. 1 . A ray path  702  indicated as a dotted line is shown exiting the LED die array  306  through the index matching gel and input aperture  204  toward upper outer prismatic facets,  701  and  702 , of the optic  102 . Ray path  702  is directed back down toward the photosensor  210  by facets  701  and  702  angled such as to reflect by total internal reflection and pass through shroud hole  504 . Since the CPC is molded as a single piece and there has to be some means of attaching the CPC portion to the outer surface of  102 , this approach serves double duty by also acting as a light sampling means. The output aperture  110  is recessed to allow the attachment of a thin (0.010 inch) diffuser such as those termed “holographic diffusers” manufactured by such companies as Luminit of Torrance, Calif., or “Engineered Diffusers” by companies such as RPC in Rochester, N.Y. The purpose of the diffuser is to increase the uniformity of the far field intensity distribution or provide an elliptical or rectangular far field by suitable surface structures. Alternatively, the diffusers could also be molded directly onto the output aperture  110  of optic  102 . Typically, diffusers are only used for far field applications and are not generally used when coupling the output of aperture  110  to an optical fiber or light pipe or when the near field is reimaged such as in projection applications. The dark arrow emitted from the LED die array  306  indicates that the light is blocked by shroud  502  from reaching photosensor  210  without passing through the CPC  202 . 
       FIG. 8  shows an enlarged close up view, with parts broken away, of the optic  102  of  FIG. 7  showing two different ray paths  702  reflecting off of facets  701  and  702  down a side wall  806 . The efficiency of this geometry was optimized through non-sequential ray tracing using ZEMAX optical design software. 
       FIG. 9  shows an alternative embodiment optic  900  of the optic  102  of  FIG. 1 . Optic  900  was designed to attach to the same LED board  104 . Optic  900  was designed specifically for use with no index matching gel. One very important aspect of the invention of  FIG. 9  is that there is no index matching gel between the LED die and the input aperture of the collection optic for the purpose of maximizing the effective source intensity by recognizing the role of the index squared portion of the Etendue (index squared, area, solid angle product). Historically, the extraction efficiency of LED die increased by approximately the square of the index of refraction of the index matching gel (about 2 times). However, due to surface extraction enhancement technology used in state of the art LED die, this is no longer generally true. In fact the shorter wavelength GaN LED die (UV through green) only increase on the order of 20% to 40% depending on the LED die manufacturer, and the longer wavelength amber to red and near infrared LED die only increase on the order of 50% to 60% when index matched. Thus, due to the index squared factor of the Etendue, brightness is enhanced by not using index matching gel, which for a fixed Etendue effectively increases the allowed area of the source allowing the LED die to run at lower current densities. Additionally, in the case of broad band white LED light, which is typically derived by coating blue LED die with a phosphor and silicone mixture, the LED is already index matched, and there is minimal increased output from using index matching gel. The phosphor would be of a type such as cerium doped YAG (Ce:YAG) that is well-known in the art or one of the alternative yellow phosphors available from companies such as Internatix of Fremont, Calif. A portion of the blue light emitted by the LED die would be absorbed by the phosphor and re-emitted as yellow light which, in combination with the scattered but non-absorbed blue light, produces the appearance of white light. Typically, the phosphor is held in place on the emitting surface of the LED die with a silicone material. In a preferred embodiment, the phosphor would be applied by a method shown in pending International Patent Application (WO 2007/064342) which describes a conformal coating process to achieve optimal color uniformity and intensity. Many prepackaged LEDs are encapsulated, which means they will result in effectively lower brightness relative to non-index matched LEDs on COB. 
     Light from the LED die or die array or die and phosphor would enter the optic  902  at an input aperture  904 . The input aperture  904  is square to optimally match the shape of the LED die or die array, which is also square. A section  906  in this example has a length on the order of 10 mm and transitions from the square cross section of input aperture  904  to the circular cross section at the CPC entrance aperture  908 . Since this is effectively a non-index matched CPC, the first section of the CPC  910  is conical as the light just inside the CPC has an angle less than 90 degrees dictated by Snell&#39;s Law (n 1  sin θ 1 =n 2  sin θ 2 ). Thus, the collection optic becomes a θ on  by θ out  concentrator also described by Winston and Welford for which a finite input angle is converted to a smaller finite output angle. The features on the bottom of optic  902  are similar to those of optic  102  of  FIG. 1 . A kinematic pin  914  interfaces to a tightly toleranced hole on LED board  104 . A pin  912  on optic  902  fits into another hole on LED board  104  locking the optic in the rotation axis. The tangential dimension of pin  912  is greater than its radial dimension to prevent any issues with fit due to manufacturing tolerance in the distance between the holes on the board versus the distance between the pins on the optic. Holes  916  on opposite sides of the bottom of optic  902  are for self tapping plastic screws which lock the LED board  104  to the optic  102 . An indented area  918  on the bottom of optic  902  provides room for the shroud  502  (See  FIG. 6 ) in the vicinity of the photosensor hole  504 . The surface of  918  is smooth to allow for the light sampled from the output aperture  110  to be directed with minimal scattering toward the photosensor.  FIG. 10  shows a bottom view of  FIG. 9 . The conical section  910  leads into the CPC section  1002  and joins the top of the optic in a circular cross section at  1004 . 
       FIG. 11  shows a cross sectional view of optic  902 . Light enters the optic at  904  from the LED die, die array, phosphor or both die and phosphor combination and is homogenized in the near field as it passes down light integrating section  906  toward an entrance  908  of the collection optic θ in  by θ out  CPC. It is important to note that a CPC is highly efficient at preserving the Etendue and thereby maintains the smallest output aperture for a given maximum extent output angle providing the highest brightness. In fact about 96% of the light that enters the CPC is emitted within the solid angle defined by Etendue matching. While the output of the CPC is very uniform both in the near and far fields for a uniformly filled input, both the near and far field can show structure if the input is not uniform. Thus, the light homogenizing effect of the light integrator section  906  results in a much reduced dependence of output near and far field uniformity on the input near field uniformity. Typically, both the output from LED die and phosphors are substantially Lambertian sources (fall off as the cosine of the input angle) so the only consideration is for near field uniformity not far field uniformity at the input of the CPC section  908 . By virtue of the draft angle between apertures  904  and  908 , the light entering aperture  908  is slightly reduced in far field angle from that just inside aperture  904  which is taken into account by opening up the input aperture of the CPC. Shaping the input aperture  904  to match the square shape of the LED die array maximizes brightness and maintains the Etendue of the LED or phosphor sources. The homogenizing effect of section  906  also allows multiple colored LED die, or the combination of LED die and phosphors, to be used with excellent near and far field uniformity at an exit aperture  1102 . For example, blue LED die and red LED die can be used at the input aperture  904  to achieve both high Color Rendering Index (CRI) and a controlled Correlated Color Temperature (CCT) simultaneously. The manufacturing process can be simplified by coating both the blue die and red die with phosphor, as the red die wavelengths are not absorbed by typical phosphors and the index matching effect of the phosphor/silicone combination used to coat the LED die compensates for losses due to back scattering. Likewise, multiple LED die colors such as red, green and blue can be used in combination to give precise control over the output spectrum without suffering from uniformity issues in the near or far fields due to the sensitivity that would otherwise exist for non-uniform input intensity distributions. The dotted line at  1108  indicates the transition between the conical section  910  of the θ in  by θ out  CPC and the parabolic section  1002 . Again the output aperture  1102  is designed to accommodate a diffuser to change the output far field if desired for specific applications. 
       FIGS. 12 and 13  show an alternative embodiment  1200  in cross section and in bottom view, respectively, to the optic of  FIG. 9 . In this case, the input and output apertures,  1202  and  1206 , of a homogenizing section  1204  are both square in cross section. Other shapes can also be used as homogenizers including rectangles, and polygons with an even number of sides. Typically, polygons with an odd number of sides are not as effective at homogenizing, and round cross sections only homogenize in the tangential, not the radial directions, but could be used as well; however, not as effectively. The unique optic section  1208  is formed by lofting the square cross section of  1206  with the round cross section of aperture  1302  such as can be done using SOLIDWORKS computer aided design software. 
       FIGS. 14 and 15  represent another alternative embodiment to the collection optic of  FIG. 9  It is designated at  1400  and is shown in cross sectional view in  FIG. 14  and in bottom view in  FIG. 15 , respectively. An optic  1404  is square at an input  1402  and circular at  1406  and is therefore similar to the section  1208  of  FIG. 12 , but runs the full length of the optic and does not contain a light integrating section. 
       FIGS. 16 and 17  show cross sectional and bottom views  1700  of another alternate embodiment to the optic of  FIG. 9 , designated at  1600  and also intended for use without index matching gel. Light enters a square cross section input aperture  1602  positioned over a similarly shaped LED die, die array, phosphor coated die or die array or combination thereof. The light is guided by total internal reflection up the square cross section optic  1604  to the output at square cross section  1606  and then through output aperture  1608 . The far fields of both the round and square tapered collection optics of  FIGS. 9 through 17  are substantially circular. Alternatively, the sides  1604  of the optic of  FIGS. 16 and 17  could be similar in profile to the sections  910  and  1002  of  FIG. 11  and would in fact have a similar cross section. In this case, however, the far field is substantially square. In fact the far field&#39;s aspect ratio would be controlled by the aspect ratio of the output in an inverse relationship according to the brightness theorem for which the product of the face dimension along a particular axis and the numerical aperture (NA which is sin(θ) of the output angle). Thus, the long dimension of a rectangular output produces the narrow dimension of the rectangular far field and the shorter dimension produces the wider far field. Note that the corner of the output aperture of the optic of  FIG. 17  overlaps the facets in the vicinity of  1702  to allow for light to be sampled back to the photosensor in the same manner as described for the optic of  FIGS. 1 and 9 . Thus, the LED array is rotated by 45 degrees along the optical axis to achieve this condition. 
       FIGS. 18 through 22  show an alternative embodiment to the LED light engine of  FIG. 1 . The alternative embodiment is designated at  1800  shown in isometric view in  FIG. 18 , cross sectional view in  FIG. 19 , bottom view in  FIG. 20 , isometric cross sectional view in  FIG. 21 , and enlarged cross sectional view in  FIG. 22 , respectively. An output aperture  1802  of an optic  1806  in  FIG. 18  shows a mirror  1804  with reflective side toward the optic centered on output aperture  1802 . An LED board  1808  and connector  1810  are similar to board  104  and connector  106  of  FIG. 1 . With reference to cross section of  FIG. 19 , light enters this non-index matched CPC at input aperture  1904  and is directed toward the output aperture  1802  and mirror  1804  either directly or by total internal reflection off the sides of optic  1902 .  FIG. 20  shows the circular cross section of optic  1902  at both the input aperture  1904  and output aperture  1906 . The ray path of a ray  2108  is shown in isometric cross sectional view of optic  1806  in  FIG. 21  indicating how it first enters aperture  1904  then passes up toward mirror  1804  which reflects it back toward the input aperture  1904  to the LED array shown just below aperture  1904  where it is subsequently scattered back through the output aperture  1802 . In this way, the brightness of the output aperture  1802  within the central aperture of mirror  1804  is increased from what it would otherwise be in the absence of mirror  1804 . This enhancement of brightness is useful in Etendue limited applications requiring very high brightness, such as endoscopic fiber bundle illumination which is typically accomplished by use of a high intensity discharge short arc Xenon or Metal Halide lamp. Due to the finite losses at reflecting or scattering interfaces, this approach necessarily reduces efficiency, but does yield higher brightness (power per unit angle per unit area) than other approaches. Typically, increases in brightness on the order of a factor of 2 can be realized by this method, but there are diminishing returns as the ratio of the optic aperture to the mirror aperture increases. Typical reflectivity for state of the art LED die is on the order of 80% for UV through green wavelengths and the order of 60% or better for amber through near infrared (NIR). Phosphors such as Ce:YAG and others made specifically for LEDs typically have quantum efficiencies near unity, so they work extremely well in reflection. The light reflected in the blue spectrum thus has a chance to get partially reabsorbed and emitted as yellow light with most of the non-absorbed light being reflected back toward the output aperture. In this way, there is a yellow shift (toward lower CCT) between the bare phosphor coated LED and the output with the optic with mirrored aperture. Thus, the thickness of the phosphor coating on the LED is reduced from what it would be in the absence of this light feedback for a given CCT specification and thereby reducing the light scattered back toward the LED die as it originally exits the die. This can offset some of the efficiency loss do to the reflective and scattering efficiency described previously. If more than one color of LED die or the combination of LED die and phosphors with different colored LED die are used there is the additional benefit of this approach in that it tends to increase the mixing or homogenization of the light in the near field. The mirror  1804  could be made out of a number of materials. For example, 3M markets a reflective sheet under the product name Vikuiti™ Reflective Display Film with reflectivity on the order of 98%. The film can be attached by adhesive or by other optically transparent cements, epoxies or adhesives. 
     The shape of the optic  1902  in the cross-sectional view of  FIG. 22  is analogous to that of the optic of  FIG. 11  in that the optic  1902  is designed to work without index matching gel and is, therefore, comprised of the conical input section  2102 , which is tangent to the CPC parabolic shape  2104  at the interface between them as indicated by the dotted line. The section  2106  is also conical and tangent to the section  2104  at the dotted line between these two sections and is done for the purpose of maintaining sufficient draft angle toward the top of the optic to make it easier for the optic to be released from the mold. The basic design approach for this type of optic is to design the input aperture as if it were to be index matched and thus a standard CPC with no conical section, taking proper account of the index of refraction of the dielectric medium and the surrounding medium which is typically air. One then opens the input aperture up to what it would be for a similar exiting Etendue (diameter and angle) and requires the conical section to be tangent to the parabolic section of the CPC, thus determining the angle and length of the conical section  2102 . The slight draft of section  2106  has only a minor effect on the output diameter and angle, but can be accounted for in design optimization by the use of non-sequential design software such as TRACEPRO, ZEMAX, FRED, ASAP, and LIGHT TOOLS. Alternatively the input and output apertures of this optic could both be square or rectangular in cross section to match the shape of the die for smaller apertures. Note, however, that in the case of using phosphor, the area surrounding the LED die would be diffusely reflective assuming it was coated with phosphor. 
       FIGS. 23 through 28  show an alternative embodiment to the optical configuration of the optic of  FIG. 18 .  FIGS. 23 and 24  show an isometric view of an optic holder  2302 , a collection optic  2404  with output aperture  2306 , and a reflective interface component  2406  with aperture  2310 , side reflective wall  2308  and top aperture  2304 .  FIG. 24  shows greater detail of the system of  FIG. 23  in cross sectional view. The optic  2404 , which could have a shape analogous to that of optic  1902  of  FIG. 21 , is held into interface component or optic holder  2406  which, in turn, is held into surrounding holder  2302 . The inside lip  2408 , which interfaces to optic output face  2306 , can be reflective so that any light incident on it has an opportunity to be reflected back to the LED array below input aperture  2402  and be subsequently scattered back through optic  2404  and out through apertures  2310  and  2304 . This type of method for holding the optic is particularly useful for glass versions of the optic which would be used for high power density short wavelength optics that would otherwise be negatively affected by aging affects due to yellowing if they were made of polymers or melting due to high temperature operation. While it is feasible to mold the entire component out of glass, it is not very easily manufactured. The optic holder  2302  would optimally be made out of a high temperature material with an expansion coefficient closely matching that of the optic so that the distance between the input aperture and the LED die array did not change significantly with changes in temperature. Otherwise, the optic could potentially be damaged at extreme temperatures or the wire bonds could be compromised. 
       FIGS. 25 and 26  show a system very similar to  FIGS. 23 and 24  for which the only modification is the ratio of the optic diameter to the output aperture diameter of the reflective optic holder with output aperture  2502  indicated in the isometric view of  FIG. 25 .  FIG. 26  shows the system of  FIG. 25  in cross sectional view indicating ray path  2606 , which starts at the LED die below the input aperture  2402 , reflects off the inside face of mirrored holder  2602  back toward the LED array and back out through aperture  2502  and  2506 . Again, the inner wall  2504  of holder  2602  is mirrored so that any light hitting it is substantially redirected out of aperture  2506  and is not significantly scattered or absorbed. The system of  FIGS. 25 and 26  has an analogous affect as the system of  FIG. 21 . The material of holder  2602  is preferably a high temperature plastic or could be metal as well. It could be designed with slits along the side walls within the counter bore where the optic goes which could be forced by the shape of holder  2302  to bend in and push against the tapered walls of the optic  2404  and thereby act as a means of holding all three components together. Otherwise a retaining ring could be used to hold the optic and holder  2406  into holder  2302 . 
       FIGS. 27 and 28  show cross sectional views of the systems of  FIGS. 23 and 25 , respectively. The optic  2404  of the system of  FIG. 27  has sections  2702 ,  2704  and  2706 , which are analogous to sections  2102 ,  2104 , and  2106  of  FIG. 22  respectively. 
       FIGS. 29 through 38  show another embodiment  2900  of a LED light engine incorporating a mirrored aperture and designed to interface to an optical fiber or fiber bundle for applications such as endoscopic illumination, which can now replace Xenon arc lamp based systems with much improved lifetime, lower cost, and reduced input power. Furthermore, these systems do not require a high voltage to start them such as is necessary for arc lamps which eliminates EMI (electromagnetic interference) associated with many operating room illumination systems presently in use.  FIG. 29  shows LED light engine  2900  with holder  2902  attached to LED board  2904  with light being emitted from attached fiber optic  2908 , thermistor temperature sensor  2910  and surface mount connector  2906 .  FIG. 30  shows a cross sectional view of the system of  FIG. 29  showing internal components including the optic  3004 , mirrored aperture  3010 , fiber  2908 , optic retainer  3008 , and fiber ferrule  3014 . A bevel near the bottom of the fiber holder allows for a set screw through the side of holder  2902  to retain the fiber  2908  in close proximity to the output face of the collection optic  3004  at aperture  3006 . Kinematic pins in the holder are shown projecting into holes in the metal core LED board  2904 . The bottom side of mirrored aperture  3010  is coated with a high reflectivity coating such as protected aluminum or silver or alternatively, a multilayer reflective dielectric stack coating. Alternatively, a material such as was described above from 3M comprising reflective film could be attached to the bottom of mirror  3010 . The fiber  2908  extends out beyond the lower face of the ferrule  3014  so that it can extend through the aperture in  3010 . If the inner wall of the hole in the mirror is specularly reflective, then the fiber would not need to be directly against the output aperture  3006  of optic  3004 . Again, the LED die, die array or die/phosphor combination would be located on the LED board  2904  centered below the input aperture  3002  of the optic  3004 . The function of the mirror of system  3000  is analogous to that of mirror  1802  of  FIG. 18 .  FIG. 31  shows an isometric cross sectional view of the system of  FIG. 30  providing a better perspective on the inner components, particularly the optic retainer  3008 , which is centered by similar kinematic pins on the bottom of holder  2902 .  FIG. 32  shows a bottom isometric cross sectional view of the system of  FIG. 29  without the board or attached components, with the exception of the LED die at the input aperture  3002  or the optic  3004 . Ray path  3202  shows a ray emitted from the LED array reflected off the bottom side of the reflective mirror aperture  3010  at surface  3204 , directed by specular reflection back to the LED array or phosphor and then scattered back out the aperture  3006  and into optical fiber  2908 . 
       FIG. 33  shows a detail of the system of  FIG. 32  with the holder  2902  removed and in cross sectional view in  FIG. 34 .  FIGS. 35 through 38  show detailed views of the optic  3004  of the system of  FIG. 33  indicating the square dimensions of input face  3002  relative to square LED array  3502 . The output face  3006  is also square in cross section. The corners of the optic have been radiused to make the glass optic easier to release from the mold. There is no loss in performance due to the radiused edge up to a point as the small amount of light lost at the corners of the LED die is made up for by decreasing the ratio of the exit face area to the fiber diameter area, thereby increasing the efficiency of coupling light out through the exit aperture.  FIGS. 37 and 38  show the optic  3004  with the edges squared off and without radius in isometric view and cross sectional view, respectively. The sides  3706  are comprised of three sections;  3802  which is conical,  3804  which is parabolic CPC form, and  3806  which is conical in the same manner as the surfaces of optic  1902  of  FIG. 22 . 
     Finally, a system  3900  of  FIG. 39  is shown in isometric view representing an alternative embodiment of the system of  FIG. 30  for which the single LED array has been replaced with four (4) separate LED arrays  3904  at the input apertures of four individual collection optics  3906 . The apertured mirror  3908  is positioned at the output faces of the collection optics  3906  and centered on the central axis of the four arrays attached to LED board  3902 . The optical fiber or fiber bundle  3910  interfaces to the output of apertured mirror  3908 .  FIG. 40  shows a cross sectional isometric view of the system of  FIG. 39  indicating an overlap between the output apertures  4002  of the optics  3906  and the central aperture of mirrored reflector  3908 .  FIGS. 41 and 42  show an isometric bottom view of the system of  FIG. 39  without the LED board indicating the separation between the four groups of LED die and in cross sectional view, respectively. The lower side  4102  of apertured mirror  3908  is highly reflective as would be the inside of the aperture at cylindrical surface  4204 . Thus, the fiber  3910  is butt-coupled to and centered on apertured mirror  3908 . The benefit of the system of  FIG. 39  over using a single optic is thermal in nature. By virtue of increasing the distance between the heat sources (LED die) on the LED board, the heat flux is reduced, and the die can be maintained at a lower temperature for a given degree of cooling. If this improved output due to the improved thermal performance outweighs the disadvantage of a more complex system then there is a net benefit to this configuration. It is clear from the separation of the LED die groups in  FIG. 41  that the thermal situation would be improved. In particular for the inner most LED die, which in a 4 by 4 array of 16 die would be completely surrounded by other LED heat loads. 
     For all the systems indicated above it is well known in the art that the transmission efficiency could be further enhanced by the addition of anti-reflection coatings on any air-dielectric interfaces. Additionally, the optical shapes shown could be approximated by faceted optics with some loss in Etendue maintenance. Additionally, other types of emitters including organic light emitting diodes and laser diodes could be substituted for the LEDs. Other variations will occur to those skilled in the art and are intended to be within the scope of the appended claims.