An illumination source includes a number of light emitting diodes (LEDs) operating at a first wavelength. Light from the LEDs illuminates a phosphor material that generates light at a second wavelength. A reflective polarizer transmits light at the second wavelength in a first polarization state and reflects light at the second wavelength in a second polarization state orthogonal to the first polarization state. The light at the second wavelength reflected by the reflective polarizer is directed back towards the phosphor material without an increase in angular range. In some embodiments the LEDs, having a conformal layer of phosphor material, are attached directly to the first surface of a liquid cooled plate. A liquid coolant contacts a second surface of the plate.

FIELD OF THE INVENTION

The present invention relates to illumination systems that may be used in image projection system. More specifically, the invention relates to illumination systems that include an array of light emitting elements, such as light emitting diodes (LEDs) to generate polarized light.

BACKGROUND

Illumination systems may be found in many different applications, including image projection display systems, backlights for liquid crystal displays and the like. Projection systems usually use a source of light, illumination optics to pass the light to one or more image-forming devices, projection optics to project the image(s) from the image-forming device(s) and a projection screen on which the image is displayed. The image-forming device(s) are controlled by an electronically conditioned and processed video signal.

White light sources, such as high pressure mercury lamps, have been, and still are, the predominant light sources used in projection display systems. In a three-panel image-projection system, the white light beam is split into three primary color channels, red, green and blue, and is directed to respective image-forming device panels that produce the image for each color. The resulting primary-colored image beams are combined into a full color image beam that is projected for display. Some other projection systems use a single imager panel, and so rotating color wheels, or some other type of time-sequential color filter, is used to filter the white light so that light at one primary color is incident on the image-display device at any one time. The light incident at the panel changes color sequentially to form colored images synchronously with the incident light. The viewer's eye integrates the sequentially colored images to perceive a full color image.

More recently, light emitting diodes (LEDs) have been considered as an alternative to white light sources. In some cases, different illumination channels are powered by respectively colored LEDs, or arrays of LEDs. For example, blue LEDs are used to illuminate the blue channel and red LEDs are used to illuminate the red channel. Some types of image display device, such as a liquid crystal display (LCD), employ polarized light, whereas the LEDs produce unpolarized light, and so only half of the generated light is usable by the LCD. Furthermore, LEDs that operate in the green region of the visible spectrum are known to be relatively inefficient, compared to blue and red LEDs, and so many systems require more green LEDs than blue or red LEDs. This problem of inefficiency in the green portion of the spectrum is compounded when the light is required to be polarized.

There remains a need for a solid state light source that efficiently generates green polarized light.

SUMMARY OF THE INVENTION

One embodiment of the invention is directed to an illumination source that includes an arrangement of one or more light emitting diodes (LEDs) capable of generating light at a first wavelength. A phosphor material disposed proximate the one or more LEDs, the phosphor material emitting light at a second wavelength when illuminated by the light at the first wavelength. The source also includes a light collecting/focusing unit having at least a tapered optical element. A reflective element is disposed to reflect light at the first wavelength that has passed through the phosphor material. A reflective polarizer is disposed to transmit light at the second wavelength in a first polarization state and to reflect light at the second wavelength in a second polarization state orthogonal to the first polarization state. Light at the second wavelength reflected by the reflective polarizer is directed back towards the phosphor material without an increase in angular range.

Another embodiment of the invention is directed to an illumination source that includes an array of one or more light emitting diodes (LEDs) capable of emitting light at a first wavelength. There is a first light collecting/focusing unit to form at least some of the light at the first wavelength into a telecentric beam. A phosphor is capable of generating light at a second wavelength when illuminated by light at the first wavelength. The telecentric beam is directed to the phosphor. A reflective polarizer is disposed to transmit light at the second wavelength, received from the phosphor, in a first polarization state and to reflect light at the second wavelength in a second polarization state back to the phosphor.

Another embodiment of the invention is directed to an illumination source that includes an array of one or more light emitting diodes (LEDs). The LEDs are attached directly to a first surface of the a liquid cooled plate. A liquid coolant contacts a second surface of the liquid cooled plate. A phosphor layer is conformally disposed on the one or more LEDs.

Another embodiment of the invention is directed to a method of manufacturing an illumination source. The method includes providing one or more (LED) dies having a metallic layer on respective LED lower surfaces and placing the LED dies in thermal contact with a first surface of a plate whose temperature is controllable by flowing a fluid past a second surface of the metal plate. A heated fluid is passed by the second surface of the plate so as to melt the metallic layer. The metallic layer is cooled so that the metallic layer solidifies, thereby attaching the LED dies to the first surface of the liquid cooled plate.

The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The following figures and detailed description more particularly exemplify these embodiments.

DETAILED DESCRIPTION

The present invention is applicable to illumination systems, and is more particularly applicable to illumination system for displaying images, for example projection systems such as may be used in projection televisions and displays, monitors and the like.

It is well known that green light emitting diodes (LEDs) are less efficient than LEDs operating in the blue and red regions of the visible spectrum. Consequently, LED-based illumination systems require more green LEDs than blue and red LEDs to achieve desired levels of brightness and color balance. Instead of generating green light directly with an LED, another approach is to generate light at a first wavelength, for example blue or UV wavelengths, and to convert the light at the first wavelength to a green wavelength.

One exemplary approach that may be useful for wavelength converting light from an LED to generate green light is schematically illustrated inFIG. 1. An exemplary illumination system100has an array of one or more LEDs102mounted on a baseplate104. The baseplate104may be used for providing electrical power to the LEDs102and also for extracting heat from the LEDs.

At least some of the light106from the LEDs102is collected in a first light collecting/focusing unit107. In the illustrated embodiment, the light collecting/focusing unit107includes a light pipe108having an input110and an output112.

A side view of an array of LEDs102mounted on a baseplate104is schematically illustrated inFIG. 2. Some commercially available LEDs that may be used in the illumination system emit light through the upper surface, facing the light pipe108. Other types of commercially available LEDs, as shown, emit light out of angled faces202of the LED dies.

In some exemplary embodiments, a reflective element114may be disposed close to the LED array. The reflective element114surrounds at least part of the input110to reduce the amount of light that leaks away from the input110of light pipe108. The reflective element114may be desired, for example, where the input110is separated from the LEDs102by a small distance due to interference of the wirebonds204used to make electrical connection to the top of the LEDs102. This configuration, with the reflective element114, allows a reduced number of LEDs102to be used, thus reducing cost and power consumption, while still filling the light pipe108. The reflective element114may include a metalized or multilayered reflective coating.

In some exemplary embodiments, the light pipe108is a tapered solid rectangular prism located directly over LEDs102. The input110of the light pipe108may be made small so as to prevent an increase in the étendue of the system. The étendue is the product of the area of the light beam at the light source times the solid angle of the light beam. The étendue of the light cannot be reduced but can be increased by the optical system. This reduces the total brightness of the light illuminating the display, since the brightness is given by the optical flux divided by the étendue. Thus, if the area of the light beam is increased, for example to cover the active area of the imager device, it is sufficient that the angular range of the beam be reduced proportionally in order to conserve the étendue of the light beam. By conserving the étendue, the brightness of the illumination light incident at the imager device is maintained at, or close to, the highest achievable level.

In the exemplary embodiment shown inFIG. 1, the maximum flux per étendue is obtained when the LEDs102are non-encapsulated LED dies, emitting light106into the air with no additional epoxy, silicone or other intervening material so that LEDs102are separated from the input110of the light pipe108by an air gap. This configuration may improve the reliability of LEDs102by eliminating organic and polymer layers that might be degraded by high temperatures and light flux. In some embodiments, it may be desired to include some encapsulation between the LEDs102and the input110, for example for environmental protection.

It has been found that a light collection efficiency in the range of 80%-90% can be obtained by a light pipe108in gathering the light emitted by LEDs102. In some exemplary embodiments, the light pipe108may have a length that is between two and ten times longer than its width at the output, although the light pipe108may also operate outside this range. As the length of light pipe108is increased, the uniformity of the light at the output112increases. If the light pipe108becomes too long, however, the system becomes more bulky and expensive, and less light exits from the light pipe108dues to losses within the light pipe108. Other configurations of light pipe108may also be used, such as a hollow tunnel rather than a solid light pipe.

In some embodiments, the light collecting/focusing unit107may also include a focusing optic116, such as a lens, at the output112. The focusing optic116may be separate from the light pipe108or may be integrated with the light pipe108.

The light118output from the light collecting/focusing unit107may be substantially telecentric. The term “telecentric” means that the angular range of the light is substantially the same for different points across the beam. Thus, if a portion of the beam at one side of the beam contains light in a light cone having a particular angular range, then other portions of the beam, for example at the middle of the beam and at the other side of the beam contain light in substantially the same angular range. Consequently, the light beam is telecentric if light at the center of the beam is directed primarily along an axis and is contained within a particular cone angle while light at the edges of the beam is also directed along the axis and has substantially the same cone angle. If the light pipe108is sufficiently long, then the light118at the output112may be sufficiently telecentric without the need for a focusing optic116. Use of a focusing optic116permits the light collecting/focusing unit107to be shorter while still producing a telecentric output. The fraction of the light that is subsequently concentrated at the phosphor for frequency conversion is increased when the light is telecentric.

In some exemplary embodiments, the focusing optic116may be integrated with the light pipe108, or may be separate from the light pipe108. In other exemplary embodiments, the light pipe108may be provided with curved sidewalls that perform a focusing function.

The light118is passed into a polarizing beamsplitter (PBS)120. The PBS120may be any suitable type of PBS, for example a MacNeille-type PBS or a multilayer optical film (MOF) PBS, such as an MZIP PBS as described in U.S. Pat. Nos. 5,962,114 and 6,721,096, incorporated herein by reference. Other suitable types of PBS include wire grid and cholesteric PBSs. The PBS120typically contains a polarization selective layer122disposed between the hypotenuse faces of two right-angled prisms124aand124b, although other configurations may be used. The polarization selective layer122reflects light in one polarization state and transmits light in the orthogonal polarization state. The PBS120may also include a reflecting film123disposed between the polarization selective layer122and the second prism124b. The reflecting layer123reflects the light from the LEDs102that is transmitted through the polarization selective layer122. The reflecting film123is reflective at the first wavelength of light generated by the LEDs102and is transmissive at the second wavelength of light generated by the phosphor: this configuration of reflector may be referred to as a long pass reflective filter.

As will become apparent below, in this particular embodiment, the PBS120is used for polarizing the light at the second wavelength generated by the phosphor, and the effects of the PBS120on the light118at the first wavelength may be essentially ignored. For example, in some embodiments, the polarization selective layer122may be designed to be essentially transparent for both polarizations of the light118at the first wavelength. In such a case, the reflecting film123reflects both polarization states of the light118at the first wavelength. In other embodiments, the polarization selective layer122may reflect the light at the first wavelength in one polarization state, in which case the reflecting film123reflects the light118at the first wavelength in the second polarization state that is transmitted through the polarization selective layer122.

The light126at the first wavelength reflected by the PBS120is directed to a color converting phosphor128. The phosphor128contains a material that absorbs the light126generated by the LEDs102and generates light at a second wavelength, typically longer than the first wavelength. In some exemplary embodiments, the phosphor128may convert blue or UV light to green light. One particularly suitable example of a phosphor material is Eu-doped strontium thiogallate (SrGa2S4:Eu), although other types of phosphor materials may also be used, for example rare earth doped nitrides and oxy-nitrides, such as europium doped silicon aluminum oxy-nitride (SiAlON:Eu) and rare-earth doped garnets, such as cerium doped yttrium aluminum garnet (Ce:YAG).

The light126may pass through a second light collecting/focusing arrangement130on the way to the phosphor128. The second light collecting/focusing arrangement130may be configured like the first light collecting/focusing arrangement107, having a focusing optic132and a light pipe134, or may be configured differently. The focusing optic132and light pipe134concentrate the light126on the phosphor128.

The phosphor128may be mounted on a baseplate136that, in some exemplary embodiments, operates as a heatsink for removing excess heat. The light138at the second wavelength (dashed lines) is directed back through the second light collecting/focusing arrangement130to the PBS120, which transmits the p-polarized light140as useful output142and reflects the s-polarized light144. Some element behind the phosphor128may be used to reflect light at the second wavelength that originally is generated traveling in a direction away from the PBS120. For example, the baseplate136itself may be reflective, or an optional reflector152may be disposed between the phosphor128and the baseplate136. One example of a suitable reflector152includes a metal coating on the baseplate136, for example a silver coating. Another example of a reflector152includes Enhanced Specular Reflector (ESR) film available from 3M Company, St. Paul, Minn.

A reflective filter146, transmissive at the first wavelength and reflective at the second wavelength, may be disposed between the PBS120and the first light collecting/focusing arrangement107to reflect the s-polarized light144back to the phosphor128via the PBS120. The reflected light150may subsequently be re-reflected, for example by the phosphor128, the baseplate136or the reflector152, back towards the PBS120.

A polarization converter148may be disposed between the PBS120and the phosphor128so that at least some of the light150reflected back to the phosphor128is subsequently returned to the PBS120in the polarization state that is transmitted as useful output142.

One characteristic of the system100illustrated inFIG. 1is that the light126at the first wavelength is incident at the phosphor with an étendue substantially similar to that of the light106emitted by the LEDs102. Consequently, the étendue of the output light142at the second wavelength is similar to what would have been achieved by generating the light at the second wavelength directly using the appropriate LEDs. This permits the output light142to be efficiently used in an illumination application, for example illuminating an LCD imager device.

Another exemplary embodiment of an illumination system300that includes a phosphor for converting light wavelength and that produces a polarized output is schematically illustrated inFIG. 3. In this embodiment, the light from the LEDs102is passed through a light collection/focusing unit307that includes a light pipe308having an input310and an output312. The light pipe308in this particular embodiment has curved sidewalls so that the light314at the output312is substantially telecentric. The light314passes to a dichroic beamsplitter320that has the property of reflecting light at the first wavelength and transmitting light at the second wavelength. The light324at the first wavelength that is reflected by the dichroic beamsplitter is directed through a second light collecting/focusing unit326to the phosphor128. The second light collecting/focusing unit326may also comprise a light pipe328having curved sidewalls, or may comprise an optical arrangement different from that of the first light collecting/focusing unit307.

The light329at the second wavelength passes through the second light collecting/focusing arrangement326and is transmitted through the dichroic beamsplitter320. A polarizer330, for example a wire grid polarizer, a MOF polarizer or a cholesteric polarizer, transmits light332in one polarization state as useful output and reflects light334in the orthogonal polarization state back to the phosphor128. A polarization control element336, for example a quarter-wave retarder, may be positioned between the polarizer330and the dichroic beamsplitter320. The reflected light334is incident once again at the phosphor128and is reflected back towards the polarizer330by the phosphor128, the baseplate136or the reflector152. The polarization control element336is used to rotate the polarization of at least some of the light that is recycled back to the polarizer330.

Additionally, at least some of the light at the first wavelength that is not converted by the phosphor128to the second wavelength may be returned to the LEDs102via reflection at one of the phosphor128, reflector152or baseplate136, and reflection at the dichroic beamsplitter320. Such reflected light at the first wavelength may be recycled to the phosphor128by reflection from the baseplate104or the LEDs102.

In another exemplary embodiment, not illustrated, the dichroic beamsplitter may transmit the light at the first wavelength and reflect the light at the second wavelength. In such a configuration, the LEDs and phosphor are typically positioned on opposing sides of the dichroic beamsplitter.

In some exemplary embodiments, the phosphor may be disposed close to the LEDs, or the LEDs may even be conformally coated with the phosphor material. Such a configuration may lead to a reduction in the number of elements used in the illumination system. Also, in some cases, the LEDs are formed of a material, such as silicon carbide, which is effective at transferring heat from the phosphor to the baseplate.

One exemplary embodiment of an illumination system400in which the phosphor is disposed close to the LEDs is schematically illustrated inFIG. 4. The system includes an array of one or more LEDs102and a light collecting/focusing unit107. The light collecting/focusing unit may be configured differently from the illustrated embodiment, for example using a light pipe with an integrated focusing element or without a focusing element. In addition, the sidewalls may be straight or curved. In the exemplary embodiment the phosphor428is positioned close to, or even on, the LEDs102. A reflective filter430may be placed at the output of the light collecting/focusing unit107to reflect light106at the first wavelength and to transmit light432at the second wavelength, generated by the phosphor428.

The light432at the second wavelength is incident on a PBS420, which transmits light434in one polarization state as useful output and reflects light436in the orthogonal polarization state. A reflector438reflects the light440back to the PBS420, where it is reflected back towards the phosphor428. The light440may subsequently be reflected back towards the PBS420by the phosphor428, the LEDs102, the baseplate104or some other reflecting element. A polarization rotation element442, such as a quarter-wave retarder, may be positioned between the PBS420and the phosphor428to rotate the polarization of the reflected light440, so as to increase the amount of light extracted by the PBS420as useful output434.

In an alternative configuration, the light that is reflected by the PBS may be used as the useful output while the reflector is positioned to reflect the light that is transmitted by the PBS.

Another exemplary embodiment of an illumination system500is schematically illustrated inFIG. 5A. This exemplary system is similar to the system400illustrated inFIG. 4, except that the PBS420and reflector438are replaced with a reflecting polarizer layer520, for example a MOF polarizer, a wire grid polarizer or a cholesteric polarizer. The reflecting polarizer layer520transmits light in one polarization state as useful output534and may reflect light536in the orthogonal polarization state for recycling.

Another exemplary embodiment of an illumination system550is schematically illustrated inFIG. 5B. This system550is similar to that illustrated inFIG. 5A, except that the focusing optic116is omitted and the reflective filter552and reflecting polarizer554are both curved. In some embodiments, it may be desired that the centers of curvature of both the reflective filter552and the reflecting polarizer554are approximately at the phosphor428, which reflects the light at both the first and second wavelengths. This configuration increases light106at the first wavelength reflected back to the LEDs102and the amount of light536at the second wavelength reflected back towards the phosphor428. The centers of curvature may, of course, be located elsewhere. The polarization rotation element442may be curved to match the curve of the reflecting polarizer554, or may be straight. Curved reflecting elements may also be used in the other embodiments described above. For example, in the system400schematically illustrated inFIG. 4, the reflective filter430and the reflector438may each be curved.

One characteristic of the illumination system that increases the amount of the light reflected back for recycling, be it light at the first or second wavelengths, is that reflection of the light for recycling does not substantially increase the angular range of the incident light upon reflection. This is explained further with reference toFIG. 5C, which shows the direction of light rays at various points across a non-telecentric light beam propagating along an axis560. At the center of the beam, the center ray562is parallel to the axis560, and rays564,566propagate at angles al relative to the center ray562. The rays564,566represent the rays whose light intensity is a specified fraction of the intensity of the ray of maximum intensity, in this case the on-axis ray562. For example, where the light beam has an f/number of 2.4, the light beam is generally accepted as having a cone half angle, α1, of ±11.7°, where practically all the light, at least more than 90%, is contained within the ±11.7° cone.

The dashed line570, at the edge of the beam, is parallel to the axis560. Ray572, representing the direction of the brightest ray at the edge of the beam, propagates at an angle θ relative to the axis560. Rays574and576propagate at angles of α2relative to ray572. Ideally, the value of α2is close to the value of α1, although they need not be exactly the same.

Reflection of beam562by a flat mirror568, aligned perpendicular to the axis560, results a reflected beam that propagates parallel to the axis560. Reflection of the beam572by the flat mirror568, on the other hand, results in a reflected beam that propagates at an angle of 2θ relative to the axis560. Thus, reflection of the non-telecentric light by a flat mirror results in an increase in the angular range of the light.

On the other hand, if the light were telecentric, then beams562and572would be parallel, and reflection by the flat mirror568would not increase the angular range of the incident light.

Also, reflection of the non-telecentric light by a curved mirror580, as schematically illustrated inFIG. 5Dmay result in no increase in the angular range of the light where the beams562and572are each normally incident at the mirror580.

One exemplary embodiment of a projection system600that may use an illumination source of the type described above is schematically illustrated inFIG. 6. The system600comprises a number of differently colored light sources602a,602b,602cthat illuminate respective image-forming devices604a,604b,604c, also referred to as image-forming panels. Each light source602a,602b,602cmay include a number of light emitting elements, such as light emitting diodes (LEDs), and produces an output light beam having a particular color. One or more of the light sources602a,602b,602cmay include a phosphor for converting the wavelength of the light emitted by the LEDs, a light collecting/focusing arrangement for maintaining the étendue of the light beam and a polarizer for selecting a desired polarization state. In some embodiments, the illumination light sources602a,602b,602cgenerate respective red, green and blue illumination light beams.

The image-forming devices604a,604b,604cmay be any suitable type of image-forming device. For example, the image-forming devices604a,604b,604cmay be transmissive or reflective image-forming devices. Liquid crystal display (LCD) panels, both transmissive and reflective, may be used as image-forming devices. One example of a suitable type of transmissive LCD image-forming panel is a high temperature polysilicon (HTPS) LCD device. An example of a suitable type of reflective LCD panel is the liquid crystal on silicon (LCoS) panel. The LCD panels modulate an illumination light beam by polarization modulating light associated with selected pixels, and then separating the modulated light from the unmodulated light using a polarizer. Another type of image-forming device, referred to as a digital multimirror device (DMD), and supplied by Texas Instruments, Plano, Tex., under the brand name DLP™, uses an array of individually addressable mirrors, which either deflect the illumination light towards the projection lens or away from the projection lens. While the illumination light sources may be used with both LCD and DLP™ type image-forming devices, there is no intention to restrict the scope of the present disclosure to only these two types of image-forming devices and illumination systems of the type described herein may use other types of devices for forming an image that is projected by a projection system. Also, it is recognized that many systems that include a DLP™ type image-forming device do not need polarized illumination light. The illustrated embodiment includes LCD-type image-forming devices for purposes of illustration only, and is not intended to limit the type of image projection system in which the illumination source is used.

The illumination light sources602a,602b,602cmay include beam steering elements, for example mirrors or prisms, to steer any of the colored illumination light beams606a,606b,606cto their respective image-forming devices604a,604b,604c. The illumination light sources602a,602b,602cmay also include various elements such as polarizers, integrators, lenses, mirrors and the like for dressing the illumination light beams606a,606b,606c.

The colored illumination light beams606a,606b,606care directed to their respective image forming devices604a,604band604cvia respective polarizing beamsplitters (PBSs)610a,610band610c. The image-forming devices604a,604band604cpolarization modulate the incident illumination light beams606a,606band606cso that the respective, reflected, colored image light beams608a,608band608care separated by the PBSs610a,610band610cand pass to the color combiner unit614. The colored image light beams608a,608band608cmay be combined into a single, full color image beam616that is projected by a projection lens unit611to the screen612.

In the illustrated exemplary embodiment, the colored illumination light beams606a,606b,606care reflected by the PBSs610a,610band610cto the image-forming devices604a,604band604cand the resulting image light beams608a,608band608care transmitted through the PBSs610a,610band610c. In another approach, not illustrated, the illumination light may be transmitted through the PBSs to the image-forming devices, while the image light is reflected by the PBSs.

One or more power supplies620may be coupled to supply power to the illumination light sources602a,602b,602c. In addition, a controller622may be coupled to the image forming devices604a,604b,606c, for controlling the image projected image. The controller622may be, for example, part of a stand-alone projector, or part of a television or a computer.

It may be desired in some embodiments to use a densely packed array of LEDs, for example to achieve efficient and economic generation and collection of light. Such an array may be arranged to have an aspect ratio that is similar to that of the imaging device being illuminated and to have an étendue at least as large as that of the imaging device being illuminated.

One of the major challenges with packing LEDs densely in an array is the management of the heat flux. To help manage this heat load, the LEDs702may be attached directly to a liquid cooled plate704, as is schematically illustrated inFIG. 7. The cooled plate704may be, for example, a liquid cooled, microchannel cold plate, having an input706and an output708for the liquid coolant. The number of LEDs702mounted to the cold plate704may be different from that shown in the figure. One suitable type of cold plate704plate is a Normal flow microchannel Cold Plate (NCP) available from Mikros Technologies, Claremont, N.H.

An important feature of such an arrangement is to reduce the thermal resistance from p-n junction temperature of the LEDs702to the liquid medium as far as possible by attaching the LEDs702directly to the cold plate704. The LEDs702may be attached directly to the liquid cooled plate704using any suitable method, for example, a flux eutectic die attach method or a conductive epoxy.

The flux eutectic method used to attach the LED dies702to the cold plate704offers low electrical resistance, low thermal resistance and good mechanical and electrical integrity. It is accomplished by placing a carefully controlled amount of tacky flux on the cold plate. Next, an LED die702is precisely positioned on the cold plate through the tacky flux. The LED die702is supplied with a metal coating on its lower surface. The metal coating may be, for example, a mixture of 80:20 Au/Sn. The assembly700is heated above the melting point of the metal coating, so that the metal reflows, thereby attaching the LED die702to the cold plate704. In some embodiments, the heating is only performed for a short period, for example reaching a temperature of about 305° C. for 5-8 seconds.

Traditionally, a reflow process is performed by directly heating the attachment substrate (e.g.: using a hot plate) or by using a stream of hot gas aimed at the top of the die. These conventional methods are not well suited to attaching the LEDS702to the cold plate704, however. It is difficult to heat the substrate up to reflow temperatures and cool down again quickly enough to avoid excess dwell time at or near the process temperature. Leaving the die at the reflow temperature for too long can cause the Au/Sn to flow further than desired, and the metal may wick up the side of the LED die, resulting in an undesirable shunt or Schottky contact. Additionally, excess dwell time may cause the die to fully or partially separate from the substrate, resulting in poor electrical and thermal contact.

The conventional hot gas method can be used to heat the die and proximate substrate for a carefully controlled time, minimizing the likelihood of shunt formation or die separation. This method is used one die at a time, however, requiring 5-8 seconds of direct heating per die. This process can be quite time consuming for arrays containing many dies.

Another approach to providing the heat for the reflow process is to control the flow of hot inert gas or hot liquid through the cold plate. This method permits the entire plate704to be heated simultaneously, which allows batch processing for vastly improved manufacturing throughput. Also, since the full thermal mass of the cold plate704is not being heated externally, it is easier to control the dwell time at the reflow temperature, thus minimizing quality defects associated with excess time at the reflow temperature.

This process enables the placement of LEDs directly onto a cold plate, which is quite desirable. In conventional approaches, LEDs are mounted on an intermediate substrate that is then mounted to a heat sink. This introduces additional thermal resistance due to the extra layer of material of the intermediate substrate, and an extra thermal interface. At high flux densities, this extra resistance can substantially increase the p-n junction temperature (Tj) of the LEDs702in the array.

The omission of the intermediate substrate and thermal interface reduces the thermal resistance between the diode junction and the coolant, and so the junction operates at a cooler temperature. This decrease in operating temperature offers at least two advantages. First, the lifetime of the LEDs is increased while operating at high power, since the lifetime is related to Tj. Keeping the LED die cooler, therefore, increases the reliability. Secondly, higher values of Tjadversely affect the amount of light output by the LED. By keeping the Tjlower, the brightness from the LED array is higher for a given input power.

One example of an array of LEDs attached directly to a cold plate includes 84 LEDs arranged in a 12×7 array. Each LED is a type 460 XT 290 blue LED, supplied by Cree Inc., Durham, N.C. Each LED is 300 μm square and they are mounted with a center-to-center spacing of 325 μm. Thus, the array has a dimension of approximately 3.9×2.25 mm: The LED die are relatively thin, around 110 μm in height, although taller LEDs may also be used. For example, Cree Type XB900 LED die, 900 μm square×250 μm high, may also be used. A wirebond wire of 25-50 μm diameter is attached to the top of each LED to provide electrical connection. The wire may be formed of any suitable material, such as gold. The cold plate serves as the common ground for all the LEDs.

The phosphor may be conformally coated over the LEDs. The phosphor material may be coated on using any suitable method. Some suitable “wet” methods include spraying the phosphor material on the LEDs and dipping the LEDs in a slurry. Other methods of applying the phosphor, such as vacuum coating methods, may be used.

Since the wavelength conversion of the phosphor is often less efficient at higher temperatures, it is important to keep the phosphor temperature low, as well as Tj. The configuration where the phosphor is conformally coated over the LEDs may enhance the phosphor cooling: the particular Cree LEDs discussed above are made of silicon carbide, which has a relatively high thermal conductivity, thereby reducing the thermal resistance of the heat path between the phosphor and the cold plate. Thus, another advantage of reducing the thermal resistance between the LEDs and the liquid coolant is that the phosphor may be thermally coupled to the cooling system via the LEDs.

This array is then placed as closely as possible to a tapered light pipe. Where the wire bonding is of the ‘wedge’ type, rather than the ‘ball’ type, the height of the wire bonding is reduced, and so the input face of the tapered light pipe may be placed as close as approximately 100 μm from top surface of the LED dies. The input end of the tapered light pipe may have input dimensions of approximately 2.25 mm×3.9 mm. The length of the tapered light pipe may be in the range 50 mm-60 mm long, although other lengths may also be used. In an example where the output face has sides that are 1.8 times larger than the input face, the output face has a size of approximately 7.05 mm×4.1 mm.