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By Robert Crase and David Gray. Deposition Sciences, Inc. (DSI)

SSL is one of the fastest-growing areas in the lighting market. As the efficiency of these systems improves, light-emitting diodes (LEDs) will gain a larger share of the lighting market. Gains in efficiency have been made at an ever-increasing rate during the past several years. As impressive as these gains have been, they were made largely through improvements at the chip level and were developed with a semiconductor approach to the problem. This article will discuss several proposed optical coating-based solutions to develop and extend improvements in SSL.

LEDs can emit light from every direction, even from the bottom of the chip. Presently, a common practice is to deposit a metal coating on the bottom of the chip to reflect and redirect some of this light to the front of the LED. This metal coating, typically aluminum, is not a very effective reflector of visible light, but it is an extremely efficient reflector of heat. If this aluminum coating could be replaced by dielectric coatings, which would have higher reflection in the visible spectrum and would also transmit in the infrared spectrum, it would allow the LED to run much cooler.

Heat dissipation can be an issue with SSL systems. The current state of the art is to use conduction and a large radiating surface to remove the heat from the LED. The conduction at the first interface can be improved with the deposition of an index-matching stack at the chip interface. Using dielectriccoated mirrors (as proposed earlier), the last layer can be made of a material with superior thermal conductive properties. This LED package with the cold mirror is bonded to the heat sink, improving the conduction of heat away from the LED.