Measurement of temperature profiles on visible light-emitting diodes by use of a nematic liquid crystal and an infrared laser.

We present a new technique for measuring the temperature profiles of visible LED chips by use of a nematic liquid crystal with IR laser illumination. The LEDs studied have a multi-quantum-well InGaN/GaN/sapphire structure. New features in this technique are the use of a high-power IR laser beam as the sensing light and the insertion of a color filter in the optical path to block the high-intensity LED light. For the LEDs measured, the conversion efficiency decreases by 70% when the junction temperature rises from 25 to 107 degrees C. This technique is a valuable tool for studying the performance of LEDs as a function of junction temperature.