Accurate measurement of LED lens surface temperature

Radiant power emitted by high power light-emitting diodes (LEDs) have been steadily increasing over the past decade. High radiation, especially short wavelength, can increase the temperature and negatively affect the primary lens performance of high-power LEDs. In this regards, assessment of lens temperature during operation is important. Past studies have shown large errors when thermocouples are used for measuring temperature in high radiant flux environments. Therefore, the objective of this study was to understand the problem in using thermocouples to measure LED lens surface temperature and to find a solution to improving the measurement accuracy. A laboratory study was conducted to better understand the issue. Results showed that most of the error is due to absorption of visible radiant energy by the thermocouple. In this study, the measurements made using an infrared (IR) thermal imaging system were used as the reference temperature because the IR imaging system is unaffected by radiant flux in the visible range. After studying the thermocouple wire metallurgy and its radiation absorption properties, a suitable material was identified to shield the thermocouple from visible radiation. Additionally, a silicone elastomer was used to maintain the thermal interface between the lens surface and the thermocouple junction bead. With these precautions, the lens temperature measurements made using the J-type thermocouple and the IR imaging system matched very well.