On the thermal characterization of an RGB LED-based white light module

The study aims at evaluating the thermal characteristics of a red-green-blue (RGB) light emitting diode (LED)-based white light module in natural convection through infrared (IR) thermography and forward voltage measurements. The light output and packaging efficiencies of these three color types of LEDs are first characterized using a spherical integrating photometer employing an integrating sphere and an optical analysis program, respectively. These two efficiency data give an estimate of the power fraction not converted into emitting light, by which the temperature distribution of the module is calculated using thermal finite element (FE) modeling. To facilitate the IR thermography measurement, the coefficient of emissivity of the transparent molding compound as the LED optical lens is explored. The results of the measurements are compared with those of the thermal FE modeling and thermocouple measurement. Moreover, the limitation of the IR thermography measurement in LED thermal characterization is addressed, and enhancement of the measurement accuracy is achieved through a proposed temperature correction procedure. Besides, the uncertainty in the forward voltage measurements is also assessed. Results show that the maximum junction temperature of the RGB LED-based white light module can reach about 100 degrees C even at a low ambient temperature of 25 degrees C, and the low thermal conductivity of the transparent molding compound is one of the major causes of the poor thermal performance. In addition, IR thermography tends to overrate the surface temperature of the LEDs, and the transparency of the molding compound could be the direct consequence of the overestimate. It also turns out that the IR thermography can be an effective tool for LED surface temperature measurement after modification using the proposed temperature correction procedure. At last, the uncertainty analysis reveals that the uncertainty in the measured junction temperature using the forward voltage method is about 4-8%, depending on the color of the LEDs, and can it be greatly improved by increasing the accuracy of the voltage measurement. (C) 2012 Elsevier Ltd. All rights reserved.