Efficiency measurement of white LED devices

This paper presents two methods for the efficiency measurement of a white light emitting diode (LED) without optical sensors for measuring light intensity. The proposed methods are based on the measurement of the power loss of a white LED. Because all the power loss of a white LED is transferred into heat, it is possible to evaluate the power loss by measuring the temperature of the heat sink attached to the LED. The first method is referred to direct method, in which the precise thermal characteristics of the heat sink were required for the evaluation of the power loss of an LED. The second method is referred to differential method, in which two same heat sinks were used. One was installed with the LED to be measured and the second one was installed with a power MOSFET as a heater. A temperature control system was developed for the second heat sink with the heater so that the temperature was controlled equal to the heat sink with LED. In steady state, the power loss of an LED was evaluated based on the power consumption of the heater. The two methods were tested to measure the efficiency of a 9 W Cree© XLamp white LED. The experiments show that the differential method is more effective to provide accurate measuring results.