Experimental study of measuring LED's temperatures via thermocouple

For light-emitting diode (LED) packages, temperatures, especially junction and phosphor temperature, can greatly affect the optical performance. Thus, measuring the temperatures accurately is of significance for the performance estimation. Among the methods of measuring temperature, physical contact method, represented by thermocouple method, is most convenient and economical. In this paper, we measured the junction and phosphor temperature of LED packages by thermocouple. In the experiments to measure junction temperature of a blue LED module, we simplified the judgment by measuring the air temperature above the bare chip and it was found that the test result was much higher than that obtained by another recognized method. The causes were ascribed to two factors, i.e. thermal resistance of the air gap and the light energy absorption. In the experiments to measure phosphor temperature, plug-in measurement was adopted to eliminate the influence of air gap's thermal resistance. However, the tested value turned out to be even higher. Then the main cause of the inaccurate measurement by thermocouple was attributed to the light energy absorption and conversion. And an experiment was carried out to support the explanation. As a whole, to accurately measure the LED's temperatures by thermocouple, the light absorption effect needs to be considered.