Thermal characterization issues of LEDs during reliability testing

Measurement of the radiated power is a key step in the thermal characterization of power LEDs. However, as the LEDs age, the light transmittance of the lens may decrease, which blurs the results of the thermal measurements. The combined thermal and radiometric characterization techniques of power LEDs does not take into account any aging effect of the LEDs, while the most widely accepted lifetime testing procedures seem to ignore the thermal aspects. Consequently, the results of measurements performed blindly in accordance with the standards will be ambiguous and the operator performing the measurement will not even be aware of this fact.