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To reduce testing time and enhance the accuracy of the lifetime estimation of light emitting diode (LED) packages, a new complex-stress accelerated lifetime test (ALT) is proposed, which is based on the combination of thermal stress and optical stress. An in situ monitoring of electrical, optical and thermal characteristics took place during the ALT. The transient response of voltage change in the LED chips was used to measure the thermal properties such as junction temperature and thermal resistance to consider the thermal stress. The relative optical power and luminous flux, monitored during the ALT, were used as the optical stress and a measure of lifetime prediction. Compared to a conventional temperature-stress ALT model, the proposed ALT model describes an actual optical degradation and thus enables accurate estimation of the lifetime of LED packages.