LED light sources: a survey of quality-affecting factors and methods for their assessment

Nowadays, after nearly half a century of rapid and sustained development of light-emitting diodes (LEDs), conventional light sources such as incandescent and fluorescent lamps are beginning to be replaced by LEDs. Therefore, understanding and assessing all the relevant factors that affect the quality of LEDs is becoming increasingly important for design, production and maintenance of various LED products. The most adverse quality-affecting factors are the initial variability of the optical and electrical properties in a batch of LEDs, temperature and electrical dependence and temporal degradation with corresponding variability. In this paper, we survey the most important quality-affecting factors and corresponding methods for their assessment. First, initial variability of the optical and electrical properties in a new batch of LEDs and the corresponding assessment methods are outlined. Next, the temperature stability of optical and electrical properties of LEDs is discussed. Moreover, the most frequently studied methods for spectral degradation and corresponding degradation variability prediction are reviewed according to the accuracy, applicability and specificity. Finally, the advantages and disadvantages of the established models of part stress analysis are pointed out. In this way, all the major factors affecting the quality of LED products are summarized and the corresponding methods for their assessment are outlined.