What is useful life for white light LEDs?

The goal of this paper is to initiate a discussion within the lighting community regarding standardized measurement procedures and a definition for useful life for light emitting diode (LED) technology. In general, LEDs do not fail catastrophically, but instead their light Output slowly decreases over their operating period. Presently, some manufacturers use a 50 percent light output level as the criterion for LED life. Although 50 percent light loss might be acceptable for non-critical signage applications using monochromatic LEDs, it might not be acceptable for general lighting applications. It is important to develop a method for rating lamp life and a definition of "useful life" for LEDs so, when reported by manufacturers, the lighting community can compare LEDs to traditional light sources. The "useful life" definition for LEDs should consider light loss and color shift. Therefore, an experimental study was conducted to investigate light loss and color shift patterns of white LEDs as a function of operating time. The 5 mm type white InGaN +YAG LEDs evaluated in this experiment, representing technology commercially available in 1999, exhibited high light output degradation rates and color shifts as a function of operating time. It is further shown, using a simple mathematical fit to the data gathered during a short life-test study, and extrapolating it to predict the life of white LEDs, depends on the initial data collection period. Therefore, an alternate method for projecting LED life is investigated by overdriving the LEDs at different currents. Using their degradation patterns at higher drive currents, the life of these LEDs was predicted at normal drive current values. The results show excellent correlations between predicted light loss and actual measured losses at 20 and 30 mA drive currents for the LEDs tested. The authors believe this technique is applicable for accurately predicting life of any type of LED and hope to verify this using future configurations. This study adds information to the knowledge needed for the lighting community to develop standardized measurement procedures and a definition for useful life for LED technology.