What is the life of LEDs?
The lighting industry presently does not have a standard definition for LED lamp life (Narendren et al. 2001a). The lamp life definition for traditional light sources is the time at which 50% of the test samples have burned out (Rea 2000). LEDs generally do not fail by burning out but will slowly reduce in light output over time; as solid state devices they will continue to operate even after 100,000 hours, continuing to use electrical power even if they produce very little useful light. A comparison of lumen maintenance near the end of rated life for traditional light sources (e.g., incandescent, fluorescent, high intensity discharge) (Figure 11) shows that with the exception of metal halide lamps, these lamps usually have at least 80% of their initial light output by the time they have operated 10,000 hours. Furthermore, even when light level reductions occur over a few minutes, people tend not to notice them until the light level reaches 80% of the initial value (Kryszczuk and Boyce 2002). For these reasons, it may be appropriate to consider this criterion as a basis for “useful” life for LED sources used in general lighting.
|Figure 11. Lumen maintenance curves for different light sources out to 10,000 hours.|
Indicator-type white LEDs that were on the market as recently as 2000 and 2001 reach 80% light output within 1000 to 2000 hours when operated at rated current in laboratory conditions (Figure 11) (Narendran et al. 2000a, 2001b). The high degradation rate in these LEDs is mainly due to yellowing of the clear epoxy material caused by high temperature in the semiconducting element. Newer, high-power, white LEDs have improved lumen maintenance, and therefore could have much longer lamp life (greater than 25,000 hours). Recent measurements by the Lighting Research Center have shown that these devices maintain their light output for significantly longer periods than indicator-type LEDs using earlier technology (Narendran and Deng 2002a). This is largely due to improved thermal management and heat sinking characteristics.