Lighting Research Center Lighting Research Center
    Volume 7 Issue 3
May 2003    
beam angle - The angle at which luminous intensity is 50 percent of the maximum intensity. bin - To sort or classify light sources (such as light emitting diodes) into groups according to their luminous intensity or color appearance. conduction - The process of removing heat from an object via physical contact with other objects or materials, usually metals. convection - The process of removing heat from an object through the surrounding air. cosine distribution - A property of a light source such that its luminous intensity in a particular direction is proportional to the cosine of the angle from the normal to the source. driver - For light emitting diodes, a device that regulates the voltage and current powering the source. heat sinking - Adding a material, usually metal, adjacent to an object in order to cool it through conduction. illumination - The process of using light to see objects at a particular location. indication - The process of using a light source as something to be seen as in signaling. junction temperature - For light emitting diodes, the temperature of the light-emitting portion of the device (see PN junction), which is inversely correlated with its light output. lumen maintenance - The lumens produced by a light source at any given time during its operating life as a percentage of its lumens at the beginning of life. monochromatic - For light, consisting of a single wavelength and having a very saturated color. PN junction - For light emitting diodes, the portion of the device where positive and negative charges combine to produce light. pulse-width modulation - Operating a light source by very rapidly (faster than can be detected visually) switching it on and off to achieve intermediate values of average light output; the frequency and the duty cycle (percentage of time the source is switched on) are important parameters in the modulation. semiconductor - A material whose electrical conductivity is between that of a conductor and an insulator; the conductivity of most semiconductors is temperature dependent. spectral power distribution (SPD) - A representation of the radiant power emitted by a light source as a function of wavelength. substrate - For light emitting diodes, the material on which the devices are constructed. tri-phosphor - A mixture of three phosphors to convert ultraviolet radiation to visible light in fluorescent lamps; each of the phosphors emits light that is blue, green or red in appearance with the combination producing white light.
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.


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