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How To Measure The Lifespan Of White LEDs
Lifespan in LEDs, especially in White LEDs, is a very misunderstood concept. This is mainly because of the two ideas behind a LEDs true lifespan: Lumen Depreciation and Useful Life. Lumen Depreciation is the decrease in lumen output that occurs as a lamp operates. Whether for incandescents, halogen, fluorescents or LEDs this measures the decrease in the amount of light they emit over time. 
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The second critical idea is Useful Life. Unlike other light sources, LEDs don't "burn out;" they simply get dimmer over time. Although there is not yet an official industry standard defining "life" of an LED, for a common application such as general lighting in an office or home environment, research has shown that the majority of occupants in a space will accept light level reductions of up to 30 per cent with little notice, particularly if the reduction is gradual.
Therefore, a level of 70 per cent of initial light level could be considered a threshold of useful life for general lighting. Based on this research, the Alliance for Solid State Illumination Systems and Technologies (ASSIST), a group led by the Lighting Research Center (LRC) in the USA recommends defining useful life as the point at which light output has declined to 70 per cent of initial lumens (abbreviated as L70) for general lighting and 50 per cent (L50) for LEDs used for decorative purposes.
Using this definition, at normal operating temperatures, the best white LEDs have been found to have a useful life of around 35,000-50,000 hours (that's between four and six years of continuous operation). For comparison, a 75-watt incandescent light bulb lasts about 1,000 hours; a comparable CFL lasts 8,000 to 10,000 hours. However, LED lifetime depends greatly on operating temperature. An increase in operating temperature of 10 °C can cut the useful life of an LED in half.
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