One of the main advantages of LEDs is their potentially very long life; however, determining the lifetime of LED light bulbs is trickier than determining the life of other light sources.
We’ve all heard the small “pop” as an incandescent lamp fails. It’s the sound of the tungsten filament finally breaking as the electric current hits it. This makes it easy to recognize the end of life for an incandescent light source. With fluorescent light bulbs, end of life may involve flickering, or the bulb may simply not activate when the switch is turned on. With LEDs, outright failure of the device is less likely, although it can happen due to component failure. Instead, the LED’s light output slowly declines over time.
So, how are LED lifetimes rated? A life testing procedure for LEDs is currently under development by the Illuminating Engineering Society of North America (IESNA). The proposed method is based on the idea of “useful life,” i.e., the operating time in hours at which the device’s light output has declined to a level deemed to no longer meet the needs of the application. For example, for general ambient lighting, the level might be set at 70% of initial lumens. Useful life would be stated as the average number of hours that the LED would operate before depreciating to 70% of initial lumens. LEDs in decorative applications, on the other hand, may be able to have a longer useful life, operating until they reach 50% of their initial lumens because decorative applications do not need as much light. For many LED light bulbs, this results in a useful life of 25,000-50,000 hours.
Electrical and thermal design of the LED system or fixture determine how long LEDs will last and how much light they will provide. Driving the LED at higher than rated current will increase relative light output but decrease useful life. Operating the LED at higher than design temperature will also decrease useful life significantly.
How do the lifetime projections for LEDs compare to traditional light sources?