Determining the lifetime of light-emitting diode (LED) lighting products is not straightforward. LEDs can continue to produce light over periods of time much longer than most other light sources. Barring failure of other components (such as the driver, electrolytic capacitor, or other parts), LED light output typically declines slowly over time. Useful life is usually defined in terms of the number of operating hours until the light output has decreased to a given percentage of initial output, often 70% (denoted as L70), but it may differ depending on the needs of the lighting application.
The challenge is how to verify LED lumen maintenance without undertaking full life testing; 25,000 hours of continuous operation takes nearly three years. An industry standard test procedure, Illuminating Engineering Society (IES) LM-80-08, “Approved Method for Measuring Lumen Maintenance of LED Light Sources,” provides a method for LED packages to be tested for at least 6,000 hours, with light output and chromaticity measurements made at least every 1,000 hours. An accompanying document, Technical Memorandum (TM)-21-11, “Projecting Long Term Lumen Maintenance of LED Light Sources,” provides a method for estimating lumen maintenance for a period up to six times the LM-80-08 test period.
At the time the L Prize requirements were developed, LM-80-08 was only recently approved and TM-21-11 did not exist. The scope of LM-80-08 is lumen and chromaticity maintenance of LED packages operated under specific temperature conditions, and testing is not conducted on end-use products such as integral replacement lamps or complete luminaires. Therefore, a methodology for evaluating full-lamp lumen and chromaticity maintenance over time was developed by DOE specifically for the L Prize contest.