Given the need, and recognizing the limitations, the DOE and NGLIA recommend the “standard” or default lifetime of an LED luminaire (or lamp) be defined only in terms of lumen output and be specified as the time when half the product population has fallen below 70 percent of average initial light output for any reason.
To be clear, while emphasizing lumen output, the definition above goes beyond lumen depreciation to include any mechanisms that lower the light output. It encompasses gradual lumen depreciation of the LED sources, depreciation due to interaction with other components or materials in the luminaire, and catastrophic failure of any component or subsystem, ranging from total failure with no light output to the failure of a subset of the LEDs leading to luminous flux below a specified threshold.
Whatever the stated lifetime of an LED lighting product, it is a statistical measure of the performance of a given design, in this case the mean time to failure because of low (or no) light output. In the specification above, the standard reference level of 70 percent of LED initial output is referred to as “L70”; the time at which half the LEDs have fallen below this level is “B50” and is the same as conventional rated lamp life.
The minimum threshold (70 percent) chosen as the “default” failure level by which we define a “standard” lifetime may not exactly suit every application. For example, in certain safety situations depreciation of this magnitude may be unacceptable, or in some cost-sensitive, non-critical situations a higher level of depreciation may be acceptable. Nonetheless, the definition above is recommended as a standard life designation by which different products may be compared to one another.
Additional specifications, as discussed in the next section, may be added to cover particular applications. These additional specs may cover other aspects of light quality that affect product usefulness over time, or they may provide what amounts to a different estimate of useful life such as B50/L50, for example. Generally this additional information would be most useful for professionally designed lighting solutions.