Subsequent measurements usually show a small increase in light output for the first 1,000 hours or so, followed by a very slow decline. The rate of change is generally related to temperature. At higher temperatures the light output will fall more quickly than at lower temperatures. This is why good thermal management is so important for luminaire performance.
An LM-80 report will usually show data collected over 6,000 to 10,000 hours. However, when many lighting applications require the life of an LED to be many times this it is necessary to project the data into the future to determine how it might perform after several years of use. There are many ways that data can be projected, some of which would be downright misleading. To avoid this and enable the data of different LEDs and manufacturers to be compared the IES has written another standard, TM-21. This defines precisely what arithmetic formulae are to be used to project data gathered over, say 9,000 hours, out to 30,000 or more hours.
Manufacturers will often talk of L80 or L70. These are the points in the life of an LED when the light output has fallen to 80% or 70% respectively of the initial light output.
At NVC Lighting when we state the rated life of an LED we mean the running hours taken to reach L70.
After L70 has been reached, light output will continue to fall gradually. In general, LEDs do not fail to emit any light as they age; rather their light output and efficiency (lumens/watt) very gradually both fall.