What is L70?
Three things can kill the LED. Heat, dirt and moisture can adversely affect LED life. In fact, even if the LEDs can last forever, their lumen output will gradually decrease over time until they no longer function as a useful light source. We call this the "lumen maintenance rate." The industry has determined that LEDs are no longer a useful source of light when lumen output is reduced to 70% of its initial lumens. This is called L70.
The L70 is a life measurement standard developed by IESNA (Illuminating Engineering Society of North America) to evaluate the life of an LED luminaire until the expected output hours of light output is reduced to 70% of the initial level. Or when the lumen output is 70% of its initial output. For example, if an LED starts illuminating at 100 lumens and after 40,000 hours it only emits 70 lumens (70% of the original output) then the L70 estimated life of this LED is 40,000 hours.
Since LEDs do not fail and "burn out" like other light sources; instead, they gradually decrease until they no longer produce useful light. It is generally determined that the human eye is only sensitive to lumen degeneration of 30% or higher. Therefore, the L70 lifetime is defined by the Lighting Engineering Society Standard LM-80-08, entitled "Methods for Measuring Lumen Maintenance of LED Light Sources Approved by IES".
The life of the L70 depends on many variables, such as operating temperature, drive current, and the technology and materials used to build the product.