L70 is a lifetime measurement criteria developed by IESNA (Illuminating Engineering Society of North America) to evaluate the useful lifetime of an LED luminaire in terms of the expected number of operating hours until the light output has diminished to 70% of initial levels. It is generally established that the human eye is only sensitive to lumen depreciation of 30% or more. Therefore, L70 lifetime is defined by Illuminating Engineering Society Standard LM-80-08, entitled "IES Approved Method for Measuring Lumen Maintenance of LED Light Sources. L70 lifetime is dependent upon many variables, such as the operating temperature, drive current, and the technology and materials used to construct the products.
The IESNA has also developed the "LM80-08 Approved Method for Measuring Lumen Maintenance of LED Light Sources" (LM80) to present a basic understanding of how an LED lighting system will perform over time in regard to lumen maintenance. The LED lumen maintenance is measured under conditions in which the case temperature of the LED, forward voltage to the LED, and forward current to the LED are controlled. The LM80 standard also requires lumen maintenance data for at least 6,000 hours of constant DC mode operation. Lighting manufacturers use the LM-80 test results as part of their submissions to the ENERGY STAR program. An ENERGY STAR qualification provides a degree of confidence to consumers when purchasing LED lighting products.
TM-21-2011 (Projecting Long Term Lumen Maintenance of LED Light Sources), generally known as "TM-21", was established as a companion method to estimate LED lumen maintenance and service life in which light output is measured at a minimum of every 1,000 hours over a minimum period of 6,000 hours use.. TM-21 introduced the use of "L" values such as L70, L50, L30, etc.
The "LM-79-08 Approved Method: Electrical and Photometric Measurements of Solid-State Lighting (SSL) Products" test allows an end user to evaluate the suitability of the solid state lighting system, for instance a lighting fixture, for its use in a specific application or to compare SSL systems with one another. LM79 provides data for total luminous flux, electric power, and efficacy and chromaticity. As such, test results based on the LM79 and LM80 standards offer an accurate estimate of lumen depreciation in a LED lighting system.