Knowledge What is lumen maintenance?

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Lumen maintenance or luminous flux maintenance refers to the remaining luminous flux over the initial flux (typically expressed as a percentage) at any selected elapsed operating time. LED packages rarely fail catastrophically (i.e., instantaneously stop emitting light), but rather experience parametric failures such as declining light output and shifts in color point (chromaticity coordinates). The useful life of an LED package or LED lighting system is generally expressed using the term "lumen maintenance life". The lumen maintenance life is defined as the elapsed operating time over which an LED package or LED lighting system maintains a given percentage of its initial light output. It is measured in Lp, where p is the percentage of its initial light output.

It's generally accepted that a light output of 70 percent of initial lumens constitutes the lowest limit for most lighting applications. The useful life of an LED package, therefore, is often cited using the L70 metric. Nevertheless, L80 or L90 data may be required in some applications where a 30% loss of light could be easily distinguished.

L70 = time in hours to which the lumen output has degraded to 70 percent of initial lumens
L80 = time in hours to which the lumen output has degraded to 80 percent of initial lumens
L90 = time in hours to which the lumen output has degraded to 90 percent of initial lumens

It should be noted that it's impractical to perform a complete lifetime test of every LED. The Lp value is extrapolated from a shorter duration test. IES has documented a forecasting procedure, IES TM-21, for projecting the lumen maintenance of LED light sources from the data obtained by LM-80 testing.

It should also be noted that LM-80 measurements are taken with the LED packages operating continuously in a temperature-controlled environment. The lumen maintenance life of LED light sources does not reflect the lumen maintenance life of LED lighting systems. In real-world operating conditions, LED lighting systems are affected by operational and environmental variables such as operating cycle, drive current, thermal management, and ambient temperature. For that reason IES TM-28 has been developed as a guidance for projecting lumen maintenance of LED lamps and LED luminaires from the data obtained by LM-84 testing.
 
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