The most important feature of LED light source is the long life, can reach 50,000 ~ 100,000 hours, a long time to monitor its light decay is impractical, therefore, this standard will predict the life of LED lighting fixtures through a method of accelerated life test.
Scope of application
This standard applies to all types of LED lighting fixtures life test, does not include the test of the power supply part of the lamps and lanterns.
Technical Requirements
The light output of the LED luminaire is 70% of the initial light output as the indicator of life judgment failure.
Test Method I
Junction temperature is an important cause of LED light decay. The increase of junction temperature will make the LED light decay quickly, LED working at high current will generate more heat, thus accelerating the aging.
This test method uses different driving currents and selects 5 LED luminaires with different currents for accelerated life test at 25℃ ambient temperature to derive the mathematical model of light output decay.
Mathematical Models
y=exp(-αt)
α=m × exp(nI)
Where “y” represents the relative light output, “α” represents the attenuation constant, “t” is the lighting time, “m” and “n” are constants, and “I” is the test current.
According to the test data about “y” and t obtained at different currents, the final attenuation coefficient “α” is derived about different lamps, which leads to the life time value of t70% for the luminous flux maintenance rate at 70%.
Test Method II
Using temperature as a constant accelerating stress, the expected life of the LED luminaire was derived at 70% of the failure criterion at 25°C.
Selected five LED lamps of the same specifications, adjust the temperature of the oven, respectively, at 50 ℃, 80 ℃, 100 ℃, 120 ℃, 150 ℃ conditions, in the rated current, constant current conditions as the conditions of constant acceleration.
Record all parameters of the 5 groups of LED reliability test process: light output (illumination or light intensity or luminous flux), test time, current, power, junction temperature, etc.
P=P0exp(-βt)
β=β0IFexp(-Ea/kTj)
Where P0 is the initial light output, P is the light output after heating and charging for t time, β is the attenuation coefficient at a certain temperature, t is the working time of charging at a certain temperature, β0 is a constant, Ea is the activation energy, k is the Boltzmann constant, IF is the working current, and Tj is the junction temperature.
After the equation transformation and experimental data
Ea=[K×In(t2/t1)]/(1/Tj2-1/Tj1)
t2=t1×exp[Ea/K(1/Tj2-1/Tj1)
This results in a failure judgment of 70% life expectancy at 25°C.
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