How to predict the life of LED?
In general description, LED can be used for more than 50000 hours. Even some manufacturers promote their LED can operate 100000 hours or so. The main problem in this aspect is that the LED is not simply not working, and its rated service life can not be calculated by the measurement of the traditional lamp. In fact, when we test LED service life, nobody would be staying at the side of the lamp waiting it stop working. However, there are still other ways to measure the service life of LED. The reason why LED can work longer is because it won’t cause the problem of filament fuse. LED does not work directly, but it will gradually degenerate as time goes by. There are predictions show that the high quality LED can maintain the original lamp brightness of more than 60% after 50000 hours of continuous operation. Assuming that LED has reached its rated service life, actually it may still shine, just the light is very weak.
If we do not consider the power and drive failures, the service life of LED reflects as its decay, that is its brightness getting darker with the time, until finally extinguished. Usually defined the time decay to 30% as its life span.
So the life of LED can predict or not?
1.LED Light Decay
Most of the white LED is obtained by blue LED shining the yellow phosphor. The reasons lead to the LED light decay are mainly the following two, one is the blue LED itself light decay, blue LED decay even faster than red light, yellow light, and green light LED. Another reason is the fluorescent phosphor attenuation; fluorescent powder may decay seriously at high temperature.
Diagram1. LED Decay Curves
We may see from the diagram that LED light attenuation related with its junction temperature. So-called junction temperature is the junction temperature of the semiconductor pn. Decay may happen earlier when junction temperature gets higher, that is life gets shorter. We may see from the picture, if the junction temperature is 150 degrees, the service life is only 10 thousand huors when the brightness gets to 70%; if the temperature is 95 degree, then the life gets to 20 thousand hours; if the temperature decline to 75 degree, the life gets to 50 thousand hours; when the junction temperature decreases to 65 degrees, the life can be extended to 90 thousand hours. So the key to prolong the life is to reduce the junction temperature. However, these data are only suitable for LED of Cree, not for the LED from other companies. For example, the decay curves of LuxeonK2 as shown in diagram 2.
Diagram1. LED Decay Curves
When the junction temperature increases from 115°to 135°, the life span can be reduced from 50000 to 20000 hours.
2. How to extend the life of LED?
It is concluded that the key to extend its life span is to reduce its junction temperature. And the key to reduce the junction temperature is to have a good heat radiator. Be able to exhaust the heat generated by LED timely.
3. How to predict the life of lamp.
Predict the life from junction temperature seems to be very simple. As long as check the curves in the diagram 1, it may seems clear that the 20 thousand hours LED life corresponding the junction temperature of 95 degrees. However, this method may not very reliable when applied to the indoor LED lamps. If applied to outdoor LED lamps, especially high-power LED street lamps, where there are a lot of uncertain factors. The biggest problem is that the cooling efficiency of the radiator of LED street lamp decreases with time. This is due to the accumulation of dust, bird feces, so that the heat dissipation efficiency is reduced. Also because there is a very strong ultraviolet radiation outdoor can still reduce the life of LED. Ultraviolet radiation may has a great effect on the aging of the epoxy resin, if using silicone, this problem can be improved. The UV also has some bad effects on the aging of the phosphor, but it is not very serious.
LEDLM-80PL LED Lumen Maintenance and Aging Life Test System developed by LISUN is produced by the requirements of IES-LM-80, IES0LM-82, TM-21, GB2312-80 and CIE, IEC standards. It can record the change curve of luminous flux VS time and the color parameters VS time. Through test and record of the attenuation data records in a short period of time to calculate the life time of LED
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