Currently, LED light source life tests are internationally based on the IES TM21-11 test standard. The specific operation is to take N sample light sources and light them up for 1 hour under three test temperature conditions of 85°C, 55°C and a self-selected temperature. After reaching thermal stability, the initial luminous flux value is obtained. Then read the luminous flux value every 1000 hours and test continuously for 6000 hours. Compare it with the initial luminous flux and draw the luminous flux attenuation curve to calculate the life time of the LED light source L70.
In practice, we found that those LED light source products whose L70 has been tested for 50,000 hours in the Optoelectronics Center Standard Laboratory experienced serious light degradation after less than 20,000 or even 10,000 hours of actual installation and operation. It is a well-known fact that there is a large deviation between the L70 life value of the LED light source according to the IES TM21-11 test standard and the actual value.
After in-depth research, I believe that the IES TM21-11 test standard needs to make major corrections to the initial luminous flux test so that the laboratory test life of the LED light source can basically match the actual operating life. According to the LED temperature and luminous flux curves provided by LED chip component manufacturers OSRAM and CREE, the real initial luminous flux of the LED should be the luminous flux just after it is lit for 1 second at room temperature. At that time, the LED junction temperature is 25°C. The luminous flux curve Relatively straight. As the lighting time increases, the LED junction temperature increases, and the LED luminous flux curve climbs downward. The higher the temperature, the greater the curvature. When the LED junction temperature reaches 135°C, the curve has become very steep.
The initial luminous flux of the IES TM21-11 test standard is not the real initial luminous flux, but the luminous flux displayed by the LED chip components after the initial thermal stabilization of the LED light source under different packaging process conditions. If we compare the luminous flux after thermal stabilization with the luminous flux at the initial 1 second, we will find that the light has decayed by about 20% (when the junction temperature is 135°C). I call this light decay the packaging structure of the LED light source. Sexual light attenuation, the higher the level of LED light source packaging technology, the smaller the structural light attenuation of the package.
I think 8% of the structural light attenuation of LED light source packaging is an ideal value for practical applications. Because the actual L70 of this type of LED light source has truly reached 50,000 hours, and the requirements for heat dissipation devices are not too stringent. During the test, if the structural light attenuation of the LED light source is 20% after thermal stabilization, the LED junction temperature at this time is the operating temperature during actual operation. From the perspective of LED life testing, the LED light source under this packaging process condition The lifespan of L80 is only 1 hour (if the LED light source has a light decay of 20% within 1 hour of thermal stabilization time).
According to the LED temperature and life, temperature and luminous flux curves provided by Philips, OSRAM, and CREE companies, if the luminous flux of the packaged LED light source product has been thermally stabilized by 8%, then the LED junction temperature will be around 85°C at this time. ; If the luminous flux of the LED light source decreases by 20% after thermal stabilization, then the LED junction temperature will be around 135°C at this time. It can be seen from the CREE chip junction temperature light decay curve of 135°C: L80 tested by IES TM21-11 is actually L64 of the tested LED light source (80%*80%=64%). The L70 tested by IES TM21-11 is actually the L56 of the tested light source (80%*70%=56%).
It can be seen from the LED temperature and life, temperature and luminous flux curves provided by CREE that at 25°C, the curvature of the light decay curve is gentle. Under different junction temperature conditions, the higher the temperature, the greater the light attenuation curvature. At 135°C, the curvature of the light attenuation curve becomes steeper. If the LED light source packaging process is poor and the junction temperature is as high as 135°C, the IES TM21-11 test L70 curve is actually the line segment where the LED light source decays from L80 to L56.
Because the curvature of the L80 point is larger than the starting point, the light attenuation curve from L80 to L56 will be steeper. The inferred LED light source life should be shorter, which is consistent with the actual situation of current LED street light engineering applications. The initial luminous flux tested by IES TM21-11 is calculated from the assumed zero curvature of the gentle curve at 25°C, ignoring the fact that light decay has occurred when the LED light source is thermally stable, and the light decay curvature has become larger.
The L70 lifespan is calculated from this Because of the error in the initial curvature, the theoretical value deviates from the actual value. It can be seen from the CREE chip junction temperature light decay chart and the ENERGY STAR life requirement chart of the US Energy Star LM-80 that the tested light decay curve misrepresents the LED light source under three different temperature conditions of 25°C, 55°C, and 85°C. The package structural light attenuation is regarded as the real initial light attenuation of the same LED component, which is obviously inconsistent with the facts, because the initial point position of the curve after thermal stabilization is different, and the initial point curvature is naturally different.
The current internationally accepted IES TM21-11 test standard for LED light sources has played an important role in the initial stage of the promotion and application of LED lighting for more than ten years, and has played a very misleading role – making the life of LED light sources with simple packaging technology testing It also became very long. Nowadays, with the vigorous development of LED lighting applications, the original IES TM21-11 is obviously outdated. The result is that the LED light source life test conclusions seriously deviate from reality. Low-quality LED light source products and large machining light parts are widely regarded as high-quality products, which increases global LED lighting maintenance costs and causes a huge waste of social resources.
Only by improving the IES TM21-11 initial luminous flux test standard can LED Outdoor Lighting source manufacturers seriously improve the packaging process level, making the LED light source thermally stable, with less light attenuation and longer life. Taking the LED temperature and life, temperature and luminous flux curves provided by Philips, OSRAM, and CREE companies as examples, it can be inferred that only LED light source products whose luminous flux can still maintain more than 92% after thermal stabilization have an LED node operating temperature of 85°C At about 50,000 hours, the LED light source life span L70 can reach a real 50,000 hours.