By BizLED Bureau
September 23, 2015: Testing lumen maintenance of LED lights is very important to find out the life of LEDs. This is because LED light output slowly decreases with time, very unlike traditional light sources. LEDs operationally do not fail completely.
Why lumen maintenance tests are critical
Besides the LEDs, the long term lumen maintenance also depends on other components used in the LED lighting system, like the LED lamp, optical components, luminaire, driver, thermal management devices, etc. With time, these components also undergo degradation. For example, the plastic elements that are used in the optics change in many ways due to light transmittance; it may become haze and change colours. As a result, the light output of LED lighting gets reduced beyond the normal lumen degradation. Hence, it is necessary to conduct lumen maintenance tests.
IESNA LED testing system
Lumen maintenance of LEDs usually vary from manufacturer to manufacturer and even between one LED package to another of the same manufacturer.
There are two test methods developed by the Illuminating Engineering Society of North America (IESNA). These are the LM 80 and TM 21. These tests the lumen maintenance of LED lights. While LM 80 is a test to measure the lumen depreciation of LEDs, TM 21 is a technical specification that shows how to extrapolate LM 80 data to make long term projections regarding lumen maintenance. TM 21 also helps to find out when the lifespan of an LED has been reached to its end, which means the light emitted from an LED reduces to a level when it is no longer useful for any application as the light is not considered sufficient.
What is LM 80 test?
Lumen maintenance compares the amount of light emitted from an LED or from an LED light when it is new to the amount of light emitted at a time after a certain period of usage. For example, if an LED emitted 1000 lumens of light when it was new and then emitted 600 lumens of light after 25,000 hours, it means that it has a lumen maintenance of 60 per cent at 25,000 hours. This is also known as lumen depreciation, which in this case would be 25 per cent from the original light output. Lumen maintenance is considered as L50, L70, L80, or L90. Here, L stands for lumen maintenance and the number is the percentage of the remaining light output.
LM 80 test measures the lumen maintenance of LED packages, arrays and modules. This test is performed to measure lumen maintenance while controlling the case temperature of LEDs, DC current and DC forward voltage to the LED source. This test aims to find out the depreciation in the LED light output over a period of time. The lumen maintenance test usually target the requirements of the application specified by the customers. Since we cannot usually detect a depreciation in light output until it is less than 30% depreciation, L70 is essential as per the requirements of an application.
Why lumen maintenance tests are essential for LED lights
Beyond the LED, other components in the LED lighting system can also impact the long term lumen maintenance. These components include LED lamps, engines, luminaires, drivers, thermal management devices and optical components. Over time, these components may experience some change or degradation. In particular, the plastic elements used in the optics may change in several ways in the areas of light transmittance, a haze and undesired colours. In turn, the overall light output of LED lighting products will be further reduced?beyond the normal lumen degradation of the LED source. Therefore, it may be necessary to conduct lumen maintenance tests at the LED lighting system level.
The procedure of the tests
A minimum operating period of 6000 hours at rated current and voltage is required to measure the components as well as the whole LED system under LM 80. The initial seasoning period for the LEDs is the first 1000 hours, and the next 5000 hours is for actual light output measurement. The light output data that is collected between 1000 and 6000 hours is then used for measuring or estimating the time needed to reach 70% and 50% lumen maintenance.
The tests are performed at 55°C, 85°C and another temperature chosen by the manufacturer of the LED syatem (for up to 6000 hours). Sophisticated life racks with at high temperature chamber are required for LM 80 test. A measure and control system is also essential for the test. Lumen maintenance cannot be detected over a short testing period, it has to be about 6000 hours of testing. Hence, an alternate test that can be conducted over a shorter period of is required, experts say.
Is LM 80 an adequate test?
In 2010, IESNA Testing Procedures Committee (TPC) stated that LED lumen maintenance data obtained through LM 80 and TM 21 tests may not be sufficient to show the changes in an LED system?s light output levels over time. Hence, it is developing another standard for lumen maintenance tests for LED lamps, engines and luminaires.
The performance of LED lighting is impacted by several variables like its operating cycles, airflow, ambient temperatures, orientation, etc. These conditions cannot be checked through LM 80 test. Hence, it is necessary for IES to provide other tests, and test methods that would have to be designed to give comparable results. However, experts believe that any recommended testing procedure by IES must be practical, keeping the future conditions in mind.
TPC working group is still debating the testing duration, and there are several suggestions under consideration. One option is to specify the testing durations in the test report based on the parameters that the users are looking for. This may help the test data to apply as it does with the TM 21. But if the test is done to obtain the lumen maintenance of the LED products, they should be tested for the entire claimed lumen maintenance life.
However, experts feel that LM 80 is a sufficient standard for the time being. Being the the first standard for lumen maintenance, L80 is good enough, despite certain issues that come up. But in future, other improved standards need to be developed.
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