Lumen output testing is needed for reliability
By BizLED Bureau
Aug 17, 2017: While buying an LED light, a consumer may not be aware of lumen maintenance, but a designer or a manufacturer knows how important it is to measure lumen maintenance to enhance the life of LED lighting products, as their basic goal is to meet the right lighting needs of the end users, and giving them the desired amount of light output from the product.
What is lumen maintenance?
Lumen maintenance compares the amount of light produced by a light source or a luminaire when it is brand new to the amount of light output at a specific time in the future. As the lifetime of LED packages is often tens of thousands of hours, it is usually not realistic to measure the lumen output of LED products before at least 6,000 hours or 10,000 hours.
For example, if a luminaire produces 1000 lumens of light when it was new and after 30,000 hours produces 800 lumens of light, then it is said that the luminaire produces 80% lumen at 30,000 hours. In other words, it can be said that 30% lumen depreciation has taken place over 30,000 hours. Lumen maintenance is often specified as L50, L70, L80, or L90. Here, L stands for lumen maintenance and the number is the percentage of light output remaining.
Usually, a particular lumen maintenance is suggested to a customer based on her application and requirements. Since the human eye generally cannot detect a change in light output until there has been 30% depreciation, L70 is often suggested for an application. For example, Philips Lumiled claims that its Luxeon LEDs generally ?takes tens of thousands of hours before it will lose 30% of its initial light output.?
Why LM-80 is critical?
Illuminating Engineering Society of North America (IESNA) has developed two test methods to check lumen maintenance of LEDs in LED products?LM-80 and TM-21 tests.
LM-80 is a standard or a method to measure lumen depreciation of LED packages, modules and arrays. It allows customers to compare lumen maintenance of LED components of different company products.
Since LM-80 measures only one component of the whole LED luminaire?the LED light source?it is not an ultimate method to measure the performance or reliability of the whole LED system. But LM-80 is important for luminaire and lamp manufacturers and lighting designers as they should know how quickly the light output of an LED will depreciate and make a product.
LM-80 also helps manufacturers to earn an Energy Star rating for their products. In fact, those who want to sell LED lamps in the US should obtain an Energy Star certification for an LED lamp and for that the lamp should undergo LM-80 testing.
For LED sources, LM-80 testing is done in three case temperatures. The LEDs that are tested are placed in temperature-controlled thermal chambers with sophisticated active-cooling systems. These set the LED case temperatures to meet test requirements, with each test running for a minimum of 6000 hours at each case temperature.
However, LM-80 test cannot detect lumen maintenance over short testing period; it requires a minimum of 6000 hours of testing.
TM-21 is equally important
TM-21, on the other hand, uses LM-80 data to make consistent lifetime projections beyond the testing period. TM-21 standard, in fact, picks up from where LM-80 has left off. It says which values can be used in the calculation based on the sample size, number of hours and intervals tested, and test suite temperature. The accurate range based on TM-21 is up to 36,000 to 60,000 hours of useful life.
Limitations of LM-80 and TM-21
While these industry standards have provided tremendous help to the manufacturers and designers, they have some limitations as well. According to industry experts, LED lumen-maintenance information obtained from LM-80 and TM-21 tests may not be enough to judge the performance of the whole system because the whole lighting system needs to be tested.
The performance of LED lighting system is affected by variables such as operating cycle, ambient temperature, airflow, and orientation, as well as conditions imposed by auxiliary equipment and fixtures. These conditions cannot be tested by LM-80 or TM-21 tests. As a result, IES is designing more test methods to test all the other components of the LED lighting system.
A lumen-maintenance test standard for LED lamps, engines and luminaires is still being developed, which will test temperature conditions, operational duty cycle and test duration.
LM-80 also fails to test cycling which can affect the driver electronics. Hence, experts are trying for an operational cycle with certain periods of on time and off time. This cycling condition is intended to have a closer relationship to real-life LED system applications.
Experts also recommend LED lighting systems to be tested in a condition as close as possible to the installation orientation and mounting method, which in many cases is room temperature (25°C) with tolerances.
The duration of testing is another variable to consider, where there is still no standards.
The industry is, therefore, looking forward for IESNA to come up with some more necessary standards and tests that will help implementation and adoption of LED lighting products faster.