TM-21-19 OR TM-21-21 testing

In 2011, the Illuminating Engineering Society (IES) published the TM-21 guidelines titled “Lumen degradation lifetime estimation method for LED light sources.” TM-21 is the illuminating Engineering society of North America (IESNA) approved method for taking LM-80 data and making useful LED lifetime projections from it. The standards apply to lifetime projection of LED package, array or module alone. The result may be used for interpolating the lifetime of an LED light source within a system (luminaire or integrated lamp) using the in-situ LED source case temperature. To make sense of TM-21, you must understand LM-80, which refers to a method of measuring the lumen depreciation of solid-state light sources, such as LED packages, arrays and modules. Before the advent of LM-80, manufacturers of LED components reported lumen maintenance data using their own varied and disparate systems. To avoid confusions among the customers, members of the IES came together to create a standard methodology which makes it possible for customers to evaluate and compare the lumen maintenance of Led components from different companies, giving birth to LM-80. 

LM-80 is typically a 6000hr test (can be 10, 000 hrs.) which depicts depreciation and chromaticity shift over the period at given operating temperatures; 55 degrees, 85 degrees, and a third manufacturer defined temperature, say 105 degrees. Therefore, TM-21 is not a test, but, a mathematical method based on LM-80 collected data and among other things, it considers;

While LED light fixtures are known to have quite a long-life compared to the conventional lighting fixtures, sometimes these characteristics can be distorted. The trick has been how to measure or estimate the longevity to provide assurance to users of this technology`s reliability compared to other options. While estimating this longevity, it`s important to understand that the overall reliability of a complete LED lighting fixture can be affected by the reliability of individual product components (driver, lens, diodes, etc.) and ought to be accounted for in lifetime estimations. LEDs, unlike the antique lighting products, do not have filament burn-out that conveniently announces the end of life. Additionally, the rapid development of the technology and the desire to bring products to the market in a timely manner does not allow for actual testing verification of then long lives claimed (40,000 or even 65,000 hours). The life and performance of industrial LED lighting products also greatly depends on excess heat retained at the diode, which explains why LEDs need testing at different temperatures. Therefore, when a source is installed in a luminaire, it is possible to measure it`s actual temperature and derive the product`s lumen depreciation. 

As such, for serving the solid-state lighting industry, the IES developed appropriate tests applied in rating the longevity of the LED light fixture or light bulb products. The initial need was a measure of the basic lumen degradation of LED source components identified by package, module or array of diodes, which came in the form of LM-80. LM-80 only specifies how to measure lumen depreciation to minimum of 6000 hours (with recommendation of testing to 10,000 hours or longer). LM-80 stops short of using that data for the estimation of any depreciation after that, which is where TM-21 comes in. The TM-21 working group evaluated projection options beginning with an analysis of mathematical, engineering-based models to provide effective depreciation fit and a useful projection method. The analysis showed that the LED lumen depreciation trends often change after 6000 hours and no consistent and reliable approach exists to predict trends from 6000-hours data points. 

TM-21 is important in providing a projection of the lumen maintenance of an LED source based on data collected according to LM-80. With this projection information, it is possible to project the expected lumen degradation of the light source as part of a complete system (fixture). It (TM-21) also provides a suggested sample size of 20 LED packages, modules or arrays. Based on the evaluation of the uncertainty of measurement at different sample sizes, a larger sample size (30), does not significantly increase the uncertainty and the smaller size (10) would significantly reduce the uncertainty of the degradation estimates.  

The TM-21 provides a projected lifetime for the LED luminaire or system, at each tested temperature. Typically, life projections for LED luminaires are typically 70% of the initial light output (L70). Life notation results will then use the following standardized nomenclature: Lp (Yk)    

P: Lumen maintenance percentage. For LED luminaire, L70 is considered to be the standard. After about 30% lumen depreciation, the system is considered as not performing its duty well and ought to be replaced. 

Y: length of LM-80 data period in thousands of hours. Example, L70 (6K) = 36,000 hours. 

As earlier stated, TM-21 is a method of projecting long term lumen maintenance of an LED light source based on 6,000 hours (or more) of lumen depreciation data collected per LM-80 test. The information can be quite technical as it involves complex mathematical calculations and equations to arrive at these hours at different temperatures of testing. 

As a client, looking for the TM-21 in your LED lighting fixtures and understanding how the figures were acquired could be mentally exhausting.   The important thing is that buying from a reputable Brand you can be sure the IES reports and photometric layouts convey correct and honest projections.   

Typically, when lighting designers, specifiers, builders and contractors evaluate or implement LED lighting products, just as with any other lighting technology, they are interested in knowing the longevity of these LED lighting products. More specifically, their interest is in knowing the duration of time it will take, in terms of hours or years, until the light output of the products is reduced to a level where they call for replacement. Ideally, the users wish to know how to predict LED lighting lumen maintenance. It is therefore a bigger concern for the LED fixtures makers to understand the concept of TM-21. For the consumer, the information is summarized as half-life of the fixture in hours. 

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