LM80 TM21 – LED luminaire lifetime predictions

I mentioned previously that I’d expand on the LM80/TM21 LED theme and what it means in relation to a luminaire life.  The short answer is…absolutely nothing!  LM80 and TM21 refer, only, to the predicted lifetime of LEDs and COBs.  How the LEDs are mounted, cooled and driven are the key factors affecting luminaire life.

Having said it means nothing, that’s not totally true.  What the data is intended for is to provide a luminaire manufacturer with a way to directly compare LEDs/COBs allowing for the best choice of component to achieve a good luminaire life.

So, what does it mean, in simple terms?

IES LM80-80-2008 “Measuring Lumen Maintenance of LED Light Sources”  is the industry standard method for testing LEDs to determine lumen depreciation over time and is carried out over a 6000- 10000 hour period.  At 1000 hour intervals the luminous flux is taken.  As a typical example (CREE, because we use them) let’s say the depreciation is 3%. That means a 97% maintenance of light output. (and, perhaps, at 10000 hours, 6% depreciation or 94% maintenance) Historically that was it and LED manufacturers could draw their own curve through the test points and boldly quote a 50000 hour lumen maintenance of 70% (L70) output while others would quote 90% (L90).   – Before TM21 came along, there was no agreed standard as to how to predict the end of useful life. This was not helpful for us as luminaire manufacturers or you as customers.

IES TM-21-2011 “Projecting Long Term Lumen Maintenance of LED Light Sources” recommends a method to use the LM80 data to predict the lumen maintenance of an LED.  Basically, an exponential curve is drawn between the 1000 hour test points on a graph plotting lumen maintenance from 70% against a time line of up to 100,000 hours.

This could well give a calculated figure of L70 = 50000 hours

from CREE X-lamp lumen maintenance

However, it is also stipulated in TM21 that the maximum life that may be quoted is 6x the actual test duration. So, if the test duration is 6000 hours the maximum life that can be quote is 36000 hours and would be quoted

L70 (6K) >36000 hours if 6000 hours of testing have been carried out or L70 (10K )>60,000 hours if 10000 hours of testing has been done.

It’s obviously more complicated than this and the tests are actually carried out at 3 different LED case temperatures; 55deg C, 85deg C and a manufacturer selected temperature.  (The meaningful one is 85deg C as this represents practical conditions).

Don’t forget,this is just the LED in lab conditions with controlled cooling and drive current and doesn’t take account of ambient temperatures or power supply life time.

Beware, therefore, what is quoted as a luminaire life.  I’ve seen lifetimes of >100,000 hours to L90 at 25deg C quoted in big type on luminaire datasheets and, in small print, this is with a junction temp Tj  and is the LED manufacturers data. Rather misleading but looks good on the spec sheet.

Earlsmann uses the CREE COB CXA 2530 or 2540. TM21 predictions for these is L90 (6k) >36,300 hours with Tc 105deg and 808mA drive current.  We optimise for a lower case temp of 85deg C. which can only improve the life expectancy.