Predicting LED Lifetimes

Recently, a corporate client asked me to specify only LED fixtures with a lifetime of 100,000 hours, and preferred fixtures with a life of 200,000 hours.  I don’t know where they came up with these numbers, but my reply was that an L70 of 100,000 hours or more cannot be validated through standard testing procedures.  Here’s why.

To begin with, LEDs themselves don’t experience catastrophic failure the way incandescent and fluorescent lamps do. The don’t stop making light, but their output declines over time.  Today the generally accepted calculation of the life of an LED is called L70, which is the length of time before the light output has fallen to 70% of initial output.

The IES approved lifetime calculation method begins by collecting data using the procedure described in LM-80 (ANSI/IES LM-80 Measuring Maintenance of Light Output Characteristics of Solid-State Light Sources). Please note that LM-80 measures “LED packages, arrays, and modules” not fully fabricated fixtures, and there’s some dispute about whether or not testing bare modules is appropriate.  However, it does permit module manufacturers to test once and derive a lifetime, rather than every fixture manufacturer testing every fixture with every module they want to offer, which would be incredibly burdensome and expensive.

LM-80 requires a minimum collection time of 6,000 hours (250 days) but sets no upper limit.  If manufacturers want to use the data they’ve collected and project future performance they use the calculation procedure in TM-21 (ANSI/IES TM-21 Projecting Long-Term Luminous, Photon, and Radiant Flux Maintenance of LED Light Sources).  Importantly, TM-21 only permits data to be projected to six times the LM-80 data collection time period.  This is because of uncertainties involved with longer predictions (see PS-10-08 IES Position on LED Product Lifetime Prediction at https://www.ies.org/advocacy/position-statements/ps-10-18-ies-position-on-led-product-lifetime-prediction/).  So, an L70 of 50,000 hours is based on at least 8,333 hours of LM-80 testing.  That’s 347 days.

Thus, to say that an LED has an L70 100,000 hour life would require a data collection period of 16,667 hours (695 days), or 1,390 days (3.8 years of continuous testing) for a life of 200,000 hours.  Today, no LED manufacturer conducts LM-80 tests for that extended period of time because the lifetime of a given LED product is too short.  By the time you’ve finished a 4 year long test, the LED being tested is out of production and replaced by something new.  In the future, when the LED industry has matured and we’re no longer seeing continuous improvements in efficacy, color rendering, etc., they may test for that long, but not now.

Where do these 100,000 hour and longer lifetimes come from?  It seems that some manufacturers are using an internally generated prediction to get to these numbers.  The thing is, we don’t know what’s involved in that prediction, which means we can’t validate it or compare it to any other prediction.  We just have to take their word for it. With the LM-80/TM-21 procedure, on the other hand, we know that testing labs, regardless of who or where, are using the same procedure and their results should be consistent and repeatable.  That allows us to reliably, confidently compare fixtures by any number of manufacturers.

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