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a better understanding about expected lifetime of a LED light

04-05-2018
typical lifetime of different lights technologies
People who works in the line of led lights for some time knows this number: 50,000 hours. When asked about the expected lifetime of a LED lamp, anyone  probably will answer that. It is also a digit that needs a lot of background information.

50,000 is such a large number that it is helpful to explain it into more familiar terms; 50,000 hours is about five years and nine months. Has someone actually lit the LED lamps in question continuously for five years and nine months? Probably not. Think about it: The LED lighting industry is changing so quickly that by the time someone tested an LED light for almost six years, that type of LED would be out-of-date.

If LED lights are not usually tested for 50,000 continuous hours, how do we have that number? The answer is a mix of testing and mathematical modeling. The Illuminating Engineering Society (IES) has published standard methods for "endurance testing" of LED lamps and for interpreting the data from those tests — most important, standard methods LM-80 and TM-21. In an LM-80 test, LED lights are lit continuously, usually for either 6000 or 10,000 hours, while being "cooked" at high temperature — usually 55°C and 85°C (130°F and 185°F), although higher temperatures may also be used. Every 1000 hours, the lighting output and color of the LEDs are recorded.

The testing result is a few data points, and the rest is math — exactly exponential least-squares curve fitting. The lighting output data is fit to a curve that can be used to predict, for example, when the LED lamps will have only 70% of their initial light output (L70). The problem is that these methods generate only 6 data points for a 6000-hour test, and 10 data points for a 10,000-hour test. That is a small volume of data, and there is only so far one can take a mathematical estimation before it becomes a false story. Actually, we say that LED lights have lifetimes of 50,000 or 60,000 hours because that is about as far out as one can effectively estimate using these methods.

While this kind of endurance testing is not useless, it does not help much, either — at least not for most users. The real world is full of complexities that no laboratory test can fully predict. Obviously, most people do not install plain LEDs in an oven, as in the laboratory test. Most people install  finished lighting fixtures that include LEDs along with lots of other parts: printed circuit board (PCB), power conversion circuits or drivers, optics, outer casings, diffusers, etc. Strangely, though, even with all those parts that could possibly break, we still focus on the performance of the LEDs themselves.

We need to change our thinking way. We need to look at the entire lighting fixture and how we use it. Instead of fixing on 50,000, instead of talking about terms like LM-80 and TM-21, we should be asking simpler, more complete questions: How long will this entire led lamp last? What will break first? How can I make the led lamp last as long as possible?

The answers to those questions are both complex and simple. LED lighting can go anywhere and do things that traditional incandescent and fluorescent lighting cannot. It has violated tiring ideas of the possible and been a beacon for the creative. However, LED lights do have shortcoming, and the more that we understand and mention those shortcomings, the longer a led lighting fixture will last.

Temperature limits are very important as we squeeze LED lamps into ever-smaller spaces. Heat melts plastic, overloads drivers, cooks phosphors, and kills LEDs. It can even make safety problems, both for the careless persons who touch hot surfaces and for the space itself. Yet we sometimes do not endure the heat that electronic components generate. A little ventilation goes a long way.

Power limits also an major issue to consider. A driver loaded to its maximum power output is more likely to fail. Drivers should never be loaded to the edge of failure, and in harsh environments, they should be turned down, loaded only to a small part of their rated maximum load. Good documentation and newer generations of small drivers specifically designed for LEDs may help.

Using a single number to describe the lifetime of a complex product is simply too reductive. I try every day to provide explanations, to make others think about the whole led fixture. Yet old habits die hard, and old numbers still have their helpfulness. Sometimes, when nothing else I answer will satisfy, I say it too: 50,000 hours.
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