Terry K. McGowan, FIES, LC
ALA Director of Technology & Engineering
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Newsletter Sponsor
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A recurring question in lighting, and especially in residential lighting, concerns the life of the bulb or tube and, now, the LED. How long will these products last once they are installed and used on a daily basis in typical applications? Lamp companies tell me that questions about bulb life have been, and continue to be, the subject of most of the complaint letters and emails that they receive from customers with, of course, the complaint being that the lamp in question didn't last for the published, rated life period.
Part of the problem is the definition of lamp life itself. For incandescent and fluorescent lamps, rated life is the time, measured in operating hours, when half the lamps in a statistical sample have failed. So, rated life may not have much to do with the actual life of any particular lamp but rather it's the average of a large group of lamps. If 100 lamps that have a life rating of 2,000 hours are operated for exactly those 2,000 hours, 50 will have burned out and 50 will still be burning. As you can imagine, it's those first 50 lamps that generate consumer complaints, not the 50 that are still burning at the 2,000-hour point.
Common fluorescent and incandescent lamp types are made on high-speed production machinery that is designed to make lamps with little variation lamp-to-lamp; but variations in materials and processing do occur. More importantly, once lamps are put into service, they can be subjected to application and use conditions that can significantly affect lamp life. Socket voltage, physical shock and vibration strongly affect the life of incandescent bulbs while fluorescent lamps are sensitive to the type of ballast used as well as the number of starts, meaning that frequent on/off cycles can shorten their life.
However, as incandescent bulbs disappear and CFLs and LED bulbs are increasingly used as more-efficient screw-in replacements, our ideas about lamp life are due to change as well. CFL bulbs are typically rated for 8,000 to 12,000 hours of life and they fail in a way we understand. They go out. Lamp failures sometimes include a puff of smoke or a burning smell because often a component of the electronic ballast fails rather than the fluorescent tube.
Screw-in LED bulbs also have integral electronics or "drivers" that can fail just like the ballast of a CFL. However, the LED itself is a very long-lived semiconductor device compared to a fluorescent tube or incandescent filament. In fact, LEDs don't often fail in the usual sense; they just emit less light output over time and that time can be quite long.
You have probably seen, for example, the LED life claims, which can range from 15,000 to more than 60,000 hours. Manufacturers now talk about "useful life" instead of rated life where useful life is defined as the time it takes a lamp to reach 70 percent or 50 percent of its initial light output.
A group organized by the Lighting Research Center (Troy, N.Y.) called the Alliance for Solid State Illumination Systems and Technology (ASSIST) developed the idea based upon their research as to when lighting users begin to notice a reduction in light output. They have suggested that, for commercial lighting systems, the useful life of an LED system is when the light output drops 30 percent while for decorative and accent applications, useful life is when the drop is 50 percent.
For now, however, consider LED life ratings as estimates. Some are based upon factors such as loss of light output; others, including products with integral drivers, may take component failure into account. Ask lamp manufacturers what they mean by the "rated life" numbers that they publish. There's still much to learn and data to gather and analyze before some kind of industry definition is established and broadly accepted.
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Sincerely,
ALA Director of Engineering & Technology |
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