The hype touting LEDs as having a useful life of 100,000 hr is incorrect. So say engineers at Philips Lumileds Lighting Co. in San Jose, Calif.
“The technology for 100,000-hr, high-brightness LEDs doesn’t exist,” says Philips Application Engineer Pat Goodman. “We have published what is probably the longest database on lifetime data and it only goes out to 9,000 hours. To extrapolate from 9,000 to 100,000 hours is quite a stretch.”
Goodman thinks he knows where the 100,000-hr figure originated. “A Hewlett- Packard engineer in the 1980s determined that a 5-mm LED lamp would electrically still look like a diode at 100,000 hours,” says Goodman. “It might put out little useful light but it would have the same forward and reverse voltages, and so on. He published that information without qualifying what the 100,000-hr lifetime meant. He was right, but the question is whether the diode would be useful as a light.”
The LED lighting industry is now trying to define what “useful life” really means for LEDs. It turns out that a concept called lumen depreciation plays a key role. It basically refers to a fall-off in light output. “Lumen depreciation is widely understood in the lighting community and is not unique to LEDs. But it doesn’t come into play when you have a light source that only lasts hundreds or thousands of hours, as with incandescents. On average, incandescents fail before the eye notices a difference in their output,” says Philips Marketing Communications Director Steve Landau.
The industry is leaning toward defining LED lifetimes in terms of how long the device puts out a prescribed amount of light in an intended application. For example, “People in an office can’t tell there is a change in illumination until after a 30% drop in output. So it is not objectionable to wait until the LED is at 70% of its original spec before you replace the lamp,” says Goodman. “On the other hand, the eye doesn’t notice a change in outdoor light until after a 50% drop in illumination. That’s when you need to think about replacing an outdoor lamp.”
Nevertheless, LEDs still last a lot longer than other kinds of lighting. “I am not going to say 100,000 hours of life isn’t realistic. It is just a myth because we don’t have the data to prove it,” says Goodman. “We are gathering that data. In the future, under specific operating conditions those figures may be possible. But right now they are unsubstantiated.”
Philips Lumileds Lighting Co.,