“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.”
Contact
Philips Lumileds Lighting Co.,
(408) 964-2900,
lumileds.com/