On Wed, 2 May 2007 16:17:54 +0000 (UTC), Dylan Smith
wrote in
:

On 2007-05-02, Larry Dighera wrote:
LEDs lamps are 10 to 50 times more energy-efficient than conventional
lights, which can reduce operating costs by up to 90%. LEDs are even
more efficient than fluorescent lamps!

They aren't really quite there yet - I bought a few LED lamps to swap
out for halogen downlighters. The current crop is nowhere near bright
enough for general room lighting (they are fine for supplimental
lighting) unless you covered the ceiling with a huge array of them. They
are also a slightly odd colour - the white ones are really very pale
violet rather than white.

There's a good article here
http://environment.newscientist.com/...hold-bulb.html
that explains the reason for the color.

The most significant changes to the way we light our homes are
likely to come when LEDs become cheap and reliable enough to
provide ordinary diffuse white light. This is because CFLs, while
much more efficient than incandescent bulbs, still only emit
around 15 per cent of the electrical energy fed into them as
light, or up to 30 per cent in "tube" form. This compares with 30
per cent for existing white LEDs, with a target of up to 70 per
cent. "It will be CFLs first, but LEDs may eventually bypass
them," says Colin Humphreys, a pioneer of LEDs at the University
of Cambridge.

LEDs are semiconductor devices that emit light when a voltage is
applied across them. Each LED is typically a stack of five very
thin layers of the semiconductor indium-gallium-nitride, separated
by gallium nitride layers, and measures just 1 millimetre square.
By varying the amounts of indium, engineers can alter the colours
produced. For example, 10 per cent indium gives blue light, and 20
per cent gives green. To produce white light, blue LEDs are coated
with phosphor, which generates yellow light. This merges with the
blue light from the LED to create a somewhat harsh white light.

Already, some LED-based domestic light sources are appearing. Last
month Philips unveiled a globe-like lamp based on four LEDs - two
red, one blue and one green. By varying the intensity of the LEDs
it's possible to create mood lighting in up to 16 million
different colours. Launched in the Netherlands, the lamp, called
LivingColors, is operated with a simple remote control. Philips
stresses that this is a long way from the LED-based "bulb" that
people can simply screw into existing sockets. But it's a start.

Most white LEDs for the home are likely to appear first in sharp,
functional lighting such as desk lamps. Earlier this month
Siemens's subsidiary Osram unveiled an LED spotlight called Ostar,
which the company says can easily illuminate desks from a height
of 2 metres. The lights should also last 50 times longer than
incandescent lamps, and five times as long as CFLs.

To produce LEDs that can replace incandescent bulbs, the challenge
is to develop devices that create a warmer white light.
Humphreys's team and others around the world are tackling this by
coating individual LEDs with red, blue and green phosphors. "In
principle, we can mimic the quality of sunlight," says Humphreys.
"We're not there yet, but we're getting close," he says.

If white LEDs are ever going to be used as light bulbs, they will
also have to get much cheaper. Nowadays, a single LED lamp costs
up to $60, mostly because indium-gallium-nitride wafers have to be
grown on expensive sapphire crystals. Humphreys is confident LEDs
can be grown on silicon instead. This would cut the cost
drastically, as a 5-centimetre sapphire substrate costs $40,
compared to just $5 for silicon. His team has already grown blue
LED structures on 5-centimetre silicon wafers in the laboratory.

Humphreys and his collaborators have also banished a gremlin that
was causing LEDs to fail early. The units stopped working after
just 400 to 500 hours of use because of heat trapped by the
transparent epoxy resin dome that caps and protects the chip. By
exchanging epoxy for a type of silicone, Humphreys stopped the
LEDs overheating, vastly prolonging their working lives. Once
they're cheap enough, LEDs could last the entire life of a
lighting unit, he says.

Whenever LEDs are ready to take over, one thing is certain: the
incandescent light bulb is finally on its way out. "It's amazing
it's lasted this long," says Humphreys.


From issue 2597 of New Scientist magazine, 02 March 2007, page
26-27



They will get there in the end though, but we're still a few years off
general purpose LED house lighting.