Re: efficacy of LED's

On Thu, 15 Jun 2006 14:17:30 -0000, "David Lee"
<davidlee_malvern@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

Victor Roberts wrote...
By 1997 LEDs had already improved to the point of being three times
more efficacious than compact flourescents and 30 (yes 30!) times more
efficacious than 'standard' incandescents.

see

http://www.homepower.com/files/ledlight.pdf?search=led%20lighting

How is it that we have gone backwards since then?

I blame the price of oil, global warming, the Iraq war, George Bush,
the price of cheese, my dog Charlie, .......

The author states that his meter is only measuring the light
in a small area directly beneath the lamp, and then he adds
that ridiculous headline to the start of the article.

There is also a common confusion about the meaning of "Efficiency" in LEDs.
The QUANTUM efficiency of an LED is indeed close to 100%. However that
simply means that every electron passing into the junction region will give
rise to the emission of a photon of light - it says nothing about the amount
of power dissipated in moving the electron around the circuit. So efficiency
as optical-power-out divided by electrical-power-in is generally a much
smaller value.

I don't think there is as much confusion here as there is
misrepresentation. LED supporters would like to think there
is something magical about LEDs. They don't discuss the
quantum efficiency of fluorescent lamps, which is about 70%,
or the fact that the radiation efficiency of many types of
incandescent lamps is virtually 100%. It just that most of
the radiation they generate is not visible to the human eye.
Another problem is getting the light out of a semiconductor
die once it has
been emitted - although this was much greater in the mid IR LEDs that we
were developing because of the very high refractive index - resulting in a
big hit in external efficiency. I don't think that it's a serious problem
with optical wavelength materials.

Discharge lamps also have high "internal " efficiency if we
choose to forget about the problem of getting those darn
photons out of the lamp.

The only fair way to compare visible light sources is to
measure the radiant power that exits the device, weighted
and converted into lumens by the CIE Photopic Eye Response
curve, divided by the line plug power input. All these
other numbers are just smoke and mirrors.

--
Vic Roberts
http://www.RobertsResearchInc.com
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