Re: efficacy of LED's

In article <ZIudnXty7tF_wAzZnZ2dnUVZ8qudnZ2d@xxxxxxxxxxxxxx>, David Lee 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.

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.

It's often a major problem with visible wave length materials. Lumileds
managed a significant gain in "extraction efficiency" from InGaAlP dice
(presumably GaP substrate) by making the shape of the die a truncated
inverted pyramid instead of square.

In addition, I see quantum efficiency of visible LEDs usually varying
significantly with current. With white LEDs, the photon output / electron
throughput in my experience is often about 20% higher at some reduced
current than it is at "characterizing current" in my experience so far.

- Don Klipstein (don@xxxxxxxxx)
.

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