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Re: Anyone experimenting with LED lighting?

The Eng Family <engfam at axion_net> wrote:

>So is anyone experimenting with LED's (light emitting diodes)?

I looked into LEDs as a possible source of light for planted aquaria. 
In spite of it promising future, though, I found that the technology 
falls short of what we require, at least the existing, commercialy
available technology. General consensus in the illumination community 
is that widespread use of high intensity white LEDs for general 
illumination purposes is still several years away. This is recent info 
that I gathered, among other sources, at the last SPIE conference in 
June 2001.

LED efficiency falls short of fluorescent and MH technology by a large
factor. The best white, phosphor based LEDs can emit about
10 lumen / Watt. Compare this with the 70 - 90 lumen / Watt of MH and

The main difference is that LED output is highly directional, while 
fluorescent and MH output is omnidirectional. Thus a LED basically 
makes up for it lower transducing efficiency by directing _all_ its 
output into a narrow beam. That's why they are so good in applications
where one stares _into_ the light beam, such as in brake and traffic 
lights, indicator lights, etc. In other words, a LED-based fixture does 
not require a reflector to direct its output into a narrow beam. Even 
so, the LED lower light output per Watt still will result in lower _lux_ 
at the receiving surface, if we compare a bundle of LEDs with a 
fluorescent or MH bulb fitted with a suitable high efficiency reflector, 
both feeding on the same amount of electricity. 

Beam angles from LED fixtures are narrow. This may pose a problem with 
uneven illumination in the tank. A typical figure seems to be around 
20 - 30 degrees. This corresponds to a 7 - 10 inch diameter spot at the 
bottom of a 20 inch deep tank. 

LED efficiencies are expected to increase about fivefold until reaching
"hard" limits imposed by heat dissipation and other issues. At that
point they may be comparable to fluorescent and MH, but not better
by any large margin.

There is also the lumen maintenance issue. Contrary to the claims we 
usually see in ad literature, research shows that white phosphor-based 
LEDs seem to degrade considerably along their lifetime, maybe as much 
as or even more than fluorescent bulbs. There seems to be scant data 
in this regard, and even the manufacturers do not seem to have reached 
an agreement on how to quantify the ageing process. In a recent 
research paper addressing LED ageing, I saw figures like a 35% decay 
in 4,000 hours of operation for a GaN LED with ytrium aluminum phosphor 
driven at 20 ma, not a large current by any measure.

Given the above, and the generally high cost of existing white LED fixtures, 
I decided for abandoning the idea for now and revisiting it maybe in
five years or so. But if someone is willing to spend money and effort
on experimenting with LED technology, I would be the first to applaud,
and be interested in seeing the results !

- Ivo Busko
  Baltimore, MD