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LEDs as a Light Source

Contents:

  1. Anyone experimenting with LED lighting?
    by busko/stsci.edu (Ivo Busko) (Fri, 24 Aug 2001)
  2. Anyone experimenting with LED lighting?
    by Bill Wichers <billw/waveform.net> (Fri, 24 Aug 2001)
  3. LED
    by Thomas Barr <tcbiii/earthlink.net> (Fri, 24 Aug 2001)

Anyone experimenting with LED lighting?

by busko/stsci.edu (Ivo Busko)
Date: Fri, 24 Aug 2001

The Eng Family <engfam@axion.net> wrote:

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

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
fluorescents.

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


Anyone experimenting with LED lighting?

by Bill Wichers <billw/waveform.net>
Date: Fri, 24 Aug 2001

>So is anyone experimenting with LED's (light emitting diodes)?
>Pros: Low power usage, low heat and long bulb life (100K hours)
>Cons: Expensive & lots of LED's required to make sufficient light.
>Seems like an interesting idea if it works.

>I was going to attempt to make a small lighting array but I can't find the
>instructions to make them anymore.  I remember reading a thread (in either
>reefcentral or reefs.org) about using LED's to light a tank but I can't
>seem to find the thread and the website that was on it seem to be down for
>the last week.

It's very easy. Just wire the LED's in series (and they are polarity
sensitive) until they get to an easy voltage to deal with (12 or 24 volts
usually), and then wire groups of LEDs like that in parallel. That allows
you to avoid the dropping resistors since the LEDs will usually want around
2.1v or so, although that varies with the LEDs - check your spec sheet.

You can get the LEDs from DigiKey (http://www.digikey.com), but the white
ones are expensive -- $3-5/each in small quantities if I remember right.
Surplus LEDs are cheaper but they are usually surplus due to color
variations that prevent them from being sold through regular channels.

>I was thinking of buying a 144 LED array (with super bright white LEDs,
>which are full spectrum 6500K) that the manufacturer says is equal to 150
>watts of light but it costs $200 (it may pay off really quick though
>because it will last 22 years of 12hr usage and the lower power consumption
>about 1/10th of conventional lights).

Hmmm. Might be an interesting expieriment. I had no idea anyone made an LED
array that was that potent. Chances are it is very, very directional as
well, which would actually be an advantage in an aquarium since that's what
you'd be trying to accomplish with the reflector of a normal light.

The only problem I can think of is the fact that LEDs tend to emit light in
very narrow bands (several peaks at certain wavelengths and almost nothing
everywhere else). The white LEDs I have are super-blue shifted too,
comparable to the 10000K MH lights around the perimeter with a lower-temp
white spot in the middle. Nice for a flashlight, but might look kinda wierd
on a tank.

     -Bill

*****************************
Waveform Technology
UNIX Systems Administrator


LED

by Thomas Barr <tcbiii/earthlink.net>
Date: Fri, 24 Aug 2001

> So is anyone experimenting with LED's (light emitting diodes)?
> 
> Pros: Low power usage, low heat and long bulb life (100K hours)
> 
> Cons: Expensive & lots of LED's required to make sufficient light.
> 
> Seems like an interesting idea if it works.
> 
> I was going to attempt to make a small lighting array but I can't find the
> instructions to make them anymore.  I remember reading a thread (in either
> reefcentral or reefs.org) about using LED's to light a tank but I can't
> seem to find the thread and the website that was on it seem to be down for
> the last week.

They make a moon light that LED and larger.
I'm totally interested in it though.

> 
> I was thinking of buying a 144 LED array (with super bright white LEDs,
> which are full spectrum 6500K) that the manufacturer says is equal to 150
> watts of light but it costs $200

So! That's still quite good!

> (it may pay off really quick though
> because it will last 22 years of 12hr usage and the lower power consumption
> about 1/10th of conventional lights).

And be easy to slim line applications. I like it.
Regards, 
Tom Barr
> 
> Cheers,
> Victor Eng     Vancouver, British Columbia, Canada
> engfam@axion.net


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