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Re: Lighting

Bill wrote:

It seems that we need some way to measure "effective" illumination, that
the amount of light that reaches the bottom of the aquarium.

I thought that lux did that, or came close to it.   Does it?  Is there a
formula that can compute lux based on wattage and the distance of the
source from the bottom of the tank?

Maybe some kind of measurement could be developed that would be an
improvement over watts per gallon, and maybe it could start here!

There are two issues here. One is determining what sort of plants a
given light fixture will be able to grow and the other is measuring
light levels in an existing aquarium. The latter is pretty simple. You
can jsut take a measurement with a lux meter and then multiply the
result by a corection factor that takes into account the spectrum of the
lamp you are using. Those correction factors are available in Ivo
Busko's article as well as a couple of other places. Ivo's correction
factors are not based on empirical data but they should be pretty
accurate provided the lamp manufacturer did not publish bad data. You
can also just look at the plants and fairly easily determine if more
light is required or not.

The first issue is more dificult but doable by the method I described.
The aquarium fixture efficiency introduces quite a bit of error but not
so much that the method is not useful. The lamp spectrum's efficiency at
growing plants is also not known for aquatic plants. I suspect the
photopic response of aquarium plants is a lot flatter than people think.
This is the case for non-aquatic plants. Individual plant species have
have a strong response at particular frequencies but when you average
the response of many plant species together the curve is quite smoothed
out. I think it doesn't matter a whole lot unles you just have one type
of plant. As long as a good amount of blue light and red light are in
the spectrum the plants seem to do fine. How else to explain the fact
that triphosphors do just as well as full spectrum lamps. I think the
biggest obstacle to determining how much light a fixture produces is
determining the aquarium fixture efficiency. What I mean by that is how
much light produced by the lamps passes through the water surface and
into the water. It is not that hard to measure actually. There is an
article in the Krib by Ron Woszniak that describes a method of doing
just this. If I had a lux meter and a spare 55 gallon tank I would do it