# [APD] Re: How much light? -- or - reflections on bright replies

```Sorry, Scott, I should have sprinkled a few smileys in there...

anyway...

Scott retorts:
>Well, if you you get over, say, about 18" or so, even 2-3
>watts can be inadequate for some of the foreground stuff
>that's so far away from the bulb. So with tall tanks you
>might want to use more wpg than with shorter tanks. That's
>all I meant. Maybe that goes without saying.

Yes, I knew what you meant. More smileys next time, I promise.

"So far away from the bulb"??

In all seriousness, I disagree with that. There seems to be a
misconception that water depth by itself requires more light. I think you
would be hard-pressed to demonstrate that, in a bare tank, the light
intensity 18" below the surface is much different than the light intensity
at 24".

Certainly the light at 12 feet would be less than at 3 feet in a natural
body of water, but we aren't talking about much depth differential or a
natural body of water.

The commonly used "Inverse Square Law" doesn't apply very well here. First
of all, it's only true with a point source in free space.  A single MH
bulb comes close to a point source but FL bulbs are a linear source. In
free space, FL bulbs will loose intensity proportional to distance from
the bulb if you don't use a reflector. If you use a good reflector, the
light is not radiating away from the bulb in all directions (the reasoning
behind the Inverse Square Law) but is directed towards the tank. In this
case, there are few common rules that apply.

Secondly, once light gets into the water, the aquarium walls make darn
good reflectors, bouncing any light at shallow angles back into the water.
I think someone (Wright?) likened this to a light pipe of sorts. Once
light gets into the tank, a lot of it stays there.

I had a chance to measure the lighting in a bare 29g tank once with a
luxmeter. With water, of course. I found that the light at lower levels
was actually a little more intense than that just below the surface! I
checked that a couple of time since it was so counter-intuitive.

IMHO, taller tanks require more light IFF there are tall plants that are
blocking the light from short foreground plants.  Yes, the same end
results but let's get the reasoning accurate to avoid confusion.

George Booth in Ft. Collins, CO (gbooth at frii dot com)
The website for Aquatic Gardeners by Aquatic Gardeners
http://aquaticconcepts.thekrib.com/  (mirror)
http://www.frii.com/~gbooth/AquaticConcepts/

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