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**To**:**Aquatic-Plants at actwin_com****Subject**:**Re: Light intensity****From**:**Steve Lacy <slacy at tivo_com>**- Date: Thu, 13 Jul 2000 14:46:31 -0700

Shouldn't we be talking about the amount of light that strikes the bottom of tank per area per day? That would lead me to a formula like this: light factor = (wattage/depth^2) / (area of bottom) * (hours of light per day) * 100 This is barring other factors like efficency of the reflector, spectra of the bulbs, etc. (which matter quite a bit) It also leads me to ask the question: Can't I just adjust the duration of lighting to get what I need, no matter what my actual wattage is? This formula might be total B.S., so lets do some checks: A 10g tank: bottom area of ~210 in^2. Depth to bottom is ~10in. I've got 26 watts over that tank, running 12 hours a day. That gives me a light factor of 1.4857 Two 10g tanks under one 80W shoplight: Light factor of 2.28 A 60 gallon 24" cube tank under 250W halide: Light factor of 0.904 The same tank with 175W instead of 250 has a light factor of 0.6329 A "standard" 50g tank has a bottom area of 36*15 in^2 and a depth of 21". With 100W lighting @ 12h, we get a light factor of 0.50. At 175W, it rises to 0.88. According to my new "light factor" formula, the smaller tanks have extremely high lighting, mostly because of their shallow depth. I'd believe that. Is 250W too much light for a 60g tank thats 24" deep? Hard to tell. Its not as high a factor as the smaller tanks, but still nearly twice what the 50g tank has at 2 watts/gallon. (the reason I ask is that I'm currently setting up a tank like that, and after hearing the discussions here, I think I have too much light) I'm not sure that aquarium lighting falls off with the square of the distance, because there is quite a bit of internal reflection of the lights off the sides of the aquarium. It may be something like depth^1.5, but I really don't know. Thoughts? Steve

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