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Re: [APD] How much light actually gets down here?

There some easy answers, but they are at best just rules of thumb. 
Answer 1) Measure it  with a PAR meter and numerous locations at the bottom, especially locations not directly under the bulb.
Answer 2) Figure that the spectral output is reasonably good for aquatic plants and, after having gotten a lux value (which measure the lightfall on a given area) from the manufacturer or by measurement, roughly figure that the lux will drop proportionally with the distance (or as a square of the distance if the light was a point source instead of a line source). You have to figure roughly since a variety of factors come into play, including reflected light from the sides.
Answer 3) Use watts per gallon as an ersatz standard and figure that changes in wpg will yield generally equivalent changes in how much light reaches any given area in the aquarium -- double the wpg, you'll double the light, other things being equal. When using wpg, figure that roughly 2wpg is medium, 3 or 4 high and less than 2 is low. But those are only guidelines since your actual results will depend on a wide variety of factors including, ambient temperature near the bulb(s), ballast, reflector quality, bulb spectral output, input voltage to the ballast, water clarity, the amount of etching that has occurred on the cover glass, et al.
Answer 4) Watch your plants and gauge accordingly. Try more light if you want faster growth or to grow some of the plants that stubbornly require a lot of light. But keep you eye on the plants -- if you increase the light, be sure to keep tabs on CO2 and other nutrients -- they will be used up faster and if you don't keep up, you could provoke a burst of algae.

Lastly,  65w should work well for many plants in most circumstances on a 37g tank. Put the more demanding plants towards the center and under the light source, other locations will tend to receive less light.
good luck, good fun,

----- Original Message ----
From: Erin Poythress <anang3 at yahoo_com>
To: aquatic-plants at actwin_com
Sent: Thursday, May 3, 2007 9:45:04 AM
Subject: [APD] How much light actually gets down here?

Hi everyone,

Anyone know a reliable way to estimate how much of my
65W PCF reaches to the bottom of my 37G? The tank is
22" high and the legs of the light have it sit 1.5"
above the glass lid (which is a necessity because of
two very curious kitties). I tried to sit it flat on
the lid, but the plastic joint got too hot for my

Low-light plants grow reasonably well in there, but
I'm considering an upgrade and wondered how much light
would reasonably reach the bottom.
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