AQUAMYTH #1 More Light for Deep Tanks -- NOT!

This is one of a (proposed) series of posts aimed at exploding (or at 
least questioning) some of the mythology that is frequently repeated 

I'll start in my area of expertise, for I have been involved heavily in 
optical engineering for over 30 years. Your flame opportunities will go 
up dramatically, as I shift to subjects that are not so close to home. 

If the lamp housing is on, or close to, the top of the tank, the amount 
of light reaching the bottom does not depend on depth.

We aren't talking the Monterey aquarium here, but a tank 1m deep and a 
tank 0.5m deep will have virtually identical illumination at the 
substrate.  Check it out with any light meter, keeping the 
lamp-to-water distance constant, changing only depth.

A distributed source, like fluorescents, couples light into the water. 
with inside-the-tank propagation within what is called the critical 
angle. Light hitting the sides of the tank is totally reflected, for it 
is beyond the critical angle. It is absorbed or reflected by objects in 
the tank, but keeps going until it finds such objects, like a huge 
light pipe.

The situation is only slightly different for a pendant MH hanging well 
above the water, in that the the total light in the tank depends 
greatly (by roughly 1/R^2) on the lamp-to-water distance. Once the 
light is inside the water, however, the propagation is still in a light 
pipe, and only absorption and scatter cause light to be lost. The water 
absorption, at normal aquarium depths, can usually be ignored, unless 
you are horribly addicted to overfeeding. <g> There is a little more 
absorption of the valuable red light in thick-wall glass tanks, but 
this is mostly a secondary effect of concern only in FW tanks.

It would be nice if some of the old-timers would quit advising that 
more light is needed for deeper tanks. In general it isn't. I recognize 
that heavy plant growth absorbs a lot of light, and a really thickly 
planted tank may use some more Watts (NOT lumens, by the way), but 
that's the subject of another missive. The point is that it is 
certainly not the depth that calls for more light.



Wright Huntley    o  (408) 248-5905  Santa Clara, CA  huntley at ix_netcom.com
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