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Re: AQUAMYTH #1 lights in deep tanks

Wright Huntley wrote:
> Subject: 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 
> here.

Good! I always enjoy a nice explosion (as long as it's in good taste! ;-)
From time to time a too near detonation can send a few bits of our egos
splattering (Doom-style) against the upholstery. Often a little shaking
is exactly what we need and remember, as we pick ourselves up and dust
off the shards of the video display, to take comfort that the scientific
method always involves questioning and we are friends here for a bit of
fun, diversion and learning. I just wanted to say that; no particular
reason. ;-)

> 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.

In an tank with only water (no plants or algae), I'll go along with this.
In a tank with some degree of turbidity (lack of clarity) there would be
a diminishing but perhaps not as much as we'd think. As well, the
absorption coefficient of water would depend on the light frequency
as well. Can you give us any data on absorption vs. light color Wright?
I have a feeling you're ready to counter on this point. ;-)

> Light hitting the sides of the tank is totally reflected, for it 
> is beyond the critical angle. 

Another way to illustrate this is to hold a piece of paper near the glass
at the bottom of the aquarium. Hmmm... no light comes from within with
MH pendant ~18" from the water surface of a 28"deep (24"x24" base).
You can exploit this to your advantage by putting a high light plant at
the corners of such a tank where you get good lighting. Those plants
at the centre have the advantage of a considerable reduction in
intensity complements of the much TALLER plants which can surround.
There don't seem to be any distinct shadows even with very large leaves
like E cordifolius. This is due in part to having a diffusion lens in
my pendant as well as a parabolic reflector but also to the refraction/
reflection effect sending light into the "shadowy" parts of the tank!

> 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. 

Where Neil Frank can increase his lighting effectiveness by dropping
his lights, I can diminish mine by raising the lights.  The parabolic
reflector tends to make the dependency not 1/R^2 but I can still spill
lots of light. A proper reflector can do a good job of sending an almost
parallel wide beam of light straight down.

You have a very good point Wright. This particular bit of aquarium lore
is mentioned in The Optimum Aquarium (Horst & Kipper) p143 where
they quote 500-1000 Lux at the surface & 20-30 Lux at the substrate.
I suspect we reinforce some of the misconceptions when we observe the
results in deep vs. shallow tanks where all the factors have to be taken
into consideration. I think H&K must have been talking about a very
wide aquarium (perhaps the 2500g) where the 1/R^2 law is more accurate
due to the lack of close walls to provide reflection!

 Steve