Lighting Myths, Revisited!
> From: JOlson8590 at aol_com
> Thus far, I have seen no mention of the Law of Inverse Squares as it affects
> lighting. Very briefly, the intensity of light (any light) shining on a
> surface varies inversely as the square of the distance from the light to the
> surface. This is true, regardless of the efficiency of the reflector, etc.
> (Unless, of course, you have a coherent source of light, such as a laser, but
> they are really lousy lights for growing plants. :-D )
Actually, it ain't necessarily so, specially for us. Wright Huntley and
others have brought in a lot of interesting discussions on this about a
1. The inverse square law only applies to POINT sources of light. Since
fluorescent fixtures are long skinny things, they're more apt to be
considered "line sources", which drop off light as 1/R (inverse), and if
you've packed a lot of them in, then in the middle of the tank it will
behave as a "plane source", in which there is NO dropoff. Reflectors will
heavily influence this behavior as they are able to focus light into a
2. You also have the effect of the side walls, which if clean will reflect
nearly all the light in the tank back into the water due to an effect
called TOTAL INTERNAL REFLECTION. A little light gets out at the top, but
not much. This means that in theory almost all the light will stay in the
tank, irregardless of depth! This only applies to the light once it has
hit the water, so it's ideal (as the original poster said) to position the
source so as much light as possible enters the water, i.e. close to the
surface, or for MH, focused on the surface.
3. On the other hand, you also have other effects that reduce your
intensity: Plants, Algae, decorations and fish all absorb most of the
light that touches them. This is sort of a "Duh", but have to mention it,
because it modifies effect 2 to say "get enough light so that it can be
distributed over the countour area of plants"... If you have plants that
grow in a sort of "bowl" shape, up the sides of your tank, you will need
more light than if they are all low-growing on just the bottom part.
4. Crud in the water (humic acids, unicellular algae, etc) absorbs
light exponentially (intensity = e^ [- (K * depth)]. So for real-life
cases, depth does matter.
Erik D. Olson amazingly, at home
eriko at wrq_com