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Lighting, from another angle
There has been a very interesting thread about lighting. Lumens per Watt has
been well explained. Believe it! You will see that the folks explaining
this are basically telling us that there is no free lunch. They are telling
us, quite accurately, that No single light source is all that dramatically
brighter per watt than the others, and T-8 bulbs are a darned good choice for
several reasons. They are economical, they are good in lumen per watt
efficiency, many of them have built in reflectors, and you can pack a whole
lot of them in a hood! (They are also readily available and pretty darned
cheap, compared to some of the other choices! :-) )
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 )
This Law of Physics also applies to other sources of energy, such as radio
waves, etc. (Anybody out there brave enough to get into explaining how radio
waves and light visible to humans light are part of the same spectrum of
energy, which at one end eventually ends up as Gamma Rays? Not ME!!! :-) )
For a simple quickie, lets put the fluorescent tube 2 inches from the water
surface, and then raise it to 12 inches above the water surface. 2 x 2 is 4,
12 x 12 is 144. (squaring the distances) 4 / 144 is 1 / 36. The intensity of
the light at the surface is 36 TIMES AS BRIGHT when the light is 2 inches
from the surface as it is when the same light is 12 inches from the surface.
On the tanks here at the Aquarium Center (Des Moines, Iowa) the difference
in light intensity at the BOTTOM of the tank (the gravel) is (rounded) two
and a half times as bright at the 2 inch from the surface position as it is
at 12 inches from the surface. An incandescent light works just exactly the
same. We pay 15 cents per KWh for electricity for some 90 odd exhibit tanks,
and you can see why I am busy lowering lights and cutting the number of bulbs
down.
The reason for mentioning this is not to stir up problems and lead to
complicated calculations. I just wanted to stress the fact that you want
your light sources to be AS CLOSE TO THE SURFACE as is practical. A really
good reflector can almost double the intensity at the BULB. Aluminum foil is
quite good, but a good coat of white paint isn't all that bad. You can lose
a lot of the effectiveness of your expensive lights by just having them a bit
too high.
Hope this will be helpful