Demystifying lighting

IMHO you guys are making this difficult for non-scientific folks reading
this list to understand. In typical aquariums with sufficient lighting
to grow plants, the 1/r**2 approximation for light intensity does NOT give
a good approximation of how the light varies in relation to the distance
from the source. There are a number of factors which come into play
including the reflectivity of plants, the shape of any reflectors in the
hood. Since the sides of a tank of typical dimensions does cause about 95% 
of the light striking the sides to be reflected internally, in a tank empty
of plants, the light intensity is determined by the total light flux
entering the tank divided by the surface area of the bottom. When plants
are in a tank, they create shadows within the tank, effectively using
up a good portion of the light shining into the tank. The amount of
plants you can put into a tank (ie. the surface area of plants) is
roughly proportional to the VOLUME of the tank and this is why we give
the empirical rule 3-4 watts of fluorescent (or equivalent) lighting
per gallon of tank capacity as a MINIMUM. With proper precautions for
algae control, you can safely exceed this lighting level by using
metal halide (MH), halogen and even direct sunlight! Plants grow
much, MUCH better when they have more intense light.

If the tank has insufficient lighting from a linear source like a single
tube over a large tank, then you can't approximate light intensity in
the way I described above because the 1/r**2 effect becomes more
predominant. 1/r**2 needs to be considered when the width and length of
the tank is greater than the depth of the tank and there is only a
single light source, not rows of FL bulbs across the entire top of
the aquarium.

If you have a tank with tall plants that are shading out the lower
plants then you have a different kind of problem. This would be a
poor way to aquascape your tank because you don't want significant
shadows on the leaves of plants which are growing and which you
don't want to turn yellow and drop off. You could easily have this
if you planted a tank densely with a stem plant like Hygrophila
and let it grow right up to the surface of the water. BTW, this
partly explains why a nice bushy bunch of H poly looks much nicer
than a tall gangly stand which hasn't been pruned regularly.

A little mind puzzle for you to test if you understand how internal
reflection works: Two tanks, Tank A is 4' long, 12" front to back
and 18" in height. It has 5 FL tubes at 40 watts each for a total
of 200 watts in a 50 gal tank. It is planted with short Crypts
which don't grow more than 3 inches in height at this time. Tank B
has the same volume but is 18" front to back and 12" in height
and is also lit by 5 40 watt FL tubes. It is also planted in an
identical way to tank A but since the tank is a little wider,
we can plant 50% more Crypts. However the tank isn't quite so
deep as tank A so this shouldn't be a problem. Both tanks have
nice clear water (this is important).

Which tank has a higher light intensity on the leaves of the 
plants? Don't reply to the APD with your answer. I'll post it
in a day or two. If you disagree with my explanation then we can
quibble later.

Steve P              Vancouver BC CANADA