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More reflector measurements

A few days ago I posted some measurements that compare different types
of reflectors 

These measurements where made "in air". Then I got hold of a spare 20 
gallon tank that enabled me to re-do the measurements under actual, or 
almost, conditions found in aquaria. This test aquarium was kept bare. 
It has a black acrylic bottom pane where I put a white china saucer to 
act as a diffusive target. The aquarium was filled to the top, which 
resulted in a depth of 13" between the target and the water surface. 
The same hood I used before was lined with the same materials, top, 
front and back internally. The hood was put on top of the aquarium such 
that the distance between the light bulb and the water surface was 1"3/4. 
Measurements were taken in the same way as before, using my SLR camera. 

Here are the measured illuminances, in lux:

no reflector:        500
white paper:         910
aluminum foil:       840
Triton Enhancer:    1680

So again we see the same behavior as before, somewhat altered by the
effects of total internal reflection inside the tank. All four measurements
are significantly brighter than their in-air counterparts, even at a
14"3/4 target-bulb distance, as opposed to the minimum 11" distance used
for the in-air measurements. So we may conclude that internal reflection
_does_ take place, and it _is_ an important effect. However, the tank was 
clean and bare, and in real tanks the effects of plants and dirty glass may 
subdue internal reflection effects in way that might be very difficult to

The gain factors provided by the different reflectors when compared with
the no-reflector (or black reflector if you wish) case are similar in
the in-air and in-water cases, given the measurement precision. The only
one that is a bit off is the aluminum foil reflector. However, this could 
be explained by the fact it is almost impossible to build consistent 
reflectors out of this material. Even small unavoidable wrinkles seem to
lead to very different reflecting properties. 

Anyway, the fact that in-air and in-water measurements show the same 
relative gain factors for the tested reflectors suggests that any
"optical coupling" effect that might exist between the light fixture and
the water-filled tank, and which would favor diffusive reflectors, is 
minimal. In other words, specular reflectors that are able to redirect 
most of the light into a parallel beam perpendicular to the water surface, 
offer a large gain over reflectors that spread the light over all directions, 
such as diffusive reflectors, even when these are kept close to the water 
surface. I would even dare to suggest that specular but plane reflectors 
do not offer a significant improvement over diffusive ones, just because 
they do not form a parallel reflected beam, but reflect light in a pattern 
similar to the pattern of reflection created by a Lambertian diffusive 

The problem I find with those parabolic mirrors is that they only work 
well with a single bulb. Move the bulb a little off from the focus
position, and the reflected beam completely breaks down. It remains to be 
seen by how much the performance of one of these reflectors degrades with 
say two tubes. More measurements on the way, I' afraid...

-Ivo Busko
 Baltimore, MD