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Reflector measurements (long)

OK, at last I got light measurements to compare different kinds of
reflectors for use in an aquarium fluorescent hood. Despite the 
excellent material already available at the krib, in this archive, and
other sources, I wanted to see it by myself, since there are some
inconsistent results out there. I was particularly interested in seeing
if adding an expensive polished aluminum parabolic reflector would
be worth the cost of it, as compared with cheaper techniques such
as white paint or aluminum foil on a flat surface.

For the test I used my 20 gal aquarium hood, which is equipped with a 
single 20 Watt T12 GE Plant & Aquarium bulb about 6 months old, driven 
by a magnetic ballast. The hood was laid on a table (in a darkened room) 
resting on its side, so the light points sidewise instead of down. At a 
certain distance of the hood I hanged vertically a small piece of white 
paper, aligned with and facing the center of the hood. This paper acted 
as a diffuse target. I took readings of the target brightness using an
Olympus OM-2 SLR camera with a 200 mm lens. I also covered the entire 
area around the target with a piece of black cloth to avoid reflections 
from surrounding surfaces that could spoil the measurements.

I got readings at four different lamp-target distances, and using four
types of reflectors:

- the hood is entirely made of black acrylic and its internal surface 
  is pretty rough, an almost perfect "black matte". So the hood by 
  itself, with just the light bulb in place, acted as the "no reflector" 

- lining the entire hood internally with white paper (regular A4 sheets
  of Xerox paper) simulated a diffusive reflector. It can be argued 
  that white paint would make for a somewhat better reflector, and that's
  probably true.

- I also lined the hood with regular Reynolds aluminum foil wrap, shiny 
  side outwards. It is almost impossible to make a good specular reflector
  out of this material, it gets wrinkled very easily. I attempted to lay
  it out so the surface was as smooth as possible.

- lastly, I used a Triton Enhancer reflector ($25), made of polished 
  aluminum sheet bent into a paraboloid shape.

So here are the results (in lux):

Distance     No refl.    White     Alum.    Triton
in inches                paper     foil

   29         100        180       120        350    
   22         180        290       210        550     
   16         290        460       350        960     
   11         460        760       570       1560 

A photographic photometer is not a very precise device. By repeating each
measurement several times, each one with a different focusing ring 
adjustment and/or slightly changing the framing of the target, I estimated 
that I can tell apart light levels that differ by 1/4 of f/stop. That
translates to about 18% precision, or equivalently a plus-or-minus error 
of approximately +-10%. 

Even with that modest precision, the differences among the different
reflectors are large enough to stand out clearly. There are a couple of 
interesting effects that we can see if we plot the data as a function of 
the inverse distance. If anyone is interested, e-mail me off-list. 

The important point I want to emphasize here is the large gain provided by 
the expensive reflector. It more than triples the light, as compared with 
a no-reflector situation. A white diffusive reflector does provide a 
more modest gain, of the order of 1.6-1.8, not bad if we consider its 
cost (nothing). The aluminum foil reflector, on the other hand, is barely
worth the trouble. Aluminum foil would possibly make a good reflector only if 
it can be laid out smoothly as a polished mirror. The wrinkles on the foil 
turn it out effectively into a diffusive reflector, and a bad one...

This suggests me that the use of reflective flexible materials such as
Mylar and similar ones does not provide a significant gain above what one 
can get with a simple white paint. They can be even worse than the white
reflector, as in my test. The reason is the difference between specular 
and diffusive reflection. Unless the reflective material can be laid out 
with a smoothness comparable with a true mirror, it will tend to act as 
a diffusive reflector. And will most likely perform below the "perfect" 
diffusive reflector, a white matte surface. Stretching this reasoning a 
little, *any* shinyness in the reflective material is prone to *decrease* 
the reflector effectiveness, unless it is *perfectly* smooth as a mirror. 
That is, unless you can see your image clearly reflected when looking into 

An important point to consider though, is that with this particular 
Triton reflector, probably the large gain only applies when the reflector 
is used with a single tube. The dimensions of this reflector and its matched 
lamp holders suggest that it is optically optimized to work with a single 
T12 bulb. Nevertheless, the manufacturer claims that the reflector can 
be used with two or even three tubes. With two tubes, the gain would not be 
that impressive. The tubes cannot be placed precisely at the parabola focus 
anymore, and that completely breaks down the reflector optical properties. 
Also, the amount of self-shading and restrike gets large with two T12 tubes. 
With three tubes, just forget it. The effect will probably be to *decrease*
the light as compared with a no-reflector situation, due to almost complete
self-shading and tube overheat.

Of course, these are "dry" measurements and in a real situation with water,
things can be quite different. However, my goal here wasn't to get absolute
values of light level at the inside of a water-filled tank, but to get a 
*relative* comparison. Most likely, the relative performance of these 
reflectors will still hold when used over a water-filled tank. I would
expect a slight decrease in performance of the diffusive reflectors as
compared with the Triton, due to large angle of incidence effects at the 
water surface.

Btw, I cannot use the 20 gal aquarium to get these measurements, because 
it is configured as a half-filled paludarium with lots of branching 
driftwood and plants between the lamp and the water surface. I guess I'm
gonna need an extra 20 gal to continue this...

-Ivo Busko
 Baltimore, MD