[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
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"
reference.
- 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
it.
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