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Re: [APD] Pseudo AH Supply Reflector
Date: Sat, 19 Feb 2005 12:25:52 -0800
From: Vaughn Hopkins <hoppy1 at surewest_net>
Subject: [APD] Pseudo AH Supply Reflector
I now have the light fixture completed. Below are three more pictures
of the final product, on my 29 gallon tank, on the stand I built, but
the stand still needs to be finished. I used Luan Door Skins, from
Home Depot for the outer surface of both the stand and canopy. The
canopy is made of 1/4" hardboard, with a layer of the Luan glued on,
and the stand is made of 2 X 3's with the Luan on the outer surfaces.
This is really cheap "mahogany", but it looks good either stained or
just clear coated, which I am doing.
Looks really pretty, Vaughn. Nice job.
I have a couple of minor optical comments. These are general, and not
intended as a critique of your nice work. I'm sure your lights will grow
plants like mad!
The wrinkles will do little harm with the design you have. Most light
loss will be restrike, and wrinkles are just about as likely to reflect
light into the water as back onto the tubes.
Your structure isn't even close to the AH design, as it is a basic
flat-top with sloping sides. The AH design gets a lot of its efficiency
from the skinny (T5) tubes and the straight-line approximation of the
MacDonald's double arches cross section. With T8s, the flat-top
reflector will dump nearly half the light right back onto the tubes. In
the AH design, there is no flat spot above the tube reflecting light
directly back at the tube.
I didn't think that space blankets are mylar. It is usually, even when
quite thin, too stiff for such applications. The wrinkles tend to prove
me correct, as mylar is pretty easy to apply flat and mostly resists
wrinkling like that. The main consequence would be that the material
might not last forever. OTOH, I had potato-chip-bag reflectors that
lasted for years if I kept heat away from them (i.e., good ventilation
in the hood). Mine had lots of wrinkles, too. :-)
Unfortunately, aluminum is a rather poor reflector when compared to
silver or Al with enhanced-reflectivity overcoatings to get it above
about 85%.
You can easily test a piece of flat reflector material with a small
solar cell, a milliammeter, and a red laser pointer. Shine the pointer
on the cell and record the current.[Note: read current, not voltage.]
Shine it just past the edge of the cell and reflect it back with the
test material so the reflected spot again falls entirely inside the
cell. Read that current, and divide it by the first reading to get the
amount of reflected light to incident light ratio. Multiply by 100 to
get reflectivity percentage. Aluminum will give you about 80-85%, and
silver about 95%. That higher reflectivity may seem to be a small
difference but if a lot of light gets bounced off it 2 or 3 times, such
as water reflections or in a flat lid, it adds up fast. 80% of 80% of
80% is 51%. 95% of 95% of 95% is 86%. That can be a big difference and
is one reason the AH reflectors are about 160% better than any others --
superior reflectivity combined with an effective design for twin-tube CFs.
Wright
--
Wright Huntley - Rt. 001 Box K36, Bishop CA 93514 - whuntley at verizon_net
760 872-3995
...frontier society offered ‘the most civilized type of association’
because it had ‘the absolute minimum of external regulation’ and
therefore ‘the maximum of voluntary civility and morality.’
------ Isabel Paterson
http://www.libertarianism.com/
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