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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