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Re: [APD] Pseudo AH Supply Reflector

I agree that my reflector is nowhere near an AH clone. I refer to it as a pseudo AH reflector only because I started my canopy design to use the AH reflector, then got fascinated by the challenge of making my own. A bit of sketching took me back to my college physics class days and ray tracing. So, I took the AH sketch that they use to show how their reflector works and enlarged it to scale, tried to duplicate it for a three tube set up and gave up that idea. Next I just played around with various flat plate angles using ray tracing to see if I could get what I wanted. My goal was to make the light intensity be great enough over the whole top of the aquarium that the plants wont all lean towards one area of the light. I used the "space blanket" just because it was cheap and is claimed to reflect 80% of the incident light. (I expect the blanket will soon need to be replaced, since some of the aluminum coating has already started to tarnish or separate.) I figure my 60 watts of light is adequate for a 29 gallon tank even if I get no reflection at all. I didn't save any money at all over the AH Supply kit, and if I were starting over I might just buy the kit, but I did have a lot of fun. Fun is my goal in any hobby!

On Sunday, February 20, 2005, at 10:16 AM, Wright Huntley wrote:

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 Huntley - Rt. 001 Box K36, Bishop CA 93514 - whuntley at verizon_net
760 872-3995

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