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Some Notes On Fresnel Lens
Hello good people

I have been doing some research on fresnel lens, and how we use them to start fire.

1. There are two kinds of lenses, one focuses to a line, the other to a spot. The spot is more intense, and a higher temperature. Most of the smaller one that are sold are spot types.

2. Face the ridged side toward the sun. If you have the flat side pointed to the sun, and the ridged side down, the focus will be a bit scattered. This is due to something called spherical aberration, and lens is made to compensate for it.

3. Size matters, multiplying factor does not. The larger the lens, the more light is collects.

4. Older rear projection TVs have better fresnel lens then the newer rear projection TVs. When salvaging lens for old rear projection TVs, you may find the line type or the spot type. The older the TV, the better the chances that it is spot type. The older lens were made a bit thicker too. This is my experience based on salvaging two TV lens. An easy way to judge the age, the older TV shells were mostly made with wood, the newer ones more plastic. There are three high quality glass magnifying lens in the TV as well, make sure to salvage them too.

5. They are dangerous. Make sure use eye protection, I use welding goggles. Store them in a safe manner, covered.

There is a lot more to learn about these wonderful tools. I would like to encourage all of you to salvage a rear projection TV when you can get them for free. Look on Craigs list, people are trying to get rid of them.

Good information, thanks Bill.

Hopefully the S won't HTF and I pray every day that it won't. It would not be fun.

I have a high art..I wound with cruelty, all who wound me...Archillocus; 650 B.C.

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