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Fresnel lens

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Fresnel lens

Postby Lynn LeFey » Fri Feb 03, 2012 12:01 pm

Some time back, I had some old electronics and metal scrap I wanted to have hauled off. I called a local scrap guy, and while talking to him, happened to ask if he had any fresnel lenses from the front of old broken big-screen projection TVs. He did. So he brought his lens and picked up my junk.

This lens is about 2'6" by 3'3". It will instantly ignite wood, and flash boil liquid in full sunlight. I don't have a frame on the lens (which makes it a little hard to hold a single good heat point, since it's a little 'floppy') so I haven't brought any significant volume of water to boil, but I feel confident in saying that in a stainless steel container painted black with oven-safe paint, it'd boil water faster than any stove.

I urge you all to look into local scrap dealers, or on Craigslist, because these big old projection TVs are on there ALL the time, it seems. Odds are you can pick these things up super-cheap or free.

IF you want some ideas as to the power of these things, look up 'Greenpowerscience fresnel lens' on youtube. The guy that runs the channel has done a ton of stuff with them.
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Re: Fresnel lens

Postby Onegreenguy » Fri Feb 03, 2012 12:53 pm

Oh yeah...haven't got one yet but it's pretty high up on my list. They have a lot of uses some will even melt certain metals!
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Re: Fresnel lens

Postby Lynn LeFey » Fri Feb 03, 2012 2:29 pm

I just checked with a local electronics recycling company, and the owner told me he'd set them aside for me. When I asked what he wanted for them, he said 'nothing'. If I can get a few of them, I'd be willing to barter or sell them pretty cheap, if others don't have a supply available.

I just checked e-bay, and there are ones a bit larger than mine running for $30 (I didn't look at shipping cost). The claim on that one (about 40" by 30") was 12 oz. boiled in 7 minutes. So mine may not actually be faster than any stove. I've seen esbits boil in (if I remember right) just under 5 minutes. Still, Fresnels just need sunlight to work, and are about as reliable a piece of technology as I could ever think of (no parts, won't rot, etc). I wouldn't leave them out in the weather though.
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Re: Fresnel lens

Postby karlsgunbunker » Fri Feb 03, 2012 7:08 pm

I have the Credit Card Fresnel Lens' in my prep kits as fire starters.
I also have the 8 1/2" x 11" size.
If I can calculate the focal length and build a frame so they all focus on the same point it could be "Interesting". :D
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Re: Fresnel lens

Postby MrDanB » Fri Feb 03, 2012 10:00 pm

I have been interested in picking one up for a while now! Living in the Seattle area, I may be looking at much longer times to boil water, but for keeping shower water warm it might just be a handy tool! The price can't be beat either!
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Re: Fresnel lens

Postby Lynn LeFey » Sat Feb 04, 2012 2:25 pm

I watched a bunch of Greenpowerscience videos on youtube, and wondered how much these things would cost to acquire. I simply couldn't believe it when I found out that folks have no idea what these things are good for, and don't care in the least about them, so they give them away for free. People who own the big-screen TVs consider the TVs complete garbage when they no longer produce a picture, and the folks who get the broken TVs for scrap aren't interested in the lenses, since they apparently have no scrap value.

It really IS just as easy as asking to get one. I seriously urge folks to try finding them, either on your local Craigslist (which might be your nearest major metro area), or see if there's a freecycle group near you. Barring that, you can try recycling places, or maybe put an electronic want add in local papers (if they have an online edition). Curb surfing also works. Sometimes folks just set them out to be picked up by garbage trucks.

Karisgunbunker,
Because of the way the beams focus (which is directly away from the sun on any given lens), you can't get multiple lenses to join their focal points, at least not without additional mirrors or some such. If you wanted to go that route, I'd consider parabolic mirrors, which CAN have multiple focal points converged. Parabolic mirrors can be 'aimed'. While those are awesome, I can't find them free/cheap.
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Re: Fresnel lens

Postby RadioHeadPrepper » Wed Apr 04, 2012 5:33 pm

Lynn LeFey wrote:Some time back, I had some old electronics and metal scrap I wanted to have hauled off. I called a local scrap guy, and while talking to him, happened to ask if he had any Fresnel lenses from the front of old broken big-screen projection TVs. He did. So he brought his lens and picked up my junk.

This lens is about 2'6" by 3'3". It will instantly ignite wood, and flash boil liquid in full sunlight. I don't have a frame on the lens (which makes it a little hard to hold a single good heat point, since it's a little 'floppy') so I haven't brought any significant volume of water to boil, but I feel confident in saying that in a stainless steel container painted black with oven-safe paint, it'd boil water faster than any stove.


I did this only with different results. My TV projection lens produced a stripe, not a spot so a solar death ray it is not. Perhaps some TV's have different lens configurations but after taking a rear projection set apart you can see where a stripe not a spot is what they need since the RBG guns are in a line and not on top of each other.

I guess my mileage did vary. :o
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