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Ugly Duck
07-20-2010, 01:07 PM
I saw this Youtube video of a guy melting a padlock into a puddle using a giant Fresnel lens from an old rear projection TV. Thought that was kewl, so I went to the recycle center & took out the lens from a discarded big screen & focused the sunlight onto a new penny. At 5 seconds the zinc melted out of the copper, at 7 seconds the penny started to vaporize (burned a hole right through it). Any longer than 7 secs & there wouldn't have been anything left. This thing ignites wood into flame in a fraction of a second.

Try it! But don't leave the lens laying around uncovered or you'll start a fire. Even the light coming in from a window can be enough to burn your house down. Who wudda thunk TVs had such destructive powers?


http://i426.photobucket.com/albums/pp348/UglyDuck2/destructo.jpg?1279652435

(video is boring until the 5:00 mark - jump ahead)
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Radar Chief
07-20-2010, 01:20 PM
Thatís not a fresnel lens but its still cool. :thumb:

mikeyis4dcats.
07-20-2010, 01:23 PM
that's a bit scary

Ugly Duck
07-20-2010, 01:28 PM
That’s not a fresnel lens but its still cool. :thumb:

Same principle as the Fresnel lens in a lighthouse, but instead of separate segments focused onto a spot there are concentric grooves cut into acrylic that refract the light onto a spot.

http://fresneloptic.com/images/solar_fresnel_lens.jpg

seclark
07-20-2010, 01:35 PM
you're wasting good money, ud. you should be putting those pennies on a train track and smashing them. much better investment.
sec

Dave Lane
07-20-2010, 01:46 PM
Is that the size the lens is?

cdcox
07-20-2010, 02:24 PM
So if you mounted one of those inside a giant frisbee you could fling it through the air and leave scorched earth in the path of flight.

Detoxing
07-20-2010, 02:27 PM
So if you mounted one of those inside a giant frisbee you could fling it through the air and leave scorched earth in the path of flight.

Or attach it to a parasailing donkey.

Ugly Duck
07-20-2010, 02:39 PM
Is that the size the lens is?

The lens I used is 2.5 x 3.5 ft, but there's even bigger ones out there. Generally, the older the TV, the better the lens. After the late '90s they learned how get better video out of cheaper plastic. Also, it looks like the ones with lower aspect ratios tend to be the hotter ones.

Hog Farmer
07-20-2010, 04:34 PM
Seems as though there would be an application with that thing to heat your water and your home in the winter.

AustinChief
07-20-2010, 04:53 PM
Seems as though there would be an application with that thing to heat your water and your home in the winter.

no kidding...

vailpass
07-20-2010, 04:57 PM
This is very cool, my boys would dig the shit out of doing this. When you say 'lens' do you mean the actual TV screen or something behind the screen?

Hog Farmer
07-20-2010, 05:04 PM
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Ugly Duck
07-20-2010, 05:16 PM
This is very cool, my boys would dig the shit out of doing this. When you say 'lens' do you mean the actual TV screen or something behind the screen?

There's 3 panels to the screen. The outer one is a protective sheet, the middle one has a buncha vertical lines... its the inner one that is the Fresnel lens - has concentric grooves cut into it.

Another BIG word of caution... don't look directly at the focal spot! Its so bright it will burn your retinas - think arc welding x 2. I use welder's goggles with flip-down #14 lenses.


Hey HogFarmer.... check out what happens when you point it at a Sterling engine:

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vailpass
07-20-2010, 05:44 PM
Thanks Duck. While searching for Fresnel lenses I came across this cool site, at least it's cool if you have boys or are still a boy at heart...

http://www.sciplus.com/index.cfm

Gadzooks
07-20-2010, 05:52 PM
That first clip is like the light that comes through my office window every morning, (I would be the lock).

Hog Farmer
07-20-2010, 06:22 PM
Just think what terrorists could do with these. If they got them positioned at night in the right spots, when the sun comes up they would ignite fires, blow up fuel tanks, all kinds of bad stuff. I wonder if homeland security is aware of this ?

Hog Farmer
07-20-2010, 06:23 PM
I think I'm gonna get one just to try to light my wifes cigarrette.

Ugly Duck
07-20-2010, 07:03 PM
I think I'm gonna get one just to try to light my wifes cigarrette.

Oh - I get it..... You would both light her cigarette and give her a nose job at the same time - very frugal!

More penny destruction:

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sandynme
07-20-2010, 07:45 PM
duhhhhh......

teedubya
07-20-2010, 10:30 PM
what kind of TV's should I be looking for? I want one of these. LOL.

Think how easy it would be to start a fire in nature.

'Hamas' Jenkins
07-20-2010, 10:33 PM
Triple Rainbow?

CosmicPal
07-20-2010, 10:37 PM
Broncos fans just found a cure for their genital warts.

:D

teedubya
07-20-2010, 11:13 PM
Triple Rainbow?

almost

Ugly Duck
07-20-2010, 11:21 PM
what kind of TV's should I be looking for? I want one of these. LOL.

Think how easy it would be to start a fire in nature.

Remember back when "big screen TVs" were rear-projection setups? The ones with the three different-color lenses inside? Those are the ones. Big CRTs or flat-screens are no good. I gave a dude at a recycle center a six pack & he let me take six Fresnels. The wide screen ones don't seem to burn as hot as the ones that are more like a square.

Some guy on eBay is slapping a cheap wood frame on them & selling them for about $100. Some are $250+ on his website. Unemployed individuals could probably make rent money doing that.