Parabolic Lens and TV

OK I was talking with a friend about generating electricity, and the subject came up of using lenses to create heat. Maby run a sterling engine with one.

He mentioned a friend had removed the parabolic lenses from inside of a television. The lenses acording to him are strong enough to burn a hole on cloth and other materials.

Well I have to have one. I am always in the need of new toys. I have a few questions before smashing up some tvs for fun.

Ok first of all is there such a lens inside a tv tude.

If so

The burough land fill almost always has at least one tv hanging around and come christmas there will be plenty with all the flat screen upgrades. So the supply wont be a problem.

I have heard the dangers of latent electricity inside of a tv and the dangers even if it is off. I am aware of the dangers of the phosporus coating on the inside that is toxic. I am aware of the vacuume and resulting implosion/explosion if the tube is shattered. The air and resulting dust my be danerious as well.

I am still gonna get me some of the lenses though. If they are in there.

Has anyone here ever done this? Is there a way to get inside without damaging the lens? Do you have any good recomindations?

What is a parabolic lens?

I think he means parabolic mirrors and, as far as I’m aware, they are not required in a CRT; the focusing and beam manipulation is done electromagnetically.

It may be that some of the glass surfaces in the CRT are curved so as to function approximately as a concave mirror, but I don’t think that’s going to help.

BTW another significant danger in dismantling CRT devices is that some of the large capacitors can retain a lethal charge (reportedly) long after the set is disconnected from the mains.

Also, if you want a large, cheap, rough-and-ready convex reflector, there are easier and less dangerous ways to make one; here’s one I’ve often wanted to try:

Get a rigid container with a large diameter
Run a bead of silicone sealant around the rim
Stretch a large sheet of aluminised Mylar across the top (like a drum skin)
Fold over the edges and secure them to the sides of the container
Evacuate some of the air from inside the ‘drum’ - the mylar skin should take up a convex shape.

You could always build one of these :smiley:

I have no idea what you’re talking about – I don’t know of parabolic lenses or mirrors inside TV tubes.
If you just want to burn holes in things there’s no need to get a parabolic as opposed to a simple spherical mirror (or lens). And if the radius of curvature is large compared to the size of the item, you’re going to be hard-pressed to tell the difference anyway with optical testing.

Plenty of places sell large-area mirrors and lenses. Try Edmund Optics (formerly Edmund Scientific), or surplus houses.

If you’re really set on making a parabolic mirror, you can do what Robert Wood did a hundred years ago – rotate a big container full of mercury. (The idea was used in Raymond Z. Gallun’s SF story “Old Faithful” back in the 1930s. I note that there are several sites devoted to such liquid mirrors on the 'net). Except that people don’t like to be around mercury anymore.

There might be 3-4" parabolic MIRRORS inside a rear-projection TV. There certainly won’t be any inside a CRT-based TV.

Older projection TVs had a couple of large fresnel lenses in them. The new projection TVs probably have a large traditional lens, but I’ve not taken one apart to know for sure.

Traditional CRTs have no lenses in them.

A lens won’t create heat. It will concentrate radiated heat from the sun though, which is what I think you are trying to do.

Spot on. You need to find an old projection TV and fish out the fresnel lenses. Or you can buy them - this site lists a few handy places. Haven’t checked any of them out though.

An ebay auction for a 6" by 10" fresnel lens finished a couple of days ago. Keep a lookout!

I was going to say, maybe the friend’s friend didn’t use anything from inside a TV, he might have used the parabolic dish satellite antenna, lined with something reflective.

But Seven beat me to it with his cool reference .

An old-fashioned 8’ disk could probably smelt ore.

I wonder if the OP’s friend is talking about the front glass of an old-fashioned ‘goldfish bowl’ TV - these were pretty uniformly convex, so the reverse side would make a fairly good concave mirror, if lined with foil or similar (assuming you could separate it cleanly from the rest of the tube without breaking it, or killing or injuring yourself.