Armed Polite Society
Main Forums => The Roundtable => Topic started by: Brad Johnson on November 18, 2015, 11:34:14 AM
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Friend of mine is moving and had an older rear projection big screen TV he was going to toss. I pulled the optics and panel before Dempsterizing commenced.
Okay, big whoopie. But wait....
What that means is I now have in my greedy, sweaty, evil-genius hands is a giant Fresnel lens - a 3'x4' magnifying glass. Muhahahaha!! I can zap ants in China with this thing.
I also have three 3" projection quality multicoated glass lens groups that I'll disassemble for the convex optics. Always did want a really nice magnifying glass. Might see if I can cobble up a retro style handle/holder for them.
Brad
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Swing by my office....
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Keep an watch on your private message folder, for grid coordinates [ar15]
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If you don't disassemble them, those projection lenses can make pretty fine microscopes. I've got a couple of lenses out of slide projectors and overhead projectors which I use at least weekly for close examination of various things.
The image is fuzzy around the edges, but in the center it's fine. Quite surprisingly powerful. And great for the kids to use. (Might be able to make a science fair project out of this, no? Build a simple Leeuwenhoek microscope for comparison and let the "audience" look through both.)
After WWII, my Pop used to haunt the numerous surplus stores and buy all sorts of technical goodies, One of them was a huge aerial camera lens, about 10" in diameter, maybe a foot. That thing would melt concrete, I think. Weighed a lot.
I also had an 8" or so reflector from a Brownie camera flash unit, focal length about 10" to a foot. Took either 40-sized flashbulbs or with an adaptor, M2-sized. That thing would make tissue paper burn almost instantly.
I almost had an accident with it the first time I tried it in sunlight, since the target tissue started up so suddenly and flared so quickly I had to drop it and started a secondary fire in the normal scattering of dried leaves in the fall. Stamped it out, but not without some panic arising in my little fifteen year old brain. Mom wanted to know how come my new sneakers got so black so soon after purchase.
Moral 1: Don't use tissue paper or other light kindling for a focal target while 'speriminking.
Moral 2: Don't 'sperimink outdoors around other flammables, expecially in the Fall of the year, and especially with a light breeze going.
Moral 3: Don't wear your new sneakers while conducting these initial 'speriminks.
Terry, 230RN
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My college has Joseph Priestly's "burning glass," which he used in his experiments that led to his discovery of oxygen.
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I've got a couple of crystal balls and it turns out the focal length is about a third of the diameter away from the surface, more or less. (There's a formula for that.)
There are several documented cases where they've started fires, especially if they're on a windowsill, where the sun's image can impinge on the drapery.
There's also a (documented) case where one started a fire while sitting on a couch and the sunlight coming in the window struck the crystal ball.
So I often wonder when I see glass figurines of any kind or glass vases in a window whether they're a fire hazard. Unlikely, with the broken up focal lengths of an irregular glass figure, but Murphy lurks everywhere.
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Tried the internal optic group on a piece of scrap wood today. The wood charred and began smoking the moment I hit focal length. The big lens is gonna be even more fun. >:D
Brad
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What that means is I now have in my greedy, sweaty, evil-genius hands is a giant Fresnel lens - a 3'x4' magnifying glass. Muhahahaha!! I can zap ants in China with this thing.
What's the focal length? I'm thinking a frame and stand for it, adjustable to track the sun, would be useful; essentially the focal point becomes your new forge.
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All these promises of death rays, yet no photos. Hmmmm.
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Oh, OK, here's my small 80mm crystal ball.
See, it's sort of like a round ball made of glass. :facepalm: :rofl:
I've also got a 4" magnifying glass, real convex/convex, but part of it is a Fresnel lens. About 10" focal length. Don't know why part is Fresnel, but most of it is a true convex/convex. If I can dig it up I'll post a pic.
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Oh, OK, here's my small 80mm crystal ball.
I saw that coming.
I was actually wanting to see what Brad has cobbled together.
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(https://armedpolitesociety.com/proxy.php?request=http%3A%2F%2Funitednuclear.com%2Fimages%2Flasersign.jpg&hash=e66313788ad0d052c5031d327923e2e19fd09ffd)
http://unitednuclear.com/index.php?main_page=product_info&cPath=27_82&products_id=585&zenid=e382e3a50c39ff724f67b333e94b9b10 (http://unitednuclear.com/index.php?main_page=product_info&cPath=27_82&products_id=585&zenid=e382e3a50c39ff724f67b333e94b9b10)
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What's the focal length? I'm thinking a frame and stand for it, adjustable to track the sun, would be useful; essentially the focal point becomes your new forge.
[/quote]
Might be a good use for a astro-tracker.
http://www.bhphotovideo.com/c/search?=iOptron+SkyTracker+Camera+Mount+with+Polar+Scope+%28Black%29&N=10979344&InitialSearch=yes&sts=pi (http://www.bhphotovideo.com/c/search?=iOptron+SkyTracker+Camera+Mount+with+Polar+Scope+%28Black%29&N=10979344&InitialSearch=yes&sts=pi)
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Something like this. The TV you briefly see at the beginning is the same brand and model I got my parts from.
https://youtu.be/jrje73EyKag
Brad
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Couple that with a couple of those lenses for maximum focus and ya will have yer deth rey fer sure!
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Holy smoke !
(Somebody had to say it, so I volunteered.)
Hmmm....
I read somewhere that you can figure on 1000 watts from the sun per square meter at temperate latitudes.
Figuring that lens was about 0.6 sq meter, and the image was, say, 5 cm in diameter, call it 0.002 square meters, that spot must be getting roughly:
0.6/.002 X 1000 watts = 300,000 watts.
That don't sound right nohow. Where did I go wrong?
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Assuming no loss in the optics, the spot is getting 300,000 watts-per-0.6-meters, or 500KW/sqMeter, but the spot itself is only getting a total of 1000 watts.
Also, sunlight is about half-infrared, a bit under half visible, and the rest UV. Are those wavelengths all going to have the same focal point? Probably lose quite a good bit there from them focusing differently, and loss in the optics.
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Oh, OK, here's my small 80mm crystal ball.
See, it's sort of like a round ball made of glass. :facepalm: :rofl:
Still doesn't help. Got any schematics?
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All these promises of death rays, yet no photos. Hmmmm.
He keeps bursting into flames every time the flash goes off.
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That part-Fresnel-part convex/convex lens I mentioned above.
Focal length ~10"
Fresnel engraving only on a portion within the "frame."
Anyone have a clue what it might be for?
Sorry, hard to eliminate lamp reflections on a convex surface, and I messed up the color balance a little.
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Anyone have a clue what it might be for?
Ants, duh.
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^ :rofl:
Okay, let me rephrase that.
Anyone have a clue what it might be from?
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Looks almost like the screen to a 1952 Philco...
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Guesses and questions from a techno-dummy: Does the frez lens spread the light over the entire convex surface, thereby concentrating the energy? Could that magic hotspot be focused onto a mirror and therefore redirected as a "spot" of intense energy?
Another glass of red wine and I'll solve all the world's problems!
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Think Thin version of a regular lens by use of "steps."
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Guesses and questions from a techno-dummy: Does the frez lens spread the light over the entire convex surface, thereby concentrating the energy? Could that magic hotspot be focused onto a mirror and therefore redirected as a "spot" of intense energy?
All the parallel rays of light hitting the fresnel lens get bent so they all get together (are concentrated, converge) on one spot at the focal distance.
This is the same as a regular lens, except they brought the curved surface of a regular lens down to a flat surface with concentric rings, each ring reproducing a "ring" of the original lens:
http://80.36.73.149/almacen/medicina/oftalmologia/enciclopedias/duane/graphics/figures/v1/0520/001f.gif
Another example of how the curvatures of a regular lens are brought down to a flat surface:
http://off-camera-flash.com/wp-content/uploads/2014/08/Fresnel-lens-01.jpg
The simplest mechanics of a lens, that is, two prisms slapped together as a "lens":
http://www.niiler.com/phy140/doubleprism.png
Incidentally, there are Fresnel mirrors, too. The last versions of the Polaroid insta-cameras had a Fresnel mirror in them.
Terry
(Credits in URLs.)
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Another example of how the curvatures of a regular lens are brought down to a flat surface:
http://off-camera-flash.com/wp-content/uploads/2014/08/Fresnel-lens-01.jpg
Very rarely will the individual facets of a Fresnel lens be progressively curved as illustrated here - usually the pitch will be fixed at a set number of grooves per inch and the facet tilt will vary to approximate a surface which is tangential to the curved surface at that fractional aperture. This limits the potential image quality.
Of course, the risers will also have a little bit of draft built in to facilitate manufacture . . . and they may be "frosted" to produce a "low glare" Fresnel.
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^ True. Bob F. asked about it. Trying to keep it basic.