So you don't have access to your own rail gun or military space laser....but never fear, we'll use the 1000 Watts/m2 of free sunlight in your backyard! But how?
Seriously. A solid glass lens that size would be silly, but instead we can use a 4 foot wide Fresnel lens. You know, those clear, flat things with the ridges, you find them on overhead projectors and rear windows on some buses? The idea is pretty simple: a Fresnel lens is just a normal curved lens chopped into thousands of little rings, but just as effective.
The Project
This instructable (my second) chronicles my progress over the last month or so on this Fresnel deathray. Each step was figured out in real time, but the general idea is this: once you have your giant Fresnel lens, all that remains is to build a frame to keep it straight, and hold perpendicular to the sun. While you can stop here and enjoy the blinding energy of the nickel-sized spot you get at the focus, I went further and attempted to collimate the light into a straight beam. I ordered a focusing lens online and constructed a scaffold to hold it in place, but ultimately found the Fresnel lens to have imperfections standing in the way of proper functionality.
Disclaimer: This device is extremely dangerous, and will INSTANTLY set things on fire! It's extremely cool, but I'm not responsible for anything that happens if you decide to ignite yourself, your house, the forest, or anything else. Also, if you decide to skip the eye protection step, I hope you like braille.
....A 13 square foot magnifying glass!
Seriously. A solid glass lens that size would be silly, but instead we can use a 4 foot wide Fresnel lens. You know, those clear, flat things with the ridges, you find them on overhead projectors and rear windows on some buses? The idea is pretty simple: a Fresnel lens is just a normal curved lens chopped into thousands of little rings, but just as effective.
The Project
This instructable (my second) chronicles my progress over the last month or so on this Fresnel deathray. Each step was figured out in real time, but the general idea is this: once you have your giant Fresnel lens, all that remains is to build a frame to keep it straight, and hold perpendicular to the sun. While you can stop here and enjoy the blinding energy of the nickel-sized spot you get at the focus, I went further and attempted to collimate the light into a straight beam. I ordered a focusing lens online and constructed a scaffold to hold it in place, but ultimately found the Fresnel lens to have imperfections standing in the way of proper functionality.
Disclaimer: This device is extremely dangerous, and will INSTANTLY set things on fire! It's extremely cool, but I'm not responsible for anything that happens if you decide to ignite yourself, your house, the forest, or anything else. Also, if you decide to skip the eye protection step, I hope you like braille.
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Signing UpStep 1: Acquire The Lens
For many future scientists, the destructive power of magnifying glasses provide hours of fun in the backyard (although I do not believe in burning living creatures, whatever the size). But everybody already has a magnifying glass. Where are we gonna get a 60-inch Fresnel lens?
They can be had online, but only for substantial piles of cash (from $80-$150 on Ebay), which is why few people ever enjoy these devices. Traditionally, the actual lens is by far the biggest cost in a project like this, with lumber and hardware being almost nothing if you already have the tools. And now, I will impart to you the ultimate source of FREE giant Fresnel lenses:
Every rear projection TV uses a Fresnel lens the exact size of the screen to focus the image. The screen has several layers:
1. Craigslist! Go to the free section on your local Craigslist community, and you'll probably find dozens of massive, usually broken projection TVs being given away. Say Billy has a TV from about 10 years ago, and when it breaks, Billy decide to upgrade to a newer technology. Big-screen TVs usually weigh 200-400 pounds, so all Billy wants is someone to make it disappear. If you have a truck and at least one strong friend, this is a great option especially if you don't like option 2.
2. The Dump. If your local dump recycles TVs, you may be fortunate enough to find a pile of TVs sitting around there. My dump doesn't allow scavenging, so we just made sure there was no one around, and helped ourselves to the front parts of TVs and scored 3 giant lenses.
Once you have your TV screen, peel the layers apart (you may need to cut some tape along the top) and extract the precious Fresnel. Admire your plunder, and dispose/recycle the TV carcass.
They can be had online, but only for substantial piles of cash (from $80-$150 on Ebay), which is why few people ever enjoy these devices. Traditionally, the actual lens is by far the biggest cost in a project like this, with lumber and hardware being almost nothing if you already have the tools. And now, I will impart to you the ultimate source of FREE giant Fresnel lenses:
...Rear Projection TVs.
Every rear projection TV uses a Fresnel lens the exact size of the screen to focus the image. The screen has several layers:
- Outer cover (optional) - Some TVs have a clear layer on the very outside....keep it, it could be useful in another project.
- Lenticular lens - This is the hideous outer screen with 1000s of vertical lines. The purpose of the lines is to spread each pixel outward so you can see the screen from the side. It will probably rip apart as you separate the layers.
- Fresnel lens - this is the innermost layer - clear with millions of circular ridges on one side. The crown jewel of the TV.
1. Craigslist! Go to the free section on your local Craigslist community, and you'll probably find dozens of massive, usually broken projection TVs being given away. Say Billy has a TV from about 10 years ago, and when it breaks, Billy decide to upgrade to a newer technology. Big-screen TVs usually weigh 200-400 pounds, so all Billy wants is someone to make it disappear. If you have a truck and at least one strong friend, this is a great option especially if you don't like option 2.
2. The Dump. If your local dump recycles TVs, you may be fortunate enough to find a pile of TVs sitting around there. My dump doesn't allow scavenging, so we just made sure there was no one around, and helped ourselves to the front parts of TVs and scored 3 giant lenses.
Once you have your TV screen, peel the layers apart (you may need to cut some tape along the top) and extract the precious Fresnel. Admire your plunder, and dispose/recycle the TV carcass.
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No fresnel in this type. I am looking for things I can do with those monster lenses.
I am sure I could put my smart phone in the right spot and have a large screen phone but since I cannot use touch screen that way, the only point would be to make a holder for the phone and perhaps use it to watch streaming movies on the wall in a dark room. ( But then I have HD TV for that too ! )
Any other good ideas to use them?
Love this site, and its participants.
KILL IT WITH FIRE!
http://www.newscientist.com/article/dn4009-optic-fibre-delivers-solar-surgery.html
http://www.newscientist.com/articleimages/dn4009/0-optic-fibre-delivers-solar-surgery.html
Polarized lens are so cool to work with.
http://cgi.ebay.com/Pro-Rider-IR3-Welding-Wraparound-Safety-Glasses-Z87-1-/330526133677?pt=LH_DefaultDomain_0&hash=item4cf4e48dad#ht_6224wt_1139
The "floor stand" can be screwed right to the plastic bezel.
Plus, it looks like my DIY fresnel apparatus is sponsored by Sony :)
1) Not all big boxy televisions are rear-projection! Smaller and especially older ones may simply be CRT (not rear-projection) which means there is no screen, just a metal cathode inside a solid glass tube. On the inside of the front face of the tube is a very thin and very toxic shiny phosphor coating. There is no lens, other than--perhaps--the glass itself. I repeat: what you are looking at--the outermost layer of the display--in a CRT television is the solid glass tube itself.
2) UNDER NO CIRCUMSTANCES BREAK ANY GLASS IN THE TUBE
If you have to break glass or ceramic sealant, you're doing it wrong.
The toxic phosphors will flake off and start floating around like dust, it *will* get everywhere.
Yttrium (in the red pixels), for one, causes lung disease.
Some tubes have a band of metal glued to the tube which looks like it is holding a screen to the tube. They are not. While it is possible to remove it, you will find nothing more behind it than a solid glass seam which is part of the main tube, not a seam holding the tube to a screen.
If you can, then please send it to my blog:elektritsaabtasuta.blogspot.com or mail me sullivanbrendan35@yahoo.ie
All CRTs that I've seen have glass screens.
DLP tvs have them in their light engine. It is basically two lenses at each end of a square tube that has mirrors lining the inside.
Would that work?
And can anyone tell me what is the difference between fresnel & lenticular lens?
P.S. a fresnel lens is a series of concentric rings, which has a single focal point. Lenticular lenses are a 1 dimensional pattern like a bunch of rows of crops. It's pretty much useless except for image screens, such as projection TVs and certain holograms.
You could purpose that heat for electricity in 1 of 2 ways.
Stirling engine, Steam turbine !!
I wonder if the lens from old seeing glasses could also be used for the focal lens in a pinch .