Giant Fresnel Lens Deathray: An Experiment in Optics

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?

....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.

Step 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:

...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.

Two excellent sources of free rear-projection TVs:

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.

Comments

[stuffdone]

I took apart an old projection set about a year ago and just stashed the parts for eventual toy's. The one I had used three CRT's, Three hefty projection lenses and a front surface mirror ( which a friend broke! ). Lot of other stuff like a blower and hoses used to keep the center tube cool, the screen and lots of other parts of various types.

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?

[stuffdone]

Here is image of one of the lenses. All three same. They include the aluminum mounting flange that mated to the front of the CRT. Weigh 8LB each. About 5" Dia. Overall assembly is about 8.5". Lens screws in/out to focus.

[ezetuloveth]

what an interesting webpage can i view this from portal.unn.edu.ng

[sleepydog]

Great job! The build detail, thought process and references are outstanding. I'm working on a related project and this helps me immensely. Thank you for taking the time and effort to share your work.

[FirstSpear]

OK, the big electricity companies will poo poo it, but I'm thinking that this could be a cheap approach to desalination by condensation. The steam could simultaneously be used to supply the water vapour, and also spin a generator to pump cold sea water through the condenser in one swell foop. Stopping at night is irrelevant as the output is storeable.

Love this site, and its participants.

[Deadalus]

Were you ever able to work out the problem with the secondary lens ?  It ,may be that some form of lasing element would work.  The guts of small laser pointers which are almost free may be worth trying.

[mgalyean]

Trying to be nice here, but what part of physics and engineering are you actually basing this comment upon?

[Infinite_minus_zero]

This would be awesome with bugs crawling everywhere.
KILL IT WITH FIRE!

[Bersa]

Just a, maybe wrong, idea: what about adding a reverse Fresnel lens (as small as the focusing) at the focusing point? Won't it reverse the light in a straight path (concentrating the light)?

[spamattakky]

Did you see the article where the Israelites concentrated the beam into a glass fiber optic cable and was able to use in in surgery?

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

[Ev]

Here's how I found the focal point...  Get a trigger pump type of fluid sprayer, either a glass cleaner sprayer or you can buy empty plant misting devices at a hardware store. Put the lens in the sun and spray water behind the lens. You'll see the rays converge and diverge. I guess you could measure it with a metal tape. 

[spamattakky]

I really like the misting idea, would be great in demonstrations with my Fresnel lens cooker.

[spamattakky]

I use a regular polarized sunglasses and wear the Real circular polarized 3D glass in front of the other pair, they darken up quite nicely and I always have sunglasses on when you lift the Real to undarken you view. This is the only way for the two polarized glasses will work, put them in any other order or angle and it will not get any darker.

Polarized lens are so cool to work with.

[cacci]

What type of glasses are recommended for a fresnel lens? Would these be adequate? Please answer only if you actually know, I would prefer to keep the remainder of my eyesight.

http://cgi.ebay.com/Pro-Rider-IR3-Welding-Wraparound-Safety-Glasses-Z87-1-/330526133677?pt=LH_DefaultDomain_0&hash=item4cf4e48dad#ht_6224wt_1139

[DrSimons]

Firstly, I don't have much formal welding training and I can't tell you the specific ratings to look for. What I can tell you is that I used goggles designed for gas welding (ie oxyacetylene), and I still saw a lot of vision spots after staring at the focal point for a little while. It depends a lot on how much exposure you get. If you really want to be able to stare continuously at it, go with something rated for arc welding. Usually you can't see anything through these other than the actual bright spots. The goggles in your link definitely aren't dark enough for that, they're definitely better than nothing.

[kgillette]

Why reinvent the wheel? The TV manufacturer has designed and built a nice frame for the front screen to hold it nice and flat. If you harvest the lens from the TV in a controlled manner (put away the BFH!), you can retain the bezel from the TV's cabinet as a frame. Once you get the bezel/screen assembly off the front of the TV, simply unscrew the clamps holding the screen in the frame, delaminate the fresnel, and re-clamp. I did that with my old 42" Sony. Worked great!
The "floor stand" can be screwed right to the plastic bezel.
Plus, it looks like my DIY fresnel apparatus is sponsored by Sony :)

[dslkjdflkjsdlkjds]

--NOTE!!:
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.

[bsullivan-2]

Could you show photo how looks CRT television the solid glass tube,please?

If you can, then please send it to my blog:elektritsaabtasuta.blogspot.com or mail me [e-mail]

[Silence]

Identify rear projection TVs quick by a flat, gray or black screen surface (the Lenticular lens as described above) with lines. You'll know it when you see it.
All CRTs that I've seen have glass screens.

[BOOM5601]

Who cares? if you get a crt by mistake, you've just gotten yourself a flyback!

[jcksparr0w]

i totally agree, and a big one at that

[xirekaj]

Basically, if you can't tell the difference between a CRT and a rear projection tv then this and almost every other project on this site is not for you.

[snmfield]

my thoughts exactly!

[Unit042]

I once read a book about many different science projects, including one just like this instructible: using a fresnel lens to melt a penny, but I figured the lens would be too expensive to acquire. Now that I know where to look, I will be melting metal, experimenting with thermal collection experiments etc. in no time! Thank you for putting up the rear-projection TV tip! :)

[Cobalt59]

Kipkay made something like this.......

[davoww]

How about using a light tunnel to align the beam after the fresnel lens.
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?
 

[DrSimons]

I've thought about using some kind of mirror tube. It would work to funnel the light down the tube, but it would diverge like crazy at the other end. I think the problem with a glass lens is with such a high intensity of light, a ton of it is reflected away or comes in at the wrong angle due to dispersion.

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.

[static]

Dan Rojas of green power science seems to get his kicks using the Sun to catch wood afire for demonstration purposes of course. In that he end with lenses is already clearly heavier, why the holes to lighten that rear cross member?

[DrSimons]

I mainly put the holes there for effect...looks more "engineered" than a solid plank. However, it helped by shifting the center of gravity back towards the fresnel lens.

[dasic002]

briiliant!! the penny turned gold because the copper fused with zinc to form an alloy - "Brass", which is gold coloured. that is very cool, was this done in winter or early spring?

[Mike McGill]

Hi, This is very impressive. I reckon it would make an extremely clean method of melting metal for casting, using a cast iron crucible at the focal point. I remember reading years ago about an experiment in Switzerland, where a hillside was covered with parabolic mirrors controlled and focussed by computer. They burnt a hole through a six inch steel plate in less than a minute.

[John S]

It may not work on a solar panel but it it should enhance the efficiency of the passive solar heater I have been thinking of building to help heat my house. By building a metal frame with soda pop cans stacked inside and a clear front and mounting a lens like this in front of the glass front I should multiply the amount of heat collected from the air flowing past the stack of solar heated aluminum cans. I just realized it would only be in focus a small part of the day since the solar heater box will be mounted on the south wall of my house. Still it shouldn't hurt anything. I might have to make the back of the box out of heaver metal.

[Helioskannon]

Be careful with that, Aluminum melts at 1200 F... (Copper at 2000, and it _destroyed_ the penny) It could well work if it's not in perfect focus, just be careful and don't burn a hole in your house. Though, a well designed solar furnace to melt metal would be awesome. If I can find one of those lenses anywhere, I'll see what I can do, and if it works, post an instructable.

[Silence]

Thats some intense heat.
You could purpose that heat for electricity in 1 of 2 ways.
Stirling engine, Steam turbine !!

[Shiftlock]

Both of these ideas have been done. There's a Youtube user called GREENPOWERSCIENCE that has posted videos of these in action. He plays with all kinds of solar. In his most recent video, he cuts a beer bottle in half by lightly scoring it, then using a small parabolic mirror to stress the glass. It's neat.

[badideasrus]

i found a source of the lenses. craigslist. search for rear projection tvs. most are free for the haulin, and over 55 inches. start the burnin!

[BtheBike]

Yes . I got 2 from curbside garbage also. Right now with black friday ,these things are all over the place like old CRT's . Maybe there should be a mandate to collect these lenses for 'green' industrial use ,or ship to 3rd world countries for use.

I wonder if the lens from old seeing glasses could also be used for the focal lens in a pinch .

[badideasrus]

that was a similar idea that i had. use fresnel lens to heat metal ffor forging and smelting. no need for coal or wood. just stick the metal bar under the light for a few seconds and then continue forging. i've heard of parabolic mirrors being turned out of alluminum (lathed.) why? it would be simpler to just take molten metal and pour it on on a spinnin disk. the liquid metal would take a parabolic shape, and you would not need an insanely large lathe to make it. i'm seeing 12 foot mirrors made with an old car motor.

[qualia]

please elaborate on your pouring molten metal on a spinning disc idea, whichever way i picture this, i always end up with molten metal being thrown out from the edges of your parabolic shape towards faces, via inertia and gravity.

[badideasrus]

the disk would have to have edges.... like a pie pan shape. my comment does sound kind of silly, doesnt it?