Instructables

Giant Fresnel Lens Deathray: An Experiment in Optics

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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.

stuffdone1 year ago
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?
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.
Projection TV Lense.jpg
--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.
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.
jfb15 years ago
Sharp rear projection TV, 50" fresnels are not very clear.
matroska5 years ago
I'm having big troubles finding a fresnel lens around here.. So far I've checked my local classified ads services, but everyone is selling a working television for big bucks. Seems like nobody broke one yet around here. I've droped a wanted ad with these services, but I doubt I'll get any response in my region...
I was hoping that I could just edit my last post. Here is a REALLY big lens.

http://www.sciplus.com/singleItem.cfm/terms/13603