....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.
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
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.