Before the flat screen revolution in televisions, many people had big screen projection televisions to watch their favorite movies and tv shows. But now that technology has improved and allowed for lighter, thinner, high definition televisions, the old projection televisions are becoming obsolete.
But don't get rid of that old television just yet; they have a fresnel lens on the front that can be turned into a Solar Death Ray! All it takes is a little demolition, a solid frame and stand and some good old sunlight!
Do you have an old projection tv in the basement? If so, you need to build a Solar Death Ray today - you won't be disappointed!
What is a fresnel lens?
A fresnel lens is similar to a magnifying glass in the sense that it focuses a wide angle of light into one small point of light. However, it differs from a magnifying glass because it is a series of angles that focuses the light instead of a curved piece of glass. They were invented by a French physicist, Augustin-Jean Fresnel, to be used in lighthouses. A torch (or later a light bulb) was placed in the lighthouse with a fresnel lens standing up vertically next to it (like the walls of a house placed around a fireplace in the center of the home). This allowed the light to be focused into a point and then cast out across the sea to alert passing ships that they were near the coastline. Fresnel lenses are common in older lighthouses even today.
The fresnel lens in the television is actually a sheet of plastic that has ridges cut into it that bend light. The projector in the back of the television shines onto a mirror that bounces the light toward the fresnel lens (and toward the viewer) and is bent into a larger image.
The Solar Death Ray (as I like to call it) takes the opposite approach. The sunlight shines toward the lens, and it bends light into a small point. Because light is energy, this point of light contains a lot of energy which is why it can easily burn wood, pop cans, marshmallows and a whole lot more!
Where can I get a fresnel lens?
The best place to find a large fresnel lens is on CraigsList.org. Search for "projection television" and see if you can find a free one. A lot of people will allow you to have it for free if you are willing to move it out of their basement!
You can purchase fresnel lenses online. Ebay has some large lenses and there is a selection of smaller lenses at Amazon (click here), which are much more affordable.
How do I build a solar cooker from a fresnel lens?
Watch the video to find out and find the full instructions and materials listed below.
Instructions for building a solar cooker from a fresnel lens
4 - 1in x 1in boards (8 feet long) - this will vary depending on how large your lens is
2 - 1/2in x 1/2in boards (8 feet long)
2 - 1/4in lag bolts
4 - 1/4in washers
2 - 1/4in wing nuts
Remove the fresnel lens from the front of the television (see video above for instructions)
Set the lens on carpet or soft cloth
Frame the outside with 1in x 1in strips of wood
Attach the wood pieces together so they frame the lens tightly
Using strips of wood (rip 1/2 inch strips from the 1x1 inch boards) add an internal frame to "pinch" the fresnel lens into the larger frame you made in the last step (see video). Attach to larger frame with small nails
Build two legs for your stand and cut two small boards to support the stand (see video)
Drill holes every 6 inches on both legs and a hold in the center of the frame
Insert lag bolt into hole in frame and into hole on each leg. Secure with washers and wing nut.
Change the height of the fresnel lens to determine the optimal focal length to reach maximum temperature.
Burn things...with great care!
How hot does a fresnel lens solar cooker get?
Hot enough to melt aluminum and cause a soda can to explode (see infrared heat photo)!
There are a lot of variables that determine how hot your solar cooker will get; things like: solar intensity, time of year, size of lens, angle, time of day, and the list goes on. I have done a fair amount of research and it appears that a 50 inch class fresnel lens can produce anywhere from 1500 deg F to 3000 deg F! This is a phenomenal amount of heat. Rest assured, the Beals Science team is working on a definitive answer! But first we need to finish burning anything we can get our hands on inside our solar death ray - then we will get back to the science!
Keep on Learning! ~Craig Beals
Special thanks for FLIR infrared camera team who shot video of the can exploding!
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