The other day a friend gave me a Fresnel lens that he removed from a projection television. It is huge, it measures 50 inches by 37 inches. We took and installed it in a wooden frame and have been experimenting with it. Now many of you may be familiar with the small Fresnel lenses that come in many survival kits as a fire starter. They measure approximately 3 ¼ x 1 ½ inches and are very close in size to a business card.
I have had small one for years in a pocket survival kit that I often carry. If the sun is out, they will start a fire. Now Fresnel lens can be made much thinner than a comparable conventional lens and can be made flat.
The big ones have many uses that partially depend on the type that you have. I understand that both types come in TVs and you don’t know what you have until you try them out. The two types are
A spot Fresnel lens uses multiple flat segments, arranged in a circle, thus focusing light on a small spot. This type does not produce a sharp image, but has application in solar power, such as focusing sunlight on a solar panel.
A linear Fresnel lens uses multiple flat segments, arranged linearly, thus focusing light on a narrow band. This type does not produce a sharp image, but has application in solar power, such as focusing sunlight on a pipe, to heat the water within:
You have to be very careful with these lenses; some of the spot lenses will produce enough heat to melt a penny, and can very easily burn your hand. Some of the linear are safer to use, since they defuse the heat. The big one I have is a linear. The small ones that come in survival kits are spot.
I have been playing with mine in the backyard. The first thing you need to do is to determine its focal length. The focal length is the distance from the lens to the point at which it generates the most heat. Since mine is a linear it forms a line and not a spot. This shows you how far you need to set the object you want to heat from the lenses.
Since plastic Fresnel lenses can be made larger than glass lenses, as well as being much cheaper and lighter, they are used in industrial applications to concentrate sunlight for heating in solar cookers, in solar forges, and in solar collectors used to heat water for domestic use. Some Fresnel lenses can concentrate sunlight (with a ratio of almost 500:1) onto solar cells.
So far, we have been able to heat water on a cool sunny day just by setting a pan on the ground and focusing the lens on it. In the next few days, we plan to build a frame to make it more practical to use.
Now there are some dangers in working with these. Mine will make a line so bright you need welder’s goggles, and so hot, it can almost instantly char a piece of plywood. You have to be careful where you store these, people have burned their houses down by leaving them where the sun can reach them.
In the near future, we plan on doing a bit of experimenting with mine to determine the limits of it capabilities. I can see possible use for them in solar cooking, purifying water and in melting glass and some metals such as lead and copper.
We will post more on these in the future.