In our area I've bought cheap fresnel lenses in the form of $1 flexible (or $4 stiff) page magnifiers. The optical quality of the $1 lenses were better, ironically. A local office supply store might have them for people wanting to make solar stoves from them (if that's possible) or whatever. The other thing is that magnifying glasses can be used to start fires by focusing the beam on a pile of leaves. I know the regular large round kind (or any that can focus the sun into a very sharp point) can start a fire pretty quick. A super-large large fresnel lens can heat surfaces to thousands of degrees, and even the smaller $1.50 flimsy page magnifiers will burn stuff if the sun hits it. So be careful, but if you have a sunny day you could probable liquify metal with one of those large $100 2x3 foot lenses. So if you use one on your window beware, if the sun happens to pass by it the right way you might cook something. If you look at such a window with even a dim sun on the other side, and the focal point happens to land on your retina, you can kiss your eyesight goodbye.
Offered by Joe.
I've actually had some experience with one of the 2x3 ones. It would disfigure pennys (mostly separate the zinc from the copper) but not do much else. It would start fires quite nicely with dry grass! You cannot see through it and it is quite hazardous with the little ones hanging around (they tend to want to put their hands in the beam - ouch!).
Offered by Roger.
With in the 20 year period of gloom following the pole shift I can't think of a good way to use these lenses asa supplement to light bulbs that can't be done more efficiently in another manner. For example if you need to use it for a plant - why not move the light bulb closer to the plant. You won't have the light loss going through the lens.
Offered by Mike.