Friday, January 13, 2017

fresnel lens

I have a large flat plastic Fresnel lens hung between two benches that are the same height, more or less. The benches are pushed together so that the sheet is sandwiched between them. The sheet is supported on wires from hooks in the ceiling. I can vary the height by adjusting the wires.

I have two of these sheet lenses, but they are different. I am trying to measure the focal length of each, the distance from the lens to the point at which the light converges. A Frensel lens is like a magnifying glass, except in thin lightweight plastic sheet form.

Mine both came out of discarded rear projection televisions. It does take a bit of effort to free one of these from the set and it isn't for the faint of heart. I generated quite another pile of waste by taking out every part from the frame but perhaps my separation of the waste helps with the ultimate recycling effort?

Both lenses came from the sets as an assembly of two sheets, the Frensel and another sheet with very fine vertical stripes, like a diffraction grating. The two sheets were taped together anong their edges and held in plastic frames. I removed the frames and tape to get the assembly apart to free the Fresnel sheet.

At any rate, I have the sheet hung from the ceiling so that the center is approximately at the table height. I am using a laser pointer to see the direction that the beam is bent as it passes through the sheet.

This is a series of pictures showing the bending at the back of the sheet. The metal ruler is perpendicular to the back of the lens lined up with the main axis (at the center of the sheet).

I have marked lines parallel to the laser beam near both edges of the sheet with a ruler and I find that the lines converge at a point that is 30 inches from the sheet. This is the Focal Length of this sheet.

The sheets are different and seem to give different results depending on which side of the sheet faces the laser pointer.

Fresnsel lenses have application in concentrating photovoltaics with some interesting examples shown by Green Rhino Energy.

Fresnel lenses can also be used as solar concentrators to reach very high temperatures, like this.

Not sure what I am going to do with mine yet.

Thank for your interest

George Plhak
Lion's Head, Ontario, Canada

The donor of one of my lenses, a Sony KP-43T70, made in Westmoreland, PA in February 2000.

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