Backing up Ouch!

[DWPC]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">What is that exactly? </div></BLOCKQUOTE>

A fresnel lens is a thin lens that uses a grooves in the surface rather than a curved surface to magnify or reduce images. One side is usually clear, the other covered with small grooves to suit the desired image.

They're used a lot in lighting (pre-LED traffic lights use them) and photo processing equipment. They will make a point of light fill a large area evenly. On a flat window, one will produce a fish-eye effect of almost 180 degrees view, though distorted. As I said, you often will see them stuck on the rear window of RVs. I've seen them sold in auto supply stores.