A recent Slashdot article was a rather dull one about fun things you could do with a large Fresnel lenses. I had seen stuff like this before, but in scanning over the responses as I often do, I found a reference to a patent for a spiral Fresnel reflector. You can review this the patent here. It’s rather clever. Basically you can plot a spiral onto a flat sheet of reflective material, and then wind it around according to a particular formula (equation 2 in the patent) which causes the slats to tilt at the appropriate angle to bring light to the desired focus.

I thought it was a cool idea, I’ll have to work up a program to plot the design for these, and then use it to make an immense solar hot dog cooker. Or something.