Re: Fresnel Lens

[Hugh Haskell <hhaskell@MINDSPRING.COM>]

> I would like to know how those flat plastic Fresnel lens (those floppy
> transparent things with concentric rings)can actually behave like ordinary
> lens.  What is the physics behind this.

Imagine taking an ordinary lens (flat on one side and convex on the
other), and cutting it into a whole bunch of very thin concentric
rings. The cross=section of each ring will be a little trapezoid,
flat on the bottom and tilted on the top, with the degree of tilt
increasing with the radius of the ring. Now grind each ring's
cross-section down until the cross-section is just the triangular
part on top of the trapezoid and reassemble the rings in the order
they were originally. Voilla! A Fresnel lens!

Since the lens effect occurs at the surface where the refraction
takes place, all that extra material behind the curved surface is
just extra weight to haul around. The Fresnel lens is just a way to
get rid of the extra weight. This is especially valuable for large
diameter lenses. But creating those concentric rings does degrade the
optical quality of the image because of all the little
discontinuities in the surface. But you can also play around with the
tilt angles on the rings and make it a great collimating lens. They
are used for that property in the light system that defines landing
glide-paths for the aircraft landing on aircraft carriers, and also
for those traffic lights that are not supposed to be seen except by
the people in the lane the light is intended for.

Hugh
--

Hugh Haskell
<mailto://hhaskell@mindspring.com>

Let's face it. People use a Mac because they want to, Windows because they
have to..
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