Re: Fresnel Lens

[Ludwik Kowalski <KowalskiL@MAIL.MONTCLAIR.EDU>]

Leigh Palmer wrote:

> ... M. Fresnel invented these devices to collimate lighthouse
> beacons, not to make high resolution optical images, and they
> don't. ...

Let me add that the main purpose was to reduce the weight of
the lens (and parhaps to save some francs on the amount of glass).
A typical diameter was 1/2 m with about 5 to 10 steps. I doubt
that the wave aspect of light played any role in the original design.
I remember reading Fresnel's manuscripts (when the history of
physics library were in New York City) about 20 years ago.
Ludwik Kowalski




> The rings of the lens are cut the way they are, closer together at the
>
> edges, in order to make them more or less the same average
> thickness overall. For a plastic lens this means it can be formed
> from sheet stock. (This makes the poorest quality lenses.) If you
> check out an automobile taillight lens you will probably find that
> part of it is a Fresnel lens. Sealed beam headlights have another
> variant for collimating light, a multi-prismatic lens.
>
> Leigh