The concept of a fresnel lens is really rather simple. Think of a
typical convex -convex lens 2 feet in diameter (or larger in a
lighthouse).

One, it's very thick and two, it's very heavy! Now slice through that
lens in concentric circles from circumference to centre, making a
series of glass rings. Slice again across the diameter.  Then, working
from the outside in, shave the excess glass of the back of each ring
until when laid flat all the rings now have the same height when laid
upon a flat surfave. .  The upper surface will be a series of ridges
which once formed the convex curve of the lens. The unnecessary mass
under the curve is removed saving enormous weight. Optically it is
essentially the same lens but physically it is greatly reduced in depth
and mass.

They are also used in theatrical lighting for essentially the same
reasons.

On Saturday, August 16, 2003, at 01:26 PM, Automatic digest processor
wrote:

>
> You prol'ly mean Fresnel Lense. Oriigially developed by a Frenchman and
> applied to light houses. The were fantastic for collecting the light
> from a lantern, concentrating it into a beam that could be projected
> for
> miles across the water. Most Fresnel lenses were made up of glass
> pieces
> that has a triangular cross section. The pieces were organized into
> concentric circles and the angle of the glas set to collect light from
> onw directtion and through the prism effect of the triangular cross
> section, concentrate the light and project it as narrow beam of light.
> The Fresnel mirror is doing the same thing, but just collecting
> naturally reflected light which it projects for you to see as an image
> it the miror.
>