OK, so in order to be called a Fresnel lens, the ridges are large relative
to the light's wavelength to avoid quantum effects (but still small from a
human scale), the shape of the ridge approaches a flat triangle as the size
goes down, and the angle of said triangle is critical.

A diffraction grating or Fenman-style quantum reflector, made of closely
spaced opaque marks on the scale of the frequency of light, could certainly
be made to focus monochromatic light by varing the refraction angle at
different points of the "lens", but that would =not= be a Fresnel lens.

Since it depends on the frequency, a bunch of closely-spaced marks will
cause a rainbow effect on white light, which is what we normally think of as
a diffraction grating as looking like.  It could only act as a controlled
bender of coherient images for monochromatic light.

Then again, lenses based on prism effects (normal refractive media like
glass) have the same problem, in that they are frequency dependant.  The
effect is small, and can be corrected by using multiple lenses stacked
together ... I wonder if the same thing could be done with a diffraction
grating based lens?

--John

Mark Wagner <mar### [at] gtenet> wrote in message
news:37ccb3d3@news.povray.org...
>
> John M. Dlugosz wrote in message <37cc8b6c@news.povray.org>...
> >I've seen a description of Fresnel's invention a short time ago, how it
was
> >designed for lighthouses.  I agree, it's a normal lens with the "stair
> >steps" taken out.
> >
> >I've seen the scratch thing called Fresnel as well, but I don't know if
> >that's correct, or if the name is now applied to any kind of flat lens.
>
>
> It's still a Fresnel lens, just even more compressed than the larger
lenses.
> The nice thing about Fresnel lenses is that you can make them quite flat
and
> the lens will still work.
>
> Mark
>
>

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