Re: Fresnel Lenses

[brian whatcott <inet@INTELLISYS.NET>]

At 14:43 4/15/00 -0400, John Denker wrote:
> 2) The cast-versus-ground argument is very weak.  For years it has been
> possible to cast optically-perfect surfaces.  Many eyeglass lenses are
> cast, and there are other important technological applications for
> precision casting.

I assert that cast eyeglasses are not glass, in general.
When the optician extolls the virtue of his lightweight spectacles
 - read 'plastic'.

> > Any old piece of lumpy glass with a smooth surface (a cast drinking
> > glass, for example) ... After all, one can still see an image through a
> > cast drinking glass, though a distorted one.
>
> An excellent point.  But I wonder how much of that has to do with the
> adaptive focusing powers of the eye?  When I try to use a lumpy drinking
> glass to form a recognizable image on a passive image-plane (like an index
> card), it doesn't work very well.

There's a well known technique in star-gazing when the seeing is bad,
that involves setting a stop in front of the tube.
This would be a means of providing mediocre image quality from a
 lumpy glass, essentially by restricting the variability to a
small patch of glass.

[John]
> > > > For a plastic lens this means it can be formed from sheet stock.
> > >
> > > If the plastic is good enough to implement micron-scale
> > > figuring _within_ a
> > > given ring, is it not good enough to implement micron-scale control over
> > > the step height?
[Leigh]
> > Well, that is a question you can think about if you know something
> > about creep or thermal expansion, or if you have ever seen a
> > Fresnel lens made of a flexible sheet material that, because it is
> > nonrigid, can't possibly be held to micron tolerances across its
> > lateral extent.

Here Leigh's response falters somewhat in my view: polycarbonate and
such engineering plastics cannot be said to be floppy though their E
is evidently lower than the metals.
 They make perfectly adequate lenses, as you can
demonstrate when you buy a cheap import inspection 'glass' of four or
more inches diameter.

 This is of quite reasonable optical quality, (if you get lucky).
 Better to recall that objectives need to handle off axis rays
gracefully,and not deflect a proportion of the strays into the active
path - the bugbear of the optical quality Fresnel.



brian whatcott <inet@intellisys.net>
Altus OK