Re: filmscanners: Anti-Newton Rings powder

["Karl Schulmeisters" <karlsch@earthlink.net>]

My understanding of "Newton Rings" is that they came from the same source as
the rainbow on an oil-slick or a thin prism put on a reflector.  Namely you
are getting 1/2 wave interference patterns from the light reflected at each
"boundary layer" - a "boundary layer" is where the optical density, (or
transmissivity etc ie the "C" of nu = C/lambda aka Freq = C/Wavelength),
changes.  At all such boundary layers,  light gets reflected in proportion
to how steep the gradient is (this is why lenses are coated to improve
contrast - the coatings reduce the gradient).

So the way to reduce Newton rings is to insure more than 1-2 wavelenths of
separation between these reflective regions.  Putting some translucent dust
of sufficient diameter, between the surfaces in effect acts as a separator.
But such dust would also act as a diffuser, reducing contrast and acuity
----- Original Message -----
From: "Jim Snyder" <jimsnyder@insight.rr.com>
To: <filmscanners@halftone.co.uk>
Sent: Wednesday, August 29, 2001 8:49 PM
Subject: Re: filmscanners: Anti-Newton Rings powder


> on 8/29/01 10:37 PM, SKID Photography at skid@bway.net wrote:
>
> >
> > LAURIE SOLOMON wrote:
> >
> >> They might also use an Anti-Newton Rings powder on the
> >> glass between it and the subject being scanned.
> >
> > Sorry for my ignorance....What is 'Anti-Newton Rings powder'?
> >
> When you place two flat surfaces together, you get a form of banding known
> as Newton Rings that represent the different transference of light between
> the surfaces due to the pressure. Glass slides, and even enlarger glass
has
> been coated for years with a transparent substance that prevents flat/flat
> contact, spacing the two flat surfaces just far enough apart to prevent
the
> pressure banding.
>
> Jim Snyder
>