Re: ATM Foucalt testing

[Dwight Elvey <elvey@hal.com>]

Donovan Buck <donovan@bravoz.com> wrote:
> 
> I have a simple yes or no question to test my understanding of my 
> Foucalt tester/tests.
> 
> Is contrast in the shadow pattern inversely proportional to the slit width?

Hi
 As far as I know, there is no special advantage to using
a narrow slit for the light source. Many ( including me )
use what is called a slit-less that only uses one defining
edge at the source. The darker parts of the shadow pattern
are not any darker with a narrow slit. The only thing that
a narrow slit might give you is reduced stray light. Still,
at the distances to the mirror, I don't think this has
any advantage. I think there is a little difference in
the ease of locating the center of axis with a slit as
compared to the slit-less but I don't think this is vary
significant. 
 Probably the main reason for narrow slits is that they tend to
make the light a little more uniform at the various angles across the
mirror face because of diffraction, at the slit, but most
people use diffusers at the light source to make the light
more uniform at angles. This was more important before
simple diffuse light sources like LED's became available.
The Foucault test depends on the light intensity, from the
source, being uniform across the face of the mirror.
Dwight