Re: ATM Computerized Foucault Test

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

Marcus,

>With some people attaching video cameras to their Foucault tester, I had an
>idea: Basically, in the classic Foucault test, one has to compare the grey
>level of two areas opposite of one another. 

Harold Suiter has written about it in ATMJournal, and Mike Peck has a webside of it. They 
use slightly different ways to find where the intensities "cross", that is the radius 
where the intensities at both sides are equal. 

Simulations suggest that there is some uncertainty near center (of less importance) and 
also close to the edge (possibly more important). 

With Couder masks, you know the zonal radius very exactly and need to compare intensities 
accurately, without a mask it you need to compare intensities quite as accurately and 
also measure the zonal radius, introducing some extra uncertainty.... Besides, a Couder 
screen will balance the diffraction effects  that are unbalanced near the edge of an 
unmasked mirror.

But on the whole, the idea ought to give useful results, even without Couder mask, and it 
is likely technically easier to set the KE position and read the "shadow position" than 
moving the KE in search of equal illumination.


>Of course to make it work there are some details that need to be solved: The
>computer needs to find the center of the mirror in the image. Finding the
>radius with the lowest variance might not be as exact as it sounds in
>theory, there might be several local minima, the minimum might be very
>broad, etc. Some smoothing might be necessary.

The smoothing seems indeed to be necessary - I believe using not too narrow a slit and 
using non-monochromatic light (such as from LEDs) will achieve this. In simulations, 
monochrome and very narrow slit seems to cause significant confusing effects. Hope to 
find out more specific info about what is needed here.


Nils Olof