Re: [ATM] zone one challenge

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Michael Lindner wrote:
> Interesting work, and an interesting challenge. FWIW I do not use the "center 
> of the zone must be 50%" method for foucault testing. One method I use is to 
> look for the edges of the zones to be equally illuminated, which leads me to 
> pick "J" every time.

Hmmmm.... I adjusted my monitor settings and now J looks the best. 
Interesting.  I'm using a CRT - wonder what it would look like on an LCD.

> If this were a "dynamic" foucault tester, I would tap the tester and look for 
> the shadows to completely darken/lighten the zone in unison. Heretical, but I 
> find it easier than judging brightness. This latter method I have found 
> introduces a bias, but if all the readings are done that way, it's the same 
> bias, so...

So if I twiddle the brightness/contrast knobs on the monitor, maybe 
that's like tapping the tester.  ;)

Personally, I look at several (similar) brightnesses, mostly with the 
knife edge about halfway from "first cut" to "last extinction".  If 
first cut is a brightness of 10 and last extinction is a 0, I look at 
4, 5 and 6, roughly.

At these KE positions, I look for the same range of dark to light in 
both openings.  For narrower outer zones, this implies that the zones 
look the same, in terms of their brightness and gradient.  I can 
generaly find a reading where this happens for all the intermediate 
gray levels.  (The KE is on the optical axis, or very close - I don't 
test at the extremes of KE insertion.)  My slightly wider inner zones 
don't typically have the brightness gradient show in Steve's example.

	Mike Lockwood



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