Re: [ATM] Improving my focault tester / testing skills

[Jim Burrows <burrjaw@earthlink.net>]

At 2008-03-31 00:18 +0200, Emil Johnsen wrote:

>My problem with the tester is that when I do zonal relading with a
>couder mask I find it very hard to se which side becomes dark first. I
>can tell when I'm some distance inside or outside ROC, but there is a
>wide range where the two sides appear to gray out at the same time.

What you want to do is make the reading at the point the two mask 
holes are equally grey.  Warren Smith, "Modern Optical Engineering", 
p. 108, says, "[The eye] is an excellent instrument for comparison 
purposes, and can be used to match the brightness or color of two 
adjacent areas with a high degree of precision."

>How important is the size and brightness of the light source?

On page 109, he says, more or less, the brighter the better - you can 
get 2% matching precision (if the holes were adjacent, which, 
unfortunately, they aren't), and the surround (the mask) should also 
be lighted.

>Would a light source emitting mostly one color be better?

I don't think that makes much difference as long the colors of the 
two mask holes are the same (diffraction isn't giving you a pretty rainbow).

         -- Jim Burrows
         -- http://home.earthlink.net/~burrjaw
         -- mailto:burrjaw@earthlink.net
         -- Seattle N47.4723 W122.3662 (WGS84)

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