From: John Swenson 
CC: ATM List 
Reply-To: John Swenson 


James,
the problem with the photo LWT is you need to know the transverse
measurement of the shadow when it is in the center of the mirror(from the
digital indicator or whatever). The test is almost useless unless your
transverse measurements are referenced to the center. Its easy to get the
center of the shadow measurements, just count pixels. But if you don't have
a picture with the shadow in the center what do you use as the transverse
measurement offset?

Thats why I do two pictures with the shadow close to center on each side of
center, then use the ratio between pixel locations relative to pixel center
to interpolate the transverse measurements to calculate the transverse
measurement center. I did some simulations and this interpolation close to
the center doesn't mess things up too bad.

John S.

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