Re: [ATM] Phase contrast and the 10 incher.

[Dominic-Luc Webb <dlwebb@canit.se>]

On Tue, 24 Apr 2007, Arjan te Marvelde wrote:

> surface shape. What you normally see as structures on the surface (usually
> larger scale) are deviations from the surface shape.

What you (keyword, "mainly") see is the surface. Having worked thinner
plate glass pieces, in some tests, I have also seen the back surface,
and this is light that has passed twice through the glass and any
defects inside the glass.

Now let us say that in Dale's piece, the defects are in some cases
close to the surface, I can imagine light striking those surfaces
and scattering, whereas at other more homogeneous points, the light
either passes right through, or reflects back towards to the
detector, perhaps striking any defects within the glass a second
time.

These testers are not confocal systems. They really do have depth,
and all the complications associated with depth. It would not
surprise me if someone has come up with a confocal (or maybe
quasi-confocal/deconvolution) imaging technique for mirror surface
testing, but I am not aware of it.

Dominic

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