Re: (ATM) Polishing mirrors

Bill Marriott (btk@ix.netcom.com)
Tue, 22 Aug 1995 21:51:21 -0700

You wrote: >
>I would be interested in tapping the collective wisdom of the list in
regards to determining when an optical surface is considered polished. >I have been inspecting the surface with a microscope. When I cannot
see any pits out to the edge with the microscope, I consider the surface polished. More recently I have also been shining a light at the surface to see if the light passes through or is reflected from the surface. If the light passes through, I consider the surface pretty good. I always back it up with the microscope however. >
>Any other suggestions? I would also be interested in how professional
shops make the determination as to surface quality. >
>
Well, in the optic shop we do final inspection in two ways. Most of the optics we make are windows, and are usually 'blocked' with wax or pitch on larger (6-8-10-12" pyrex) discs. But its the same as if it was a large single part. We use an Olympus Metalological (sp?) scope, with most work using 40x. At this power we can see any pitting left. We know this because some parts we manufacture are for high power lasers, and if the surface are not defect free, power loss occurs. Using hydrofluric acid, surfaces can be tested for residual subsurface damage, which can be present, even if no visable defects are present. I guess what I'm saying is if you can't see it at 40x, you shouldn't worry about it. I prefer the bright field format, as the polished surface appears white, with pits or scratches being black.

We make a sweep from center to edge, looking for pitting. Since most of our work is machine polishing, it is easy to polish out pitting completly, in hand polishing mirrors, it takes a bit of work to to remove the last few pits, but if properly ground, it'll happen, and as pits stand out much more after aluminizing, take that extra time. Though as we all know, a few pits won't affect the overall quality of a mirror, why not go for perfection.

Finished windows are QA'd using a high intensity source light, and are spec'd by the scratch dig callout for the specified part. This has to do with the permitable defect size, which has to do with ratios of defect size, number of defects, etc, in porportional to the size of the part. 80-50 scratch/dig is commercial(ie crap) surfaces. 10-5 is typical of a laser quality surface.

In more sophisticated optical shops this is read and digested and spit out via computers...

Bill Marriott btk@ix.netcom.com