Found this 20" diameter 3" thick glass disc. Is it an Optical Flat?

I recently bought some surplus items from NIST (National Institute of Science and Technology) and one item was a wooden storage crate with 3 glass discs. All are 20" in diameter, one is 3" thick and two are 3 1/2" thick. Look to be very heavy! The 3" disc is polished both sides, and the 3 1/2" thick discs have about a 1/2" wide groove around the perimeter and are silver coated on one side. The thicker discs seem to have some small internal bubbles, and the 3" thick disc is optically clear with no obvious internal flaws.

These came with a Universal Measuring Machine a Inspection Comparator, a Gage Comparator and some other measuring machines.

I am thinking the 3" disc could be an optical flat, and the others are mirrors of some type.

Any ideas as to what these are would be appreciated.

I appreciate your thoughts!

Dan

Sounds like a reasonable guess. Pics? Could be that one is just an unfinished (not coated yet) mirror also.

junkdealer said:

I appreciate your thoughts!

Dan

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You might want to reconsider your AKA if you want to be taken seriously.

The glass disks with the internal bubbles are made from Pyrex low thermal expansion glass. A quick check with a flashlight will reveal what the focal length of the mirrors are.

The glass disk that is defect free and finished on both sides is either a very large optical flat or a window. The optical flats used for measuring surface defects and geometry are made from fused quartz. A quartz flat larger than 12 inch diameter would be unlikely. There are other optical measurements setups that use a window to split a light beam. The accuracy requirements may not be as high as those used in a optical flat and they are made with a low hardness low melting point glass.
The windows sometimes have a antireflection coating on one surface.

The set of two mirrors plus a window all of the same diameter in a single storage box suggests that the optics were part of a interferometer setup for checking lenses.

I would agree with some of Robert R's observations, although I would not necessarily think that fused quartz is not a material for the 3" thick part, especially if the components were coming from NIST possession. On the other hand, the clear disk doesn't have the cross-section ratio that would normally be expected for a high-accuracy optical flat; that would typically be somewhere in the 6:1 range or less (diameter:thickness). The 3" thick part as noted could very well be a window of some sort, employed in some way as a common path component in an interferometric arrangement.

junkdealer said:

I recently bought some surplus items from NIST (National Institute of Science and Technology) ...

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National Institute of Standards and Technology