(thing) by baffo Mon Feb 23 2004 at 17:31:34
A piece of high quality glass lapped flat on one side. Used in laboratories to check the flatness of machined surfaces, though a process that uses a monochromatic light source to create interference figures, usually in the form of Newton rings.

Also, a very flat and clear disc of glass used in the cinema industry to protect expensive lenses or filters.

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