1) Optical device
Found on
https://www.crosswordclues.com/clue/optical-flat
An optical flat is an optical-grade piece of glass lapped and polished to be extremely flat on one or both sides, usually within a few millionths of an inch (about 25 nanometres). They are used with a monochromatic light to determine the flatness of other optical surfaces by interference. When an optical flat is placed on another surface and illum...
Found on
http://en.wikipedia.org/wiki/Optical_flat
<microscopy> Usually, a glass or quartz plate or disk, the thickness of which should be at least 1/10 of its diameter. It is ground until any remaining unevenness can be measured only by interferometric methods. Their maximum departure from flatness usually is less than 1/10 of the sodium doublet (589.3 nm). ... (05 Aug 1998) ...
Found on
http://www.encyclo.co.uk/local/20973
A piece of glass with one or both surfaces polished flat. Also known as a test plate, test glass or reference flat.
Found on
https://www.encyclo.co.uk/local/20687
A high quality glass lapped flat on one side and used with a monochromatic light to read seal face flatness.
Found on
https://www.encyclo.co.uk/local/21120
No exact match found.
