Lapping
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Lapping is a machining process in which two surfaces are rubbed together with an abrasive between them, by hand movement or using a machine.
Small lapping plate made of cast iron
Small lapping machine
Logitech lapping machine and retention jig
Optical flats in a wooden case
Optical flat
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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 tens of nanometres. They are used with a monochromatic light to determine the flatness of other surfaces, by means of wave interference.
A λ/20 optical flat that has been coated with aluminum, making a first-surface mirror
Two optical flats tested using 589 nm laser-light. At 2 inches (5.1 cm) in diameter and 0.5 inches (13 mm) thick, both surfaces are flat to within 1/10 of the wavelength of the light (58.9 nm), as indicated by the perfectly straight fringes.
Testing the flatness of surfaces with optical flats. The lefthand surface is flat; the righthand surface is astigmatic, with curvatures in two orthogonal directions.