A disk made of crystal or glass with one surface finished to extremely precise flatness.
Contact with the workpiece surface that has been precisely mirror finished causes interference fringes for light waves that are observed and the degree of flatness of the mirror-finished surface can be determined. It is ideal for inspecting the flatness of the measuring surfaces of block gauges, micrometers and snatch gauges.

Specifications
Diameter: 60mm (made of glass)/130mm (made of glass)
Thickness: 15mm/27mm
Flatness: 0.1um 0.1um