Both surfaces are finished to perfect flatness and in parallel.
y observing light wave interference fringes caused by the contact surface, the degree of flatness and parallelism of the measurement surfaces of measurement instruments and certain measurement tools can be checked.

Diameter: 30mm
Thickness: 12mm, 12.12mm, 12.25mm, 12.37mm
Flatness: within 0.1um
Parallel level: within 0.2um