(heterodyne interferometer at 633nm)
z<300 nm peak to peak
spatial frequency between few mm-1 and few 100 mm-1
noise = few 0,001 nm RMS
Measure on a 1mm long circular path.
Mecanical adaptation is needed to keep the surface perpendicular to the beam in case of spherical surface.
Plan, concave, convex, traited or not.