Different possible interferometers for surface roughness measurements are described. Advantages and disadvantages of optical techniques compared to stylus instruments are discussed. Important features of phase-shifting techniques for surface roughness measurements are given. The WYKO NCP-1000M optical profiler is described along with techniques for measuring surfaces having an rms roughness on the order of 0.1 nm.
James C. Wyant,
"Optical Profilers For Surface Roughness", Proc. SPIE 0525, Measurement and Effects of Surface Defects & Quality of Polish, (3 September 1985); doi: 10.1117/12.946361; https://doi.org/10.1117/12.946361