This paper is a theoretical presentation of a two wavelength holographic method of measuring the roughness of a surface. This method extends the range of surface roughness which can be measured to arbitrarily rough surfaces. This paper also discusses the factors that limit the accuracy of the proposed method.
William B. Ribbens, "Surface Roughness Measurement by Two Wavelength Holographic Interferometry," Appl. Opt. 13, 1085-1088 (1974)