If a rough surface is illuminated by a coherent light wave of wavelength Al, it is not possible to determine the surface profile from the phases of the speckle field formed by the scattered light. However, if the rough surface is illuminated by an additional coherent wave of wavelength A2, the phase differences between the two speckle fields do contain information about the macro-scopic surface profile even though subject to a statistical error. We present the pertinent statistical properties of dichromatic speckle fields and show that (1) the macroscopic surface profile may be determined from the phase differences if the effective wavelength A = Al X2/I Al - A21 is sufficiently larger than the standard deviation of the microscopic profile of the illuminated surface and (2) the statistical error is reasonably small if the phase measurements are obtained from speckles with sufficient intensity.
A. F. Fercher,
"Two-Wavelength Speckle Interferometric Technique For Rough Surface Contour Measurement," Optical Engineering 25(5), 255623 (1 May 1986). https://doi.org/10.1117/12.7973875