Limitations in the densitometric analysis of moire contour patterns have been considered. A fringe analysis method based on heterodyne detection is described. The heterodyne method is shown to be comparatively insensitive to variations in the light-scattering properties of the surface and to allow automatic differentiation between concavity and convexity.
G. T. Reid, G. T. Reid,
"Surface Form Measurement By Heterodyne Moire Contouring", Proc. SPIE 0376, Optical Sensing: Techniques, Benefits, Costs, (10 August 1983); doi: 10.1117/12.934718; https://doi.org/10.1117/12.934718