Flatness nanometrology of large optically rough areas

A grating type interferometer uses a grazing reflection on the testing surface. The grazing reflectance could be very large and the projected roughness height on the incident light directions is small. We built such an interferometer using a diffractive optical element (DOE) realized in National Institute of Microtechnology. We obtain resolvable quality fringes for different materials from industrial quality metal pieces to wood. Contour map of a testing flatness metallic proof is obtained followng a 4 step algorithm: smoothing for both direction of interferogram, skeletonizing interference fringes, computing heights that correspond to the deviation from a plan surface for each fringe point; representing 3D contour lines of equal (400 nm) height.