We present an interferometry using a far-infrared light as a tool for surface topography measurement of rough reflective surfaces. The method is based on the optical configuration of classical Fizeau interferometry, but we achieve roughness tolerance by using a long-wavelength infrared light (λ = 10.6 µm). The method is called far-infrared Fizeau interferometry. We conducted a rigorous mathematical analysis to describe the true intensity distribution of fringe patterns while considering multiple reflections and surface roughness. The mathematical derivation is verified with experimental data obtained from specimens with various values of reflectivity and roughness. The effect of reflectivity and roughness on fringe contrast is discussed.
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