Fizeau interferograms are evaluated by two-beam phase-stepping algorithms. The sinusoidal intensity profile pattern needed for the application of these algorithms is obtained by low-pass filtering the moiré pattern provided by the superposition of the interferogram onto a Ronchi grid. The phase modulation is performed by the in-plane displacement of the grid by means of a motorized stage. In this research the measurement range of the evaluation method as a function of the maximum derivative of height surfaces is calculated. A spherical surface is measured in different configurations in order to verify the obtained results. # 1997 Optical Society of America
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