Abstract: The zone-plate interference fringe pattern is analyzed by FFT. The spectrum obtained by FFT contains information on the shape error. To select the information on the shape error from the spectrum of the fringe pattern is difficult, because the resolution of the spectrum is low. An FFT with over- sampling is applied to increase the spectrum resolution. A method is proposed to select the necessary information from the spectrum of the fringe pattern. Experimental results obtained by the proposed method agree well with those obtained by Fizeau interferometer. !3
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