The optimum condition of fringe spacing in a flatness test using digital fringe analysis is described. This condition is based on information theory and maximizes the amount of measurement information in a fringe image. First, the relationship between resolution and data sampling number is discussed. Then the optimum fringe spacing is derived using the concept of average information—entropy. Numerical analysis to evaluate fringe spacing in FFT fringe analysis is performed using the derived condition. An experiment is carried out with a liquid level flatness tester and its results show high space frequency resolution superiority at the condition.
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