Two-dimensional Fourier transform algorithm analyzing the interferogram and the fringe shift

A 2-D fast Fourier transform (FFT) algorithm for analyzing interferograms and the fringe shift is described. The phase errors caused by nonlinear response of a detector and by a random noise are analyzed theoretically. From the analysis, it is concluded that (1) the phase error due to the nonlinear response of a detector can be canceled by the proper filter window in the transform plane, and (2) the 2-D transform permits better separation of the desired information components from unwanted components than a 1-D transform. The relationship of 2-D FFT algorithm accuracy with factors such as the quantization of grey levels, spatial carrier frequency, spatial scanning direction, pixel array, form of the wavefront to be tested, etc., are discussed by analyzing a simulated ideal interferogram. An example of analyzing an actual interferogram and measuring the displacement of a piezoelectric transducer (PZT) device is given. In principle, the 2-D FFT algorithm can attain to an accuracy of (lambda) /100 approximately (lambda) /200 under optimum parameter conditions.