Evaluation steps of the fringe observation system. The carrier-frequency method is applied to a region of interest in the camera image (indicated by the red rectangle). The main frequency peak of the 2D FFT is shifted to the center of the Fourier space and all other frequencies are set to zero. After inverse 2D FFT, the phase can be calculated from the imaginary part and the real part. The wavefront offset, which corresponds to the fringe movement of the fringe pattern, then can be calculated by the mean value.