Abstract: The signal to noise ratio in non-destructive testing or evaluation of a sample (1) by in phase stepped optical inspection systems such as the optical shearography system using a shearing interferometer involves production of correlated speckle images successively stepped in phase by illuminating with coherent radiation a sample in an unstressed state, illuminating the sample with coherent radiation, stressing it incrementally at predetermined stress increments, and generating and capturing correlated speckle images successively stepped in phase of the incrementally stressed sample at the predetermined stress increments, using the phase stepped speckle images of the unstressed and incrementally stressed sample to calculate the phase before and after stressing and differencing the images to extract the incremental phase change, inspecting the magnitude and sign of each incremental phase change and if the phase difference between successive measurements increases by more than &pgr; subtracting 2 &pgr; from the m