Method for increasing the signal-to-noise ratio in non-destructive testing

A method for increasing the signal-to-noise ratio in non-destructive testing or evaluation of a sample by a phase-stepped optical inspection system in which,

• correlated speckle images successively stepped in phase are generated and captured of a sample to be tested, by illuminating the sample, while in a static unstressed state, with coherent radiation,
• the sample is illuminated with coherent radiation, stressed incrementally at predetermined stress increments and correlated speckle images successively stepped in phase are generated of the incrementally stressed sample at the predetermined stress increments and captured,
• the phase-stepped speckle images of the unstressed sample and the phase-stepped speckle images of the incrementally stressed sample are used to calculate the phase before and after stressing and differenced to extract the incremental phase change,
• the magnitude and sign of each incremental phase change is inspected, and if the phase difference between successive measurements increases by more than π, 2 π is subtracted from the measurement values, or if the phase difference between successive measurements decreased by more than π, 2 π is added to the measurement value, and
• the phase at each point in each speckle, image is calculated with improved accurancy by summing the nearest neighbor phase differences in the image that are weighted by the square of their respective modulations, where the modulation is a measure of intensity variation with phase variation, and the result of this calculation is normalized by dividing by the sum of the modulations.