Noise Analysis Directly on Shearography Wrapped Phase Maps

Shearography is a contactless, full-field, and laser-based optical nondestructive testing (NDT) method capable of measuring deformation fields. However, due to its interferometric nature, it is susceptible to environmental noise factors. Therefore, quantifying the amount of noise contained in the interferograms is a leading factor in increasing the robustness of shearography systems. In addition, noise measurement can be used as a determining parameter for the feasibility of shearographic inspections, allowing the establishment of practical limits conducive to reliable and accurate shearography inspections. This work introduces a novel, streamlined, and efficient method for evaluating random noise. To the best of our knowledge, this is the first demonstration of a noise evaluation routine directly on wrapped phase difference maps, eliminating the necessity for phase unwrapping algorithms. This not only reduces one step in the entire procedure but also mitigates potential errors associated with the unwrapping step, thereby simplifying and refining the entire process. The proposed method was initially applied to simulated wrapped phase difference maps containing Gaussian noise, leading to relative errors smaller than 1% for synthetic maps with noise levels below 62 gray levels. An upper limit of 65 gray levels was observed, as the image are then completely degraded. Finally, the proposed method was applied to real phase maps acquired experimentally with a one-shot shearography device, where results showed that the method is capable of measuring noise with amplitudes up to
$0.07 \mu \text{m}$
(70 nm).