Evaluation of noise in measurements with speckle shearography

Speckle shearography has the advantage of allowing non-contact, full-field and high resolution measurements. Nevertheless, due to its nature and to its sensitivity to ambient perturbations, the measurements may present high levels of noise. The evaluation of the experimental noise is needed in order to assess the degree of accuracy of the measurements. This assessment is not straightforward since the measurements will involve resolving simultaneously the phase discontinuities and the phase fluctuations produced by the noise which are 2π wrapped. The phase discontinuities can be easily removed if the phase map is smooth and the phase jumps are not greater than 2π rad. However, this procedure cannot be directly applied for resolving the random phase fluctuations produced by the noise. In this work, a method is proposed to efficiently evaluate the noise in measurements with speckle shearography whenever it is in the range [−π, π]. The limits of the proposed method are assessed by applying it to phase maps with added noise, which are obtained through the numerical simulation of speckle shearography. The results show a very good evaluation of noise with low and moderate amplitudes. Finally, the proposed method is used to evaluate the experimental noise in rotation fields of a free-free aluminum beam measured with speckle shearography.