Pattern Position Correction for Measuring Large Deformation in Speckle Interferometry

We propose a pattern position correction method for obtaining a fringe pattern in speckle interferometry to measure deformations of both small enough displacements and large displacements which exceed the limit in the same observation region. Intensity values at each point of the deformed state speckle pattern illuminated by dual beam are shifted to initial state location. In this method, not only speckle images illuminated by dual beam, but also speckle images illuminated by single beam are captured at the initial state and the deformed state. Shift amounts for each point are obtained by local least squares for data obtained by digital image correlation using the initial and the deformed state speckle images illuminated by single beam. Shift amounts vary continuously on each point. Intensity values at each point of the corrected speckle image are obtained by bilinear interpolation. Image subtraction is performed to the dual illuminated initial state spackle image from the corrected deformed state image. An interference fringe can be observed on not only the region that the displacement is small enough, but also the region that the displacement exceeds the range of conventional spackle interferometry, signifying the successful application of our proposed technique.