Fringe scanning speckle-pattern interferometry

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Abstract
Digital speckle-pattern interferometry systems for automatic measurement of deformations of a diffuse object are presented, which are based on a fringe scanning method with phase-shifted speckle interferograms. A digital speckle pattern before deformation of an object is recorded in the mass storage device of a computer facility. After deformation, four digital speckle patterns are recorded as changing the phase of reference light such as 0, π/2, π, and 3π/2, respectively. Four speckle interferograms, whose phases are shifted by 0, π/2, π, and 3π/2, are generated by calculating the square of the differences between speckle patterns before and after deformation. These interferograms are low-pass filtered to reduce speckle noise. The calculation of the arctangent with four phase-shifted speckle interferograms gives the optical path difference which is proportional to the deformation. A correction of the discontinuity of the calculated phase gives the numerical data of the deformation in the whole object area. Some experimental results for the measurement of out-of-plane, in-plane, and 3-D deformations are presented.