Recent advances in temporal phase-shifting speckle interferometry

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Temporal phase-shifting speckle interferometry (TPSSI) is a technique based on the recording of a sequence of speckle interferograms throughout the entire deformation history of an object. To calculate the phase map, a controlled phase change is introduced in the reference beam as a function of time. As temporal phase unwrapping involves only one-dimensional signals, this procedure is generally easier to carry out than 2-D spatial unwrapping. TPSSI also allows large-object absolute displacement fields to be obtained. This paper provides an overview of recent developments and applications of temporal phase-shifting speckle interferometry. These include the performance of different phase-shifting algorithms when a TPSSI system is used in the presence of harmonic and random vibrations and also for measuring mechanical vibrations. A novel adaptive phase-shifting algorithm is also presented that searches for the peak of the windowed Fourier transform of the modulated intensity signal and evaluates the phase at that frequency instead of at the carrier frequency. The presentation is illustrated with an application of TPSSI to the detection of sub-surface delamination defects in carbon fiber specimens submitted to vacuum loading. It is shown that re-referencing the temporal phase unwrapping algorithm after different time intervals allows the measurement of time-varying displacements in excess of 100 μm without the limit normally imposed by speckle decorrelation.