Adaptive phase-shifting algorithm for temporal phase evaluation

Most standard temporal phase-shifting algorithms evaluate the phase by computing a windowed Fourier transform (WFT) of the intensity signal at the carrier frequency of the system. However, displacement of the specimen during image acquisition may cause the peak of the transform to shift away from the carrier frequency, leading to phase errors and even unwrapping failure. We present a novel TPS method that searches for the peak of the WFT and evaluates the phase at that frequency instead of at the carrier frequency. The performance of this method is compared with that of standard algorithms by using numerical simulations. Experimental results from highspeed speckle interferometry studies of carbon fiber panels are also presented

Depth-resolved whole-field displacement measurement using wavelength scanning interferometry

We describe a technique for measuring depth-resolved displacement fields within a 3-dimensional (3-D) scattering medium based on wavelength scanning interferometry. Sequences of 2-dimensional interferograms are recorded whilst the wavelength of the laser is tuned at constant rate. Fourier transformation of the resulting 3-D intensity distribution along the time axis reconstructs the scattering potential within the medium, and changes in the 3-D phase distribution measured between two separate scans provides one component of the 3-D displacement field. The technique is illustrated with a proof-of-principle experiment involving two independently controlled reflecting surfaces. Advantages over the corresponding method based on low coherence interferometry include a depth range unlimited by mechanical scanning devices, and immunity from fringe contrast reduction when imaging through dispersive media

Measurement of sub-surface delaminations in carbon fibre composites using high-speed phase-shifted speckle interferometry and temporal phase unwrapping

A high-speed phase-shifted speckle interferometer has been developed recently for studying dynamic events. Speckle interferograms are continuously recorded by a CCD camera operating at 1 kHz with temporal phase shifting carried out by a Pockels cell running at the same frequency. Temporalphase unwrapping through sequences of more than 1000 frames allows the determination of time-varying absolute displacement maps. This paper presents the application of this speckle interferometry system to the detection and measurement of subsurface delamination defects in carbon fibre specimens. The influence of re-referencing the temporal phase unwrapping algorithm after different time intervals is analysed to reduce the random phase errors produced by speckle decorrelation and vibration. The performance of severalphase-shifting algorithms to minimize the influence of the vibration noise caused by the vacuum pump used to load the specimen is also investigated

Effects of random vibration in high-speed phase-shifting speckle pattern interferometry

The influence of random vibrations on a dynamic phase shifting speckle pattern interferometer, in which phase difference evaluation is performed using temporal phase shifting and temporal phase unwrapping, is investigated by means of experiments and numerical simulations. A well-defined velocity spectral density function, typical of the spectra found under non-vibration-isolated conditions, is used throughout. Five phase-shifting formulae are studied, with camera framing rates (1,2 and 4 kHz) typical of current dynamic speckle pattern interferometers. Two main aspects were evaluated: firstly the unwrapping reliability, and secondly the noise induced in the phase maps by the vibration. The former was found to be a significant constraint, even for peak velocities well below the Nyquist velocity limit of the interferometer, and is therefore likely to be more important than the latter in many applications. Three analytical criteria for determining the expected unwrapping success rate are proposed and their predictions compared with the measured values. It is demonstrated that shorter sampling windows and higher framing rates are preferred in order to increase the unwrapping success rate, but that longer windows reduce the root mean square error in the phase change maps due to the vibration