Unwrapping of Digital Speckle-Pattern Interferometry Phase Maps by use of a Minimum L^0-Norm Algorithm (original) (raw)
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Unwrapping of digital speckle pattern interferometry phase maps using a minimum L0-norm algorithm
Proceedings of SPIE - The International Society for Optical Engineering
The performance of a minimum L0-norm unwrapping algorithm is investigated using computer-simulated digital speckle pattern interferometry (DSPI) wrapped phase maps. This algorithm estimates its own weights in order to mask inconsistent pixels. Particular features usually included in DSPI wrapped phase maps such as shears, speckle noise, fringe cuts, object physical limits, and superimposed fringe patterns are analyzed. Some adequate approaches to solve these features are discussed. It is shown that a complex case in which shears and fringe cuts coexist in the wrapped phase map cannot be successfully solved by the minimum L0-norm algorithm on its own. In order to cope with this problem, a new scheme is proposed.
Applied Optics, 1998
Inasmuch as current fringe analysis techniques used in digital speckle-pattern interferometry ͑DSPI͒ yield a phase map modulo 2, phase unwrapping is the final step of any data evaluation process. The performance of a recently published algorithm used to unwrap DSPI phase maps is investigated. The algorithm is based on a least-squares minimization technique that is solvable by the discrete cosine transform. When phase inconsistencies are present, they are handled by exclusion of invalid pixels from the unwrapping process through the assignment of zero-valued weights. Then the weighted unwrapping problem is solved in an iterative manner by a preconditioned conjugate-gradient method. The evaluation is carried out with computer-simulated DSPI phase maps, an approach that permits the generation of phase fields without inconsistencies, which are then used to calculate phase deviations as a function of the iteration number. Real data are also used to illustrate the performance of the algorithm.
EPJ Web of Conferences, 2010
Holographic interferometry (HI) techniques are powerful tools for experimental mechanics which can be used to assess displacement fields on a loaded object surface. Used with no contact and high resolution HI can be combined with image methods for data analysis. Nevertheless, the speckle should be fully resolved by the image detector to digitally record an interferogram. This leads to a noisy recording which difficult it's post-processing. Each measurement with HI ends normally in a phase map computation representing the object displacement. Phase calculation algorithms are based on spatial or temporal phase modulation and lead to wrapped phase maps. To obtain the continuous phase distribution several solutions have been implemented. This paper describes the main filtering and unwrapping algorithms available and compares it´s performance to solve discontinuities in noisy phase maps.
Applied Optics, 2003
We evaluate the use of a smoothed space-frequency distribution ͑SSFD͒ to retrieve optical phase maps in digital speckle pattern interferometry ͑DSPI͒. The performance of this method is tested by use of computer-simulated DSPI fringes. Phase gradients are found along a pixel path from a single DSPI image, and the phase map is finally determined by integration. This technique does not need the application of a phase unwrapping algorithm or the introduction of carrier fringes in the interferometer. It is shown that a Wigner-Ville distribution with a smoothing Gaussian kernel gives more-accurate results than methods based on the continuous wavelet transform. We also discuss the influence of filtering on smoothing of the DSPI fringes and some additional limitations that emerge when this technique is applied. The performance of the SSFD method for processing experimental data is then illustrated.
Applied Optics, 1999
The compensation of large in-plane motions in digital speckle-pattern interferometry ͑DSPI͒ with the use of digital speckle photography ͑DSP͒ is demonstrated. Ordinary recordings of DSPI are recombined and analyzed with DSP. The DSP result is used to compensate for the bulk speckle motion prior to calculation of the phase map. This results in a high fringe contrast even for deformations of several speckle diameters. In addition, for the case of an in-plane deformation, it is shown that the absolute phase change in each pixel may be unwrapped by use of the DSP result as an initial guess. The principles of this method and experiments showing the in-plane rotation of a plate and the encounter of two rounded plates are presented.
Optics express, 2006
Digital speckle pattern interferometry (DSPI) and digital shearography (DS) are well known optical tools for qualitative as well as quantitative measurements of displacement components and its derivatives of engineering structures subjected either static or dynamic load. Spatial phase shifting (SPS) technique is useful for extracting quantitative displacement data from the system with only two frames. Optical configurations for DSPI and DS with a double aperture mask in front of the imaging lens for spatial phase shifting are proposed in this paper for the measurement of out-of-plane displacement and its first order derivative (slope) respectively. An error compensating four-phase step algorithm is used for quantitative fringe analysis.
Phase-shifting speckle interferometry
Applied Optics, 1985
Speckle patterns have high frequency phase data, which make it difficult to find the absolute phase of a single speckle pattern; however, the phase of the difference between two correlated speckle patterns can be determined. This is done by applying phase-shifting techniques to speckle interferometry, which will quantitatively determine the phase of double-exposure speckle measurements. The technique uses computer control to take data and calculate phase without an intermediate recording step. The randomness of the speckle causes noisy data points which are removed by data processing routines. One application of this technique is finding the phase of deformations, where up to ten waves of wavefront deformation can easily be measured. Results of deformations caused by tilt of a metal plate and a disbond in a honeycomb structure brazed to an aluminum plate are shown.
Applied Optics, 2011
A method to construct an unweighted quality map for phase extraction and phase unwrapping is proposed, based on an object image pattern. The object image pattern must be recorded under the same conditions as that of the corresponding interference patterns, except that the lights coming from the reference arm of the interferometer are hidden. An unweighted quality map that can represent the valid and invalid regions in the interference patterns is completed successfully, based on two factors: the fact that the object region in the object image pattern is homologous with the valid region (i.e., the interference region) in the interference patterns, and on distinguishing between the object and background regions in the object image pattern using neighbor window threshold filtering and fitting the boundary of the object image. The application of the proposed method to the real measurement shows its feasibility and correctness. This paper might provide an alternative method for constructing an unweighted quality map for phase extraction and phase unwrapping.
Applied Optics, 2000
A method for measurement of continuous displacements that uses phase-shifting speckle interferometry is presented. The initial random phase of the speckle pattern is evaluated by phase shifting before deformation. The changing phase thereafter is evaluated from only one image at a time by a leastsquares algorithm. The technique can be used for measuring transients and other dynamic events, heat expansion as well as other phenomena, for which it is difficult to accomplish phase shifting during deformation. Theory along with computer simulations and experimental results are described.