Guillermo H. Kaufmann

Phase-shifted dynamic speckle pattern interferometry at 1 kHz

We describe a phase-shifting out-of-plane speckle interferometer operating at 1 kHz for studying dynamic events. The system is based on a Pockels cell that is synchronized to a high-speed video camera to ensure that the phase shifting occurs between frames. Phase extraction is performed by use of a standard four-frame algorithm, and temporal phase unwrapping allows sequences of several hundred absolute (rather than relative) displacement maps to be obtained fully automatically. The maximum theoretical surface velocity of 67 microm s(-1) is a factor of 40 greater than can be achieved with a speckle interferometer based on a conventional video camera. We test the system using a target that is displaced with constant speed in a direction normal to its surface by means of a piezoelectric transducer. The systems performance in a practical situation is illustrated with measurements on a thin plate undergoing out-of-plane deformation.

Comparative study of wavelet thresholding methods for denoising electronic speckle pattern interferometry fringes

This paper presents a comparative study of different thresholding methods for speckle noise reduction in electronic speckle pattern interferometry fringes using several wavelet bases. An approach based on the removal of the wavelet subbands of the transformed image is presented. The performance of this method is analyzed and compared with the results obtained with the denoising techniques that use wavelet shrinkage. It is shown that the wavelet subband removal method reduces speckle noise and maintains image features more effectively than the wavelet shrinkage techniques based on soft and hard thresholding.

SPECKLE NOISE REDUCTION IN TELEVISION HOLOGRAPHY FRINGES USING WAVELET THRESHOLDING

Recently, a novel wavelet method to reduce speckle noise in synthetic aperture radar images was presented. The method, based on the thresholding of the wavelet coefficients of the transformed image, is computationally efficient and maintains sharp image features. The application of a similar method is explored to reduce speckle noise in TV holography fringes, which is a very important problem in the quantitative analysis of transient phenomena. Several thresholding approaches are used to test the noise reduction algorithm on computer-simulated fringes and results are assessed through the evaluation of two comparative parameters: the image fidelity and the speckle index. It is shown that the method achieves the highest speckle reduction when speckle grains have the same average size of the image pixels.

Determination of residual stresses using hole drilling and digital speckle pattern interferometry with automated data analysis

A digital speckle pattern interferometry and hole drilling combined system is developed to determine the magnitude of the residual stress in a aluminum thin plate subjected to an uniform uniaxial tensile load. Performing automated fringe analysis, the optical data contained in the speckle interferograms are quickly converted into values of residual stress. The evaluation is carried out through the measurement of the in-plane displacement field generated by the introduction of the small hole. The displacement field is determined from the calculation of the optical phase distribution by means of a phase shifting method. The magnitude of the residual stress is finally evaluated through a least-squares calculation and compared with the stress value applied to the specimen.

Phase measurement in temporal speckle pattern interferometry: comparison between the phase-shifting and the Fourier transform methods

The measurement of dynamic displacements by use of speckle pattern interferometry and temporal phase unwrapping allows for the evaluation of large-object displacement fields without the propagation of spatial unwrapping errors. If a temporal carrier is introduced in one of the beams of the interferometer, phase data for whole-object displacement can be retrieved by use of a temporal phase-shifting method or a temporal Fourier transformation approach. We present a comparison between both methods of temporal phase measurement in terms of precision and execution speed. We performed the analysis by using computer-simulated speckle interferograms, an approach that allowed us to know precisely the original phase distribution and also to determine the spatial rms phase error as a function of the phase change introduced between two consecutive speckle interferograms. The performance of both methods to process experimental data is also illustrated by use of the results from a high-speed speckle interferometry study of a carbon fiber panel.

Noise reduction in digital speckle pattern interferometry using bidimensional empirical mode decomposition

We propose a bidimensional empirical mode decomposition (BEMD) method to reduce speckle noise in digital speckle pattern interferometry (DSPI) fringes. The BEMD method is based on a sifting process that decomposes the DSPI fringes in a finite set of subimages represented by high and low frequency oscillations, which are named modes. The sifting process assigns the high frequency information to the first modes, so that it is possible to discriminate speckle noise from fringe information, which is contained in the remaining modes. The proposed method is a fully data-driven technique, therefore neither fixed basis functions nor operator intervention are required. The performance of the BEMD method to denoise DSPI fringes is analyzed using computer-simulated data, and the results are also compared with those obtained by means of a previously developed one-dimensional empirical mode decomposition approach. An application of the proposed BEMD method to denoise experimental fringes is also presented.

Normalization of fringe patterns using the bidimensional empirical mode decomposition and the Hilbert transform

We evaluate a data-driven technique to perform bias suppression and modulation normalization of fringe patterns. The proposed technique uses a bidimensional empirical mode decomposition method to decompose a fringe pattern in a set of intrinsic frequency modes and the partial Hilbert transform to characterize the local amplitude of the modes in order to perform the normalization. The performance of the technique is tested using computer simulated fringe patterns of different fringe densities and illumination defects with high local variations of the modulation, and its advantages and limitations are discussed. Finally, the performance of the normalization approach in processing real data is also illustrated.

Evaluation of the continuous wavelet transform method for the phase measurement of electronic speckle pattern interferometry fringes

Universidad Nacional de RosarioInstituto de Fi´sica de Rosario(CONICET-UNR)andDepartamento de Fi´sicaFacultad de Ciencias ExactasIngenieri´a y AgrimensuraBoulevard 27 de Febrero 210 bisS2000EZP RosarioArgentinaAbstract. We present an evaluation of the continuous wavelet trans-form method when it is used to measure the phase distribution encodedby electronic speckle pattern interferometry (ESPI) fringes. The evalua-tion is performed using computer-simulated fringes, an approach thatallows knowing precisely the phase map contained in the pattern. It isshown that only ESPI fringes that verify the stationary phase approxima-tion and its analytic asymptotic limit can be analyzed with the continuouswavelet transform method. The influence of the filtering process tosmooth the ESPI fringes and the method used to extend the fringe pat-tern edges is also analyzed. Finally, additional drawbacks that emergewhen this phase evaluation method is applied are discussed.

Numerical processing of speckle photography data by Fourier transform

Using a 1-D analysis of a double-exposed specklegram, we explore the influence of the diffraction halo removal in the numerical processing of data when it is done via discrete Fourier transform. Relative errors in displacements appear if the removal is not done, and they increase as fringe visibility and fringe density decrease. These errors are <0.5% for fringe densities larger than six fringes within the diffraction halo.

Phase measurement in temporal speckle pattern interferometry using the Fourier transform method with and without a temporal carrier

The combination of speckle pattern interferometry and temporal phase unwrapping allows the measurement of large-object dynamic displacements without the propagation of spatial unwrapping errors. For retrieving phase data for whole-object displacement, the temporal Fourier transform method is a commonly used approach. As the temporal Fourier transform method can be applied with or without introducing a temporal carrier in one of the beams in an interferometer, this paper presents a comparison of both phase measurement techniques. The analysis is performed using computer-simulated speckle interferograms, an approach that allows to know precisely the original phase distribution and also to evaluate the spatial rms phase error as a function of the phase change introduced between two consecutive speckle interferograms. The performance of the temporal Fourier transform method to process experimental data modulated by a temporal carrier is also illustrated using results obtained from the study of a metal plate subjected to thermal loading.