Using speckle interferometry to study longitudinal surface displacements in metallic beam samples

It is shown that the speckle nature of electronic speckle pattern interferometry images can be reduced in quasireal time with the use of only three consecutive video images. By use of additive-subtractive phase modulation processing the technique is essentially insensitive to environmental noise, and distortion of fringes does not occur. By further removing the random speckle phase we show, using speckle statistics, that the number of dark speckles in the resulting fringe pattern is close to zero.

Optimization of wavefield intensities in phase-shifting speckle interferometry

Video-based speckle interferometric methods such as electronic speckle pattern interferometry (ESPI) allow us to measure full-field surface deformation of a diffuse object. In this paper we show, in a first step, that the susceptibility of ESPI to noise can be substantially reduced [1] by synchronizing the optical interferometer and the object stressing system with the CCD image acquisition and processing system, and by performing what amounts to a repetitive sequence of rapid ESPI tests. In this manner, a stable fringe pattern can be obtained as long as the ambient noise is of sufficiently lower frequency than the video acquisition rate (typically 30 Hz). This scheme will be referred in this paper as reference-updating subtractive correlated ESPI to distinguish it from classical ESPI. We then take this issue further with a hybrid additive-subtractive decollated ESPI technique that will freeze out unwanted environmental noise of frequencies much higher than video rates [2], The basic principle of this proposed technique can be summed up as follows: (i) acquire speckle images containing information about the same two deformed object states in every frame of the image acquisition sequence, (ii) decorrelate the speckles between every frame, and (iii) compare every consecutive pair of speckle images to extract visible fringes corresponding to the two deformed states of the test object.

The reconstruction of surfaces from speckle interferometry data is a demanding data-analysis task that involves edge detection, edge completion, and image reconstruction from noisy data. We present an approach that makes optimal use of the experimental information to minimize the hampering influence of the noise. The experimental data are then analyzed with a combination of wavelet transform and Bayesian probability theory. Nontrivial examples are presented to illustrate the proposed technique.

A theoretical and numerical investigation of the systematic phase errors in phase-shifting speckle interferometry is presented. The theoretical investigation analyzes the behavior of some systematic error induced by intensity variations in two cases of data-computing techniques. The first case deals with the technique in which the phase change is computed, unwrapped, and then linearly filtered; the second case deals with the technique in which the data are linearly filtered before the arctangent calculation and then unwrapped. With the first filtering technique it is shown that it is preferable when the phase change is of relatively low spatial frequency, leading to a particularly accurate method. With the second case it is demonstrated that an important parameter of speckle interferometry is the modulation factor of the interference frame that induces phase errors when the data are filtered before the arctangent calculation. We show that this technique is better than the first when the phase change is composed of high-spatial-frequency variations. The theoretical investigation of the two techniques is compared with numerical simulations, considering the frequency characteristics of the phase change, and this shows a good match between theory and simulations.

TV-Holography and holographic interferometry: Industrial applications

Y. M. HeHuazhong University of Science andTechnologyDepartment of MechanicsHuazhong 430074ChinaC. J. TayH. M. ShangNational University of SingaporeDepartment of Mechanical and ProductionEngineering10 Kent Ridge CrescentSingapore 119260E-mail: mpetaycj@nus.edu.sgAbstract. An electronic speckle shearing interferometry method for thefull field measurement of the slope of an object is presented. The objectunder study is placed on a rotating platform and illuminated by an ex-panded laser beam. Its image is recorded by a shearing CCD cameraand stored directly into a computer. By recording two images before andafter giving the object a small angle of rotation, the resulting correlationfringe pattern that represents the object surface slope can be obtained.The theory of the new method and some experimental results are pre-sented.

The interference between a uniform reference wave and a speckle object wave results in variable fringe contrast and background level. Taking these variations into account, we optimize system parameters of phase-shifting speckle interferometry. The results show that the optimal reference intensity should always be equal to one fourth of the detector’s saturation level. The optimal reference to the object-intensity ratio shows an increase from one up to, in most practical cases, six as a chosen interference dynamic range increases from its minimum value. The dependence of a maximal fraction of acceptable measurements on the dynamic range is calculated. Numerical examples indicate that we may hope for a readout accuracy in the range of 1/50th to 1/100th of a fringe period and still cover more than half of the image area with acceptable data. These data are taken without spatial averaging and have maximum resolution.

Carburizing is widely used to improve wear resistance and fatigue life of high duty machine parts. Fatigue performance of the carburized components is greatly dependent on the residual stress state in the surface layer. The aim of this paper is to measure the depth profiles of residual stresses in the carburized steels by electronic speckle laser interferometry (ESPI) assisted hole-drilling, and to compare the results with those measured by X-ray diffraction technique. To comprehend the differences in the residual stress state, the low-C steel components were carburized, and then, tempered in the range of 180–600 $$^{\circ }$$ C. Microstructural investigations and hardness measurements were also conducted. The results obtained from both techniques gave identical results, and showed that the beneficial compressive residual stresses exist at the surface after carburizing, and their magnitudes decrease with increasing tempering temperature. It was concluded that ESPI assisted hole-drilling, with optimized drilling and stress calculation parameters, is suitable for determining the residual stress state of the carburized and tempered steels.

Electronic Speckle Pattern Interferometry (ESPI) is becoming a very common test and measurement tool among industrial users. In its present form it rivals any current techniques for speed and ease of use. With improvements in image quality it has the potential to become the definitive tool for non-destructive testing.

The mean, the variance, and the correlation function of the intensity pattern resulting from an incoherent source that has propagated through a complex ABCD optical system are derived. The number of speckle correlation cells (or modes, N) within an effective measurement area is presented and discussed. Optical field decorrelation effects with respect to secondary fringe formation in phase-shifting speckle interferometry are discussed. In particular, the correlation coefficient resulting from load-induced object tilt, in-plane translations, and displacements parallel to the optic axis as a function of the number of modes propagating through the optical system is derived and discussed. In contrast to previous research, in which the calculation of speckle interferometric decorrelation effects was restricted to direct imaging systems and the special case where N → ∞, the correlation coefficients that we derive are valid for arbitrary complex ABCD systems (i.e., nonimaging configurations) and for all finite values of N ≥ 1.

The work is devoted to the application of an artificial neural network (ANN) to analyze eigenfrequencies of personal tires of different construction. Experimental measurements of personal tire eigenfrequencies by electronic speckle interferometry (ESPI) are compared with those previewed by ANN. Very good agreement of both data sets is presented.

The present paper describes a fitting procedure capable of providing a smooth approximation of experimental data distributed on a bidimensional domain, e.g. the typical output of an interferometric technique. The procedure is based on the optimization of an analytical model defined on the whole domain by the B-spline formulation. In the paper rectangular, circular and polygonal convex domains are considered in details, but, according to the need of the operating conditions, the procedure can be extended to domains of different shapes. The proposed procedure was initially calibrated by an analytical case study: a thin square plate simply supported along the edges and loaded by a uniform pressure. Subsequently, by the operative parameters defined by the analyses carried out on the analytic data, the fitting procedure was applied on experimental data obtained by phase shifting speckle interferometry.

Full-field vibration amplitude recovery from high-resolution time-averaged speckle interferograms and digital holograms by regional inverting of the Bessel function

A novel interferometric method for contour mapping of optically rough surfaces