Extended-range temporal electronic speckle pattern interferometry

In recent years the availability of high-speed digital video cameras has motivated the study of electronic speckle pattern interferometry (ESPI) in the time domain. To this end a properly sampled temporal sequence of N-fringe patterns is used to analyze the temporal experiment. Samples of temporal speckle images must fulfill the Nyquist criteria over the time axis. When the transient phenomena under study are too fast, the required sampling frequency over time may not be fulfilled. In that case one needs to extend the measuring range of the algorithm used to extract the modulating phase. We analyze how to use short laser pulses or short video acquisition times with fairly long temporal separation among them to estimate the modulating phase of a dynamic ESPI experiment. The only requirement is that the modulating phase being estimated be properly sampled in the spatial domain.     

Multiwavelength electronic speckle pattern interferometry for surface shape measurement

Profilometry by electronic speckle pattern interferometry with multimode diode lasers is both theoretically and experimentally studied. The multiwavelength character of the laser emission provides speckled images covered with interference fringes corresponding to the surface relief in single-exposure processes. For fringe pattern evaluation, variations of the phase-stepping technique are investigated for phase mapping as a function of the number of laser modes. Expressions for two, three, and four modes in four and eight stepping are presented, and the performances of those techniques are compared in the experiments through the surface shaping of a flat bar. The surface analysis of a peach points out the possibility of applying the technique in the quality control of food production and agricultural research.     

Contrast enhancement of electronic speckle pattern interferometry addition fringes

The electronic speckle pattern interferometer in the double-pulse addition mode can be used to measure physical parameters in unstable environmental conditions. Owing to additive optical noise, however, the fringe patterns obtained have poor contrast. Some methods that use subtraction of addition double-pulsed fringe patterns improve fringe visibility but impose a limitation in measurement time ranges. To increase this range, to be limited by only the pulse separation, the contrast enhancement of double-pulsed addition-fringe patterns with a spatial filter based on local-standard-deviation measurements is investigated. Computer simulations and experimental results are presented.     

Phase extraction from electronic speckle pattern interferometry addition fringes

Addition fringes are obtained in real time from electronic speckle pattern interferometry (ESPI) by use of a twin-pulsed laser when two pulses are fired during a single field of a CCD camera. This enables object deformations to be studied in harsh environmental conditions. However, the fringe patterns have poor visibility because optical noise is additive. To our knowledge automatic phase extraction from addition fringes has not previously been achieved: Low-pass filtering to suppress random speckle noise also eliminates the fringes because of their low visibility. Two phase-stepping algorithms that calculate phase from ESPI fringes without the need for a preprocessing filter are presented. In the first ESPI subtraction fringes are considered, for which an improvement in accuracy is seen, and in the second ESPI addition fringes are considered, which, we believe, has enabled the phase to be extracted for the first time. The algorithms are demonstrated with theoretical data and with experimental ESPI fringepatterns recorded with a cw laser. As presented, they form the first step toward a procedure that can beused with twin-pulsed ESPI.     

Digital processing of electronic speckle pattern interferometry addition fringes

A digital image-processing method for analyzing double-pulsed electronic speckle pattern interferometry addition fringes is described. The procedure consists of three steps, forming a combination particularly suited to addressing some important practical limitations of the measurement system. In the first step it is shown that in certain cases fringe visibility may be enhanced by subtraction of a reference interferogram, so that a pattern with a quality similar to that of a subtraction one is obtained. In the second step noise is reduced by the application of a spectral subtraction image-restoration method. The third step concerns the calculation of the wrapped phase by means of a Fourier transform method with bandpass filtering. Preliminary experimental results that illustrate the performance of this approach are presented.     

Theoretical investigations on dual-beam illumination electronic speckle pattern interferometry

Contrary to what is found in most of the existing scientific literature, where a specific frame is developed, the theory of speckle interferometry is (conveniently) presented here as a particular case of the more general theory of holographic interferometry. In addition to the intellectual benefit of dealing with a single unified theory, this brings about many advantages when it comes to discuss fundamental topics such as the three-dimensional evolution of the complex amplitude of the diffuse optical wavefronts, the degree of approximation of the leading formulas, the loss of fringe contrast, the decorrelation effects, the real influence of the terms generally neglected in out-of-focus regions. In the same way, the statistical properties of the speckle fields, usually treated as a separate subject matter, are also integrated in the theory, thus providing a comprehensive knowledge of the qualitative features of speckle interferometry methods, otherwise difficult to understand.     

Vibration analysis of logs with electronic speckle pattern interferometry

Electronic speckle pattern interferometry combined with phase-shifting techniques for vibration analysis of logs is presented. Changes in the vibration pattern were followed on a television monitor at the video rate. We determined resonant vibrations by scanning through the frequency range of interest and by changing the point of excitation. We observed bending and torsional modes of vibration by changing the support and point of excitation. The vibrational modes reflect the structural properties of the material. The longitudinal modulus of elasticity and the shear modulus were calculated, giving valuable information for the strength grading of logs.     

Analysis of coherent symmetrical illumination for electronic speckle pattern shearing interferometry

In an effort to find a non-contact technique capable of providing measurements of in-plane strain, a speckle shearing interferometer was designed using symmetrical coherent illumination. This paper presents an analysis of the sensitivity to displacement and strain of this interferometer, together with an analysis of the phase-stepping of the resultant fringe patterns. New notation is introduced alongside this analysis to define the interference components in speckle shearing interferometers using multiple illumination beams. Experimental results show fringe patterns and phase stepping in support of the theoretical analysis.     

Measurement of in-plane strains using electronic speckle and electronic speckle-shearing pattern interferometry

Abstract

This letter reports the application of the electronic speckle and electronic speckle-shearing pattern interferometry for determining the first-order partial derivatives of in-plane displacements. Two possibilities of obtaining the in-plane strain contours are described. Experimental results showing the in-plane strain and the in-plane shearing strain contours on a tensile test specimen are presented.     

Optimum determination of speckle size to be used in electronic speckle pattern interferometry

Characteristics of the fringe pattern detected by an electronic speckle pattern interferometer, in conditions in which a test object deforms in an arbitrary direction and the speckle intensity is detected over a pixel area in the TV camera to be used, have been investigated from two aspects: speckle noise reduction and fringe contrast. The main result is that the fringes are obtained with high contrast and low speckle noise, if the speckle size is selected by the optical system so as to be smaller than the pixel size. This result is applicable to highly accurate measurements of the out-of-plane displacements of the test object, whose in-plane displacement is small.     

Depth-resolved whole-field displacement measurement by wavelength-scanning electronic speckle pattern interferometry

We show, for the first time to our knowledge, how wavelength-scanning interferometry can be used to measure depth-resolved displacement fields through semitransparent scattering surfaces. Temporal sequences of speckle interferograms are recorded while the wavelength of the laser is tuned at a constant rate. Fourier transformation of the resultant three-dimensional (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 provide the out-of-plane component of the 3-D displacement field. The principle of the technique is explained in detail and illustrated with a proof-of-principle experiment involving two independently tilted semitransparent scattering surfaces. Results are validated by standard two-beam electronic speckle pattern interferometry.     

Computer-aided speckle pattern interferometry

An on-line computer system for measuring the deformation of a diffuse object with a speckle interferometer is presented. Methods for evaluating a speckle interferogram using digital image processing techniques are also discussed. The system consists of an interferometric optical setup and a computer-TV image processing facility. A speckle interferogram is generated arithmetically between two digitized speckle patterns before and after deformation of the object. The information about the deformation is extracted by two procedures in analyzing the interferogram: (a) automatic analysis using digital image processing techniques such as gray scale modification, linear spatial filtering, thresholding, and skeletoning; (b) man-machine interactive method for simple high-speed processing of the interferogram using a light pen. The determined fringe order numbers are interpolated and differentiated spatially to give strain, slope, and bending moment of the deformed object. Some examples of processed patterns are presented.     

Increased sensitivity to in-plane displacements in electronic speckle pattern interferometry

We describe an optical arrangement that increases the sensitivity to in-plane displacement in an electronic speckle-pattern interferometric system. This is accomplished by oblique illumination and observation along the direction of illumination. An anamorphic prism placed in front of the object is used to correct for the eccentricity in the image caused by the oblique observation. The sensitivity to in-plane displacement can be increased to a maximum of approximately λ/2. Experimental results including phase stepping are presented.     

Vibration measurement by the time-averaged electronic speckle pattern interferometry methods

Three different image-processing methods based on the time-averaged technique were compared by the electronic speckle pattern interferometry (ESPI) technique for vibration measurement. The three methods are the video-signal-addition method, the video-signal-subtraction method, and the amplitude-fluctuation method. Also, errors introduced by using the zero-order Bessel function directly into the analysis of the fringe pattern were investigated. The video-signal-addition method has been the most generally used ESPI technique for vibration measurement. However, without additional image and/or signal-processing procedures, the fringe pattern obtained directly by the video-signal-addition method is rather difficult to observe. The reason for poor visibility of the experimentally obtained fringe pattern with this method is explained. To increase the fringe pattern's visibility without additional image and/or signal processes, we tried two video-signal-subtraction methods. One of the two methods is the video-signal-subtraction method that has normally been used in the static applications. The other method, called the amplitude-fluctuation method, and its associated theory are reported here.     

Analysis of mutually incoherent symmetrical illumination for electronic speckle pattern shearing interferometry

A speckle shearing interferometer has been designed using symmetrical mutually incoherent illumination, in an effort to provide measurements of in-plane strain. An analysis of the sensitivity to displacement and strain of this interferometer is presented, together with analysis of the optical phase extraction of the resultant fringe pattern. This interferometer is an improvement on previous designs as it provides information on the in-plane strain separated from components of the displacement. Experimental results provide qualitative data in the form of fringe patterns in support of the theoretical analysis.     

Fringe analysis method for electronic speckle pattern interferometry using only speckle patterns before and after deformation

Electronic speckle pattern interferometry is employed in many industrial fields as a useful deformation measurement method. However, two speckle patterns obtained before and after the deformation are necessary for measurement. Furthermore, at least three speckle patterns are required for high resolution measurement using ordinary fringe scanning technologies. In this paper, a novel method that can measure high speed deformations using a limited number of speckle patterns without using high speed cameras is proposed. The method enables application to dynamic deformation analysis because the method involves analysis using only two speckle patterns obtained before and after the deformation. A novel optical system that can record some spatial information into each speckle is set up for the method. In experimental results, it is confirmed that the out-of-plane deformation measurement can be precisely performed by the method and that the resolution power is almost equivalent to that of the ordinary method.     

Measuring mixing dynamics of transparent fluids with electronic speckle pattern interferometry

Electronic speckle pattern interferometry (ESPI) combined with phase-shifting techniques is used for studying the mixing dynamics of transparent fluids. The potentials of the technique for studying fluid mixing are illustrated for simple examples of water flow and moving water droplets in water. With ESPI we could actually follow water droplets moving in water and water flowing in water on a television monitor at the video rate. A He-Ne laser (lambda(0) = 632.8 nm) was used as the light source, and phase stepping was applied, giving an interferometric sensitivity better than lambda(0)/10. The observed phase changes are due to changes in the refractive index caused by small temperature differences between the droplets and the surrounding water. Temperature differences of less than 0.1 K are detectable for droplets of a diameter of 4 mm.