Phase measurement improvement in temporal speckle pattern interferometry using empirical mode decomposition

When the Hilbert transform method is used to recover the phase distribution in temporal speckle pattern interferometry, the influence of the fluctuations of the bias and the modulation intensities on the calculated phase must be driven heuristically. In this paper we show that the Empirical Mode Decomposition method can overcome these drawbacks and consequently introduces an improvement in the evaluation of the phase distribution. An example is used to illustrate the phase measurement improvement that can be obtained by the application of the proposed approach.     

Mode-shape measurement of piezoelectric plate using temporal speckle pattern interferometry and temporal standard deviation

This study proposes an image processing method to improve the quality of interference fringes in mode-shape measurement using temporal speckle pattern interferometry. A vibrating piezoelectric plate at resonance was investigated, and the full-field optical information was saved as a sequence of images. According to derived statistical properties, an algorithm was developed to remove noise from both the background and disturbance, resulting in high-resolution images of excellent quality. In addition, the resonant frequency and mode shape obtained using the proposed algorithm demonstrate excellent agreement with theoretical results obtained by the finite element method.     

Low-frequency harmonic vibration analysis with temporal speckle pattern interferometry

In this paper, the time sequence phase method (TSPM) has been applied to measure the displacement caused by low-frequency vibration in temporal speckle pattern interferometry (TSPI). The principle is that by capturing the temporal speckle patterns related to the object vibration, the whole-field displacement responses (amplitude and phase) of the vibrating object can be calculated through scanning these fluctuations. Thus, quantitative measurement can be carried out using a conventional ESPI system without a camera synchronized to the object vibration or a phase shifting system. The elaboration on the method is given and experimental results are presented.     

Electronic speckle pattern interferometry using vortex beams

We show that it is possible to perform electronic speckle pattern interferometry (ESPI) using, for the first time to our knowledge, vortex beams as the reference beam. The technique we propose is easy to implement, and the advantages obtained are, among others, environmental stability, lower processing time, and the possibility to switch between traditional ESPI and spiral ESPI. The experimental results clearly show the advantages of using the proposed technique for deformation studies of complex structures.     

Electronic speckle pattern interferometry using optical fibers

Initial experiments using electronic speckle pattern interferometry with fiberoptic imaging and illumination are described.     

Vibration mode analysis using electronic speckle pattern interferometry

The use of electronic speckle pattern interferometry as a non-destructive testing technique has been widely reported for measuring a variety of objects. When used for vibration mode analysis, the only information presented to the operator was at the nodal area. The instrument has been developed so that, with the use of a microcomputer, the operator can now produce an isometric view of an object vibrating in a resonant mode, making the results easier to interpret. The instrument provides a real-time, non-contact alternative to other mode analysis equipment and can detect high-order modes as easily as low-order modes.     

Measurement of non-monotonous phase changes in temporal speckle pattern interferometry using a correlation method without a temporal carrier

Recently, a phase evaluation method was proposed to measure nanometric displacements by means of digital speckle pattern interferometry when the phase change introduced by the deformation is in the range [0,π) rad. This method is based on the evaluation of a correlation coefficient between two speckle interferograms generated by both deformation states of the object. In this paper, we present a novel technique to measure non-monotonous displacements in temporal speckle pattern interferometry using a correlation method without a temporal carrier. In this approach, the sign ambiguity is resolved automatically due to the introduction of a function that determines the correct sign of the displacement between two consecutive speckle interferograms. The rms phase errors introduced by the proposed method are determined using computer-simulated speckle interferograms. An application of the phase retrieval method to process experimental data is also presented.     

Quantitative modal analysis using electronic speckle pattern interferometry

Existing modal analysis techniques based on pointwise methods such as accelerometers and laser vibrometers, suffer from the compromises required to infer whole field vibration behaviour from a predetermined number of discrete measurement data points. The measurement grid is normally chosen before the most sensitive areas of the test piece have been identified and this can lead to a requirement for further grid refinement, with the consequent spatial and temporal disadvantages. Whole-field modal analysis using electronic speckle pattern interferometry (ESPI) has been developed, and focuses on the manipulation of optical information to provide a grid of data points which is transferred to a modal analysis software package for comparison with traditional point wise modal data sets. The advantages of obtaining the initial whole-field vibration picture in real time and subsequently overlaying a chosen measurement data grid, are demonstrated. Modal analysis of a steel plate is presented, which demonstrates single displacement vector measurements. Data is transferred into modal analysis software allowing ESPI mode shape information to be directly compared with measurements using other pointwise techniques and finite element analysis (FEA), through a modal assurance criterion (MAC) calculation. Further results are presented for multiple displacement vector analysis of high-power ultrasonic components, demonstrating the ability to accurately measure the modal characteristics of complex dynamic components.     

Denoising in digital speckle pattern interferometry using wave atoms

We present an effective method for speckle noise removal in digital speckle pattern interferometry, which is based on a wave-atom thresholding technique. Wave atoms are a variant of 2D wavelet packets with a parabolic scaling relation and improve the sparse representation of fringe patterns when compared with traditional expansions. The performance of the denoising method is analyzed by using computer-simulated fringes, and the results are compared with those produced by wavelet and curvelet thresholding techniques. An application of the proposed method to reduce speckle noise in experimental data is also presented.     

Comparison of phase recovery methods in spiral speckle pattern interferometry correlation fringes

Spiral interferometry can be used as a solution to the problem of sign ambiguity presented in the conventional speckle pattern interferometric technique when the optical phase needs to be reconstructed from a single closed fringe system. Depressions and elevations of the topography corresponding to the object deformation are distinguished by the direction of rotation of the local spiral fringe pattern. In this work, we implement and compare several methods for optical phase reconstruction by analyzing a single image composed of spiral speckle pattern interferometry correlation fringes. The implemented methods are based on contour line demodulation, center line demodulation, Spiral Phase Quadrature Transform and the 2D Riesz transform with multivector structure. Contour line and center line demodulation approaches are exclusively dedicated to images containing a fringe system with spiral structure. The others are based on the 2D Riesz transform, these being well known approaches in conventional interferometry. We examine simulated experiments and analyze some of the emerging drawbacks for solving the phase reconstruction problem by using different mean values of speckle size and background noise levels. We also discuss several numerical procedures that may well improve the efficiency and robustness of the presented numerical implementations. The performance of the implemented demodulation methods is evaluated by using a universal image quality index and therefore a quantitative comparison is also presented.     

Pulsed lasers in electronic speckle pattern interferometry

This paper describes the incorporation of a pulsed ruby laser into an electronic speckle pattern interferometer. A technique is described for observing vibrational and transient events with a double pulsed laser and some typical results are given. Results of the application of the interferometer to non-destructive testing are included.     

Measurement of poisson''s ratio using real-time digital speckle pattern interferometry

This paper describes an application of Digital Speckle Pattern Interferometry (DSPI) to the measurement of Poisson's ratio. This real-time technique provides several advantages over conventional holographic methods. A suitable operator interactive algorithm has been developed to evaluate the fringe pattern and then calculate the Poisson's ratio. Experimental results have been presented for a few specimens.     

In-plane electronic speckle pattern shearing interferometry

An analysis of the out-of-plane shearing interferometer has been performed which shows that production of in-plane strain partial derivatives is possible, which are not affected by out-of-plane displacement function components. The in-plane data are represented as subtraction correlation fringes. This interferometer employs a single diverging illumination beam and is applicable to object plane stress and plane strain loading conditions. The interferometer was tested and compared using a compact tension crack specimen and the results are correlated with finite element software predictions of strain distributions across modelled specimens. This experimental validation was chosen because we had an existing test rig and finite element models which had been independently verified.     

Temporal heterodyne shearing speckle pattern interferometry

Shearing speckle pattern interferometry is a full-field speckle interferometric technique used to determine surface displacement derivatives. In this paper, a new measurement system of real-time heterodyne shearography interferometry is presented. This system combined with heterodyne measurement, shearography interferometry and time domain signal processing technology can dynamically detect the out-of-plane displacement gradient. The principles and system arrangement are described. Using the Jones matrix, the mathematical expression of light intensity distribution passing through this system is deduced. A preliminary experiment was performed to demonstrate the performance of this new device, and simulations were conducted using the finite element method. Comparison of results shows that quantitative measurement of the displacement derivative has been achieved.     

Non-destructive testing of overlap shear joints using electronic speckle pattern interferometry

The electronic speckle pattern interferometry (ESPI)¹ which was used is capable of detecting debonded areas with certain limitations. These depend on the size of the defect and the thickness of metal used.Prepared specimens with debonded areas in the middle of adhesive joints and on the periphery of an overlap shear joint were tested using various loading techniques. It was found that the mechanical loading and vacuum chamber method gave the best results when using the ESPI system for detecting debonded areas.The aim of the present work was to develop a reliable method for the testing of an environmental degradation of an adhesive joint which would progressively deteriorate the joint around the periphery and progress inwards, thus weakening the joint.     

Accurate measurement of composite laminates deflection using digital speckle pattern interferometry

The modern digital speckle pattern interferometry (DSPI) technique is applied for the accurate measurement of the full-field deflection distribution of a bent composite laminates. Two kinds of powerful phase-shifting methods, phase of differences method (PDM) and difference of phases method (DPM), are described briefly and employed in DSPI to quantitatively extract the phase information, respectively. A comparison of the deflection distributions measured by DPM and PDM indicates that the former has a better measurement accuracy than the latter in laminate's static test experiment.     

Measuring velocity of speckle fields with digital speckle pattern interferometry

We investigate a digital method for detecting the velocity of a diffusing object. The technique is based on Digital Speckle Pattern Interferometry (DSPI). A set of reference fringes is generated externally through the reference beam in a digital interferometer. As the object moves, subsequent frames are acquired and subtracted according to the normal DSPI procedure and stored. By means of the theory of first order speckle statistics applied to speckle intensity correlation, we relate the visibility variations in the reference fringes with the object velocity. Thus, by measuring the fringe visibility variation in the resulting DSPI stored frames the mean object velocity can be obtained. The theoretical results are experimentally verified.     

Holography and electronic speckle pattern interferometry in geophysics

Geophysical applications of holography and ESPI are reviewed. First, laboratory experiments of rock deformation and failure with holographic interferometry and holographic in situ stressmeters are briefly summarized. Then, holographic measurements of tunnel deformations made in Japan are described. The holographic recording system, consisting of an He---Ne gas laser and associated optical elements, was installed in a tunnel at the Amagase Crustal Movement Observatory, Japan in 1984. Tunnel deformations caused by tidal and tectonic forces have been precisely determined using the ‘real-time’ technique of holographic interferometry. Finally, some attempts to apply ESPI to geophysical measurements are introduced.     

Optimal signal processing in electronic speckle pattern interferometry

A theoretical investigation of the signal processing by ESPI is presented. Both electronic and optical noise are taken care of, and the optimum of ESPI with regard to the reference/object-ratio is found. In the limit of small electronic noise contributions this optimum is reached when the noise on the reference beam is equal to the noise on the object beam.