Digital Speckle Pattern Interferometry System Research Articles

A reference material for evaluating full-field out-of-plane deformation measurement system

Optical techniques such as Digital Speckle Pattern Interferometry(DSPI) and Digital Image Correlation (DIC) have been widely used in deformation measurement for their advantages of full-field, non-contact and high accuracy. To exploit the advantages of those optical techniques and enhance the confidence in the data from the optical system, a physical reference material (PRM) that can generate a deformation field with submicron uncertainty is proposed. The PRM is a circular plate of composite structure including a central rod and the peripheral annular plate clamped at the edge, , the central rod of which has merely displacement and no deformation under displacement loading. The displacement of the central area of the PRM is used for calibration, which provides connection to the national standard for length. Through traceability comparison, the expanded uncertainty of full field deformation of the PRM is 0.03μm, with a coverage factor of K=2. The PRM is used to calibrate a digital speckle pattern interferometry system. The combined standard uncertainty (CSU) of the full-field out-of-plane deformation measurement is 0.090μm. Both theoretical analysis and experiments results show that the proposed device can be applied as a PRM to calibrate and evaluate optical full-field out-of-plane deformation measurement system to a submicron level of uncertainty.

Adaptive local threshold segmentation for Fourier spatial filtering in automatic analysis of digital speckle interferogram

Automated extraction of the +1 term spectrum can greatly benefit the real-time measurement of dynamic deformation in the digital speckle pattern interferometry (DSPI). However, most spatial filtering methods are not satisfactory for automatic analysis because they need manual intervention or are unable to accurately extract multiple +1 term spectrums in one frequency distribution image. We propose an adaptive local threshold segmentation method for the Fourier spatial filtering of a three-dimensional DSPI system. This method first uses global thresholding to coarsely recognize the spectral regions and then adopts local thresholding to accurately determine the spatial filtering windows for each spectrum. Experimental result shows that the proposed adaptive local threshold segmentation can accurately extract multiple desired +1 term spectrums even if the intensities of different spectral regions differ greatly. In addition, experiments of the comparison between the automatic and manual spatial filtering and automatic analysis of dynamic deformation demonstrate that the proposed method can be used for the real-time measurement.

大视场空间载波数字散斑干涉系统

大视场空间载波数字散斑干涉系统

Effect of the Speckle Size on the Quality of Speckle Pattern in DSPI System

In order to detect the deformation and strain of materials accurately, the key is to obtain the phase information caused by dynamic loading in digital speckle pattern interferometry (DSPI). In this paper, the evaluation method of quality of the speckle pattern in DSPI system is proposed, and the influence of the size of speckle grain on the stability and contrast of speckle pattern is discussed. And then, the strain detection experiments of inactive and bioactive materials are provided with different aperture slit size under the same detection conditions. The size of speckle grain has an important influence on the quality of speckle pattern. For strain detection of inactive materials, using the small size of speckles can obtain higher quality speckle pattern under the condition of satisfying the Nyquist theorem and spectral separation. For active biomaterials, non-structural factors easily induce the instability of speckle pattern, which leads to the de-correlation of between pre-deformation and post-deformation speckle pattern. So the compromise between the stability and the information capacity of speckle images should be considered in the selection of speckle size. Experiments show that the optimum size of speckles used for strain detection active biomaterials is larger than that of inactive biomaterials under the same conditions in the same DSPI system.

Spatial carrier color digital speckle pattern interferometry for absolute three-dimensional deformation measurement

It is difficult to measure absolute three-dimensional deformation using traditional digital speckle pattern interferometry (DSPI) when the boundary condition of an object being tested is not exactly given. In practical applications, the boundary condition cannot always be specifically provided, limiting the use of DSPI in real-world applications. To tackle this problem, a DSPI system that is integrated by the spatial carrier method and a color camera has been established. Four phase maps are obtained simultaneously by spatial carrier color-digital speckle pattern interferometry using four speckle interferometers with different illumination directions. One out-of-plane and two in-plane absolute deformations can be acquired simultaneously without knowing the boundary conditions using the absolute deformation extraction algorithm based on four phase maps. Finally, the system is proved by experimental results through measurement of the deformation of a flat aluminum plate with a groove.

Characterization and optimization of illumination vector for contouring surface form and feature using DSPI.

Surface defect or damage is one of the critical factors leading to the failure of engineering materials and structures. The methodologies for the measurement of surface shape and feature or defect have been extensively explored and developed over the past few decades, including both contact and non-contact methods. Speckle pattern interferometry, as a non-contact optical method, has been demonstrated to effectively contour the surface shape through adjusting the illumination vector. However, few studies have been made to investigate the effect of the initial position of the illumination source as well as the source translation direction. In this paper, we report to carry out a study of measuring the surface form and feature using digital speckle pattern interferometry system via a slight translation of illumination source. Through theoretically analyzing the sensitivity factor along with the experimental validation, it is shown that the contouring fringe is more sensitive to the surface height with an off-axis illumination than the paraxial illumination. It is also found that translating the source along axial and lateral direction can be both used for the surface shape re-construction.

Synchronous triple-optical-path digital speckle pattern interferometry with fast discrete curvelet transform for measuring three-dimensional displacements

Digital speckle pattern interferometry (DSPI) is a well-established and widely used optical measurement technique for obtaining qualitative as well as quantitative measurements of objects deformation. The simultaneous measurement of an object's surface displacements in three dimensions using DSPI is of great interest. This paper presents a triple-optical-path DSPI based method for the simultaneous and independent measurement of three-dimensional (3D) displacement fields. In the proposed method, in-plane speckle interferometers with dual-observation geometry and an out-of-plane interferometer are optimally combined to construct an integrated triple-optical-path DSPI system employing the phase shift technique, which uses only a single laser source and three cameras. These cameras are placed along a single line to synchronously capture real-time visible speckle fringe patterns in three dimensions. In addition, a pre-filtering method based on the fast discrete curvelet transform (FDCT) is utilized for denoising the obtained wrapped phase patterns to improve measurement accuracy. Finally, the simultaneous measurement of the 3D displacement fields of a simple beam and a composite laminated plate respectively subjected to three-point and single-point bend loading are investigated to validate the feasibility and effectiveness of the proposed method.

Simultaneous Measurement of In-Plane and Out-of-Plane Deformations Using Dual-Beam Spatial-Carrier Digital Speckle Pattern Interferometry

Digital speckle pattern interferometry (DSPI) is an advanced technique for both in-plane and out-of-plane deformation measurements of diffuse surfaces in nanoscale. It has been widely used in aerospace engineering and other high-tech industries due to the advantages of non-contact, high-accuracy and full-field measurement. Traditionally, DSPI uses temporal phase shifting method to achieve precise deformation measurement, but it is only suitable for quasi-static deformation. Spatial-carrier method is another effective phase retrieval method used in DSPI and its validity has been verified in some DSPI setups. DSPI with spatial-carrier method enjoys the advantages of simple optical arrangement, easy operation, and above all, high-speed measurement of deformation. This paper introduces a dual-beam spatial-carrier digital speckle pattern interferometry system, with which in-plane and out-of-plane deformations can be measured simultaneously as well as quickly. In the optical setup, two lasers are employed to illuminate the measured object with different illumination angles, and two single-mode fibers server as carriers to transmit the reference beams. In-plane and out-of-plane deformations can be obtained by combining the phase maps of both channels. Theoretical discussion and experimental analysis are both presented.

A Novel Optical Method for Real-Time Control of Bolt Tightening

A digital speckle pattern interferometry (DSPI) system is developed for the real-time measurement and monitoring of the out-of-plane surface deformation around a preloaded bolt head or nut. The proposed system is specifically developed for the dynamic control of the bolt tightening process by continuously monitoring the out-of-plane joint surface deformation that will have been independently correlated to the bolt preload. Spatial phase shifting is employed to quantitatively determine the distribution of phase data by introducing a spatial carrier fringe pattern to the speckle interferogram. This is achieved by leading the object and reference beams through two separate apertures. The configuration is also suitable for collecting the real-time surface deformation during bolt tightening. The experimental DSPI system is set-up with optical components on a vibration-isolation table. A MATLAB software is developed for image acquisition and phase data calculations that yield the out-of-plane surface deformation caused by the bolt preload. The test fixture uses an M12 steel fastener and aluminum joint. For miniature screw applications, however, a plastic joint is used.

Simultaneous measurement of out-of-plane displacement and slope using a multiaperture DSPI system and fast Fourier transform

The simultaneous quantitative measurement of out-of-plane displacement and slope using the fast Fourier transform method with a single three-aperture digital speckle pattern interferometry (DSPI) arrangement is demonstrated. The method coherently combines two sheared object waves with a smooth reference wave at the CCD placed at the image plane of an imaging lens with a three-aperture mask placed in front of it. The apertures also introduce multiple spatial carrier fringes within the speckle. A fast Fourier transform of the image generates seven distinct diffraction halos in the spectrum. By selecting the appropriate halos, one can directly obtain two independent out-of-plane displacement phase maps and a slope phase map from the two speckle images, one before and the second after loading the object. It is also demonstrated that by subtracting the out-of-plane displacement phase maps one can generate the same slope phase map. Experimental results are presented for a circular diaphragm clamped along the edges and loaded at the center.

Measurement of strain distributions in mouse femora with 3D-digital speckle pattern interferometry

Bone is a mechanosensitive tissue that adapts its mass, architecture and mechanical properties to external loading. Appropriate mechanical loads offer an effective means to stimulate bone remodeling and prevent bone loss. A role of in situ strain in bone is considered essential in enhancement of bone formation, and establishing a quantitative relationship between 3D strain distributions and a rate of local bone formation is important. Digital speckle pattern interferometry (DSPI) can achieve whole-field, non-contacting measurements of microscopic deformation for high-resolution determination of 3D strain distributions. However, the current system does not allow us to derive accurate strain distributions because of complex surface contours inherent to biological samples. Through development of a custom-made piezoelectric loading device as well as a new DSPI-based force calibration system, we built an advanced DSPI system and integrated local contour information to deformation data. Using a mouse femur in response to a knee loading modality as a model system, we determined 3D strain distributions and discussed effectiveness and limitations of the described system.

Digital speckle pattern interferometry (DSPI) with increased sensitivity: Use of spatial phase shifting

This paper presents a real-time digital speckle pattern interferometry system with twofold increase in sensitivity for the measurement of in-plane displacement and first order derivative of out-of-plane displacement (slope). Spatial phase shifting technique has been used for quantitative fringe analysis. The system employs a double aperture arrangement in front of the imaging system that introduces spatial carrier fringes within the speckle for spatial phase shifting. For in-plane displacement measurement, the scattered fields from the object are collected independently along the direction of illumination beams, and combined at the image plane. For slope measurement, a shear is introduced between the two scattered fields. Experimental results on an edge clamped circular plate subjected to in-plane rotation for in-plane displacement measurement and central loading for slope measurement are presented.

A least-squares method to cancel rigid body displacements in a hole drilling and DSPI system for measuring residual stresses

This paper presents the evaluation of a method to cancel rigid body displacements that can be introduced when a hole drilling and digital speckle pattern interferometry (DSPI) combined system is used to measure residual stresses. The proposed method is based on a least-square calculation of three correction parameters determined from two evaluation lines located near the edge of the phase map where the displacement field generated by the drilling process is supposed to be negligible. The errors introduced by the method for different residual stress levels and rigid body displacements are analysed using a numerical simulation. An application of the method to experimental data is also presented.

Measurement of the near-tip displacement field in a fatigue damaged steel plate by digital speckle pattern interferometry

A digital speckle pattern interferometry system with automatic data analysis is used to measure the near-tip in-plane displacement field in a stainless steel specimen previously cracked by a fatigue test. In-plane displacements over a zone of 3.15 mm×3.15 mm are determined through the evaluation of the optical phase distribution. A theoretical solution developed for elasto-plastic materials is used to evaluate the obtained experimental results.

Impact-induced transient deformation analysis by means of digital speckle pattern interferometry

Transient in-plane displacements generated in a metal plate subjected to impact loading are measured using a pulsed digital speckle pattern interferometry system. Two separated speckle patterns produced by a ruby laser that freezes the object motion are recorded using a CCD camera and stored in a frame grabber. Transient displacements are evaluated by digital analysis of the fringes generated by the subtraction of these speckle patterns. The computer used for image processing is also utilized to control image acquisition and to synchronize the laser pulses to the CCD camera and the object position. Experimental results are compared with numerical calculations obtained using the finite-element method.