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Multi-camera stereo-digital image correlation (stereo-DIC) methods have gained popularity as non-contact methods for measuring high-spatial-resolution deformation of large engineering structures. Common multi-camera stereo-DIC methods can be classified into two types: target-based and calibration-based methods. Considering the wide application prospects of multi-camera stereo-DIC methods, it is important to study and compare the performances of these two types of multi-camera stereo-DIC methods. A four-point bending test of reinforced seawater sea-sand coral concrete (Coral-SWSSC) beams was conducted in this study to investigate the performance of multi-camera stereo-DIC in deformation measurement of slender components, and the measurement accuracy, robustness, and applicability of the two types of multi-camera stereo-DIC methods were compared and discussed. Furthermore, deformation analysis of reinforced Coral-SWSSC beams based on high-resolution multi-camera stereo-DIC methods was performed. The analysis showed that beams strengthened with basalt fiber-reinforced polymer wrapped steel tube and ultra high performance concrete (UHPC) can fully exploit the properties of Coral-SWSSC; the two strengthening materials demonstrated a higher compatibility, which compensates for the low strength defect of the Coral-SWSSC, making it a viable strengthening method. This study may promote the application of multi-camera stereo-DIC methods to measuring high-resolution deformation of large engineering structures.

In fringe projection profilometry (FPP), non-sinusoidal fringes due to the gamma effect of a projector will cause measurement errors. To solve this problem, a binary defocusing technique has been introduced in recent years, yet the appropriate defocusing is hard to evaluate quantitatively. An innovative approach to quantify binary defocusing in real-time is proposed. No matter how the projector and camera are arranged in FPP system, the fringe period in a captured defocusing fringe image is strictly varying; therefore, unlike previous methods, the proposed method uses a limited window for defocusing evaluation. This not only improves the accuracy of defocusing evaluation, but also greatly improves the evaluation speed, thus in turn enabling real-time evaluation. In addition, numerical differentiation is introduced on the window using a modified five-interval-point algorithm to effectively improve the sensitivity to improper (inadequate or excessive) defocusing. The difference between the numerical differentiation and its fundamental harmonic extracted by Levenberg–Marquardt iterative is taken as an evaluation value of binary defocusing. The smaller the difference, the more appropriate is the defocusing. By minimizing this difference, the most appropriate defocusing can be obtained quantitatively. Both numerical simulations and experiments validate the high sensitivity and high speed of the proposed method.

With the increase in digital image correlation (DIC) applications, it is an important task to realize the fully automatic measurement and provide an automatic deformation measurement sensor. Keeping this in mind, we propose an automatic speckle region selection method for DIC. This method can be divided into three main steps: initial contour selection, active contour computation, and morphological operation. The sum of modulus of the local intensity gradient vector of the local image is used to determine the initial contour, the active contour computation is driven by improved automatic local Gaussian distribution fitting energy, and the morphological closing operation is finally conducted to achieve better robustness. Experimental results show that the root mean squared error of the localization is less than 0.8260 pixels. We further integrate the proposed method into an open-source DIC software, and the fully automatic deformation analysis is realized. The fully automatic deformation measurement will further widen the application scope of DIC, especially in the field of smart robotics and smart city.

Measuring surface deformation of objects with natural patterns using digital image correlation (DIC) is difficult due to the challenges of the pattern quality and discriminative pattern matching. Existing studies in DIC predominantly focus on the artificial speckle patterns while seldom paying attention to the inevitable natural texture patterns. We propose a recursive-iterative method based on salient features to measure the deformation of objects with natural patterns. The method is proposed to select salient features according to the local intensity gradient and then to compute their displacements by incorporating the inverse compositional Gauss–Newton (IC-GN) algorithm into the classic image pyramidal computation. Compared with the existing IC-GN-based DIC technology, the use of discriminative subsets allows avoidance of displacement computation at pixels with poor spatial gradient distribution. Furthermore, the recursive computation based on the image pyramid can estimate the displacements of the features without the need for initial value estimation. This method remains effective even for large displacement measurements. The results of simulation and experiment prove the method’s feasibility, demonstrating that the method is effective in deformation measurement based on natural texture patterns.

A dual-biprism-based stereo camera system, with an imaging model derived from geometrical optics analysis, is proposed for detection of defects on the external and internal surfaces of pipelines. The proposed system, which comprises two biprisms, a lens, a single camera, and common light sources, overcomes the difference in the camera specifications and synchronization mechanism of conventional two-camera systems. Moreover, different fields of views can be obtained by adjusting the angles of the two prisms and the distance between the dual-biprism and the camera. Measurements can be made even for regions in narrow and small areas using a coaxial illumination and speckle-based three-dimensional digital image correlation method. After a typical experiment performed to verify the efficacy of the proposed system, three different types of defects were accurately detected: (1) a local indentation on the external pipe wall; (2) a weld defect in the pipeline that was measured, and the shape and location retrieved after image processing; and (3) a crack on the internal surface of the pipe determined based on strain calculation. These results validate the proposed system as an efficient and convenient tool for defect detection.

The strain errors in stereo-digital image correlation (DIC) due to camera calibration were investigated using precisely controlled numerical experiments and real experiments. Three-dimensional rigid body motion tests were conducted to examine the effects of camera calibration on the measured results. For a fully accurate calibration, rigid body motion causes negligible strain errors. However, for inaccurately calibrated camera parameters and a short working distance, rigid body motion will lead to more than 50-μϵ strain errors, which significantly affects the measurement. In practical measurements, it is impossible to obtain a fully accurate calibration; therefore, considerable attention should be focused on attempting to avoid these types of errors, especially for high-accuracy strain measurements. It is necessary to avoid large rigid body motions in both two-dimensional DIC and stereo-DIC.

Multiple digital image correlation (DIC) systems can enlarge the measurement field without losing effective resolution in the area of interest (AOI). However, the results calculated in substereo DIC systems are located in its local coordinate system in most cases. To stitch the data obtained by each individual system, a data merging algorithm is presented in this paper for global measurement of multiple stereo DIC systems. A set of encoded targets is employed to assist the extrinsic calibration, of which the three-dimensional (3-D) coordinates are reconstructed via digital close range photogrammetry. Combining the 3-D targets with precalibrated intrinsic parameters of all cameras, the extrinsic calibration is significantly simplified. After calculating in substereo DIC systems, all data can be merged into a universal coordinate system based on the extrinsic calibration. Four stereo DIC systems are applied to a four point bending experiment of a steel reinforced concrete beam structure. Results demonstrate high accuracy for the displacement data merging in the overlapping field of views (FOVs) and show feasibility for the distributed FOVs measurement.

Noninvasive, three-dimensional (3-D), full-field surface deformation measurements of the human body are important for biomedical investigations. We proposed a 3-D noninvasive, full-field body sensor based on stereo digital image correlation (stereo-DIC) for surface deformation monitoring of the human body in vivo. First, by applying an improved water-transfer printing (WTP) technique to transfer optimized speckle patterns onto the skin, the body sensor was conveniently and harmlessly fabricated directly onto the human body. Then, stereo-DIC was used to achieve 3-D noncontact and noninvasive surface deformation measurements. The accuracy and efficiency of the proposed body sensor were verified and discussed by considering different complexions. Moreover, the fabrication of speckle patterns on human skin, which has always been considered a challenging problem, was shown to be feasible, effective, and harmless as a result of the improved WTP technique. An application of the proposed stereo-DIC-based body sensor was demonstrated by measuring the pulse wave velocity of human carotid artery.

The bulge test has been utilized extensively in determining the mechanical properties of thin film materials. We develop a bulge-test system that consists of three main components, including an optical system, a loading system, and a control system. A telecentric lens is adopted as an imaging camera in an optical system to provide high imaging quality along with a constant angle of view across the entire field of view. The out-of-plane deformation is obtained by projecting a sinusoidal fringe pattern onto a specimen surface and analyzing recorded images with a digital image correlation algorithm. A least-squares polynomial fitting is presented to solve the problem that the out-of-plane calibration coefficient is not a constant. Experiments were performed to validate the availability and reliability of this proposed bulge-test system in the measurement of a dynamic microscopic deformation of membrane. The measuring range of deformation is from several microns to hundreds of microns with an accuracy of 0.6 μm.

In order to have an efficient design methodology and achieve high-performance inertial instruments, it is important to analyze the dynamic characteristics of micromechanical vibratory gyroscopes. In this paper, a novel optical method based on a high-speed CMOS camera and digital image spectrum correlation is developed to measure the motion of the sensitive elements. Unlike digital image correlation (DIC) in the spatial domain, correlation in the spectral domain gives direct access to the whole image under translational motion, which retains all the advantages of DIC and provides an expanded range of measurement. In addition, the proposed method is much faster in computation and capable of calculating dynamic temporal serial images in real time. The proposed method is employed to obtain the displacement of temporal series images and to extract dynamical parameters such as resonant frequency and quality factor at atmospheric pressure. During dynamic testing, the discrete wavelet transform is employed to eliminate the low-frequency components, and the continuous wavelet transform (CWT) for identification of the frequency and damping ratio. Experimental results show that the proposed method is a direct inspection tool and a practical alternative to the conventional electrical measurements of micromechanical vibratory gyroscopes.

An optical technique for evaluating Young's modulus of a vi- brating cantilever beam is presented. The technique utilizes two charge- coupled device (CCD) cameras to record and process the images of a vibrating beam. Equations relating the natural frequency with the imag- ing time are obtained, and Young's modulus as a function of the natural frequency is subsequently determined. The Young's modulus values are compared with those obtained using the conventional strain gauge method. The results show good agreement. The proposed technique is relatively simple and could be used to determine the Young's modulus of a wide variety of beam materials. © 2003 Society of Photo-Optical Instrumentation Engineers. (DOI: 10.1117/1.1602089)

We describe a stereoscopic measurement method to study the coplanarity of solder balls on a ball grid array (BGA) packaging. The optical system consists of two long-focal-length lenses and a two- channel digital image grabber. The proposed technique uses image rec- ognition and digital correlation, which simplify the calculation procedure. The computation requires only a few seconds to calculate the height of a few hundred balls. The technique is suitable for in situ measurement in BGA manufacturing. © 2001 Society of Photo-Optical Instrumentation Engineers.