Real-time 3D digital image correlation method and its application in human pulse monitoring

High-accuracy optical coherence elastography digital volume correlation methods to measure depth regions with low correlation

The decreasing correlation of optical coherence tomography (OCT) images with depth is an unavoidable problem for the depth measurement of the digital volume correlation (DVC) based optical coherence elastography (OCE) method. We propose an OCE-DVC method to characterize biological tissue deformation in deeper regions. The method proposes a strategy based on reliability layer guided displacement tracking to achieve the OCE-DVC method for the deformation measurement in deep regions of OCT images. Parallel computing solves the computational burden associated with the OCE-DVC method. The layer-by-layer adaptive data reading methods are used to guarantee the parallel computing of high-resolution OCT images. The proposed method shown in this study nearly doubles the depth of quantitative characterization of displacement and strain. At this depth, the standard deviation of displacement and strain measurements is reduced by nearly 78%. Under nonuniform deformation field, OCE-DVC method tracked the displacement with large strain gradient in depth region.

Digital image correlation assisted absolute phase unwrapping

This paper presents an absolute phase unwrapping method for high-speed three-dimensional (3D) shape measurement. This method uses three phase-shifted patterns and one binary random pattern on a single-camera, single-projector structured light system. We calculate the wrapped phase from phase-shifted images and determine the coarse correspondence through the digital image correlation (DIC) between the captured binary random pattern of the object and the pre-captured binary random pattern of a flat surface. We then developed a computational framework to determine fringe order number pixel by pixel using the coarse correspondence information. Since only one additional pattern is used, the proposed method can be used for high-speed 3D shape measurement. Experimental results successfully demonstrated that the proposed method can achieve high-speed and high-quality measurement of complex scenes.

DIC measurement for large-scale structures based on adaptive warping image stitching

As a representative method of optical non-interference measurement, digital image correlation (DIC) technology is a non-contact optical mechanics method that can measure the displacement and deformation of the whole field. However, when the measurement range of the field is too large, the existing DIC method cannot measure the full-field strain accurately, which limits the application of the DIC measurement in the case of a large size and wide-field view. To address this issue, a DIC measurement method for large-scale structures based on adaptive warping image stitching is proposed in this paper. First, multiple adjacent high-resolution images are collected at different locations of large-scale structures. Secondly, the collected images are stitched by applying the adaptive warping image stitching algorithm to obtain a panoramic image. Finally, the DIC algorithm is applied to solve the whole deformation field. In the experiments, we first verify the feasibility of the proposed method for image matching and fusion through the numerical simulation of a rigid body translation experiment. Then the accuracy and robustness of the proposed method in practical application are verified by rigid body translation and a three-point bending experiment. The experimental results demonstrate that the measurement range of DIC is improved significantly with the adaptive warping image stitching algorithm.

Three-dimensional shape and deformation measurement on complex structure parts

Stereo digital image correlation technique (stereo-DIC or 3D-DIC) has been widely used in three-dimensional (3D) shape and deformation measurement due to its high accuracy and flexibility. But it is a tough task for it to deal with complex structure components because of the severe perspective distortion in two views. This paper seeks to resolve this issue using a single-camera system based on DIC-assisted fringe projection profilometry (FPP). A pixel-wise and complete 3D geometry of complex structures can be reconstructed using the robust and efficient Gray-coded method based on a FPP system. And then, DIC is just used to perform the temporal matching and complete full-field pixel-to-pixel tracking. The in- and out-of-plane deformation are obtained at the same time by directly comparing the accurate and complete 3D data of each corresponding pixel. Speckle pattern design and fringe denoising methods are carefully compared and chosen to simultaneously guarantee the measuring accuracy of 3D shape and deformation. Experimental results demonstrate the proposed method is an effective means to achieve full-field 3D shape and deformation measurement on complex parts, such as honeycomb structure and braided composite tube, which are challenging and even impossible for the traditional stereo-DIC method.

Global Mechanical Behavior Characterization of Uniaxially Loaded Rock Specimen Based on Its Structural Evolution

Characterizing global mechanical behavior accurately is important for a detailed understanding of the deformation mechanism of rock material. In this paper, a new characterization model of the global mechanical behavior of rock is proposed, based on the structural characteristics of rock deformation. Uniaxial compression tests were carried out using the digital image correlation method and acoustic emission to obtain the interrelationship between mechanical behavior and deformation evolution. The test results show that the appearance of deformation localization leads to non-linear evolution of global mechanical behavior in a rock specimen. Further, due to the gradual evolution of deformation localization bands, the rock specimen evolves from a complete whole to a rock structure with a “weak interlayer”. Thus, the global mechanical behavior of the rock specimen depends heavily on the structural evolution process, especially when close to failure. A simplified characterization model was established according to the deformation process. The finite element method was used to verify the rationality of the proposed structural model. The verification result showed that under uniaxial compression, the structural model can reproduce the global mechanical behavior evolution process of the rock specimen.

A 117 Line 2D Digital Image Correlation Code Written in MATLAB

Digital Image Correlation (DIC) has become a popular tool in many fields to determine the displacements and deformations experienced by an object from images captured of the object. Although there are several publications which explain DIC in its entirety while still catering to newcomers to the concept, these publications neglect to discuss how the theory presented is implemented in practice. This gap in literature, which this paper aims to address, makes it difficult to gain a working knowledge of DIC, which is necessary in order to contribute towards its development. The paper attempts to address this by presenting the theory of a 2D, subset-based DIC framework that is predominantly consistent with state-of-the-art techniques, and discussing its implementation as a modular MATLAB code. The correlation aspect of this code is validated, showing that it performs on par with well-established DIC algorithms and thus is sufficiently reliable for practical use. This paper, therefore, serves as an educational resource to bridge the gap between the theory of DIC and its practical implementation. Furthermore, although the code is designed as an educational resource, its validation combined with its modularity makes it attractive as a starting point to develop the capabilities of DIC.

Real-time matching strategy for rotary objects using digital image correlation

Real-time monitoring of structural health conditions for rotary objects is of importance for safety assessments. In this work, an efficient algorithm based on digital image correlation is presented to achieve accurate rotational matching in real time. The proposed algorithm measures rotation in object motion with an integer pixel search followed by a subpixel correlation refinement. In the integer pixel search, the reference subset is rotated inversely to facilitate the correlation computation between the reference and target subsets. Then an independent and global integer pixel search for each point of interest is performed by applying the particle swarm optimization algorithm. Finally, a modified iterative registration algorithm is introduced to refine the displacement in the subpixel level by considering both the rotation angle and displacement components. Simulation and rotation experiments demonstrate that the proposed method achieves rapid and accurate measurements and is an effective method for retrieving the rotation data of rotating structures.

Digital image correlation with improved efficiency by pixel selection

With the increase in digital image correlation (DIC) applications, the computational efficiency of DIC is becoming increasingly important. In previous studies, real-time DIC was realized with a relatively small subset. However, a small subset does not always include sufficient gray gradient information. In this paper, a pixel selection strategy is proposed to improve the computational efficiency of DIC further, allowing a real-time deformation measurement with a large subset. Within the subset, zero weight is assigned to unreliable pixels as a way of pursuing maximum efficiency. The modulus of the local intensity gradient vector of each pixel in the reference image is used as the criterion for reliability. Numerical and real experiments conducted to validate the feasibility and effectiveness of the strategy showed that the computational speed of DIC could be improved about 2 times.

Prediction of Fracture Damage of Sandstone Using Digital Image Correlation

Investigation on the deformation mechanism of sandstone is crucial to understanding the life cycle patterns of pertinent infrastructure systems considering the extensive adoption of sandstone in infrastructure construction of various engineering systems, e.g., agricultural engineering systems. In this study, the state-of-the-art digital image correlation (DIC) method, which uses classical digital photography, is employed to explore the detailed failure course of sandstone with physical uniaxial compression tests. Four typical points are specifically selected to characterize the global strain field by plotting their corresponding strain–time relationship curves. Thus, the targeted failure thresholds are identified. The Hill–Tsai failure criterion and finite element simulation are then used for the cross-check process of DIC predictions. The results show that, though errors exist between the experimental and the theoretical values, overall, they are sufficiently low to be ignored, indicating good agreement. From the results, near-linear relationships between strain and time are detected before failure at the four chosen points and the failure strain thresholds are almost the same; as low as 0.004. Failure thresholds of sandstone are reliably determined according to the strain variation curve, to forecast sandstone damage and failure. Consequently, the proposed technology and associated information generated from this study could be of assistance in the safety and health monitoring processes of relevant infrastructure system applications.

Digital Assisted Image Correlation for Metal Sheet Strain Measurement

Current methods of correlation and point matching between stereoscopic images produce large errors or are completely inefficient when the surface has a repetitive, non-isotropic, low contrast pattern. In this article a new method of Digital Assisted Image Correlation (DAIC) is presented to match specific points in order to estimate the deformation of the surface in the metal sheets used in the automotive industry. To achieve this, it is necessary to stamp the surface to be measured with a regular pattern of points, then a digital image processing is done to obtain the labels of the circles of the pattern. After this, a semi-automatic search is made in the labels of both images to correlate all of them and perform the triangulation. DIC is used to corroborate the correspondence between points and verify the accuracy and efficiency of the developed method. This allows the 3D reconstruction of the sheet with a minimum of information and provides more efficiency and a great benefit in computational cost. Deformation is calculated by two methods, which show similarity between the values obtained with a digital microscope. It is assumed that quality of marks stamping, lighting, and the initial conditions, also contribute for trustworthy effects.

Self-calibration approach to stereo cameras with radial distortion based on epipolar constraint

In this paper, we propose a self-calibration approach to stereo cameras with radial distortion from stereo image pairs of a common 3D scene. Based on the epipolar constraint in the stereo image pair, the intrinsic and extrinsic parameters of stereo cameras are estimated synchronously with a minimum number of nine image point correspondences. It is significant within a random sample consensus (RANSAC) scheme to cope with the outliers of feature matches efficiently and robustly. Then the inliers of the stereo image pair that have been determined after RANSAC are used to optimize the calibration parameters of stereo cameras. Furthermore, more accurate calibration results can be achieved with the joint optimization of multiple stereo image pairs. Both synthetic and real data are used to evaluate the performance of the proposed method, demonstrating that our method can calibrate stereo cameras with radial distortions efficiently and accurately.

Accuracy analysis of an orthogonally arranged four-camera 3D digital image correlation system

To reduce the uncertainty region of a three-dimensional (3D) position, a four-camera 3D digital image correlation (3D-DIC) system was built by orthogonally arranging two sets of two-camera DIC systems. The theoretical model proposed herein revealed the relationship between 3D coordinates and system parameters and the propagation of the matching error to the position error. Numerical simulation and experiment were conducted to verify the theory. The simulation and experimental results indicated that the 3D position error of the four-camera system was smaller than that of the two-camera DIC system. The present contribution proves the feasibility of using four-camera DIC systems to improve measurement accuracy.

DIC/Moiré hybrid method based on regular patterns for deformation measurement

Digital Image Correlation (DIC) is a superior optical method to measure the surface deformation with a high accuracy. Currently, most researches on DIC are based on random patterns. In this paper, A DIC/Moiré hybrid method using regular patterns is proposed for deformation measurement. In this method, a Moiré fringe technique based on correlation coefficient is developed to provide accurate initial deformation estimation for DIC. Experimental results indicate a higher computational efficiency by the proposed method than the conventional DIC method. It is also found that the calculation accuracy increases using regular patterns. The advantage of obtaining accurate initial estimation by the DIC/Moiré hybrid method may enable potential application in deformation measurements.

Digital image correlation with reduced bias error based on digital signal upsampling theory

Based on digital signal upsampling theory, a new computing strategy has been proposed to reduce the bias error in digital image correlation (DIC) caused by intensity interpolation. For each subset, before subpixel image matching, the subimage around the target subset was processed by increasing the sampling rate with an integer factor. The increase of the sampling rate is realized by resampling in the digital domain. The combination of digital signal upsampling processing with DIC can greatly reduce the interpolation bias error. The measurement accuracy of the proposed computing strategy was investigated in this study. Both numerical experiments and real-world experiments have been conducted in order to verify the effectiveness of the proposed computing strategy. The results indicate that the bias error can be significantly reduced without sacrificing the standard deviation error. With the proposed computing strategy, high-accuracy DIC measurement with near-negligible bias error is expected.

A novel vision-based system for quantitative analysis of abdominal aortic aneurysm deformation

BACKGROUND

In clinical diagnostics, combination of different imaging techniques is applied to assess spatial configuration of the abdominal aortic aneurysm (AAA) and deformation of its wall. As deformation of aneurysm wall is crucial parameter in assessing wall rupture, we aimed to develop and validate a Non-Invasive Vision-Based System (NIVBS) for the analysis of 3D elastic artificial abdominal aortic models. 3D-printed elastic AAA models from four patients were applied for the reconstruction of real hemodynamic. During experiments, the inlet boundary conditions included the injection volume and frequency of pulsation averaged from electrocardiography traces. NIVBS system was equipped with nine cameras placed at a constant distance to record wall movement from 360^o angle and a dedicated set of artificial lights providing coherent illumination. Additionally, self-prepared algorithms for image acquisition, processing, segmentation, and contour detection were used to analyze wall deformation. Finally, the shape deformation factor was applied to evaluate aorta's deformation. Experimental results were confronted with medical data from AngioCT and 2D speckle-tracking echocardiography (2DSTE).

RESULTS

Image square analyses indicated that the optimal distance between the camera's lens and the investigated object was in the range of 0.30-0.35 m. There was approximately 1.44% difference observed in aneurysm diameters between NIVBS (86.57 ± 5.86 mm) and AngioCT (87.82 ± 6.04 mm) (p = 0.7764). The accuracy of developed algorithm for the reconstruction of the AAA deformation was equal to 98.56%. Bland-Altman analysis showed that the difference between clinical data (2DSTE) and predicted wall deformation (NIVBS) for all patients was 0.00 mm (confidence interval equal to 0.12 mm) for aneurysm size, 0.01 mm (confidence interval equal to 0.13 mm) and 0.00 mm (confidence interval equal to 0.09 mm) for the anterior and posterior side, as well as 0.01 mm (confidence interval equal to 0.18 mm) and 0.01 mm (confidence interval equal to 0.11 mm) for the left and right side. The optimal range of camera's lens did not affect acquired values.

CONCLUSIONS

The NIVBS with proposed algorithm that reconstructs the pressure from surrounding organs is appropriate to analyze the AAAs in water environment. Moreover, NIVBS allowed detailed quantitative analysis of aneurysm sac wall deformation.

New Four Points Initialization for Digital Image Correlation in Metal-Sheet Strain Measurements

Nowadays, the deformation measurement in metal sheets is important for industries such as the automotive and aerospace industries during its mechanical stamping processes. In this sense, Digital Image Correlation (DIC) has become the most relevant measurement technique in the field of experimental mechanics. This is mainly due to its versatility and low-cost compared with other techniques. However, traditionally, DIC global image registration implemented in software, such as MATLAB 2018, did not find the complete perspective transformation needed successfully and with high precision, because those algorithms use an image registration of the type “afine” or “similarity”, based on a 2D information. Therefore, in this paper, a DIC initialization method is presented to estimate the surface deformation of metal sheets used in the bodywork automotive industry. The method starts with the 3D points reconstruction from a stereoscopic digital camera system. Due to the problem complexity, it is first proposed that the user indicates four points, belonging to reference marks of a “Circle grid”. Following this, an automatic search is performed among the nearby marks, as far as one desires to reconstruct it. After this, the local DIC is used to verify that those are the correct marks. The results show reliability by reason of the high coincidence of marks in experimental cases. We also consider that the quality of mark stamping, lighting, and the initial conditions also contribute to trustworthy effects.

A Cross-Dichroic-Prism-Based Multi-Perspective Digital Image Correlation System

A robust three-perspective digital image correlation (DIC) system based on a cross dichroic prism and single three charge-coupled device (3CCD) color cameras is proposed in this study. Images from three different perspectives are captured by a 3CCD camera using the cross dichroic prism and two planar mirrors. These images are then separated by different CCD channels to perform correlation calculation with an existing multi-camera DIC algorithm. The proposed system is considerably more compact than the conventional multi-camera DIC system. In addition, the proposed system has no loss of spatial resolution compared with the traditional single-camera DIC system. The principle and experimental setup of the proposed system is described in detail, and a series of tests is performed to validate the system. Experimental results show that the proposed system performs well in displacement, morphology, and strain measurement.

Soft tissue elastography via shearing interferometry

Early detection of cancer can significantly increase the survival chances of patients. Palpation is a traditional method in order to detect cancer; however, in minimally invasive surgery the surgeon is deprived of the sense of touch. We demonstrate how shearing elastography can recover elastic parameters and furthermore can be used to localize stiffness imhomogenities even if hidden underneath the surface. Furthermore, the influence of size and depth of the stiffness imhomogenities on the detection accuracy and localization is investigated.

Camera array-based digital image correlation for high-resolution strain measurement

Digital image correlation (DIC) is a well-known technique for non-contact, non-destructive, full-field deformation measurement in experimental solid mechanics. Although DIC has been widely used in science and engineering, the resolution of strain measurement with DIC is limited by imaging resolution and is much lower than that obtained with a strain gauge. To achieve a breakthrough in strain measurement using DIC, a camera array-based DIC method is proposed herein for high-resolution strain measurement. Twenty-five industrial cameras were assembled into a plane array, with each camera capturing a part of the specimen. A novel calibration-based image stitching method is proposed and was applied to these images and their corresponding displacement fields. The strain field was then calculated based on the stitched displacement fields. The use of the camera array greatly improved the measurement spatial resolution of DIC and made high-resolution strain measurement possible. Both static error analysis and four point-bending experiments were performed to demonstrate the feasibility and effectiveness of the proposed method, and a full-field strain resolution of 10 μ ε was achieved.

Whole-field macro- and micro-deformation characteristic of unbound water-loss in dentin hard tissue

High-resolution deformation measurements in a functionally graded hard tissue such as human dentin are essential to understand the unbound water-loss mediated changes and their role in its mechanical integrity. Yet a whole-field, 3-dimensional (3D) measurement and characterization of fully hydrated dentin in both macro- and micro-scales remain to be a challenge. This study was conducted in 2 stages. In stage-1, a stereo-digital image correlation approach was utilized to determine the water-loss and load-induced 3D deformations of teeth in a sagittal section over consecutively acquired frames, from a fully hydrated state to nonhydrated conditions for a period up to 2 hours. The macroscale analysis revealed concentrated residual deformations at the dentin-enamel-junction and the apical regions of root in the direction perpendicular to the dentinal tubules. Significant difference in the localized deformation characteristics was observed between the inner and outer aspects of the root dentin. During quasi-static loadings, further increase in the residual deformation was observed in the dentin. In stage-2, dentin microstructural variations induced by dynamic water-loss were assessed with environmental scanning electron microscopy and atomic force microscopy (AFM), showing that the dynamic water-loss induced distention of dentinal tubules with concave tubular edges, and concurrent contraction of intertubular dentin with convex profile. The findings from the current macro- and micro-scale analysis provided insight on the free-water-loss induced regional deformations and ultrastructural changes in human dentin.

Precise Detection of Wrist Pulse Using Digital Speckle Pattern Interferometry

Pulse diagnosis is one of the four diagnostic methods of traditional Chinese medicine. However it suffers from the lack of objective and efficient detection method. We propose a noncontact optical method to detect human wrist pulse, aiming at the precise determination of the temporal and spatial distributions of pulse. The method uses the spatial-carrier digital speckle pattern interferometry (DSPI) to measure the micro/nanoscale skin displacement dynamically. Significant improvements in DSPI measurement have been made to allow the DSPI to detect the comprehensive information of the arterial pulsation at locations of Cun, Guan, and Chi. The experimental results prove that the spatiotemporal distributions of pulse can be obtained by the proposed method. The obtained data can be further used to describe most of the pulse parameters such as rate, rhythm, depth, length, width, and contour.

Comparative Analysis of Warp Function for Digital Image Correlation-Based Accurate Single-Shot 3D Shape Measurement

Digital image correlation (DIC)-based stereo 3D shape measurement is a kind of single-shot method, which can achieve high precision and is robust to vibration as well as environment noise. The efficiency of DIC has been greatly improved with the proposal of inverse compositional Gauss-Newton (IC-GN) operators for both first-order and second-order warp functions. Without the algorithm itself, both the registration accuracy and efficiency of DIC-based stereo matching for shapes with different complexities are closely related to the selection of warp function, subset size, and convergence criteria. Understanding the similarity and difference of the impacts of prescribed subset size and convergence criteria on first-order and second-order warp functions, and how to choose a proper warp function and set optimal subset size as well as convergence criteria for different shapes are fundamental problems in realizing efficient and accurate 3D shape measurement. In this work, we present a comparative analysis of first-order and second-order warp functions for DIC-based 3D shape measurement using IC-GN algorithm. The effects of subset size and convergence criteria of first-order and second-order warp functions on the accuracy and efficiency of DIC are comparatively examined with both simulation tests and real experiments. Reference standards for the selection of warp function for different kinds of 3D shape measurement and the setting of proper convergence criteria are recommended. The effects of subset size on the measuring precision using different warp functions are also concluded.

Technique for two-dimensional displacement field determination using a reliability-guided spatial-gradient-based digital image correlation algorithm

This paper proposed a novel in-plane displacement field measurement algorithm using an optical flow strategy. We built a linear illumination model between images before and after deformation to guarantee intensity invariability. We used image upsampling and a reliability-guided strategy to find the matching points accurate to 0.5 pixels in the reference and deformed images. The criterion to determine the reliability is zero-mean normalized cross-correlation coefficient. Afterward, we used the brightness constancy constraint combined with Lucas-Kanade optical flow constraint in a specific image region to obtain an overdetermined linear equation. We applied the noniterative least-squares algorithm to solve the equations and to achieve the displacement offsets. This research utilized multithread calculation to handle the complete cracking applications. We estimated the computing efficiency and calculation precision of the proposed method through a series of experimental speckle patterns. All results demonstrated the correctness, effectiveness, and robustness of the proposed method.

Optimized digital speckle patterns for digital image correlation by consideration of both accuracy and efficiency

The technique of digital image correlation (DIC), which has been widely used for noncontact deformation measurements in both the scientific and engineering fields, is greatly affected by the quality of speckle patterns in terms of its performance. This study was concerned with the optimization of the digital speckle pattern (DSP) for DIC in consideration of both the accuracy and efficiency. The root-mean-square error of the inverse compositional Gauss-Newton algorithm and the average number of iterations were used as quality metrics. Moreover, the influence of subset sizes and the noise level of images, which are the basic parameters in the quality assessment formulations, were also considered. The simulated binary speckle patterns were first compared with the Gaussian speckle patterns and captured DSPs. Both the single-radius and multi-radius DSPs were optimized. Experimental tests and analyses were conducted to obtain the optimized and recommended DSP. The vector diagram of the optimized speckle pattern was also uploaded as reference.

Efficient Background Segmentation and Seed Point Generation for a Single-Shot Stereo System

Single-shot stereo 3D shape measurement is becoming more popular due to its advantages of noise robustness and short acquisition period. One of the key problems is stereo matching, which is related to the efficiency of background segmentation and seed point generation, etc. In this paper, a more efficient and automated matching algorithm based on digital image correlation (DIC) is proposed. The standard deviation of image gradients and an adaptive threshold are employed to segment the background. Scale-invariant feature transform (SIFT)-based feature matching and two-dimensional triangulation are combined to estimate accurate initial parameters for seed point generation. The efficiency of background segmentation and seed point generation, as well as the measuring precision, are evaluated by experimental simulation and real tests. Experimental results show that the average segmentation time for an image with a resolution of 1280 × 960 pixels is 240 milliseconds. The efficiency of seed point generation is verified to be high with different convergence criteria.

3D shape, deformation, and vibration measurements using infrared Kinect sensors and digital image correlation

Consumer-grade red-green-blue and depth (RGB-D) sensors, such as the Microsoft Kinect and the Asus Xtion, are attractive devices due to their low cost and robustness for real-time sensing of depth information. These devices provide the depth information by detecting the correspondences between the captured infrared (IR) image and the initial image sent to the IR projector, and their essential limitation is the low accuracy of 3D shape reconstruction. In this paper, an effective technique that employs the Kinect sensors for accurate 3D shape, deformation, and vibration measurements is introduced. The technique involves using the RGB-D sensors, an accurate camera calibration scheme, and area- and feature-based image-matching algorithms. The IR speckle pattern projected from the Kinect projector considerably facilitates the digital image correlation analysis in the regions of interest with enhanced accuracy. A number of experiments have been carried out to demonstrate the validity and effectiveness of the proposed technique and approach. It is shown that the technique can yield measurement accuracy at the 10 μm level for a typical field of view. The real-time capturing speed of 30 frames per second makes the proposed technique suitable for certain motion and vibration measurements, such as non-contact monitoring of respiration and heartbeat rates.

Noninvasive, three-dimensional full-field body sensor for surface deformation monitoring of human body in vivo

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.

A Wearable and Highly Sensitive Graphene Strain Sensor for Precise Home-Based Pulse Wave Monitoring

Profuse medical information about cardiovascular properties can be gathered from pulse waveforms. Therefore, it is desirable to design a smart pulse monitoring device to achieve noninvasive and real-time acquisition of cardiovascular parameters. The majority of current pulse sensors are usually bulky or insufficient in sensitivity. In this work, a graphene-based skin-like sensor is explored for pulse wave sensing with features of easy use and wearing comfort. Moreover, the adjustment of the substrate stiffness and interfacial bonding accomplish the optimal balance between sensor linearity and signal sensitivity, as well as measurement of the beat-to-beat radial arterial pulse. Compared with the existing bulky and nonportable clinical instruments, this highly sensitive and soft sensing patch not only provides primary sensor interface to human skin, but also can objectively and accurately detect the subtle pulse signal variations in a real-time fashion, such as pulse waveforms with different ages, pre- and post-exercise, thus presenting a promising solution to home-based pulse monitoring.

Shape measurement with modified phase-shift lateral shearing interferometry illumination and radial basis function

Three-dimensional shapes of objects were evaluated with modified phase-shift lateral shearing interferometry illumination and radial basis function. A simple optical system was developed to create the fringe pattern based on the Murty interferometer. The phase shift was generated only by moving a plane-parallel plate along an in-plane parallel direction. A novel moving radial basis function method was presented to improve the quality of fringe patterns. And the proper calculation window size was given based on numerical simulation. Three-dimensional shapes of two kinds of objects were determined to verify the feasibility and effectiveness of the proposed method, and the reconstructed height distributions were in good accordance with the referenced data.

Self-calibration single-lens 3D video extensometer for high-accuracy and real-time strain measurement

The accuracy of strain measurement using a common optical extensometer with two-dimensional (2D) digital image correlation (DIC) is not sufficient for experimental applications due to the effect of out-of-plane motion. Although three-dimensional (3D) DIC can measure all three components of displacement without introducing in-plane displacement errors, 3D-DIC requires the stringent synchronization between two digital cameras and requires complicated system calibration of binocular stereovision, which makes the measurement rather inconvenient. To solve the problems described above, this paper proposes a self-calibration single-lens 3D video extensometer for non-contact, non-destructive and high-accuracy strain measurement. In the established video extensometer, a single-lens 3D imaging system with a prism and two mirrors is constructed to acquire stereo images of the test sample surface, so the problems of synchronization and out-of-plane displacement can be solved easily. Moreover, a speckle-based self-calibration method which calibrates the single-lens stereo system using the reference speckle image of the specimen instead of the calibration targets is proposed, which will make the system more convenient to be used without complicated calibration. Furthermore, an efficient and robust inverse compositional Gauss-Newton algorithm combined with a robust stereo matching stage is employed to achieve high-accuracy and real-time subset-based stereo matching. Tensile tests of an Al-alloy specimen were performed to demonstrate the feasibility and effectiveness of the proposed self-calibration single-lens 3D video extensometer.

Multicamera system extrinsic stability analysis and large-span truss string structure displacement measurement

A novel technique for measuring the displacements of large-span truss string structures that employs multicamera systems is proposed. The coordinates of the stereo-vision systems are unified in a single global coordinate system by employing 3D data reconstructed using close-range photogrammetry. To estimate the camera's attitude motions during an experiment, an instantaneous extrinsic rectification algorithm was developed. Experiments in which a camera was rotated and translated were conducted to verify the accuracy and precision of the developed algorithm. In addition, the proposed multicamera systems were employed to analyze a large-span truss string structure. The displacement results obtained from numerical simulations and experiments using pre-calibration and rectification methods are compared in this paper, and the stability of the camera's extrinsic parameters is discussed.

Three-dimensional displacement measurement based on the combination of digital image correlation and optical flow

This paper proposes what we believe is a novel simultaneous three-dimensional (3D) displacement measurement technique based on a combination of digital image correlation (DIC) and optical flow (OF). In this method, both the in-plane and out-of-plane displacements can be accurately extracted from only two continuous interferograms. DIC estimates the velocity field between two consecutive frames. According to the optical flow constrained equation, we can then obtain the whole-field out-of-plane displacement map by the estimations of the in-plane displacement components and the local frequency of the original image. The proposed method's operation is simple compared with other phase demodulation methods. Moreover, the new method works perfectly in areas with dense fringes. To verify its effectiveness, we applied a new algorithm to simulated and experimental interferograms. The results of our simulation and experiment show that the new method can demodulate the out-of-plane component of the deformation-phase from the visible in-plane velocity field without an unwrapping process. Further, the proposed algorithm provides a new approach to measure whole-field 3D displacement and dynamic deformation.

Advanced video extensometer for non-contact, real-time, high-accuracy strain measurement

We developed an advanced video extensometer for non-contact, real-time, high-accuracy strain measurement in material testing. In the established video extensometer, a "near perfect and ultra-stable" imaging system, combining the idea of active imaging with a high-quality bilateral telecentric lens, is constructed to acquire high-fidelity video images of the test sample surface, which is invariant to ambient lighting changes and small out-of-plane motions occurred between the object surface and image plane. In addition, an efficient and accurate inverse compositional Gauss-Newton algorithm incorporating a temporal initial guess transfer scheme and a high-accuracy interpolation method is employed to achieve real-time, high-accuracy displacement tracking with negligible bias error. Tensile tests of an aluminum sample and a carbon fiber filament sample were performed to demonstrate the efficiency, repeatability and accuracy of the developed advanced video extensometer. The results indicate that longitudinal and transversal strains can be estimated and plotted at a rate of 117 fps and with a maximum strain error less than 30 microstrains.