This article is a personal review of the historical developments of digital image correlation (DIC) techniques, together with recent important advances and future goals. The historical developments ...of DIC techniques over the past 35 years are divided into a foundation-laying phase (1982-1999) and a boom phase (2000 to the present), and are traced by describing some of the milestones that have enabled new and/or better DIC measurements to be made. Important advances made to DIC since 2010 are reviewed, with an emphasis on new insights into the 2D-DIC system, new improvements to the correlation algorithm, and new developments in stereo-DIC systems. A summary of the current state-of-the-art DIC techniques is provided. Some further improvements that are needed and the future goals in the field are also envisioned.
In digital image correlation (DIC), the iterative spatial domain cross-correlation algorithm using high-order B-spline interpolation algorithms has been strongly recommended for accurate sub-pixel ...displacement measurement. However, the magnitude of the position-dependent bias error increases with the increase of noise level, which dramatically reduces the registration accuracy of DIC for real experimental images. In this paper, a simple method, based on pre-smoothing the speckle images with a 5×5pixels Gaussian low-pass filter prior to correlation analysis, is proposed for reducing the bias error in measured displacements. Both numerical simulations and real experiments reveal that the proposed technique is capable of reducing the bias error in measured displacement to a negligible degree for both noisy and noiseless images, even though a simple bicubic interpolation is used.
•The effect of noise on DIC measurements is investigated in this work.•Gaussian pre-filtering of speckle images is proposed for bias error reduction in DIC.•Numerical and real experiments verify the effectiveness of the proposed technique.
Optical metrology practitioners ought to embrace deep learning with an open mind, while devote continuing efforts to look for its theoretical groundwork and maintain an awareness of its limits.
Single-camera stereo-digital image correlation （stereo-DIC） techniques have gained increasing attentions and demonstrated excellent prospects in the experimental mechanics community owing to their ...prominent advantages of cost-effectiveness, compactness, and the avoidance of the complicated camera synchronization. Using additional optical devices, e.g. a diffraction grating, a bi-prism or a set of planar mirrors, pseudo stereo images of a test sample surface can be recorded with a single camera. By correlating these stereo images using DIC, full-field three-dimensional （3D） shape and deformation can be retrieved. This review comprehensively summarizes the historical development, methodologies, strengths and weaknesses of the diffraction grating-based, prism-based, four-mirror-adaptor-based single-camera stereo-DIC techniques, and the recently proposed novel full-frame single color camera-based stereo-DIC technique for full-field 3D shape and deformation measurement. The optical arrangements, principles and calibration procedures of these single-camera stereo-DIC techniques are described in detail. Since high-speed deformation measurement is efficiently achieved by combining the single-camera stereo-DIC with one high-speed camera, single-camera stereo-DIC techniques show great potential in impact engineering, vibration and other dynamic tests.
The nonradiative recombination of electrons and holes has been identified as the main cause of energy loss in hybrid organic–inorganic perovskite solar cells (PSCs). Sufficient built‐in field and ...defect passivation can facilitate effective separation of electron–hole pairs to address the crucial issues. For the first time, we introduce a homochiral molecular ferroelectric into a PSC to enlarge the built‐in electric field of the perovskite film, thereby facilitating effective charge separation and transportation. As a consequence of similarities in ionic structure, the molecular ferroelectric component of the PSC passivates the defects in the active perovskite layers, thereby inducing an approximately eightfold enhancement in photoluminescence intensity and reducing electron trap‐state density. The photovoltaic molecular ferroelectric PSCs achieve a power conversion efficiency as high as 21.78 %.
A homochiral molecular ferroelectric was incorporated into a perovskite film to enlarge the built‐in electric field of the perovskite solar cell (PSC), thereby facilitating charge separation and transportation. The molecular ferroelectric component of the PSC passivates the defects in the perovskite active layers to induce an approximately eightfold enhancement in photoluminescence intensity and a reduction in electron trap‐state density.
Dysregulated prefrontal control over amygdala is engaged in the pathogenesis of psychiatric diseases including depression and anxiety disorders. Here we show that, in a rodent anxiety model induced ...by chronic restraint stress (CRS), the dysregulation occurs in basolateral amygdala projection neurons receiving mono-directional inputs from dorsomedial prefrontal cortex (dmPFC→BLA PNs) rather than those reciprocally connected with dmPFC (dmPFC↔BLA PNs). Specifically, CRS shifts the dmPFC-driven excitatory-inhibitory balance towards excitation in the former, but not latter population. Such specificity is preferential to connections made by dmPFC, caused by enhanced presynaptic glutamate release, and highly correlated with the increased anxiety-like behavior in stressed mice. Importantly, low-frequency optogenetic stimulation of dmPFC afferents in BLA normalizes the enhanced prefrontal glutamate release onto dmPFC→BLA PNs and lastingly attenuates CRS-induced increase of anxiety-like behavior. Our findings thus reveal a target cell-based dysregulation of mPFC-to-amygdala transmission for stress-induced anxiety.
Brain systems that promote maintenance of homeostasis in the face of stress have significant adaptive value. A growing body of work across species demonstrates a critical role for the amygdala in ...promoting homeostasis by regulating physiological and behavioral responses to stress. This review focuses on an emerging body of evidence that has begun to delineate the contribution of specific long-range amygdala circuits in mediating the effects of stress. After summarizing the major anatomical features of the amygdala and its connectivity to other limbic structures, we discuss recent findings from rodents showing how stress causes structural and functional remodeling of amygdala neuronal outputs to defined cortical and subcortical target regions. We also consider some of the environmental and genetic factors that have been found to moderate how the amygdala responds to stress and relate the emerging preclinical literature to the current understanding of the pathophysiology and treatment of stress-related neuropsychiatric disorders. Future effort to translate these findings to clinics may help to develop valuable tools for prevention, diagnosis, and treatment of these diseases.
As a practical and effective tool for quantitative in-plane deformation measurement of a planar object surface, two-dimensional digital image correlation (2D DIC) is now widely accepted and commonly ...used in the field of experimental mechanics. It directly provides full-field displacements to sub-pixel accuracy and full-field strains by comparing the digital images of a test object surface acquired before and after deformation. In this review, methodologies of the 2D DIC technique for displacement field measurement and strain field estimation are systematically reviewed and discussed. Detailed analyses of the measurement accuracy considering the influences of both experimental conditions and algorithm details are provided. Measures for achieving high accuracy deformation measurement using the 2D DIC technique are also recommended. Since microscale and nanoscale deformation measurement can easily be realized by combining the 2D DIC technique with high-spatial-resolution microscopes, the 2D DIC technique should find more applications in broad areas.
An advanced video deflectometer using actively illuminated LED targets is proposed for remote, real-time measurement of bridge deflection. The system configuration, fundamental principles, and ...measuring procedures of the video deflectometer are first described. To address the challenge of remote and accurate deflection measurement of large engineering structures without being affected by ambient light, the novel idea of active imaging, which combines high-brightness monochromatic LED targets with coupled bandpass filter imaging, is introduced. Then, to examine the measurement accuracy of the proposed advanced video deflectometer in outdoor environments, vertical motions of an LED target with precisely-controlled translations were measured and compared with prescribed values. Finally, by tracking six LED targets mounted on the bridge, the developed video deflectometer was applied for field, remote, and multipoint deflection measurement of the Wuhan Yangtze River Bridge, one of the most prestigious and most publicized constructions in China, during its routine safety evaluation tests. Since the proposed video deflectometer using actively illuminated LED targets offers prominent merits of remote, contactless, real-time, and multipoint deflection measurement with strong robustness against ambient light changes, it has great potential in the routine safety evaluation of various bridges and other large-scale engineering structures.
An advanced video deflectometer using off-axis digital image correlation is proposed for real-time, non-contact and targetless measurement of vertical deflection of bridges. To achieve real-time ...displacement tracking with sub-pixel accuracy, an efficient inverse compositional Gauss–Newton algorithm is employed. The detected image displacements in pixels are converted to physical displacements in millimeters using an easy-to-implement yet accurate calibration technique with the aid of a laser rangefinder. Real translation tests with precisely controlled motions were performed to examine the accuracy of the proposed technique. Real-time deflection monitoring of a railway bridge subjected to train pass-by is also demonstrated to show the practicality, accuracy and application potential of the proposed technique.
•Real-time, non-contact tracking of bridge deflection using an advanced video deflectometer is described.•The algorithm details of the efficient digital image correlation method for real-time motion tracking are provided.•A practical calibration method is proposed to convert image motions in pixels to physical displacements in millimeters.•Real translation tests with precisely controlled vertical motions verified the accuracy of the video deflectometer.•Real-time deflection monitoring of a railway bridge subjected to train pass-by demonstrated its practicality and accuracy.
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