Zhaoyang Wang

Study on subset size selection in digital image correlation for speckle patterns

Digital Image Correlation (DIC) is a flexible and effective technique to measure the displacements on specimen surfaces by matching the reference subsets in the undeformed image with the target subsets in the deformed image. With the existing DIC techniques, the user must rely on experience and intuition to manually define the size of the reference subset, which is found to be critical to the accuracy of measured displacements. In this paper, the problem of subset size selection in the DIC technique is investigated. Based on the Sum of Squared Differences (SSD) correlation criterion as well as the assumption that the gray intensity gradients of image noise are much lower than that of speckle image, a theoretical model of the displacement measurement accuracy of DIC is derived. The theoretical model indicates that the displacement measurement accuracy of DIC can be accurately predicted based on the variance of image noise and Sum of Square of Subset Intensity Gradients (SSSIG). The model further leads to a simple criterion for choosing an optimal subset size for the DIC analysis. Numerical experiments have been performed to validate the proposed concepts, and the calculated results show good agreements with the theoretical predictions.

Equivalence of digital image correlation criteria for pattern matching.

In digital image correlation (DIC), to obtain the displacements of each point of interest, a correlation criterion must be predefined to evaluate the similarity between the reference subset and the target subset. The correlation criterion is of fundamental importance in DIC, and various correlation criteria have been designed and used in literature. However, little research has been carried out to investigate their relations. In this paper, we first provide a comprehensive overview of various correlation criteria used in DIC. Then we focus on three robust and most widely used correlation criteria, i.e., a zero-mean normalized cross-correlation (ZNCC) criterion, a zero-mean normalized sum of squared difference (ZNSSD) criterion, and a parametric sum of squared difference (PSSD(ab)) criterion with two additional unknown parameters, since they are insensitive to the scale and offset changes of the target subset intensity and have been highly recommended for practical use in literature. The three correlation criteria are analyzed to establish their transversal relationships, and the theoretical analyses clearly indicate that the three correlation criteria are actually equivalent, which elegantly unifies these correlation criteria for pattern matching. Finally, the equivalence of these correlation criteria is further validated by numerical simulation and actual experiment.

High-temperature digital image correlation method for full-field deformation measurement at 1200 ?C

A simple, easy-to-implement yet effective high-temperature digital image correlation (DIC) method is established for non-contact full-field deformation measurement at elevated temperatures. The technique employs a bandpass optical filter to eliminate the influence of black-body radiation of high-temperature objects on the intensity of captured images. With the bandpass filter, high-quality digital images of an object at high temperatures up to 1200 °C can be easily acquired and directly compared with the reference image recorded at room temperature using the DIC technique to extract full-field deformation information with high fidelity. To verify the performance of the proposed technique, a chromium-nickel austenite stainless steel sample was heated from room temperature to 1200 °C using an infrared heating device, and the surface images at various temperatures were captured using the bandpass filter imaging system. Afterwards, full-field thermal deformation and coefficient of thermal expansion of the sample were determined using the DIC technique. Experimental results indicate that the proposed high-temperature DIC method is easy to implement and can be applied to practical full-field high-temperature deformation measurement with high accuracy.

Generic gamma correction for accuracy enhancement in fringe-projection profilometry

Fringe-projection profilometry is one of the most commonly used noncontact methods for acquiring the three-dimensional (3D) shape information of objects. In practice, the luminance nonlinearity caused by the gamma effect of a digital projector and a digital camera yields undesired fringe intensity changes, which substantially reduce the measurement accuracy. In this Letter, we present a robust and simple scheme to eliminate the intensity nonlinearity induced by the gamma effect by combining a universal phase-shifting algorithm with a gamma correction method. First, by using three-step and large-step phase-shifting techniques, the gamma value involved in the measurement system can be detected. Then, a gamma pre-encoding process is applied to the system for actual 3D shape measurements. With the proposed technique, high accuracy of measurement can be achieved with the conventional small-step phase-shifting algorithm. The validity of the technique is verified by experiments.

Advanced continuous wavelet transform algorithm for digital interferogram analysis and processing

An advanced continuous wavelet transform algorithm for digital interferogram analysis and processing is proposed. The algorithm is an extension of the traditional wavelet transform; the mother wavelet and normalization parameter are selected based on the characteristics of optical interferograms. To reduce the processing time, a fast Fourier transform scheme is employed to implement the wavelet transform calculation. The algorithm is simple and is a robust tool for interferogram filtering and for whole-field fringe and phase information detection. The concept is verified by computer simulation and actual experimental interferogram analysis.

Genuine full-field deformation measurement of an object with complex shape using reliability-guided digital image correlation.

Digital image correlation (DIC) is an easy-to-implement yet powerful optical metrology for deformation measurement. The technique measures the displacement of a point of interest by matching the subsets surrounding the same point located in the reference image and the deformed image. Although the technique is simple in principle, the existing DIC technique has several deficiencies. For example, for the points located near or at the boundaries of a specified region of interest (ROI), the selected square subsets surrounding these points may contain unwanted or foreign pixels from background image or other regions. In the existing DIC method, these points are either intentionally excluded from calculation or automatically removed after calculation, and leads to the absence of deformation information for the boundary points. Besides, existing DIC technique is prone to yield erroneous measurement for specimen with geometric discontinuities. In this paper, two approaches are developed to overcome the deficiencies of existing DIC technique. First, a modified Zero-mean Normalized Sum of Squared Differences (ZNSSD) criterion is defined for the correlation analysis of subsets surrounding the boundary points. Second, considering the possible complex shape of the ROI, a scanning strategy guided by the correlation coefficients of computed points is proposed to ensure reliable computation between consecutive points. With these two measures, the deformation of all the points including those located near or at the ROI boundaries can be automatically, reliably, and accurately determined. The improved DIC technique is universally applicable to the genuine full-field deformation measurement of objects with complex or arbitrary shapes. Two typical experimental image pairs are processed to evaluate the performance of the proposed method, and the results successfully demonstrate its effectiveness and practicality.

Three-dimensional shape measurement with a fast and accurate approach

A noncontact, fast, accurate, low-cost, broad-range, full-field, easy-to-implement three-dimensional (3D) shape measurement technique is presented. The technique is based on a generalized fringe projection profilometry setup that allows each system component to be arbitrarily positioned. It employs random phase-shifting, multifrequency projection fringes, ultrafast direct phase unwrapping, and inverse self-calibration schemes to perform 3D shape determination with enhanced accuracy in a fast manner. The relative measurement accuracy can reach 1/10,000 or higher, and the acquisition speed is faster than two 3D views per second. The validity and practicability of the proposed technique have been verified by experiments. Because of its superior capability, the proposed 3D shape measurement technique is suitable for numerous applications in a variety of fields.

On Moisture Diffusion Modeling Using Thermal-Moisture Analogy

Thermal-moisture analogy schemes for a moisture diffusion analysis are reviewed. Two schemes for practical applications are described using the governing equations of heat and mass diffusions: (1) direct analogy and (2) normalized analogy. The schemes are implemented to define valid domains of application. The results corroborate that the direct analogy is valid only for single-material systems, but the normalized analogy can be extended to multimaterial systems if thermal loading conditions are isothermal, spatially as well as temporally.

Real-time, high-accuracy 3D imaging and shape measurement

In spite of the recent advances in 3D shape measurement and geometry reconstruction, simultaneously achieving fast-speed and high-accuracy performance remains a big challenge in practice. In this paper, a 3D imaging and shape measurement system is presented to tackle such a challenge. The fringe-projection-profilometry-based system employs a number of advanced approaches, such as: composition of phase-shifted fringe patterns, externally triggered synchronization of system components, generalized system setup, ultrafast phase-unwrapping algorithm, flexible system calibration method, robust gamma correction scheme, multithread computation and processing, and graphics-processing-unit-based image display. Experiments have shown that the proposed system can acquire and display high-quality 3D reconstructed images and/or video stream at a speed of 45 frames per second with relative accuracy of 0.04% or at a reduced speed of 22.5 frames per second with enhanced accuracy of 0.01%. The 3D imaging and shape measurement system shows great promise of satisfying the ever-increasing demands of scientific and engineering applications.

Advanced Geometric Camera Calibration for Machine Vision

In many machine vision applications, a crucial step is to accurately determine the relation between the image of the object and its physical dimension by performing a calibra- tion process. Over time, various calibration techniques have been developed. Nevertheless, the existing methods cannot satisfy the ever-increasing demands for higher accuracy per- formance. In this letter, an advanced geometric camera cali- bration technique which employs a frontal image concept and a hyper-precise control point detection scheme with digital image correlation is presented. Simulation and real experi- mental results have successfully demonstrated the superior of the proposed technique. C 2011 Society of Photo-Optical Instrumen-