Kemao Qian

Two-dimensional digital image correlation for in-plane displacement and strain measurement: a review

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.

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.

Windowed Fourier transform method for demodulation of carrier fringes

The Fourier transform method for demodulation of carrier fringes has been extensively developed and widely used in optical metrology. However, the Fourier transform being a global operation, it has poor ability to localize the signal properties and hence the result of FTM is not ideal. A windowed Fourier transform method is thus proposed, with advantages of signal localization and noise filtering. An example demonstrates the improved result compared to the traditional Fourier transform.

Phase-shifting windowed Fourier ridges for determination of phase derivatives

Determination of the phase or phase derivative from interferometric fringe patterns is an important task in optical interferometry. The use of wavelet ridges was recently shown to be an effective method for phase retrieval from a single fringe pattern. One necessary requirement in this method is the need for carrier frequency. In cases when carrier frequency is not available, the novel phase-shifting windowed Fourier ridges method can be used. Phase derivatives with the proper sign can be directly retrieved even in the presence of noise. An application for curvature determination from speckle shearographic fringes demonstrates the effectiveness of the method.

Algorithm for directly retrieving the phase difference: a generalization

Determination of phase difference is important in many applications of optical metrology. Algorithms for temporal phase difference measurement using the Fourier transform and phase shifting methods are available. These algorithms are first shown to be equivalent and then are generalized for spatial phase difference measurements and a spatial carrier phase shifting method. The efficiency of the algorithm is verified by two examples for slope and curvature determination.

Two-dimensional windowed Fourier frames for noise reduction in fringe pattern analysis

The two-dimensional continuous windowed Fourier transform has been shown to be effective for fringe pattern analysis in our previous work. In this paper, we first estimate the sampling intervals, using frame theory, to discretize the transform. Suitable sampling intervals are esti- mated as 1/x and 1/y, which is verified by simulations. Noise reduc- tion using windowed Fourier frames is then investigated and compared with that using the orthogonal wavelet transform. Due to the coherence of its kernels and fringe patterns and its redundancy, windowed Fourier frames are able to reduce noise more effectively, which is verified by processing both simulated and experimental fringe patterns. The relative errors are reduced by half, in various simulations, from those with or- thogonal wavelet filtering.

Fault detection by interferometric fringe pattern analysis using windowed Fourier transform

Windowed Fourier transform (WFT), a tool of spatial-frequency analysis, is able to characterize the local frequency at any location in a fringe pattern. Changes in fringe frequency characterize defects in optical interferometric-based systems. Hence, the WFT is suitable for fault detection and condition monitoring in optical NDT. In this paper, a WFT-based algorithm is described and theoretically analysed for fault detection. This is followed by demonstrations using both simulated and real fringe patterns. Comparisons with the traditional Fourier transform approach and normalized cross correlation approaches are also carried out.

Generalized three-dimensional windowed Fourier transform for fringe analysis

A 3D windowed Fourier transform is proposed for fringe sequence analysis, which processes the joint spatial and temporal information of the fringe sequence simultaneously. The 2D windowed Fourier transform in the spatial domain and the 1D windowed Fourier transform in the temporal domain are two special cases of the proposed method. The principles of windowed Fourier filtering and windowed Fourier ridges are developed. Experimental verification shows encouraging results despite a longer processing time.

Review of Confocal Fluorescence Endomicroscopy for Cancer Detection

The cancer burden is increasing worldwide and there is a need to develop new technologies for cancer diagnosis. Confocal laser endomicroscopy (CLE) is a minimally invasive optical technique that enables in vivo confocal imaging of tissue structures. With the use of fluorescent dyes, the technique allows confocal fluorescence endomicroscopy of tissue from surface to subsurface layers. CLE has been applied to the surveillance and diagnosis of cancer in numerous clinical studies recently, and also holds potential for optical and guided biopsy procedures. The first part of this mini review is focused on the application of CLE for cancer detection and surveillance. The second part is focused on the application of CLE to imaging of the oral cavity. We have previously demonstrated the potential of CLE for diagnostic imaging of oral cavity lesions. To move toward real-time 3-D imaging, we interfaced an endomicroscope to an embedded computing system. The prototype system is capable of automated image acquisition and real-time volume rendering. Rendering results provide topographical and depth information. Our aim is to achieve a real-time 3-D fluorescence imaging system that can be used for diagnostic imaging and guided biopsy procedures of oral cavity lesions in a clinical setting.

Embedded Computing for Fluorescence Confocal Endomicroscopy Imaging

A novel clinic protocol of virtual histology using an in vivo cellular imaging and real-time processing system is being developed. Main ideas of photoactivation and miniaturized confocal image scanning are presented. Technical innovation in embedded real-time image processing, feature detection and visualization system is demonstrated.