Dr. Xiaolei Li Profile

Dr. Xiaolei Li

at Tianjin Univ

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Publications (3)

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Proceedings Article | 20 December 2019 Paper

Robust weighted least-squares phase-unwrapping algorithm

Canlin Zhou, Shuchun Si, XiaoLei Li, Zhenkun Lei, YanJie Li

Proceedings Volume 11209, 112091U (2019) https://doi.org/10.1117/12.2547663

KEYWORDS: Reliability, Fringe analysis, Algorithm development, Cameras, Phase measurement, Physics, Mechanics, Phase shifting

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Phase unwrapping is one of the key steps of optical interferogram analysis.However,implementing phase-unwrapping algorithm(PUA) can be challenging,and the accuracy may be low when it is used to handle fringe patterns containing complicated singularities, such as noise, shadow, shears and surface discontinuity. When weighted least-squares unwrapping approach is adopted to unwrap the phase,the weighting coefficients are designed and defined to distinguish the singularities regions and normal regions in wrapped phase patterns.The weighting coefficients corresponding to the singularities are given zero-weighted, the singularities will not affect the unwrapping.But weighted least-squares unwrapping approach has the smooth effect on the final phase map.In view of this problem, a Robust Weighted leastsquares phase-unwrapping algorithm is proposed in this paper, which is based on combining the phase correction operation and weighted least-squares(WLS) method.In our method, Two partial derivatives of the wrapped phase are obtained from the fringe patterns. The phase singularities are determined using reliability, and the weighting coefficient is obtained from the binary mask from the reliability mask generated by thresholding method. The Weighted leastsquares phase-unwrapping was solved using the preconditioned conjugate gradient (PCG) method,The result of WLS is used to estimate the fringe order directly.The spatial phase unwrapping algorithm is applied to correct the phase with residual wraps by. Experiments are conducted to prove the validity of the proposed method. Results are analyzed and compared with the other least-squares methods, The experimental results demonstrate that the performance of proposed method.

Proceedings Article | 24 July 2018 Paper

Modified Fourier transform profilometry based on digital time-multiplexing technique

Canlin Zhou, XiaoLei Li, Shuchun Si, Zhenkun Lei, YanJie Li

Proceedings Volume 10827, 108271S (2018) https://doi.org/10.1117/12.2500390

KEYWORDS: Fringe analysis, Fourier transforms, Phase shifting, 3D metrology, Projection systems, Cameras, Digital filtering, Computer simulations, CCD cameras

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In order to avoid frequency aliasing,improve the spatial resolution of the phase map in Fourier transform Profilometry(FTP),an approach based on the digital time-multiplexing technique is proposed to remove the background component from the deformed fringe pattern. Firstly, a sinusoidal fringe pattern is projected onto the tested object by digital-light-processing( DLP) projector, the fringe pattern modulated by the object’s surface is captured by a CCD camera.Secondly, apply Fourier transform to the captured fringe pattern to obtain the spectrum. Thirdly, rotate the specimen’s fringe pattern 90-deg to obtain the rotated fringe pattern , then obtain the new spectrum corresponding to the rotated fringe pattern.Fourthly,the new spectrum is subtracted from the original spectrum ,clip the negative going values in the resultant spectrum by digital manipulation.Fifthly, filter out the first-order spectrum from the resultant spectrum by the band filter,apply inverse Fourier transform to the selected spectrum to obtain complex fields,then retrieve the phase, unwrap the wrapped phase map by the phase unwrapping algorithm.Finally, the simulation and experimental evaluations are conducted to prove the validity and performance of the proposed method. The results are analyzed and compared with those of the conventional method.The effectiveness and superiority of the proposed method have been demonstrated and verified.

Proceedings Article | 11 January 2005 Paper

Application of the wavelet transform in simulating digital moiré

Xiaolei Li, Can Zhou

Proceedings Volume 5642, (2005) https://doi.org/10.1117/12.575016

KEYWORDS: Wavelet transforms, Image processing, Phase shifting, Photonics, Metrology, Fringe analysis

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The Moiré method is a well-known technique which can be used to measure in-plane and out-of-plane deformations. The basis of all moiré methods is gratings, which for metrological applications are primarily an array of dark and bright lines or a set of crossed array of lines or dots. Moiré is a process where the intensity distributions of two dissimilar grid patterns are combined, for example by superimposition. The direct approach to generating moiré patterns is utilizing the aliasing phenomenon, which is generally avoided in traditional image processing, now can be used to analyze the deformed specimen grating for deformation studies. An alternate approach, the digital moiré method, is to use logical operations for generating moiré patterns. Accepted as a novel mathematical tool, wavelet transform has a great advantage for image processing. In this pager, we apply it to simulate the aliasing phenomenon. In analysis of moiré fringes, phase shifting is by far the most powerful of fringe analysis techniques. For digital moiré, phase shifting can be achieved simply by translating the software generated gratings by the appropriate fractional steps. An application for large deformation of rubber material is given; the result shows that the method is feasible.

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