Key Laboratory of Opto Electronics Information Technology of Ministry of Education

Tianjin, China

Key Laboratory of Opto Electronics Information Technology of Ministry of Education

Tianjin, China
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Tian Q.,Tianjin University | Tian Q.,Key Laboratory of Opto Electronics Information Technology of Ministry of Education | Li Y.,Tianjin University | Li Y.,Key Laboratory of Opto Electronics Information Technology of Ministry of Education | And 4 more authors.
Transactions of Tianjin University | Year: 2017

Conventional camera calibration that employs calibration targets is a commonly used method to acquire a camera’s intrinsic and/or extrinsic parameters. The calibration targets are usually designed as periodic arrays of simple high-contrast patterns that provide highly accurate world coordinate system points and the corresponding image pixel coordinate system points. The existing pixel coordinate extraction algorithms can reach a sub-pixel level; however, they treat each single pattern in one image as an independent individual, which makes it difficult to further improve extraction accuracy. In this paper, a novel method is proposed by utilizing the periodic arrangement characteristics of the calibration target pattern as a global constraint to improve the calibration accuracy. Based on a camera’s pinhole model, the intersection point of two fitted curves is used as an optimized pixel point to replace the initial one. Following the pixel coordinate optimization procedures, experiments were performed using real data from a 3D laser line scanner and a dynamic precision calibration target. Our results show that the relative errors of camera homography matrix elements obtained by the proposed optimization method were reduced compared with the commonly used method. The average coordinate measurement accuracy can be improved by nearly 0.05 mm. It is shown that the proposed optimization method can enhance the camera calibration accuracy, especially when the extracted pixels are of poorer precision. © 2017 Tianjin University and Springer-Verlag Berlin Heidelberg


Wu D.,Tianjin University | Wu D.,Key Laboratory of Opto Electronics Information Technology of Ministry of Education | Lu Q.-N.,Tianjin University | Lu Q.-N.,Key Laboratory of Opto Electronics Information Technology of Ministry of Education | And 5 more authors.
Guangdianzi Jiguang/Journal of Optoelectronics Laser | Year: 2017

According to extracting the center coordinate of spot in an oil film thickness measurement system with differential laser triangulation, based on the analysis on spot image features of ceramic gauge block and oil film, a method of spot positioning is presented with cross correlation and improved Gaussian fitting. The smooth and feature reserved spot image is obtained through the correlation operation of the spot image and a Gaussian mask image. The center coordinate of the spot image is obtained by improved Gaussian fitting with the ascending edge and descending edge data of the correlation results extracted for fitting. To verify the performance of the method, experimental measurement is carried out with different spot center extraction methods. Compared with square weighted centroid method, Gaussian fitting method, improved Gaussian fitting method and cross correlation Gaussian fitting method, the precision of the presented mothod is higher. The experiment of oil film spot center extraction is carried out. The results show that the algorithm presented is feasible for oil film thickness measurement system with differential laser triangulation. © 2017, Science Press in China. All right reserved.


Pan L.,Tianjin University | Pan L.,Key Laboratory of Opto Electronics Information Technology of Ministry of Education | Ge B.,Tianjin University | Ge B.,Key Laboratory of Opto Electronics Information Technology of Ministry of Education | And 2 more authors.
Journal of Quantitative Spectroscopy and Radiative Transfer | Year: 2017

The laser diffraction method is widely used to measure particle size distributions. It is generally accepted that the scattering angle becomes smaller and the angles to the location of the main peak of scattered energy distributions in laser diffraction instruments shift to smaller values with increasing particle size. This specific principle forms the foundation of the laser diffraction method. However, this principle is not entirely correct for non-absorbing particles in certain size ranges and these particle size ranges are called anomalous size ranges. Here, we derive the analytical formulae for the bounds of the anomalous size ranges and discuss the influence of the width of the size segments on the signature of the Mie scattering kernel. This anomalous signature of the Mie scattering kernel will result in an indetermination of the particle size distribution when measured by laser diffraction instruments in the anomalous size ranges. By using the singular-value decomposition method we interpret the mechanism of occurrence of this indetermination in detail and then validate its existence by using inversion simulations. © 2017 Elsevier Ltd


Wu D.,Tianjin University | Wu D.,Key Laboratory of Opto Electronics Information Technology of Ministry of Education | Lu Q.,Tianjin University | Lu Q.,Key Laboratory of Opto Electronics Information Technology of Ministry of Education | And 2 more authors.
Zhongguo Jiguang/Chinese Journal of Lasers | Year: 2014

According to the measurement of oil film thickness on the sea, an oil film thickness sensor based on buoy is developed using vertical incidence differential laser trigonometry. The composition and principle of sensor is introduced and the process of the system integration and debugging is presented. Ceramic grade 0 gauge block is measured as a standard thickness and high order curve is fitted based on least square method for the calibration of the sensor. 1.4~9.4 mm gauge blocks are measured to verify the precision. The results show that the measurement relative error is less than 1%.


Geng Y.,Tianjin University | Geng Y.,Key Laboratory of Opto Electronics Information Technology of Ministry of Education | Chen X.,Tianjin University | Jin W.,Tianjin University | And 4 more authors.
Zhongguo Jiguang/Chinese Journal of Lasers | Year: 2015

The analysis on the seawater refractive index which increases the error of oil film thickness measured by differential laser triangulation are taken to discover the relationship between the change of refractive index and error. Accordingly a method based on two-dimensional curved surface fitting is proposed to compensate the error. The data of thickness are measured in the water of different refractive indexes and the quadratic polynomial curved surface fitting is adopted to formulate the error, thickness and refractive index. Then the measurement of thickness is compensated by using the above the formula and the refractive index. The experiments taken on different refractive indexes show the thickness measurement compensated is closer to the true value and the error is minimized. ©, 2015, Science Press. All right reserved.


Qingguo T.,Tianjin University | Qingguo T.,Key Laboratory of Opto Electronics Information Technology of Ministry of Education | Xiangyu Z.,Tianjin University | Xiangyu Z.,Key Laboratory of Opto Electronics Information Technology of Ministry of Education | And 4 more authors.
Pattern Recognition | Year: 2016

Light stripe centerline extraction is the basic and key procedure in line-structured laser three-dimensional (3D) scanner. Based on the fact that light stripe's contour is approximately parallel to its centerline, a novel contour polygon segmentation method is proposed for extracting and optimizing centerline. Different light stripe segments are identified in images by contour tracking, and then each of them is segmented into several parts using contour polygonization. Interior angle is defined to trim open light stripe polygon and contour to make sure that centerlines extracted from open light stripes do not include superfluous points. Taking advantage of polygon segmentation, piecewise polynomial fitting method and self-adaptive interpolating strategy are adopted to acquire smoother and evenly spaced centerline points. Simulated experiments show that the proposed method can calculate centerlines from images robustly with a 0.309 pixel average accuracy. Point clouds and surface models of different objects acquired by a line-structured laser scanner demonstrate that the proposed method can produce more complete and smoother 3D models compared to other classical methods. Processing time for the proposed method is approximately positive proportional to the number of pixels in image. Quantitative analysis of time used for each sub-procedure puts forward an improvement direction for the proposed method. © 2016 Elsevier Ltd.


Pan L.,Tianjin University | Pan L.,Key Laboratory of Opto Electronics Information Technology of Ministry of Education | Zhang F.,Tianjin University | Zhang F.,Key Laboratory of Opto Electronics Information Technology of Ministry of Education | And 8 more authors.
Journal of Quantitative Spectroscopy and Radiative Transfer | Year: 2016

Use of laser diffraction is considered as a method of reliable principle and mature technique in measurements of particle size distributions. It is generally accepted that for a certain relative refractive index, the size of the scattering pattern (also called Airy disk) of spherical particles monotonically decreases with increasing particle size. This fine structure forms the foundation of the laser diffraction method. Here we show that the Airy disk size of non-absorbing spherical particles becomes larger with increasing particle size in certain size ranges. To learn more about this anomalous change of Airy disk (ACAD), we present images of Airy disk and curves of Airy disk size versus particle size for spherical particles of different relative refractive indices by using Mie theory. These figures reveal that ACAD occurs periodically for non-absorbing particles and will disappear when the absorbing efficiency is higher than certain value. Then by using geometrical optics (GO) approximation, we derive the analytical formulae for the bounds of the size ranges where ACAD occurs. From the formulae, we obtain laws of ACAD as follows: (1) for non-absorbing particles, ACAD occurs periodically, and when the particle size tends to infinity, the period tends to a certain value. As the relative refractive index increases, (2) the particle size ranges where ACAD occurs shift to smaller values, (3) the period of ACAD becomes smaller, and (4) the width of the size ranges where ACAD occurs becomes narrower. In addition, we can predict from the formulae that ACAD also exists for particles whose relative refractive index is smaller than 1. © 2015 Elsevier Ltd.


Ge B.,Tianjin University | Ge B.,Key Laboratory of Opto Electronics Information Technology of Ministry of Education | Pan L.,Tianjin University | Pan L.,Key Laboratory of Opto Electronics Information Technology of Ministry of Education | And 3 more authors.
Guangxue Xuebao/Acta Optica Sinica | Year: 2013

Laser particle sizer which based on Mie scattering theory is one of the most widely used instruments in particle size measurement. Generally the smaller the particles are, the bigger the scattering angle is. The main peak position of Mie scattered energy distribution received by the detector array moves outward. However, for some particles of relative index of refraction in certain size ranges, the main peak position of scattered energy distribution moves toward the inside of the detector array as the particle size decreases, which can be called the abnormal moving of scattered energy distribution. Based on Mie scattering theory, the patterns of such abnormal moving and abnormal particle size interval of different relative indexes of refraction are obtained, the effect on particle size analysis is analysed, a method to reduce the effect is proposed. An actual sample is measured and compared. The results show that this method can reduce the effect of abnormal moving on particle size analysis.

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