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Accurate calibration method for camera and projector in fringe patterns measurement system

APPLIED OPTICS 2016年第16期55卷 4293-4300页

作者： Chen, Rui Xu, Jing Chen, Heping Su, Jianhua Zhang, Zonghua Chen, Ken Tsinghua Univ State Key Lab Tribol Beijing 100084 Peoples R China Tsinghua Univ Dept Mech Engn Beijing 100084 Peoples R China Beijing Key Lab Precis Ultra Precis Mfg Equipment Beijing 100084 Peoples R China SW Texas State Univ Ingram Sch Engn San Marcos TX 78666 USA Chinese Acad Sci Inst Automat Beijing 100190 Peoples R China Hebei Univ Technol Mech Engn Tianjin 300130 Peoples R China

The 3D measurement system based on fringe patterns is widely applied in diverse fields. The measurement accuracy is mainly determined by camera and projector calibration accuracy. In the existing methods, the system is calibrated by a dot calibration board with traditional image process algorithms. In this paper, an improved calibration method is proposed to increase camera and projector calibration accuracy simultaneously. To this end, first, a subpixel edge detection method is proposed to improve the detection accuracy of reference features for coarse calibration;second, an iterative compensation algorithm is developed to improve the detection accuracy of the reference feature centers for fine calibration. The experimental results demonstrate that the proposed calibration method can improve the calibration accuracy and measurement accuracy. (C) 2016 Optical Society of America

关键词： Camera calibration Digital micromirror devices Edge detection image processing algorithms Laser scanning Point spread function

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Automatic detection of microaneurysms in diabetic retinopathy fundus images using the L*a*b color space

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JOURNAL OF THE OPTICAL SOCIETY OF AMERICA A-OPTICS image SCIENCE AND VISION 2016年第1期33卷 74-83页

作者： Navarro, Pedro J. Alonso, Diego Stathis, Kostas Univ Politecn Cartagena Div Sistemas & Ingn Elect Cartagena 30202 Spain Royal Holloway Univ London Dept Comp Sci Egham TW20 0EX Surrey England

We develop an automated image processing system for detecting microaneurysm (MA) in diabetic patients. Diabetic retinopathy is one of the main causes of preventable blindness in working age diabetic people with the presence of an MA being one of the first signs. We transform the eye fundus images to the L*a*b* color space in order to separately process the L* and a* channels, looking for MAs in each of them. We then fuse the results, and last send the MA candidates to a k-nearest neighbors classifier for final assessment. The performance of the method, measured against 50 images with an ophthalmologist's hand-drawn ground-truth, shows high sensitivity (100%) and accuracy (84%), and running times around 10 s. This kind of automatic image processing application is important in order to reduce the burden on the public health system associated with the diagnosis of diabetic retinopathy given the high number of potential patients that need periodic screening. (C) 2015 Optical Society of America

关键词： Color spaces Digital image processing Fourier transforms image processing image processing algorithms Wavelet transforms

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Laser induced ultrasonic phased array using full matrix capture data acquisition and total focusing method

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OPTICS EXPRESS 2016年第19期24卷 21921-21938页

作者： Stratoudaki, Theodosia Clark, Matt Wilcox, Paul D. Univ Bristol Dept Mech Engn Bristol BS8 1TR Avon England Univ Nottingham Fac Engn Div Elect Syst & Opt Univ Pk Nottingham NG7 2RD England

Laser ultrasonics is a technique where lasers are employed to generate and detect ultrasound. A data collection method (full matrix capture) and a post processing imaging algorithm, the total focusing method, both developed for ultrasonic arrays, are modified and used in order to enhance the capabilities of laser ultrasonics for nondestructive testing by improving defect detectability and increasing spatial resolution. In this way, a laser induced ultrasonic phased array is synthesized. A model is developed and compared with experimental results from aluminum samples with side drilled holes and slots at depths of 5 - 20 mm from the surface. (C) 2016 Optical Society of America

关键词： High power lasers image processing algorithms image quality Laser arrays Laser sources Phased arrays

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Self-calibration single-lens 3D video extensometer for high-accuracy and real-time strain measurement

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OPTICS EXPRESS 2016年第26期24卷 30124-30138页

作者： Shao, Xinxing Eisa, Mohammed Mokhtar Chen, Zhenning Dong, Shuai He, Xiaoyuan Southeast Univ Dept Engn Mech Nanjing 210096 Jiangsu Peoples R China

The accuracy of strain measurement using a common optical extensometer with two-dimensional (2D) digital image correlation (DIC) is not sufficient for experimental applications due to the effect of out-of-plane motion. Although three-dimensional (3D) DIC can measure all three components of displacement without introducing in-plane displacement errors, 3D-DIC requires the stringent synchronization between two digital cameras and requires complicated system calibration of binocular stereovision, which makes the measurement rather inconvenient. To solve the problems described above, this paper proposes a self-calibration single-lens 3D video extensometer for non-contact, non-destructive and high-accuracy strain measurement. In the established video extensometer, a single-lens 3D imaging system with a prism and two mirrors is constructed to acquire stereo images of the test sample surface, so the problems of synchronization and out-of-plane displacement can be solved easily. Moreover, a speckle-based self-calibration method which calibrates the single-lens stereo system using the reference speckle image of the specimen instead of the calibration targets is proposed, which will make the system more convenient to be used without complicated calibration. Furthermore, an efficient and robust inverse compositional Gauss-Newton algorithm combined with a robust stereo matching stage is employed to achieve high-accuracy and real-time subset-based stereo matching. Tensile tests of an Alalloy specimen were performed to demonstrate the feasibility and effectiveness of the proposed self-calibration single-lens 3D video extensometer. (C) 2016 Optical Society of America

关键词： Digital image processing image processing algorithms image sensors Strain measurement Three dimensional imaging Three dimensional measurement

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Noncontact detection of Teflon inclusions in glass-fiber-reinforced polymer composites using terahertz imaging

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APPLIED OPTICS 2016年第36期55卷 10215-10222页

作者： Zhang, Jin Wang, Jie Han, Xiaohui Cui, Hong-Liang Shi, Changcheng Zhang, Jinbo Shen, Yan Jilin Univ Coll Instrumentat & Elect Engn Changchun 130061 Peoples R China Chinese Acad Sci Chongqing Inst Green & Intelligent Technol Key Lab Multiscale Mfg Technol Res Ctr Terahertz Technol Chongqing 400714 Peoples R China Harbin FRP Inst Harbin 150036 Peoples R China Chongqing Polycomp Int Corp Chongqing 400082 Peoples R China

We employed terahertz (THz) time-domain spectroscopy (TDS) imaging technology, a new nondestructive testing method, to detect the inclusions of glass-fiber-reinforced polymer (GFRP) composites. The refractive index and absorption coefficient of two types of GFRP composites (epoxy GFRP composites and polyester GFRP composites) were first extracted, and GFRP composites with Teflon inclusions were examined, including an epoxy GFRP solid panel with a smaller Teflon inclusion hidden behind a larger Teflon inclusion, and polyester GFRP solid panels with Teflon inclusions of various sizes, at different depths. It was experimentally demonstrated that THz TDS imaging technology could clearly detect a smaller inclusion hidden behind a larger inclusion. When the reflected THz pulse from the inclusion did not overlap with that from the front surface of the sample, removal of the latter before Fourier transform was shown to be helpful in imaging the inclusions. With sufficiently strong incident THz radiation, inclusion insertion depth had little impact on the ability of the THz wave to detect inclusions. However, as the thickness of the inclusion became thinner, the inclusion detection ability of the THz wave deteriorated. In addition, with a combination of reflected C-scan imaging and B-scan imaging using the reflected time-domain waveform, both the lateral sizes and locations of the inclusions and the depths and thicknesses of the inclusions were clearly ascertained. (C) 2016 Optical Society of America

关键词： Absorption coefficient image processing image processing algorithms Imaging techniques Refractive index Terahertz imaging

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Qualitative and quantitative interpretation of SEM image using digital image processing

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JOURNAL OF MICROSCOPY 2016年第1期264卷 102-124页

作者： Saladra, Dawid Kopernik, Magdalena AGH Univ Sci & Technol Al Mickiewicza 30 PL-30059 Krakow Poland

The aim of the this study is improvement of qualitative and quantitative analysis of scanning electron microscope micrographs by development of computer program, which enables automatic crack analysis of scanning electron microscopy (SEM) micrographs. Micromechanical tests of pneumatic ventricular assist devices result in a large number of micrographs. Therefore, the analysis must be automatic. Tests for athrombogenic titanium nitride/gold coatings deposited on polymeric substrates (Bionate II) are performed. These tests include microshear, microtension and fatigue analysis. Anisotropic surface defects observed in the SEM micrographs require support for qualitative and quantitative interpretation. Improvement of qualitative analysis of scanning electron microscope images was achieved by a set of computational tools that includes binarization, simplified expanding, expanding, simple image statistic thresholding, the filters Laplacian 1, and Laplacian 2, Otsu and reverse binarization. Several modifications of the known image processing techniques and combinations of the selected image processing techniques were applied. The introduced quantitative analysis of digital scanning electron microscope images enables computation of stereological parameters such as area, crack angle, crack length, and total crack length per unit area. This study also compares the functionality of the developed computer program of digital image processing with existing applications. The described pre- and postprocessing may be helpful in scanning electron microscopy and transmission electron microscopy surface investigations.

关键词： Coatings digital image image processing algorithms micromechanical test scanning electron microscopy

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Demonstration of 136 dB dynamic range capability for a simultaneous dual optical band CAOS camera

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OPTICS EXPRESS 2016年第26期24卷 29427-29443页

作者： Riza, Nabeel A. La Torre, J. Pablo Univ Coll Cork Sch Engn Coll Rd Cork Ireland

For the first time, proposed and demonstrated is a simultaneous dual optical band coded access optical sensor (CAOS) camera design suited for extreme contrast multispectral bright target scenarios. Deploying a digital micromirror devices (DMDs)-based time-frequency agile pixels CAOS-mode within a two point detector spatially and spectrally isolating framework, this imager simultaneously and independently detects pixel selective image information for two different broad spectral bands that further undergo independent spectral image data extraction via finer-tuned wavelength filtering using all-optical or CAOSmode electronic filters. A proof-of-concept visible-near infrared band CAOS imager is successfully demonstrated using a target scene containing LEDs and engaging narrowband optical filters. In addition, using the CAOS-mode, demonstrated is the RF domain simultaneous color content monitoring of a white light LED image pixel. Also proposed is the use of a higher bit count analog-to-digital converter (ADC) with both range and sampling duration parameter control along with a larger data set electronic DSP to extract higher DSP gain and realize additional noise suppression. Using a 16-bit ADC and 2,097,152 point fast Fourier transform (FFT) digital signal processing (DSP) for a 633 nm laser engaged test target scene that is subject to nearly 7 decades (10(7)) of gradual optical attenuation, the experimental camera demonstrates an agile pixel extreme dynamic range of 136 dB, which is a 56 dB improvement over the previous CAOS-imaging demonstrations. (C) 2016 Optical Society of America

关键词： Digital micromirror devices image processing image processing algorithms Multispectral imaging Optical wireless communication Variable optical attenuators

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Effective star tracking method based on optical flow analysis for star trackers

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APPLIED OPTICS 2016年第36期55卷 10335-10340页

作者： Sun, Ting Xing, Fei Wang, Xiaochu Li, Jin Wei, Minsong You, Zheng Tsinghua Univ Dept Precis Instrument Beijing 100084 Peoples R China Beijing Informat Sci & Technol Univ Sch Instrument Sci & Opto Elect Engn Beijing 100192 Peoples R China China Acad Space Technol Qian Xuesen Lab Space Technol Beijing 100094 Peoples R China

Benefiting from rapid development of imaging sensor technology, modern optical technology, and a high-speed computing chip, the star tracker's accuracy, dynamic performance, and update rate have been greatly improved with low power consumption and miniature size. The star tracker is currently one of the most competitive attitude measurement sensors. However, due to restrictions of the optical imaging system, difficulties still exist in moving star spot detection and star tracking when in special motion conditions. An effective star tracking method based on optical flow analysis for star trackers is proposed in this paper. Spot-based optical flow, based on a gray gradient between two adjacent star images, is analyzed to distinguish the star spot region and obtain an accurate star spot position so that the star tracking can keep continuous under high dynamic conditions. The obtained star vectors and extended Kalman filter (EKF) are then combined to conduct an angular velocity estimation to ensure region prediction of the star spot;this can be combined with the optical flow analysis result. Experiment results show that the method proposed in this paper has advantages in conditions of large angular velocity and large angular acceleration, despite the presence of noise. Higher functional density and better performance can be achieved;thus, the star tracker can be more widely applied in small satellites, remote sensing, and other complex space missions. (C) 2016 Optical Society of America

关键词： image analysis image processing algorithms image sensors Imaging noise Optical flow Remote sensing

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Method for out-of-focus camera calibration

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APPLIED OPTICS 2016年第9期55卷 2346-2352页

作者： Bell, Tyler Xu, Jing Zhang, Song Purdue Univ Sch Mech Engn W Lafayette IN 47907 USA Tsinghua Univ Dept Mech Engn Beijing 10084 Peoples R China

State-of-the-art camera calibration methods assume that the camera is at least nearly in focus and thus fail if the camera is substantially defocused. This paper presents a method which enables the accurate calibration of an out-of- focus camera. Specifically, the proposed method uses a digital display (e.g., liquid crystal display monitor) to generate fringe patterns that encode feature points into the carrier phase;these feature points can be accurately recovered, even if the fringe patterns are substantially blurred (i.e., the camera is substantially defocused). Experiments demonstrated that the proposed method can accurately calibrate a camera regardless of the amount of defocusing: the focal length difference is approximately 0.2% when the camera is focused compared to when the camera is substantially defocused. (C) 2016 Optical Society of America

关键词： Calibration Camera calibration Diffraction theory image processing image processing algorithms Phase unwrapping

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Real-time thermal imaging of solid oxide fuel cell cathode activity in working condition

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APPLIED OPTICS 2016年第25期55卷 7142-7148页

作者： Montanini, Roberto Quattrocchi, Antonino Piccolo, Sebastiano A. Amato, Alessandra Trocino, Stefano Zignani, Sabrina C. Lo Faro, Massimiliano Squadrito, Gaetano Univ Messina Dipartimento Ingn I-98166 Messina Italy Tech Univ Denmark Dept Engn Management Anker Engelunds Vej 1 DK-2800 Lyngby Denmark CNR ITAE Via Santa Lucia Contesse 5 I-98126 Messina Italy

Electrochemical methods such as voltammetry and electrochemical impedance spectroscopy are effective for quantifying solid oxide fuel cell (SOFC) operational performance, but not for identifying and monitoring the chemical processes that occur on the electrodes' surface, which are thought to be strictly related to the SOFCs' efficiency. Because of their high operating temperature, mechanical failure or cathode delamination is a common shortcoming of SOFCs that severely affects their reliability. Infrared thermography may provide a powerful tool for probing in situ SOFC electrode processes and the materials' structural integrity, but, due to the typical design of pellet-type cells, a complete optical access to the electrode surface is usually prevented. In this paper, a specially designed SOFC is introduced, which allows temperature distribution to be measured over all the cathode area while still preserving the electrochemical performance of the device. Infrared images recorded under different working conditions are then processed by means of a dedicated image processing algorithm for quantitative data analysis. Results reported in the paper highlight the effectiveness of infrared thermal imaging in detecting the onset of cell failure during normal operation and in monitoring cathode activity when the cell is fed with different types of fuels. (C) 2016 Optical Society of America

关键词： Computed tomography image processing algorithms Infrared imaging Real time imaging Thermal infrared imaging Visible light

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