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Optics and Lasers in Engineering

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Updated: 20 weeks 6 days ago

Structured light stereo catadioptric scanner based on a spherical mirror

Sun, 04/15/2018 - 06:03
Publication date: August 2018
Source:Optics and Lasers in Engineering, Volume 107

Author(s): S. Barone, P. Neri, A. Paoli, A.V. Razionale

The present paper describes the development and characterization of a structured light stereo catadioptric scanner for the omnidirectional reconstruction of internal surfaces. The proposed approach integrates two digital cameras, a multimedia projector and a spherical mirror, which is used to project the structured light patterns generated by the light emitter and, at the same time, to reflect into the cameras the modulated fringe patterns diffused from the target surface. The adopted optical setup defines a non-central catadioptric system, thus relaxing any geometrical constraint in the relative placement between optical devices. An analytical solution for the reflection on a spherical surface is proposed with the aim at modelling forward and backward projection tasks for a non-central catadioptric setup. The feasibility of the proposed active catadioptric scanner has been verified by reconstructing various target surfaces. Results demonstrated a great influence of the target surface distance from the mirror's centre on the measurement accuracy. The adopted optical configuration allows the definition of a metrological 3D scanner for surfaces disposed within 120 mm from the mirror centre.





Categories: Optics news

Low-speed-camera-array-based high-speed three-dimensional deformation measurement method: Principle, validation, and application

Sun, 04/15/2018 - 06:03
Publication date: August 2018
Source:Optics and Lasers in Engineering, Volume 107

Author(s): Ran Chen, Zhongwei Li, Kai Zhong, Xingjian Liu, Yuh Jin Chao, Yusheng Shi

High-resolution imaging devices are of interest in the development of accurate 3D vision systems. However, it is challenging to achieve a balance between the image capturing speed and resolution. The image capturing speed is relatively low for high-resolution imaging devices, which restricts their applications in high-speed 3D measurements. Therefore, a low-speed-camera-array imaging method for high-speed 3D deformation measurements is proposed. Compared with existing methods using high-speed imaging devices, it has the advantages of low cost and high flexibility achieved by combining low-speed cameras into a stereo camera array high-speed imaging system. In order to achieve accurate 3D measurements, a bundle-adjustment-principle-based system calibration method is proposed. Four experiments, including an accuracy experiment, repeatability experiment, vibration measurement of a plastic board, and out-of-plane displacement measurement of rotating blades, demonstrated the accuracy and effectiveness of the proposed method.





Categories: Optics news

Absolute phase retrieval methods for digital fringe projection profilometry: A review

Sun, 04/15/2018 - 06:03
Publication date: August 2018
Source:Optics and Lasers in Engineering, Volume 107

Author(s): Song Zhang

This paper provides a review for absolute phase recovery methods that are applicable for digital fringe projection (DFP) systems. Specifically, we present two conventional absolute phase unwrapping methods: multi-frequency or -wavelength phase-shifting methods, and hybrid binary coding and phase-shifting methods; and also introduce some non-conventional methods that are specific for DFP systems: multiview geometry methods with additional camera(s) or projector(s), DFP system geometric constraint-based phase unwrapping method, and pre-knowledge (e.g., computer-aided-design, CAD, model) based phase unwrapping method. This paper also briefly overviews hybrid methods including phase coding, composite, and pre-defined markers based absolute phase unwrapping methods. This paper explains the principle behind each individual absolute phase unwrapping method; and finally offers some practical tips to handle common phase unwrapping artifact issues.





Categories: Optics news

High-dynamic-range 3D shape measurement utilizing the transitioning state of digital micromirror device

Sun, 04/15/2018 - 06:03
Publication date: August 2018
Source:Optics and Lasers in Engineering, Volume 107

Author(s): Vignesh Suresh, Yajun Wang, Beiwen Li

This paper introduces a novel method for 3D shape measurement of high-contrast surfaces in real-time by taking advantage of the transitioning state of digital micromirror device. We propose to project defocused 1-bit binary patterns for sinusoidal generation to bypass the rigid camera-projector synchronization requirement. Each pattern is capture twice in one projection cycle to obtain two fringe images with different brightness: the first fringe is captured when the camera is totally exposed within the projector’s bright period; the second fringe is captured when part of the projector’s dark period is within the camera exposure. The two fringes will be combined to alleviate the saturation problem. Experiments demonstrate the success of the proposed method by showing the real-time 3D shape measurement capability (image acquisition rate: 166 Hz; 3D frame rate: 14 Hz). Moreover, since 1-bit binary patterns are used, it is potentially applicable to high-speed measurements.





Categories: Optics news

A method of the direct measurement of the true stress–strain curve over a large strain range using multi-camera digital image correlation

Sun, 04/15/2018 - 06:03
Publication date: August 2018
Source:Optics and Lasers in Engineering, Volume 107

Author(s): Junrui Li, Guobiao Yang, Thorsten Siebert, Ming F. Shi, Lianxiang Yang

A quick method to directly determine the true stress–strain curve over a large strain range is proposed. The true stress–strain curve includes the post-necking strain, is directly measured via a special multi-camera digital image correlation (DIC) system. A tension test with the standard dog-bone shape specimens is used in the system. The cross-section area of the specimen throughout the test is measured by the DIC, as well as the entire field true strain. The true stress is calculated based on the measured cross-section area and the load. A dual-phase steel with a minimum 780 MPa tensile strength (DP780), as a typical advanced high-strength steel (AHSS), is used in the test to demonstrate the method and to validate the result. The true stress–strain curve including the post-necking data of DP780 is presented. The test result is validated with the theoretical calculation as well as the experimental method. Furthermore, the measured true stress–strain curve is approximated by the power law equation to identify the material constants. The methodology, experimental test, and the evaluated result are described in detail.





Categories: Optics news

An optical watermarking scheme with two-layer framework based on computational ghost imaging

Sun, 04/15/2018 - 06:03
Publication date: August 2018
Source:Optics and Lasers in Engineering, Volume 107

Author(s): Sui Liansheng, Cheng Yin, Tian Ailing, Anand Krishna Asundi

A two-layer watermarking scheme based on computational ghost imaging and singular value decomposition is proposed. In the first layer, the original watermark is encoded into a significantly small number of measured intensities in the process of computational ghost imaging to constitute a new watermark. In the second layer, the significant blocks chosen from the host image based on spatial frequency are combined to the reference image, which is used to embed the new watermark by using the singular value decomposition. Differing from other watermarking schemes, the information of original watermark can be verified without clear visualization via calculating the nonlinear correlation map between the original one and the reconstructed one. Besides optical parameters such as wavelength and propagation distance, a series of phase-only masks are used as security keys, which can enlarge the key space and enhance the level of security. The results illustrate the feasibility and effectiveness about the proposed watermarking mechanism, which provides an effective alternative for the related work.





Categories: Optics news

A novel CT reconstruction algorithm for incomplete projection based on information repairment

Sun, 04/15/2018 - 06:03
Publication date: August 2018
Source:Optics and Lasers in Engineering, Volume 107

Author(s): Yu Xiao, Feng Xu, Kang Shen, Bo Dong, Xiaofang Hu, Tiqiao Xiao

An incomplete projection reconstruction algorithm was proposed in this study. In synchrotron radiation computed tomography (SRCT) of mechanical loading process, shielded projection due to the loading device is the main problem of high-resolution CT reconstruction. Thus, the proposed algorithm was developed based on information repairment considering the continuity of sinogram and slice. The truncated sinogram was estimated using orientation total variation minimization (TVM), which was based on the continuity of projections at adjacent angles in sinogram. Slice rectification was conducted to restrain image distortion, which was based on the continuity of matter contribution. Then, the algorithm was validated by a series of numerical reconstruction experiments. Results showed that the new algorithm could repair the lost boundary and restrain the artifacts. Quantitative comparison of evaluation parameters indicated that the new algorithm worked well when the shielded angle was less than 60° and that satisfactory results could be obtained by approximately 200 times of iteration. Finally, the new algorithm was validated using actual SRCT data and demonstrated effective improvement of reconstruction quality. Therefore, the proposed algorithm may be a suitable method for the in situ SRCT studies of mechanical loading processes.





Categories: Optics news

Influence of no-core fiber on the focusing performance of an ultra-small gradient-index fiber probe

Sun, 04/15/2018 - 06:03
Publication date: August 2018
Source:Optics and Lasers in Engineering, Volume 107

Author(s): Shubo Bi, Chi Wang, Jun Zhu, Zhiwen Yuan, Yingjie Yu, Sergiy Valyukh, Anand Asundi

The light-beam expansion effect of a no-core fiber on the focusing performance of an ultra-small gradient-index fiber probe is investigated with a view to optimizing the optical performance of such probes. By taking the variable relationship between the focusing performance (including the working distance and the focusing spot size) of the probe and the length of the no-core fiber as the criterion, the effective beam expansion length of the no-core fiber in the ultra-small gradient-index fiber probe is calculated based on the basic properties of the Gaussian beam. Verification and analysis are done by numerical calculations and experimental measurements, respectively. The obtained results show that the working distance of an ultra-small gradient-index fiber probe can be increased effectively by adding a no-core fiber; however, this will lead to increasing the focusing spot size. For the parameters of the fiber probe studied here, the effective beam expansion length of the no-core fiber spacer is less than 0.357 mm.





Categories: Optics news

100-m standoff detection of a QCL-induced photo-vibrational signal on explosives using a laser vibrometer

Sun, 04/15/2018 - 06:03
Publication date: August 2018
Source:Optics and Lasers in Engineering, Volume 107

Author(s): Yu Fu, Huan Liu, Jiecheng Xie

Laser Doppler vibrometry is a key technology in the field of photomechanics. It allows pointwise noncontact measurements of dynamic effects. In recent years, it has been applied as a sensor in photoacoustic/photothermal spectroscopy to detect hazardous chemicals or explosive substances in an open environment. In photoacoustic spectroscopy, normally the analysis has to be conducted in a sealed space to enable signal detection, either through a high-sensitivity microphone, or a piezo-sensor coupled with a lock-in amplifier. In addition, the requirement for sample preparation prior to analysis is an impediment preventing practical adoption of the technique in real-time, standoff chemical detection deployments in an open environment. The research team has previously demonstrated the feasible use of a laser interferometric technique to detect photo-vibrational signals of trace chemicals because it was excited by an intensity-modulated quantum cascade laser (QCL). In this study, our long distance laser Doppler vibrometer developed in house was demonstrated to be able to detect the vibration amplitude and frequency of trace explosives at long standoff distances of 100 m in an open environment. Three explosives of 10 µg/mm2 concentration were tested, and the photo-vibrational spectrum obtained by scanning the output wavelength of the QCL through the mid-IR range, coincides well with the corresponding spectrum obtained using typical FTIR equipment. The experiment demonstrated that the detection of hazardous chemicals or explosive substances at a safe standoff distance can be realized through a system utilizing capable interferometric sensors like the laser Doppler vibrometer, coupled with a QCL excitation source.





Categories: Optics news

Review of phase measuring deflectometry

Sun, 04/15/2018 - 06:03
Publication date: August 2018
Source:Optics and Lasers in Engineering, Volume 107

Author(s): Lei Huang, Mourad Idir, Chao Zuo, Anand Asundi

As a low cost, full-field three-dimensional shape measurement technique with high dynamic range, Phase Measuring Deflectometry (PMD) has been studied and improved to be a simple and effective manner to inspect specular reflecting surfaces. In this review, the fundamental principle and the basic concepts of PMD technique are introduced and followed by a brief overview of its key developments since it was first proposed. In addition, the similarities and differences compared with other related techniques are discussed to highlight the distinguishing features of the PMD technique. Furthermore, we will address the major challenges, the existing solutions and the remaining limitations in this technique to provide some suggestions for potential future investigations.





Categories: Optics news

Fundamental study of the bulge structure generated in laser polishing process

Sun, 04/15/2018 - 06:03
Publication date: August 2018
Source:Optics and Lasers in Engineering, Volume 107

Author(s): Chen Chen, Hai-Lung Tsai

Laser polishing is an innovative part-finishing process to reduce the surface roughness by melting a thin layer of material on the part surface. The polished surface quality is influenced by many factors including initial surface condition, properties of material, laser power, scan speed, focal offset, beam shape, percentage of overlap, etc. In addition to removing the original asperities, the laser polishing process may also introduce new asperities including bulges, ripples, undercuts, etc. In this paper, a fundamental study is carried out on Ti-6Al-4 V alloy slabs to investigate the formation of bulge structure and the influence of processing parameters (the laser power, scan speed, focal offset and successive track displacement) on the bulge structure through parametric analyses. The formation of bulge structure is mainly caused by the phase transition in the heat-affected zone and the mass transport of the fluid flow in melting pool. The parameters of the laser power, scan speed and focal offset have significant influences on the width and depth of polished tracks, the amplitude of bulge structures and the relative volume expansion by the bulge structures. Two different melting modes and a maximum track width can be found in the parametric study of focal offset. The scan speed has a significant influence on the relative volume expansion at low speeds. By reducing the successive track displacement, the overlapped bulge amplitude can be reduced rapidly, but the finer solidification-induced surface structure would get intensified.





Categories: Optics news

Optical hyperspectral image encryption based on improved Chirikov mapping and gyrator transform

Sun, 04/15/2018 - 06:03
Publication date: August 2018
Source:Optics and Lasers in Engineering, Volume 107

Author(s): Hang Chen, Camel Tanougast, Zhengjun Liu, Walter Blondel, Boya Hao

We present an optical hyperspectral image cryptosystem using improved Chirikov mapping in the gyrator transform domains. This optical encryption scheme can hide the spatial and spectrum information simultaneously. First, the original hyperspectral image is converted into binary format and then extended into a one dimensional array. Subsequently, improved Chirikov mapping is performed to generate a position array. Here, the binary array of the image can be scrambled by the position sequence. Finally, three thin cylinder lenses are controlled using a PC to implement the gyrator transform, and the amplitude and phase information in the output plane is considered to be the encrypted information. Some numerical simulations verify the validity and capability of the proposed cryptosystem.





Categories: Optics news

Deciphering dentin tissue biomechanics using digital moiré interferometry: A narrative review

Sun, 04/15/2018 - 06:03
Publication date: August 2018
Source:Optics and Lasers in Engineering, Volume 107

Author(s): Fang-Chi Li, Anil Kishen

The functional integrity of human teeth to different thermo-mechanical stresses relies on the characteristics of dentin. Dentin is a biocomposite and biologically adapted material that is composed of mineral, organic and water phases. It displays conspicuous gradients in its ultrastructural, physical and mechanical characteristics, which experiences further variations with age and disease. Thus the biomechanical behavior of tooth/dental hard tissues under different functions is considered extremely complex. Photomechanical techniques utilize optical principles to study the biomechanical response of biological tissues under functional forces. They are largely non-destructive techniques, which provide high-sensitive, and whole-field information of specimens in situ. They have been applied extensively in dentistry to understand the biomechanical principles underlying the responses of dental hard tissue, tooth-bone relationship and restorative appliances to different forces generated within the mouth. In this line, moiré interferometry is an established method to study the deformation patterns in tooth and dental hard tissues under water-loss/thermo-mechanical loads. This article aims to provide a comprehensive review on the application of moiré interferometry to understand the biomechanical response of teeth/dentin hard tissue.





Categories: Optics news

Comparative analysis for combination of unwrapping and de-noising of phase data with high speckle decorrelation noise

Sun, 04/15/2018 - 06:03
Publication date: August 2018
Source:Optics and Lasers in Engineering, Volume 107

Author(s): Haiting Xia, Silvio Montresor, Pascal Picart, Rongxin Guo, Junchang Li

Unwrapping and de-noising are key processes for the restoration of phase data in the presence of high speckle decorrelation noise. Usually, there are two strategies to deal with noisy wrapped phase: de-noising before unwrapping, or unwrapping before de-noising. This paper aims at comparing the robustness and efficiency of the strategies. Six combinations which belong to different strategies are compared in this paper. Ten simulated phase maps with progressive noise standard deviations are generated based on the realistic speckle decorrelation noise to evaluate the performances of the approaches. The results of simulation show that de-noising with windowed Fourier transform filtering before unwrapping with the algorithm based on least-squares and iterations which belongs the first strategy has the best accuracy and acceptable computation speed for the restoration of high noisy phase data. Application of selected methods to experimental phase data from digital holography validated the analysis.





Categories: Optics news

Highly sensitive magnetic field sensor using tapered Mach–Zehnder interferometer

Sun, 04/15/2018 - 06:03
Publication date: August 2018
Source:Optics and Lasers in Engineering, Volume 107

Author(s): Hao Chen, Zhihua Shao, Xuan Zhang, Yongxin Hao, Qiangzhou Rong

A fiber-optic Mach–Zehnder interferometer is proposed and experimentally demonstrated for magnetic field measurement. The device comprises an in-line sandwiched structure, i.e., a multi-single-multi mode structure that is sealed in a tube with a magnetic fluid (MF). The diameter of the single mode fiber is tapered down to 4.2 µm, which effectively improves the coupling and recoupling of the cladding-to-radiation modes and increases the interferometer's sensitivity to the refractive index change in the MF. As a result, the interference spectrum is highly sensitive to a magnetic field ranging from 0.24 to 2.0 mT with a wavelength-referenced magnetic sensitivity of −2.886 nm/mT and an intensity-referenced sensitivity of −8.0 dB/mT.





Categories: Optics news

Toward 3D integral-imaging broadcast with increased viewing angle and parallax

Sun, 04/15/2018 - 06:03
Publication date: August 2018
Source:Optics and Lasers in Engineering, Volume 107

Author(s): A. Dorado, S. Hong, G. Saavedra, J.C. Barreiro, B. Javidi, M. Martinez-Corral

We propose a new method for improving the observer experience when using an integral monitor. Our method permits to increase the viewing angle of the integral monitor, and also the maximum parallax that can be displayed. Additionally, it is possible to decide which parts of the 3D scene are displayed in front or behind the monitor. Our method is based, first, in the direct capture, with significant excess of parallax, of elemental images of 3D real scenes. From them, a collection of microimages adapted to the observer lateral and depth position is calculated. Finally, an eye-tracking system permits to determine the 3D observer position, and therefore to display the adequate microimages set. Summarizing, it is reported here, for the first time we believe, the application of eye-tracking technology to the display of integral images of 3D real scenes with bright background. Although we are reporting here only a proof-of-concept experiment, this result could have direct application in a close future for the broadcasting of 3D videos recorded in professional studio, for videoconferences or for on-line professional meetings.





Categories: Optics news

A multi-camera speckle interferometer for dynamic full-field 3D displacement measurement: Validation and inflation testing of a human eye sclera

Sun, 04/15/2018 - 06:03
Publication date: August 2018
Source:Optics and Lasers in Engineering, Volume 107

Author(s): Luigi Bruno, Gianfranco Bianco, Massimo A. Fazio

We developed and optimized a custom dynamic Electronic Speckle Pattern Interferometer (d-ESPI) for measuring time-dependent three-dimensional (3D) surface displacements during inflation testing of ocular tissues. The 3D displacement field was resolved under dynamic loading conditions for a simplified test case of a rubber phantom and a real-world test case of the posterior sclera of a human eye. We present the optical layout and calibration procedure of the d-ESPI, and demonstrate how the displacement field can be accurately resolved for specimen deformation rates up to 6 μm/s.





Categories: Optics news

Investigation of the humping formation in the high power and high speed laser welding

Sun, 04/15/2018 - 06:03
Publication date: August 2018
Source:Optics and Lasers in Engineering, Volume 107

Author(s): Yuewei Ai, Ping Jiang, Chunming Wang, Gaoyang Mi, Shaoning Geng, Wei Liu, Chu Han

High power and high speed laser welding is as an attractive way to achieve the higher productivity in practical production. The weld periodical appearance defect, humping, is frequently generated in the high speed welding process. The weld quality is sequentially deteriorated severely due to the formed humping defect, which greatly limits the improvement of welding efficiency. In this paper, the keyhole and weld pool dynamic behaviors in the normal welding and high power high speed welding are investigated in the welding process of 304 stainless steel using 3–9 kW fiber laser at different welding speeds. The complex keyhole evolution, metal flow and metallic vapor are recorded by the high speed video camera and described in schematic illustration. The weld pool behaviors in normal welding and high power welding are compared and their effects on the humping formation are discussed in details. The results show that the tilted angle of keyhole, narrow and long molten pool, collision of fluid flow are the main factors for humping formation. Moreover, both of the microstructure characteristics of pre-humping and humping welds induced by molten pool behaviors are further analyzed and compared based on the SEM images, which is essential for reducing welding defects and achieving high quality welding with high efficiency.





Categories: Optics news

Single-shot color image encryption based on mixed state diffractive imaging

Sun, 04/15/2018 - 06:03
Publication date: August 2018
Source:Optics and Lasers in Engineering, Volume 107

Author(s): Xiaoliang He, Hua Tao, Cheng Liu, Jianqiang Zhu

We propose a method for color image encryption based on mixed state diffraction imaging. The three channels of the color image, R, G, B, are encrypted together into a single grayscale image by applying the classical optical double random phase mask encryption system, and mixed state decomposition algorithm is employed for the color image decryption, distinctively reducing the complexity of color image encryption with other methods which require to encrypt the color image into three separated channels. To the best of our knowledge, this is the first time that a color image is directly encrypted into a single grayscale encoded image. We have demonstrated the feasibility and high security of the proposed method with computer simulations.





Categories: Optics news

Lensless phase-shifting point diffraction interferometer for spherical mirror measurement

Sun, 04/15/2018 - 06:03
Publication date: August 2018
Source:Optics and Lasers in Engineering, Volume 107

Author(s): Zhongming Yang, Jiantai Dou, Qun Yuan, Zhishan Gao

We demonstrate a lensless phase-shifting point diffraction interferometer with a tiny pinhole to measure spherical mirror surface figures. The interferograms used to reconstruct the surface figure are formed without imaging optics. Fresnel diffraction calculations with a coordinate transformation are studied as a means of reconstruction method for wavefront amplitude and phase. The Radon transform is used to determine the distance from the tiny pinhole to the CCD target, which guarantees the accuracy of the diffraction calculations. Our lensless phase-shifting point diffraction interferometer not only retains the higher measurement precision by the ideal reference wavefront, but also overcome the fabrication and mounting limitation of the imaging optics. The simulations and experiments have validated the accuracy and feasibility of the spherical mirror measurement by proposed lensless phase-shifting point diffraction interferometer.





Categories: Optics news

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