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Optics and Lasers in Engineering

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Updated: 20 weeks 5 days ago

A novel dual-wavelength iterative method for generalized dual-wavelength phase-shifting interferometry with second-order harmonics

Sun, 04/15/2018 - 06:03
Publication date: July 2018
Source:Optics and Lasers in Engineering, Volume 106

Author(s): Xiaoqing Xu, Yawei Wang, Ying Ji, Yuanyuan Xu, Ming Xie, Hao Han

To address dual-wavelength interferograms with arbitrary phase shifts and second-order harmonics, a novel dual-wavelength iterative method (DWIM) based on the least-squares algorithm is proposed. In generalized dual-wavelength phase-shifting interferometry, to compensate for the phase-shift errors consisting of systematic and random phase-shift error, the wrapped phases of single-wavelength with high accuracy can be simultaneously obtained from generalized dual-wavelength interferograms without second-order harmonics. In addition, this method is also employed to deal with randomly phase-shifted dual-wavelength interferograms with the second-order harmonics, and then the effects of the fringe number in interferogram and the number of interferograms used on the accuracy of phase extraction are investigated by numerical simulations. Based on theoretical analysis and simulation results of DWIM, we present the basic relationship between the number of wavelengths, the second-order harmonics and the requirement of the minimum number of interferograms. Finally, the effectiveness of this method is proved by the simulation results of the spherical cap, and its applicability is verified with the results of the micro-sphere, the HeLa cell and the red blood cell, respectively.





Categories: Optics news

Experimental study on microsphere assisted nanoscope in non-contact mode

Sun, 04/15/2018 - 06:03
Publication date: July 2018
Source:Optics and Lasers in Engineering, Volume 106

Author(s): Jinzhong Ling, Dancui Li, Xin Liu, Xiaorui Wang

Microsphere assisted nanoscope was proposed in existing literatures to capture super-resolution images of the nano-structures beneath the microsphere attached on sample surface. In this paper, a microsphere assisted nanoscope working in non-contact mode is designed and demonstrated, in which the microsphere is controlled with a gap separated to sample surface. With a gap, the microsphere is moved in parallel to sample surface non-invasively, so as to observe all the areas of interest. Furthermore, the influence of gap size on image resolution is studied experimentally. Only when the microsphere is close enough to the sample surface, super-resolution image could be obtained. Generally, the resolution decreases when the gap increases as the contribution of evanescent wave disappears. To keep an appropriate gap size, a quantitative method is implemented to estimate the gap variation by observing Newton's rings around the microsphere, serving as a real-time feedback for tuning the gap size. With a constant gap, large-area image with high resolution can be obtained during microsphere scanning. Our study of non-contact mode makes the microsphere assisted nanoscope more practicable and easier to implement.





Categories: Optics news

Localized analysis of paint-coat drying using dynamic speckle interferometry

Sun, 04/15/2018 - 06:03
Publication date: July 2018
Source:Optics and Lasers in Engineering, Volume 106

Author(s): Daniel Sierra-Sosa, Myrian Tebaldi, Eduardo Grumel, Hector Rabal, Adel Elmaghraby

The paint-coating is part of several industrial processes, including the automotive industry, architectural coatings, machinery and appliances. These paint-coatings must comply with high quality standards, for this reason evaluation techniques from paint-coatings are in constant development. One important factor from the paint-coating process is the drying, as it has influence on the quality of final results. In this work we present an assessment technique based on the optical dynamic speckle interferometry, this technique allows for the temporal activity evaluation of the paint-coating drying process, providing localized information from drying. This localized information is relevant in order to address the drying homogeneity, optimal drying, and quality control. The technique relies in the definition of a new temporal history of the speckle patterns to obtain the local activity; this information is then clustered to provide a convenient indicative of different drying process stages. The experimental results presented were validated using the gravimetric drying curves





Categories: Optics news

Super-sensitive two-wavelength fringe projection profilometry with 2-sensitivities temporal unwrapping

Sun, 04/15/2018 - 06:03
Publication date: July 2018
Source:Optics and Lasers in Engineering, Volume 106

Author(s): Manuel Servin, Moises Padilla, Guillermo Garnica

Since the early 1970s, optical two-wavelength phase-metrology (TWPM) has been used in a wide variety of experimental set ups. In TWPM one may compute the phase-sum and the phase-difference of two close phase measurements. Early TWPM optically computed the phase difference and phase sum by double exposure holography. However soon after, TWPM became almost synonymous to calculating the phase-difference only. The more sensitive phase-sum was largely forgotten. The standard application for phase-difference TWPM is to extend the phase measurement depth without phase-unwrapping for discontinuous phase-objects. This phase-difference, while non-wrapped, decreases however the signal-to-noise ratio (SNR) of the estimated phase. On the other hand, the phase-sum increases the phase sensitivity, and the SNR of the estimated phase. In spite of these two great advantages, the use of the phase-sum in TWPM has been almost ignored. In this paper we review and set the stage for digital TWPM for super-sensitive phase-sum estimation. This is coupled with two-sensitivity phase-unwrapping to obtain extended-range super-sensitive fringe-projection profilometry estimations. Here we mathematically prove, and experimentally show that using the phase-sum one obtains a huge increase in SNR with respect to using the phase-difference alone. The pioneer works on double exposure TWPM holography that uses the phase-difference and phase-sum are also properly acknowledged. Finally, two experimental results from fringe-projection profilometry that clearly show the huge SNR gain of the phase-sum, with respect to the phase-difference is now mathematically well established.





Categories: Optics news

Wavefront reconstruction for multi-lateral shearing interferometry using difference Zernike polynomials fitting

Sun, 04/15/2018 - 06:03
Publication date: July 2018
Source:Optics and Lasers in Engineering, Volume 106

Author(s): Ke Liu, Jiannian Wang, Hai Wang, Yanqiu Li

For the multi-lateral shearing interferometers (multi-LSIs), the measurement accuracy can be enhanced by estimating the wavefront under test with the multidirectional phase information encoded in the shearing interferogram. Usually the multi-LSIs reconstruct the test wavefront from the phase derivatives in multiple directions using the discrete Fourier transforms (DFT) method, which is only suitable to small shear ratios and relatively sensitive to noise. To improve the accuracy of multi-LSIs, wavefront reconstruction from the multidirectional phase differences using the difference Zernike polynomials fitting (DZPF) method is proposed in this paper. For the DZPF method applied in the quadriwave LSI, difference Zernike polynomials in only two orthogonal shear directions are required to represent the phase differences in multiple shear directions. In this way, the test wavefront can be reconstructed from the phase differences in multiple shear directions using a noise-variance weighted least-squares method with almost no extra computational burden, compared with the usual recovery from the phase differences in two orthogonal directions. Numerical simulation results show that the DZPF method can maintain high reconstruction accuracy in a wider range of shear ratios and has much better anti-noise performance than the DFT method. A null test experiment of the quadriwave LSI has been conducted and the experimental results show that the measurement accuracy of the quadriwave LSI can be improved from 0.0054 λ rms to 0.0029 λ rms (λ = 632.8 nm) by substituting the DFT method with the proposed DZPF method in the wavefront reconstruction process.





Categories: Optics news

Dual demodulation interferometer with two-wave mixing in GaAs photorefractive crystal

Sun, 04/15/2018 - 06:03
Publication date: July 2018
Source:Optics and Lasers in Engineering, Volume 106

Author(s): Zhang Zhenzhen, Jia Zhongqing, Ji Guangrong, Wang Qiwu

A dual demodulation interferometer with two-wave mixing (TWM) in the GaAs photorefractive crystal (PRC) is proposed and experimentally demonstrated. The GaAs PRC has tiny temperature change under high voltage thus not requiring thermoelectric cooler (TEC) to stabilize the temperature, and adaptive to low frequency fluctuation below 200 Hz. The system is an unbalanced TWM interferometer, which could demodulate the phase change both space variation and wavelength shift induced by strain. Two demodulation modes' formulas are provided in theory respectively. Experimental results have been tested and compared with theoretical analysis, demonstrating that it is a practical and flexible system for detection of mechanical vibration or structure health monitoring (SHM) in engineering by selecting different demodulation mode.





Categories: Optics news

Structured light system calibration with unidirectional fringe patterns

Sun, 04/15/2018 - 06:03
Publication date: July 2018
Source:Optics and Lasers in Engineering, Volume 106

Author(s): Vignesh Suresh, Jace Holton, Beiwen Li

For the calibration of a structured light system, one type of highly accurate calibration method was developed by treating the projector as an inverse camera. This type of method typically creates pixel-to-pixel mapping between a camera point and a projector point using fringe patterns and resultant phase maps in orthogonal directions. However, requiring orthogonal patterns limits its feasibility of implementation on systems where the illumination device (e.g. grating projectors, interferometers, etc.) only supports fringe projection in one direction. This paper introduces a novel calibration method that only uses patterns in a single direction. We have theoretically proved that there exists one degree-of-freedom of redundancy in conventional calibration methods, making it possible to reduce the requirement of using orthogonal fringe patterns. Experiments show that under a measurement range of 200 mm(X) ×  150 mm (Y) × 120 mm (Z), our measurement results are comparable to the results obtained using conventional calibration method. Evaluated by repeatedly measuring a sphere with 147.726 mm diameter, our measurement accuracy on average can be as high as 0.20 mm with a standard deviation of 0.12 mm.





Categories: Optics news

In-situ infrared thermography measurements to master transmission laser welding process parameters of PEKK

Sun, 04/15/2018 - 06:03
Publication date: July 2018
Source:Optics and Lasers in Engineering, Volume 106

Author(s): M. Villar, C. Garnier, F. Chabert, V. Nassiet, D. Samélor, J.C. Diez, A. Sotelo, M.A. Madre

The temperature field along the thickness of the specimens has been measured during transmission laser welding. Polyetherketoneketone (PEKK) is a very high performance thermoplastic with tunable properties. We have shown that this grade of PEKK can be turned to quasi-amorphous or semi-crystalline material, due to its slow kinetics of crystallization. Its glass transition temperature is 150 °C. The effect of its crystalline rate directly impacts its optical properties: the transmittance of quasi-amorphous PEKK is about 60% in the NIR region (wavelength range from 0.4 to 1.2 µm) whereas it is less than 3% for the semi-crystalline material. The welding tests have been carried out with an 808 nm laser diode apparatus. The heat field is recorded during the welding experiment by infrared thermography with the camera sensor perpendicular to the lasersheet and to the sample's length to focus on the welded interface. The study is divided in two steps: firstly, a single specimen is irradiated with an energy density of 22 J.mm−²: the whole sample thickness is heated up, the maximum temperature reaches 222 ± 7 °C. This temperature corresponds to about Tg + 70 °C, but the polymer does not reach its melting temperature. After that, welding tests were performed: a transparent (quasi-amorphous) sample as the upper part and an opaque (semi-crystalline) one as the lower part were assembled in static conditions. The maximum temperature reached at the welded interface is about 295 °C when the upper specimen is irradiated for 16 s with an energy density of 28 J.mm−². The temperature at the welded interface stays above Tg during 55 s and reached the melting temperature during 5 s before rapid cooling. These parameters are suitable to assemble both polymeric parts in a strong weld. This work shows that infrared thermography is an appropriate technique to improve the reliability of laser welding process of high performance thermoplastics.





Categories: Optics news

Image reconstruction through thin scattering media by simulated annealing algorithm

Sun, 04/15/2018 - 06:03
Publication date: July 2018
Source:Optics and Lasers in Engineering, Volume 106

Author(s): Longjie Fang, Haoyi Zuo, Lin Pang, Zuogang Yang, Xicheng Zhang, Jianhua Zhu

An idea for reconstructing the image of an object behind thin scattering media is proposed by phase modulation. The optimized phase mask is achieved by modulating the scattered light using simulated annealing algorithm. The correlation coefficient is exploited as a fitness function to evaluate the quality of reconstructed image. The reconstructed images optimized from simulated annealing algorithm and genetic algorithm are compared in detail. The experimental results show that our proposed method has better definition and higher speed than genetic algorithm.





Categories: Optics news

A holistic calibration method with iterative distortion compensation for stereo deflectometry

Sun, 04/15/2018 - 06:03
Publication date: July 2018
Source:Optics and Lasers in Engineering, Volume 106

Author(s): Yongjia Xu, Feng Gao, Zonghua Zhang, Xiangqian Jiang

This paper presents a novel holistic calibration method for stereo deflectometry system to improve the system measurement accuracy. The reconstruction result of stereo deflectometry is integrated with the calculated normal data of the measured surface. The calculation accuracy of the normal data is seriously influenced by the calibration accuracy of the geometrical relationship of the stereo deflectometry system. Conventional calibration approaches introduce form error to the system due to inaccurate imaging model and distortion elimination. The proposed calibration method compensates system distortion based on an iterative algorithm instead of the conventional distortion mathematical model. The initial value of the system parameters are calculated from the fringe patterns displayed on the systemic LCD screen through a reflection of a markless flat mirror. An iterative algorithm is proposed to compensate system distortion and optimize camera imaging parameters and system geometrical relation parameters based on a cost function. Both simulation work and experimental results show the proposed calibration method can significantly improve the calibration and measurement accuracy of a stereo deflectometry. The PV (peak value) of measurement error of a flat mirror can be reduced to 69.7 nm by applying the proposed method from 282 nm obtained with the conventional calibration approach.





Categories: Optics news

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