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A special issue of Sensors (ISSN 1424-8220). This special issue belongs to the section "Optical Sensors".

Deadline for manuscript submissions: closed (30 September 2024) | Viewed by 15335

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Special Issue Editors

Dr. Xiangchao Zhang

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Guest Editor

Department of Optics, Fudan University, Shanghai 200433, China
Interests: precision optical measurement; computational imaging; surface metrology

Dr. Jian Wang

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Guest Editor

Huazhong University of Science and Technology, Wuhan 430074, China
Interests: precision optical measurement; 3D optical imaging; surface/film measurement

Prof. Dr. Yajun Wang

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Guest Editor

College of Electronics and Information Engineering, Sichuan University, Chengdu 610065, China
Interests: optical metrology; 3D imaging; computer vision; structured light; phase retrieval
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Computer-vision-based optical metrology methods, including triangulation, fringe projection and deflectometry, have attracted intensive interest and have widespread applications. With increasing requirements for measurement accuracy and sensing efficiency, evolutions and revolutions continuously occur in terms of measuring setup, fringe coding/decoding, light modulation, error compensation, etc. The impacts of environmental factors, machine tools, measuring procedures or even operators need to be investigated and addressed carefully in combination with recently emerging technologies such as artificial intelligence, robots, digital twin, photonics and so on.

In order to establish a highly transdisciplinary discussion forum among scientists and engineers on precision optical metrology and intelligent sensing, this Topical Collection is proposed to share novel ideas, recent developments and literature reviews. The topics include, but are not limited to:

3D optical sensing;
Computer vision;
Quantitative phase imaging;
Fringe projection;
Deflectometry;
Computational imaging.

Dr. Xiangchao Zhang
Dr. Jian Wang
Prof. Dr. Yajun Wang
Guest Editors

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Published Papers (11 papers)

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Editorial

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4 pages, 155 KiB

Open AccessEditorial

Precision Optical Metrology and Smart Sensing

by Xiangchao Zhang, Jian Wang and Yajun Wang

Sensors 2024, 24(21), 6816; https://doi.org/10.3390/s24216816 - 23 Oct 2024

Viewed by 561

Abstract

Optics, renowned for its non-contact measurement capabilities, versatility, and high sensitivity, is recognized as a powerful tool driving the technological evolution of precision metrology [...] Full article

(This article belongs to the Special Issue Precision Optical Metrology and Smart Sensing)

Research

Jump to: Editorial

21 pages, 5469 KiB

Open AccessArticle

Θ-Net: A Deep Neural Network Architecture for the Resolution Enhancement of Phase-Modulated Optical Micrographs In Silico

by Shiraz S. Kaderuppan, Anurag Sharma, Muhammad Ramadan Saifuddin, Wai Leong Eugene Wong and Wai Lok Woo

Sensors 2024, 24(19), 6248; https://doi.org/10.3390/s24196248 - 26 Sep 2024

Cited by 1 | Viewed by 653

Abstract

Optical microscopy is widely regarded to be an indispensable tool in healthcare and manufacturing quality control processes, although its inability to resolve structures separated by a lateral distance under ~200 nm has culminated in the emergence of a new field named fluorescence nanoscopy, while this too is prone to several caveats (namely phototoxicity, interference caused by exogenous probes and cost). In this regard, we present a triplet string of concatenated O-Net (‘bead’) architectures (termed ‘Θ-Net’ in the present study) as a cost-efficient and non-invasive approach to enhancing the resolution of non-fluorescent phase-modulated optical microscopical images in silico. The quality of the afore-mentioned enhanced resolution (ER) images was compared with that obtained via other popular frameworks (such as ANNA-PALM, BSRGAN and 3D RCAN), with the Θ-Net-generated ER images depicting an increased level of detail (unlike previous DNNs). In addition, the use of cross-domain (transfer) learning to enhance the capabilities of models trained on differential interference contrast (DIC) datasets [where phasic variations are not as prominently manifested as amplitude/intensity differences in the individual pixels unlike phase-contrast microscopy (PCM)] has resulted in the Θ-Net-generated images closely approximating that of the expected (ground truth) images for both the DIC and PCM datasets. This thus demonstrates the viability of our current Θ-Net architecture in attaining highly resolved images under poor signal-to-noise ratios while eliminating the need for a priori PSF and OTF information, thereby potentially impacting several engineering fronts (particularly biomedical imaging and sensing, precision engineering and optical metrology). Full article

(This article belongs to the Special Issue Precision Optical Metrology and Smart Sensing)

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10 pages, 6522 KiB

Open AccessArticle

Standard-Deviation-Based Adaptive Median Filter for Elimination of Batwing Effects in Step Microstructure Measurement Using Digital Holography

by Jiasi Wei, Junjie Wu and Chen Wang

Sensors 2024, 24(18), 5928; https://doi.org/10.3390/s24185928 - 12 Sep 2024

Cited by 1 | Viewed by 549

Abstract

Digital holography has transformative potential for the measurement of stacked-chip microstructures due to its non-invasive, single-shot, full-field characteristics. However, significant light scattering and diffraction at steep edges in step microstructures cause the batwing effect, leading to measurement errors. Herein, we propose a standard-deviation-based adaptive median filter to eliminate batwing effects in step microstructure measurement using digital holography. The standard deviation determines the positions of the steps and the range of the batwing effect. During filtering, the filter window size varies: it adjusts according to the center’s position within the batwing effect range and reduces outside this range to prevent distortion in other regions. Filtering weights are set to maintain information integrity while using larger filter windows. Experiments on the Standard Resolution Target USAF 1951 and the standard step height target show that our method successfully eliminates batwings while preserving the integrity of the remaining profile. Full article

(This article belongs to the Special Issue Precision Optical Metrology and Smart Sensing)

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13 pages, 1930 KiB

Open AccessArticle

A Reflected-Light-Mode Multiwavelength Interferometer for Measurement of Step Height Standards

by Dariusz Litwin, Kamil Radziak, Adam Czyżewski, Jacek Galas, Tadeusz Kryszczyński, Narcyz Błocki, Robert Szumski and Justyna Niedziela

Sensors 2024, 24(16), 5082; https://doi.org/10.3390/s24165082 - 6 Aug 2024

Cited by 1 | Viewed by 778

Abstract

The article is dedicated to measuring the thickness of step height standards using the author’s version of the variable wavelength interferometer (VAWI) in the reflected-light mode, where the interference pattern is created by the combination of two Wollaston prisms. The element of novelty consists in replacing the traditional search for the coincidence of fringes in the object and background with a continuous measurement of their periods and phases relative to the zero-order fringe. The resulting system of sinusoids is then analyzed using two methods: the classical one and the second utilizing the criterion of uniform thickness. The theory is followed by simulation and experimental parts, providing insight to the metrological potential of the VAWI technology. Full article

(This article belongs to the Special Issue Precision Optical Metrology and Smart Sensing)

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13 pages, 2755 KiB

Open AccessArticle

Thermal Deformation Measurement of the Surface Shape of a Satellite Antenna Using High-Accuracy Close-Range Photogrammetry

by Kaifeng Ma, Guiping Huang and Junzhen Meng

Sensors 2024, 24(14), 4722; https://doi.org/10.3390/s24144722 - 20 Jul 2024

Cited by 1 | Viewed by 1048

Abstract

To determine both the size of a satellite antenna and the thermal deformation of its surface shape, a novel high-accuracy close-range photogrammetric technique is used in this study. The method is also applied to assess the performance of the antenna in orbit. The measurement principle and solution method of close-range photogrammetry were thoroughly investigated, and a detailed measurement test scheme was developed. A thermal deformation measurement of the surface shape of a satellite antenna was then carried out. The results show that the measurement error using close-range photogrammetry was smaller than 0.04 mm, which meets the accuracy requirement. Thanks to the high accuracy, it was discovered that both the surface shape and the rib precision of the satellite antenna deteriorate with decreasing temperature. The accuracy of the surface shape and ribs was lowest when the temperature node was −60 °C. The maximum root mean square errors (RMSEs) reached 0.878 mm and 0.761 mm, respectively. This indicates that the surface shape deformation error of the antenna caused by high and low temperatures is relatively high. However, the requirement for the technical design index (RMSE ≤ 1 mm for the surface shape accuracy of the antenna) is still met. Furthermore, for temperature differences of 40 °C and 80 °C, the measured RMSEs for the surface shape deformation were 0.216 mm and 0.411 mm, respectively. Overall, the technical design indicators (RMSE ≤ 0.3 mm and RMSE ≤ 0.5 mm, respectively) for the surface shape deformation of the antennas are met. Full article

(This article belongs to the Special Issue Precision Optical Metrology and Smart Sensing)

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22 pages, 597 KiB

Open AccessArticle

Understanding the Nonlinear Response of SiPMs

by Víctor Moya-Zamanillo and Jaime Rosado

Sensors 2024, 24(8), 2648; https://doi.org/10.3390/s24082648 - 21 Apr 2024

Cited by 1 | Viewed by 1041

Abstract

A systematic study of the nonlinear response of Silicon Photomultipliers (SiPMs) was conducted through Monte Carlo (MC) simulations. The MC code was validated against experimental data for two different SiPMs. Nonlinearity mainly depends on the balance between the photon rate and the pixel recovery time. Additionally, nonlinearity has been found to depend on the light pulse shape, the correlated noise, the overvoltage dependence of the photon detection efficiency, and the impedance of the readout circuit. Correlated noise has been shown to have a minor impact on nonlinearity, but it can significantly affect the shape of the SiPM output current. Considering these dependencies and a previous statistical analysis of the nonlinear response of SiPMs, two phenomenological fitting models were proposed for exponential-like and finite light pulses, explaining the roles of their various terms and parameters. These models provide an accurate description of the nonlinear responses of SiPMs at the level of a few percentages for a wide range of situations. Full article

(This article belongs to the Special Issue Precision Optical Metrology and Smart Sensing)

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15 pages, 9277 KiB

Open AccessArticle

Stitching Locally Fitted T-Splines for Fast Fitting of Large-Scale Freeform Point Clouds

by Jian Wang, Sheng Bi, Wenkang Liu, Liping Zhou, Tukun Li, Iain Macleod and Richard Leach

Sensors 2023, 23(24), 9816; https://doi.org/10.3390/s23249816 - 14 Dec 2023

Cited by 2 | Viewed by 1177

Abstract

Parametric splines are popular tools for precision optical metrology of complex freeform surfaces. However, as a promising topologically unconstrained solution, existing T-spline fitting techniques, such as improved global fitting, local fitting, and split-connect algorithms, still suffer the problems of low computational efficiency, especially in the case of large data scales and high accuracy requirements. This paper proposes a speed-improved algorithm for fast, large-scale freeform point cloud fitting by stitching locally fitted T-splines through three steps of localized operations. Experiments show that the proposed algorithm produces a three-to-eightfold efficiency improvement from the global and local fitting algorithms, and a two-to-fourfold improvement from the latest split-connect algorithm, in high-accuracy and large-scale fitting scenarios. A classical Lena image study showed that the algorithm is at least twice as fast as the split-connect algorithm using fewer than 80% control points of the latter. Full article

(This article belongs to the Special Issue Precision Optical Metrology and Smart Sensing)

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13 pages, 4751 KiB

Open AccessArticle

Flexible Measurement of High-Slope Micro-Nano Structures with Tilted Wave Digital Holographic Microscopy

by Xinyang Ma, Rui Xiong, Wei Wang and Xiangchao Zhang

Sensors 2023, 23(23), 9526; https://doi.org/10.3390/s23239526 - 30 Nov 2023

Cited by 2 | Viewed by 1008

Abstract

Digital holographic microscopy is an important measurement method for micro-nano structures. However, when the structured features are of high-slopes, the interference fringes can become too dense to be recognized. Due to the Nyquist’s sampling limit, reliable wavefront restoration and phase unwrapping are not feasible. To address this problem, the interference fringes are proposed to be sparsified by tilting the reference wavefronts. A data fusion strategy including region extraction and tilt correction is developed for reconstructing the full-area surface topographies. Experimental results of high-slope elements demonstrate the validity and reliability of the proposed method. Full article

(This article belongs to the Special Issue Precision Optical Metrology and Smart Sensing)

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14 pages, 20301 KiB

Open AccessCommunication

Dynamic 3D Measurement without Motion Artifacts Based on Feature Compensation

by Guoce Hu, Jun Wang, Huaxia Deng, Mengchao Ma and Xiang Zhong

Sensors 2023, 23(16), 7147; https://doi.org/10.3390/s23167147 - 13 Aug 2023

Cited by 2 | Viewed by 1160

Abstract

Phase-shift profilometry (PSP) holds great promise for high-precision 3D shape measurements. However, in the case of measuring moving objects, as PSP requires multiple images to calculate the phase, the movement of the object causes artifacts in the measurement, which in turn has a significant impact on the accuracy of the 3D surface measurement. Therefore, we propose a method to reduce motion artifacts using feature information in the image and simulate it using the six-step term shift method as a case study. The simulation results show that the phase of the object is greatly affected when the object is in motion and that the phase shift due to motion can be effectively reduced using this method. Finally, artifact optimization was carried out by way of specific copper tube vibration experiments at a measurement frequency of 320 Hz. The experimental results prove that the method is well implemented. Full article

(This article belongs to the Special Issue Precision Optical Metrology and Smart Sensing)

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14 pages, 6084 KiB

Open AccessArticle

Three-Dimensional Shape and Deformation Measurements Based on Fringe Projection Profilometry and Fluorescent Digital Image Correlation via a 3 Charge Coupled Device Camera

by Wei Sun, Zhongda Xu, Xin Li, Zhenning Chen and Xinqiao Tang

Sensors 2023, 23(15), 6663; https://doi.org/10.3390/s23156663 - 25 Jul 2023

Cited by 5 | Viewed by 1386

Abstract

We propose a novel hybrid FPP-DIC technique to measure an object’s shape and deformation in 3D simultaneously by using a single 3CCD color camera, which captures the blue fringe patterns and red fluorescent speckles within the same image. Firstly, red fluorescent speckles were painted on the surface of the specimen. Subsequently, 12 computer-generated blue fringe patterns with a black background were projected onto the surface of the specimen using a DLP projector. Finally, both the reference and deformed images with three different frequencies and four shifted phases were captured using a 3CCD camera. This technique employed a three-chip configuration in which red–green–blue chips were discretely integrated in the 3CCD color camera sensor, rendering independent capture of RGB information possible. Measurement of out-of-plane displacement was carried out through the implementation of Fringe Projection Profilometry (FPP), whereas the in-plane displacement was evaluated using a 2D Digital Image Correlation (DIC) method by leveraging a telecentric-lens-based optical system. In comparison to the traditional FPP-DIC hybrid methodology, the present approach showed a lower incidence of crosstalk between the fringe patterns and speckle patterns while also offering a corrective for the coupling of the in-plane displacement and out-of-plane displacement. Experimental results for the in-plane cantilever beam and out-of-plane disk comparisons with the traditional 3D-DIC method indicated that the maximum discrepancy obtained between FPP-DIC and 3D-DIC was 0.7 μm and 0.034 mm with different magnifications, respectively, validating the effectiveness and precision of the novel proposed FPP-DIC method. Full article

(This article belongs to the Special Issue Precision Optical Metrology and Smart Sensing)

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15 pages, 9618 KiB

Open AccessArticle

An Efficient Method for Laser Welding Depth Determination Using Optical Coherence Tomography

by Guanming Xie, Sanhong Wang, Yueqiang Zhang, Biao Hu, Yu Fu, Qifeng Yu and You Li

Sensors 2023, 23(11), 5223; https://doi.org/10.3390/s23115223 - 31 May 2023

Cited by 4 | Viewed by 2832

Abstract

Online monitoring of laser welding depth is increasingly important, with the growing demand for the precise welding depth in the field of power battery manufacturing for new energy vehicles. The indirect methods of welding depth measurement based on optical radiation, visual image and acoustic signals in the process zone have low accuracy in the continuous monitoring. Optical coherence tomography (OCT) provides a direct welding depth measurement during laser welding and shows high achievable accuracy in continuous monitoring. Statistical evaluation approach accurately extracts the welding depth from OCT data but suffers from complexity in noise removal. In this paper, an efficient method coupled DBSCAN (Density-Based Spatial Clustering of Application with Noise) and percentile filter for laser welding depth determination was proposed. The noise of the OCT data were viewed as outliers and detected by DBSCAN. After eliminating the noise, the percentile filter was used to extract the welding depth. By comparing the welding depth determined by this approach and the actual weld depth of longitudinal cross section, an average error of less than 5% was obtained. The precise laser welding depth can be efficiently achieved by the method. Full article

(This article belongs to the Special Issue Precision Optical Metrology and Smart Sensing)

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