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Photonics
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Optical Imaging and Measurements
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Optical Imaging and Measurements

A special issue of Photonics (ISSN 2304-6732).

Deadline for manuscript submissions: closed (15 July 2024) | Viewed by 21775

Special Issue Editors

Dr. Zixin Zhao

E-Mail Website
Guest Editor
State Key Laboratory for Manufacturing Systems Engineering, School of Mechanical Engineering, Xi’an Jiaotong University, Xi’an, China
Interests: computational optical imaging; interferometry; holography
Special Issues, Collections and Topics in MDPI journals
Dr. Feifei Gu

E-Mail Website
Guest Editor
Shenzhen Institute of Advanced Technology, CAS, Shenzhen, China
Interests: machine vision; computational 3D vision; structured light; optical image processing; optical measurement; dynamic reconstruction
Special Issues, Collections and Topics in MDPI journals
Dr. Gaopeng Zhang

E-Mail Website
Guest Editor
Xi'an Institute of Optics and Precision Mechanics, CAS, Xi’an, China
Interests: machine vision; optical measurement; optical image processing
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

An optical wave carries plenty of information by modulating its amplitude, phase, polarization or coherence. As a result, different types of imaging and measurement techniques have been developed. Traditional optical imaging is two-dimensional, and thus, what you see is what you get. Modern optical imaging is multidimensional, meaning what you compute is what you get. Computational optical imaging restores image information by accurately characterizing multidimensional light fields and using advanced modulation and demodulation techniques. It provides a new way to break through the limitations of traditional imaging technology. A large number of exciting research developments are helping to continuously improve the performance of these optical imaging and measurement techniques under different situations. Hence, real-time imaging and dynamic measurements with high resolution and accuracy are increasingly becoming a reality.

The objectives of this Special Issue are to report on the advances in optical imaging and measurements. Topics of interest include, but are not limited to:

  • 3D imaging;
  • Phase imaging;
  • Polarization imaging;
  • Computational optical imaging;
  • Optical coherence tomography;
  • Digital holography;
  • Speckle interferometry;
  • Optical triangulation measurements.

Prof. Dr. Zixin Zhao
Dr. Feifei Gu
Dr. Gaopeng Zhang
Guest Editors

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Photonics is an international peer-reviewed open access monthly journal published by MDPI.

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Related Special Issue

Published Papers (15 papers)

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Research

16 pages, 4942 KiB  
Article
Three-Shot Dual-Frequency Fringe Scheme Based on Spatial Computer-Generated Moiré Fringe
by Hechen Zhang, Jin Zhou, Dan Jia, Jinlong Huang and Jing Yuan
Photonics 2024, 11(8), 758; https://doi.org/10.3390/photonics11080758 - 14 Aug 2024
Viewed by 811
Abstract
A highly robust dual-frequency hierarchical temporal phase unwrapping (DHTPU) based on the novel spatial computer-generated Moiré profilometry (SCGMP) is proposed. The method requires only three patterns: a high-frequency fringe to provide robust surface information, a multi-period low-frequency fringe to eliminate the 2π-phase ambiguities, [...] Read more.
A highly robust dual-frequency hierarchical temporal phase unwrapping (DHTPU) based on the novel spatial computer-generated Moiré profilometry (SCGMP) is proposed. The method requires only three patterns: a high-frequency fringe to provide robust surface information, a multi-period low-frequency fringe to eliminate the 2π-phase ambiguities, and a flat pattern to remove the average intensity of the two fringes. In decoding, different from traditional Moiré profilometries that rely on spectrum filters, SCGMP only employs spatial-domain calculations to extract the wrapped phase, thereby preserving more detailed information. Furthermore, we fully explore SCGMP’s capability to significantly alleviate phase ambiguity and provide an algorithm to determine the maximum measurable height range for a fixed system, enabling the direct extraction of the continuous basic phase from the multi-period low-frequency fringe. Consequently, the proposed basic phase exhibits an enhanced signal-to-noise ratio, compared to the traditional basic phase derived from the single-period fringes, effectively releasing the high-frequency restriction in the traditional DHTPU. The experimental results verify that the proposed DHTPU method has considerable accuracy and great potential for high-speed measurements, due to there being only three shots required. Full article
(This article belongs to the Special Issue Optical Imaging and Measurements)
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13 pages, 2963 KiB  
Article
Optical Detection of Underwater Propeller Wake Based on a Position-Sensitive Detector
by Guanlong Zhou, Qin Liu, Hu Wang, Liyan Li, Yan Zhou and Xinyu Chen
Photonics 2024, 11(8), 732; https://doi.org/10.3390/photonics11080732 - 6 Aug 2024
Viewed by 831
Abstract
The study of underwater vehicle wake detection is of significant importance within the field of target detection, localisation, and tracking of underwater vehicles. Given that propellers are the propellers of modern ships and underwater vehicles, the propeller wake field represents the principal target [...] Read more.
The study of underwater vehicle wake detection is of significant importance within the field of target detection, localisation, and tracking of underwater vehicles. Given that propellers are the propellers of modern ships and underwater vehicles, the propeller wake field represents the principal target source for wake detection in underwater vehicles. The objective of this paper is to propose a method for measuring the wake of an underwater propeller based on a position-sensitive detector. A theoretical model of the relationship between the laser spot displacement and the change in the refractive index of the wake field is established on the basis of the principle of laser beam deflection. A prototype experimental setup for underwater propeller wake measurement was constructed based on the aforementioned optical measurement method. Furthermore, the simulation of the propeller wake flow field with strong density stratification and linear density stratification was conducted based on the experimental setup. Furthermore, experiments were conducted to detect the flow field of a propeller wake. The experimental results indicate that the wake dissipation times of the propeller in a strong density-stratified water environment are approximately 800 s and 750 s. Following the stabilisation of the wake field density, the laser spot position is observed to be stable at 0.341 mm and 0.441 mm, respectively, with a corresponding refractive index change of 2.99 × 10−6 RIU (refractive index unit) and 3.87 × 10−6 RIU, respectively. These experimental results are found to be in general agreement with the simulation results of the propeller wake field. A comparison of the experimental wake measurements based on the device with the wake measurements based on a CTD (conductivity–temperature–depth) device reveals a consistent trend. The realisation of this detection technique is of great significance for the advancement of research in the field of optical detection of underwater vehicle wake streams. Full article
(This article belongs to the Special Issue Optical Imaging and Measurements)
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13 pages, 10876 KiB  
Article
A Segment-Based Algorithm for Grid Junction Corner Detection Used in Stereo Microscope Camera Calibration
by Junjie Liu, Weiren Zhao, Keming Li, Jiahui Wang, Shuangping Yi, Huan Jiang and Hui Zhang
Photonics 2024, 11(8), 688; https://doi.org/10.3390/photonics11080688 - 24 Jul 2024
Viewed by 1037
Abstract
Corner detection is responsible for accurate camera calibration, which is an essential task for binocular three-dimensional (3D) reconstruction. In microscopic scenes, binocular 3D reconstruction has significant potential to achieve fast and accurate measurements. However, traditional corner detectors and calibration patterns (checkerboard) performed poorly [...] Read more.
Corner detection is responsible for accurate camera calibration, which is an essential task for binocular three-dimensional (3D) reconstruction. In microscopic scenes, binocular 3D reconstruction has significant potential to achieve fast and accurate measurements. However, traditional corner detectors and calibration patterns (checkerboard) performed poorly in microscopic scenes due to the non-uniform illumination and the shallow depth of field of the microscope. In this paper, we present a novel method for detecting grid junction corners based on image segmentation, offering a robust alternative to the traditional checkerboard pattern. Model fitting was utilized to obtain the coordinates at a sub-pixel level. The procedures of the proposed method were elaborated, including image segmentation, corner prediction, and model fitting. The mathematical model was established to describe the grid junction. The experiment was conducted using both synthetic and real data and the experimental result shows that this method achieves high precision and is robust to image blurring, indicating this method is suitable for microscope camera calibration. Full article
(This article belongs to the Special Issue Optical Imaging and Measurements)
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13 pages, 6112 KiB  
Article
Automatic Defect Detection Instrument for Spherical Surfaces of Optical Elements
by Yali Shi, Mei Zhang and Mingwei Li
Photonics 2024, 11(7), 681; https://doi.org/10.3390/photonics11070681 - 22 Jul 2024
Viewed by 823
Abstract
In order to realize automatic surface defect detection for large aperture precision spherical optical elements, an automatic surface defect detection instrument has been designed. The instrument consists of a microscopic imaging system, illumination system, motion scanning system, and a software algorithm system. Firstly, [...] Read more.
In order to realize automatic surface defect detection for large aperture precision spherical optical elements, an automatic surface defect detection instrument has been designed. The instrument consists of a microscopic imaging system, illumination system, motion scanning system, and a software algorithm system. Firstly, a multi-angle channel illumination source and a coaxial illumination source were designed. Bright and dark field images of surface defects were captured by cooperating with an automatic zoom microscope. Then, algorithms for scanning trajectory planning, image stitching, and intelligent defect recognition were designed to achieve full-aperture surface image acquisition and defect quantification detection. The automated defect detection process of the instrument is summarized and introduced. Finally, the experimental platform was constructed, which can work well for the optical elements with a maximum diameter of 400 mm and a relative aperture R/D value of 1. It takes about 15 min to detect an optical element with a diameter of 200 mm in dark-field imaging mode. As a result, the minimum line width of scratch detectable is 2 μm and the minimum diameter of pitting detectable is 4 μm. Clearly, the instrument can realize the automatic detection of surface defects of spherical optical elements, and has the advantages of a high efficiency, stability, reliability, quantification, and data traceability. Full article
(This article belongs to the Special Issue Optical Imaging and Measurements)
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15 pages, 6240 KiB  
Article
Design of an Airborne Low-Light Imaging System Based on Multichannel Optical Butting
by Jianwei Peng, Hongtao Yang, Yangjie Lei, Wanrong Yu, Weining Chen and Guangdong Zhang
Photonics 2024, 11(7), 636; https://doi.org/10.3390/photonics11070636 - 3 Jul 2024
Viewed by 815
Abstract
For the purpose of achieving long-range, high-resolution, and ultra-wide-swath airborne earth imaging at extremely low-light levels (0.01 Lux), a low-light imaging system built on multi-detector optical butting was researched. Having decomposed the system’s specifications and verified its low-light imaging capability, we proposed to [...] Read more.
For the purpose of achieving long-range, high-resolution, and ultra-wide-swath airborne earth imaging at extremely low-light levels (0.01 Lux), a low-light imaging system built on multi-detector optical butting was researched. Having decomposed the system’s specifications and verified its low-light imaging capability, we proposed to employ an optical system with a large relative aperture and low distortion and achieve imaging through the field-of-view (FOV) butting facilitated by eight 1080P high-sensitivity scientific complementary metal-oxide semiconductor (SCMOS) detectors. This paper elaborates on the design concept of the mechanical configuration of the imaging system; studies the calculation method of the structural parameters of the reflection prism; provides mathematical expressions for geometric parameters, such as the length and width of the splicing prism; and designs in detail the splicing structure of six reflection prisms for eight-channel beam splitting. Based on the design and computational results, a high-resolution, wide-swath imaging system for an ambient illuminance of 0.01 Lux was developed. Exhibiting a ground sampling distance (GSD) of 0.5 m (at a flight height of 5 km), this low-light imaging system keeps the FOV overlap ratio between adjacent detectors below 3% and boasts an effective image resolution of 4222 × 3782. The results from flight testing revealed that the proposed imaging system is capable of generating wide-swath, high-contrast resolution imagery under airborne and low-light conditions. As such, the way the system is prepared can serve as a reference point for the development of airborne low-light imaging devices. Full article
(This article belongs to the Special Issue Optical Imaging and Measurements)
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14 pages, 4597 KiB  
Article
Simulation Analysis of an Atmospheric Turbulence Wavefront Measurement System
by Gangyu Wang, Laian Qin, Yang Li, Yilun Cheng, Xu Jing, Gongye Chen and Zaihong Hou
Photonics 2024, 11(4), 383; https://doi.org/10.3390/photonics11040383 - 18 Apr 2024
Viewed by 1468
Abstract
In this paper, a turbulent wavefront measurement model based on the Hartmann system structure is proposed. The maximum recognizable mode number of different lens units is discussed, and the influence of different lens array arrangements on the accuracy of turbulent wavefront reconstruction is [...] Read more.
In this paper, a turbulent wavefront measurement model based on the Hartmann system structure is proposed. The maximum recognizable mode number of different lens units is discussed, and the influence of different lens array arrangements on the accuracy of turbulent wavefront reconstruction is analyzed. The results indicate that the increase in the aberration order of the turbulent wavefront has a certain influence on the reconstruction ability of the system. Different lens arrangements and number of lens units will lead to the effective reconstruction of different final mode orders. When using a 5 × 5 lens array arrangement and a hexagonal arrangement of 19 lenses, the maximum order of turbulent wavefront aberrations allowing for effective reconstruction was 25. When the sparse arrangement of 25 lenses or the sparse arrangement of 31 lenses was used, the maximum order allowing for effective reconstruction was 36. If the aberration composition of the turbulent wavefront contained higher-order aberrations, the system could not accurately measure the turbulent wavefront. When the order of the aberrations of the turbulent wavefront was low, the turbulent wavefront could be measured by the lens arrangement with fewer lens units, and the wavefront reconstruction accuracy was close to the measurement results obtained when more lens units were used. Full article
(This article belongs to the Special Issue Optical Imaging and Measurements)
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13 pages, 12542 KiB  
Article
A Deep Learning-Based Preprocessing Method for Single Interferometric Fringe Patterns
by Xueliang Zhu, Di Zhang, Yilei Hao, Bingcai Liu, Hongjun Wang and Ailing Tian
Photonics 2024, 11(3), 226; https://doi.org/10.3390/photonics11030226 - 29 Feb 2024
Viewed by 1208
Abstract
A novel preprocessing method based on a modified U-NET is proposed for single interference fringes. The framework is constructed by introducing spatial attention and channel attention modules to optimize performance. In this process, interferometric fringe maps with an added background intensity, fringe amplitude, [...] Read more.
A novel preprocessing method based on a modified U-NET is proposed for single interference fringes. The framework is constructed by introducing spatial attention and channel attention modules to optimize performance. In this process, interferometric fringe maps with an added background intensity, fringe amplitude, and ambient noise are used as the input to the network, which outputs fringe maps in an ideal state. Simulated and experimental results demonstrated that this technique can preprocess single interference fringes in ~1 microsecond. The quality of the results was further evaluated using the root mean square error, peak signal-to-noise ratio, structural similarity, and equivalent number of views. The proposed method outperformed U-NET, U-NET++, and other conventional algorithms as measured by each of these metrics. In addition, the model produced high-quality normalized fringes by combining objective data with visual effects, significantly improving the accuracy of the phase solutions for single interference fringes. Full article
(This article belongs to the Special Issue Optical Imaging and Measurements)
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16 pages, 18828 KiB  
Article
Dark Light Image-Enhancement Method Based on Multiple Self-Encoding Prior Collaborative Constraints
by Lei Guan, Jiawei Dong, Qianxi Li, Jijiang Huang, Weining Chen and Hao Wang
Photonics 2024, 11(2), 190; https://doi.org/10.3390/photonics11020190 - 19 Feb 2024
Viewed by 1826
Abstract
The purpose of dark image enhancement is to restore dark images to visual images under normal lighting conditions. Due to the ill-posedness of the enhancement process, previous enhancement algorithms often have overexposure, underexposure, noise increases and artifacts when dealing with complex and changeable [...] Read more.
The purpose of dark image enhancement is to restore dark images to visual images under normal lighting conditions. Due to the ill-posedness of the enhancement process, previous enhancement algorithms often have overexposure, underexposure, noise increases and artifacts when dealing with complex and changeable images, and the robustness is poor. This article proposes a new enhancement approach consisting in constructing a dim light enhancement network with more robustness and rich detail features through the collaborative constraint of multiple self-coding priors (CCMP). Specifically, our model consists of two prior modules and an enhancement module. The former learns the feature distribution of the dark light image under normal exposure as an a priori term of the enhancement process through multiple specific autoencoders, implicitly measures the enhancement quality and drives the network to approach the truth value. The latter fits the curve mapping of the enhancement process as a fidelity term to restore global illumination and local details. Through experiments, we concluded that the new method proposed in this article can achieve more excellent quantitative and qualitative results, improve detail contrast, reduce artifacts and noise, and is suitable for dark light enhancement in multiple scenes. Full article
(This article belongs to the Special Issue Optical Imaging and Measurements)
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18 pages, 8493 KiB  
Article
Investigation of the Space-Variance Effect of Imaging Systems with Digital Holography
by Xingyu Yang, Rong Zhao, Huan Chen, Yijun Du, Chen Fan, Gaopeng Zhang and Zixin Zhao
Photonics 2023, 10(12), 1350; https://doi.org/10.3390/photonics10121350 - 7 Dec 2023
Cited by 1 | Viewed by 1122
Abstract
In classical Fourier optics, an optical imaging system is regarded as a linear space-invariant system, which is only an approximation. Especially in digital holography, the space-variance effect has a great impact on the image quality and cannot be ignored. Therefore, it is comprehensively [...] Read more.
In classical Fourier optics, an optical imaging system is regarded as a linear space-invariant system, which is only an approximation. Especially in digital holography, the space-variance effect has a great impact on the image quality and cannot be ignored. Therefore, it is comprehensively investigated in this article. Theoretical analyses indicate that the space-variance effect is caused by linear frequency modulation and ideal low-pass filtering, and it can be divided into three states: the approximate space-invariance state, the high-frequency distortion state, and the boundary-diffraction state. Classical Fourier optics analysis of optical imaging systems only considers the first. Regarding the high-frequency distortion state, the closer the image field is to the edge, the more severe the distortion of high-frequency information is. As for the boundary-diffraction state, in addition to the distortion of high-frequency information in the margin, a prominent boundary-diffraction phenomenon is observed. If the space-variance effect of the imaging lens is ignored, we predict that no space-variance effect in image holography will occur when the hologram is recorded at the back focal plane of the imaging lens. Simulation and experimental results are presented to validate our theoretical prediction. Full article
(This article belongs to the Special Issue Optical Imaging and Measurements)
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13 pages, 3408 KiB  
Article
Analysis of Coaxiality Error Induced by the Cube Corner Retro-Reflector Geometrical and Assembly Errors of an Acquisition, Pointing, and Tracking System
by Daquan Li, Zhaoyong Mao, Lijuan Sun, Haifeng Zhang and Furui Zhang
Photonics 2023, 10(10), 1176; https://doi.org/10.3390/photonics10101176 - 23 Oct 2023
Viewed by 1234
Abstract
Satellite laser communication is a promising technology for the next-generation communication system. Its communication performance is subject to the APT beam-pointing accuracy. One of the most important problems is reducing the coaxiality error before the APT starts working. However, the coaxiality error is [...] Read more.
Satellite laser communication is a promising technology for the next-generation communication system. Its communication performance is subject to the APT beam-pointing accuracy. One of the most important problems is reducing the coaxiality error before the APT starts working. However, the coaxiality error is difficult to correct effectively owing to the lack of empirical guidance based on qualitative analysis. We study the inducement that will generate coaxiality errors. The mathematical model of the influence of the CCR dihedral angle error and planeness error on the spot centroid measurement are built, and an analysis is performed. The model of the beam-pointing error induced by the APT element’s assembly error is built, and the pointing error change rule is explored. Furthermore, the coaxiality performance simulation is performed in the presence of a CCR geometrical error while considering the assembly error. The results show that the coaxiality error has a nonlinear characteristic. The CCR planeness error has a greater influence on coaxiality deviation than that of dihedral angle error under certain conditions. This research is relevant to the design and test work of the APT system. Full article
(This article belongs to the Special Issue Optical Imaging and Measurements)
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9 pages, 5327 KiB  
Communication
Single-Shot Phase-Contrast Imaging with a Single Grating
by Xin Liu, Lang Liu, Jianheng Huang, Yaohu Lei and Ji Li
Photonics 2023, 10(9), 968; https://doi.org/10.3390/photonics10090968 - 24 Aug 2023
Cited by 2 | Viewed by 1663
Abstract
In the field of X-ray phase-contrast imaging, a time-saving approach and preservation of details are crucial factors for obtaining phase-contrast images. In this manuscript, a single grating imaging system is proposed to perform the X-ray phase-contrast imaging. Instead of the time-consuming phase-stepping method, [...] Read more.
In the field of X-ray phase-contrast imaging, a time-saving approach and preservation of details are crucial factors for obtaining phase-contrast images. In this manuscript, a single grating imaging system is proposed to perform the X-ray phase-contrast imaging. Instead of the time-consuming phase-stepping method, this system uses a single-shot algorithm to retrieve the distribution of samples’ attenuation and phase gradient. Unlike the single-shot Fourier transform algorithm, which truncates the high-frequency component of the image and reduces the spatial resolution, our method can retrieve the attenuation and phase information images with the same spatial resolution as the images acquired directly by the X-ray detector used. Furthermore, by using a large-size X-ray detector (29 cm × 23 cm), the imaging system can be configured as either a microscopic instrument or a normal large field-of-view imaging system. Finally, a series of experiments were performed to validate the feasibility of the proposed method. Full article
(This article belongs to the Special Issue Optical Imaging and Measurements)
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15 pages, 7533 KiB  
Article
Storage Duration Prediction for Long-Expired Frozen Meat Exceeding State Reserve Time via Swept-Source Optical Coherence Tomography (SS-OCT) under Low-Frequency Electric Field
by Lu Zhang, Ruoxuan Li, Xiaorong Shen, Linkai He, Jie Huang, Chi Song, Zeyu Fan, Hong Zhao, Kejia Li, Meizhen Xie, Jinfeng Peng, Pingping Jia, Xiaojun Deng and Minli Yang
Photonics 2023, 10(9), 956; https://doi.org/10.3390/photonics10090956 - 22 Aug 2023
Viewed by 1062
Abstract
Storage duration detection for frozen meat, especially meat exceeding the state reserve time several times, has always been a big challenge in food safety inspection. Under long freezing times, the physical and chemical properties of meat change complexly. In this paper, the SS-OCT [...] Read more.
Storage duration detection for frozen meat, especially meat exceeding the state reserve time several times, has always been a big challenge in food safety inspection. Under long freezing times, the physical and chemical properties of meat change complexly. In this paper, the SS-OCT detection method under a low-frequency electric field is firstly (to our knowledge) applied to the predict storage durations of long-expired frozen meat. The average normalized cross-correlation (ANCC) is put forward as a comprehensive parameter to reflect both the electric–kinetic and optical properties of meat’s biological changes. A monotonically increasing inversion rule between ANCC and the storage duration of frozen meat is found after investigating 3840 pork samples, the frozen storage durations of which were from 1 to 13 months. To verify the correctness and accuracy of our method, nine groups of long-expired frozen pork samples were investigated. The maximum relative error for their storage durations is less than 5.71%, which means that our SS-OCT method under a low-frequency electric field is promising in providing a rapid on-site storage duration detection method without any complicated laboratory pretreatments for food safety inspection. Full article
(This article belongs to the Special Issue Optical Imaging and Measurements)
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15 pages, 11326 KiB  
Article
Design of a Large-Format Low-Light Imaging System Based on the RGB Filter Wheel
by Jianwei Peng, Hongtao Yang, Xiaodong Song, Yingjun Ma, Weining Chen and Guangdong Zhang
Photonics 2023, 10(8), 953; https://doi.org/10.3390/photonics10080953 - 21 Aug 2023
Cited by 1 | Viewed by 1174
Abstract
In order to capture true-color information of distant targets under extremely low light, a large-format low-light imaging system is designed based on an RGB filter wheel. By decomposing the system indicators, this study proposes a method for acquiring low-light true-color images using a [...] Read more.
In order to capture true-color information of distant targets under extremely low light, a large-format low-light imaging system is designed based on an RGB filter wheel. By decomposing the system indicators, this study proposes a method for acquiring low-light true-color images using a large-aperture, low-distortion optical lens combined with an RGB filter wheel capable of multi-line sequential exposure. The optical field segmentation is achieved using a four-panel optical reflective prism, and the images from four high-sensitivity SCOMS detectors are stitched together to form a composite image. The working principle of the system is explained, and the low-light imaging capability is thoroughly evaluated. The dimensions and rotation speed of the filter wheel are then calculated in detail, ensuring accurate synchronization of the filter wheel’s speed and exposure time. The calculation method for the parameters of the four-panel reflective prism structure is investigated, mathematical expressions for the geometric parameters of the prism assembly are provided, and a prism assembly suitable for four-way spectral separation is designed. Based on the research and design results, a large-swath-width, low-light true-color imaging system is developed that is suitable for an environmental illuminance of 0.01 lux. The system achieves a ground pixel resolution of 0.5 m (at an altitude of 5 km) and an effective image resolution of 4 K × 4 K, and is capable of accurately reproducing target color information. Laboratory and field flight tests verified that the large-swath-width images obtained by the imaging system are clear, with high contrast and resolution. After image fusion and spectral registration, the color images exhibit full saturation and high fidelity, meeting the requirements of low-light true-color imaging under airborne conditions. The design methodology of this low-light imaging system can serve as a reference for the development of airborne low-light imaging equipment. Full article
(This article belongs to the Special Issue Optical Imaging and Measurements)
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8 pages, 1118 KiB  
Communication
High-Sensitivity Quantum-Enhanced Interferometers
by Juan Yu, Yinhua Wu, Liang Nie and Xiaojie Zuo
Photonics 2023, 10(7), 749; https://doi.org/10.3390/photonics10070749 - 29 Jun 2023
Viewed by 1475
Abstract
High-sensitivity interferometers are one of the basic tools for precision measurement, and their sensitivity is limited by their shot noise limit (SNL), which is determined by vacuum fluctuations of the probe field. The quantum interferometer with novel structures can break the SNL and [...] Read more.
High-sensitivity interferometers are one of the basic tools for precision measurement, and their sensitivity is limited by their shot noise limit (SNL), which is determined by vacuum fluctuations of the probe field. The quantum interferometer with novel structures can break the SNL and measure the weak signals, such as the direct observation of gravity wave signal. Combining classical interferometers and the optical parametric amplifier (OPA) can enhance the signal; meanwhile, the quantum noise is kept at the vacuum level, so that the sensitivity of the nonlinear interferometer beyond the SNL can be achieved. By analyzing in detail the influence of system parameters on the precision of quantum metrology, including the intensity of optical fields for phase sensing, the gain factor of OPA, and the losses inside and outside the interferometers, the application conditions of high-sensitivity nonlinear quantum interferometers are obtained. Quantum interferometer-based OPAs provide the direct references for the practical development of quantum precise measurement. Full article
(This article belongs to the Special Issue Optical Imaging and Measurements)
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16 pages, 9976 KiB  
Article
A Polarized Structured Light Method for the 3D Measurement of High-Reflective Surfaces
by Jixin Liang, Yuping Ye, Feifei Gu, Jiankai Zhang, Juan Zhao and Zhan Song
Photonics 2023, 10(6), 695; https://doi.org/10.3390/photonics10060695 - 19 Jun 2023
Cited by 6 | Viewed by 2878
Abstract
The reflection phenomenon exhibited by highly reflective surfaces considerably affects the quality of captured images, thereby rendering the task of structured light (SL) 3D reconstruction. In this paper, a polarized SL method is proposed to address the reconstruction issues on high-reflectance surfaces. The [...] Read more.
The reflection phenomenon exhibited by highly reflective surfaces considerably affects the quality of captured images, thereby rendering the task of structured light (SL) 3D reconstruction. In this paper, a polarized SL method is proposed to address the reconstruction issues on high-reflectance surfaces. The SL system we build in this paper involves a four-channel polarizing camera and a digital light processing (DLP) projector equipped with a polarizer in the lens. The built system enables the simultaneous acquisition of four groups of fringe images, each with different brightness differences. Then, a binary time-multiplexing SL method is adopted to obtain four distinct point clouds. Additionally, a fusion algorithm is proposed to merge the four point clouds into a single, precise, and complete point cloud. Several experiments have been conducted to demonstrate that the proposed method is capable of achieving excellent reconstruction outcomes on highly reflective surfaces. Full article
(This article belongs to the Special Issue Optical Imaging and Measurements)
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