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Polarimetric Synthetic Aperture Radar (PolSAR) data is inherently characterized by speckle noise, which significantly deteriorates certain aspects of the quality of the PolSAR data processing, including the polarimetric decomposition and target interpretation. With the rapid increase in PolSAR resolution, SAR images in complex natural and artificial scenes exhibit non-homogeneous characteristics, which creates an urgent demand for high-resolution PolSAR filters. To address these issues, a new adaptive PolSAR filter based on joint similarity measure criterion (JSMC) is proposed in this paper. Firstly, a scale-adaptive filtering window is established in order to preserve the texture structure based on a multi-directional ratio edge detector. Secondly, the JSMC is proposed in order to accurately select homogeneous pixels; it describes pixel similarity based on both space distance and polarimetric distance. Thirdly, the homogeneous pixels are filtered based on statistical averaging. Finally, the airborne and spaceborne real data experiment results validate the effectiveness of our proposed method. Compared with other filters, the filter proposed in this paper provides a better outcome for PolSAR data in speckle suppression, edge texture, and the preservation of polarimetric properties. [ABSTRACT FROM AUTHOR]

Models of the resolved Event Horizon Telescope (EHT) sources Sgr A* and M87* are constrained by observations at multiple wavelengths, resolutions, polarizations, and time cadences. In this paper, we compare unresolved circular polarization (CP) measurements to a library of models, where each model is characterized by a distribution of CP over time. In the library, we vary the spin of the black hole, the magnetic field strength at the horizon (i.e., both SANE and magnetically arrested disk or MAD models), the observer inclination, a parameter for the maximum ion–electron temperature ratio assuming a thermal plasma, and the direction of the magnetic field dipole moment. We find that Atacama Large Millimeter/submillimeter Array (ALMA) observations of Sgr A* are inconsistent with all edge-on (i = 90°) models. Restricting attention to the MAD models favored by earlier EHT studies of Sgr A*, we find that only models with magnetic dipole moment pointing away from the observer are consistent with ALMA data. We also note that in 26 of the 27 passing MAD models, the accretion flow rotates clockwise on the sky. We provide a table of the means and standard deviations of the CP distributions for all model parameters, along with their trends. [ABSTRACT FROM AUTHOR]

The potential of a coherent microwave radar for infrastructure health monitoring has been investigated over the past decade. Microwave radar measuring based on interferometry processing is a non-invasive technique that can measure the line-of-sight (LOS) displacements of large infrastructure with sub-millimeter precision and provide the corresponding frequency spectrum. It has the capability to estimate infrastructure vibration simultaneously and remotely with high accuracy and repeatability, which serves the long-term serviceability of bridge structures within the context of the long-term sustainability of civil engineering infrastructure management. In this paper, we present three types of microwave radar systems employed to monitor the displacement of bridges in Japan and Italy. A technique that fuses polarimetric analysis and the interferometry technique for bridge monitoring is proposed. Monitoring results achieved with full polarimetric real aperture radar (RAR), step-frequency continuous-wave (SFCW)-based linear synthetic aperture, and multi-input multi-output (MIMO) array sensors are also presented. The results reveal bridge dynamic responses under different loading conditions, including wind, vehicular traffic, and passing trains, and show that microwave sensor interferometry can be utilized to monitor the dynamics of bridge structures with unprecedented spatial and temporal resolution. This paper demonstrates that microwave sensor interferometry with efficient, cost-effective, and non-destructive properties is a serious contender to employment as a sustainable infrastructure monitoring technology serving the sustainable development agenda. [ABSTRACT FROM AUTHOR]

Nearshore ship detection has significant applications in both the military and civilian domains. Compared to synthetic aperture radar (SAR), polarimetric synthetic aperture radar (PolSAR) provides richer information for analyzing the scattering mechanisms of ships and enables better detection of ship targets. However, ships in nearshore areas tend to be highly concentrated, and ship detection is often affected by adjacent strong scattering, resulting in false alarms or missed detections. While the GP-PNF detector performs well in PolSAR ship detection, it cannot obtain satisfactory results in these scenarios, and it also struggles in the presence of azimuthal ambiguity or strong clutter interference. To address these challenges, we propose a nearshore ship detection method named ECD-PNF by integrating superpixel-level GP-PNF and refined polarimetric decomposition. Firstly, polarimetric superpixel segmentation and sea–land segmentation are performed to reduce the influence of land on ship detection. To estimate the sea clutter more accurately, an automatic censoring (AC) mechanism combined with superpixels is used to select the sea clutter superpixels. By utilizing refined eight-component polarimetric decomposition to improve the scattering vector, the physical interpretability of the detector is enhanced. Additionally, the expression of polarimetric coherence is improved to enhance the target clutter ratio (TCR). Finally, this paper combines the third eigenvalue of eigenvalue–eigenvector decomposition to reduce the impact of azimuthal ambiguity. Three spaceborne PolSAR datasets from Radarsat-2 and GF-3 are adopted in the experiments for comparison. The proposed ECD-PNF method achieves the highest figure of merit (FoM) value of 0.980, 1.000, and 1.000 for three datasets, validating the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Polarimetric imaging technology plays a crucial role in de-scattering, particularly in the field of underwater image restoration. Circularly polarized light (or the underlying circular polarization memory effect) has been proven to better preserve the polarization characteristics of detected light. Utilizing circularly polarized light as illumination can further enhance the effectiveness of polarization de-scattering techniques. After rethinking the advantages of circularly polarized light, this paper proposes a new method for underwater polarimetric imaging restoration that leverages the pre-processing of polarized sub-images and the correlation of polarization characteristics (i.e., the angle of polarization and degree of polarization). Additionally, to address the challenge of selecting scattering light parameters due to uneven light fields in target scenes, an intensity adjustment factor search algorithm is designed. This algorithm eliminates the need for the manual selection of scattering light parameters, effectively solving the problem of uneven illumination in restoration results. A series of experiments demonstrate that, compared to traditional algorithms, the proposed method offers superior detail restoration and higher robustness. [ABSTRACT FROM AUTHOR]

The Imaging X-ray Polarimetry Explorer (IXPE) has confirmed that X-ray polarimetry is a valuable tool in astronomy, providing critical insights into the emission processes and the geometry of compact objects. IXPE was designed to be sensitive in the 2–8 keV energy range for three primary reasons: (1) celestial X-ray sources are bright within this range, (2) the optics are effective, and (3) most sources across various classes were expected to exhibit some level of polarization. Indeed, IXPE is a great success, and its discoveries are necessitating the revision of many theoretical models for numerous sources. However, one of IXPE's main limitations is its relatively narrow energy band, coupled with rapidly declining efficiency. In this paper, we will demonstrate the benefits of devising a mission focused on a broader energy band (0.1–79 keV). This approach leverages current technologies that align well with theoretical expectations and builds on the successes of IXPE. [ABSTRACT FROM AUTHOR]

Drs. Weisskopf and Ramsey were the original Principal and Deputy Principal Investigators of the Imaging X-ray Polarimetry Explorer (IXPE). They outline the path to the development of IXPE and discuss the technical and programmatic history that led up to the mission, a partnership between the Italian Space Agency and NASA, and the first fully dedicated to imaging X-ray polarimetry in the 2–8 keV band. An admittedly biased, as seen through the eyes of the original and Deputy Principal Investigators, technical overview of the development of the historical and critical scientific instrumentation is provided. The outstanding, and often paradigm-shifting results are presented in the papers following this one. [ABSTRACT FROM AUTHOR]

In recent years, handheld and portable terahertz instruments have been in rapid development for various applications ranging from non-destructive testing to biomedical imaging and sensing. For instance, we have deployed our Portable Handheld Spectral Reflection (PHASR) Scanners for in vivo full-spectroscopic imaging of skin burns in large animal models in operating room settings. In this paper, we debut the polarimetric version of the PHASR Scanner, and describe a generalized calibration technique to map the spatial and spectral dependence of the Jones matrix of an imaging scanner across its field of view. Our design is based on placement of two orthogonal photoconductive antenna (PCA) detectors separated by a polarizing beam splitter in the PHASR Scanner housing. We show that as few as three independent measurements of a well-characterized polarimetric calibration target are sufficient to determine the polarization state of the incident beam at the sample location, as well as to extract the Jones propagation matrix from the sample location to the detectors. We have tested the accuracy of our scanner by validating polarimetric measurements obtained from a birefringent crystal rotated to various angles, as compared to the theoretically predicted response of the sample. This new version of our PHASR scanner can be used for high-speed imaging and investigation of heterogeneity of polarization-sensitive samples in the field. [ABSTRACT FROM AUTHOR]

This document is a guest editorial from the Journal of Biomedical Optics, introducing a special section on polarimetry in biomedical optics. The special section is split into two issues, covering topics such as mathematical and statistical characterization of polarimetric signals, development of novel instrumentation, integration of AI and machine learning in analyzing polarization-sensitive data, and diverse medical applications. The editorial highlights several research papers that demonstrate the potential of polarimetric microscopy enhancements in various biomedical applications. It also mentions upcoming conferences and resources related to polarized light and optical momentum for biomedical applications. The authors acknowledge funding support and provide brief biographies of the guest editors. [Extracted from the article]

During the process of the airborne synthetic aperture radar (SAR) system platform in space, platform attitude deflection is inevitable. However, large attitude deflection angles are unacceptable for polarimetric calibration using point targets, especially the dihedral, which is very sensitive to the pointing angle of the radar. To mitigate the impact of attitude angles on calibration accuracy, attitude compensation of the corner reflector is necessary during the calibration process. The conventional approach to attitude compensation typically maps the three-dimensional attitude angle information to the one-dimensional polarimetric orientation angle (POA) information. However, the reduction of dimension inevitably results in information loss, leading to errors that affect calibration performance when the attitude angle is large. In order to ensure the accuracy of point target calibration, this paper proposes a novel point target compensation method based on the reflection theory of electromagnetic waves. This method is based on three-dimensional attitude angle information and has higher reliability than the POA method. Finally, this paper calculates the distance between the scattering matrices obtained after compensation based on the proposed method and the POA method to obtain the difference in the performance of the two methods. Through a simulation, this paper finds that when the attitude angle is small, the results of the two schemes are approximately the same, but as the attitude angle increases, the error between the two gradually increases. This suggests that the proposed method has greater advantages in the case of attitude deflection. Furthermore, the proposed method does not require additional information supplementation compared with the equivalent POA method, making it highly practical. [ABSTRACT FROM AUTHOR]

In this paper, Mueller polarimetry is applied to study the three groups of common barley leaf samples (Hordeum vulgare) in the visible spectrum (λ = 632.8 nm): Chlorina mutant, Chlorina etiolated mutant and Cesaer varieties. These samples differed in internal leaf structure from genetic mutation or by illumination during growth. Our main concern is to discriminate and characterize these three groups of leaf samples by depolarization metrics: degree of polarization (DoP), average degree of polarization (Average DoP), depolarization index (DI(M)), and Q(M) and R(M) metrics. The results obtained show that all depolarization metrics are sensitive to the sample's polarization properties. The most effective observable is the Q(M) metric in both forward and backward scattering. The DoP metric showed presence of depolarization anisotropy, which is significantly different for forward and backward scattering for all three groups of samples. Dichroism is observed for both forward and backward scattering, with lower dichroism in forward scattering. [ABSTRACT FROM AUTHOR]

Accurate precipitation characterization relies on the estimation of raindrop size distribution (RDSD) from observations. While various techniques using centimeter-wavelength radars have been proposed for RDSD retrieval, the potential of millimeter-wavelength polarimetric radars, offering enhanced spatial and temporal resolution while capturing light to moderate rain, remains unexplored. This study focuses on retrieving the mass-weighted mean diameter Dm using a dual-frequency cloud radar. Since the differential reflectivity Zdr is ineffective for Dm retrieval at 94 GHz, and simulations demonstrate a strong dependence of the differential backscatter phase δco on Dm, the estimation of δco takes precedence in this paper. Notably, δco remains unaffected by attenuation and polarimetric calibration. Addressing the initial requirement of disentangling backscattering and propagation effects at millimeter wavelength, an automatic algorithm is proposed to detect Rayleigh plateaus in the spectral domain. Subsequently, a methodology for estimating δco and its associated error is presented. Leveraging simulation results, confidence intervals for Dm that align with δco confidence intervals are retrieved. The assessment of Dm and its confidence interval at 35 and 94 GHz is conducted employing disdrometer-derived Dm. The results demonstrate a comprehensive concordance within a margin of 0.2 mm, underscoring the cloud radar's efficacy in delineating nuanced variations in the raindrop mean diameter versus altitude. The validation process encounters difficulties for Dm below 1 mm, as the disdrometer-derived Dm may exhibit an overestimation, while the cloud-radar-derived Dm may exhibit an underestimation. The combination of 35 and 94 GHz serves to diminish the confidence interval associated with the retrieved Dm. [ABSTRACT FROM AUTHOR]

An observatory dedicated to X-ray polarimetry has been operational since 9 December 2021. The Imaging X-ray Polarimetry Explorer (IXPE), a collaboration between NASA and ASI, features three X-ray telescopes equipped with detectors sensitive to linear polarization set to 120°. This marks the first instance of a three-telescope SMEX mission. Upon reaching orbit, an extending boom was deployed, extending the optics and detector to a focal length of 4 m. IXPE targets each celestial source through dithering observations. This method is essential for supporting on-ground calibrations by averaging the detector's response across a section of its sensitive plane. The spacecraft supplies power, enables attitude determination for subsequent on-ground attitude reconstruction, and issues control commands. After two years of observation, IXPE has detected significant linear polarization from nearly all classes of celestial sources emitting X-rays. This paper outlines the IXPE mission's achievements after two years of operation in orbit. In addition, we report developments for future high-throughput X-ray optics that will have much smaller dead-times by using a new generation of Applied Specific Integrated Circuits (ASIC), and may provide 3D reconstruction of photo-electron tracks. [ABSTRACT FROM AUTHOR]

X-ray polarimetry has been suggested as a prominent tool for investigating the geometrical and physical properties of the emissions from active galactic nuclei (AGN). The successful launch of the Imaging X-ray Polarimetry Explorer (IXPE) on 9 December 2021 has expanded the previously restricted scope of polarimetry into the X-ray domain, enabling X-ray polarimetric studies of AGN. Over a span of two years, IXPE has observed various AGN populations, including blazars and radio-quiet AGN. In this paper, we summarize the remarkable discoveries achieved thanks to the opening of the new window of X-ray polarimetry of AGN through IXPE observations. We will delve into two primary areas of interest: first, the magnetic field geometry and particle acceleration mechanisms in the jets of radio-loud AGN, such as blazars, where the relativistic acceleration process dominates the spectral energy distribution; and second, the geometry of the hot corona in radio-quiet AGN. Thus far, the IXPE results from blazars favor the energy-stratified shock acceleration model, and they provide evidence of helical magnetic fields inside the jet. Concerning the corona geometry, the IXPE results are consistent with a disk-originated slab-like or wedge-like shape, as could result from Comptonization around the accretion disk. [ABSTRACT FROM AUTHOR]

Synthetic aperture radar (SAR) interferometry (InSAR) is a well-established remote sensing technique capable of providing accurate topographic information of an area. Multiple scattering occurring at different heights within a single resolution cell, however, cannot be resolved using a single baseline and results in a degradation of the height accuracy. Techniques such as polarimetric InSAR and SAR tomography tackle this problem using additional measurements to obtain the three-dimensional structure of the volume. However, polarimetry-based methods assume orthogonal and deterministic scattering mechanisms and tomography requires numerous observations. This paper presents a novel approach to retrieving the three-dimensional structure of semitransparent media using wide fractional bandwidth InSAR. The frequency dependency of the InSAR coherence is exploited to retrieve the three-dimensional structure, assuming that signals with wide fractional bandwidth are available, as is the case for unmanned aerial vehicles (UAVs) or drones. As the extinction is, in general, frequency-dependent, a third observation may improve the inversion results. Simulations of different scenarios show that the frequency profile can be inverted to obtain information about the three-dimensional scattering structure. The acquired data also enable retrieval of the phase center depth as a function of frequency. This method allows for frequent and accurate monitoring of semitransparent targets, e.g., forests and vegetation, using a swarm of two or three drones. [ABSTRACT FROM AUTHOR]

Chiral thin films showing electronic and plasmonic circular dichroism (CD) are intensively explored for optoelectronic applications. The most studied chiral organic films are the composites exhibiting a helical geometry, which often causes entanglement of circular optical properties with unwanted linear optical effects (linearly polarized absorption or refraction). This entanglement limits tunability and often translates to a complex optical response. This paper describes chiral films based on dark conglomerate, sponge-like, liquid crystal films, which go beyond the usual helical type geometry, waiving the problem of linear contributions to chiroptical electronic and plasmonic properties. First, we show that purely organic films exhibit high electronic CD and circular birefringence, as studied in detail using Mueller matrix polarimetry. Analogous linear properties are two orders of magnitude lower, highlighting the benefits of using the bi-isotropic dark conglomerate liquid crystal for chiroptical purposes. Next, we show that the liquid crystal can act as a template to guide the assembly of chemically compatible gold nanoparticles into 3D spiral-like assemblies. The Mueller matrix polarimetry measurements confirm that these composites exhibit both electronic and plasmonic circular dichroisms, while nanoparticle presence is not compromising the beneficial optical properties of the matrix. [ABSTRACT FROM AUTHOR]

The paper presents theoretical analyses and experimental investigations of broadband differential interference in planar gradient waveguides made via K+-Na+ ion exchange in BK-7 glass. This technology, due to its large polarimetric dispersion, is especially useful for applications in differential interferometry. We discuss the influence of technological parameters on the operation characteristics of the structure in terms of sensor applications. The refractive index variation in the measured external surroundings affects the modal properties of TE and TM modes and the spectral distribution at the output of the differential interferometer. The optical system described in this work has been designed specifically for use in biological systems where variations in the index of refraction need to be measured. [ABSTRACT FROM AUTHOR]

From a passive satellite remote sensing point of view, the richest set of information on aerosol properties can be obtained from instruments that measure both intensity and polarization of backscattered sunlight at multiple wavelengths and multiple viewing angles for one ground pixel. However, it is challenging to exploit this information at a global scale because complex algorithms are needed with many fit parameters (aerosol and land/ocean reflection), based on online radiative transfer models. So far, two such algorithms have demonstrated this capability at a global scale: the Generalized Retrieval of Atmosphere and Surface Properties (GRASP) algorithm and the Remote sensing of Trace gas and Aerosol Products (RemoTAP) algorithm. In this paper, we present a detailed comparison of the most recent versions of RemoTAP and GRASP. We evaluate both algorithms for synthetic observations, for real PARASOL (Polarization and Anisotropy of Reflectances for Atmospheric Science coupled with Observations from a Lidar) observations against AERONET (Aerosol Robotic Network) for common pixels, and for global PARASOL retrievals for the year 2008. For the aerosol optical depth (AOD) over land, both algorithms show a root mean square error (RMSE) of 0.10 (at 550 nm). For single scattering albedo (SSA), both algorithms show a good performance in terms of RMSE (0.04), but RemoTAP has a smaller bias (0.002) compared to GRASP (0.021). For the Ångström exponent (AE), GRASP has a smaller RMSE (0.367) than RemoTAP (0.387), mainly caused by a small overestimate of AE at low values (large particles). Over ocean both algorithms perform very well. For AOD, RemoTAP has an RMSE of 0.057 and GRASP an even smaller RMSE of 0.047. For AE, the RMSEs of RemoTAP and GRASP are 0.285 and 0.224, respectively. Based on the AERONET comparison, we conclude that both algorithms show very similar overall performance, where both algorithms have stronger and weaker points. For the global data products, we find a root mean square difference (RMSD) between RemoTAP and GRASP AOD of 0.12 and 0.038 over land and ocean, respectively. The largest differences occur over the biomass burning region in equatorial Africa. The global mean values are virtually unbiased with respect to each other. For AE the RMSD between RemoTAP and GRASP is 0.33 over land and 0.23 over ocean. For SSA, we find much better agreement over land (bias = - 0.01, RMSD = 0.043 for retrievals with AOD > 0.2) than over ocean (bias = 0.053, RMSD = 0.074). As expected, the differences increase towards low AOD, over both land and ocean. We also compared the GRASP and RemoTAP AOD and AE products against MODIS. For AOD over land, the agreement of either GRASP or RemoTAP with MODIS is worse than the agreement between the two PARASOL algorithms themselves. Over ocean, the agreement is very similar among the three products for AOD. For AE, the agreement between GRASP and RemoTAP is much better than the agreement of both products with MODIS. The agreement of the latest product versions with each other and with AERONET improved significantly compared to the previous version of the global products of GRASP and RemoTAP. The results demonstrate that the dedicated effort in algorithm development for multi-angle polarimetric (MAP) aerosol retrievals still leads to substantial improvement of the resulting aerosol products, and this is still an ongoing process. [ABSTRACT FROM AUTHOR]

The launch of the IXPE telescope in late 2021 finally made polarization measurements in the 2– 8 keV band a reality, more than 40 years after the pioneering observations of the OSO-8 satellite. In the first two years of operations, IXPE targeted more than 60 sources, including four magnetars, neutron stars with magnetic fields in the petaGauss range. In this paper we summarize the IXPE main findings and discuss their implications for the physics of ultra-magnetized neutron stars. Polarimetric observations confirmed theoretical predictions, according to which X-ray radiation from magnetar sources is highly polarized, up to ≈ 80 % , the highest value detected so far. This provides an independent confirmation that magnetars are indeed endowed with a super-strong magnetic field and that the twisted magnetosphere scenario is the most likely explanation for their soft X-ray emission. Polarization measurements allowed us to probe the physical conditions of the star's outermost layers, showing that the cooler surface regions are in a condensed state, with no atmosphere on top. Although no smoking-gun of vacuum QED effects was found, the phase-dependent behavior of the polarization angle strongly hints that vacuum birefringence is indeed at work in magnetar magnetospheres. [ABSTRACT FROM AUTHOR]

The bionic polarimetric imaging navigation sensor (BPINS) is a navigation sensor that provides absolute heading, and it is of practical engineering significance to model the measurement error of BPINS. The existing BPINSs are still modeled using photodiode-based measurements rather than imaging measurements and are not modeled systematically enough. This paper proposes a measurement performance analysis method of BPINS that takes into account the geometric and polarization errors of the optical system. Firstly, the key error factors affecting the overall measurement performance of BPINS are investigated, and the Stokes vector-based measurement error model of BPINS is introduced. Secondly, based on its measurement error model, the effect of the error source on the measurement performance of BPINS is quantitatively analyzed using Rayleigh scattering to generate scattered sunlight as a known incident light source. The numerical results show that in angle of E-vector (AoE) measurement, the coordinate deviation of the principal point has a greater impact, followed by grayscale response inconsistency of CMOS and integration angle error of micro-polarization array, and finally lens attenuation; in degree of linear polarization (DoLP) measurement, the grayscale response inconsistency of CMOS has a more significant impact. This finding can accurately guide the subsequent calibration of BPINS, and the quantitative results provide an important theoretical reference for its optimal design. [ABSTRACT FROM AUTHOR]

The detection of ships on the open sea is an important issue for both military and civilian fields. As an active microwave imaging sensor, synthetic aperture radar (SAR) is a useful device in marine supervision. To extract small and weak ships precisely in the marine areas, polarimetric synthetic aperture radar (PolSAR) data have been used more and more widely. We propose a new PolSAR ship detection method which is based on a keypoint detector, referred to as a PolSAR-SIFT keypoint detector, and a patch variation indicator in this paper. The PolSAR-SIFT keypoint detector proposed in this paper is inspired by the SAR-SIFT keypoint detector. We improve the gradient definition in the SAR-SIFT keypoint detector to adapt to the properties of PolSAR data by defining a new gradient based on the distance measurement of polarimetric covariance matrices. We present the application of PolSAR-SIFT keypoint detector to the detection of ship targets in PolSAR data by combining the PolSAR-SIFT keypoint detector with the patch variation indicator we proposed before. The keypoints extracted by the PolSAR-SIFT keypoint detector are usually located in regions with corner structures, which are likely to be ship regions. Then, the patch variation indicator is used to characterize the context information of the extracted keypoints, and the keypoints located on the sea area are filtered out by setting a constant false alarm rate threshold for the patch variation indicator. Finally, a patch centered on each filtered keypoint is selected. Then, the detection statistics in the patch are calculated. The detection statistics are binarized according to the local threshold set by the detection statistic value of the keypoint to complete the ship detection. Experiments on three data sets obtained from the RADARSAT-2 and AIRSAR quad-polarization data demonstrate that the proposed detector is effective for ship detection. [ABSTRACT FROM AUTHOR]

This document is an editorial from the journal Photonics, titled "Advanced Polarimetry and Polarimetric Imaging." It discusses the importance of polarization in light waves and its applications in various fields such as object detection, biomedical imaging, and remote sensing. The editorial highlights a special issue of the journal that focuses on advanced polarimeters and polarimetric imaging. It mentions the fourteen papers that were accepted for publication in this special issue, covering topics such as polarization measurements, metasurfaces and metamaterials, polarimetric imaging in scattering environments, and polarization lidar. The editorial concludes by emphasizing the progress and potential of polarization technology in various applications. [Extracted from the article]

Context. The Metis coronagraph is one of the remote sensing instruments of the ESA-NASA Solar Orbiter mission. The goal for the instrument is to enable the study of the solar atmosphere and solar wind by simultaneously acquiring images of the solar corona at two different wavelengths: visible light (VL), within a band ranging from 580 nm to 640 nm, and ultraviolet light, in the HI Lyα 121:6 ± 10 nm. The visible-light channel of the coronagraph includes a polarimeter with electro-optically modulating liquid crystal variable retarders to measure the linearly polarized brightness of the K-corona and derive the electron density. Aims. In this paper, we present the first in-flight validation results of the Metis polarimetric channel together with a comparison to the on-ground calibrations. This paper seeks to validate the first use of an electro-optical device, the liquid crystal-based polarimeter, in deep space and within a hard radiation environment. Methods. We used the orientation of the K-corona's linear polarization vector during the roll maneuvers of the Space Orbiter spacecraft for the in-flight calibration. Results. The Metis coronagraph on board the Solar Orbiter shows good agreement with the on-ground measurements. The in-flight validation confirms the expected performance of the visible-light channel polarimeter. Furthermore, a comparison between the first polarized brightness value obtained by Metis and the polarized brightness values obtained by the space-based coronagraph LASCO and the ground-based coronagraph K-Cor shows the consistency of the Metis calibrated results. [ABSTRACT FROM AUTHOR]

Faraday tomography through broad-band polarimetry can provide crucial information on magnetized astronomical objects, such as quasars, galaxies, or galaxy clusters. However, the limited wavelength coverage of the instruments requires that we solve an ill-posed inverse problem when we want to obtain the Faraday dispersion function (FDF), a tomographic distribution of the magneto-ionic media along the line of sight. This paper explores the use of wavelet transforms and the sparsity of the transformed FDFs in the form of wavelet shrinkage (WS) for finding better solutions to the inverse problem. We recently proposed the Constraining and Restoring iterative Algorithm for Faraday Tomography (CRAFT; Cooray et al. 2021, MNRAS, 500, 5129), a new flexible algorithm that showed significant improvements over the popular methods such as Rotation Measure Synthesis. In this work, we introduce CRAFT + WS, a new version of CRAFT incorporating the ideas of wavelets and sparsity. CRAFT + WS exhibits significant improvements over the original CRAFT when tested for a complex FDF of a realistic Galactic model. Reconstructions of FDFs demonstrate super-resolution in Faraday depth, uncovering previously unseen Faraday complexities in observations. The proposed approach will be necessary for effective cosmic magnetism studies using the Square Kilometre Array and its precursors. The code is made publicly available at 〈 https://github.com/suchethac/craft 〉. [ABSTRACT FROM AUTHOR]

Having knowledge of the drop size distribution (DSD) is of particular interest to researchers as it is widely applied to quantitative precipitation estimation (QPE) methods. Polarimetric radar measurements have previously been utilized to derive DSD curve characteristics frequently modeled as a gamma distribution. Likewise, approaches using dual frequency measurements have shown positive results. Both cases have relied on the need to constrain the relationship between the DSD parameters based on prior knowledge or assumptions of the collected data. This paper presents a methodology for retrieving the DSD parameters using the dual frequency and polarimetric nature of measurements from a unique data set taken at co-located S-band and C-band dual polarization radars. Using the reflectivity and differential phase measurements from each radar, an optimization routine employing particle swarm optimization (PSO) and T-Matrix computation of radar parameters is able to accurately retrieve the gamma distribution parameters without the constraints required in previous methods. Retrieved results are compared to known truth data collected using a network of OTT PARSIVEL disdrometers in Taiwan in order to assess the success of this procedure. [ABSTRACT FROM AUTHOR]

This paper presents a study on the temperature dependent photo-elastic coefficient in single-crystal silicon with (100) and (110) orientations at a wavelength of 1550 nm. The measurement of the photo-elastic coefficient was performed using a polarimetric scheme across a wide temperature range from 5 to 300 K. The experimental setup employed high-sensitivity techniques and incorporated automatic beam path correction, ensuring precise and accurate determination of the coefficient's values. The results show excellent agreement with previous measurements at room temperature, specifically yielding a value of d n / d σ = - 2.463 × 10 - 11 1/Pa for the (100) orientation. Interestingly, there is a significant difference in photo-elasticity between the different crystal orientations of approximately 50 % . The photo-elastic coefficient's absolute value increases by approximately 40% with decreasing temperature down to 5 K. These findings provide valuable insights into the photo-elastic properties of silicon and its behavior under varying mechanical stress, particularly relevant for optomechanical precision experiments like cryogenic gravitational wave detectors and microscale optomechanical quantum sensors. [ABSTRACT FROM AUTHOR]

The spectrometer of the Munich Aerosol Cloud Scanner (specMACS) is a high spatial resolution hyperspectral and polarized imaging system. It is operated in a nadir-looking perspective aboard the German High Altitude and LOng range research aircraft (HALO) and is mainly used for the remote sensing of clouds. In 2019, its two hyperspectral line-cameras which are sensitive to the wavelength range between 400 and 2500 nm were complemented by two 2D RGB polarization resolving cameras. The polarization resolving cameras have a large field of view and allow for multi-angle polarimetric imaging with high angular and spatial resolution. This paper introduces the polarization resolving cameras and provides a full characterization and calibration of them. We performed a geometric calibration and georeferencing of the two cameras. In addition, a radiometric calibration using laboratory calibration measurements was carried out. The radiometric calibration includes the characterization of dark signal, linearity, and noise as well as the measurement of the spectral response functions, a polarization calibration, vignetting correction, and absolute radiometric calibration. With the calibration, georeferenced absolute calibrated Stokes vectors rotated into the scattering plane can be computed from raw data. We validated the calibration results by comparing observations of the sunglint, which is a known target, with radiative transfer simulations of the sunglint. [ABSTRACT FROM AUTHOR]

As a typical Correlated Electron Material, vanadium dioxide was discovered to demonstrate metalinsulator transition at a relatively low temperature. The transition occurs from an insulating monoclinic phase to a metallic tetragonal phase (rutile structure) upon heating with the rearrangement of vanadium ions along an axis of the monoclinic lattice. The phase change corresponds to a colossal resistivity drop by over four orders of magnitude as well as other dramatic property changes, which can be reversible via a natural cooling process. Therefore, vanadium dioxide has attracted extensive attention for its applications in highly sensitive smart devices that can abruptly respond to diverse external stimuli. In recent years, rapid advancement in the fabrication and property modulation of vanadium dioxide has greatly facilitated its applications in many aspects, such as thermal sensing, thermochromics, electronics, and multiple-response mechanics. The optical properties of thin vanadium dioxide films VO2 were researched using the modulation polarimetry technique. For VO2 thin film deposition the two-step method was used. VO2 films were grown on quartz glass substrates by magnetron sputtering of the VO2 target. The films had different modifications of composition, structure, morphology, and optical properties due to the manufacturing technology. The angular dependence of the reflection coefficients of electromagnetic radiation of s- and p- polarizations and their difference for different wavelengths was measured in the paper. The polarization characteristics were simulated by a matrix transformation of the Fresnel formulas. The values of the refractive and absorption indices of the films were obtained from the condition of the best agreement between the experiment and mathematical simulation. Atomic force microscope and X-ray diffraction analysis were used as standard analytical methods. [ABSTRACT FROM AUTHOR]

This paper focuses on the experimental characterization of the polarization behavior of light backscattered through fog. A polarimetric orthogonal state contrast imager and an active, purely polarized white illuminator system are used to evaluate both linear and circular polarization signals. The experiments are carried out in a macro-scale fog chamber under controlled artificial fog conditions. We explore the effect of backscattering in each imaging channel, and the persistence of both polarization signals as a function of meteorological visibility. We confirm the presence of the polarization memory effect with circularly polarized light, and, as a consequence, the maintenance of helicity in backscattering. Moreover, the circular cross-polarized channel is found to be the imaging channel less affected by fog backscattering. These results are useful and should be taken into account when considering active polarimetric imaging techniques for outdoor applications under foggy conditions. [ABSTRACT FROM AUTHOR]

Relativistic jets from supermassive black holes or stellar mass black holes are among the most powerful astrophysical phenomena. Magnetic field plays an important role in the jet launching and propagation, as well as particle acceleration and radiation. Polarimetry is the only way to observe the magnetic field evolution. The recent launch of the Imaging X-ray Polarimetry Explorer (IXPE) has opened up the X-ray polarization window, which has revealed very interesting phenomena for relativistic jets. However, the field of MeV gamma-ray polarimetry remains largely unexplored. This paper aims to summarize key scientific potentials for MeV polarimetry for blazars and gamma-ray bursts (GRBs) from recent theoretical modeling. These predictions, which are closely related to the cosmic ray acceleration, neutrino production, radiation mechanism, and the jet evolution, can be examined by future MeV polarimeters, such as the Compton Spectrometer and Imager (COSI), the LargE Area burst Polarimeter (LEAP), and the All-sky Medium-Energy Gamma-ray Observatory eXplorer (AMEGO-X). [ABSTRACT FROM AUTHOR]

The latest established generation of weather radars provides polarimetric measurements of a wide variety of meteorological and nonmeteorological targets. While the classification of different precipitation types based on polarimetric data has been studied extensively, nonmeteorological targets have garnered relatively less attention beyond an effort to detect them for removal from meteorological products. In this paper we present a supervised learning classification system developed in the Finnish Meteorological Institute (FMI) that uses Bayesian inference with empirical probability density distributions to assign individual range gate samples into 7 meteorological and 12 nonmeteorological classes, belonging to five top-level categories of hydrometeors, terrain, zoogenic, anthropogenic, and immaterial. We demonstrate how the accuracy of the class probability estimates provided by a basic naive Bayes classifier can be further improved by introducing synthetic channels created through limited neighborhood filtering, by properly managing partial moment nonresponse, and by considering spatial correlation of class membership of adjacent range gates. The choice of Bayesian classification provides well-substantiated quality estimates for all meteorological products, a feature that is being increasingly requested by users of weather radar products. The availability of comprehensive, fine-grained classification of nonmeteorological targets also enables a large array of emerging applications, utilizing nonprecipitation echo types and demonstrating the need to move from a single, universal quality metric of radar observations to one that depends on the application, the measured target type, and the specificity of the customers' requirements. Significance Statement: In addition to meteorological echoes, weather radars observe a wide variety of nonmeteorological phenomena including birds, insects, and human-made objects like ships and aircraft. Conventionally, these data have been rejected as undesirable disturbance, but lately their value for applications like aeroecological monitoring of bird and insect migration has been understood. The utilization of these data, however, has been hampered by a lack of comprehensive classification of nonmeteorological echoes. In this paper we present a comprehensive, fine-grained, probabilistic classifier for all common types of nonmeteorological echoes which enables the implementation of a wide range of novel weather radar applications. [ABSTRACT FROM AUTHOR]

The Jones matrix is a powerful quantitative tool for polarimetric imaging and characterization, where the matrix elements represent the intrinsic optical properties of the anisotropic objects on interaction with the polarized light. Most of the state‐of‐the‐art single‐shot Jones‐matrix imaging techniques rely on interferometric methods, which makes the design complex with the use of large number of polarization optics and produces space‐bandwidth limitations. The simultaneous extraction of Jones matrix elements with a compact system design is still a technical challenge in view of its potential applications in biomedical and real‐time characterization scenarios. In this paper, the imaging features of the in‐line holography are exploited to develop a compact polarization in‐line holography (PIH) system capable of single‐shot extraction of Jones‐matrix elements. The technique achieves simultaneous orthogonal polarization state generation and corresponding polarization multiplexed in‐line hologram detection by utilizing a compact polarization geometry. The imaging compatibility and measurement accuracy of the method is experimentally validated by the real‐time synthesis of Jones‐matrix elements corresponding to custom designed polarization sensitive samples and standard birefringent resolution target. Furthermore, to demonstrate the application of the system in dynamic imaging, the transient polarization changes of a custom‐designed parallel nematic liquid crystal display (LCD) are investigated. [ABSTRACT FROM AUTHOR]

In this paper we present methods for estimating shape from polarisation and shading information, i.e. photo-polarimetric shape estimation, under varying, but unknown, illumination, i.e. in an uncalibrated scenario. We propose several alternative photo-polarimetric constraints that depend upon the partial derivatives of the surface and show how to express them in a unified system of partial differential equations of which previous work is a special case. By careful combination and manipulation of the constraints, we show how to eliminate non-linearities such that a discrete version of the problem can be solved using linear least squares. We derive a minimal, combinatorial approach for two source illumination estimation which we use with RANSAC for robust light direction and intensity estimation. We also introduce a new method for estimating a polarisation image from multichannel data and provide methods for estimating albedo and refractive index. We evaluate lighting, shape, albedo and refractive index estimation methods on both synthetic and real-world data showing improvements over existing state-of-the-art. [ABSTRACT FROM AUTHOR]

Superpixel segmentation is widely used in polarimetric synthetic aperture radar (PolSAR) image classification. However, the classification method using simple majority voting cannot easily handle evidence conflicts in a single superpixel. At present, there is no method to evaluate the quality of superpixel classification. To solve the above problems, this paper proposes a hybrid classification model based on superpixel entropy discrimination (SED), and constructs a two-level cascade classifier. Firstly, a light gradient boosting machine (LGBM) was used to process large-dimensional input features, and simple linear iterative clustering (SLIC) was integrated to obtain the primary classification results based on superpixels. Secondly, information entropy was introduced to evaluate the quality of superpixel classification, and a complex-valued convolutional neural network (CV-CNN) was used to reclassify the high-entropy superpixels to obtain the secondary classification results. Experiments with two measured PolSAR datasets show that the overall accuracy of both classification methods exceeded 97%. This method suppressed the evidence conflict in a single superpixel and the inaccuracy of superpixel segmentation. The test time of our proposed method was shorter than that of CV-CNN, and using only 55% of CV-CNN test data could achieve the same accuracy as using CV-CNN for the whole image. [ABSTRACT FROM AUTHOR]

In this paper, we report the significance of the double Stokes–Mueller polarimetry (DSMP) technique, to characterize a large size (3 × 3 × 5 mm) Potassium titanyl phosphate (KTP) crystal. The crystal undergoes second harmonic generation with type II phase matching. The study of standard KTP crystal using the DSMP technique helps to validate the efficiency of this technique. We were able to extract the crystal's double Mueller matrix, relative contribution of the susceptibility tensor components, the phase difference between the susceptibility tensor components, etc. We could determine the crystal axes orientation using this optical technique, which was not possible through a single crystal X-Ray diffraction technique for such a large size crystal for which both optic axes and crystallographic axes are the same. Axes direction determined from polarization microscope measurements and Laue diffraction measurements on KTP crystal is compared with those obtained from DSMP measurements. [ABSTRACT FROM AUTHOR]

Transitional millisecond pulsars are an emerging class of sources that link low-mass X-ray binaries to millisecond radio pulsars in binary systems. These pulsars alternate between a radio pulsar state and an active low-luminosity X-ray disc state. During the active state, these sources exhibit two distinct emission modes (high and low) that alternate unpredictably, abruptly, and incessantly. X-ray to optical pulsations are observed only during the high mode. The root cause of this puzzling behaviour remains elusive. This paper presents the results of the most extensive multi-wavelength campaign ever conducted on the transitional pulsar prototype, PSR J1023+0038, covering from the radio to X-rays. The campaign was carried out over two nights in June 2021 and involved 12 different telescopes and instruments, including XMM-Newton, HST, VLT/FORS2 (in polarimetric mode), ALMA, VLA, and FAST. By modelling the broadband spectral energy distributions in both emission modes, we show that the mode switches are caused by changes in the innermost region of the accretion disc. These changes trigger the emission of discrete mass ejections, which occur on top of a compact jet, as testified by the detection of at least one short-duration millimetre flare with ALMA at the high-to-low mode switch. The pulsar is subsequently re-enshrouded, completing our picture of the mode switches. [ABSTRACT FROM AUTHOR]

Context. The Solar Orbiter mission completed its first remote-sensing observation windows in the spring of 2022. On 2 April 2022, an M-class flare followed by a filament eruption was seen both by the instruments on board the mission and from several observatories in Earth's orbit, providing an unprecedented view of a flaring region with a large range of observations. Aims. We aim to understand the nature of the flaring and filament eruption events via the analysis of the available dataset. The complexity of the observed features is compared with the predictions given by the standard flare model in 3D. Methods. In this paper, we use the observations from a multi-view dataset, which includes extreme ultraviolet (EUV) imaging to spectroscopy and magnetic field measurements. These data come from the Interface Region Imaging Spectrograph, the Solar Dynamics Observatory, Hinode, as well as several instruments on Solar Orbiter. Results. The large temporal coverage of the region allows us to analyse the whole sequence of the filament eruption starting with its pre-eruptive state. Information given by spectropolarimetry from SDO/HMI and Solar Orbiter PHI/HRT shows that a parasitic polarity emerging underneath the filament is responsible for bringing the flux rope to an unstable state. As the flux rope erupts, Hinode EIS captures blue-shifted emission in the transition region and coronal lines in the northern leg of the flux rope prior to the flare peak. This may be revealing the unwinding of one of the flux rope legs. At the same time, Solar Orbiter SPICE captures the whole region, complementing the Doppler diagnostics of the filament eruption. Analyses of the formation and evolution of a complex set of flare ribbons and loops, of the hard and soft X-ray emissions with STIX, show that the parasitic emerging bipole plays an important role in the evolution of the flaring region. Conclusions. The extensive dataset covering this M-class flare event demonstrates how important multiple viewpoints and varied observations are in order to understand the complexity of flaring regions. While the analysed data are overall consistent with the standard flare model, the present particular magnetic configuration shows that surrounding magnetic activity such as nearby emergence needs to be taken into account to fully understand the processes at work. This filament eruption is the first to be covered from different angles by spectroscopic instruments, and provides an unprecedented diagnostic of the multi-thermal structures present before and during the flare. This complete dataset of an eruptive event showcases the capabilities of coordinated observations with the Solar Orbiter mission. [ABSTRACT FROM AUTHOR]

The world's largest macroalgae bloom (also known as green tide) has been reported since the 29th Olympic Games in 2008, which is verified as the fast reproduction of floating green macroalgae (Ulva prolifera). It is helpful to assess the biomass of macroalgae for the government of marine environment protection. In this study, the synchronization cruise experiment was firstly introduced, which aimed to investigate the biomass evaluation of Ulva prolifera in the Yellow Sea of China. The Floating Algae Index by Polarimetric SAR image (FAIPS) was then proposed. Finally, the floating algae biomass evaluation model was demonstrated and verified, which showed an exponential relationship between FAIPS and wet biomass per area (kg/m2) of macroalgae. The model proposed in this paper can be used in the biomass assessment of floating algae in the presence of polarimetric SAR images, regardless of daylight and cloud coverage over the sea surface. [ABSTRACT FROM AUTHOR]

Polarimetry, which seeks to measure the vectorial information of light modulated by objects, has facilitated bionic geolocation and navigation applications. It is a novel and promising field that provides humans with a remote sensing tool to exploit polarized skylight in a similar way to polarization-sensitive animals, and yet few in-depth reviews of the field exist. Beginning with biological inspirations, this review mainly focuses on the characterization, measurement, and analysis of vectorial information in polarimetry for bionic geolocation and navigation applications, with an emphasis on Stokes–Mueller formalism. Several recent breakthroughs and development trends are summarized in this paper, and potential prospects in conjunction with some cutting-edge techniques are also presented. The goal of this review is to offer a comprehensive overview of the exploitation of vectorial information for geolocation and navigation applications as well as to stimulate new explorations and breakthroughs in the field. [ABSTRACT FROM AUTHOR]

In this paper we describe the Kottamia Faint Imaging Spectro-Polarimeter (KFISP) that has been recently developed and designed to be mounted at the Cassegrain focus of the 1.88 m telescope at Kottamia Astronomical Observatory (KAO), Egypt. The optical design of KFISP is developed such that it can be used in various modes of operation. These are: direct imaging, spectroscopic, polarimetric imaging, and spectro-polarimetric. The KFISP is an all-refractive design to meet the polarimetric requirements and includes a focal reducer with a corrector section, collimator section, parallel beam section (containing various imaging components), and camera section. The corrector section gives an unvignetted Field-of-View of 8ʹ × 8ʹ and the collimator section has a focal length of 305 mm and matches the focal ratio of the input beam. The parallel beam section is 200 mm long and near the middle of it there is an image of the telescope pupil. The camera section includes 5 elements and has a focal length of 154.51 mm which gives an instrument effective final focal ratio of f/6.14 (acting as a telescope focal reducer of 1:2 ratio). The KFISP contains an internal calibration system which hosts the calibration light injection system, an integrating sphere equipped with the required calibration light sources. The opto-mechanical parts of KFISP contain a double-layered carbon fiber strut structure and comprises its subsystems of slit and guider assemblies, filter wheel drawer, grism wheel drawer, polarimetric components cubical box, and CCD camera which is integrated with camera optics. The CCD camera has 2048 × 2048 pixels with 13.5-micron square pixel size. The camera is cooled by liquid Nitrogen and is fixed to the KFISP through the integrated camera lens. The KFISP has been fully commissioned, mounted and is being tested in all modes of operation. In this paper we introduce the ambitious scientific goals, the optical setups of KFISP, its opto-mechanical implementation and the performance analysis of the instrument. In addition, we describe the camera system, its performance, and its software control. Finally, we present a sample of the first light observations obtained from the instrument. [ABSTRACT FROM AUTHOR]

This paper presents measurements using a dual-polarised radiometer operating at 93 GHz to detect ice or water on asphalt in laboratory conditions. The brightness temperatures of both H and V polarizations were measured for a dry surface, liquid water, and ice on asphalt at observation angles of 50° and 56°. The results presented in this paper demonstrate that the studied road conditions can be identified by the radiometer. The measurements are compared with a model and surface parameters, such as dielectric constant and roughness are fitted and compared to reference values. The experiments and results, described in this article, are the first steps towards the future installation of a polarimetric sensor on a moving vehicle for traffic safety. [ABSTRACT FROM AUTHOR]

EOS-04 (Earth Observation Satellite-04), launched on 14 February 2022, is a follow-on mission to Indian Space Research Organisation’s first synthetic aperture radar (SAR) satellite RISAT-1. It is capable of imaging in either side of the track with incidence angle range from 12.2° to 55.1°. Onboard SAR system supports variety of resolution and swath requirements and can operate in various modes like StripMap, ScanSAR and Sliding Spotlight modes with various polarization combinations like linear, hybrid and full/quad polarimetry. EOS-04 is the first spaceborne SAR mission worldwide supporting full-polarimetry (FP) data acquisitions in ScanSAR modes. Availability of FP datasets in wide swath modes (ScanSAR modes) is a big step towards scientific analysis for various applications in a timebound manner. For enabling interferometric processing, EOS-04 orbits are being maintained within the baseline requirements of 500 m. EOS-04 has 17 days repetivity resulting in less temporal decorrelation to use the datasets for various deformation studies. In addition to this, analysis ready data (ARD) product, normalized radar backscatter (NRB) Gamma0 product, are introduced to minimize the user efforts for any immediate analysis. In NRB Gamma0 products, terrain induced variations particularly on hill-slopes affecting the brightness of the radar return is normalized by applying radiometric terrain correction (RTC) on SAR images. This paper discusses about the data processing aspects related to calibration, polarimetry, interferometry and NRB Gamma0 product generation. [ABSTRACT FROM AUTHOR]

We present the first part of a spectroscopic and polarimetric study on a sample of 58 Be stars that have been measured since 1998. The aim of the study is to understand the time-scales of disc variability, formation, and dissipation as a function of the properties (mass, luminosity, and rotational velocity) of the underlying B star. In this paper, we classified the sample based on the presence of emission or absorption of the Hα line, and the shape of the peak as single or double peak, as well as noting changes between emission and non-emission states. We find a probability of ∼0.75 per cent per year that an object in the sample will undergo such a change. We also present re-derived values of the projected rotational velocities for the sample. When we compare our polarization values with those from the literature, we find that most of the stars do not show a change in the value of the polarization angle; however, a small number show significant changes which could be attributed to either disc strength (optical depth) or geometry changes. Finally, we show how, by combining the (interstellar corrected) degree of polarization and the projected rotational velocity, we can construct an inclination angle-free parameter that includes the true equatorial velocity. Using this inclination angle-independent parameter, we show that the populations of single and double peak stars are indistinguishable, giving further evidence that Be star line profiles are essentially inclination angle driven. [ABSTRACT FROM AUTHOR]

The Kodaikanal Tower Tunnel Telescope (KTT) is a three-mirror Coelostat-based Solar telescope. An inherent variable image drift is present in the KTT which is detrimental to observations particularly to slit-based spectroscopic and spectropolarimetric observations. An image Auto Guider system has recently been installed at KTT to arrest the image drift based on sensing the image limb. The image's limb position on the image plane is sensed using a two-segment photodiode sensor in North–South and East–West directions. Feedback is given to the second mirror of Coelostat, which tilts, causing the image to move on the image plane. This paper discusses instrumental setup, calibration, and testing of the Auto Guider system at KTT. The Auto Guider system is robust and capable of arresting image drifts up to 55 arcsec min − 1 , which is much higher than the typically observed drifts (maximum 7 arcsec min − 1 ) at KTT with a precision of ± ∼ 0. 9 arcsec. [ABSTRACT FROM AUTHOR]

A representative general polarimetric model-based decomposition framework was proposed by Chen et al., which implements a simultaneous full-parameter inversion by using complete polarimetric information and solves several limitations in previous decomposition methods. However, there are still shortcomings in Chen's work. Firstly, only the real part of the parameter β in the generalized surface scattering model is considered. Secondly, inappropriate initial input values may lead to local optima in the nonlinear least squares optimization algorithm. Thirdly, the volume scattering component is underestimated in the volume scattering-dominated scene, but overestimated in buildings with large orientation (LOB) areas. Finally, nonlinear optimization is time-consuming computationally. To overcome those issues, an improved generalized polarimetric model-based decomposition method is proposed in this paper. The imaginary part of the parameter β is incorporated into the decomposition framework of the proposed method. Ingeniously utilizing the internal relationship in the generic equations composed of coherent matrix elements, the model parameters can be inversed by simplifying the nonlinear equations to linear equations. Therefore, compared with Chen's method, the proposed method does not rely on the initial input values, and improves the computational efficiency. In addition, a hierarchical decomposition scheme is presented to solve the problem of underestimation or overestimation of volume scattering component mentioned above. The performance and advantages of this method are evaluated with L-band and C-band polarimetric synthetic aperture radar (PolSAR) data sets. Comparison studies are carried out with other model-based decomposition methods, demonstrating that the proposed method can further improve decomposition performance, especially in LOB areas. [ABSTRACT FROM AUTHOR]

Ramsey theory constitutes the dynamics of mechanical systems, which may be described as abstract complete graphs. We address a mechanical system which is completely interconnected by two kinds of ideal Hookean springs. The suggested system mechanically corresponds to cyclic molecules, in which functional groups are interconnected by two kinds of chemical bonds, represented mechanically with two springs k 1 and k 2 . In this paper, we consider a cyclic system (molecule) built of six equal masses m and two kinds of springs. We pose the following question: what is the minimal number of masses in such a system in which three masses are constrained to be connected cyclically with spring k 1 or three masses are constrained to be connected cyclically with spring k 2 ? The answer to this question is supplied by the Ramsey theory, formally stated as follows: what is the minimal number   R (3 , 3) ? The result emerging from the Ramsey theory is R (3 , 3) = 6 . Thus, in the aforementioned interconnected mechanical system at least one triangle, built of masses and springs, must be present. This prediction constitutes the vibrational spectrum of the system. Thus, the Ramsey theory and symmetry considerations supply the selection rules for the vibrational spectra of the cyclic molecules. A symmetrical system built of six vibrating entities is addressed. The Ramsey approach works for 2D and 3D molecules, which may be described as abstract complete graphs. The extension of the proposed Ramsey approach to the systems, partially connected by ideal springs, viscoelastic systems and systems in which elasticity is of an entropic nature is discussed. "Multi-color systems" built of three kinds of ideal springs are addressed. The notion of the inverse Ramsey network is introduced and analyzed. [ABSTRACT FROM AUTHOR]

Malignant melanoma is considered the most serious type of skin cancer. In clinical practice, the conventional technique based on subjective visual examination has a high rate of misdiagnosis for malignant melanoma and benign nevus. Polarization imaging techniques have great potential in clinical diagnosis due to the advantages of improving sensitivity to functional structures, such as microfiber. In this paper, a set of human skin tissue sections, including 853 normal, 851 benign nevus, and 874 malignant melanoma, were analyzed and differentiated using a homemade high-fidelity Mueller matrix imaging polarimeter. The quantitative result using support vector machine algorithms confirmed that, while scalar retardance yields lower accuracy rates, vectorial retardance results in greater accuracy for both the training and testing sets. In particular, the cross-validation accuracy for the training set increased from 88.33% to 98.60%, and the prediction accuracy for the testing set increased from 87.92% to 96.19%. This tackles the limitation of the examination based on clinical experience and suggests that vectorial retardance can provide more accurate diagnostic evidence than scalar retardance. Unfortunately, it is inconvenient and time-consuming to read and analyze each component of the vectorial retardance simultaneously in the qualitative assessment. To address this clinical challenge, a color-encoded vectorial retardance imaging method was implemented. This method can provide superior tissue-specific contrast and more fiber details than scalar retardance. The anisotropic microfiber variation among different skin lesions, including the orientation and distribution, can be clearly highlighted. We believe that this work will not only enable early and rapid diagnosis of skin cancer but also provide a good observation and analysis of the state of cancer progression. [ABSTRACT FROM AUTHOR]

The classical polarimetric method has been widely used in liquid crystal on silicon (LCoS) phase measurement with a simple optical setup. However, due to interference caused by LCoS cover glass reflections, the method lacks accuracy for phase uniformity measurements. This paper is aimed at mathematically analyzing the errors caused by non-ideal glass reflections and proposing procedures to reduce or eliminate such errors. The measurement is discussed in three conditions, including the ideal condition with no reflections from the LCoS cover glass, the condition with only the front reflection from the cover glass, and the condition with only the back reflection from the cover glass. It is discovered that the backward reflection makes the largest contribution to the overall measurement error, and it is the main obstacle to high-quality measurements. Several procedures, including optical alignment, LC layer thickness measurement, and phase estimation method, are proposed, making the uniformity measurement more qualitative and consistent. [ABSTRACT FROM AUTHOR]

The 100-m X-ray Test Facility of the Institute of High Energy Physics (IHEP) was initially proposed in 2012 for the test and calibration of the X-ray detectors of the Hard X-ray Modulation Telescope (HXMT) with the capability to support future X-ray missions. The large instrument chamber connected with a long vacuum tube can accommodate the X-ray mirror, focal plane detector and other instruments. The X-ray sources are installed at the other end of the vacuum tube with a distance of 105 m, which can provide an almost parallel X-ray beam covering 0.2 ∼ 60 keV energy band. The X-ray mirror modules of the Einstein Probe (EP) and the enhanced X-ray Timing and Polarimetry mission (eXTP) and payload of the Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) have been tested and calibrated with this facility. It has been also used to characterize the focal plane camera and aluminum filter used on the Einstein Probe. In this paper, we will introduce the overall configuration and capability of the facility, and give a brief introduction of some calibration results performed with this facility. [ABSTRACT FROM AUTHOR]