Polarimetric Techniques Research Articles

A Novel Detection Transformer Framework for Ship Detection in Synthetic Aperture Radar Imagery Using Advanced Feature Fusion and Polarimetric Techniques

Ship detection in synthetic aperture radar (SAR) imagery faces significant challenges due to the limitations of traditional methods, such as convolutional neural network (CNN) and anchor-based matching approaches, which struggle with accurately detecting smaller targets as well as adapting to varying environmental conditions. These methods, relying on either intensity values or single-target characteristics, often fail to enhance the signal-to-clutter ratio (SCR) and are prone to false detections due to environmental factors. To address these issues, a novel framework is introduced that leverages the detection transformer (DETR) model along with advanced feature fusion techniques to enhance ship detection. This feature enhancement DETR (FEDETR) module manages clutter and improves feature extraction through preprocessing techniques such as filtering, denoising, and applying maximum and median pooling with various kernel sizes. Furthermore, it combines metrics like the line spread function (LSF), peak signal-to-noise ratio (PSNR), and F1 score to predict optimal pooling configurations and thus enhance edge sharpness, image fidelity, and detection accuracy. Complementing this, the weighted feature fusion (WFF) module integrates polarimetric SAR (PolSAR) methods such as Pauli decomposition, coherence matrix analysis, and feature volume and helix scattering (Fvh) components decomposition, along with FEDETR attention maps, to provide detailed radar scattering insights that enhance ship response characterization. Finally, by integrating wave polarization properties, the ability to distinguish and characterize targets is augmented, thereby improving SCR and facilitating the detection of weakly scattered targets in SAR imagery. Overall, this new framework significantly boosts DETR’s performance, offering a robust solution for maritime surveillance and security.

Searching for magnetic fields in featureless white dwarfs with the DIPOL-UF polarimeter at the Nordic Optical Telescope

About 20% of the white dwarfs possess a magnetic field that may be detected by the splitting and/or polarization of their spectral lines. As they cool, the effective temperatures of the white dwarfs become so low that no spectral lines can be seen in the visible wavelength range. If their atmospheres are not polluted by the debris of a planetary system, these cool white dwarfs have featureless optical spectra. Until quite recently, very little was known about the incidence of magnetic fields in these objects. However, when observed with polarimetric techniques, a significant number of featureless white dwarfs reveal strong magnetic fields in their optical continuum spectra. Measuring the occurrence rate and strength of magnetic fields in old white dwarfs may help us to understand how these fields are generated and evolve. We report the results of an ongoing survey of cool white dwarfs with the high-precision broad-band polarimeter DIPOL-UF, which is deployed at the Nordic Optical Telescope on La Palma, Spain. This survey has led to the firm discovery of 13 new cool magnetic white dwarfs in the solar neighborhood so far, including six new detections that we report in this paper.

A handheld polarimetric imaging device and calibration technique for accurate mapping of terahertz Stokes vectors

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.

Polarimetric Imaging for Robot Perception: A Review.

In recent years, the integration of polarimetric imaging into robotic perception systems has increased significantly, driven by the accessibility of affordable polarimetric sensors. This technology complements traditional color imaging by capturing and analyzing the polarization characteristics of light. This additional information provides robots with valuable insights into object shape, material composition, and other properties, ultimately enabling more robust manipulation tasks. This review aims to provide a comprehensive analysis of the principles behind polarimetric imaging and its diverse applications within the field of robotic perception. By exploiting the polarization state of light, polarimetric imaging offers promising solutions to three key challenges in robot vision: Surface segmentation; depth estimation through polarization patterns; and 3D reconstruction using polarimetric data. This review emphasizes the practical value of polarimetric imaging in robotics by demonstrating its effectiveness in addressing real-world challenges. We then explore potential applications of this technology not only within the core robotics field but also in related areas. Through a comparative analysis, our goal is to elucidate the strengths and limitations of polarimetric imaging techniques. This analysis will contribute to a deeper understanding of its broad applicability across various domains within and beyond robotics.

Circularly Polarized Luminescence Without External Magnetic Fields from Individual CsPbBr3 Perovskite Quantum Dots.

Lead halide perovskite quantum dots (QDs), the latest generation of the colloidal QD family, exhibit outstanding optical properties, which are now exploited as both classical and quantum light sources. Most of their rather exceptional properties are related to the peculiar exciton fine-structure of band-edge states, which can support unique bright triplet excitons. The degeneracy of the bright triplet excitons is lifted with energetic splitting in the order of millielectronvolts, which can be resolved by the photoluminescence (PL) measurements of single QDs at cryogenic temperatures. Each bright exciton fine-structure-state (FSS) exhibits a dominantly linear polarization, in line with several theoretical models based on the sole crystal field, exchange interaction, and shape anisotropy. Here, we show that in addition to a high degree of linear polarization, the individual exciton FSS can exhibit a non-negligible degree of circular polarization even without external magnetic fields by investigating the four Stokes parameters of the exciton fine-structure in individual CsPbBr3 QDs through Stokes polarimetric measurements. We observe a degree of circular polarization up to ∼38%, which could not be detected by using the conventional polarimetric technique. In addition, we found a consistent transition from left- to right-hand circular polarization within the fine-structure triplet manifold, which was observed in magnetic-field-dependent experiments. Our optical investigation provides deeper insights into the nature of the exciton fine structures and thereby drives the yet-incomplete understanding of the unique photophysical properties of this class of QDs for the benefit of future applications in chiral quantum optics.

Broadband spectropolarimeter based on spatial heterodyne spectroscopy and channeled polarimetric technique

A spectropolarimeter based on spatial heterodyne spectroscopy and the channeled polarimetric technique (SSHSCP) is introduced. The SSHSCP uses a polarization grating for channel modulation, meaning that the Stokes parameter is no longer dependent on the wavenumber; this suppresses channel aliasing considerably and improves the polarization parameter reconstruction accuracy. The SSHSCP’s optical path structure is based on the Mach–Zehnder interferometer, ensuring efficient energy use. The spectral detection and polarization detection principles of the SSHSCP are derived, and a specific design example is presented. Simulations indicate that the SSHSCP can restore the target’s spectral and complete Stokes parameters with high accuracy for a broadband source and narrowband lines. The reconstructed Stokes parameter errors are of the order of 10−3 or less. The SSHSCP acquires spectral and polarization parameters in a single step and has no moving parts in a compact structure.

Kalman filtering approach to polarimetric calibration of an optical imager

The Mueller matrix of an optical instrument describes the polarimetric effects the instrument will have on the optical observations it makes in terms of the Stokes parameters. The calibration of the instrument relies on a robust characterization of the elements of this matrix. In this paper, we present what we believe is a new technique that uses Kalman filtering to characterize the Mueller matrices of optical instrumentation based on a set of lab calibration measurements. Kalman filtering is a ubiquitous statistical optimizer that works by comparing measurements and a model of the observed physical system to minimize error. Typically, this technique is applied as a filter to refine a set of observations, but it can also be used to retrieve the properties of the physical system that are not directly measured. We demonstrate the use of the Kalman approach to polarimetric calibration through simulation of measurements, where the polarimetric behavior of optical elements is represented by the Mueller matrices of individual components. The elements of these Mueller matrices are then retrieved with the uncertainty estimates using the Kalman filter approach. The results of this simulation are compared to the standard polarimetric calibration technique as a benchmark, demonstrating the superior performance of the Kalman approach. Then, both the Kalman technique and the standard technique are applied to real measurements from a multispectral polarimetric imager used for atmospheric aerosol remote sensing.

A Review on PolSAR Decompositions for Feature Extraction.

Feature extraction plays a pivotal role in processing remote sensing datasets, especially in the realm of fully polarimetric data. This review investigates a variety of polarimetric decomposition techniques aimed at extracting comprehensive information from polarimetric imagery. These techniques are categorized as coherent and non-coherent methods, depending on their assumptions about the distribution of information among polarimetric cells. The review explores well-established and innovative approaches in polarimetric decomposition within both categories. It begins with a thorough examination of the foundational Pauli decomposition, a key algorithm in this field. Within the coherent category, the Cameron target decomposition is extensively explored, shedding light on its underlying principles. Transitioning to the non-coherent domain, the review investigates the Freeman-Durden decomposition and its extension, the Yamaguchi's approach. Additionally, the widely recognized eigenvector-eigenvalue decomposition introduced by Cloude and Pottier is scrutinized. Furthermore, each method undergoes experimental testing on the benchmark dataset of the broader Vancouver area, offering a robust analysis of their efficacy. The primary objective of this review is to systematically present well-established polarimetric decomposition algorithms, elucidating the underlying mathematical foundations of each. The aim is to facilitate a profound understanding of these approaches, coupled with insights into potential combinations for diverse applications.

Review of polarization-based technology for biomedical applications

Polarimetry is a powerful optical tool in the biomedical field, providing more comprehensive information on the sub-wavelength micro-physical structure of a sample than traditional light intensity measurement techniques. This review summarizes the concepts and techniques of polarization and its biomedical applications. Specifically, we first briefly describe the basic principles of polarized light and the Mueller matrix (MM) decomposition method, followed by some research progress of polarimetric measurement techniques in recent years. Finally, we introduce some studies on biological tissues and cells, and then illustrate the application value of polarization optical method.

Analysis of the performance of polarimetric PSI over distributed scatterers with Sentinel-1 data

Sentinel−1 (S1) data enables effective monitoring of displacements using persistent scatterer interferometry (PSI). S1 includes VV and VH polarization channels, allowing us to apply polarimetric techniques to PSI. In short, polarimetric PSI (PolPSI) exploits the available polarization channels to enhance the identification and processing of measurement points including persistent scatterers (PS) and distributed scatterers (DS). Previous works have shown the benefits of using PolPSI for PS points with S1 data, but the corresponding analysis for DS is missing.DS points are processed by finding a neighborhood of statistically homogeneous pixels (SHP) and averaging the phase within that neighborhood. In this work we show how dual-polarimetric data are stricter on the selection of the SHP group than single-polarimetric data. Thanks to the information added by the second channel, different land covers are not mixed in the SHP group. As a result, the number of points in the SHP groups is generally smaller than with VV alone, but they are more reliable. The impact of this strategy on the resulting deformation estimates is also investigated in this work, showing that the deformation areas are fully preserved and the influence of nearby pixels associated with other scene elements is avoided.

Experimental Characterization of Polarized Light Backscattering in Fog Environments

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.

Analyzing the influence of oblique incidence on quantitative backscattering tissue polarimetry: a pilot ex vivo study.

Among the available polarimetric techniques, backscattering Mueller matrix (MM) polarimetry provides a promising non-contact and quantitative tool for in vivo tissue detection and clinical diagnosis. To eliminate the surface reflection from the sample cost-effectively, the non-collinear backscattering MM imaging setup always has an oblique incidence. Meanwhile, for practical organ cavities imaged using polarimetric gastrointestinal endoscopy, the uneven tissue surfaces can induce various relative oblique incidences inevitably, which can affect the polarimetry in a complicated manner and needs to be considered for detailed study. The purpose of this study is to systematically analyze the influence of oblique incidence on backscattering tissue polarimetry. We measured the MMs of experimental phantom and ex vivo tissues with different incident angles and adopted a Monte Carlo simulation program based on cylindrical scattering model for further verification and analysis. Meanwhile, the results were quantitatively evaluated using the Fourier transform, basic statistics, and frequency distribution histograms. Oblique incidence can induce different changes on non-periodic, two-periodic, and four-periodic MM elements, leading to false-positive and false-negative polarization information for tissue polarimetry. Moreover, a prominent oblique incidence can bring more dramatic signal variations, such as phase retardance and element transposition. The findings presented in this study give some crucial criterions of appropriate incident angle selections for in vivo polarimetric endoscopy and other applications and can also be valuable references for studying how to minimize the influence further.

Wave retrieval from quad-polarized Chinese Gaofen-3 SAR image using an improved tilt modulation transfer function

ABSTRACT An accurate Modulation Transfer Function (MTF) is essential for Synthetic Aperture Radar (SAR) wave spectra retrieval. This study aimed to investigate the performance of a quad-polarized wave retrieval algorithm based on fully polarimetric SAR image data using the improved tilt MTF and considering the influence of wind speed. The tilt MTF is the key factor in the wave retrieval scheme from quad-polarized (Vertical–Vertical (VV), Horizontal–Horizontal (HH), Vertical–Horizontal (VH), and Horizontal–Vertical (HV)) SAR images. In this study, the waves were inverted from more than 1300 Gaofen-3 (GF-3) images acquired in quad-polarization strip mode with a spatial resolution of 16 m and a swath coverage of 50 km. The winds were retrieved using the Geophysical Model Function (GMF) C-band SAR model for Gaofen-3 (CSARMOD-GF), which is suitable for re-calibrated GF-3 images in VV-polarization. The comparison of the wind speed yielded a Root Mean Square Error (RMSE) of 1.73 m/s and a Correlation Coefficient (COR) of 0.94. The validation of the Significant Wave Height (SWH) simulated using the Waves Nearshore (SWAN) model against Haiyang-2B (HY-2B) altimeter data yielded an RMSE of 0.56 m and a COR of 0.87. The results reveal that the SAR-derived wind and SWAN-simulated SWH are suitable for analysis of SAR wave retrieval. The full polarimetric technique was applied to the collected images, and the statistical analysis yielded a RMSE of 0.51 m, a COR of 0.75, and a Scatter Index (SI) of 0.44 compared with the SWHs retrieved using the simulations from the SWAN model. The non-polarized contribution in the Normalized Radar Cross Section (NRCS; unit: dB) caused by wave breaking was calculated using a theoretical approach that employs the VV-polarized calibrated NRCS and HH-polarized calibrated NRCS. The effect of wave breaking on the SAR retrieval waves was studied. The bias (SAR-derived minus SWAN-simulated SWH) increased as the (/) ratio (>0.4) increased, and the accuracy improved when the ratio was less than 0.4. This behavior is reasonable since the wave breaking inevitably affects the tilt modulation. Therefore, wave breaking should be considered in SAR wave retrieval using the approach proposed in this paper under extreme sea states, such as typhoons and hurricanes.

Polarimetric Images of Biological Tissues Based on the Arrow Decomposition of Mueller Matrices

Polarimetric techniques are widely used in a vast number of applications such as remote sensing, material characterization, astronomy and biological tissue inspection. In this last scenario, different polarimetric observables have proved their potential for enhancing imaging visualization. In this work we use a set of polarimetric observables derived from the arrow decomposition of the Mueller matrix for the first time: enpolarizing, retarding and depolarizing descriptors. In particular, the mean intensity coefficient and the three indices of polarimetric purity, the absolute values and Poincaré orientations of diattenuation, polarizance, entrance retardance and exit retardance vectors are considered. Results show images with enhanced visualization or even revealing invisible structures when compared to standard intensity images. In particular, thanks to these metrics, we improve the visualization of the necrotic areas of a Vitis rupestris leaf. In the case of animal samples, boundaries between different fascicles inside a tendon of an ex vivo chicken sample are revealed, as is the directionality of fiber tracts of the subcortical white matter in an ex vivo cow brain. The experimental results show the potential for biophotonics imaging and how polarimetric techniques could be useful for biomedical and botanical applications.

Discovery of strongly variable X-ray polarization in the neutron star low-mass X-ray binary transient XTE J1701−462

Context. After about 16 years since its first outburst, the transient neutron star low-mass X-ray binary XTE J1701−462 turned on again in September 2022, allowing for the first study of its X-ray polarimetric characteristics by a dedicated observing program with the Imaging X-ray Polarimeter Explorer (IXPE). Aims. Polarimetric studies of XTE J1701−462 have been expected to improve our understanding of accreting weakly magnetized neutron stars, in particular, the physics and the geometry of the hot inner regions close to the compact object. Methods. The IXPE data of two triggered observations were analyzed using time-resolved spectroscopic and polarimetric techniques, following the source along its Z-track of the color–color diagram. Results. During the first pointing on 2022 September 29, an average 2–8 keV polarization degree of (4.6 ± 0.4)% was measured, the highest value found up to now for this class of sources. Conversely, only a ∼0.6% average degree was obtained during the second pointing ten days later. Conclusions. The polarimetric signal appears to be strictly related to the higher energy blackbody component associated with the boundary layer (BL) emission and its reflection from the inner accretion disk, and it is as strong as 6.1% and 1.2% (> 95% significant) above 3–4 keV for the two measurements, respectively. The variable polarimetric signal is apparently related to the spectral characteristics of XTE J1701−462, which is the strongest when the source was in the horizontal branch of its Z-track and the weakest in the normal branch. These IXPE results provide new important observational constraints on the physical models and geometry of the Z-sources. Here, we discuss the possible reasons for the presence of strong and variable polarization among these sources.

The high-albedo, low polarization disk around HD 114082 that harbors a Jupiter-sized transiting planet

Aims. We present new optical and near-infrared images of the debris disk around the F-type star HD 114082 in the Scorpius-Centaurus OB association. We obtained direct imaging observations and analyzed the TESS photometric time series data of this target with the goal of searching for planetary companions to HD 114082 and characterizing the morphology of the debris disk and the scattering properties of dust particles. Methods. HD 114082 was observed with the VLT/SPHERE instrument in different modes – the IRDIS camera in the K band (2.0–2.3 μm) together with the IFS in the Y, J, and H bands (0.95–1.66 μm) using the angular differential imaging technique as well as IRDIS in the H band (1.5–1.8 μm) and ZIMPOL in the I_PRIME band (0.71–0.87 μm) using the polarimetric differential imaging technique. To constrain the basic geometrical parameters of the disk and the scattering properties of dust grains, scattered light images were fitted with a 3D model for single scattering in an optically thin dust disk using a Markov chain Monte Carlo approach. We performed aperture photometry to derive the scattering and polarized phase functions, the polarization fraction, and the spectral scattering albedo for the dust particles in the disk. This method was also used to obtain the reflectance spectrum of the disk and, in turn, to retrieve the disk color and study the dust reflectivity in comparison to the debris disk HD 117214. We also performed the modeling of the HD 114082 light curve measured by TESS using models for planet transit and stellar activity to put constraints on the radius of the detected planet and its orbit. Last, we searched for additional planets in the system by combining archival radial velocity data, astrometry, and direct imaging. Results. The debris disk HD 114082 appears as an axisymmetric debris belt with a radius of ~0.37″ (35 au), an inclination of ~83°, and a wide inner cavity. Dust particles in HD 114082 have a maximum polarization fraction of ~17% and a higher reflectivity when compared to the debris disk HD 117214. This high reflectivity results in a spectral scattering albedo of ~0.65 for the HD 114082 disk at near-infrared wavelengths. The disk reflectance spectrum exhibits a red color at the position of the planetesimal belt and shows no obvious features, whereas that of HD 117214 might indicate the presence of CO2 ice. The analysis of TESS photometric data reveals a transiting planetary companion to HD 114082 with a radius of ~1 RJup on an orbit with a semimajor axis of 0.7 ± 0.4 au. No additional planet was detected in the system when we combined the SPHERE images with constraints from astrometry and radial velocity. We reach deep sensitivity limits down to ~5 MJup at 50 au and ~10 MJup at 30 au from the central star.

Optical rotation in the lithium triborate nonlinear crystal

A dual-wavelength polarimetric technique at 633 and 661 nm has been used for the characterization of a nonlinear lithium triborate (LiB3O5) nonenantiomorphous biaxial crystal. The mismatch of the crystallographic and optical coordinate systems was taken into account. The optical rotatory power for light propagation along one of the optical axes is ρ = 7.06° mm−1. The gyration tensor component along the bisector between the x and y crystallographic axes has been measured as g 12 = 4.31 × 10−5. Computed values based on the crystalline structure and electronic polarizabilities are in good agreement with those obtained experimentally.

SAR Data for Detecting Landslide Phenomena: The November 26, 2022 Landslide of the Ischia Island (Southern Italy)

In this work several change detection techniques based on satellite SAR data acquired by Cosmo-SkyMed Second Generation missions have been evaluated aiming at detecting the landslide-mudflow phenomenon triggered by the strong flooding event that hit Ischia Island in November 2022. It severely impacted on Casamicciola Terme area causing damages and collapses of many buildings and unfortunately also casualties. Experimental results show how both Single- (SP) and Dual- (DP) Polarimetric techniques are able to detect the main landslide occurring along the northern flank of Mt. Epomeo and in some cases also connected phenomena such as mud accumulation. Additional analyses have been performed to quantitatively evaluate the performance of all of them showing as in this case DP techniques outperform the SP ones. The outcomes are then discussed taking into account both the features of each technique and the investigated scenario. Detecting and mapping this kind of phenomena is important for the evaluation of the affected area, especially for complex scenarios such as Ischia island, and can be very useful to support both the stakeholders for the first aid and the Civil Protection for the post-crisis management.

Exploring Fine Polarimetric Decomposition Technique for Built-Up Area Monitoring

Highly variable polarimetric signatures caused by complex structures in built-up areas make interpretation of these scattering behaviors intractable for PolSAR remote sensing. This paper proposes a fine polarimetric decomposition method and derives several products to finely simulate the scattering mechanisms of urban buildings, thus fulfilling its use for effective surveillance. First, through theoretically establishing the roll-invariant condition for a completely general scatterer, a roll-invariant cross polarization (RICP) scattering model is constructed, which characterizes the cross polarization scattering in the manner of planar structure distribution. Second, by designing a root-discriminant-based parameter inversion strategy, a fine seven-component decomposition is proposed, which achieves the complete physical interpretation of matrix elements and reasonable inversion of model parameters. Third, by analyzing the external and internal scattering difference, the derivative products, i.e., scattering contribution synthesizers are derived for built-up area monitoring. Experimental results derived from real PolSAR data confirm the superiority and effectiveness of the constructed descriptors on the one hand. On the other hand, the extensibility of fine polarimetric decomposition in specific remote sensing is also explicitly demonstrated.

Inspection of plant pathologies through pseudocolored images based on polarimetric basis

The study of the interaction of biological tissue with polarized light leads to relevant information of physical properties (dichroism, retardance and depolarization) of samples. Polarimetric analysis of different characteristics in tissues is useful for applications such us tissue classification, contrast enhancement or pathology detection. By means of polarimetric imaging techniques we can characterize the polarimetric signature of biological samples in a noninvasive and nondestructive way. We have found that depolarization information is of special interest in turbid media such as plant tissue. In this manuscript we use polarimetric observables for plant inspection. In particular, we provide enhanced visualization of certain plant pathologies by constructing depolarization based pseudocolored images of pathological leaves where the pathological areas are revealed.