This PDF file contains the front matter associated with SPIE Proceedings Volume 13050, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

In earlier work we introduced a metric based on the spatial variability of the Angle of Polarization (AoP) as a measure of the quality of the polarization signature measured by a polarimeter. In this paper, we call this the spatial angle of polarization confidence metric, or S-AoPC. The S-AoPC is computed by considering the variability of the AoP in a small region surrounding each pixel. It is based on the assumption that high spatial variance in AoP exists when the underlying Stokes parameters are essentially random. In the present paper we extend this concept to also consider temporal variation of AoP in a time angle of polarization confidence metric, or T-AoPC. We examine the S-AoPC and T-AoPC empirically for a range of polarization images and we consider applications of these metrics in polarimetry including polarimetric display, setting the parameters of the camera such as integration time, and even helping to understand the processing of hybrid polarimeter data to maximize image quality. As the name implies, the angle of polarization confidence metric can be extended to consider variability in other dimensions such as wavelength for different types of polarimeters.

In 2014, we laid out the theory for how a 2×4 microgrid polarimetric array provides superior spatial resolution when compared to a conventional 2×2 array. In this paper, we provide experimental evidence to support our claims via a prototype 2×4-patterned infrared microbolometer camera developed by Polaris Sensors Technologies. The benefits of the 2×4 array are obtained through a combination of the physical arrangement of the pattern itself and though the application of a log-based framework for reconstructing degree and angle of linear polarization directly, without calculating Stokes parameters or interpolating intensity channels as intermediate steps.

Snapshot polarimetry relies on the micro-polarizer array (MPA)—a spatial multiplexing of pixel-sized wiregrid analyzers. The limitation of MPA-based polarimetric imaging is the loss of spatial resolution and light received by each pixel. Reconstructing the degree and the angle of linear polarization (DoLP and AoLP, respectively) from MPA accurately requires the joint application of demosaicking (to demodulate the spatial modulation of the wiregrid analyzers) and denoising (to account for photon and thermal noise). We propose a wavelet-based Bayesian estimation technique for jointly demosaicking and denoising 2 × 2 MPA-sampled sensor data.

Polarization imaging can yield crucial information in multiple applications of remote sensing, such as characterization of clouds, aerosols, and the Aurora Borealis. Some applications require sub-percent polarimetric sensitivity and accuracy in determining the Stokes parameters, which can be a challenge to attain. In 2018, Sony released a low-cost CMOS-based imaging chip with integrated micro-polarizer array for general polarization measurements. We implement the calibration steps required for these Sony chips to reach sub-percent polarimetric accuracies. To analyze their performances, we have compared the characteristics of four different detector packages by three manufacturers housing either the monochromatic version or the RGB color variant. We present a comprehensive overview of the effects that these characteristics have on the polarimetric performance of the camera. They include dark noise, behavior over different gain settings, detector/pixel artifacts, and polarimetric effects determined by polarizer extinction ratios, polarizer orientations, and accuracy of polarimetric zero points due to differential pixel gains. In addition to calibrations using unpolarized light and fully linearly polarized light, we assess the polarimetric sensitivity within a tilting and rotating glass-plate set-up. We discuss the benefits of adding a rotating half-wave plate as an additional temporal modulator to generically mitigate some of the detector effects, and achieve better polarimetric sensitivity/accuracy albeit at the expense of lower temporal resolution. We conclude by presenting and discussing the polarimetric limits to which we were able to calibrate the detector effects for practical purposes. By reaching a compound absolute polarimetric uncertainty of less than a percent, these very compact, low-cost detectors are enabled for a multitude of scientific goals.

Performance fluctuations of LWIR radiometers necessitate frequent calibration which is further complicated for LWIR polarimeters. Existing compact LWIR polarization calibration methods have inherent limitations; notably, combining an unpolarized source with a wire-grid polarizer can lead to polarized reflections which depend upon the thermal environment. Furthermore, microbolometers, which have risen in popularity due to their low size, weight, and power (SWaP) and cost, have intrinsic diattenuation. This research reviews several approaches to generating a linearly polarized signal in the thermal IR waveband for use as a polarimetric calibration target. The impact of environmental reflections from wire-grid polarizers and the intrinsic polarimetric response of a low-cost uncooled microbolometer is first demonstrated. A wire-grid polarizer is place in direct contact with a heating element to analyze the emission behavior of the wire-grid polarizer. Thin wires are also known to exhibit polarized thermal emission. This phenomena is observed through application of an electrical current through nanoscale wires of a wire-grid polarizer as well as through 20[μm] diameter tungsten wires suspended in free-space. The goal of these investigations is a low SWaP polarimetric calibration source for thermal LWIR polarimeters that is robust to the environmental fluctuations.

Global Navigation Satellite Systems (GNSS) are widely used due to their easy access to outdoor GNSS signals and their spatial precision. However, such systems are sensitive to jamming and spoofing. Simple and robust navigation strategies can be found in animals deprived by essence of any GNSS system. Studies have shown that animals like bees or ants utilize the sky’s polarization pattern for navigation. We recently proposed a method inspired by migratory birds, which calibrate their magnetic compass through the celestial rotation of night stars or the daytime polarization pattern. By considering the temporal properties of the sky’s polarization pattern as a relevant navigation information, we developed a bio-inspired method to find the geographical north bearing and the observer’s latitude, requiring only skylight polarization observations during day. To reduce the noise susceptibility of our method, we added a pre-processing step using a least square method based on skylight polarization models, and a segmentation process based on a convolutional autoencoder neural network, trained with simulated data.

The underwater realm lacks GPS signal penetration, making passive navigation challenging. To overcome these hurdles, we've developed an underwater geolocalization technique that solely utilizes polarization data. This method leverages the underwater scattering phenomenon, which largely depends on the positions of the sun or moon. By training a deep neural network with over 10 million images gathered over two years, we have achieved an impressive accuracy of approximately 60 km with this approach.

In this work, we present a retrieval framework to invert aerosol microphysical parameters using ground-based hyper-angular polarimetry of the full-sky dome. Measurements are acquired with Mantis, a custom polarimeter built for the NRL by Polaris Sensor Technologies. We compare and contrast our results with Tafkaa, an at-altitude method of atmospheric correction currently used by the NRL with data collected at the Army Corps of Engineers Field Research Facility. We discuss and analyze the technique in this complex near-coastal environment and compare the derivation of remote sensing reflectance from both techniques with that measured in-situ.

Infrared polarimetry for surface spill detection has been an emerging sensing modality for the last several years. Polarimetric imagery leverages the polarization signature differences between surface slicks and water that are different from those that lead to thermal and visual signatures. Imaging of the polarization response of oil and water in a scene can lead to enhanced detection, with detection enhancement being most pronounced when the spill in a scene has the same apparent temperature as the background. The detection limit for oil thickness is around 50 micrometers or greater which corresponds to thickness of floating oil that is recoverable. The technique is also effective on sand when the oil is pooling. The sensing improvement offers the promise of automated detection of oil spills and leaks for routine monitoring and accidents with the added benefit of being able to continue monitoring at night.

The sensor is based on a DoFP architecture implemented in a microbolometer. Basic performance specifications and examples of several use cases will be given.

Polarimetric imagers (Polaris Sensor Technologies) mounted on an airplane acquired remotely sensed field data in the visible (Vis), shortwave infrared (SWIR), and longwave infrared (LWIR) bands. These airborne data focused on manmade urban materials in settings that also included natural materials such as vegetation. Initial analyses indicated that the Vis degree of linear polarization provided the greatest success in distinguishing between natural and man-made materials, that materials that differed more widely in inferred composition exhibited larger Michelson contrast, and that the Stokes parameters S1 and S2 aided separability among urban materials. The same imagers mounted on a 2 m diameter goniometer then acquired laboratory data of selected urban materials representative of the remotely sensed materials, and additional instruments characterized the compositions of the materials. Preliminary analyses of these laboratory measurements improved the statistics of multispectral polarimetric separability and exhibited dependence upon composition, while confirming and extending the field separability results.

The Spectral Polarimetric Instrument Recommendation and Evaluation (SPIRE) project compared the use of spectral and polarimetric instruments for standoff detection and discrimination of explosive hazards in varied environments. Fifteen instruments primarily comprising small fieldable focal plane polarized imagers in the visible, shortwave infrared, and longwave infrared bands were deployed in three kinds of configurations, namely on masts, on ground vehicles, and on an uncrewed aerial system, and collected many terabytes of data in nine field campaigns imaging thousands of targets and natural backgrounds in desert, arctic, temperate green, and tropical environments. In addition to the primary data from these instruments, the SPIRE team collected observations of deployability and operability in these environments as well as other field data including meteorological, radiometric, and other metadata necessary to assess technology performance and ensure robust algorithm development. These data are accumulated on a networked platform for sharing with research and development partners.

From an environmental point of view, bio-based composite material reinforcements made of flax or hemp fibers prove to be attractive alternatives to glass fibers. Since the mechanical properties of the reinforcements depend on the fiber orientation and since flax is reported to be significantly birefringent, we propose to use polarimetric imaging to study flax reinforcement structure. The characterization is made with a commercial camera equipped with an array of linear micropolarizers, therefore the circular polarization is not considered. Stokes imaging under polarized illumination provides interesting results, as well as 3x3 Mueller imaging. We study fabrics made of flax yarns gimped with thin cotton yarns and fabrics made of flax and polypropylene (PP) commingled yarns gimped with PP yarns.

Human eyes are known to exhibit birefringence due to the anisotropic structure of collagen fibrils in the cornea. This birefringence has been suggested as a feature which can be used for medical diagnoses as well as computer vision applications. However, only partial polarimetric eye measurements appear in the literature. The primary contribution of this work is full Mueller matrix images of the left eyes of 20 individuals. An existing dual-rotating retarder Mueller polarimeter was modified to address the challenges of in vivo eye measurements. These Mueller images revealed significant circular retardance that has not been previously reported. This result demonstrates the importance of full Mueller characterization as a step to identify task-relevant polarimetric features.

Accurate birefringence measurements are necessary for designing and producing optics that manipulate polarized light. This is of particular importance for waveplates, which must be polished to a specific thickness, typically with a tolerance on the order of ± 0.25 μm. In this work the birefringence of three common waveplate materials is studied: crystal quartz (over the wavelength range of 320-1650 nm), magnesium fluoride (over the wavelength range of 300-1800 nm), and synthetic sapphire (over the wavelength range of 300-1400 nm). Two independent birefringence measurement methods are reported, which agree with each other to within 5 × 10⁻⁶ for each material. These methods are compared to previously published values, which are within 1.5 × 10⁻⁵ for a large portion of the wavelength range measured for quartz and magnesium fluoride, and within 4 × 10⁻⁵ for sapphire. Additionally, dispersion formulae for each material are determined.

Liquid Crystal Polarization Rotators (LCPRs) have been developed for the Miniaturized Absolute Magnetometer (MAM) instrument in NanoMagSat project, an ESA’s SCOUT program mission. This project consists of a constellation of three nanosatellites aimed to study the Earth’s magnetic and ionospheric environment based on a 16U CubeSat-type structure. The MAM instrument is an optically pumped scalar and vector magnetometer derived from the ASM flown on the ESA Swarm mission. In this type of instruments, a device to rotate the direction of the incident linear polarization of the pumping beam injected into the helium-4 gas cell sensor is required. In NanoMagSat, the LCPRs will replace the sensor head rotor driven by a piezoelectric motor used in the ASM, allowing a very significant miniaturization of the sensor head. The LCPRs developed are miniaturized devices derived from the polarization modulators based on liquid crystals of PHI and METIS instruments on board the Solar Orbiter mission and optimized for the MAM instrument requirements. The key performance parameters of the devices have been evaluated in a validation test campaign, under the different environmental conditions expected in NanoMagSat, including the polarization rotation and the Polarization Extinction Ratio (PER) as a function of voltage, and the response times at the MAM polarization rotation scheme and will be presented in this work. Based on the results found, the LCPRs design and validation test campaign has been considered successful and they have been approved to be implemented for the NanoMagSat mission.

Our group in the Space Optics Department at INTA has been working during more than 20 years in the development of devices based on liquid crystals for optical payloads onboard space missions. Currently, there are three of our devices successfully in operation in the PHI and METIS instruments of the Solar Orbiter ESA/NASA mission. Therefore, they have the highest technology maturity level, TRL9 (Technology Readiness Level). To the best of our knowledge, we are pioneers of the use of polarization modulators based on liquid crystals in a telescope or a camera in a space platform.

Liquid crystal devices avoid using standard solutions that involve mechanisms with rotatory polarization optical parts. Instead, we use this technology that minimizes the size, mass and power consumption of the device while maximizing its useful aperture and performance. These new capabilities open up new possibilities for small satellites that were previously only attainable by larger satellites. Liquid crystal-based polarization modulator technology is highly versatile and can be configured in multiple ways to suit diverse applications. It is based on the ability of liquid crystal variable retarders to control, modify and measure the polarization state of light, be it in an image or in a spot beam.

The application fields are numerous, from Astrophysics to Earth Observation. This work will introduce some of the main instruments that we are working on: from the Vigil ESA mission for Space Weather to Quantum Communication Space Systems, and including the Miniature Absolute Magnetometer for the NanoMagSat mission of ESA’s SCOUT Program. Also, we will show the development status of other liquid-crystal devices for compact space instrumentation that we are developing as Liquid Crystal Tunable Filters (LCTFs) and Spatial Light Modulators (SLMs).

In October 2023, the INTA spinoff Eye4Sky was established to the exploitation and commercialization of this optical technology of liquid crystal devices for space applications. This deep-tech startup has been selected for the prestigious European Space Agency Business Incubation Centre (ESA BIC) program.

Using the information contained in polarimetric measurements to estimate the surface normals of an object or scene is a well-established computer vision task referred to as shape-from-polarization (SfP). Challenges in current physics-based SfP are 1) the assumption of an ideal specular or ideal diffuse polarized bidirectional reflectance distribution function (pBRDF), and 2) the fundamental 180◦ ambiguity in surface normal azimuth angle which is present even when an appropriate pBRDF is chosen. In this work, we demonstrate the use of single-view Mueller matrix imaging to address these two challenges. In prior work, Mueller characterization of a material is performed to create a realistic mixed specular and diffuse pBRDF. A depth map is estimated based on the polarimetric measurements and a priori knowledge of the source and camera positions. An unambiguous surface normal is independently estimated from this depth map. Although this surface normal from depth is not expected to be precise, it is used to disambiguate the surface normal azimuth angles. The mean angular error of our single-view Mueller SfP disambiguation method is 22.9° for a 20cm distance from the source to the center of the object and 34.2° for 90cm distance for a simulated SNR of 100. This is the first deterministic method known to the authors to disambiguate the surface normals from single-view polarimetric measurements.

Underwater imaging is crucial for many applications, from marine biology research to industrial inspections, oil and gas exploration, search-and-rescue operations, and defense and security. Yet, underwater imaging poses numerous challenges, including backscatter from suspended particles, light absorption, and distortion caused by the medium's varying optical properties, in which traditional imaging methods often fall short. To address these challenges, we exploit the polarization properties of light by integrating a unique polarization-demultiplexing metasurface with an imager. Both direct imaging using a conventional CCD camera and a time-of-flight single-photon counting camera are used. By correlating the polarization states of emitted and reflected light, our approach enables us to develop means to enhance image contrast and achieve a more accurate estimation of the true target depth.

Contemporary advancements in polarization fiber sensor technology offer the potential for substantial enhancements in the responsiveness of security systems, enabling their deployment even in hazardous environments. A significant advantage of these systems is their sensitivity. These fiber systems are faster and safer compared to other security sensors. The possibility of utilizing fiber sensors in environments with an explosion risk makes them irreplaceable in military facilities. This paper focuses on the analysis, design, and testing of a polarization fiber sensor in military applications. The design is focused on monitoring changes in temperature in order to detect object disturbances. Such a detector would act as a very fast, sensitive, and above all non-electrical security system. Further, the design also focuses on changes in pressure that can accompany occurrences such as the passage of military vehicles or the deflections of bridge and building structures in industrial Ex-proof dangerous areas. The amount of different arrangements and measured waveforms makes our work a unique study that confirms the universality and advantages of optical fiber polarization sensors.

This paper presents a comprehensive study on the evolution, applications, and impact of the Polarimetric Kalman Filter (PKF) in the fields of signal processing and remote sensing. By employing a methodological framework that integrates literature collection, screening and selection, bibliometric analysis, and synthesis, we aim to elucidate the PKF's contributions and potential research trajectories within its applications. Our findings highlight the PKF's significant role in enhancing data assimilation, improving predictive accuracy, and refining measurement techniques across various domains, particularly in meteorological research and environmental monitoring. The analysis reveals the interdisciplinary nature of PKF applications and its capability to merge theoretical advancements with practical implementations, underlining the importance of sophisticated signal processing algorithms in interpreting complex environmental data. The study also identifies emerging trends, such as the integration of PKF with machine learning and artificial intelligence, indicating future directions for research that promise to push the boundaries of current methodologies and applications. This paper underscores the PKF's pivotal role in advancing signal processing and remote sensing technologies, offering insights into its continuing development and the expanding scope of its applications.