This PDF file contains the front matter associated with SPIE Proceedings Volume 11833, including the Title Page, Copyright information, and Table of Contents

Cirrus clouds are important to the radiation energy budget due to their temporal duration and >50% global coverage.¹ The variety of ice crystal shapes and sizes in a cirrus cloud create challenges differentiating radiation insulated by the Earth's atmosphere from that reflected back to space. The optical thickness of these clouds is often too thin to be sensed using any current passive satellite radiometers. Sensitivity studies in the UV have shown that the angle of linear polarization (AoLP) of solar radiation backscattered from thin cirrus clouds and thin liquid water clouds is rotated.² Pust and Shaw also demonstrated subvisual clouds detection in degree of linear polarization (DoLP) and AoLP.³ An Ultraviolet Stokes Imaging Polarimeter (ULTRASIP) was designed and developed for optically thin clouds and sky observations in the 360 nm - 450 nm range.⁴ ULTRASIP is a time modulated polarimeter rotating a wire-grid polarizer in front of a 16-bit, water-cooled, back-illuminated CCD sensor. Polarized light scattering models will be compared in the visible and the UV to motivate measurements in this waveband.⁵

Cloud thermodynamic phase, whether a cloud is composed of spherical water droplets or polyhedral ice crystals, is crucial for understanding the role of clouds in climate change, weather, and optical propagation. Clouds, covering approximately 60% of the earth's surface at any given time, still contribute some of the largest uncertainties in climate science. Cloud thermodynamic phase is also required to properly retrieve other cloud properties, including cloud optical depth and particle size distributions. Cloud phase remote sensing is often done with passively measured radiance ratios or lidar cross-polarization measurements, but recent research shows that the sign of the S1 Stokes parameter can be used to detect cloud thermodynamic phase with a ground-based polarimeter. Our group has been developing ground-based polarimetric imagers to determine cloud thermodynamic phase, with lidar cross-polarization detection used as ground truth. However, because the cloud polarization is small, often on the order of a percent, accurate classification requires high polarization sensitivity. This paper reports preliminary measurements indicating feasibility of using a low-cost, commercial division-of-focal-plane polarization imager for cloud thermodynamic phase remote sensing.

The CloudCT project is a mission that aims to demonstrate 3D volumetric scattering tomography of clouds. A formation of ten nanosatellites will simultaneously image cloud fields from multiple directions, at ≈20m nadir ground resolution. Based on this data, scattering tomography will seek the 3D volumetric distribution of cloud properties. We quantitatively compare visible polarized imagers to other imagers considered for the mission. We investigated specifically visible light and short-wave infra-red imagers. Each possibility was considered using Large Eddy Simulation clouds. Major consideration criteria are tomographic quality in the face of sensor and photon noise, calibration errors and stray light. We check the sensitivity to unknown stray light and uncertainty in gain calibration.

Tomography aims to recover a three-dimensional (3D) density map of a medium or an object. In medical imaging, it is extensively used for diagnostics via X-ray computed tomography (CT). We define and derive a tomography of cloud droplet distributions via passive remote sensing. We use multi-view polarimetric images to fit a 3D polarized radiative transfer (RT) forward model. Our motivation is 3D volumetric probing of vertically-developed convectively-driven clouds that are ill-served by current methods in operational passive remote sensing. Current techniques are indeed based on strictly 1D RT modeling and applied to a single cloudy pixel, where cloud geometry defaults to that of a plane-parallel slab. Incident unpolarized sunlight, once scattered by cloud droplets, changes its polarization state according to droplet size. Therefore, polarimetric measurements in the rainbow and glory angular regions can be used to infer the droplet size distribution. This work defines and derives a framework for a full 3D tomography of cloud droplets for both their mass concentration in space and their distribution across a range of sizes. This gridded 3D retrieval of key microphysical properties is made tractable by our novel approach that involves a restructuring and partial linearization of an open-source polarized 3D RT code to accommodate a special two-step iterative optimization technique. Physically-realistic synthetic clouds are used to demonstrate the methodology with rigorous uncertainty quantification, while a real-world cloud imaged by AirMSPI is processed to illustrate the new remote sensing capability.

We present LOUPE, the Lunar Observatory for Unresolved Polarimetry of the Earth, a compact snapshot spectropolarimeter designed to observe the Earth from the Moon as if it were an exoplanet. Viewing the Earth as it would be seen by a faraway observer will offer novel insight into the spectropolarimetric signatures of planets harboring life, as well as a chance to refine algorithms for the retrieval of exoplanetary properties such as the presence of liquid water, clouds, vegetation, and more. LOUPE boasts a novel solid-state design based on patterned liquid crystal optics built atop the cosine HyperScout^®, a flight-proven hyperspectral imager. Uniquely to LOUPE, a microlens array creates a two- dimensional grid of unresolved Earth-images on the detector, resulting in an array of "pale (blue) dots" filtered spectrally along one direction, with polarization modulation applied in the perpendicular direction. The clever use of custom-patterned liquid crystals as a passive modulator thus replaces the need for classical dispersion elements and polarization modulation optics. This pioneering approach enables LOUPE to simultaneously obtain spectral and Stokes measurements for the entire Earth, whilst the position of the Earth-dots also has the benefit of providing input for angle-dependent spectral and polarization calibration. Here we discuss our detailed design process and the challenges involved in creating a unique, space-qualified spectropolarimeter with no moving parts and no bulky optics, whilst maintaining flexibility for different usage scenarios: rovers, landers, orbiters, and more. We present a performance trade-off and optical design informed by ray tracing with polarization effects, to prepare for the demodulation of simulated Earth observation data.

Despite recent advances, customized multispectral cameras can be challenging or costly to deploy in some use cases. Complexities span electronic synchronization, multi-camera calibration, parallax and spatial coregistration, and data acquisition from multiple cameras, all of which can hamper their ease of use. This paper discusses a generalized procedure for multispectral sensing using a pixelated polarization camera and Solc stages to create multivariate optical filters. We then describe some preliminary experimental results of a fabricated filtered camera system. Finally, classification of the imagery is achieved using either shallow or deep neural networks. We also discuss the potential of using a color red, green, and blue microgrid polarization camera to detect upwards of 12 spectral channels using readily available standard off-the-shelf components.

We propose a new class of computer generated holograms whose far fields possess designer-specified polarization response. We dub these Jones matrix holograms. We provide a simple procedure for their implementation using form-birefringent metasurfaces. Jones matrix holography generalizes a wide body of past work with a consistent mathematical framework, particularly in the field of metasurfaces, and suggests previously unrealized devices, examples of which are demonstrated here. In particular, we show holograms whose far-fields implement parallel polarization analysis and custom waveplate-like behavior. This work may suggest interesting new possibilities for polarimetry and optical systems generally.

This paper presents a 9-channel, spatially modulated partial Mueller matrix polarimeter that uses a spatial light modulator (SLM) to create the polarization state generator (PSG) modulation and a division of focal plane (DoFP) polarimeter as the polarization state analyzer (PSA) with a coventional 2×2 spatial modulation pattern. We demonstrate here how adapatation of the PSG modulation to the spectral structure of the scene can have significant benefits in reconstruction accuracy.

Many correlations exist between spectral reflectance and various phenotypic responses from plants. Of interest to us are structural characteristics; namely, how the various spectral and polarimetric components may correlate to underlying environmental, metabolic, and genotypic differences among plant varieties within a given species. In this paper, we overview a portable Mueller matrix imaging spectropolarimeter that has been optimized for field use. Key aspects to the design included minimizing the measurement time while maximizing signal-to-noise ratio with low systematic errors. These goals must be achieved while maintaining an imaging capability across multiple measurement wavelengths, spanning the blue to near-infrared spectral region. To this end, we will review our optimization procedure, simulations, and experimental results, including preliminary field data taken from our summer 2021 field trials.

Measuring the state of polarization of light is critical because it contains information about its source, including radiation, reflection, and any other interactions with matter. However, the traditional method to operate polarimetry relies on knowing the direction of wave propagation. Otherwise, one measures only the projection of the three-dimensional optical field onto the two-dimensional detector. This limitation, however, can be circumvented by using a reference vector field with a non-homogeneous spatial distribution of the state of polarization. Here we demonstrate that a “radially polarized” reference beam offers a simple and robust solution for a single-shot, omnidirectional polarimeter. The technique relies on intensity contrasts measured simultaneously in different orthogonal bases followed by an appropriate Fourier analysis to extract both direction and state of polarization of an incoming beam of light.

Monochromators are frequently used in spectral calibrations of optical systems due to their ability to sweep a narrowband output across a wide range of wavelengths. However, monochromators tend to output light with a degree of linear polarization that can vary significantly as a function of wavelength. To use a monochromator to calibrate a polarization-sensitive imager, the monochromator output is often passed into an integrating sphere to convert the linearly polarized light into randomly polarized light. In this paper, we demonstrate the ability to obtain a spectral calibration of a division-of-focal-plane (DoFP) imager by assuming subpixels of a polarization super pixel have equal spectral responses. We also characterize the polarization of the output of a monochromator as a function of wavelength using both a DoFP imager and a wire-grid polarizer mounted on a precision rotation stage with an optical power meter.

Optical remote sensing systems are often used to gather imagery of scenes containing partially polarized light. Partially polarized reflection or emission will affect the detected response if the sensor system has intentional or unintentional polarization sensitivity. As the use of optical remote sensing systems becomes more widespread, the factors affecting the response of these systems needs to be better understood. In this paper, we present the results of polarization response measurements of six hyperspectral imaging systems manufactured by Resonon Inc. The imagers included in this study cover wavelengths from approximately 350nm to 1700 nm, with various spectral sampling rates. Efforts are ongoing to model and compensate for the observed response.

In remote sensing, radiometric measurements taken in the mid-wave infrared and beyond (λ > 3μm) are commonly reported in units of Kelvin by utilizing Planck's radiation law to relate measured radiance and target brightness temperature (T_b). Thus, it is desirable to match this formalism in thermal polarimetry and report the unnormalized Stokes parameters in units of K instead of radiance (Wm^-2sr^-1). This approach also allows common performance metrics in long-wave infrared (LWIR) imaging such as Noise Equivalent Differential Temperature (NEDT) to be modified and extended to metrics of polarimetric accuracy and precision. However, since the relationship between measured radiance and T_b are non-linear, the conversion of I, Q, and U in units of radiance to T_b, T_b,Q, and T_b,U in K is ambiguous. As a solution a metric of performance for thermal linear Stokes polarimetry, the Stokes resolved differential temperature (SRDT), is introduced.

Materials commonly used for making waveplates or retarders all have measured performance deviations from what might be expected. These materials include crystals such as quartz, magnesium fluoride and sapphire, polymers and nematic and ferro-electric liquid crystals. Some deviations result from manufacturing errors but many are inherent properties of the birefringent materials used. These effects create systematic errors in polarimetry and in other applications where precise knowledge of polarization is important. We will discuss our quantitative measurements of the sometimes unexpected presence of elliptical retardance, retardance fringes and birefringence nonuniformities as well as other effects.

The complete Mueller polarimetry is particularly promising optical technique for the fast and non-contact characterization of complex media (e. g. layered, periodical, scattering, anisotropic and absorbing) as it provides information about all polarimetric properties of an object under study (diattenuation, birefringence, depolarization) and allows to characterize its scattering and absorption properties, surface topography and composition. We will present the results of our modeling and experimental studies in the domains of optical metrology, remote sensing and optical tissue diagnosis using number of custom-built Mueller polarimeters operating in both spectral and imaging modes and discuss further potential applications of Mueller polarimetry.

Although the concept of a snapshot Mueller matrix channeled spectropolarimeter was first presented in 2007, no experimental demonstration of such a system has yet been shown. We detail static and dynamic calibration procedures for snapshot Mueller matrix channeled spectropolarimeters, allowing for real-time adjustment of system parameters during sample property measurements. Using these calibration procedures, we provide the first experimental demonstration of Mueller matrix spectra obtained with one of these systems and review ongoing development work.

Are we alone? In our quest to find life beyond Earth, we use our own planet to develop and verify new methods and techniques to remotely detect life. Our Life Signature Detection polarimeter (LSDpol), a snapshot full-Stokes spectropolarimeter to be deployed in the field and in space, looks for signals of life on Earth by sensing the linear and circular polarization states of reflected light. Examples of these biosignatures are linear polarization resulting from O2-A band and vegetation, e.g. the Red edge and the Green bump, as well as circular polarization resulting from the homochirality of biotic molecules. LSDpol is optimized for sensing circular polarization. To this end, LSDpol employs a spatial light modulator in the entrance slit of the spectrograph, a liquid-crystal quarter-wave retarder where the fast axis rotates as a function of slit position. The original design of LSDpol implemented a dual-beam spectropolarimeter by combining a quarter-wave plate with a polarization grating. Unfortunately, this design causes significant linear-to-circular cross-talk. In addition, it revealed spurious polarization modulation effects. Here, we present numerical simulations that illustrate how Fresnel diffraction effects can create these spurious modulations. We verified the simulations with accurate polarization state measurements in the lab using 100% linearly and circularly polarized light.

SCExAO at the Subaru telescope is a visible and near-infrared high-contrast imaging instrument employing extreme adaptive optics and coronagraphy. The instrument feeds the near-infrared light (JHK) to the integralfield spectrograph CHARIS. The spectropolarimetric capability of CHARIS is enabled by a Wollaston prism and is unique among high-contrast imagers. We present a detailed Mueller matrix model describing the instrumental polarization effects of the complete optical path, thus the telescope and instrument. From measurements with the internal light source, we find that the image derotator (K-mirror) produces strongly wavelength-dependent crosstalk, in the worst case converting ∼95% of the incident linear polarization to circularly polarized light that cannot be measured. Observations of an unpolarized star show that the magnitude of the instrumental polarization of the telescope varies with wavelength between 0.5% and 1%, and that its angle is exactly equal to the altitude angle of the telescope. Using physical models of the fold mirror of the telescope, the half-wave plate, and the derotator, we simultaneously fit the instrumental polarization effects in the 22 wavelength bins. Over the full wavelength range, our model currently reaches a total polarimetric accuracy between 0.08% and 0.24% in the degree of linear polarization. We propose additional calibration measurements to improve the polarimetric accuracy to <0.1% and plan to integrate the complete Mueller matrix model into the existing CHARIS post-processing pipeline. Our calibrations of CHARIS’ spectropolarimetric mode will enable unique quantitative polarimetric studies of circumstellar disks and planetary and brown dwarf companions.

A near-monostatic laser polarimeter has been designed, built, and used to measure angular Mueller matrices characterizing the polarization-dependent reflectance of materials in support of research into polarization lidar. Here we explain the system, show that it has qualitatively similar results as previous complex systems, and show example measurements for red brick, concrete, sheetrock, tar shingles, milled aluminum, and plain steel.

The bidirectional reflectance distribution function (BRDF) of surfaces has deleterious effects on optical measurements. Collecting Mueller matrix BRDF (mmBRDF) measurements of a surface by conventional goniometric techniques can be time-consuming. We present a system for collecting mmBRDF measurements using optical fiber detectors that sample the hemisphere surrounding an object. The entrance to each fiber contains a polarization state analyzer (PSA) configuration which allows for the simultaneous acquisition of the Stokes vector at many altitudinal and azimuthal viewing positions. We describe the setup, calibration, and data processing for this system and present its performance as applied to mmBRDF measurements of maize leaves.

Polarization phenomenology is particularly complex at electro-optical and infrared wavelengths. The observed polarization state is dependent upon material surface characteristics, shape, chemistry, sensor geometry, relative position of any illumination sources, relative temperatures of scene and background objects, atmospheric conditions, and the presence and temperature of objects within the sensor field of view. First principles physics-based models often require full material characterization in the form of the polarimetric bidirectional reflectance distribution function (pBRDF). While pBRDF measurements can be reliably obtained for small target samples under controlled laboratory conditions, they are more challenging to obtain for many remote sensing targets of interest. Furthermore, in outdoor settings it is difficult to control pBRDF parameters such as atmospheric conditions, solar illumination position, and polarization state. The result is that pBRDF measurements are simply not available for many materials of interest at the level of fidelity required by physics-based models. Moreover, the level of accuracy capable with physics-based modeling tools is often not necessary for many tasks. For experiment or mission planning purposes, it is desirable to have a rough idea of expected polarization signatures for a given material class, time of day, and sensor look angle. Having simplified models that are capable of predicting expected polarimetric signatures, even at a low fidelity, is of high utility for many applications. Here we present the initial framework of such a model based upon empirical data measurements. Results are generated for several material classes with corresponding validation against physics-based models. We show that our measured Stokes vector and DoLP values are within expected physical bounds for 96% of the measured data and generally agree with truth results.

Recent research in developing methods for visualizing polarimetric images are implemented in Python modules now available in public repository. In addition to implementing the set of current visualization typically used for polarimetric imaging, the recently developed methods that are implemented involve depicting polarimetric variables using color coordinates from a uniform color space. These methods can be used both linear and circular polarization, and can be used to depict one, two, or three polarimetric variables in a single image. Collaboration on Github is encouraged for implementing new ideas as well as translating the modules into other programming languages.

Light in nature addresses the optical phenomena we see in our everyday lives, ranging from rainbows and auroras to insect wing colors. Explanations of these phenomena draw from fields that include optical science, physics, engineering, chemistry, biology, astronomy, photography, and art. What is light? How do the beautiful and often colorful features in nature appear? What are the best ways of observing and studying these things? How does human vision affect what we see with these phenomena? Join this virtual live event panel discussion for an exploration into light in nature.

This presentation video was recorded for the SPIE Optics + Photonics 2021 symposium

This presentation video was recorded for the SPIE Optics + Photonics 2021 symposium