This paper presents an overview of polarimetric thermal imaging for biometrics, focusing on face recognition, with a short discussion on fingerprints and iris. Face recognition has been and continues to be an active area of biometrics research, with most of the research dedicated to recognition in the visible spectrum. However, face recognition in the visible spectrum is not practical for discrete surveillance in low-light and nighttime scenarios. Polarimetric thermal imaging represents an ideal modality for acquiring the naturally emitted thermal radiation from the human face, providing additional geometric and textural details not available in conventional thermal imagery. One of the main challenges lies in matching the acquired polarimetric thermal facial signature to gallery databases containing only visible facial signature, for interoperability with existing government biometric repositories. This paper discusses approaches and algorithms to exploit polarization information, as represented by the Stokes vectors, through feature extraction and nonlinear regression to enable polarimetric thermal-to-visible face recognition. In addition to cross-spectrum feature based approaches, crossspectrum image synthesis methods are discussed that seek to reconstruct a visible-like image given a polarimetric thermal face image input. Beyond facial biometrics, this paper presents an initial exploration of polarimetric thermal imaging for latent fingerprint acquisition. Latent prints are formed when the oils and sweat from the finger are deposited onto another surface through contact, and are typically collected by first dusting with powder before being imaged and then lifted with adhesive tape. This paper presents polarimetric thermal imagery of latent prints from a nonporous glass surface, acquired without the dusting process. A brief discussion of the utility of polarimetric thermal imaging for iris recognition is also presented.

Short-wave infrared light is weakly absorbed by human epidermal and dermal tissue. Investigation of polarization characteristics of human skin taken in vivo distinguishes how depolarization index and the Mueller matrix are markers that can identify skin from a rough Lambertian surface material. A custom Near-Infrared imaging Mueller Matrix Polarimeter using an expanded, coherent 1550nm laser beam source is used to analyze the skin on the back of hands as a subject.

We report on Mueller matrix bidirectional reflectance distribution function (BRDF) measurements of a sintered polytetrafluoroethylene (PTFE) sample over the scattering hemisphere for incident angles from 0° to 75° and for four wavelengths from 351 nm to 1064 nm. The data are fit to a radiative transfer equation solution for a diffuse medium, letting parameters describing the single scattering phase function be adjusted.

Measured extinction ratio on high quality linear polarizers depends on test system geometry. The measurement becomes especially challenging for polarizers with extinction ratio expected to exceed 10⁶. We describe methods capable of measurements of high purity polarizers at and above 10⁶ extinction ratio. We discuss the geometrical factors affecting the measured results that may be pertinent in determining what performance is achievable in a users system. We describe methods for computing extinction ratios without an absolute reference perfect polarizer with infinite extinction ratio and for rank ordering performance of a set of several polarizers. Measurement results are presented for several high performance polarizers including Glan-Thompson polarizers, dichroic glass polarizers and Meadowlark Optics Ultra Broadband Polarizers.

The ability to accurately rotate the polarization of incident light while minimizing any losses in polarization purity has applications in optical switching, polarimetry, and microscopy. Polarization rotators utilizing tunable birefringent plates, such as liquid crystal (LC) devices, have the advantage of non-mechanically tuning the devices' retardance. However, these devices properly work with incident light within a very specific wavelength range. Ferroelectric liquid crystal (FLC) devices can switch between two orthogonal states of linear polarization, and offer response times much faster than their nematic liquid crystal cell counterparts. An achromatic polarization rotator can be constructed with an FLC cell between two half-wave plates that have been constructed to produce a half-wave retardance at a certain design wavelength. This results in a device that offers fast response times and high polarization purity over a broader wavelength range.

The significant effects of aerosols on public health and climate drive a growing necessity for the characterization of particulate matter in air pollutants. The earth-orbiting Multi-Angle Imager for Aerosols (MAIA) instrument will combine radiance and polarization measurements to derive features of ground-level particulate matter. The optical design requires an in-depth analysis of several key polarization factors in order to meet stringent requirements on radiometric and polarimetric accuracy. A simple two-layer optical coating, a pair of achromatic quarter-wave plates, and analysis of the polarization aberrations are used to minimize the effects of polarization errors and achieve high polarimetric accuracy.

We present an alternative technique for channeled polarimetry which uses longitudinal spatial coherence interferometry to encode a scene’s angularly dependent polarization information onto the source’s power spectrum. By spectrally resolving the output using a spectrometer, a channeled spectrum is measured. Fourier transformation of the channeled spectrum in combination with reference beam calibration techniques yields a reconstruction of the incident scene’s angular Stokes parameters. Experimental validation of the technique is demonstrated, using a Fabry-Perot etalon, for the recovery of one-dimensional linearly polarized scenes.

We report on the development of an acousto-optic tunable filter (AOTF) based novel spectrapolarimeter system operating over the visible and near-IR spectral bands to extract Stokes and Mueller matrices. Developed primarily for planetary composition and analysis applications, the wavelength tunable polarimetric system is configured with tellurium dioxide based AOTF and liquid crystal based variable retarders (LCVR) with no movable mechanical parts. Fitted with a standard silicon camera for operation up to 900 nm and an extended InGaAs camera for operation up to 2500 nm, the system is currently configured for passive operation. The AOTF, with a typical spectral bandwidth of 10 nm, is electronically tunable over a selected spectral band. The optical system with matched beam conditioning optics is mounted in a linear configuration using custom mechanical fixtures. Operational software with custom algorithms integrates the drivers of the AOTF and LCVR devices and allows for rapid display of stokes images over a selected spectral band. The operation of this spectrapolarimetric system is fully automated with an interactive and user friendly graphical user interface, and accordingly provides a snapshot polarimetric measurement capability in minutes. The system is compact, portable and battery operated, and is currently suitable for field operations and ultimately for remote sensing from space platforms. In this paper, we discuss its optical design configuration and system performance in detail along with spectrapolarimetric test results and analysis of selected mineralogical samples.

Polarization information is abundant in nature, including the underwater environment. Polarization of light in the underwater environment is due to light coming from both the sun and from the sky. Hence, the underwater polarization is primarily determined by light’s transmission from air to water and in-water scattering. In this talk, we will present a new framework to solve sun’s position (heading and elevation) using background underwater polarization information. Based on this data, the underwater geo location of an observer can be determined passively. Extensive experimental data will be presented in the talk to demonstrate the accuracy of this method.

Multi-spectral and polarization imaging have enabled and exploited a wide range of applications, from remote sensing to biomedical applications such as early cancer detection for image-guided surgery. However, state-of-the-art multispectral and polarization cameras are still based on conventional advances in optics and integrated circuits, yielding bulky form factors and poor signal reconstruction. Thus, these technologies have failed to be adopted as research or clinical imaging tools. Nature is full of examples of animals that take advantage of multi-spectral and polarization phenomena to gain an evolutionary advantage. For example, elegant low-power and compact biological visual systems, capable of multispectral and polarization imaging surpassing any man-made imaging system, can be found in the compound eyes of many arthropods. Here, we demonstrate radically novel, multi-spectral and polarization imaging sensors that function on the same fundamental principles as do the ommatidia of the mantis shrimp. Our bio-inspired imaging systems combine vertically stacked photodiodes, for single-pixel trichromatic vision, with an array of pixelated polarization filters, resulting in compact and low-power architectures. Our single-chip imager comprises of 1280-by-720 pixels, yielding a 62 dB and 48 dB dynamic range and signal-to-noise ratio, respectively, and operates at a maximum frame rate of 24 fps. This topology inherently co-registers in time and space the different spectral and polarization channels. This novel and ergonomic technology is enabling real-time in situ underwater polarization imaging as well as applications in biomedical fields.

IERUS Technologies investigated the feasibility of developing a high resolution, passive MWIR polarimetric imaging system for both day and night operation at short (1 – 5 meters) and long (1 – 2 km) range operation. The sensor system used a micro-polarizer array (MPA) over the focal plane array (FPA) in order to capture four channels of polarimetric information simultaneously. It also used an optical registration array (ORA) over the MPA in order to spatially register the polarimetric information. The MPA-ORA device is integral to the FPA, forming a drop-in-replacement, saving system size and weight relative to other polarimetric imaging technologies. A system was designed for a prototype that mitigates risk and demonstrates the utility of the ORA. The FPA employed is a MWIR array with a reticulated detector array which reduces electrical pixel-to-pixel crosstalk to zero. Polarization and radiometric performance predictions of the design will be presented.

In this paper, we purpose interpolation technique for a 3-micropolarizer design of division of focal plane (DoFP) image sensor based on polarization residuals. The super pixel of 3-micropolarizer division of focal plane (DoFP) polarization image sensor records the first three (S₀, S₁, S₂) at each frame but each specific pixel within the superpixel has a somewhat different field of view. The missing polarization information creates edges and nonconformities in the polarized images. The micropolarizer consists of a 3-micropolarizer filter array of 0°, 90°, and 135°. The DoFP image sensor output image remains 2D. We will demonstrate the performance of the algorithm on the intensity, degree of linear polarization (DoLP) and angle of linear polarization (AoP) images. We will further demonstrate that our proposed algorithm can estimate suitable missing polarization information for low-resolution images to generate high-resolution images.

We report on the design, modeling, calibration, and experimental results of a LWIR, spectrally and temporally resolved broad band bi-directional reflectance distribution function measuring device. The system is built using a commercial Fourier transform infrared spectrometer, which presents challenges due to relatively low power output compared to laser based methods. The instrument is designed with a sample area that is oriented normal to gravity, making the device suitable for measuring loose powder materials, liquids, or other samples that can be difficult to measure in a vertical orientation. The team built a radiometric model designed to understand the trade space available for various design choices as well as to predict instrument success at measuring the target materials. The radiometric model was built by using the output of commercial non sequential raytracing tools combined with a scripted simulation of the interferometer. The trade space identified in this analysis will be presented. The design was based on moving periscopes with custom off axis parabolas to focus the light onto the sample. The system assembly and alignment will be discussed. The calibration method used for the sensor will be detailed, and preliminary measurements from this research sensor will be presented.

We numerically and experimentally explore a new scanning LADAR architecture that enables Mueller matrix measurement for each point in a scene using a single ~10 ns illumination laser pulse. For the transmitter, we direct the laser pulse through an electro-optic crystal wherein a high-voltage ramp is synchronously applied. As the laser pulse propagates through the crystal the high-voltage ramp induces a time varying birefringence which results in a time varying polarization across the temporal envelope of the laser pulse. The receiver channel can be designed to measure the full Mueller matrix of the target by employing three polarization state analyzers or a subset of the Mueller matrix by employing fewer analyzers. This transmitter produces a well-defined temporal polarization variation which is used to illuminate the target. Knowing the temporal distribution of the transmitted polarization signal and using polarization analyzers in the receiver chain to measure the temporal distribution of the return signal’s polarization one can measure the entire Mueller matrix. We will introduce a model describing the transmitted and received signal polarization temporal distribution and show how the Mueller matrix can be extracted from this information. We will demonstrate the concept using a 10 ns, 1.06 μm laser pulse and a lithium tantalate electro-optic phase retarder in the transmitter and using two polarization analyzers in the receiver chain to measure a subset of the Mueller matrix in a point and shoot configuration. Measurements will be compared to theoretical reference to assess accuracy.

Multi-domain modulated polarimeters combine carriers on different domains to exploit the bandwidth of the measurement system. However, the inevitable systematic errors in polarimeters will degrade their bandwidth performance, so we developed a new type of multi-domain modulated polarimeter. Compared with conventional polarimeters and our previous separable designs, this new type of system can avoid some of the negative effects (such as the emergence of extraneous channels) caused by the systematic errors. To illustrate the advantages and disadvantages of both systems, both types of Stokes polarimeters are designed based on the same channel structure and their performance is simulated under systematic errors.

We compare four passive polarization imaging configurations by quantitatively assessing their target detection performance for different kinds of noise. Through closed-form expressions we determine the gain of these configurations compared to intensity imaging in the case of target/backrgound discrimination. For three of these configurations we show that a minimum amount of polarimetric contrast between the target and the background is required to outperform intensity imaging. We show that the only configuration that has always better performance requires to use a polarizing beamsplitter and assumes that the main source of perturbation is the background shot noise. This work has interesting perspectives for imaging architecture design.

On 21 August 2017 we measured skylight polarization during a total solar eclipse in Rexburg, Idaho, using two all-sky polarimetric imagers. The all-sky polarization images were recorded using three simultaneously operating digital singlelens-reflex (DSLR) cameras with good low-light sensitivity. Each camera was equipped with a 180° field-of-view fisheye lens to view the entire sky and each lens contained a fixed linear polarizer orientated at 0° , 60° , and 120° , respectively, to recover the first three Stokes parameters. Skylight polarization was measured from sunrise to sunset in the cameras’ blue, green, and red channels. Before and after totality, the maximum sky polarization occurred in its usual pattern with a band of maximum polarization positioned 90° from the sun. However, during totality skylight polarization became nominally symmetric about the zenith. This was observed clearly in the blue and green channels and less obviously in the red channel, which had a greatly diminished signal. At and near the observation site, we also operated an infrared cloud imager, a hand-held spectrometer to measure surface reflectance, and an AERONET solar radiometer to characterize the atmospheric aerosols. This ancillary data set provided a complete characterization of the conditions of the surrounding atmosphere and underlying surfaces.

A full sky imaging spectropolarimeter that measures spectrally resolved (~2.5 nm resolution) radiance and polarization (s₀, s₁, s₂ Stokes Elements) over approximately 2π sr between 400nm and 1000nm will be used to quantitatively characterize the spectral dependence of the polarization state of the sunlight scattered in the sky. The sensor is based on a scanning push broom hyperspectral imager configured with a continuously rotating polarizer (sequential measurement in time polarimeter). This study will help optimize sky polarimetry by offering information that can be used to select the best spectral band (or which spectra to reject) for a given application. Findings to be presented are sky maps of the angle of polarization and degree of polarization for different spectral bands, spectral dependency of degree (and angle) of polarization, and example data sets supporting each.

In almost every practical scenario the light reflected from a surface is scattered in the atmosphere before it reaches a sensor. While this effect can be a little annoying for the amateur photograph trying to take a picture, it can have disastrous consequences for unmanned autonomous vehicle navigating through fog for instance.

By employing an innovative method based on a clever combination of spectral bands and polarization analysis, coupled with advanced image processing techniques, significant improvements have been achieved on fog obscurant using the existing passive full Stokes polarization imaging camera for visible light “SALSA” (developed by Bossa Nova Technologies).

Knowing the thermodynamic phase of a cloud–whether it is composed of spherical water droplets or polyhedral ice crystals–is critical in remote sensing applications and in climate studies. We recently showed that we can determine cloud phase with visible-wavelength sky polarimetry, and in this presentation we extend that method to shortwave infrared wavelength bands near 1.6 microns. We describe the instrument, a passive, three-channel polarimeter with spectral bands at 1550 nm, 1640 nm, and 1700 nm with approximate width of 40 nm and how we are using it in experiments to discriminate between liquid-water and ice clouds. This portable polarimeter measures scattered sunlight using polarizers orientated at 0° , 45‡ , and 90° with respect to the solar vertical scattering plane. It has a 4.9° field-of-view and a motorized, computer-controlled pan-and-tilt mount that controls the positioning of the polarimeter so that it can measure any point in the sky.

A simulation study was conducted for the purpose of identifying technology improvements for an acquisition sensor for the detection of small objects in clear, sunlit cloud, fog, and mist conditions. Currently available mid-wave infrared (MWIR) and long-wave infrared (LWIR) technologies were studied. In addition, projected sensor technologies anticipated to be available in the near future, as well as idealized systems limited only by aperture size, integration time and instantaneous field of view (IFOV) were modeled. Both standard and polarimetric imaging sensors were included in the study. The Aero-Optical Prediction Tool (AerOPT) was used to model the performance of various sensors operating under the conditions of interest. Results indicate that LWIR systems may extend detection range in fog and mist environments and that polarimetry may reduce false alarm rate for sunlit cloud backgrounds. Importantly, polarimetric imaging does not appear to negatively impact detections.

Heavy fogs and other highly scattering environments pose a challenge for many commercial and national security sensing systems. Current autonomous systems rely on a range of optical sensors for guidance and remote sensing that can be degraded by highly scattering environments. In our previous and on-going simulation work, we have shown polarized light can increase signal or range through a scattering environment such as fog. Specifically, we have shown circularly polarized light maintains its polarized signal through a larger number of scattering events and thus range, better than linearly polarized light. In this work we present an active polarization imager in the short-wave infrared. We explore multiple polarimetric configurations for the imager, focusing on linear and circular polarization states. We also describe initial testing of the imager in the Sandia Fog Facility. The Sandia Fog Facility is a 180 ft. by 10 ft. chamber that can create fog-like conditions for optical testing. This facility offers a repeatable fog scattering environment ideally suited to test the imager’s performance in fog conditions.

Visualizing polarimetric imaging data is a difficult task due to its multidimensional nature, and there have been many different approaches to develop techniques for displaying this information. Currently, there is no method for producing effective visualizations, or evaluating their performance in accomplishing their intended goals. A task-based design process can be used to make sure that the unavoidable biases that occur in these visual representations match the biases required for effectively interpreting the information, relationships, and features within the data. As the field of polarimetric imaging grows and extends into other fields, some standardization of effective visualization techniques may be beneficial in communication and continued growth.

Polarimeters have broad applications in remote sensing, astronomy, and biomedical imaging to measure the emitted, reflected, or transmitted state of polarization (SOP). An Intrinsic Coincident full-Stokes Polarimeter (ICP) was previously demonstrated by our group, in a free space configuration, by using stain-aligned polymer-based organic photovoltaics (OPVs). These were tilted to avoid back-reflection cross-talk. In this paper, we present a theoretical model of a monolithic ICP which considers the back-reflection’s influence. This includes a comparison between the free space model to the new monolithic model. Experimental demonstrations yield less than 3% error between our model and the experiment data.

It is known that LADAR imaging can characterize reflective properties of a scene and provide high resolution threedimensional spatial information useful for target classification; however, scanning and processing high resolution LADAR data is extremely time and computational resource consuming. In remote sensing applications, polarization sensitive imagery can improve target-clutter discrimination of man-made objects in a natural background and anomaly detection algorithms have been shown to accurately identify areas of interest in low resolution imagery. In this paper, we investigate the possibility of enabling passively augmented LADAR for target detection by utilizing polarimetric thermal imagery to cue high resolution LADAR scans of anomalous regions of a scene. A statistical outlier detection algorithm is explored with features extracted from passive polarimetric LWIR imagery collected on an outdoor range under various conditions. The data collection process and products are discussed as well as the performance of anomaly detection algorithms for LADAR cueing. In both data collection and image processing, foliage penetration of partially hidden targets is considered. Data analysis shows polarization information of paired systems improves true positive rate and target detection rate with an acceptable false positive rate while greatly reducing LADAR scan time. As a result, a spatial clustering and anomaly ranking system is introduced to prioritize the most likely anomaly among multiple detections; minimizing time consumed performing LADAR scanning and processing.

A miniaturized long-wave InfraRed (LWIR) spectro-polarimeter is being developed as a prototype for the Compact Submm-Wave and LWIR Polarimeters (SWIRP) project. The polarimeter in development is a compact (20x20x40 cm) conical-scan instrument to measure the polarimetric radiation from ice cloud scattering at mm- submm (220 and 680 GHz) and IR (8.6, 11, and 12 m) bands. The LWIR polarimeter will provide a series of polarization measurements across the 8.5 - 12.5 micron band, measuring the full set of linear Stokes parameters (I, Q, U) as a function of wavelength. The spectro-polarimeter uses a combination of birefringent crystals, a Wollaston prism, a diffraction grating, and an uncooled microbolometer array to measure both the degree and angle of linear polarization across the spectral bandwidth by modulating the polarization flux in wavelength with a high order retarder.

Detection of targets can be difficult using thermal camera during thermal cross-over periods. Under these conditions, the target takes on the apparent temperature of objects in the foreground or background and becomes undetectable. It is commonly said that the thermal imagery is “washed-out”. When the thermal contrast of an object against its background is zero, many times a polarization contrast of the same object is non-zero. In this paper, we introduce a camera that measures thermal and polarization images in both the mid-wave infrared (MWIR) and long-wave infrared (LWIR). We also show example images and derive a simple equation that explains the conditions under which a polarization signature of an object can be expected.

Recent interest in understanding the ability of zebras to deter biting flies has generated research into understanding the mechanisms by which these flies fail to target the striped animal. Several theories exist, including the model that the polarization properties of scattered light from their coats may play a role. Here, we report on the analysis of Equus zebra hartmannae and a commercially available fly deterrent sheet, characterized in the visible spectrum. Results contribute to a quantitative understanding, analysis/interpretation of polarotaxis and the development of potential applications.