This PDF file contains the front matter associated with SPIE Proceedings Volume 8364, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

We have developed a tool to simulate reconstruction behavior of a snapshot Mueller matrix channeled spectropolarimeter in presence of noise. A shortcoming of channeled spectropolarimeters is that with a large number of channels, each channel has to be narrow, which limits the reconstruction accuracy and provides a bandlimit constraint on the object. The concept of making partial Mueller matrix measurements can be extended to a channeled system by considering polarimeter designs that make irrelevant Mueller matrix elements unreconstructable, while decreasing the number of channels and subsequently increasing the bandwidth available to each channel. This tool optimizes the distribution of the available bandwidth towards the polarization elements that we care about most. A generic linear systems model of a spectropolarimeter with four variable retarders allows us to construct a matrix that maps Mueller matrix elements into corresponding channels. A pseudo-inverse of that matrix enables the reconstruction of Mueller matrix elements from channels. By specifying a mask vector, we can control the subjective importance of each of the reconstructed elements and weigh their error contribution accordingly. Finally, searching the design space allows us to find a design that maximizes the Signal-to-Noise-Ratio (SNR) for a specific partial Mueller matrix measurement task.

Modulated imaging Stokes polarimeters require processing of acquired data to produce an estimate of the Stokes parameters from the scene. The total polarimeter operator describes the estimation of the Stokes parameters from the incident fields from the scene through reconstruction. In this discussion will shall consider the polarimeter being applied to an application where the spectral density matrix of the scene Stokes parameters and detector noise are known. The spectral density matrix of the estimated Stokes parameters is found using the known spectral density matrix of the scene to find the response of the operator to signal fluctuations. This analysis grants the ability to optimize the operator for a given application. We demonstrate an optimization of system processing algorithm that takes inspiration from the classical Wiener filter.

Most of imaging polarimeters in the field measure only a few components of the Mueller matrix or their combinations such as Stokes vector, degree of linear polarization (DOLP) and degree of circular polarization (DOCP). Our imaging polarimeter was similar in that it produced two combinations of 16 Mueller components. We upgraded our polarimeter to acquire the Mueller matrix of a scene in the field (Mueller image). Scenes consisted of flat plates mounted on a large panel, a large cylinder, and natural background such as trees and grass. We established a formula to derive Mueller images from the measurements with our instrument. Mueller images provided comprehensive information about the polarization effect on any targets in the scene, which were useful in distinguishing man-made objects from natural background. In addition, Mueller images enabled us to emulate some images by imaging polarimeters with limited capability. Comparison of those images with Mueller images provided an insight on the effectiveness and shortcomings of the associated imaging polarimeters.

A new remote sensing approach based on polarimetric wavelet fractal detection principles is introduced and the Mueller matrix formalism is defined, aimed at enhancing the detection, identification, characterization, and discrimination of unresolved space objects at different aspect angles. The design principles of a multifunctional liquid crystal monostatic polarimetric ladar are introduced and related to operating conditions and system performance metrics. Backscattered polarimetric signal contributions from different space materials were detected using a laboratory ladar testbed, and then analyzed using techniques based on wavelets and fractals. The depolarization, diattenuation, and retardance of the materials were estimated using Mueller matrix decomposition for different aspect angles. The outcome of this study indicates that polarimetric fractal wavelet principles may enhance the capabilities of the ladar to provide characterization and discrimination of unresolved space objects.

A tunable mid-wave infrared Mueller matrix scatterometer was recently developed. Initial design efforts were directed at using existing non-achromatic retarders and leveraging the flexibility of the measurement matrix method for dual rotating retarders. However, insufficient retardance existed among the non-achromatic retarders (~λ/5 at 4.35μm), resulting in high condition numbers and large errors in free-space Mueller matrix extractions. Condition number analysis and random error analysis was applied to the measurement matrix method and led to the selection of achromatic λ/3 retarders in the final design. This establishes a near-optimum configuration and the opportunity to develop and test measurement matrix method calibration techniques.

This paper describes the development of a new instrument for calibrating satellite imaging sensors - the Polarization Hyperspectral Image Projector (PHIP). The PHIP instrument is capable of producing realistic standards-based satellite imagery, simultaneously projecting spectral, spatial and polarization scenes. The feasibility study outlined here demonstrates that liquid crystal devices are capable of producing arbitrary polarization states. Boulder Nonlinear Systems is currently developing a complete spectral/spatial/polarization instrument to be delivered to NASA in 2013.

This work considers the implementation of polarimeters with liquid crystal (LC) cells as polarizing elements. Most works generally try to implement architectures with one or two pure retarding modulators such as nematic devices. In this case, rather thick LC devices able to provide a 2π retardation are generally used. Unfortunately, LC device switching speed is known to evolve as the inverse square of their thickness, which leads to practical implementations limited to a few tens of Hertz in the visible region. The alternative consisting in using much faster devices made of ferroelectric liquid crystals is not that obvious since these devices often operate in bistable mode. We show that using thinner, therefore faster nematic devices is possible with a minimal penalty in terms of performance. Therefore, several solutions can be considered. Performance evaluation will be performed through studying the system matrix condition number.

We have developed a Vector Radiative Transfer (VRT) code for coupled atmosphere and ocean systems based on the successive order of scattering (SOS) method. In order to achieve efficiency and maintain accuracy, the scattering matrix is expanded in terms of the Wigner d functions and the delta fit or delta-M technique is used to truncate the commonly-present large forward scattering peak. To further improve the accuracy of the SOS code, we have implemented the analytical first order scattering treatment using the exact scattering matrix of the medium in the SOS code. The expansion and truncation techniques are kept for higher order scattering. The exact first order scattering correction was originally published by Nakajima and Takana.¹ A new contribution of this work is to account for the exact secondary light scattering caused by the light reflected by and transmitted through the rough air-sea interface.

The calibration of a visible polarimeter is discussed. Calibration coefficients that provide a complete linear characterization of a polarimeter are represented in this paper by the analyzer vector, where sensor response in counts is given by the dot product of the analyzer vector and the incoming Stokes vector. Using the analyzer vector to represent the effect of the sensor on the incoming Stokes vector, we can include elements of the calibration Stokes vector in the fit used to estimate the analyzer vectors/calibration coefficients. This technique allows us to alleviate some of the strict requirements usually levied on the source used to generate the calibration Stokes vectors, such as source temporal stability. Data will be shown that validate the resultant analyzer vectors/calibration coefficients, using a novel technique with a tilted glass plate. A discussion of how these techniques are applied to IR sensors will also be touched on.

In imaging polarimetry, special consideration must be given to ensure proper spatial registration between frames. Edge artifacts caused by the differencing of unregistered frames has the potential to create significant spurious polarization signatures. To achieve 1/10^th pixel registration or better, a software based registration approach is often required. The focus of this paper is to present an efficient algorithm for real time sub-pixel registration in a division-of-time IR polarimeter based on a rotating polarizer. This algorithm has been implemented in a commercially available rotating polarizer LWIR imaging polarimeter offered by Polaris Sensor Technologies. This paper presents measurements of image nutation in a rotating polarizer LWIR imaging polarimeter and real-time registration of image data from that same polarimeter. The registration algorithm is based on an optimal 2D convolution. Examples of registered images are provided as well as estimates of residual misregistration artifacts.

A polarimeter capable of measuring the complete Mueller matrix of highly scattering samples in transmission and reflection from 300 to 1100 nm has been constructed and tested. Exploratory research has been conducted which may lead to the standoff detection of bio-aerosols in the atmosphere. The polarization properties of bsubtilis (surrogate for anthrax spore) have been compared to ambient particulate matter species such as pollen, dust and soot (all sampled onto microscope slides) and differentiating features have been identified. The application of this technique for the discrimination of bio-aerosol from background clutter has been demonstrated.

Polarimetric image classification is sensitive to object orientation and scattering properties. This paper is a preliminary step to bridge the gap between visible wavelength polarimetric imaging and polarimetric SAR (POLSAR) imaging scattering mechanisms. In visible wavelength polarimetric imaging, the degree of linear polarization (DOLP) is widely used to represent the polarized component of the wave scattered from the objects in the scene. For Polarimetric SAR image representation, the Pauli color coding is used, which is based on linear combinations of scattering matrix elements. This paper presents a relation between DOLP and the Pauli decomposition components from the color coded Pauli reconstructed image based on laboratory measurements and first principle physics based image simulations. The objects in the scene are selected in such a way that it captures the three major scattering mechanisms such as the single or odd bounce, double or even bounce and volume scattering. The comparison is done between visible passive polarimetric imaging, active visible polarimetric imaging and active radio frequency POLSAR. The DOLP images are compared with the Pauli Color coded image with |HH-VV|, |HV|, |HH +VV| as the RGB channels. From the images, it is seen that the regions with high DOLP values showed high values of the HH component. This means the Pauli color coded image showed comparatively higher value of HH component for higher DOLP compared to other polarimetric components implying double bounce reflection. The comparison of the scattering mechanisms will help to create a synergy between POLSAR and visible wavelength polarimetric imaging and the idea can be further extended for image fusion.

Image processing is a field of great interest for many applications. Nowadays it is very hard to name an application where image processing is not involved. Digital techniques remains the dominant ones applied to digital image processing with significant automation approaches that are built in image display, as in most digital cameras and digital TVs, to name few. Depending on the application, digital image processing techniques produces satisfactory accurate results. However, digital enhancement techniques suffer from the main constraint: slow processing speed, an inherited problem associated with any digital image processing technique. On the other hand optical image enhancement techniques such as the polarization-based ones produce satisfactory accurate results and at the same time overcome the processing time constraint associated with their digital counter ones. This paper presents a comparison between digital and polarization-based enhancement/encoding techniques with respect to their accuracy, security and processing time in automated pattern recognition applications.

Achromatic wave plates are useful in various mid-IR applications, such as analyzing or controlling the spectrum available from CO₂ and other lasers, and for the study of IR spectra from distant stars. Their production relies upon the technical skills of those who grow the required high quality crystals and upon those who fabricate the optical parts to the needed precision. Two materials are described - one useful for light in the spectral range of the visible through the near IR and another that functions well in mid-IR applications from 2.5 μm to 11.5 μm. Some limitations imposed by inherent material properties will also be discussed.

Wiregrid polarizers are commonly employed as optical components in polarization sensitive imaging systems in the infrared waveband. Achieving acceptable performance from wiregrid polarizers typically requires small feature sizes and small periods, large aspect ratios, and subtle control over duty cycle. In many cases, the metrics mentioned above can be realized with manufacturing techniques developed in the semiconductor industry. However, metrology techniques commonly utilized in the semiconductor industry are not necessarily conducive to measuring the effective performance across a large substrate. These techniques typically allow testing or inspection of only very small scale representations of the subwavelength features on the wiregrid polarizers. These techniques - for example the scanning electron micrograph, or SEM - may also damage the wiregrid polarizer. In this paper we present a non-destructive optical imaging method for measuring the performance of the entire infrared wiregrid polarizer produced on a 200mm substrate. This test method allows the users to see large scale errors present during the fabrication process that may not be visible with other metrology techniques. In addition, this technique directly correlates polarizer performance to manufacturing errors.

Existing Division-of-Focal-Plane (DoFP) polarization sensors are capable of detecting intensity and polarization information but ignore spectral information. We present a novel integrated DoFP polarimeter that can simultaneously perceive spectral, polarization and intensity information with high spatial resolution at every frame. The sensor was realized by integrating aluminum nanowire polarization filters at the focal plane of an image sensor that has a vertically stacked photodiode structure in each pixel. The sensor has been optically characterized over a range of intensities, incident polarization angles and wavelengths. Results from the optoelectronic characterization as well as real-life spectral-polarization images obtained from the sensor are presented.

We propose an interpolation algorithm for Division-of-Focal-Plane (DoFP) polarimeters based on the correlation between neighboring pixels. DoFP polarimeters monolithically integrate pixelated nanowire polarization filters with an array of imaging elements. DoFP sensors have been realized in the visible and near-infrared regime. The advantages of DoFP sensors are twofold. First, they capture polarization information at every frame. Second, they are compact and robust. The main disadvantage is the loss of spatial resolution due to the super-pixel sampling paradigm at the focal plane. These sensors produce four low-resolution images, where each image has been recorded by a linear polarization filter offset by 45 degrees. Our algorithm addresses the loss of spatial resolution by utilizing the correlation information between the four polarization pixels in a super-pixel configuration. The method is based on the following premise: if one or more of three polarization parameters (angle of polarization, degree of polarization, and intensity) are known for a spatial neighborhood, then the unknown pixel values for the 0° image, for example, can be computed from the intensity values from the 45°, 90° and 135° images. The proposed algorithm is applied to select cases and found to outperform the bicubic spline interpolation method.

A division of focal plane (DoFP) micro grid polarizer array (MGPA) has been characterized. The MGPA under test is a commercial device available from Moxtek Inc. These wire grid style polarizers use aluminum lines fabricated on a glass substrate and have opaque regions surrounding individual pixels. Our approach to testing the MGPA has been to reimage them onto a detector by placing the MGPA at an intermediate focal plane. For the purposes of characterizing the MGPA, a high magnification reimaging optical system was assembled. The oversampled MGPA pixels were examined by using an adjustable analyzing polarizer. The effects of pixel throughput and cross talk are examined as a function of both wavelength and illumination f/#. A calibration procedure has been determined for the use of such devices. The MGPA array was also examined using a scanning electron microscope (SEM). From these SEM measurements, the pitch, fill factor, and aluminum thickness were measured. In preparation for attaching the MGPA directly to a CCD, an alignment tolerance analysis was completed. The results indicate that 0.5 μm alignment of MGPA pixel center to image sensor is required to get a system with significantly low crosstalk for useful polarization imaging.

Pixel-to-pixel response nonuniformity is a common problem that affects nearly all focal plane array sensors. This results in a frame-to-frame fixed pattern noise (FPN) that causes an overall degradation in collected data. FPN is often compensated for through the use of blackbody calibration procedures; however, FPN is a particularly challenging problem because the detector responsivities drift relative to one another in time, requiring that the sensor be recalibrated periodically. The calibration process is obstructive to sensor operation and is therefore only performed at discrete intervals in time. Thus, any drift that occurs between calibrations (along with error in the calibration sources themselves) causes varying levels of residual calibration error to be present in the data at all times. Polarimetric microgrid sensors are particularly sensitive to FPN due to the spatial differencing involved in estimating the Stokes vector images. While many techniques exist in the literature to estimate FPN for conventional video sensors, few have been proposed to address the problem in microgrid imaging sensors. Here we present a scene-based nonuniformity correction technique for microgrid sensors that is able to reduce residual fixed pattern noise while preserving radiometry under a wide range of conditions. The algorithm requires a low number of temporal data samples to estimate the spatial nonuniformity and is computationally efficient. We demonstrate the algorithm's performance using real data from the AFRL PIRATE and University of Arizona LWIR microgrid sensors.

Polarimetric imaging using micropolarizers integrated on focal plane arrays has previously been limited to the linear components of the Stokes vector because of the lack of an effective structure with selectivity to circular polarization. We discuss a plasmonic micropolarizing filter that can be tuned for linear or circular polarization as well as wavelength selectivity from blue to infrared (IR) through simple changes in its horizontal geometry. The filter consists of a patterned metal film with an aperture in a central cavity that is surrounded by gratings that couple to incoming light. The aperture and gratings are covered with a transparent dielectric layer to form a surface plasmon slab waveguide. A metal cap covers the aperture and forms a metal-insulator-metal (MIM) waveguide. Structures with linear apertures and gratings provide sensitivity to linear polarization, while structures with circular apertures and spiral gratings give circular polarization selectivity. Plasmonic TM modes are transmitted down the MIM waveguide while the TE modes are cut off due to the sub-wavelength dielectric thickness, providing the potential for extremely high extinction ratios. Experimental results are presented for micropolarizers fabricated on glass or directly into the Ohmic contact metallization of silicon photodiodes. Extinction ratios for linear polarization larger than 3000 have been measured.

Software based polarimetric image generation models and hardware based infrared scene projectors commonly utilize analytical forms of polarized bi-directional reflectance distribution function and emission models. Many of these models are based in first principles physical concepts, but in practice are configured as least error fits to measured signatures. The resulting analytical model may well describe the lab measured data points, but provide erroneous results when integrated into a wide ranging radiometric simulation environment. In this work we present a methodology for characterizing the suitability of incorporating limited range lab measured data, usually through fitting to an analytical model, into a wider range modeling environment. We have found lab measured reflectance data can be fit to analytical models with parameters straying significantly from the first principles physical description of the surface. This effect may be due to over parameterization or an under sampled measurement space, resulting in radiometric anomalies when integrated into a larger scale, multi-surface, multi-material, modeling environment. Our methodology consists of a series of sanity tests that each scattering and emission model configuration must pass before confidence is had in the polarimetric optical property description.

Epsilon near zero (ENZ) structures are of increasing interest with developments initially directed at metal-dielectric material combinations and recently extended to doped semiconductor-dielectric combinations - all in an effort to drive the permittivity and wave number of the structure near zero. Of further interest is the effective theoretical characterization of these multi-layered material structures. We investigate increasing the number of layers - from one to four - of a visible ENZ design structure. Theoretical predictions are compared with experimental material properties collected from ellisometry; the region where effective medium theory breaks down and optical thin film analysis succeeds are examined.

A better understanding of the information contained in the spectral, polarized bidirectional reflectance and transmittance of leaves may lead to improved techniques for identifying plant species in remotely sensed imagery as well as better estimates of plant moisture and nutritional status. Here we report an investigation of the optical polarizing properties of several leaves of one species, Cannabis sativa, represented by a 3x3 Mueller matrix measured over the wavelength region 400-2,400 nm. Our results support the hypothesis that the leaf surface alters the polarization of incident light - polarizing off nadir, unpolarized incident light, for example - while the leaf volume tends to depolarized incident polarized light.

Active (Mueller matrix) remote sensing is an under-utilized technique for material discrimination and classication. A full Mueller matrix instrument returns more information than a passive (Stokes) polarimeter; Mueller polarimeters measure depolarization and other linear transformations that materials impart on incident Stokes vectors, which passive polarimeters cannot measure. This increase in information therefore allows for better classication of materials (in general). Ideally, material classication over the entire polarized BRDF is desired, but sets of Mueller matrices for dierent materials are generally not separable by a linear classier over elevation and azimuthal target angles. We apply non-linear support vector machines (SVM) to classify materials over BRDF (all relevant angles) and show variations in receiver operator characteristic curves with scene composition and number of Mueller matrix channels in the observation.

Imaging in scattering media with the purpose of object identification has always been a challenging task. In the ocean, and especially in coastal areas, the situation is one of the worst: absorption and scattering by suspended and dissolved particles take away most of the information and blur the image of the target to be identified. In addition, one has also to take into account the variability of the bottom which, being close to the surface, plays an important role in the resulting integrated light field. Our goal in this study is to gain insight into the effects of the variable environments on the complex polarized underwater realm. We analyze the polarized tridimensional underwater environment. The instruments deployed were an underwater hyperspectral and multi-angular polarimeter, whose accuracy and exactness of results have been previously validated by the means of different radiative transfer calculations; and a green band full-Stokes polarimetric video camera, enclosed in a custom made underwater housing. The results presented here were collected during the first field deployment of the imaging camera. An in-situ validation of the camera with the polarimeter has been obtained and the results have been used to validate the values of the Stokes elements in the images, both for the water column itself and for the underlying bottom.