Andrey Alenin

Generalized channeled polarimetry

Channeled polarimeters measure polarization by modulating the measured intensity in order to create polarization-dependent channels that can be demodulated to reveal the desired polarization information. A number of channeled systems have been described in the past, but their proposed designs often unintentionally sacrifice optimality for ease of algebraic reconstruction. To obtain more optimal systems, a generalized treatment of channeled polarimeters is required. This paper describes methods that enable handling of multi-domain modulations and reconstruction of polarization information using linear algebra. We make practical choices regarding use of either Fourier or direct channels to make these methods more immediately useful. Employing the introduced concepts to optimize existing systems often results in superficial system changes, like changing the order, orientation, thickness, or spacing of polarization elements. For the two examples we consider, we were able to reduce noise in the reconstruction to 34.1% and 57.9% of the original design values.

Adapting the HSV polarization-color mapping for regions with low irradiance and high polarization

Many mappings from polarization into color have been developed so that polarization information can be displayed. One of the most common of these maps the angle of linear polarization into color hue and degree of linear polarization into color saturation, while preserving the irradiance information from the polarization data. While this strategy enjoys wide popularity, there is a large class of polarization images for which it is not ideal. It is common to have images where the strongest polarization signatures (in terms of degree of polarization) occur in regions of relatively low irradiance: either in shadow in reflective bands or in cold regions in emissive bands. Since the irradiance is low, the chromatic properties of the resulting images are generally not apparent. Here we present an alternate mapping that uses the statistics of the angle of polarization as a measure of confidence in the polarization signature, then amplifies the irradiance in regions of high confidence, and leaves it unchanged in regions of low confidence. Results are shown from an LWIR and a visible spectrum imager.

Structured decomposition design of partial Mueller matrix polarimeters

Partial Mueller matrix polarimeters (pMMPs) are active sensing instruments that probe a scattering process with a set of polarization states and analyze the scattered light with a second set of polarization states. Unlike conventional Mueller matrix polarimeters, pMMPs do not attempt to reconstruct the entire Mueller matrix. With proper choice of generator and analyzer states, a subset of the Mueller matrix space can be reconstructed with fewer measurements than that of the full Mueller matrix polarimeter. In this paper we consider the structure of the Mueller matrix and our ability to probe it using a reduced number of measurements. We develop analysis tools that allow us to relate the particular choice of generator and analyzer polarization states to the portion of Mueller matrix space that the instrument measures, as well as develop an optimization method that is based on balancing the signal-to-noise ratio of the resulting instrument with the ability of that instrument to accurately measure a particular set of desired polarization components with as few measurements as possible. In the process, we identify 10 classes of pMMP systems, for which the space coverage is immediately known. We demonstrate the theory with a numerical example that designs partial polarimeters for the task of monitoring the damage state of a material as presented earlier by Hoover and Tyo [Appl. Opt.46, 8364 (2007)10.1364/AO.46.008364APOPAI1559-128X]. We show that we can reduce the polarimeter to making eight measurements while still covering the Mueller matrix subspace spanned by the objects.

Task-specific snapshot Mueller matrix channeled spectropolarimeter optimization

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.

Optimal bandwidth micropolarizer arrays

Currently, the best performing micropolarizer array is the 2×4 pattern introduced by LeMaster and Hirakawa. In this Letter, we extend the available set of patterns with the aim of improving reconstruction quality by leveraging the Fourier domain and designing information carriers that yield optimal bandwidth. First, the family of 2×L patterns widens the optimization space of the 2×4 pattern by facilitating variable allocation of bandwidth for channels surrounding polarization and intensity carriers. Second, the 2×2×N patterns present an intriguing option for use within a hybrid spatiotemporal modulation scheme, in which the multiple temporal measurements enable maximum theoretical spatial resolution of reconstructed Stokes parameters.

SWIR active polarization imaging for material identification

Nighttime active SWIR imaging has resolution, size, weight, and power consumption advantages over passive MWIR and LWIR imagers for applications involving target identification. We propose that the target discrimination capability of active SWIR systems can be extended further by exerting polarization control over the illumination source and imager, i.e. through active polarization imaging. In this work, we construct a partial Mueller matrix imager and use laboratory derived signatures to uniquely identify target materials in outdoor scenes. This paper includes a description of the camera and laser systems as well as discussion of the reduction and analysis techniques used for material identification.

Hyperspectral measurement of the scattering of polarized light by skin

The goal of this study is to develop a spectropolarimeter for purposes of assessing polarization signatures in skin scattering on a regional scale. Prior research has that certain skin lesions have identifiable polarization signatures;1-3 however, those studies were limited to single lesion evaluation and are not convenient for screening patients with many suspicious legions. As a precursor to the future instrument, a simple actively illuminated Stokes spectropolarimeter was constructed to gather preliminary data about expected signatures and the required performance (resolution, wavelength, polarization state, etc.). This spectropolarimeter consists of a rotating retarder and a hyperspectral camera4 that scans through wavelengths by means of a Liquid Crystal Tunable Filter (LCTF). Data is captured in a serial fashion, where LCTF scans through eight wavelengths at each of the four retarder orientations. With a single acquisition taking 23 seconds to complete, it makes the issue of image registration very important. After proper alignment, the acquired images reveal that wavelength-dependent polarization signatures exist on a regional scale. In particular, it was found that polarization factors such as Degree of Linear Polarization (DoLP) tend to suppress many uninteresting skin features like wrinkles and skin texture, while capturing information that is not necessarily apparent in the intensity image.

Focal plane filter array engineering I: rectangular lattices

Focal planes arrays (FPA) measure values proportional to an integrated irradiance with little sensitivity to wavelength or polarization in the optical wavelength range. The measurement of spectral properties is often achieved via a spatially varying color filter array. Recently spatially varying polarization filter arrays have been used to extract polarization information. Although measurement of color and polarization utilize separate physical methods, the underlying design and engineering methodology is linked. In this communication we derive a formalism which can be used to design any type of periodic filter array on a rectangular lattice. A complete system description can be obtained from the number of unit cells, the pixel shape, and the unit cell geometry. This formalism can be used to engineer the channel structure for any type of periodic tiling of a rectangular lattice for any type of optical filter array yielding irradiance measurements.

Design of channeled partial Mueller matrix polarimeters

In this paper, we introduce a novel class of systems called channeled partial Mueller matrix polarimeters (c-pMMPs). Their analysis benefits greatly by drawing from the concepts of generalized construction of channeled polarimeters as described by the modulation matrix. The modulation matrix resembles that of the data reduction method of a conventional polarimeter, but instead of using Mueller vectors as the bases, attention is focused on the Fourier properties of the measurement conditions. By leveraging the understanding of the measurements structure, its decomposition can be manipulated to reveal noise resilience and information about the polarimeters ability to measure the aspect of polarization that are important for any given task. We demonstrate the theory with a numerical optimization that designs c-pMMPs for the task of monitoring the damage state of a material as presented earlier by Hoover and Tyo [Appl. Opt.46, 8364 (2007)APOPAI0003-693510.1364/AO.46.008364]. We select several example systems that produce a fewer-than-full-system number of channels yet retain the ability to discriminate objects of interest. Their respective trade-offs are discussed.

Channeled partial Mueller matrix polarimetry

In prior work,1,2 we introduced methods to treat channeled systems in a way that is similar to Data Reduction Method (DRM), by focusing attention on the Fourier content of the measurement conditions. Introduction of Q enabled us to more readily extract the performance of the system and thereby optimize it to obtain reconstruction with the least noise. The analysis tools developed for that exercise can be expanded to be applicable to partial Mueller Matrix Polarimeters (pMMPs), which were a topic of prior discussion as well. In this treatment, we combine the principles involved in both of those research trajectories and identify a set of channeled pMMP families. As a result, the measurement structure of such systems is completely known and the design of a channeled pMMP intended for any given task becomes a search over a finite set of possibilities, with the additional channel rotation allowing for a more desirable Mueller element mixing.