Proceedings Volume 7461

Polarization Science and Remote Sensing IV

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Proceedings Volume 7461

Polarization Science and Remote Sensing IV

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Volume Details

Date Published: 11 August 2009
Contents: 10 Sessions, 34 Papers, 0 Presentations
Conference: SPIE Optical Engineering + Applications 2009
Volume Number: 7461

Table of Contents

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Table of Contents

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  • Front Matter: Volume 7461
  • Polarization in Remote Sensing Instruments
  • Atmospheric Polarization Effects
  • Phenomenology and Modeling
  • Polarization in Biological Systems
  • Imaging Polarimetry I
  • Imaging Polarimetry II
  • Polarization Metrology and Components
  • Calibration, Compensation, and Optimization
  • Poster Session
Front Matter: Volume 7461
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Front Matter: Volume 7461
This PDF file contains the front matter associated with SPIE Proceedings Volume 7461, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.
Polarization in Remote Sensing Instruments
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VIIRS polarization sensitivity testing and analysis
A requirement for the Visible/Infrared Imager Radiometer Suite (VIIRS) is that its polarization sensitivity be 3% or less for all VISNIR bands (412-865 nm). A test using a rotating polarizer sheet was performed on the sensor to validate this requirement, and though the test results show that the requirement is met, they also show a large variation in this polarization sensitivity (as much as 2%) across the field of view (FOV) in track. Though this result is unexpected, it may be the result of natural variations in the diattenuation and retardance of the VIIRS optics as a function of field angle. To test this theory, a raytracing model of the system was constructed using measured ellipsometric data from the VIIRS optics, and the polarization sensitivity of the model was computed. Using the nominal ellipsometric data, good correlation between the predicted and measured polarization sensitivity was not achieved. However, by applying small variations to the ellipsometric data as a function of position on the optics, it was possible to achieve good correlation. This paper gives the details of the sensor polarization sensitivity measurements, ellipsometric measurements, and raytracing analysis.
CIV polarization measurements using a vacuum ultraviolet Fabry-Pérot Interferometer
Edward West, G. Allen Gary, Jonathan Cirtain, et al.
Marshall Space Flight Center's (MSFC) is developing a Vacuum Ultraviolet (VUV) Fabry-Pérot Interferometer that will be launched on a sounding rocket for high throughput, high-cadence, extended field of view CIV (155nm) measurements. These measurements will provide (i) Dopplergrams for studies of waves, oscillations, explosive events, and mass motions through the transition region, and, (ii), polarization measurements to study the magnetic field in the transition region. This paper will describe the scientific goals of the instrument, a brief description of the optics and the polarization characteristics of the VUV Fabry Pérot.
Design and development of the PolZero Time Domain Polarization Scrambler
Low polarization sensitivity is critical for a number of Earth science applications, including the measurement of ocean color and ozone from satellite sensors. Polarization control places serious constraints on the optical design of these sensors, particularly for the wide field of view imaging spectrometers contemplated for future Decadal Survey applications. The PolZero Time Domain Polarization Scrambler (TDPS) is an optical component that provides significant polarization sensitivity reduction for spectrometric instruments without producing beam replication, image distortion or requiring a specialized optical path. The PolZero is implemented using a pair of photo-elastic modulators to induce a particular high frequency polarization structure onto the transmitted beam. Averaging this modulated beam over many temporal polarization cycles, that is, over a few milliseconds, produces results approximating transmission through an ideal depolarizer. Measurements have shown reduction in the transmitted degree of polarization by factors of over 30. Mueller matrices of the TDPS, measured using a Mueller matrix imaging polarimeter, are presented as a function of wavelength in the region 450-700 nm.
Measured performance for proposed depolarizer for the ocean radiometer for carbon assessment
The Ocean Radiometer for Carbon Assessment (ORCA) is a new design concept for the next generation ocean biology and biogeochemistry satellite sensor. The wavelength range will be from the near UV, through the visible and to the Short Wave infrared. The challenge in this design is to remove the polarization effects from the optical performance of this hyper spectral observing instrument. In order to remove any polarization sensitivity during observation, the design calls for a front-end depolarizer that consists of two wedged birefringent magnesium fluoride crystals. Here we discuss the polarimetry measurements performed on this polarization scrambler, the depolarizer design and compare these results with model calculations.
Atmospheric Polarization Effects
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Aerosol parameter estimation through atmospheric polarization observation with the different observation angles
K. Arai, Yui Nishimura
A method for aerosol refractive index estimation with ground based polarization measurement data is proposed. The proposed method uses a dependency of refractive index on p and s polarized down welling solar diffuse irradiance. It is much easy to measure p and s polarized irradiance on the ground with a portable measuring instrument rather than solar direct, diffuse and aureole measurements. Through theoretical and simulation studies, it is found that the proposed method show a good estimation accuracy of refractive index using measured down welling p and s polarized irradiance data with a measuring instrument pointing to the direction which is perpendicular to the sun in the principal plane. Field experimental results also show a validity of the proposed method in comparison to the estimated results from the conventional method with solar direct, diffuse and aureole measurement data.
All-sky imaging polarimeter measurements of visible and NIR skylight at Mauna Loa, Hawaii
Andrew R. Dahlberg, Nathan J. Pust, Joseph A. Shaw
An all-sky imaging polarimeter was deployed in summer 2008 to the Mauna Loa Observatory in Hawaii to study atmospheric skylight polarization. This paper describes the Mauna Loa deployment and presents an initial comparison of our data to those observed by Coulson with a zenith-slice polarimeter in the late 1970s and early 1980s. We show how the all-sky imaging technique yields additional insight to the nature of skylight polarization beyond what is observed in a single zenith scan.
Reflectance based vicarious calibration of ASTER/VNIR with aerosol refractive index and size distribution estimation using measured atmospheric polarization irradiance
A method for reflectance based vicarious calibration with aerosol refractive index and size distribution estimation using atmospheric polarization irradiance data is proposed. It is possible to estimate aerosol refractive index and size distribution with atmospheric polarization irradiance measured with the different observation angles (scattering angles). The Top of the Atmosphere (TOA) or at-sensor radiance is estimated based on atmospheric codes with estimated refractive index and size distribution then vicarious calibration coefficient can be calculated by comparing to the acquired visible to near infrared instrument data onboard satellites. The estimated TOA radiance based on the proposed method is compared to that with aureole-meter based approach which is based on refractive index and size distribution estimated with solar direct, diffuse and aureole (Conventional AERONET approach). It is obvious that aureole-meter is not portable, heavy and large while polarization irradiance measurement instruments are light and small (portable size and weight).
Phenomenology and Modeling
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Inferring the orientation of texture from polarization parameters
It is shown that once the diffusely scattered polarization properties are calibrated, the texture orientation can be calculated directly from diattenuation and retardance. Polarization scattering properties are studied for a rough aluminum surface with one-dimensional rough texture and well-defined orientation. Functions of Mueller matrix elements related to sample orientation about the normal via the arctangent function are investigated. The Mueller matrix bidirectional reflectance distribution function is measured for a linearly sanded aluminum sample. Sinusoidal fits to the Mueller matrix show that the angular orientation of the data can be recovered explicitly from its properties.
Measured comparison of the inversion periods for polarimetric and conventional thermal long-wave IR (LWIR) imagery
We report the results of a multi-day diurnal study in which radiometrically calibrated polarimetric and conventional thermal imagery is recorded in the LWIR to identify/compare the respective time periods in which minimum target contrast is achieved, e.g., thermal inversion periods are typically experienced during dusk and dawn. Imagery is recorded with a polarimetric IR sensor employing a 324x256 microbolometer array using a spinning achromatic retarder to perform the polarimetric filtering. The images used in this study include the S0, normalized S1, and normalized S2 Stokes images and the degree of linear polarization (DOLP) images of a scene containing military vehicles and the natural background. In addition, relevant meteorological parameters measured during the test period include air temperature, ambient loading in the LWIR, relative humidity, and cloud cover, height and density. The data shows that the chief factors affecting polarimetric contrast are the amount of thermal emission from the objects in the scene and the abundance of LWIR sources in the optical background. In addition, we found that contrast between targets and background within polarimetric images often remains relatively high during periods of low thermal contrast.
How good is a single-scattering model of visible-NIR atmospheric skylight polarization?
Measurements of visible-NIR skylight polarization are compared with radiative transfer calculations from a model that employs polarized single scattering. The measurements are from a full-sky imaging polarimeter under conditions ranging from very low aerosol content to thick forest fire smoke. Generally, the lack of multiple scattering in the model leads to overestimated degree of polarization in all but very clear conditions at the longest wavelengths (> 600 nm).
Polarization in Biological Systems
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Polarization signals in mantis shrimps
Thomas W. Cronin, Tsyr-Huei Chiou, Roy L. Caldwell, et al.
While color signals are well known as a form of animal communication, a number of animals communicate using signals based on patterns of polarized light reflected from specialized body parts or structures. Mantis shrimps, a group of marine crustaceans, have evolved a great diversity of such signals, several of which are based on photonic structures. These include resonant scattering devices, structures based on layered dichroic molecules, and structures that use birefringent layers to produce circular polarization. Such biological polarizers operate in different spectral regions ranging from the near-UV to medium wavelengths of visible light. In addition to the structures that are specialized for signal production, the eyes of many species of mantis shrimp are adapted to detect linearly polarized light in the ultraviolet and in the green, using specialized sets of photoreceptors with oriented, dichroic visual pigments. Finally, a few mantis shrimp species produce biophotonic retarders within their photoreceptors that permit the detection of circularly polarized light and are thus the only animals known to sense this form of polarization. Mantis shrimps use polarized light in species-specific signals related to mating and territorial defense, and their means of manipulating light's polarization can inspire designs for artificial polarizers and achromatic retarders.
Depolarization properties of the normal human fovea measured by the GDx-MM
Wai-Sze T. Lam, Russell A. Chipman, Karen M. Twietmeyer, et al.
A custom imaging Mueller matrix retinal polarimeter (the GDx-MM) is built. Mueller matrix images of normal human fovea were acquired with the GDx-MM over a 9° field at 780nm and have been analyzed for depolarization index and the variation of degree of polarization with incident polarization state. The degree of polarization (DoP) was often above 50% and varied in complex ways as a function of the incident polarization states. The depolarization properties around the macula loosely correlated with the retardance image. High spatial frequency depolarizing structures were evident throughout the fovea.
Imaging Polarimetry I
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Initial results and field applications of a polarization imaging camera
R. Chris Olsen, Michael Eyler, Angela M. Puetz, et al.
The SALSA linear Stokes polarization camera from Bossa Nova Technologies (520-550 nm) uses an electronically rotated polarization filter to measure four states of polarization nearly simultaneously. Some initial imagery results are presented. Preliminary analysis results indicate that the intensity and degree of linear polarization (DOLP) information can be used for image classification purposes. The DOLP images also show that the camera has a good ability to distinguish asphalt patches of different ages. These positive results and the relative simplicity of the camera system show the camera's potential for field applications.
High-speed imaging acquisition of Stokes linearly polarized components using a single ferroelectric liquid crystal modulator
L. Gendre, A. Foulonneau, L. Bigué
We present a high-speed ferroelectric liquid crystal based imaging polarimeter. It can evaluate the first three Stokes parameters. Contrary to previous high-speed systems, it only uses a single liquid crystal cell, driven in an optimized way in order to produce a tunable rotation of polarization. Its characterization is presented, as well as its integration in a portable implementation working at 633 nm. Preliminary results are provided.
Imaging Polarimetry II
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Examining IFOV error and demodulation strategies for infrared microgrid polarimeter imagery
For the past several years we have been working on strategies to mitigate the effects of IFOV errors on LWIR microgrid polarimeters. In this paper we present a detailed, theoretical analysis of the source of IFOV error in the frequency domain, and show a frequency domain strategy to mitigate those effects.
First high-resolution passive polarimetric images of boosting rocket exhaust plumes
We present the first high spatial resolution, passively-illuminated polarimetric images of boosting rocket exhaust plumes. The images shown here show significant linear and circular polarization, and the ability to resolve the polarization signals into images allows us to make some preliminary arguments as to their origins. Our observations are consistent with polarization caused by Rayleigh and Mie scattering (linear) and interaction with plume plasma-generated magnetic fields (circular). We also present nearly simultaneous, two-color, narrowband (633 ± 5 and 750 ± 5 nm) exhaust plume images, where significant structural differences are observed in the plumes despite a relative small difference in the two wavelengths.
Exploiting motion-based redundancy to enhance microgrid polarimeter imagery
Microgrid polarimeters are a type of division of focal plane (DoFP) imaging polarimeter that contains a mosaic of pixel-wise micropolarizing elements superimposed upon an FPA sensor. Such a device measures a slightly different polarized state at each pixel. These measurements are combined to estimate the Stokes vector at each pixel in the image. DoFP devices have the advantage that they can obtain Stokes vector image estimates for an entire scene from a single frame capture. However, they suffer from the disadvantage that the neighboring measurements that are used to estimate the Stokes vector images are acquired at differing instantaneous fields of view (IFOV). This IFOV issue leads to false polarization signatures that significantly degrade the Stokes vector images. Interpolation and other image processing strategies can be employed to reduce IFOV artifacts; however these techniques have a limit to the amount of enhancement they can provide on a single microgrid image. Here we investigate algorithms that use multiple microgrid images that contain frame-to-frame global motion to further enhance the Stokes vector image estimates. Motion-based imagery provides additional redundancy that can be exploited to recover information that is "missing" from a single microgrid frame capture. We have found that IFOV and aliasing artifacts can be defeated entirely when these types of algorithms are applied to the data prior to Stokes vector estimation. We demonstrate results on real LWIR microgrid data using a particular resolution enhancement technique from the literature.
Near-infrared simultaneous Stokes imaging polarimeter
Jason Mudge, Miguel Virgen, Peter Dean
We have designed a new near-IR imaging polarimeter which generates the complete Stokes' vector estimation simultaneously. The design is based on our first generation division of amplitude polarimeter where four images are folded on to a single focal plane detector. This gives rise to a small compact rigid instrument. The design operation wavelength is 632.8 nanometers. The new second generation design operates at a wavelength of 1550 nanometers and has three improvements over the first generation: 1) the design of the Beam-Splitter Assembly (BSA) is based on an optimization scheme where the Measurement (instrument) matrix is optimized for Stokes' vector estimation with noisy data, 2) the four individual focusing lenses positioned after the BSA have been replaced by a single lens in front of the BSA reducing differential image distortion, and 3) a reticle is placed at an intermediate image plane, providing a fiducial mark in each of the images for precise registration.
Polarization Metrology and Components
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Axisymmetrical Mueller matrix polarimeter
Mueller matrix polarimeter using the axisymmetrical polarized and analyzed optics is proposed. The axisymmetrical optics has a potential to bring out the polarization properties of a sample which kept in beam spot. A key device of this method is a detector named "a ring beam detector". The concept of the detector is a topology and/or a projective transformation and the optical configuration of the detector consists of a conical mirror and/or a conical lens, a cylindrical screen and a CCD camera. To use a Fourier transform method, we can get Mueller matrix properties from the intensity distribution of the ring beam captured by the camera without a mechnically and electrically polarization modulation. In this paper, principles of axisymmetrical Mueller matrix polarimeter, the ring beam detector and basically experimental results are shown.
Polarization and fold mirrors in application of the Leica Absolute Distance Meter
Derek Sabatke, Robert von Handorf, Joseph Sullivan
The polarization behavior of the Leica Geosystems absolute distance meter is investigated in order to understand measurement noise and dropouts sometimes observed when the instrument is used with fold mirrors. A Stokes- Mueller calculus analysis suggests retardance in the fold elements is a likely culprit. We establish the heuristic that for folds in a single plane, successful operation is obtained when the sum of the ellipsometric angle ▵ for each element in single pass is an integer multiple of 180°. The heuristic is validated experimentally, and found to have a tolerance in the range of ±12° to hold distance standard deviations to 10μm. Strategies to achieve this condition include keeping angles of incidence near 90°, tuning with angle of incidence, and judicious selection of mirror coatings.
Spectroscopic Stokes polarimeter with dual rotating retarder and analyzer for optical rotation measurement
Masanosuke Tanaka, Yoshinori Nakashima, Hideyuki Amamiya, et al.
This report proposes an optical rotation measurement using Stokes parameters with a dual rotating retarder and analyzer. The intensity captured by a spectrometer is modulated by a retarder and an analyzer in a rotating ratio of one to three. The Stokes parameters and retardance along the wavelength of the retarder are simultaneously analyzed by Fourier transform. Two standard quartz plates of 0.8 mm and 1.6 mm thickness are measured for a sample. The characteristics of optical rotation dispersion of the two standard quartz plates correspond with theoretical values at wavelengths from 550 nm to 900 nm. We succeeded to separate the optical rotation from the birefringence.
Confocal scanning Mueller polarimeter
We describe the design, construction, calibration and testing of a confocal scanning Mueller polarimeter. A polarization state generator and polarization state analyzer have been inserted into the optical path of a conventional confocal scanning imager to collect the reflectance Muller matrix of samples measuring up to 6.26 mm on a side. Four sources are available for sample interrogation using diode lasers centered at 532 nm, 635 nm, 670 nm, and 785 nm. The device captures all required imagery to calculate the Mueller matrix of each image pixel in approximately 90 s. These matrices are then reduced into polarization imagery such as the diattenuation, retardance and depolarization index. Oftentimes this polarization imagery is quite different and potentially more informative than a conventional intensity image. There are a number of fields that can benefit from alternative/enhanced imagery, most notably in the biomedical, discrimination, and target recognition communities. The sensor has been designed for biomedical applications aimed at improving the technique of noninvasive detection of melanoma lesions.
Spectroscopic Mueller matrix polarimeter by double liquid crystal phase modulators
Yukitoshi Otani, Makoto Chujo
All polarization characteristics can be described by Mueller matrix. These polarized parameters are captured by changing the states of polarization given by combination of polarization generator and polarization analyzer. We employ double liquid crystal phase modulators whose orientation is set 45 degrees by changing the rubbing direction of orientation film. A unit of LC phase modulators is set in temperature-controlled environment and calibrated spectroscopic Stokes polarimeter. A measurement method for obtaining Mueller matrix is to applied that the phase of LC phase modulators are changed with the phase modulation in the proportion of one part to five. We succeed to measure spectroscopic Mueller matrix without any mechanical movement with high accuracy.
PEPPER: polarization-encoding differential photometer and polarimeter
PEPPER is a high-speed differential Polarization-Encoded Photometer and Polarimeter developed in the Center for Astronomical Adaptive Optics at the University of Arizona, Tucson, by Dr. Dan Potter and Matthew Graham. PEPPER is capable of acting as a high-speed polarimeter by using electro-optical switching to chop between standard star and target star, and between in and out-feature bandpass filter at frequencies fast enough to suppress atmospheric variations. PEPPER is capable of either high-speed polarimetry or differential photometry using a combination of simultaneous imaging and electro-optical switching. In the differential photometry mode, PEPPER utilizes the electro-optical switching to calibrate instrumental and atmospheric photometric variation. This technique coupled with a zero-read noise photon counting detector achieves photon noise limited results demonstrated to an accuracy of less than 1 part in 105. Herein we present the design concept behind the photometer and the polarimeter mode of PEPPER, as well as, results from observations in the differential photometer mode at the Steward 90 inch telescope, at the Kitt Peak National Observatory, Tucson, Arizona. Results from the analysis of near IR polarimetry observations of young stars with circumstellar disks taken at the Gemini North Telescope with the Hokupa'a adaptive optics system are also presented.
Calibration, Compensation, and Optimization
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Retardance in three-dimensional polarization ray tracing
A three-by-three polarization ray tracing matrix P which is defined in global coordinates characterizes the polarization transformations associated with single ray through optical system. The P matrix contains both a geometrical transformation effect and the polarization characteristics of diattenuation and retardance from the optical and polarization elements. In order to separate the geometrical transformation and calculate the "physical" retardance, a non-polarizing ray tracing matrix Q is used. The diattenuation and the retardance of a dove prism are analyzed as an example.
Spectro-polarimetric BRDF determination of objects using in-scene calibration materials for polarimetric imagers
For sensing systems that characterize the spectro-polarimetric radiance reaching the camera, the origin of the sensed phenomenology is a complex mixture of sources. While some of these sources do not contribute to the polarimetric signature, many do such as the downwelled sky polarization, the target and background p- BRDF(polarimetric bi-directional reflectance distribution function), the upwelled sky polarization, and the camera Mueller matrix transfer function. In this paper we investigate candidate in-scene calibration materials potentially allowing for portions of the p-BRDF to be derived for material surfaces throughout the scene. Extraction of target p-BRDF from the sensed spectro-polarimetric energy may result in improved target detection performance in the future. Results using both synthetic and real data are presented.
Designing partial Mueller matrix polarimeters
J. Scott Tyo, Sergio J. Johnson, Zhipeng Wang, et al.
When using a MMP for a detection or identification task, a user considers certain elements of the Mueller matrix. The usual way of performing this task is to measure the full Mueller matrix and discard the unused elements. For polarimeter designs with speed, miniaturization, or other constraints it may be desirable to have a system with reduced dimensionality that measures only the important elements of the Mueller matrix as efficiently as possible. In this paper, we develop a framework that allows partial MMPs to be analyzed. Quantitative metrics are developed by considering geometrical relationships between the space spanned by a particular MMP and the space occupied by the scene components. The method is generalized to allow the effects of noise to come into the equation when noise performance is important as well.
Poster Session
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The development and assessment of a flexible inversion algorithm for aerosol property retrieval combining passive multiangle multispectral intensity and polarization measurements
Alexandra Tsekeri, Barry Gross, Fred Moshary, et al.
Quantifying aerosols on a global scale is extremely important due to their strong but anomalous impact on the global climate. Traditionally, the aerosols retrievals use only the intensity measurements of the scattered light. However, these measurements are less sensitive to aerosol type and also suffer contamination from ground surfaces. It is with these limitations in mind that we plan to improve the quality and scope of aerosol retrieval by making use of soon to be available polarimetric sensors such as the Aerosol Polarimetry Sensor (APS) on the GLORY satellite and combine them with other available datasets such as lidar data from the CALIPSO satellite for vertical profiling, and high-spatialcoverage intensity measurements from MODIS. To handle these extremely large sensor data sets, we will explore the capabilities of various statistical methods and even combine them to create inversion algorithms that will work best. Up to now, we worked with the simplest case, the single-scattering approximation and built a retrieval algorithm using multi-angular, multi-wavelength simulated measurements of intensity and polarization. The inversion techniques we used are the optimal estimator and the neural networks.
Concurrent polarimetric measurements of painted metal and illuminating skylight compared with a microfacet model
Nathan J. Pust, Joseph A. Shaw, Andrew R. Dahlberg
Previous visible-band laboratory measurements have shown that polarization data can be used to determine optical properties of materials such as the index of refraction with controlled illumination sources. For outdoor measurements, the complex illumination formed by the polarized sky for visible wavelengths makes this process considerably more difficult. This paper reports polarization measurements for horizontal painted-metal and PVC plates and the background atmosphere from a quickly changeable dual-field imaging polarimeter which provides polarization of ground-based objects nearly concurrently with full-sky polarization. A microfacet model has been developed which accounts for the polarized sky illumination and solar-reflecting and flat-reflecting microfacets. Data from this model have been used to explain the primary features of the polarization observed when viewing painted metal and PVC plates outdoors with clear skies. Future work will attempt to use this model with polarimeter data to retrieve the index of refraction of the observed plates.
Basic optical properties of the photoelastic modulator part II: residual birefringence in the optical element
Baoliang Wang, Emily Hinds, Erica Krivoy
The photoelastic modulator (PEM) is a polarization modulator that operates at the resonant frequency of its optical element. The PEM is made of isotropic optical materials, in contrast to birefringent materials used in electro-optic modulators. These two characteristics, operation at resonance and the use of isotropic optical materials, give the PEM unique optical features, such as high modulation purity and efficiency, broad spectral range, high power handling capability, large acceptance angle, large useful aperture and high retardation stability. These features make the PEM an effective polarization modulator in a variety of applications. Sometimes it is the only choice for high sensitivity applications. In an effort to characterize the PEMs more thoroughly, we are carrying out a series of tests on the basic properties of the PEM. Residual birefringence is an important property that affects the quality of a PEM. In the second paper in a series, we focus on the measurement of residual birefringence in the optical element of a PEM and maintaining the residual birefringence at a low level in the final PEM product.
Features of inverse problem arise from structure of a general pure Mueller matrix
Sergey N. Savenkov, Yevgen A. Oberemok, Vladimir N. Nikonov
Changes in the state of polarization of a beam of radiation occurring without depolarization can be described by means of a pure Mueller matrix. Pure Mueller matrix can be expressed in terms of the elements of a 2x2 Jones matrix. This results in that the pure Mueller matrix has a simple and elegant structure, which is embodied by interrelations between matrix elements. All possible interrelations for the elements of a general pure Mueller matrix are derived by Hovenier (Appl. Opt., Vol.33, No.36, pp. 8318-8324, 1994). The structure of the pure Mueller matrix enables to solve the inverse problem basing not on all sixteen matrix elements but only on certain part of them. We show that four elements which are formed each of columns and rows of the pure Mueller matrix considering them individually are dependent and the inverse problem can be solved in general case basing only on the rest of twelve matrix elements.
Evaluation of the polarization properties of a Philips-type prism for the construction of imaging polarimeters
The design and construction of wide FOV imaging polarimeters for use in atmospheric remote sensing requires significant attention to the prevention of artificial polarization induced by the optical elements. Surface, coatings, and angles of incidence throughout the system must be carefully designed in order to minimize these artifacts because the remaining instrumental bias polarization is the main factor which drives the final polarimetric accuracy of the system. In this work, we present a detailed evaluation and analysis to explore the possibility of retrieving the initial polarization state of the light traveling through a generic system that has inherent instrumental polarization. Our case is a wide FOV lens and a splitter device. In particular, we chose as splitter device a Philips-type prism, because it is able to divide the signal in 3 independent channels that could be simultaneously analyze to retrieve the three first elements of the Stoke vector (in atmospheric applications the elliptical polarization can be neglected [1]). The Philips-type configuration is a versatile, compact and robust prism device that is typically used in three color camera systems. It has been used in some commercial polarimetric cameras which do not claim high accuracy polarization measurements [2]. With this work, we address the accuracy of our polarization inversion and measurements made with the Philips-type beam divider.
Detection of disturbed earth using passive LWIR polarimetric imaging
We report test results of a study to assess the applicability for using passive polarimetric imaging in the long-wave infrared (LWIR) to detect regions of recently altered road-type surfaces, e.g., soil, gravel, asphalt, etc. The field test was conducted at the U.S. Army Research Laboratory, Adelphi, MD, on a test surface best described as a well traveled dirt road consisting of a gravel clay-soil mixture that was well compacted. During this initial proof-of-concept test, a LWIR polarimetric camera system was positioned at a slant-path of 10 degrees with respect to the line-of-site (LOS) and the natural lay of the surface, approximately 15 meters from the target test-bed. Stokes images, S0, S1, and S2, were recorded using the LWIR polarimeter that utilizes a spinning achromatic retarder design mated to Mercury Cadmium Telluride (MCT) focal plane array (FPA). Various surrogate targets were buried near the surface and great care was taken to camouflage the area to eliminate any "visible" signs of disturbance. Thermal gradients resulting from the unearthing of cool soil were allowed to dissipate. Two metrics were used to evaluate performance, i.e., conventional receiver operating characteristic (ROC) curve analysis and an effective contrast ratio between the target and background. Results showed particularly good detectability in the S2 imagery, with less in S1, and no detectability in S0, i.e., the conventional LWIR thermal image.