Proceedings Volume 6240

Polarization: Measurement, Analysis, and Remote Sensing VII

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Proceedings Volume 6240

Polarization: Measurement, Analysis, and Remote Sensing VII

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Volume Details

Date Published: 2 May 2006
Contents: 6 Sessions, 21 Papers, 0 Presentations
Conference: Defense and Security Symposium 2006
Volume Number: 6240

Table of Contents

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Table of Contents

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  • Remote Sensing and Processing I
  • Remote Sensing and Processing II
  • Scattering and Phenomenology
  • Polarimetry
  • Polarization Elements and Analysis I
  • Polarization Elements and Analysis II
Remote Sensing and Processing I
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Quantifying surface normal estimation
An inverse algorithm for surface normal estimation from thermal polarimetric imagery was developed and used to quantify the requirements on a priori information. Building on existing knowledge that calculates the degree of linear polarization (DOLP) and the angle of polarization (AOP) for a given surface normal in a forward model (from an object's characteristics to calculation of the DOLP and AOP), this research quantifies the impact of a priori information with the development of an inverse algorithm to estimate surface normals from thermal polarimetric emissions in long-wave infrared (LWIR). The inverse algorithm assumes a polarized infrared focal plane array capturing LWIR intensity images which are then converted to Stokes vectors. Next, the DOLP and AOP are calculated from the Stokes vectors. Last, the viewing angles, θv, to the surface normals are estimated assuming perfect material information about the imaged scene. A sensitivity analysis is presented to quantitatively describe the a priori information's impact on the amount of error in the estimation of surface normals, and a bound is determined given perfect information about an object. Simulations explored the impact of surface roughness (σ) and the real component (n) of a dielectric's complex index of refraction across a range of viewing angles (θv) for a given wavelength of observation.
Characterization and analysis of infrared images
Adrienne Raglin, Alan Wetmore, David Ligon
Stokes images in the long-wave infrared (LWIR) and methods for processing polarimetric data continue to be areas of interest. Stokes images which are sensitive to geometry and material differences are acquired by measuring the polarization state of the received electromagnetic radiation. The polarimetric data from Stokes images may provide enhancements to conventional IR imagery data. It is generally agreed that polarimetric images can reveal information about objects or features within a scene that are not available through other imaging techniques. This additional information may generate different approaches to segmentation, detection, and recognition of objects or features. Previous research where horizontal and vertical polarization data is used supports the use of this type of data for image processing tasks. In this work we analyze a sample polarimetric image to show both improved segmentation of objects and derivation of their inherent 3-D geometry.
Application of passive imaging polarimetry in the discrimination and detection of different color targets of identical shapes using color-blind imaging sensors
A. M. El-Saba, M. S. Alam, A. Surpanani
Important aspects of automatic pattern recognition systems are their ability to efficiently discriminate and detect proper targets with low false alarms. In this paper we extend the applications of passive imaging polarimetry to effectively discriminate and detect different color targets of identical shapes using color-blind imaging sensor. For this case of study we demonstrate that traditional color-blind polarization-insensitive imaging sensors that rely only on the spatial distribution of targets suffer from high false detection rates, especially in scenarios where multiple identical shape targets are present. On the other hand we show that color-blind polarization-sensitive imaging sensors can successfully and efficiently discriminate and detect true targets based on their color only. We highlight the main advantages of using our proposed polarization-encoded imaging sensor.
Estimating the refractive index and reflected zenith angle of a target using multiple polarization measurements
A passive polarization based imaging system records the polarization state of light reflected by objects that are illuminated with an unpolarized and generally uncontrolled source. The information conveyed by the polarization state of light has been exploited in applications such as target detection, shape extraction and material classification. In this paper we present a method to jointly estimate the refractive index and the reflected zenith angle from two measurements collected by a passive polarimeter. An expression for the degree of polarization is derived from the microfacet polarimetric bidirectional reflectance model for the case of scattering in the place of incidence. The parameters of interest are iteratively estimated from polarization measurements assumed to be collected with a polarimeter. Computer simulations are presented to demonstrate the effectiveness of the proposed method.
Remote Sensing and Processing II
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Real time extraction of polarimetric information at the focal plane
Viktor Gruev, Kejia Wu, Jan Van der Spiegel, et al.
Traditional imaging systems focus on converting light's intensity and color property into suitable electronic signals. An important property of light, polarization is ignored with these traditional imaging systems. Polarization vision contains information about the imaged environment, such as surface shapes, curvature and material properties. Real time extraction of polarization properties would further allow synergy with traditional adaptive spatiotemporal image processing techniques for synthetic imaging. Therefore, we have developed an image sensor with real-time polarimetric extraction capability at the focal plane using low power analog circuits. This novel imaging system is the first of its kind to compute Stokes parameters at the focal plane in real-time. In order to fully describe the polarization state of light in nature, three linear polarized projections or two linear polarized projections in combination with the total intensity are required. We have fabricated a two layer micro polarizer array with total thickness of around 20μm. The micro polarizer array is mounted on top of the imaging sensor. The image sensor is composed of a 256 by 256 photo pixel array, noise suppression circuitry and analog processing circuitry for polarimetric computation. The image sensor was fabricated in 0.18μ process with 10μm pixel pitch and 75% fill factor. Block-parallel pixel read out is employed in order to compute Stokes parameters on a neighborhood of 2 by 2 pixels. The Stokes parameters are presented together with the noise suppressed intensity image. Experimental data from the polarimetric imaging system is also presented.
Imaging spectropolarimetry of cloudy skies
The polarization state of atmospheric radiance varies with cloudiness and cloud type. We have developed a dual-field-of-view imaging spectro-polarimeter for measuring atmospheric polarization in five spectral bands from 450 to 700 nm. This instrument improves the acquisition time of past full-sky digital camera designs to 400 ms using liquid crystal variable retarders (LCVRs). The system can be used to measure polarization with either fisheye or telephoto optics, allowing studies of all-sky and target polarization. We present and describe measurements of sky polarization with clear and variably cloudy sky conditions. In clear skies, we observe a slight upward trend of the degree of polarization with wavelength, in agreement with previous observations. Presence of clouds generally reduces both cloudy sky and surrounding clear sky degree of polarization. The polarization measured from a cloud often reflects only the Rayleigh scattering between the instrument and the cloud, but some of our recent data shows partially polarized cloud scattering.
Material classification based on multiband polarimetric images fusion
Yongqiang Zhao, Quan Pan, Hongcai Zhang
Polarization imparted by surface reflections contains unique and discriminatory signatures which may augment spectral target-detection techniques. With the development of multi-band polarization imaging technology, it is becoming more and more important on how to integrate polarimetric, spatial and spectral information to improve target discrimination. In this study, investigations were performed on combining multi-band polarimetric images through false color mapping and wavelet integrated image fusion method. The objective of this effort was to extend the investigation of the use of polarized light to target detection and material classification. As there is great variation in polarization in and between each of the bandpasses, and this variation is comparable to the magnitude of the variation intensity. At the same time, the contrast in polarization is greater than for intensity, and that polarization contrast increases as intensity contrast decreases. It is also pointed out that chromaticity can be used to make targets stand out more clearly against background, and material can be divided into conductor and nonconductor through polarization information. So, through false color mapping, the difference part of polarimetric information between each of the bandpasses and common part of polarimetric information in each of the bandpasses are combined, in the resulting image the conductor and nonconductor are assigned different color. Then panchromatic polarimetric images are fused with resulting image through wavelet decomposition, the final fused image have more detail information and more easy identification. This study demonstrated, using digital image data collected by imaging spectropolarimeter, multi-band imaging polarimetry is likely to provide an advantage in target detection and material classification.
Fuse spectropolarimetric imagery by D-S reasoning
Yongqiang Zhao, Quan Pan, Hongcai Zhang
The detection of low signature objects in cluttered backgrounds is a crucial problem in remote sensing. In the past few years, imaging spectral and polarimetric sensors have been evaluated for this application. As the reflection or emission spectral signatures depend on the elemental composition of objects residing within the scene. The polarization state of radiation is sensitive to surface features such as relative smoothness or roughness. But each character (spectral, polarimetric or spatial character) giving an incomplete representation of an object of interest, it expected that the combination of complementary and redundant characters would be contributed to reduce the false alarm rate, improve the confidence in the target identification and the quality of the scene description as a whole. Imaging spectropolarimetry provides effective mean to acquire spatial, spectral and polarimetric information of scene. This paper presents a study of spectropolarimetric image data set recorded from imaging spectropolarimeter located on top of building. The low probability detection algorithm was separately applied to polarimetric data sets of each band (Stokes images, degree of polarization image and angle of polarization image ) to obtain a series of two dimensional map of objects and false detection. As there are some conflictions among these maps, D-S reasoning is used to combine these maps to improve the detection rate and low false rate. Through experiment and simulation, we conclude that this fusion algorithm can be well applied to enhance the detection performance.
Scattering and Phenomenology
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Polarization patterns and symmetries as a manifestation of helicity preserving characteristics of scattering media
Polarization patterns which are observed in backscattering from random media are shown to be connected with the helicity preserving characteristics of the scattering particles. The properties of the spatially resolved Mueller matrix can be derived in a simple manner and it is shown that they are related to scattering trajectories that preserve the total angular momentum of light.
Reflective and polarimetric characteristics of urban materials
Douglas G. Jones, Dennis H. Goldstein, Jonathan C. Spaulding
A spectropolarimetric reflectometer is used to measure the monostatic bidirectional reflectance distribution function (mBRDF) and the complete Mueller matrix of a number of urban type materials over a broad spectral region. Derivative features from these measurements are computed and stochastic models of each material are constructed. The models are then used to generate data for separability studies by implementation of a kernel-based linear discriminant classification technique. The purpose for this study, selection of materials, measurements process, analysis techniques, and concluding results are all presented.
Monostatic Mueller matrix measurement and theory
Changhyuk An, Dennis H. Goldstein, Jonathan Spaulding, et al.
Monostatic Mueller matrix measurements of aluminum plates of various roughnesses are presented using a Mueller matrix polarimeter with a dual rotating retarder. The measurements are compared with a theoretical Mueller matrix model derived from the vector Kirchhoff diffraction equation. The wavelength of the laser is 1.55 μm. The rms roughness depths are provided by surface profilometer measurements and the roughness correlation length is estimated by finding the best match between the measured and the model reflectance for varying roughness correlation length. Except one smooth surface, all other aluminum samples studied have roughness ratio ( = roughness correlation length/rms roughness depth) less than 5. We compare the Mueller matrices between the lab measurement and the theoretical model. The model results show that the off-diagonal elements of the matrix have a symmetry relation and the magnitudes of diagonal elements are nearly 1, implying negligible depolarization for angles less than 30°. The lab measurements show that the off-diagonal elements have a symmetry relation for a smooth sample but the symmetry relation is weaker for rougher samples (lower roughness ratios). The lab data also show that depolarization is about 2% for the smooth sample but larger than 25% for the rougher samples for angles near 0°. The smooth surface shows reasonable agreement between the lab data and the model result except higher depolarization shown by the lab data for angles larger than 30°. On the other hand, the rough samples do not show similar agreement as the smooth surface shows. Possible causes of discrepancies are discussed and improvements for the lab measurement and model are suggested.
Polarimetry
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Image processing methods to compensate for IFOV errors in microgrid imaging polarimeters
Bradley M. Ratliff, James K. Boger, Matthew P. Fetrow, et al.
Long-wave infrared imaging Stokes vector polarimeters are used in many remote sensing applications. Imaging polarimeters require that several measurements be made under optically different conditions in order to estimate the polarization signature at a given scene point. This multiple-measurement requirement introduces error in the signature estimates, and the errors differ depending upon the type of measurement scheme used. Here, we investigate a LWIR linear microgrid polarimeter. This type of instrument consists of a mosaic of micropolarizers at different orientations that are masked directly onto a focal plane array sensor. In this scheme, each polarization measurement is acquired spatially and hence each is made at a different point in the scene. This is a significant source of error, as it violates the requirement that each polarization measurement have the same instantaneous field-of-view (IFOV). In this paper, we first study the amount of error introduced by the IFOV handicap in microgrid instruments. We then proceed to investigate means for mitigating the effects of these errors to improve the quality of polarimetric imagery. In particular, we examine different interpolation schemes and gauge their performance. These studies are completed through the use of both real instrumental and modeled data.
Evaluation and display of polarimetric image data using long-wave cooled microgrid focal plane arrays
Recent developments for Long Wave InfraRed (LWIR) imaging polarimeters include incorporating a microgrid polarizer array onto the focal plane array (FPA). Inherent advantages over typical polarimeters include packaging and instantaneous acquisition of thermal and polarimetric information. This allows for real time video of thermal and polarimetric products. The microgrid approach has inherent polarization measurement error due to the spatial sampling of a non-uniform scene, residual pixel to pixel variations in the gain corrected responsivity and in the noise equivalent input (NEI), and variations in the pixel to pixel micro-polarizer performance. The Degree of Linear Polarization (DoLP) is highly sensitive to these parameters and is consequently used as a metric to explore instrument sensitivities. Image processing and fusion techniques are used to take advantage of the inherent thermal and polarimetric sensing capability of this FPA, providing additional scene information in real time. Optimal operating conditions are employed to improve FPA uniformity and sensitivity. Data from two DRS Infrared Technologies, L.P. (DRS) microgrid polarizer HgCdTe FPAs are presented. One FPA resides in a liquid nitrogen (LN2) pour filled dewar with a 80°K nominal operating temperature. The other FPA resides in a cryogenic (cryo) dewar with a 60° K nominal operating temperature.
Scanning linear polarimeter for aerosol sensing
Derek Sabatke, Bryan Karpowicz, Shelley Petroy, et al.
An imaging polarimeter for sensing of aerosol scattering and other atmospheric phenomena has been constructed and tested. The instrument is a testbed for a multispectral system architecture, in which spectral channels are added in a modular fashion using dichroic beamspltters and dedicated detectors. The testbed operates in a pushbroom scanning mode, with two co-boresighted optical trains. Each optical train features a narrow-band filter, an intermediate image at a slit, collimating optics, an appropriately oriented Wollaston prism, and two linear detector arrays. Consequently, the testbed is capable of determining the first three Stoke components (linear polarization) at a single wavelength. We describe calibration and field testing and present preliminary data analysis results.
Polarization Elements and Analysis I
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Visible Stokes imaging using programmable waveplate based on PLZT ceramic
Sebastien Breugnot, Philippe Clemenceau, Maxence De Geuser, et al.
Along with intensity and spectrum, the polarization of light carries abundant information. Polarization imaging has established strong interest for visual appearance measurement, based on its ability to analyze scattered light and for defense applications, thanks to its performances in term of object detection/identification. Image contrast enhancement and information on the objects (natural, man-made, detection of water bodies, 3D shape...) composing the scene can also be derived from the polarization analysis. In this paper, we will present an innovative polarization component based on ceramic PLZT and its integration in an imaging system. It will lead to a passive polarization camera that will measure the 4 Stokes parameters for each pixel of the image, in real-time, without any mechanical rotation and at high frame per second. Based on PLZT ceramic, we will present the design and the manufacturing of a rotatable and programmable waveplate. It will be the key component of a passive polarization imaging system. The component will be optimized, fabricated and integrated into a passive polarization camera. The performances of the polarization camera will be demonstrated in the laboratory. Measurement of Stokes vector for each pixel of the image will allow precise polarization measurement, leading to accurate analysis of the scattered light. Various parameters (gloss, color...) and images (polarization degree, surface scattering, volume scattering...) will be calculated from the Stokes parameters.
Characterization of commercial sheet polarizer material
Dennis H. Goldstein, Douglas G. Jones
Sheet polarizers were invented by Land in the 1920s. The fabrication of the type of sheet polarizers we use today, i.e. H-sheet polarizers, was described in the basic H-sheet patent issued in 1948. Single polarizer transmittance, and parallel pair and crossed pair transmittance are typically quoted for these polarizers. In this paper we describe spectropolarimetric measurement results for a variety of commercial sheet polarizer and sheet retarder materials. The measurements cover the nominal spectral region for the polarization elements but also describe performance well beyond the advertised range. Mueller matrices for the elements were measured, and diattenuation and retardance for both polarizers and retarders are presented.
All-glass broadband VIS-NIR linear polarizer for specialized applications
Rachid Gafsi, Katherine R. Rossington, Peter A. Schrauth
In this paper, the processes and results will be presented for all glass polarizer covering the wavelength range of visible (VIS) to the near Infra-Red (IR) band. Currently, Corning Incorporated manufactures and commercializes several polarizers for nominal wavelengths ranging from 633 nm to 2100 nm. For each of these polarizers, the polarization bandwidth is about 80 nm, except for Polarcor W I D E Band product, which has a polarization bandwidth of 370 nm. The all-glass absorptive polarizer discussed in this paper has a polarization bandwidth of at least 400 nm and high Polarization Extinction Ratio (PER) over the bandwidth 600 nm to 1100 nm. The polarizer is monolithic, hence free of epoxy or optical cement. The polarizer spectrum of PER and % transmission without anti-reflection coating will be presented and discussed. This new polarizer offers excellent optical properties, as well as high durability and consistency, which will offer several advantages and benefits when it is deployed in optical components, devices and systems. The applications for this polarizer may include: polarization dependent optical isolators, polarimetry systems, ellipsometers, electro-optics and liquid crystal modulators, and many other polarization based devices and systems. The new polarizer can be used in specialized governmental applications where Polarcor glass polarizers are presently used.
Air-grid polarization splitter for infrared imaging systems
Takayuki Nakano, Yasuhisa Tamagawa
Polarimetric sensing is adequate for detection of man-made targets, which generally have artificial smooth surfaces. In this paper, we propose an infrared polarization imaging system for detection of man-made targets in natural backgrounds. In order to obtain the polarization properties, images in different polarization states are required. Sequential capturing of polarization images is not suitable for practical systems because the movements of targets and backgrounds cause errors in polarization estimation. The proposed system is capable of capturing simultaneous polarization images. A Brewster angle polarization splitter (BAPS) is used for the polarization imaging system to separate the polarization components of the incident light. We introduce a new type of BAPS structure, called air-grid structure. The air-grid structure is composed of a series of parallel cavity lines on a single plane and shows structural birefringence. As a result, the refractive indices of the air-grid structure satisfy the Brewster angle condition and the total reflection for orthogonal polarization components at the same incident angle. This new structure enhances the field of vie of the BAPS and is ideal for imaging systems.
Polarization Elements and Analysis II
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Transmission ellipsometry of transparent-film transparent-substrate systems: closed-form inversion for the film optical constant
A. R. M. Zaghloul, M. Elshazly-Zaghloul, Y. A. Zaghloul
A closed-form formula for the film optical constant is presented. The derivation of the formula itself is not presented to save the reader lots of involved transformations and algebra. The formula in itself is algebraically accurate and does not introduce errors. The effects of experimental errors, random and systematic, are presented. The results are very accurate. The inversion process is very fast, stable, and resilient, does not need a guessed close-to-solution or any starting value, always provides the correct answer with no divergence in any case, and is not iterative in nature. Clearly, those are important advantages over the widely used, manufacturer supplied, fitting routines. It provides for real-time applications in research and industry.
Polarization effects aboard the Space Interferometry Mission
For precision displacement measurements, laser metrology is currently one of the most accurate measurements. Often, the measurement is located some distance away from the laser source, and as a result, stringent requirements are placed on the laser delivery system with respect to the state of polarization. Such is the case with the fiber distribution assembly (FDA) that is slated to fly aboard the Space Interferometry Mission (SIM) next decade. This system utilizes a concatenated array of couplers, polarizers and lengthy runs of polarization-maintaining (PM) fiber to distribute linearly-polarized light from a single laser to fourteen different optical metrology measurement points throughout the spacecraft. Optical power fluctuations at the point of measurement can be traced back to the polarization extinction ratio (PER) of the concatenated components, in conjunction with the rate of change in phase difference of the light along the slow and fast axes of the PM fiber. Thermal variations are one of the major contributors to this change and can lead to tight spacecraft design requirements. In this presentation, we will discuss our experimentally-validated model which predicts the polarization behavior for various distribution designs, as well as present the thermal performance of various PM components and how this levies thermal control requirements on the spacecraft.
Transmission polarization devices using an unsupported film/pellicle: closed-form design formula
A. R. M. Zaghloul, M. Elshazly-Zaghloul
We present a comprehensive study of the polarization characteristic of an unsupported film/pellicle in the transmission mode through the use of the transmission ellipsometric function τ. We determine the behavior of the unsupported film. In addition, we identify all possible transmission polarization devices using a pellicle. We present for the first time analytically-derived closed-form design formulae for each and every device, in addition to a unified closed-form design formula that works for the design of any and all possible devices. It also works for any general transmission polarization device. Due to the limited size of the paper, we only present a limited number of device characteristics and performance.