Showing papers on "Polarimetry published in 1992"

The Hubble Space Telescope Northern-Hemisphere Grid of Stellar Polarimetric Standards

Abstract: Results are presented from a comprehensive program of optical polarimetry of strongly polarized and null-polarization standard stars. Potential sources of systematic error either instrumental in origin or due to absolute calibration are critically discussed. Although the project emphasized the requirements of a spaceborne optical-UV observatory (HST), the resulting grid of standards represents a distinct improvement over previous such tabulations and offers an opportunity to reference future multiwavelength polarimetry to a common system

Noninvasive optical polarimetric glucose sensing using a true phase measurement technique

Abstract: The development and testing of a noninvasive true phase optical polarimetry sensing system to monitor in vivo glucose concentrations is described. To demonstrate the applicability of this optical sensor for glucose movement, the authors calibrate the system and then test it in vitro using both a glass test cell filled with glucose solution in the physiologic range, with a path length of 0.9 cm to approximate the 1-cm path length present in the anterior chamber of the eye, and then on an excised human eye. The technique used helium neon laser light which was coupled through a rotating linear polarizer along with two stationary linear polarizers and two detectors to produce reference and signal outputs whose amplitudes varied sinusoidally with a frequency of twice the angular velocity of the rotating polarizer, and whose phase was proportional to the rotation of the linear polarization vector passing through the glucose solution. >

Advanced Stokes polarimeter: a new instrument for solar magnetic field research

Abstract: A new Stokes polarimeter for high spatial resolution quantitative measurement of magnetic fields at multiple heights in the solar atmosphere has been constructed by the National Center for Atmospheric Research and the National Solar Observatory. The instrument uses the Vacuum Tower Telescope at Sunspot, New Mexico, and its existing horizontal spectrograph, universal birefringment filter, and image motion stabilization system. The polarimeter uses a rotating retarder polarization modulator with polarization calibration optics. Multiple paired CCDs are used for detection followed by video processing to produce spatial maps of the full state of polarization in restricted regions of the solar spectrum. Two spectral regions encompassing lines sensitive to the Zeeman effect, which form in the photosphere and low chromosphere, are recorded simultaneously. Significant developments include: construction of the new telescope post focus optical arrangement, creation of a polarization model for the telescope, construction of high-speed, low-noise solid state cameras, and construction of computer hardware for receiving and processing high-rate 12-bit digital data.

Uniqueness of Target Decomposition Theorems in Radar Polarimetry

Abstract: A key question in radar polarimetry is the existence of a target dichotomy ie. the existence of more than one target decomposition theory. In this paper we show, by using a coherency matrix approach, that there is no target dichotomy and subsequently only one unique target decomposition. We illustrate its usefulness by giving two examples; the scattering of light by small anisotropic particles and rough surface scattering under physical optics.

Mott electron polarimetry

Abstract: Electron polarimeters based on Mott scattering are extensively used in atomic and molecular, solid state, nuclear, and high‐energy physics. This use stems from the increasing realization that much additional information concerning many physical processes can be obtained through spin‐dependent measurements. In this review we discuss the basic physics and application of Mott polarimetry. A number of different Mott polarimeter designs are described that illustrate the wide range of operating energies (10 eV–1 MeV) and geometries that can be used in such instruments. The calibration of Mott polarimeters is discussed together with the potential sources of systematic error that can arise and that can limit measurement accuracies. The aim is to present a comprehensive practical guide to Mott polarimetry and the capabilities of the technique.

Polarimetric discriminators for SAR images

Abstract: A method is developed for optimizing the degree of polarization of a partially polarized wave reflected by a nonstationary scattering object. The method permits, for a scattered wave and a given target characterized by its Mueller matrix, analytic computation of the maximum and minimum values of the degree of polarization, and the corresponding transmitted polarizations. A procedure for the optimization of the scattered wave intensity is also proposed. The degree of polarization and the total scattered intensity extrema are then analyzed experimentally on JPL data. It is shown that several entities such as the received intensity extrema, the coefficient of variation, the fractional polarization and the span, which are currently used for target discrimination, can be deduced from combinations of the maximum and minimum values of the degree of polarization and the scattered wave intensity. Finally, a classification of the San Francisco image, based on these indices, is conducted for a better understanding of the specific physical meaning of each index. >

Antennas for radar and communications : a polarimetric approach

Abstract: Arrays, Broadband Antennas, and Noise Representations of Wave Polarization Polarization Matching of Antennas Polarization Characteristics of Some Antennas Polarization Changes by Reflection and Transmission Partial Polarization Polarization Measurements Target Detection Appendices Index.

Direct and inverse methods in radar polarimetry

Abstract: Basic polarimetric radar theory polarimetric scattering theories polarimetric metrology and systems calibration polarimetric vector signal processing target vs clutter discrimination vector (polarization) diffraction tomography and environmental sensing polarimetric radar systems design and operation polarimetric synthetic aperture radar (POL-SAR) and inverse SAR (POL-ISAR) systems polarimetric radar meteorology and oceanography final reporting of working discussion group activities final technical program outline.

Basic Concepts of Radar Polarimetry

Abstract: A comprehensive overview of the basic principles of radar polarimetry is presented. The relevant fundamental field equations are first provided in order to introduce the polarization state formulations of electromagnetic waves in the frequency domain, including the Jones and the Stokes vector formalism and its presentation on the Poincare sphere and on relevant map projections. In a next step, the scattering matrices [S] and [M] are given together with change of polarization bases transformation operators, where upon the optimal (characteristic) polarization states are determined for the coherent and partially coherent cases, respectively. This chapter is concluded with a set of simple examples.

Fifteen chord FIR polarimetry system on MTX

Abstract: A far‐infrared polarimeter diagnostic has been added to an existing fifteen chord interferometer on the Microwave Tokamak Experiment (MTX). The polarimeter utilizes a new technique for determination of the Faraday rotation angle based on phase measurements of a rotating polarization ellipse. This technique allows the rotation angle to be determined even in the presence of signal amplitude variations caused by refraction. The implementation of this instrument requires no new detectors and minimal optics, making it quite inexpensive to add on to existing multichord interferometers. The MTX polarimeter has been operating for about a year and has achieved a resolution of ≤0.2° with a bandwidth of ≂1 kHz and a chord spacing of 1.5 cm. Typical Faraday rotation angles on MTX are in the range of 5°–15°. To obtain the poloidal field, the line‐integrated density and Faraday rotation profiles are inverted in a manner consistent with the Grad–Shafranov equilibrium to first order in the inverse aspect‐ratio expansion. Profile measurements during normal ohmic operation are presented.

Instrumental polarization in the focal plane of telescopes

Abstract: A technique to model the instrumental polarization in the focal plane of a telescope is presented. It takes into account that different rays of an incoming beam suffer different variation of the original polarization in their paths through the system. It also considers that the net effect depends on the way in which the different rays interfere with each other. It is shown that the Mueller matrix which describes this instrumental polarization greatly simplifies if the polarimetric measurements have poor spatial resolution

Galileo Photopolarimeter/Radiometer Experiment

Abstract: The Photopolarimeter/Radiometer (PPR) is a remote sensing instrument on the Galileo Orbiter designed to measure the degree of linear polarization and the intensity of reflected sunlight in ten spectral channels between 410 and 945 nm to determine the physical properties of Jovian clouds and aerosols, and to characterize the texture and microstructure of satellite surfaces. The PPR also measures thermal radiation in five spectral bands between 15 and 100 μm to sense the upper tropospheric temperature structure. Two additional channels which measure spectrally integrated solar and solar plus thermal radiation are used to determine the planetary radiation budget components. The PPR photopolarimetric measurements utilize previously flown technology for high-precision polarimetry using a calcite Wollaston prism and two silicon photodiodes to enable simultaneous detection of the two orthogonal polarization components. The PPR radiometry measurements are made with a lithium tantalate pyroelectric detector utilizing a unique arrangement of radiometric stops and a scene/space chopper blade to enable a warm instrument to sense accurately the much colder scene temperatures.

Problem of polarization degree in spectroscopic photometric ellipsometry (polarimetry)

Abstract: The significance of the degree of polarization on reflection from the sample in photometric ellipsometry is examined. The degree of polarization was calculated from the four Stokes parameters that were measured by photometric ellipsometry in the IR region, including an additional retarder in the experimental setup. Examples are given for the polarization-degree spectra in the IR region for film-covered surfaces and polished surfaces as well as for the excitation of surface polaritons and the Berreman effect [ Phys. Rev.130, 2193 ( 1963)]. In some cases the polarization effects can be explained by an averaging process of the ellipsometric phase over the investigated region.

Solar speckle polarimetry

Abstract: The combination of a polarimeter with real-time frame selection and differential speckle imaging results in diffraction-limited magnetograms that provide new insight into the morphology of solar small-scale magnetic fields. The method to record diffraction limited narrow-band filtergrams of solar features is based on two cameras taking simultaneous short exposure images through a broad-band and a narrow-band filter, respectively. Speckle imaging reconstructs the image in the broad-band channel. This reconstruction determines the instantaneous optical transfer function (OTF) for each individual broad-band exposure

Determination of the characteristic polarization states of the radar target scattering matrix [S(AB)] for the coherent monostatic and reciprocal propagation space by using the complex polarization ratio ρ transformation formulation

Abstract: We consider a problem originating in radar polarimetry for which the radar target is to be characterized by the properties of its coherent polarization state, given complete coherent backscattering matrix data sets at one frequency and for one target aspect angle. First, the Jones vector formalism for the coherent monostatic case and Sinclair’s backscattering matrix [S(AB)] for the general polarization basis (AB) are introduced. Using the unitary change of polarization state transformation, we present the concept of the characteristic polarization states of a scatterer, which was introduced by Kennaugh and Huynen. The generalized unitary transformation matrix formulation under the change-of-basis transformation, expressed in terms of the generalized polarization ratio ρ(AB), is developed for emphasizing the unique properties of the interrelation among the existing characteristic polarization states. For the monostatic reciprocal case (SAB = SBA) treated here, it is shown that there exist in total five pairs of characteristic polarization states: the orthogonal cross-polarization null and the copolarization maximum-state pairs, which are identical and share one main circle with the copolarization null and the orthogonal cross-polarization maximum-state pairs, the latter being at right angles to the cross-polarization null pairs; and a newly identified pair, the orthogonal cross-polarization saddle-point extrema that are normal to the plane (main circle) spanned by the other four pairs. With this complete and unique mathematical description of Huynen’s polarization fork concept, it is possible to study the problem of polarimetric radar target optimization more rigorously. Various examples are provided and interpreted by comparing the unique result with the results of previous incomplete analyses. In conclusion, the relevance of these canonical results to optical polarimetry are highlighted and interpreted.

Polarimetric Speckle Filters For SAR Data

Abstract: The usual polarimetric speckle filters ophally combine the polarization channels into a single image (Novak and Burl) or only restore the radiometric information (Lee et al.), i.e. the 3 Ihh, I, Ihv intensities in the r e c i p r d case. So the phase differences and the polarization channel correlation coefficients are not restored in the fdtered data. This implies a loss of information compared to the initial data, which contain in the reciprocal case 5 independent real parameters plus 1 absolute phase for 1 look scattering matrix format and 9 independent parameters for multi-look data. In this paper we develop a polarimetric minimum mean square error (MMSE) filter and a polarimetric maximum a posteriori (MAP) filter. For each pixel, one obtains on output of the filtering process either a complex "unspeckled" scattering matrix and 3 local correlation coefficients between the polarization channels for 1 look, or the 9 real parameters of the covariance matrix for multi-look images.

Linear polarization sensitivity specifications for spaceborne instruments

Abstract: The radiometric accuracy of space-borne instruments such as radiometers and spectroradiometers which make measurements of the earth and other celestial objects can be compromised by the linear polarization sensitivity (LPS) induced by the optical system. Most of these optical systems contain optical elements whose reflectance or transmission is polarization dependent, such as diffraction gratings, folding and scanning mirrors, dichroic filters, and optical fibers. Optical system incorporating such elements generally display linear polarization sensitivity; different linear polarization states incident with equal radiometric power will be measured as different power levels. If the incident polarization state is unknown, the linear polarization sensitivity cannot be compensated during the data reduction. The light reflected from the earth and other planets and moons is usually partially linearly polarized, but in a random distribution. Thus, to make accurate radiometric measurements of these bodies, a radiometer or spectrometer should have a low level of linear polarization sensitivity. This paper contains a mathematical description of LPS, contains references to systems which have imposed a LPS specification, describes some of the sources of LPS, describes how to model LPS by polarization ray tracing, and discusses methods to reduce the LPS of an optical system.

Polarization aberration analysis of the advanced x-ray astrophysics facility telescope assembly

Abstract: The Advanced X-ray Astrophysics Facility (AXAF) Telescope consists of six concentric paraboloid-hyperboloid pairs of mirrors that operate near grazing incidence. Because of the substantial polarization effects at large angles of incidence there has been concern regarding the feasibility of doing polarimetry near the telescope focal plane. The primary mirror is here shown to act as a tangentially directed half-wave linear retarder and almost completely to depolarize the linearly polarized component of the light. The secondary mirror introduces and additional half-wave of linear retardance. The two-mirror telescope assembly acts as a tangentially directed one-wave linear retarder. Each mirror depolarizes alone but together the two-mirror assembly preserves the polarization state. The net instrumental polarization effects are small and polarimetry is feasible with AXAF.

On Polarimetric Signatures

Abstract: Formulae describing polarimetric signatures are derived from the polarization synthesis approach using the covariance matrix concept. This derivation is preferred over the Stokes matrix concept, since it is shown that polarimetric signatures are comprised of a limited set of weighting functions of the various elements of the covariance matrix. The graphical representation of the weighting functions, which are dependent on the polarization angles only, enables a quick quantitative analysis of the polarimetric signature of point targets as well as distributed targets. Using the results of the analysis presented, the influence of polarimetric calibration on the polarimetric signature can be rapidly shown. To overcome the, often ambitious, use of polarimetric signatures for determining the scattering characteristics of natural targets, the elements of the covariance matrix must be studied in a more direct fashion. Furthermore, some polarimetric parameters that are often reported in literature, like the coefficient of variation, can be directly expressed in terms of the elements of the covariance matrix. It will be shown that also the use of these parameters is not unambitious in the case of natural targets. The analysis is illustrated with data derived from the polarimetric 1989 MAESTRO-1 JPGAIRSAR imagery obtained over the Dutch Flevoland agricultural test site.

Measurement and Analysis in Radar Polarimetry

Abstract: In this paper we consider methods for the measurement and analysis of polarimetric target information. We concentrate on three main areas; the design of a single channel real time Stokes receiver, conditions for physical realisability of the Stokes reflection matrix and a generalisation of the optimum polarisation theory of Kennaugh and Graves to the case of stochastic targets.

Precision polarimetry of polarization components

Abstract: Polarization elements are inadequately characterized for many applications. For retarders, usually only the retardance is specified. For polarizers, usually only the two principal transmittances or the extinction ratio are given. For polarization elements used in critical applications, this level of characterization is woefully inadequate. Defects of polarization elements are described, and the Mueller calculus suggested as the most appropriate measure of performance. Examples of the characterization of polarization elements as a function of angle of incidence, and as a function of wavelength are provided.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Analysis of polarization signature of immersed targets

Abstract: Polarimetry has become an indispensable analysis tool, in remote sensing as well as in radar metrology. In the same way, it is possible to characterize immersed targets illuminated by a laser beam, by using signal polarization. Here our purpose is to discriminate them in relation with their depolarizing and diffusing properties. Using the Stokes-Mueller formalism and Gil- Bernabeu depolarization criterion, we have synthesized two different types of images from which we build analysis methods. In this paper, we present different results for some characteristic samples.

On The Application Of Multifrequency Polarimetric Radar Observations To Sea-ice Classification

Abstract: The use of multifrequency polarimetric radar imagery to enhance the ability to separate different sea-ice types using single-frequency, single-polarization synthetic aperture radar (SAR) data is investigated. Backscatter characteristics of six radiometrically and polarimetrically distinct sea-ice types are selected in an unsupervised range-dependent analysis of multifrequency polarimetric SAR data using the maximum a posteriori (MAP) polarimetric classifier. Maximum ice discrimination is achieved with combined C- and L-band full polarimetry, and collocated passive microwave imagery suggests greater than 90 percent classification accuracy. C-band VV-pol alone achieves only 68 percent relative accuracy because it confuses multiyear and rough compressed first year ice. L-band, relative classification accuracy is 75 percent, 83 percent, and 85 percent, using HH-pol, HH- and VV-combined, or the full polarimetry, respectively. P-band is less accurate. Combinations of two frequencies at a single polarization show the greatest improvement over a single channel.

Target Detection and Classification with Polarimetric High Range Resolution Radar

Abstract: This paper describes the signal processing theory of a polarimetric radar having very high resolution along range. The radar is meant to be used for surveillance to detect and classify targets against a clutter background. A very high range resolution radar (VHR3) is characterised by a very short pulse which cuts the target into many pieces from which separate echoes can be collected. This radar concept allows to obtain better detection and classification performance with respect to conventional low resolution radar transmitting a pulse which encompasses the whole target extension.

Magnetic structure determination using zero-field neutron polarimetry

Abstract: A simple interpretation of the formulae which predict the polarisation of elastically scattered neutrons and a pictorial representation of the polarisation directions before and after the scattering process are presented. Some results from recent zero-field neutron polarimetry experiments are used to demonstrate the theory.