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Fymat, A. L.

Publications and source records attributed to Fymat, A. L..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

High resolution Fourier interferometer-spectrophotopolarimeter

A high-resolution Fourier interferometer-spectrophotopolarimeter is provided using a single linear polarizer-analyzer the transmission axis azimuth of which is positioned successively in the three orientations of 0 deg, 45 deg, and 90 deg, in front of a detector; four flat mirrors, three of which are switchable to either of two positions to direct an incoming beam from an interferometer to the polarizer-analyzer around a sample cell transmitted through a medium in a cell and reflected by medium in the cell; and four fixed focussing lenses, all located in a sample chamber attached at the exit side of the interferometer. This arrangement can provide the distribution of energy and complete polarization state across the spectrum of the reference light entering from the interferometer; the same light after a fixed-angle reflection from the sample cell containing a medium to be analyzed; and the same light after direct transmission through the same sample cell, with the spectral resolution provided by the interferometer.

Fymat, A. L.↗
NASA NTRS1976

Inverse atmospheric radiative transfer problems - A nonlinear minimization search method of solution

The paper studies the inversion of the radiative transfer equation describing the interaction of electromagnetic radiation with atmospheric aerosols. The interaction can be considered as the propagation in the aerosol medium of two light beams: the direct beam in the line-of-sight attenuated by absorption and scattering, and the diffuse beam arising from scattering into the viewing direction, which propagates more or less in random fashion. The latter beam has single scattering and multiple scattering contributions. In the former case and for single scattering, the problem is reducible to first-kind Fredholm equations, while for multiple scattering it is necessary to invert partial integrodifferential equations. A nonlinear minimization search method, applicable to the solution of both types of problems has been developed, and is applied here to the problem of monitoring aerosol pollution, namely the complex refractive index and size distribution of aerosol particles.

Fymat, A. L.↗
NASA NTRS1976

Remote probing of atmospheric particulates from radiation extinction experiments: A review of methods

The existing methodology for reconstructing the particle size distribution and inferring the refractive index of absorbing and scattering atmospheric particulates is critically reviewed. Emphasis is placed on method capabilities and shortcomings and, wherever possible, on achievable accuracy. The nature of the associated remote probing problem is analyzed with regard to the effects of the particulates on EM wave propagation in the atmosphere. The parameterization of size distribution is studied within the unifying framework of Pearson's distribution curves. The inversions of extinction measurements and their ratios are considered separately, and the potentialities of each type of measurement are identified. Work lacking in each of the methods reviewed is indicated. A method of determining both the effective complex refractive index and size distribution model parameters from the same data is also presented. Lastly, determination from extinction ratio data of the complex refractive index independent of size distribution is discussed and error analyzed.

Fymat, A. L.↗
NASA NTRS1975

Minimization search method for data inversion

Technique has been developed for determining values of selected subsets of independent variables in mathematical formulations. Required computation time increases with first power of the number of variables. This is in contrast with classical minimization methods for which computational time increases with third power of the number of variables.

Fymat, A. L.↗
NASA NTRS1975

A new approach to radiative transfer theory using Jones's vectors. I Fundamentals

Radiative transfer of partially polarized radiation in an anisotropically scattering, inhomogeneous atmosphere containing an arbitrary polydispersion of particles is described using Jones's (1941) amplitude vectors and matrices. This approach exploits the close analogy between the quantum-mechanical states of spin 1/2 systems and the polarization states of electromagnetic radiation described by Jones's vector. The complete equivalence between the transport equation for Jones's vectors and the classical radiative-transfer equation for Stokes's intensity vectors is demonstrated in two independent ways after deriving the transport equations for the polarization coherency matrices and for the quaternions corresponding to the Jones's vectors. A compact-operator formulation of the theory is provided and used to derive the necessary equations for both a local and a global description of the transport of Jones's vectors. The integro-differential equations for the amplitude reflection and transmission matrices are derived and related to the usual corresponding equations.

Fymat, A. L.↗
NASA NTRS1975

Mean effective size and refractive index of transparent atmospheric particulates

It is demonstrated that the scattering ratio-principle originally restricted to exact forward and exact backward scattering, and to values of the size parameter and of the product (refractive index x size parameter) less than 0.8 has a much wider applicability. After relaxing these stringent conditions, it is also shown that this principle can be retained as the basis of an experimental technique for retrieving the mean effective size and refractive index of transparent atmospheric particulates.

Fymat, A. L.↗
NASA NTRS1975

Remote monitoring of environmental particulate pollution - A problem in inversion of first-kind integral equations

The determination of the microstructure, chemical nature, and dynamical evolution of scattering particulates in the atmosphere is considered. A description is given of indirect sampling techniques which can circumvent most of the difficulties associated with direct sampling techniques, taking into account methods based on scattering, extinction, and diffraction of an incident light beam. Approaches for reconstructing the particulate size distribution from the direct and the scattered radiation are discussed. A new method is proposed for determining the chemical composition of the particulates and attention is given to the relevance of methods of solution involving first kind Fredholm integral equations.

Fymat, A. L.↗
NASA NTRS1974

Instrumentation optimization in Fourier spectroscopy. I - Far infrared beam splitters

Computations of the reflectivity, transmissivity, and efficiency properties for TE, TM, and T45 deg waves of far-infrared beam splitters and of the polarizations induced at both reflection and transmission are described. Effects of variations in the state of polarization, orientation, pointing accuracy, and wavelength of the incident light, as well as variations in refractive index and thickness of the beam splitters, are discussed. These results apply directly to Fourier interferometer-spectrometers. They can be used for optimizing the performance of these instruments. They indicate, in particular, that some advantages may be gained by the use of incident polarized light (angle of polarization smaller than about 45 deg or negative elliptical polarization) or light of large incidence angle (larger than approximately 60 deg) or both.

Deuze, J. L.↗
NASA NTRS1974

Polarization in astronomical spectra - Theoretical evidence

Theoretical evidence for the existence and behavior of polarization in astronomical spectra is provided. The theory for the study of spectral multiple scattering of arbitrarily polarized light is first developed, and the detailed and integrated spectropolarimetry of a planetary atmosphere is then studied for cases in which the spectra are formed in the presence of either very small nonspherical particles (Rayleigh-Cabannes scattering) or large polydisperse spherical particles (Mie scattering). It is shown in both cases that polarization is indeed present; it increases with the line strength but decreases afterwards as the line becomes very strong and tends to saturation. A polarization reversal is also predicted during latitudinal (pole-to-equator) scan and possibly also during longitudinal (terminator-to-limb) scan of the planet. The reversal happened at all phase angles considered. Our companion article (Forbes and Fymat) will provide observational substantiation to these theoretical predictions.

Fymat, A. L.↗
NASA NTRS1974

Astronomical Fourier spectropolarimetry

Spectra of the Stokes polarization parameters of Venus (resolution 0.5 per cm) are presented. They were obtained at the Cassegrain focus of the 154-cm telescope of the National Mexican Observatory, Baja California, Mexico, July 12 and 13, 1972, with the Fourier Interferometer Polarimeter (FIP). A preliminary, limited analysis of four spectral features and of the CO2 rotational band structures at 6080 and 6200 per cm has demonstrated that spectral polarization is indeed present. These experimental results, confirmed by two series of observations, provide substantiation for this theoretically predicted phenomenon. They also tend to show that the FIP represents a novel astronomical tool for variable spectral resolution studies of both the intensity and the state of polarization of astronomical light sources.

Forbes, F. F.↗