Biomedical subjects

George W Kattawar

Publications and source records attributed to George W Kattawar.

11 recordsLinked to original sources

PubMed2006

Integrating cavities: temporal response.

The temporal response of an integrating cavity is examined and compared with the results of a Monte Carlo analysis. An important parameter in the temporal response is the average distance d between successive reflections at the cavity wall; d was calculated for several specific cavity designs--spherical shell, cube, right circular cylinder, irregular tetrahedron, and prism; however, only the calculation for the spherical shell and the right circular cylinder will be presented. A completely general formulation of d for arbitrary cavity shapes is then derived, d =4V/S where V is the volume of the cavity, and S is the surface area of the cavity. Finally, we consider an arbitrary cavity shape for which each flat face is tangent to a single inscribed sphere of diameter D (a curved surface is considered to be an infinite number of flat surfaces). We will prove that for such a cavity d =2D/3, exactly the same as d for the inscribed sphere.

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PubMed2006

Internal dipole radiation as a tool for particle identification.

A numerical approach for the calculation of the internal dipole radiation associated with particles of arbitrary morphology is investigated by using the discrete-dipole approximation (DDA) method. The DDA and analytical solutions for the total radiated power and radiation pattern are compared in the case of spherical host particles. It is shown that the DDA can be quite accurate under the condition that m <or approximately 2, and mkd<0.5, where m is the refractive index of the host particle, k=2pi/lambda is the wavenumber in vacuum, and d is the distance between two adjacent dipoles in the DDA cubic dipole array. Furthermore, the DDA solutions for the dipole radiation patterns associated with nonspherical host particles are compared with their corresponding counterparts obtained from the finite-difference time-domain method. Excellent agreement between the two results is noted. The DDA method is also applied to the computation of the internal dipole radiation associated with simulated nonspherical sporelike particles. The results suggest that the internal dipole radiation patterns contain a great deal of information about the morphology and composition of the host particle.

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PubMed2006

Design and analysis of a flow-through integrating cavity absorption meter.

We present a design for a flow-through integrating cavity absorption meter. This instrument, in principle, is capable of measuring the spectral optical absorption coefficient of natural waters in situ independently of scattering effects. Monte Carlo simulations are used to determine the design parameters and evaluate instrument performance. We investigate both detector response and the distribution of radiant energy inside the instrument and present empirical equations describing these quantities as a function of the absorption coefficient. The effects of changing the instrument geometry are illustrated. Finally, we discuss the effects of scattering on the instrument performance and verify that they are negligible for natural waters.

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PubMed2005

Scattering and absorption property database for nonspherical ice particles in the near- through far-infrared spectral region.

The single-scattering properties of ice particles in the near- through far-infrared spectral region are computed from a composite method that is based on a combination of the finite-difference time-domain technique, the T-matrix method, an improved geometrical-optics method, and Lorenz-Mie theory. Seven nonspherical ice crystal habits (aggregates, hexagonal solid and hollow columns, hexagonal plates, bullet rosettes, spheroids, and droxtals) are considered. A database of the single-scattering properties for each of these ice particles has been developed at 49 wavelengths between 3 and 100 microm and for particle sizes ranging from 2 to 10,000 microm specified in terms of the particle maximum dimension. The spectral variations of the single-scattering properties are discussed, as well as their dependence on the particle maximum dimension and effective particle size. The comparisons show that the assumption of spherical ice particles in the near-IR through far-IR region is generally not optimal for radiative transfer computation. Furthermore, a parameterization of the bulk optical properties is developed for mid-latitude cirrus clouds based on a set of 21 particle size distributions obtained from various field campaigns.

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PubMed2005

Application of the symplectic finite-difference time-domain method to light scattering by small particles.

A three-dimensional fourth-order finite-difference time-domain (FDTD) program with a symplectic integrator scheme has been developed to solve the problem of light scattering by small particles. The symplectic scheme is nondissipative and requires no more storage than the conventional second-order FDTD scheme. The total-field and scattered-field technique is generalized to provide the incident wave source conditions in the symplectic FDTD (SFDTD) scheme. The perfectly matched layer absorbing boundary condition is employed to truncate the computational domain. Numerical examples demonstrate that the fourth-order SFDTD scheme substantially improves the precision of the near-field calculation. The major shortcoming of the fourth-order SFDTD scheme is that it requires more computer CPU time than a conventional second-order FDTD scheme if the same grid size is used. Thus, to make the SFDTD method efficient for practical applications, one needs to parallelize the corresponding computational code.

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PubMed2004

Comparison of Cartesian grid configurations for application of the finite-difference time-domain method to electromagnetic scattering by dielectric particles.

Two grid configurations can be employed to implement the finite-difference time-domain (FDTD) technique in a Cartesian system. One configuration defines the electric and magnetic field components at the cell edges and cell-face centers, respectively, whereas the other reverses these definitions. These two grid configurations differ in terms of implication on the electromagnetic boundary conditions if the scatterer in the FDTD computation is a dielectric particle. The permittivity has an abrupt transition at the cell interface if the dielectric properties of two adjacent cells are not identical. Similarly, the discontinuity of permittivity is also observed at the edges of neighboring cells that are different in terms of their dielectric constants. We present two FDTD schemes for light scattering by dielectric particles to overcome the above-mentioned discontinuity on the basis of the electromagnetic boundary conditions for the two Cartesian grid configurations. We also present an empirical approach to accelerate the convergence of the discrete Fourier transform to obtain the field values in the frequency domain. As a new application of the FDTD method, we investigate the scattering properties of multibranched bullet-rosette ice crystals at both visible and thermal infrared wavelengths.

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PubMed2004

Effect of particle asphericity on single-scattering parameters: comparison between Platonic solids and spheres.

The single-scattering properties of the Platonic shapes, namely, the tetrahedron, hexahedron, octahedron, dodecahedron, and icosahedron, are investigated by use of the finite-difference time-domain method. These Platonic shapes have different extents of asphericity in terms of the ratios of their volumes (or surface areas) to those of their circumscribed spheres. We present the errors associated with four types of spherical equivalence that are defined on the basis of (a) the particle's geometric dimension (b) equal surface area (A), (c) equal volume (V), and (d) equal-volume-to-surface-area ratio (V/A). Numerical results show that the derivations of the scattering properties of a nonspherical particle from its spherical counterpart depend on the definition of spherical equivalence. For instance, when the Platonic and spherical particles have the same geometric dimension, the phase function for a dodecahedron is more similar than that for an icosahedron to the spherical result even though an icosahedron has more faces than a dodecahedron. However, when the nonspherical and spherical particles have the same volume, the phase function of the icosahedral particle essentially converges to the phase function of the sphere, whereas the result for the dodecahedron is quite different from its spherical counterpart. Furthermore, the present scattering calculation shows that the approximation of a Platonic solid with a sphere based on V/A leads to larger errors than the spherical equivalence based on either volume or projected area.

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PubMed2004

Implementing the near- to far-field transformation in the finite-difference time-domain method.

When the finite-difference time-domain (FDTD) method is applied to light-scattering computations, the far fields can be obtained by means of integrating the near fields either over the volume bounded by the particle's surface or on a regular surface encompassing the scatterer. For light scattering by a sphere, the accurate near-field components on the FDTD-staggered meshes can be computed from the rigorous Lorenz-Mie theory. We investigate the errors associated with these near- to far-field transform methods for a canonical scattering problem associated with spheres. For a scatterer with a small refractive index, the surface-integral approach is more accurate than its volume counterpart for computation of the phase functions and extinction efficiencies; however, the volume-integral approach is more accurate for computation of other scattering matrix elements, such as P12, P32, and P43, especially for backscattering. If a large refractive index is involved, the results computed from the volume-integration method become less accurate, whereas the surface method still retains the same order of accuracy as in the situation for the small refractive index.

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PubMed2004

Geometrical-optics solution to light scattering by droxtal ice crystals.

We investigate the phase matrices of droxtals at wavelengths of 0.66 and 11 microm by using an improved geometrical-optics method. An efficient method is developed to specify the incident rays and the corresponding impinging points on the particle surface necessary to initialize the ray-tracing computations. At the 0.66-microm wavelength, the optical properties of droxtals are different from those of hexagonal ice crystals. At the 11-microm wavelength, the phase functions for droxtals are essentially featureless because of strong absorption within the particles, except for ripple structures that are caused by the phase interference of the diffracted wave.

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PubMed2003

Mueller matrix imaging of targets in turbid media: effect of the volume scattering function.

Detecting objects in turbid media by use of just radiance signals has been a subject of study for many years. The use of Mueller matrix imaging methods has only recently been used as a tool for target detection. We will show not only that can targets still be detected by Mueller matrix methods even after their detection has escaped normal radiance schemes but also that their surface features can also still be distinguished. We will also show how the shape of the volume scattering function as well as the target and medium albedo strongly influences various elements of the Mueller matrix. One of the more interesting features of Mueller matrix imaging is that the diagonal elements are sensitive to perturbations in the environment surrounding the target. This implies that targets can be detected far beyond their geometric cross section. The methods presented here will have applications to submersible object detection, remote sensing in the atmosphere, and the detection of inhomogeneities in tissue.

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PubMed2003

Sensitivity of the backscattering Mueller matrix to particle shape and thermodynamic phase.

The Mueller matrix (M) corresponding to the phase matrix in the backscattering region (scattering angles ranging from 175 degrees to 180 degrees) is investigated for light scattering at a 0.532-microm wavelength by hexagonal ice crystals, ice spheres, and water droplets. For hexagonal ice crystals we assume three aspect ratios (plates, compact columns, and columns). It is shown that the contour patterns of the backscattering Mueller matrix elements other than M11, M44, M14, and M41 depend on particle geometry; M22 and M33 are particularly sensitive to the aspect ratio of ice crystals. The Mueller matrix for spherical ice particles is different from those for nonspherical ice particles. In addition to discriminating between spherical and nonspherical particles, the Mueller matrix may offer some insight as to cloud thermodynamic phase. The contour patterns for large ice spheres with an effective size of 100 microm are substantially different from those associated with small water droplets with an effective size of 4 microm.

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