Light scattering and absorption by soot in the presence of sulfate aerosols

The radiative properties of aerosol-soot mixtures, both internal and external, are determined in the visible and near-infrared bands by use of exact indirect mode-matching solutions to electromagnetic-wave scattering from a sphere with an eccentric spherical inclusion and from a cluster of spheres. Spherical sulfate droplets are assumed to represent aerosol particles. Soot particles are represented by volume-equivalent carbon spheres, the size distribution of which is obtained from the number distribution of the primary carbon particles that aggregate into soot grains. The mean gram-specific absorption cross section and the mean albedo of aerosol-soot mixtures are obtained by integration of the corresponding characteristics of composite sulfate-carbon particles over the size range of carbon spheres. Enhanced absorption of light by soot in aerosol-soot mixtures, a result of lensing by sulfate droplets, is highlighted by maps of the electromagnetic field in a sulfate-carbon particle.     

Light scattering and absorption by fractal-like carbonaceous chain aggregates: comparison of theories and experiment

This study compares the optical coefficients of size-selected soot particles measured at a wavelength of 870 nm with those predicted by three theories, namely, Rayleigh-Debye-Gans (RDG) approximation, volume-equivalent Mie theory, and integral equation formulation for scattering (IEFS). Soot particles, produced by a premixed ethene flame, were size-selected using two differential mobility analyzers in series, and their scattering and absorption coefficients were measured with nephelometry and photoacoustic spectroscopy. Scanning electron microscopy and image processing techniques were used for the parameterization of the structural properties of the fractal-like soot aggregates. The aggregate structural parameters were used to evaluate the predictions of the optical coefficients based on the three light-scattering and absorption theories. Our results show that the RDG approximation agrees within 10% with the experimental results and the exact electromagnetic calculations of the IEFS theory. Volume-equivalent Mie theory overpredicts the experimental scattering coefficient by a factor of approximately 3.2. The optical coefficients predicted by the RDG approximation showed pronounced sensitivity to changes in monomer mean diameter, the count median diameter of the aggregates, and the geometric standard deviation of the aggregate number size distribution.     

Light scattering by wood fibers

We consider the scattering of light by single wood fibers both theoretically and experimentally. We describe the size and the shape distributions and the internal structure and chemical composition of the wood fibers. We have modeled the random shape of the hollow, cylindrical wood fiber by using multivariate lognormal statistics. We have computed wood-fiber absorption and scattering cross sections, asymmetry parameters, and scattering phase matrices in the ray-optics approximation. Finally, we have provided experimental results from angular scattering measurements for wood fibers and present what we believe is the first comparison between these measurements and ray-optics computations for Gaussian random wood-fiber models. In spite of the complicated internal structure of the wood fiber, our model together with the ray-optics treatment explains the scattering measurements surprisingly well.     

Light scattering by a multilayer sphere

The recurrence algorithm for calculating electromagnetic scattering from a multilayer sphere, which was described recently by Wu and Wang [Radio Sci. 26, 1393, (1991)], is derived in a slightly modified form and extended to include a calculation of the internal field and the absorption cross sections of the individual layers. The original algorithm calculates the scattering by a recurrence procedure that propagates the log derivatives of the Debye potentials outward from the core to the outer layer. The extended algorithm then continues the calculation by an inward recurrence procedure that propagates the Debye potentials from the outer layer to the core. Concurrent with the inward propagation, a separate algorithm calculates the absorption cross sections of the imbedded concentric spheres. The results of several example calculations are presented, including the differential cross section and internal electric field of a Luneburg lens.     

Light absorption by black sand dust

Light absorption by fine and coarse aerosols derived from Hawaiian black sand was determined by light transmission. The real part of the refractive index of this material was measured directly, and the imaginary part was estimated. The measured light absorption coefficients (B(ap)) were adjusted for multiple scattering artifacts by use of absorption-to-extinction ratios estimated with Mie theory. The best agreement between calculated and measured fine and coarse B(ap) was achieved with a value of 0.014 for the imaginary part of the refractive index. The corresponding absorption efficiencies for the fine and coarse black sand aerosols were 0.22 and 0.09 m(2)/g, respectively.     

Light scattering by multiple red blood cells

The interaction of light with multiple red blood cells was systematically investigated by the finite-difference time-domain method (FDTD). The simulations showed that the lateral multiple scattering between red blood cells is very weak and that the polarization has an almost insignificant influence on the distribution of the scattered light. The numerical results of the FDTD method were compared with the results from the Rytov approximation and the discrete dipole approximation (DDA). The agreement with the DDA was excellent.     

Light scattering by a core-mantle spheroidal particle

A solution of the electromagnetic scattering problem for confocal coated spheroids has been obtained by the method of separation of variables in a spheroidal coordinate system. The main features of the solution are (i) the incident, scattered, and internal radiation fields are divided into two parts: an axisymmetric part independent of the azimuthal angle ? and a nonaxisymmetric part that with integration over ? gives zero; the diffraction problems for each part are solved separately; (ii) the scalar potentials of the solution are chosen in a special way: Abraham's potentials (for the axisymmetric part) and a superposition of the potentials used for spheres and infinitely long cylinders (for the nonaxisymmetric part). Such a procedure has been applied to homogeneous spheroids [Differential Equations 19, 1765 (1983); Astrophys. Space Sci. 204, 19, (1993)] and allows us to solve the light scattering problem for confocal spheroids with an arbitrary refractive index, size, and shape of the core or mantle. Numerical tests are described in detail. The efficiency factors have been calculated for prolate and oblate spheroids with refractive indices of 1.5 + 0.0 i, 1.5 + 0.05 i for the core and refractive indices of 1.3 + 0.0 i, 1.3 + 0.05i for the mantle. The effects of the core size and particle shape as well as those of absorption in the core or mantle are examined. It is found that the efficiency factors of the coated and homogeneous spheroids with the volume-averaged refractive index are similar to first maximum.     

Light scattering by acoustic phonons in ferroelectric and piezoelectric crystals

The characteristic features of light scattering by acoustic phonons in ferroelectric and piezoelectric crystals are briefly reviewed. The influence of piezoelectric and electro-optical effects and optical activity on scattering is discussed. Anomalies in scattering at ferroelectric phase transitions are considered.     

Light scattering by a multilayered spheroidal particle

The light scattering problem for a confocal multilayered spheroid has been solved by the extended boundary condition method with a corresponding spheroidal basis. The solution preserves the advantages of the approach applied previously to homogeneous and core-mantle spheroids, i.e., the separation of the radiation fields into two parts and a special choice of scalar potentials for each of the parts. The method is known to be useful in a wide range of the particle parameters. It is particularly efficient for strongly prolate and oblate spheroids. Numerical tests are described. Illustrative calculations have shown that the extinction factors converge to average values with a growing number of layers and how the extinction varies with a growth of particle porosity.     

Light scattering by aggregated red blood cells

In low flow rates, red blood cells (RBCs) fasten together along their axis of symmetry and form a so-called rouleaux. The scattering of He-Ne laser light by a rouleau consisting of n (2 < or = n < or = 8) average-sized RBCs is investigated. The interaction problem is treated numerically by means of an advanced axisymmetric boundary element--fast Fourier transform methodology. The scattering problem of one RBC was solved first, and the results showed that the influence of the RBC's membrane on the scattering patterns is negligible. Thus the rouleau is modeled as an axisymmetric, homogeneous, low-contrast dielectric cylinder, on the surface of which appears, owing to aggregated RBCs, a periodic roughness along the direction of symmetry. The direction of the incident laser light is considered to be perpendicular to the scatterer's axis of symmetry. The differential scattering cross sections in both perpendicular and parallel scattering planes and for all the scattering angles are calculated and presented in detail.     

Light scattering by ellipsoids in a physical optics approximation

A physical optics approximation based on Presnel's laws is developed to calculate the intensity of light scattered by a three-axis ellipsoid of any orientation and any refractive index. Some results concerning totally reflecting spheres and dielectric spheroids are presented. An approach suitable for large scatterers is particularly good for small scattering angles. The angular intensities, i(1) and i(2), are then plotted versus θ for large axially oriented ellipsoids of various thicknesses. Theoretical small-angle light-scattering patterns are also presented and discussed. The data from one of them correspond to red cells in a shear flow.     

Light scattering in liquid helium

A quantum-mechanical formulation of light scattering in a dielectric fluid is applied to liquid helium. To lowest order in the (small) dielectric susceptibilityn, the theory reproduces the usual expression for Brillouin scattering. Two-roton processes occur in both first and second order inn, and the relevant ground-state matrix elements are evaluated with Feynman wave functions in the convolution approximation. The resulting two-roton differential cross section is significantly smaller than Stephen's prediction, in good agreement with the experiments of Greytak and Yan. The depolarization is somewhat reduced from that of Stephen; present techniques should be able to detect this difference.Research sponsored by the Air Force Office of Scientific Research, Office of Aerospace Research, U.S. Air Force, under AFOSR Contract No. F44620-71-C-0044.Alfred P. Sloan Foundation Research Fellow.     

Light scattering by randomly rough isotropic dielectric surfaces

Abstract

An experimental investigation of the angular distribution of the light scattered by randomly rough, two-dimensional, isotropic dielectric surfaces is presented. The surfaces, whose profiles constitute good approximations to Gaussian random processes with Gaussian correlation functions, are fabricated in a photoresist and characterized by means of a mechanical profilometer. The substrates employed in the fabrication of the samples consist of thick parallel plates of filter glass that absorb the incident light and whose refractive index is close to that of the photoresist. This allows us to approximate experimentally a situation in which the light is scattered by a randomly rough interface between two semi-infinite dielectric media, illuminated from the air side. The results display features that can be attributed to multiple scattering. In particular, significant amounts of cross-polarized scattered light, as well as an enhanced backscattering peak, were observed in the scattering measurements.     

海水光谱吸收/散射系数测量仪的研制

在克服了由于海水的光谱吸收和散射很小的难点下,研制成功具有高分辨和高精度能力的室外现场测试仪器,其结构紧凑合理,测量装置构思精巧,测试过程完全自动化,克服环境和测试条件的多变性带来的影响,实现了海水吸收系数与散射系数的瞬态测量。现场海水光谱吸收/散射系统测量装置为国内首创,填补了国内空白。