Asymptotic solutions for optical properties of large particles with strong absorption

The transverse wave condition is not applicable to the refracted electromagnetic wave within the context of geometrical optics when absorption is involved. Either the TM or the TE wave condition can be assumed for the wave to locally satisfy the electromagnetic boundary condition in a ray-tracing calculation. The assumed wave mode affects both the reflection and the refraction coefficients. As a result, nonunique solutions for these coefficients are inevitable. In this study the appropriate solutions for the Fresnel reflection-refraction coefficients are identified in light-scattering calculations based on the ray-tracing technique. In particular, a 3 x 2 refraction or transmission matrix is derived to account for the inhomogeneity of the refracted wave in an absorbing medium. An asymptotic solution that completely includes the effect of medium absorption on Fresnel coefficients is obtained for the scattering properties of a general polyhedral particle. Numerical results are presented for hexagonal plates and columns with both preferred and random orientations.

Scattering from nonspherical Chebyshev particles. 3: Variability in angular scattering patterns

We study shape-induced variability in the scattered intensity from randomly oriented nonspherical particles. Up to 21 different Chebyshev shapes contribute to defining a shape-induced standard deviation about each of the mean nonspherical intensity vs angle curves shown in part 2 of this series. Bands of shape-induced variability (defined as plus and minus one standard deviation) for six size intervals within the size parameter range 1 </= x </= 20 are compared with corresponding spherical intensities. Averaging spherical intensities over narrow size ranges produces effects qualitatively similar to mildly distorting a single sphere. Nevertheless, among all shapes, the sphere is often the most anomalous scatterer; nonspherical scattered intensities tend to be closer to one another than to corresponding spherical intensities. For Chebyshev particles which are neither small nor large compared to the wavelength, shape-induced variability is often comparable to the mean. Furthermore, outside the forward-scattering reg ion, this variability is large relative to the deformation from a sphere. The standard deviation is up to 50% of the mean scattered intensity for particles with an average deformation of only ~10%. This exaggerated sensitivity to shape will make it difficult to define representative angular scattering curves for many real-world nonspherical scattering problems which involve imperfect shape information.

Scattering from nonspherical Chebyshev particles. 2: Means of angular scattering patterns

The calculated angular scattering properties of over 250 randomly oriented nonspherical Chebyshev particles are examined for the effect of three factors: size; concavity vs convexity; and amount of deformation from a sphere. Both shape and size averaging are performed to reveal general features of the angular scattering not discernible for particular shapes and sizes. Comparisons with a comparably extensive experimental study published by Zerull in 1976 reveal remarkable qualitative similarities, even though Zerull used greatly different shapes from ours. This augurs well for the eventual development of a general theory of nonspherical scattering, although such a theory must account for concavity in addition to the amount of deviation from a sphere; and it cannot be entirely deterministic, as the third paper in this series will argue.

Forward optical glory

Forward optical glory effects in Mie scattering are displayed here for the first time to our knowledge. These effects include regular oscillations in Mie efficiency factors and characteristic deviations from zero polarization in near-forward scattering, which are observable for real refractive indices near radical2 and 2. Complex angular momentum theory predicts the period of oscillation correctly and shows the important role played by surface waves with shortcuts through the sphere. Three possible types of experiment for detecting the forward glory are proposed, involving measurements of extinction, radiation pressure, and polarization in near-forward scattering.

Improved Mie scattering algorithms

Scattering of electromagnetic radiation from a sphere, so-called Mie scattering, requires calculations that can become lengthy and even impossible for those with limited resources. At the same time, such calculations are required for the widest variety of optical applications, extending from the shortest UV to the longest microwave and radar wavelengths. This paper briefly describes new and thoroughly documented Mie scattering algorithms that result in considerable improvements in speed by employing more efficient formulations and vector structure. The algorithms are particularly fast on the Cray-1 and similar vector-processing computers.