Prediction of light scattering by plane parallel media

Abstract

The primary motivation of this research is to recognize and establish features of particle deposit reflectivity and absorptivity which have potential for remote detection and characterization of the deposit. In order to interpret measurements generated by remote sensing instruments, one needs to have a detailed understanding of the scattering phenomena and its dependency to physical and chemical properties of the target. The diversity in the particle size, shape, refractive index, and packing density which compose natural and manufactured materials can affect the observable radiometric and polarimetric properties. In this regard, the main purpose of this dissertation is to use numerical simulation techniques to understand how the deposition of particulate material, with known physical, optical, and chemical properties alters the electromagnetic scattering characteristic of the deposit.

This research examines exact formulations for multiple particle scattering, coupled with up–to–date computational techniques, to directly simulate the absorption and scattering properties of particle deposits. The particulate deposits are numerically simulated using Monte Carlo methods with known distribution functions and microscopic configurations of films. One method used for predicting simulated deposit radiative properties is based on an exact superposition solution to Maxwell’s time harmonic wave equations for a deposit of spherical particles that are exposed to a plane incident wave. We use a FORTRAN90 implementation of this solution (the Multiple Sphere T Matrix (MSTM) code), coupled with parallel computational platforms, to directly simulate the reflection from particle layers via a configurational averaging strategy. Another method used for the simulation is the Plane Wave Plane Parallel (PWPP) formulation, which also provides an accurate solution to Maxwell’s time harmonic wave equations for discretely inhomogeneous plane parallel media. The PWPP method, that is based on the discrete dipole approximation (DDA), can directly simulate the polarimetric scattering properties of plane parallel layers of random particulate media. In this technique the medium is modeled as a periodic lattice of unit cells extending infinitely in the lateral directions with finite or infinite depth. The PWPP method is also used to perform comprehensive computational examinations of the effect of pigment particle physical structure, chemical composition, and volume concentration on the spectral reflectance and transmittance properties of pigment–binder coatings. We also used the phenomenological radiative transfer theory (RTT) to predict scattering properties of electromagnetic radiation in media composed of randomly and sparsely distributed particles. Analytical studies of the phenomenological radiative transfer equation (RTE) have formed a separate branch of electromagnetic theory. The adding and doubling method is used as the numerical solution to the scalar and vector RTE for a plane—parallel configuration. Mie theory is used in our RTE model to predict the extinction coefficient, albedo, and scattering phase function of the particles.