| Summary: | This thesis presents a method to extract complex refractive index (optical constants), m(A) = n(A) + ik(A) in the ultra-violet, visible and near-infrared wavelength regions, of particles in suspensions at concentrations where multiple scattering is significant. Optical constants can provide important information required for process and quality control of suspensions of particles, slurries, and biological suspensions. Moreover, accurate values of n(A) and k(A) are required to estimate particle size and size distribution using light scattering methods. The optical constants are obtained by inverting measurements of total diffuse reflectance and transmittance. The inversion method is based on solving the Radiative Transfer Equation (RTE), using the adding-doubling method, to account for multiple scattering along with single scattering theories to describe single-particle scattering characteristics. The method was initially implemented to estimate the optical constants of spherical particles in the UV-Vis-NIR region using the exact Mie theory for describing single scattering and taking polydispersity into account. This was applied to polystyrene and poly(methylmethacrylate) suspensions. The effect of particle size and concentration on the estimated optical constants were investigated. The inversion of multiple-scattered light measurements to extract optical constants is computationally intensive. Different approximations for computing the optical properties (absorption and scattering cross-section, anisotropic factor, and phase function) so as to significantly accelerate the calculations without introducing large inaccuracies were investigated. A comparison between the inverse method and the Kramers-Kronig relations, which extracted n(A) from known values of k(A), was also performed. In practice, we have to deal with systems of non-spherical particles as biological suspensions. An extension of the method to extract the optical constants that takes into account particle shape using the Rayleigh-Gans-Debye approximation and the T-Matrix method for non-spherical particles was implemented. This method was applied to a suspension ofBacillus subtilis spores to obtain the optical constants in the UV-Vis-NIR region.
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