Inverse radiation problem for determination of optical constants of fly-ash particles

In this paper, a technique for determining the optical constants of fly-ash particles from spectral transmittance measurements is presented. Combined with the precise Mie theory and the Kramers-Kronig (KK) relation, the complex refractive index was estimated by the spectral transmittance distribution of a cloud of fly-ash particles in potassium bromide (KBr) pellets. The unique and multi-value problems are thoroughly investigated. Particular attention is given to the error analysis of the inverse procedure including the influence of the experimental precision, the numerical simulation of the KK relation, scattering theory, and the single-scattering approximation. Good agreement was obtained between the inverse simulation results and the known optical properties of ash particles available in the literature. At the same time, an experimental analysis for two kinds of ash particles is also presented, and their spectral complex refractive indices in the range (1.0-25 μm) are determined by the inverse model used in the present work.