The spectral variation of the optical properties of soot particles is determined by combining classical and dynamic light scattering measurements with the Kramers-Krönig relations. Particle size and number densities are determined from scattering/extinction and autocorrelation measurements at the wavelength of 0.488 μm. This information is then combined with the spectral extinction measurements in the wavelength range 0.2 to 6.4 μm to determine the spectral variation of the refractive indices of flame soot. Results are presented for a premixed propane-oxygen flame with a fuel equivalence ratio ϕ = 1.8. The sensitivity of the technique and its advantage over the previous methods are discussed.

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An exact formulation for the internal field equation of an assembly of small spherical particles irradiated by an electromagnetic wave is presented. It is demonstrated that the formulation is reduced to the existing solutions by suitable approximations of the self-contribution field term and satisfies the energy conservation (optical theorem) requirement. Further analysis shows that the Jones solution, the Purcell-Pennypacker PP and the Iskander-Chen-Penner ICP solutions violate the energy requirement whereas the Goedecke-O'Brien GO solution was found to be the lowest order compatible approximation of the exact solution, and as such satisfies the conservation of energy. Furthermore, the obtained field equation is invariant to coordinate transformation and yields an invariant T matrix which accounts correctly for multiple scattering effects. Comparisons of the results were carried out with the Jones solution, the PP, ICP and GO solutions. In addition, the applicability of the solution to the inversion of light scattering data, especially from sooting flames, is assessed.

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