Light scattering and absorption characteristics of optically soft particles

Abstract

Scattering techniques have been used to infer optical properties of macroscopic particles in various scientific disciplines for many years. To be able to deduce this information about the scatterer it is necessary to have an underlying theory for the analysis of measurements. Unfortunately, the exact analytic solutions are unknown, except in some simplest and most idealized cases. For a number of shapes numerical procedures are a possibility. But in many applications even a numerical approach still proves to be impracticable or even impossible and one needs to resort to approximation methods.Many particles of interest in nature are such that their refractive index m (relative to the surrounding medium) is close to unity, i.e., |m − 1| 1. These are called soft scatterers. A number of approximations have been developed for predicting optical properties of such particles. The purpose of this article is to review the situation regarding soft particle scattering and absorption approximations and study their optical characteristics.Parameters often employed to describe the validity domains in light scattering approximations are the size parameter x = πd/λ and a parameter 2x|m − 1| which can be looked upon as a measure of interaction strength. Here λ is the wavelength of the incident radiation and d is the characteristic size of the scatterer (e.g., the diameter of a spherical particle). Two domains may be identified for the purpose of this article. (i) 2x|m − 1| ≤ 1 (small phase shifts) and (ii) arbitrary 2x|m − 1| (arbitrary phase shifts). Small phase shifts Rayleigh-Gans-Debye approximationIf we denote by E i = E 0 exp(ik i .r − iωt) as the incident electromagnetic field, the four scattering amplitudes S 1 , S 2 , S 3 and S 4 at a large distance r from the scatterer may be defined by the following relation

Approximate method for fast retrieval of particle size distribution in spectral extinction technique

“…The GEA method can be employed to study the light scattering of arbitrarily shaped particles at small scattering angles. The method is to generalize the linear propagator in the eikonal approximation (EA) by adding two adjustable parameters, which compensate for the phase change and edge effect neglected in the EA method [6,7]. The extinction efficiency for a spherical particle is:…”