Examination of the Phenomena Concerning Optical Particle Measurements Using the Direct Finite-Difference Solution of Maxwell's Equation: Influence of Multiple Scattering and Three-Dimensionality of Scatterer on Diffraction

At first, the accuracy of the scattering patterns predicted for typical examples by a direct finite-difference solution technique was examined by comparing them with the analytical solutions. Comparisons with regard to the scattering patters formed by a single cylinder, a pair of cylinders or a single true sphere showed good agreement. Based on this confirmation of accuracy of the direct finite-difference solution method, the influence of multiple-scattering and three-dimensionality of scatterer on the diffraction was discussed. Even if the volume fraction of particles is same, larger number density of scatterers leads to more remarkable distortion of scattering pattern and subsequent erroneous measurement of particle size distribution. When a cylindrical scatterer is long in the direction of optical axis, its scattering pattern is remarkably different from that formed by a true sphere of the same diameter. That is, the diffraction pattern is affected by the three-dimensionality of scatterer.