Mie and Fraunhofer: use the correct approximation model

The particle size distribution in laser diffraction instruments is calculated from the pattern of the scattered light produced by a dispersed system of particles when the laser beam passes through it. The relationship between particle size and distribution pattern of light is provided by two approximation modes: the Fraunhofer diffraction theory and Mie scattering theory. For the choice of the correct mathematical model, knowledge of particles optical properties such as refractive index and absorption index is necessary. The important factors for the model selection and the effect of optical properties on the calculated particle size distribution (PSD) are described in this white paper.


The interaction of light and matter produces four inherently related scattering phenomena: diffraction, refraction, reflection, and absorption. Diffraction describes the bending of light by the edges of an object, refraction is related to changes that occur when light crosses the boundary between an object and its surrounding medium, reflection occurs as the light returns from an object's surface and absorption is the attenuation of light by the object.

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