The Mie theory is used for evaluating the measurements of particles whose diameters are not significantly larger than the wavelength of the light used.

This theory, developed by Gustav Mie at the beginning of the 20th century, is the complete solution of the Maxwell equations for the scattering of electromagnetic waves on spherical particles. It can be used to analyse the characteristic intensity distributions for even very small particles which, contrary to the Fraunhofer theory, are not limited to scattering angles less than 90° (forward/sideward direction), but which also occur for scattering angles greater than 90° (backward direction).
In order to make use of the determined intensity distribution for calculating the particle size, the refraction index and the absorption index of the sample material need to be known for the Mie theory as opposed to the Fraunhofer theory. The FRITSCH software MaS control provides a comprehensive database for that which contains the refraction index of numerous materials.

FRITSCH MaS control Software
The actual result of a particle size measurement is provided only after the evaluation with the supplied FRITSCH software MaS control. Depending on the particle characteristics and requirements, two common analysis theories are applied: the Fraunhofer theory for larger particles whose optical parameters are not known and the Mie theory for smallest particles with known optical parameters.