What are the Mie scattering and Fraunhofer diffraction theories?

The Mie scattering theory is the Mie solution to Maxwell’s electromagnetic equations that accurately describes the scattering of light by homogeneous spheres. In the Mie theory, parameters such as refractive indices and absorption coefficients of the particle and the medium are used to calculate the particle size. Computers in the 1970s were only powerful enough to compute diffraction data with the simpler Fraunhofer approximation. Mie has been widely used since the 1980s with the evolution of the computer, and the Mie theory is a rigorous solution for the light scattering pattern and particle size distribution. The results obtained by the Mie theory are accurate since all the optical properties are used and considered. Therefore, particle size measurement using the Mie theory delivers more reliable and robust results for particles whose sizes range from the sub-micron to the millimeter level. For this reason, the Mie scattering theory is widely used in modern laser diffraction particle size analyzers.

The Fraunhofer diffraction theory, on the other hand, is a simple approximate expression of the Mie scattering theory, which describes the maximal and minimum light scattering angles as a function of an object’s size at any given wavelength. However, it does not require any knowledge of the optical properties and is preferred used for measurement when the particles are large and opaque with small light scattering angles.

Laser diffraction particle size analyzers from Bettersize offer both optical theories for particle size analysis, while Mie is the default, and Fraunhofer is used for measuring particles accurately if their sizes are greater than 25 microns.

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