Hence, it is taking into account not only the diffraction at the particle’s contour, but also the refraction, reflection and absorption phenomena within the particle and at its surface. ![]() The Mie theory is based on measuring the scattering of electromagnetic waves on spherical particles. For samples of known optical properties, Fraunhofer theory should only be applied for particles of an expected diameter at least 10 times larger than the light source’s wavelength, and/or to opaque particles. Hence is it typically applied to samples of unknown optical properties, or to mixtures of different materials. Its main advantage is that it does not require any knowledge of the optical properties ( complex refractive index) of the particle’s material. Fraunhofer theory only takes into account the diffraction phenomena occurring at the contour of the particle. The Mie scattering model, or Mie theory, is used as alternative to the Fraunhofer theory since the 1990s.Ĭommercial laser diffraction analyzers leave to the user the choice of using either Fraunhofer or Mie theory for data analysis, hence the importance of understanding the strengths and limitations of both models. The angle of the laser beam and particle size have an inversely proportional relationship, where the laser beam angle increases as particle size decreases and vice versa. Laser diffraction analysis is originally based on the Fraunhofer diffraction theory, stating that the intensity of light scattered by a particle is directly proportional to the particle size. ![]() ![]() Particles moving through the spread parallel laser beam
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