Static Laser Light Scattering
Static laser light scattering is also called laser diffraction, laser diffractometry, Fraunhofer diffraction or Mie scattering.
During the interaction of the laser light with particles, diffraction, refraction, reflection and absorption result in light scattering patterns characteristic for the particle size.
If the particle size is considerably bigger than the wavelength of the incident light (for particle sizes from several micrometers upwards), this preferentially results in light scattering patterns caused by diffraction. The information about the particle size is contained in the small diffraction angles. This phenomenon is described by the Fraunhofer theory and is also called Fraunhofer diffraction.
Laser diffraction devices for the determination of particle size used this model first of all by measuring the intensity distribution in the forward direction for small angles (<35°) and used this information for calculating the particle size distributions.
If the particle size is similar or smaller than the wavelength of the incident light, the light is increasingly scattered with large angles to the side and backwards. The Mie theory describes this phenomenon considering the optical properties (refraction and absorption) of the particles. The smaller the particles are the higher is the contribution of refraction and absorption to the light scattering pattern.
For the measurement of such particle size distributions, therefore, the scattering pattern should be detected over the entire angle range. The interpretation of light scattering patterns by means of the Mie theory applies to all particle size distributions, including the Fraunhofer diffraction as a special case. If all of the particles in the sample are larger than the wavelength of the incident light, the Fraunhofer part contained in the Mie theory dominates for the calculation of the particle size distribution. The calculation according to Mie therefore allows the use of only a single evaluation method for the entire size spectrum.