The determination is based on the non-invasive technology of dynamic light scattering (DLS) emitted by a laser. The particle is assimilated with a sphere shape in constant Brownian motion due to statistical collisions with liquid molecules. In this motion, the small particles will move faster than the large particles. 

The particle size distribution obtained by DLS depends on the relative intensity of the light scattered by particles of different dimensional classes. The time dependence of the intensity fluctuations is measured to determine the translational diffusion coefficient (D), respectively the hydrodynamic diameter (DH). The result is an intensity distribution that can be converted to a volume or mass distribution if the optical properties of the particles are known (Mie Theory). 

Mie theory allows a rigorous calculation of the particle size distribution from laser diffraction data, but this prediction of the size of all particles, small or large, transparent or opaque, is conditioned by the following approximations:

- The particles to be measured are spherical.

- The particle suspension is diluted.

- The optical properties of particles and the environment are known.

- The particles are homogeneous.