In Photon Correlation Spectroscopy (PCS) or quasi-elastic light scattering (QELS), the Brownian motion (movement in random direction) of sub-micron particles is measured as a function of time. The technique is based on the principle that smaller particles move with higher velocity than larger particles.

A laser beam is diffracted by sub-micron particles in suspension. The diffusion of particles causes rapid fluctuations in scattering intensity of the laser around a mean value at a certain angle (varying from 10 to 90°) and this depends on particle size.
The calculated correlation function results in a diffusion coefficient for a given temperature and viscosity which can be converted into particle size. The technique is used for determination of the average particle size in a range between 3 and 3000 nm. The measurements are performed on a Coulter N4 or an ALV 5000 particle size analyzer.

The measurements result in an average and mode effective hydrodynamic diameter and polydispersity index, which is a measure for the width of the distribution.