Dynamic Light Scattering:
This measurement technique is using the Doppler shift of the light, which is being scattered by moving particles. The apparatus analyses the Doppler shift between the light signal which is reflected at the interface fiber optic/colloidal medium and the signal which has been back-scattered from the moving particle. The different thermal Brownian motion leads to different Doppler shift, which allows to calculate the particle size (by calculating the particle velocity, and hence - via Einstein's Brownian motion formula - the particle size).
