measurement of the time-dependent fluctuations in the scattered intensity of
coherent monochromatic light. These fluctuations arise because the particles
are small enough to undergo random thermal (Brownian, q.v.) Motion, and the
distance between them is therefore constantly varying. The measurement yields
the diffusion coefficient of the particles from which, via the
Stokes-Einstein equations, and knowing the viscosity
of the medium, the hydrodynamic radius (or diameter) can be calculated.