The Stokes-Einstein equation was published in 1905 by Albert Einstein and around the same time independently by William Sutherland. It describes how small spherical particles diffuse through a fluid with constant temperature by undergoing Brownian motion:

 

where D is the translational diffusion coefficient, k_B is Boltzmann's constant, T is the absolute temperature, h is the solvent viscosity, and r is the radius of the particle.

From dynamic light scattering measurements with Prometheus Panta the translational diffusion coefficient can be obtained. If the temperature and the viscosity of the solvent at this temperature are accurately known, the diffusion coefficient can be converted into the hydrodynamic radius (rH) of the solvated particles.

Please note that the Stokes-Einstein equation is generally only valid for dilute solutions and spherical molecules.