Determination of particle size distribution by NPTA makes use of the Brownian motion and light scattering properties of particles suspended in liquids. Irradiation of the sample leads to light scattering by objects with a refractive index that is different from that of the surrounding medium. Light scattered from each particle is collected by magnifying optics and visualized by way of a suitable detector, such as a Complementary Metal Oxide Semiconductor (CMOS) camera. By recording a series of sequential images, the instrument’s software tracks positions of particles as a function of time, allowing analysis of their movement.

By tracking individual particles, undergoing random Brownian motion, from frame to frame, the average spatial displacement of the particles per unit time can be calculated, and this displacement can be related to the hydrodynamic diameter of the particles through the Stokes-Einstein equation.