Fig. 7. Diagram illustrating the principle of NTA measurements using the Stokes–Einstein equation. Particles are illuminated by a laser light and movements of the particles are recorded through the scattered light via a CCD by a microscope. The software tracks the Brownian motion of each particle by determining the diffusion coefficient, and then calculates the size as the mean square of the particle path using the Stokes–Einstein equation.⁹⁹.