As shown in the top view, above, of the optical setup for zeta potential measurements in the SZ-100, the particles are illuminated with laser light and, therefore,the particles scatter light. A second beam of light (the reference beam) is mixed with the scattered beam in order to sensitively extract the frequency shift in the scattered light. The measured magnitude of the frequency shift is then used to determine the particle velocity. Equation 1 is used to calculate the electrophoretic mobility (µ) using the measured frequency shift.