If a light beam hits a liquid that is clouded by small, undissolved particles, part of the light is scattered to the side of the incoming beam. The phenomenon is called the Tyndall effect.

The scattering depends on the way the light hits the particle, but also on its shape.

With nephelometric measurement, the light scattered laterally by the particles is collected and directed to a photodetector (nephelometer) via a lens system. The measurement is directly proportional to the light intensity. The related method of turbidimetry, on the other hand, measures the reduction in light intensity that passes through the liquid, caused by light scattering.

Nephelometric measuring devices measure the scattered light at an angle of 90° to the direction of incidence.

Precisely defined results are required in most standardized regulations. In order for these to be achieved, the measuring device must be calibrated and adjusted accordingly. In the case of nephelometric measuring devices, the basis for this is formed by formazine as the basis for the primary or reference standard. The results are reported in FNU units, Formazine Nephelometric Units.