A nephelometer (sometimes referred to as a turbidimeter) is a stationary or portable instrument for measuring suspended particulates in a liquid or gas colloid. A nephelometer measures suspended particulates by employing a light source and a light detector set to one side (often 90°) of the source beam. Particle density is then a function of the light reflected into the detector from the particles.
A more popular term for this instrument in water quality testing is a turbidimeter. However, there can be differences between models of turbidimeters, depending upon the arrangement (geometry) of the source beam and the detector. A nephelometric turbidimeter always monitors light reflected off the particles and not attenuation due to cloudiness.