A nephelometer is an instrument for measuring the concentration of suspended particulates in a liquid or gas colloid.
Nephelometer measures suspended particulates by employing a light beam (source beam) 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.
To some extent, how much light reflects for a given density of particles is dependent upon properties of the particles such as their shape, color, and reflectivity.
Nephelometers are calibrated to a known particulate, then use environmental factors (k-factors) to compensate lighter or darker colored dusts accordingly.
K-factor is determined by the user by running the nephelometer next to an air sampling pump and comparing results. There is a wide variety of research-grade nephelometers on the market as well as open source varieties.