From AMS Glossary
The general term for instruments used to make direct measurements of visual range in the atmosphere or of the physical characteristics of the atmosphere that determine the visual range.
Visibility meters may be classified according to the quantities that they measure. Telephotometers and transmissometers measure the transmissivity or, alternatively, the extinction coefficient of the atmosphere. Nephelometers measure the scattering function of the particles suspended in the atmosphere. A third category of visibility meters makes use of an artificial "haze" of variable density that is used to obscure a marker at a fixed distance from the visibility meter.