A formal definition by WMO: Meteorological visibility by day is the greatest distance at which a black object of suitable dimensions located near the ground can be seen and recognized when observed against a scattering background of fog, sky etc.

Most visibility sensors actually measure the meteorological optical range. This is the length of path in the atmosphere required to reduce the luminous flux in a collimated beam from an incandescent lamp at a color temperature to 5% of its original value.

Most instruments: transmissiometers, measuring the extinction coefficient. Others are actually measuring the scatter coefficient.

Visibility sensors are often combined with other sensors and sold as "present weather sensors".

Specialized manufacturers are: Envirotech Sensors Vaisala, ScTi, Impulsphysik, Belfort, MTECH Systems and Biral.

The optical distrometer manufactured by by PMTech, can also be used as a present weather sensor and visibility sensor.