Meteorological optical range
urn:aero:airm:1.0.0:ContextualModel:ATMBusinessTerms:Meteorological_optical_range
The length of the path in the atmosphere required to reduce the luminous flux in a collimated beam from an incandescent lamp, at a colour temperature of 2 700 K, to 0.05 of its original value, the luminous flux being evaluated by means of the photometric luminosity function of the International Commission on Illumination (CIE) (metre (m) or kilometre (km)).
Source:
ICAO