| length of the path in the atmosphere that is required to attenuate by 95% the luminous flux in a collimated beam from a light source at a colour temperature of 2700 K
NOTE 1 – The value of the attenuation has been chosen so that the term provides an approximate measure of the commonly used concept of (meteorological) visibility, which is the greatest distance at which a black object of suitable dimensions can be recognized by day against the horizon sky.
NOTE 2 – The meteorological optical range ν is related to the atmospheric transmissivity T, assumed to be uniform, by the formula
X
where d0 is the length specified in the definition of T.
These formulas are sometimes written
X
it being understood that v is the numerical value of the meteorological optical range ν measured with the "unit" d0 and T the numerical value of T.
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