| law relating the apparent contrast Cd of an object against a sky background, at a given distance of observation d, to the inherent contrast C0 and to the atmospheric transmissivity T, which is assumed to be uniform
Cd = C0 · Td/d0
where d0 is the length specified for the definition of T
Note 1 – The formula is sometimes written
Cd = C0 · Td
d being the numerical value of d measured with d0 as "unit".
Note 2 – Taking into account the relationship given in 845-11-20 between T and the meteorological optical range ν, this law can also be written as
Cd = C0 · 0,05d/ν
Note 3 – The contrast is taken to be the ratio of the difference between the luminance of the object and the luminance of the background to the luminance of the background.