[] In signal lighting: The atmospheric transmittance constant (which is conventionally taken as 5%) over a distance equal to the daylight visual range (the meteorological visibility), whatever the density of a fog or mist. It follows that, if the meteorological visibility distance is V, the atmospheric transmittance for unit distance, or the transmissivity, T1, is given by:

(T1)V = 0.05  ,

and the atmospheric transmittance Td for some other distance d may be conventionally calculated from:

Td = (T1)d = (0.05)d/V

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