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optical thickness, <of the atmosphere>
optical depth, <of the atmosphere>
where 
is the attenuated radiant flux of that beam reaching the surface of the Earth
Note 1 to entry: See also "spectral optical thickness".
Note 2 to entry: The optical thickness has unit one.
Note 3 to entry: This entry was numbered 845-09-86 in IEC 60050-845:1987.
Note 4 to entry: This entry was numbered 17-850 and 17-844 in CIE S 017:2011.