quantity defined by the formula
δ(ε) = — In (Φ'e/Φe)
where Φe is the radiant flux of a collimated beam entering the upper limit layers of the atmosphere at an angle ε to the vertical, and Φ'e the attenuated radiant flux of that beam reaching the surface of the Earth
unit : 1
NOTE 1 – See also 845-04-80.
NOTE 2 – In English, the term optical depth is sometimes used instead of optical thickness
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