${\displaystyle \delta (\epsilon )=-\ln \frac{{{\Phi }^{\prime }}_{\text{e}}}{{\Phi }_{\text{e}}}}$

where Φ_e is the radiant flux of a collimated beam entering the upper limit layers of the atmosphere at an angle ε to the vertical, and ${{\Phi }^{\prime }}_{\text{e}}$ 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.