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Napierian spectral absorption coefficient, <of a homogeneous non-diffusing layer>
quotient of the natural (Napierian) logarithm of the reciprocal of the spectral internal transmittance, 
where
Note 1 to entry: The Napierian spectral absorption coefficient is expressed in metre to the power minus one (m−1).
Note 2 to entry: This entry was numbered 845-04-85 in IEC 60050-845:1987.
Note 3 to entry: This entry was numbered 17-799 in CIE S 017:2011.