| Definition: | quotient of the natural (Napierian) logarithm of the reciprocal of the spectral internal transmittance, τi(λ), of a layer of the medium and the length, l, of the path of a beam of radiation traversing that layer an(λ)=−lnτi(λ)l=logτi(λ)lln(10)=An(λ)l where An(λ) is Napierian spectral absorbance. 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.
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