Logarithm of the ratio of incident to transmitted radiant power through a sample (excluding the effects on cell walls). Depending on the base of the logarithm a decadic and Napierian absorbance are used. This quantity is sometimes called extinction, although the term extinction, better called attenuance, is reserved for the quantity which takes into account the effects of luminescence and scattering as well. [IUPAC]
Absorbance is a quantity facet that quantifies the absorbance as logarithm to the base 10 (linear absorbance) of the incident (prior to absorption) spectral radiant power, divided by the transmitted spectral radiant power. [IUPAC]