The inverse of the absorption coefficient. If the decadic absorption coefficient, \(a\), is used, the depth of penetration (\(\frac{1}{a}\)) is the distance at which the radiant power, \(P_{\lambda }\), decreases to one tenth of its incident value, \(P_{\lambda }^{0}\). If the Naperian absorption coefficient, \(\alpha \), is used, the depth of penetration (\(\frac{1}{\alpha } = \beta \) in this case) is the distance at which the radiant power decreases to \(\frac{1}{\rm{e}}\) of its incident value.