## Wikipedia - Mathematical descriptions of opacity (en) depth of penetration

of light
https://doi.org/10.1351/goldbook.D01605
The inverse of the @[email protected] If the decadic @[email protected], $$a$$, is used, the depth of penetration ($$\frac{1}{a}$$) is the distance at which the @[email protected], $$P_{\lambda }$$, decreases to one tenth of its incident value, $$P_{\lambda }^{0}$$. If the Naperian @[email protected], $$\alpha$$, is used, the depth of penetration ($$\frac{1}{\alpha } = \beta$$ in this case) is the distance at which the @[email protected] decreases to $$\frac{1}{\text{e}}$$ of its incident value.
See:
absorbance
,
attenuance
Source:
PAC, 1996, 68, 2223. (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2235 [Terms] [Paper]