depth of penetration

of light

The inverse of the absorption coefficient. If the decadic absorption coefficient, , is used, the depth of penetration
() is the distance at which the radiant power, , decreases to one tenth of its incident value, . If the Naperian absorption coefficient, , is used, the depth of penetration
(
in this case) is the distance at which the radiant power decreases to
of its incident value.

Source:

PAC, 1996, *68*, 2223*
(Glossary of terms used in photochemistry (IUPAC Recommendations 1996))
* on page 2235