depth of penetration of light

The inverse of the absorption coefficient. If the decadic absorption coefficient, $a$ , is used, the depth of penetration ( $1 a$ ) is the distance at which the radiant power, $P λ$ , decreases to one tenth of its incident value, $P λ 0$ . If the Naperian absorption coefficient, $α$ , is used, the depth of penetration ( $1 α = β$ in this case) is the distance at which the radiant power decreases to $1 e$ of its incident value.

See: absorbance, attenuance

Source:

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