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.