$\delta \left(\lambda \right)={\displaystyle {\int }_{x_1}^{x_2}\mu \left(x\text{,}\lambda \right)}\text{d}x$

where μ(x,λ) is the spectral linear attenuation coefficient at the location of dx

unit : 1

NOTE 1 – The spectral radiant flux Φ_{e, λ} (x₁, λ) of the beam at the point x₁ is reduced to the value of Φ_{e, λ} (x₂, λ) at the point x₂ according to the formula

${\Phi }_{\text{e}\text{,}\lambda }\left(x_2\text{,}\lambda \right)={\Phi }_{\text{e}\text{,}\lambda }\left(x_1\text{,}\lambda \right)\cdot {\text{e}}^{-\delta \left(\lambda \right)}$

so that

${\displaystyle \delta \left(\lambda \right)=-\text{ln}\frac{{\Phi }_{\text{e},\lambda }\left(x_2,\lambda \right)}{{\Phi }_{\text{e},\lambda }\left(x_1,\lambda \right)}}$

NOTE 2 – For a homogeneous non-diffusing layer, δ(λ) is the Napierian spectral internal transmittance density.