coefficient of attenuation (at a point in a given direction), µ

[7.6.2] The decrement in flux per unit distance in a given direction within a medium. It is defined by the relationship Φ_x = Φ_oe^‑μ_x , where Φ_x is the flux at any distance x from a reference point having flux Φ_o. More generally:

\Phi _{x}= \Phi _{0}exp\left [ -\int_{0}^{x}\mu (x) dx \right ] ,

where the coefficient varies from point to point; µ = µ(x) along the path.