[7.6.2] The decrement in flux per unit distance in a given direction within a medium. It is defined by the relationship Φx = Φoe‑μx , where Φx is the flux at any distance x from a reference point having flux Φo. 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.

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