[7.6.2] coefficient of attenuation, μ
(at a point in a given direction)
The decrement in flux per unit distance in a given direction within a medium. It is defined by the relationship \Phi_{x} = \Phi_{0}e^{-\mu x} where \Phi_{x} is the flux at any distance x from a reference point having flux \Phi_{0}. More generally:
\Phi _{x}= \Phi _{0}exp\left [ -\int_{0}^{x}\mu (x) \mathrm{d}x \right ] ,
where the coefficient varies from point to point; \mu = \mu(x) along the path.
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