Linear Absorption Coefficient
The Linear Absorption Coefficient is a quantity that characterizes how easily a material or medium can be penetrated by a beam of light, sound, particles, or other energy or matter.
Applicable units are those of quantitykind:LinearAbsorptionCoefficient
http://en.wikipedia.org/wiki/Attenuation_coefficient
The Linear Absorption Coefficient is a quantity that characterizes how easily a material or medium can be penetrated by a beam of light, sound, particles, or other energy or matter.
$\alpha(\lambda) = \frac{1}{\Phi_\lambda(\lambda)}\frac{d\Phi_\lambda(\lambda)}{dl}$, where $\frac{d\Phi}{\Phi}$ is the relative decrease, caused by absorption, in the spectral radiant flux $\Phi$ of a collimated beam of electromagnetic radiation corresponding to the wavelength $\lambda$ during traversal of an infinitesimal layer of a medium and $dl$ is the length traversed.