From WebRef.org

A measure of the space rate of attenuation of any transmitted electromagnetic radiation. The attenuation coefficient is defined by

*dI = -aI _{0}dx*

or

*I = I _{0}e^{-ax}*

where *I* is the flux density at the selected point in space; *I _{0}* is the flux density at the source;

*x*is the distance from the source; and

*a*is the attenuation coefficient.

*In general, the attenuation coefficient is specified only when the attenuation is known to be due to both absorption and scattering, or when it is impossible to determine which is the cause.*