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(symbol
 ) A measure of the space rate of attenuation of any transmitted
electromagnetic radiation. The attenuation coefficient is defined by
where I is the flux density at the selected point in space;
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.
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