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

dI = -aI0dx

or

I = I0e-ax

where I is the flux density at the selected point in space; I0 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.