(or extinction coefficient), a quantity inverse to the distance at which the radiation flux forming a parallel beam is attenuated as a result of the joint action of the absorption and scattering of light in a medium by a specified factor. When the factor is 10, the coefficient is called the decimal attenuation coefficient, and when the factor is e, it is called the natural attenuation coefficient. The attenuation coefficient is the sum of the absorption coefficient and the scattering coefficient of the medium. These coefficients depend on the set of frequencies v, or wavelengths λ, that characterize the initial flux. The value of the attenuation coefficient for the limiting case of a single frequency v is called the monochromatic attenuation coefficient. Like the absorption and scattering coefficients, the attenuation coefficient can be divided by the volume of the medium or by the mass of the attenuating substance; we shall call these two ratios the volume and mass attenuation coefficients, respectively.