(kv), the inverse of the distance at which a monochromatic radiation flux that has frequency ν and forms a parallel beam is attenuated—owing to absorption in a substance—to 1/e (natural absorption coefficient) or 1/10 (decimal absorption coefficient) of its incident intensity. In the case of 1/e, the Bouguer-Lambert law is written
I = I0e-kvI
In the case of 1/10, the law is written
I = I010-kvI
The absorption coefficient is measured in cm-1 or m-1, and it is traditionally called the absorption factor in spectrography and some other branches of applied optics.