Note: 'Extinction coefficient' or 'molar absorptivity' have been widely used for the molar absorption coefficient, unfortunately often with values given in ill-defined units. Use of these terms has been discouraged since the 1960s, when international agreement with non-chemical societies reserved the word where the absorption is linearly proportional to the amount of absorber in unit area 'extinction' for diffusion of radiation, i.e., the sum of the effects of absorption, scattering and luminescence.