Absorbance is a number that measures the attenuation of the transmitted radiant power in a material. Attenuation can be caused by the physical process of "absorption", but also reflection, scattering, and other physical processes. Absorbance of a material is approximately equal to its attenuance when both the absorbance is much less than 1 and the emittance of that material (not to be confused with radiant exitance or emissivity) is much less than the absorbance. Indeed,
where
*  is the radiant power transmitted by that material,
*  is the radiant power attenuated by that material,
*  is the radiant power received by that material, and
*  is the radiant power emitted by that material.
This is equivalent to
where
*  is the transmittance of that material,
*  is the  of that material,
*  is the emittance of that material.
According to the Beer–Lambert law, , so
and finally