Spectral absorbance in frequency and spectral absorbance in wavelength of a material, denoted  and  respectively, are given by
where
*  is the spectral radiant flux in frequency  by that material;
*  is the spectral radiant flux in frequency  by that material;
*  is the spectral transmittance in frequency of that material;
*  is the spectral radiant flux in wavelength  by that material;
*  is the spectral radiant flux in wavelength  by that material; and
*  is the spectral transmittance in wavelength of that material.
Spectral absorbance is related to spectral optical depth by
where
*  is the spectral optical depth in frequency, and
*  is the spectral optical depth in wavelength.
Although absorbance is properly unitless, it is sometimes reported in "absorbance units", or AU. Many people, including scientific researchers, wrongly state the results from absorbance measurement experiments in terms of these made-up units.