text: The absorbance of a material, denoted , is given by
  where
  *  is the radiant flux  by that material,
  *  is the radiant flux  by that material, and
  *  is the transmittance of that material.
  Absorbance is a dimensionless quantity. Nevertheless, the absorbance unit or AU is commonly used in ultraviolet–visible spectroscopy and its high-performance liquid chromatography applications, often in derived units such as the milli-absorbance unit (mAU) or milli-absorbance unit-minutes (mAU×min), a unit of absorbance integrated over time.
  Absorbance is related to optical depth by
  where  is the optical depth.