Optical depth of a material is also related to its attenuation coefficient by:where
*l is the thickness of that material through which the light travels;
*α(z) is the attenuation coefficient or Napierian attenuation coefficient of that material at z,
and if α(z) is uniform along the path, the attenuation is said to be a linear attenuation and the relation becomes:
Sometimes the relation is given using the attenuation cross section of the material, that is its attenuation coefficient divided by its number density:where
*σ is the attenuation cross section of that material;
*n(z) is the number density of that material at z,
and if  is uniform along the path, i.e., , the relation becomes: