Optical depth measures the attenuation of the transmitted radiant power in a material. Attenuation can be caused by absorption, but also reflection, scattering, and other physical processes. Optical depth of a material is approximately equal to its attenuation 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 optical depth:
where
*Φ is the radiant power transmitted by that material;
*Φ is the radiant power attenuated by that material;
*Φ is the radiant power received by that material;
*Φ is the radiant power emitted by that material;
*T = Φ/Φ is the transmittance of that material;
*ATT = Φ/Φ is the attenuation of that material;
*E = Φ/Φ is the emittance of that material,
and according to the Beer–Lambert law,
so: