An Absorption Coefficient is a measure of how much Electromagnetic Radiation (EMR) (e.g., light) a substance absorbs as it passes through the substance. They are used for modeling the effect of gas on the EMR passing through. It is a ratio of the EMR absorbed to that entering the substance over a standard length through the substance. It will depend upon the substance and its density. Also, an Absorption Coefficient may apply to EMR passing in a particular direction, or may be applied to all the EMR emitted from a source.
Similarly, a Scattering Coefficient gives such a ratio indicating how much of the EMR will be scattered rather than pass through, and an Attenuation Coefficient indicates the affect of both. An Absorption coefficient is the inverse of an Emission Coefficient (j).
An Absorption Cross Section characterizes the same measure as the Absorption Coefficient, but for a single absorbing particle. They are related by:
σ = α / N
Emission Coefficient (j)
Equation of Radiative Transfer (RTE)
Source Function (S)