The source function is a characteristic of a stellar atmosphere, and in the case of no scattering of photons, describes the ratio of the emission coefficient to the absorption coefficient. It is a measure of how photons in a light beam are removed and replaced by new photons by the material it passes through.
The source function is an attribute of stellar atmospheres, and under conditions without photon scattering, it denotes the ratio between emission and absorption coefficients. It assesses how photons within light beams are diminished and renewed as they pass through substances.