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.
Its units in the cgs-system are erg s−1 cm−2 sr−1 Hz−1 and in SI are W m−2 sr−1 Hz−1 .
Putting this into the equation for radiative transfer we get where s is the distance measured along the path traveled by the beam.
The minus sign on the left hand side shows that the intensity decreases as the beam travels, due to the absorption of photons.
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