A source function (often indicated as S) is a measure of the effect of a material (e.g., gas or plasma) on electromagnetic radiation (e.g., visible light) passing through, specifically how much is contributed (emission coefficient) divided by how much is removed (absorption coefficient). It has the same units as intensity, and the intensity of a beam of light tends to change toward and approach the source function, virtually matching it if the beam passes through sufficient distance with a constant source function. An equation of radiative transfer may be written in terms of a source function:
1 dIλ - ——— ——— = Iλ-Sλ κλρ ds
The Eddington-Barbier relation relates the source function to the flux leaving a star, indicating it matches the source function at a vertical optical depth of 2/3, which is a basis for defining a photosphere as beginning at that depth.