A source function (often indicated as S) is a measure of the effect of a material (e.g., gas or plasma) on electromagnetic radiation (EMR, e.g., visible light) passing through, specifically the rate (per distance) that EMR is contributed (emission coefficient) divided by the ratio removed (also per distance, the absorption coefficient). The resulting source function 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. It has considerable variation by wavelength, and is often specified for a specific wavelength. An equation of radiative transfer may be written in terms of a source function:
1 d - ——— —— Iλ = 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.