Non-LTE stellar atmosphere (${\sum }_j{n}_j{P}_{ji}=n_i{\sum }_j{P}_{ij}$)

When radiative rates compete with or dominate collisions, level populations need statistical equilibrium coupled to radiative transfer, rather than thermal population laws at the local temperature. Radiation from other depths can make the local radiation field differ from the Planck function. This changes ionization and spectral-line strengths, so a geometrically thin atmosphere may still require non-LTE abundance modelling.