The Rosseland Mean Opacity (or simply Rosseland Mean) is a weighted average of the Opacity of a material, e.g., gas or Plasma through which Electromagnetic Radiation is passing. It is designed to be useful for models that try to do a single calculation covering all Wavelengths rather than treating each wavelength individually, and given the complexity of the effect of a gas on a spectrum, to make such models tractable. It is:
1 1 ∞ Iν ——— = — ∫ —— dν <κ> I 0 κν
A model Stellar Atmosphere using this short cut is called a Gray Atmosphere.