Effect of continuum emission on line flux. Case (a) shows an emission line on a cold continuum S_C = 0. Case (b) is an emission line on a warm continuum S_L > S_C, and case (c) is an absorption line where the continuum is hot S_L < S_C. The area A is the flux of photon energy [erg s⁻¹ cm⁻²] carried away by line photons escaping from the object (after continuum extinction and line self-absorption) and is the same in cases (a), (b) and (c). However, the objective is to derive the area O, which is the line flux as measured by observers. For a warm continuum, clearly O < A, and for an absorption line O < 0 (although A > 0).