Abstract
The authors derive an expression for the spectrum of fluorescence by a two-state atom in a perturber gas irradiated by nearly resonant monochromatic light of arbitrary intensity. The result is fully expressed in terms of the absorption profile that would be measured in a conventional low-intensity absorption experiment and of the rate of radiative and collisional transitions. The present result generalises previous work and the authors demonstrate that expressions obtained before in special cases are recovered by taking the appropriate limits.