Emission and absorption lines of Fell frequently dominate the optical and UV spectra of LBVs. Many luminous blue stars possess a rich optical Fell emission line spectrum, which can be used as diagnostic of their extended atmospheres (Viotti 1976a). Since the launch of IUE several LBVs were observed in UV, and in many cases (n Car, AG Car, P Cyg, Hubble Sandage variables, etc.) their spectra were found dominated by a large amount of Fell absorptions, presumably formed in cooler envelopes. Often these absorptions are so strong as to seriously affect the UV energy distribution. In order to study line formation outside LTE of such a complex ion, the Fell line intensities in luminous stars were analyzed using the Self Absorption Curve (SAC) method by Friedjung and Muratorio (1987). The fitting of the emission line fluxes in different multiplets to theoretical SACs obtained with different wind models, leads to the determination of the relative upper and also relative lower level populations. The overlapping of the individual multiplet curves gives the observational SAC which provides an estimate of the wind parameters (column density, acceleration parameter a, mass loss rate). This method of analysis was applied to the optical and UV high resolution spectra of several luminous stars. For instance we found that in AG Car the relative population of the Fell levels from 1.6 to 8.6 eV can be fitted to the same Boltzmann distribution with an excitation temperature of Tex«=7200+/-350K, whereas for the W Cep star KQ Pup the linearity extends from 0 to about 9 eV with Tex=6700 K (Viotti et al. 1988).