Derivation of physical parameters of chromospheric structures assuming a constant and a varying source function

H alpha profiles of a rosette region consisting of several bright and dark mottles were obtained with the Multichannel Subtractive Double Pass (MSDP) spectrograph mounted on the 50 cm `Tourelle' refractor of the Pic du Midi O bservatory. These profiles have been analysed in terms of the classical clo ud model which assumes a constant source function and holds only for optica lly thin structures. A new technique is also described which enables the de termination of various physical parameters of chromospheric features taking into account the variation of the source function with the optical depth. This method can be applied, in contrast to the classical cloud model, to al l lines whether optically thin or optically thick and gives a good fit to t he entire profile from the core to the wings. A comparison of the results g iven by the two different methods is also performed.