Hydrogen line profiles at low densities

A physical discussion of the validity conditions of the different approximations allows to compare the Model Microfield Method (MMM) which takes into account the collective dynamical effects resulting from simultaneous collisions and the impact approximation valid for weak or strong non overlapping collisions. Neglecting the level splitting effects, the profile of Lyman and Balmer lines is Lorentzian in the line centre and an analytical formula for the width is given. Then, fine structure and radiative lifetime effects are analysed