However, the measured linewidth is always greater than the actual linewidth, which is derived from the natural lifetime.
We have to take into account a further broadening of the linewidth, i.e., a reduction in the lifetime, by collision, stimulated emission(as in lasers, cf. section3.4.5) or by Doppler shift as caused by the relative motion of the molecules and the observer. Broadening of the linewidth by collision and induced emission, as homogeneous processes, lead to a Lorentzian shape, while Doppler broadening, as an inhomogeneous process, yields a Gaussian line shape. The combination of both line shapes is described by the so-called ‘Voigt’ profile.