Basic relationship of the rest-frame frequencies of spectral lines (ν_CMF) to the observer’s frame frequency (ν_obs) for one particular ray in the z direction of the spherically symmetric geometry. Shown are two spectral lines of different opacity χ that are shifted across the observer’s frame frequency by the (homologous) velocity field in the ejecta. The dots represent the stepping points of the adaptive microgrid used in solving the transfer equation in the radiative line transfer. The method employed is an integral formulation of the transfer equation using an adaptive stepping technique on every ray in which the radiation transfer in each micro-interval is solved as a weighted sum on the microgrid (cf. Pauldrach et al. 2001).