[en] The effects of optical thickness, velocity gradients and photon redistribution in frequency and direction, as described by the redistribution function Rsub(II), on emission line profiles from simple slab geometry atmospheres are examined. It is found that the optical thickness and differential expansion are the dominant factors affecting photon escape and hence the characteristics and magnitude of the line emission. The importance of frequency and angular photon redistribution as photon escape mechanisms are found to increase with increasing optical thickness and differential expansion. (author)