An analytic collisional-radiative model incorporating non-LTE and optical depth effects J. A. Pomarico, D. I. Iriarte and H. O. Di Rocco Eur. Phys. J. D, 19 1 (2002) 65-72 Published online: 15 April 2002 DOI: 10.1140/epjd/e20020056