Zhu, X., 1988: An improved Voigt line approximation for the calculations of equivalent width and transmission. Journal of Quantitative Spectroscopy and Radiative Transfer, 39(6), 421-427.

Abstract: An improved approximation for the Voigt profile is proposed to calculate the equivalent width and the transmission functions for three different random models in terms of closed formulas. The accuracy of the calculations is significantly improved, while the computation required is comparable in magnitude to the previous approximation suggested by Fels.