Escape factors for a uniformly excited expanding/shrinking sphere are calculated using a Monte Carlo method for different Voigt profiles. The effect of the spectral shift of the absorption profile relative to the emission profile due to velocity gradients is taken into account for the case where the differential motion is described by velocity gradients that vary linearly with radius. Results are presented in a parameterized form.
Glouchkov, D., Koshelev, K., & Schulz, A. (2003). Monte Carlo simulation of photon transport for optically thick, differentially moving plasmas: II. Escape factors for differentially moving spheres. Journal of Quantitative Spectroscopy and Radiative Transfer, 81(1–4), 191–197. https://doi.org/10.1016/S0022-4073(03)00071-2