The laboratory data from (Mennella et al. 1995, (coloured curves with small data points)) plotted as (αE)^0.5 vs. energy, Tauc plot (upper), and also versus wavelength (lower). The red lines show the modelled fits to the data using Eqs. (8) and (9), with the addition of an “inverse Gaussian tail” at high energies. The red squares delineate the portion used to determine B and the extrapolation to E_g (dashed red lines). The black squares indicate the transition energies to the Urbach tail, E₁. The lines without data points are the optEC_(s) model data.