The laboratory data from Smith (1984) plotted as (αE)^0.5 vs. energy (upper plot), coloured curves with small data points, i.e., a Tauc plot, and also versus wavelength (lower plot). The red lines show the modelled fits to the data using Eqs. (8) and (9), with an “inversed-Gaussian” wing at high energies. The red squares delineate the portion used to determine B and the extrapolation to the Tauc gap, E_g. The black squares indicate the transition energies to the Urbach tail, E₁.