Equivalent widths (W_λ) of Lyα at 1240 (Col. 1) and 1500 Å (Col. 2) and He ii λ1640 (Col. 3) for a primordial metallicity nebula at different densities and ionization parameters (marked with different symbols as defined in the inset). The red symbols show an upper limit for W_λ(Lyα) since they are calculated using only stellar (black body) continuum. Blue symbols – the most realistic predictions – show the results obtained for the corresponding stellar population SEDs (models with the same P in the case of Lyα and with the same log (Q(He ⁺ ) / Q(H)) for the He ii analysis, for simplicity only models with log (U) = −1 are shown) and the pink ones their case B predictions. Notice the dependence of the He ii λ1640 equivalent width on the ionization parameter and the discrepancy between its case B and photoionization model predictions for real SEDs.