Sequence of time-dependent radiative transfer calculations for ejecta model he6p0 from 200 to 428 d after explosion. To cancel the drop in luminosity, due to both the exponential decrease in decay power emitted and the enhanced γ-ray escape with time, we normalize all spectra at 6330 Å in the optical (this wavelength corresponds roughly to the peak of [O I] λλ 6300,  6364) and at 1.083 μm in the near-infrared. Other ways to normalize are possible but this choice shows the near constancy of the flux ratio of the [O I] λλ 6300,  6364 and [Ca II] λλ 7291,  7323 doublets and the progressive drop in Fe emission below 5500 Å.