Comparison of SN 2015H and our N3def models with 0.01%, 0.22%, and 36% helium abundances. Optical and infrared spectra have been normalised to the median flux from 5000 Å–7000 Å and 10 000 Å–12 000 Å, respectively. The spectra of SN 2015H are approximately +16 days after r-band maximum and have been corrected for galactic extinction only. Our N3def model is shown at +15 days after bolometric maximum light. Panels a and b: shaded regions represent He I λ5876, λ6678, λ7065 and λ10830 at rest and blue-shifted by the maximum velocity of the model (∼10 400 km s⁻¹). Panels c–f: zoom-ins of the regions surrounding He I λ5876, λ6678, λ7065, and λ10830, respectively. Rest wavelengths are denoted by a vertical dashed line.