Comparison of the 5–15 μm kernel spectra (Fig. 2) with the observations and components from a blind signal decomposition (BSS) of the reflection nebula NGC 7023. Left panel: comparison of the spectrum of NGC 7023 (blue) and the kernel spectrum resulting from the 1000 random mixtures of 548 computed PAH spectra. Middle panel: comparison between the astronomical neutral PAHs (PAH⁰) spectrum extracted from the observations of NGC 7023 (blue) using blind signal separation techniques (Rapacioli et al. 2005; Berné et al. 2007; Berne et al. 2010; Appendix D) and the kernel spectrum (red) resulting from random mixtures of computed neutral PAH spectra. Right panel: same as the middle panel but now for cation PAH species (PAH⁺). In all panels, the solid black line indicates the pure rotational line of the molecular hydrogen molecule at 9.7 μm.