Upper panels: images of the Chamaeleon (translucent) and Scutum (diffuse) lines of sight studied in this article, observed with Spitzer-IRAC (Werner et al. 2004a; Fazio et al. 2004). Positions of the spectroscopic observations are indicated with white crosses. The images show IRAC colours red = 8.0 μm, green = 4.5 μm, and blue = 3.6 μm. Inserts show the HI and CO emission spectra (McClure-Griffiths et al. 2009; Dame et al. 2001; Boulanger et al. 1998). Lower panels: mid-infrared spectra towards the studied lines of sight are represented. The vertical lines indicate the positions of C₆₀ (red) and C (black) bands observed in the NGC 7023 reflection nebula (Sellgren et al. 2010; Berné et al. 2013). The band at 7.0 μm observed in the diffuse line of sight is due to [ArII].