Upper figure: the aromatic component contribution to the imaginary part of the refractive index, k, for the optEC_(s) model materials with E_g = 0,0.5,1.0,1.5,2.0 and 2.5 eV (dark grey, pink, brown, yellow, blue and purple lines, from top to bottom, respectively). The dark grey long-dashed curve indicates the adopted C₆ band profile peaking at 6.5 eV, as indicated. Lower figure: the same data presented as (αE)^0.5 but with the low energy extrapolation now included. See text for details.