The smooth, coloured lines show the optEC_(s) model-derived imaginary (top) and real (bottom) parts of the refractive index (large band gap, violet, to low band gap, grey). The lines with data points are the laboratory-measured and model-derived data (see references earlier for the sources). The upper black lines with data points show the data for graphite (Draine & Lee 1984) and the purple and violet lines with data points show the data for diamond (Edwards & Philipp 1985; Lewis et al. 1989).