Normalized extinction, A(λ) /A(V), of representative lines of sight from Fitzpatrick & Massa (2007; solid purple and black). Laboratory spectra for an a-C:H re-normalized to the optical depth of the observed 3.4 μm band (dotted green) and renormalized laboratory soot spectra to its V-band optical depth (dotted blue). Calculated extinction curves from laboratory derived optical constants for an a-C:H (dashed green curve) and a soot (dashed blue).