Figure 9: (a) Relationship between the ratio of 6.2 μm emission line feature to the FIR continuum and the ratio of 6.2 μm continuum to FIR continuum for typical galactic objects such as HII regions, RNe, and the diffuse ISM. The dashed line indicates the PAH continuum ratio of 0.72; see text for details. (b) Ratio of 6.2 μm emission line feature to the FIR continuum as a function of the ISRF, , for a range of galactic objects. Figures adapted from Peeters et al. .