The 17.4 μm / 18.9 μm versus 7.0 μm / 18.9 μm band ratios for NGC 7023, IRAS 01005+7910, and Tc 1. The effective temperatures of their excitation sources are given in brackets. The closed boxes represent the predictions of UV-excitation model by Sellgren et al. (2010). The average photon-energy is given within each box. The curve marked with lozenges represents the predictions of thermal model. The A-values for both models are taken from the same literature (Choi et al., 2000).