3.4/3.3 ratio as a function of G₀. The black dot shows the band ratio in PDR NGC 7023 (based on Pilleri et al. 2015). Stars show the band ratio for each disc and different distances to the central star. For the disc, the G₀ has been estimated from a blackbody associated with the parameters of the central star (temperature and luminosity). The error bars for the 3.4/3.3 ratio are the dispersion of ratio values for the considered distance. The error bars for G₀ correspond to the values for the considered distance inside the VLT/NACO beam.