The same average surface brightness as in Fig. 10, but plotted against the UV luminosity of the central engine, defined as 6 νL_ν(5500 Å) (Kishimoto et al. 2007). The red and green symbols correspond to 13 and 8.5 μm, respectively. The surface brightness at 2.2 μm is shown in purple for those with the near-IR interferometry data from Kishimoto et al. (2011). The luminosity dependence is quite clear at least for the surface brightness in the mid-IR.