Average surface brightnesses νI_ν in the rest frame, defined as , are plotted against rest wavelengths. Overplotted are the Planck functions with temperatures 1400, 700, and 350 K (dotted, dashed, and dash-dot, respectively). For NGC 4151 and IRAS13349, which have near-IR Keck interferometry data, the average surface brightness at 2.2 μm is calculated in the same way but using thin-ring radii. For the objects without near-IR interferometry, it is calculated assuming R_1/2 = (1.5 ± 0.5)   R_in and indicated by dashed lines. See further details in Sect. 4.1.