Different τ = 1 lines for a disc in quiescence (panels a and c) and in outburst (panels b and d) shown atop the underlying temperature structure for the model FULL. In panels a and b, the dashed blue lines mark the τ_vert(ν) = 1 surfaces for different wavelengths, which are indicated in the labels in units of μm. The dashed lines in panels c and d show the locations of the τ_rad(ν) = 1 transitions for the stellar irradiation for several representative wavelength bins with the respective central wavelengths in μm marked in the labels. The colour of the dashed lines indicates the relative spectral weight of the bin. Additionally, each panel includes the dust sublimation front as the green line and the τ_rad,tot = 1 line for the total radial optical depth in red.