Radial disc flux profile (from one side of the disc) at 2500 Å for M_BH = 10⁸ M_⊙, ṁ_Edd = 0.2, θ = 40°, a^* = 0 and 1 (left and right panels, respectively). We consider different radial profiles: simple approximations and the proper NT radial temperature dependence (red and dotted blue lines); the radial profile when we include the temperature colour correction factor of Done et al. (2012) (solid blue line); and when all relativistic effects are included (black lines). The latter are the profiles observed at infinity. The vertical lines show the location of R_1/2, mod for the various radial profiles (see Sect. 3 for details).