Schematic view of the photon emission pattern of an accretion disk around a black hole. A distant observer is at the bottom of the figure and sees the disk face-on. For the classical SS model, photon trajectories (red lines) go straight to a distant observer; they are not affected by any relativistic effect. For the KERRBB model, photon trajectories (dotted black lines) are affected by light-bending and change their paths towards different directions with respect to the SS case (the region where light-bending is stronger is inside the dashed blue circle). Along with a lower efficiency, this explains why the KERRBB with the same spin (a = 0) is dimmer than the SS model when observed face-on. The photons emitted in the outer region of the disk are not affected by relativistic effects since they are far from the black hole influence, hence theSS model and the KERRBB model are similar. If the black hole is maximally spinning (a = 0.9982), the disk moves closer to the black hole and relativistic effects become stronger, which along with the larger efficiency make the disk brighter than the previous cases, even if the disk is edge-on with respect the observer. The top axis is the logarithmic distance from the central black hole in units of Rg: R_ISCO and radius R_PEAK (radius at which the SS emitted flux is maximized) are shown as well.