Left: illuminating flux, ℱ(r), as defined in Eq. (4) for a spectral index α = 1 and three different altitudes of the lamp, z = 1.8 M (magenta), z = 3 M (red), and z = 10 M (blue). Radius is in M units and flux in arbitrary units. This figure is extremely similar to Fig. 2, lower panel, of Dauser et al. (2013). Right: illuminating mean intensity in cgs units, ℐ(r), computed in a Newtonian spacetime (solid black and circles), a Schwarzschild spacetime (thick red) and a Kerr spacetime with spin parameter a = 0.98 (thin red). The black circles are Newtonian illuminating fluxes as computed by GYOTO, while the solid black line is the analytical expression of the Newtonian mean intensity.