Left: geometry of the lamp post model. The velocity v^ϕ is the Keplerian velocity of the corotating observer. The triad (e₁, e₂, e₃) is the local rest frame of the observer. The angle i lies between the direction of the local normal and direction of the lamp as seen by the observer. Right: image (map of the quantity g^{3 + α} as defined in Eq. (2)) of the lamp as observed by a Keplerian observer rotating at r = 40 M. The total field of view is of 0.9 rad. The lamp is not strictly speaking point-like so the angle i is equal to the average of all the directions on sky connecting the observer to the lamp. Also, the lamp appears shifted with respect to the local normal because of the special relativistic effect linked to the high velocity of the observer. The displacement is in the direction of the observer’s motion.