Sensing apparatus includes a transmitter, which emits a beam comprising optical pulses toward a scene, and a receiver, which receives reflections of the optical pulses and outputs electrical pulses in response thereto. Processing circuitry is coupled to the receiver so as to receive, in response to each of at least some of the optical pulses emitted by the transmitter, a first electrical pulse output by the receiver at a first time due to stray reflection within the apparatus and a second electrical pulse output by the receiver at a second time due to the beam reflected from the scene, and to generate a measure of a time of flight of the optical pulses to and from points in the scene by taking a difference between the respective first and second times of output of the first and second electrical pulses.