I. Introduction
Single-Photon Avalanche-Diode (SPAD) image sensors have experienced a tremendous evolution in the last years. The performance of the detector itself has been constantly improved thanks to customized CMOS processes optimized for this kind of photodiodes [1]. Their ability to precisely measure the time-of-flight has fueled a diversity of applications, among them 3D vision or 3D ranging. Nowadays smart pixels based on SPADs are becoming a leading technology for ToF imagers. They have been approved even by a very demanding sector such as automotive [2].