Single photon avalanche diodes (SPAD) are solid-state photodetectors and belong to the same family as photodiodes and avalanche photodiodes (APDs). While APDs show their strengths in the undistorted amplification of low-intensity optical signals, the behavior of SPADs is more closely related to the basic diode behavior. SPADS enable the direct counting of pulses and thus provide an indication of the optical intensity of the input signal, while the pulses can simultaneously trigger timing circuits to enable precise measurements of the time of arrival. These sensors are used in particular in time-of-flight 3D imaging, LIDAR technology, PET scanning, TCSPC, fluorescence lifetime microscopy and optical communication, especially in quantum key distribution. You might also be interested in HPDs.