Editing Single-photon avalanche diode (section)

===Photon counting and saturation===
The intensity of the input signal can be obtained by counting ([[photon counting]]) the number of output pulses within a measurement time period. This is useful for applications such as low light imaging, PET scanning and [[Fluorescence-lifetime imaging microscopy|fluorescence lifetime microscopy]]. However, while the avalanche recovery circuit is quenching the avalanche and restoring bias, the SPAD cannot detect further photon arrivals. Any photons, (or dark counts or after-pulses), that reach the detector during this brief period are not counted. As the number of photons increases such that the (statistical) time interval between photons gets within a factor of ten or so of the avalanche recovery time, missing counts become statistically significant and the count rate begins to depart from a linear relationship with detected light level. At this point the SPAD begins to saturate. If the light level were to increase further, ultimately to the point where the SPAD immediately avalanches the moment the avalanche recovery circuit restores bias, the count rate reaches a maximum defined purely by the avalanche recovery time in the case of active quenching (hundred million counts per second or more<ref name="Eisele">Eisele, A.; Henderson, R.; Schmidtke, B.; Funk, T.; Grant, L.; Richardson, J.; Freude, W.: [http://www.imagesensors.org/Past%20Workshops/2011%20Workshop/2011%20Papers/R43_Eisele_SPAD139dB.pdf ''185 MHz count rate, 139 dB dynamic range single-photon avalanche diode with active quenching circuit in 130 nm CMOS technology''] Intern. Image Sensor Workshop (IISW'11), Hokkaido, Japan; Paper R43; June 2011</ref>). This can be harmful to the SPAD as it will be experiencing avalanche current nearly continuously. In the passive case, saturation may lead to the count rate decreasing once the maximum is reached. This is called paralysis, whereby a photon arriving as the SPAD is passively recharging, has a lower detection probability, but can extend the dead time. It is worth noting that passive quenching, while simpler to implement in terms of circuitry, incurs a 1/e reduction in maximum counting rates.