Chief among the reasons to use photon counting (PC) is its increased sensitivity compared to typical linear EMCCD acquisitions (linear mode – LM). All imaging technologies using gain (*e.g.* avalanche diodes, EMCCD detectors) suffer from what is called the excess noise factor (ENF). Gain is a stochastic process and electrons have a certain probability to be multiplied at each stage of the multiplication register so even for a set gain, there is a pixel by pixel uncertainty on the exact number of electrons at the output of multiplication. The ENF is a representation of this phenomenon and has the same effect on the signal-to-noise ratio (SNR) as dividing your camera’s quantum efficiency in half. More details on this subject are available in EMCCD noise sources.

Using photon counting eliminates the effect of ENF; the same impact as multiplying your signal two-fold. This increased sensitivity makes a crucial difference when imaging low signal levels.

It can be observed that the benefits of photon counting do not extend beyond a signal of 1 photon/pixel/frame. That is because photon counting produces binary images, with each pixel either being a 0, where no photon was detected, and 1, where a photon was detected. As such, if a pixel receives more than 1 photon, this information is not registered by the image which results in a coincidence loss.