Required Conditions for Photon-Counting Image Sensors

The required conditions of photon counting and single-photon detection are theoretically derived for four different approaches: 1) photon counting and single-shot imaging; 2) photon counting and multiple-shot imaging; 3) single-photon detection and single-shot imaging; and 4) single-photon detection and multiple-shot imaging. The most important parameter for all approaches is effective quantum efficiency (EQE), which is defined as QE multiplied by the temporal aperture ratio. To realize photon counting and single-shot imaging, EQE should be ~1. Even if EQE = 0.95, it cannot be realized with a 90% confidence level, whereas single-photon detection and single-shot imaging can be realized. When objects are stationary scenes or repeated phenomena, multiple-shot imaging is effective. The signal-to-noise ratio for multiple-shot imaging is increased with the square root of the shot number. The input-referred noise in the number of detected photoelectrons is expected to be less than 0.3e to realize electron counting, which is required for photon counting and single-photon detection. single-photon avalanche photodiodes and electron multiplication CCDs have achieved it, and “normal” image sensors, which do not use avalanche multiplication, can also realize it.