A single photon counting detector system has a layer of photosensitive material and an N×M array of photo-detector diodes. Each photo-detector diode has a bias potential interface and a diode output interface. The bias potential interface is connected to bias potential. An N×M array of high gain, low noise readout unit cells is provided, one readout unit cell for each photo-detector diode. Each readout unit cell has an input interface connected to the diode output interface, a high-gain voltage amplifier with an integration capacitor at least two parallel lines of digital counters, each having a comparator with an individually selectable threshold and a gateable section to determine the counting intervals of the digital counters. A multiplexer allows access to the readout cell unit either on a per pixel basis or for several pixels in parallel to read out the digital counter to a data processor transferring the data off chip.