Abstract

A solid-state focal-plane imaging system comprises an N×N array of high gain, low-noise unit cells, each unit cell being connected to a different one of photovoltaic detector diodes, one for each unit cell, interspersed in the array for ultralow level image detection and a plurality of digital counters coupled to the outputs of the unit cell by a multiplexer (either a separate counter for each unit cell or a row of N of counters time shared with N rows of digital counters). Each unit cell includes two self-biasing cascode amplifiers in cascade for a high charge-to-voltage conversion gain (>1 mV/e-) and an electronic switch to reset input capacitance to a reference potential in order to be able to discriminate detection of an incident photon by the photoelectron (e-) generated in the detector diode at the input of the first cascode amplifier in order to count incident photons individually in a digital counter connected to the output of the second cascode amplifier. Reseting the input capacitance and initiating self-biasing of the amplifiers occurs every clock cycle of an integratng period to enable ultralow light level image detection by the array of photovoltaic detector diodes under such ultralow light level conditions that the photon flux will statistically provide only a single photon at a time incident on any one detector diode during any clock cycle.