a solid - state image sensor with very high sensitivity approaching the single - photon limit is realized with three modular building blocks : a pixel with a photo - site , intermediate photo - charge storage capability as used for correlated double sampling , and an electronic circuit for signal buffering or amplification , a column or row signal line to which a plurality of such pixels is connected using transistor switches , incorporating a low - pass filter , and a readout circuit to which the row signal lines are connected , consisting of a plurality of analog amplifiers with an analog multiplexer . photo - generated signals are read out and the reset level is subtracted either in the analog or in the digital domain , to implement a correlated - double - sampling method .