An apparatus having a rectangular imaging array characterized by a plurality of pixel sensors and a plurality of readout lines is disclosed. The apparatus has a plurality of column processing circuits, each column processing circuit being connected to a corresponding one of the readout lines and a plurality of signal injectors, one signal injector being connected to each of the readout lines. Each signal injector causes one of a predetermined number of voltages to be coupled to that readout line. An exposure for each of the pixel sensors is determined during image recording periods. The signal injectors inject a plurality of calibration voltages into the readout lines during calibration periods, and determines a gain function of an amplifier in one of the column processing circuits by measuring an output of the amplifier for the plurality of calibration voltages, the calibration period is between the imaging recording periods.