An apparatus for photon counting is disclosed that comprises a sensor element (5) comprising a photon detector (8) and a capacitance (2) for applying a potential difference across the detector (8). The apparatus also comprises switching circuitry (6) for connecting the sensor element (5) operatively to a voltage source (4) during a refresh period to charge the capacitance (2) to achieve a predetermined potential difference across the detector (8) above a threshold value, and for substantially isolating the sensor element (5) from the voltage source (4) during a sensing period in which the charged capacitance (2) maintains a potential difference across the detector (8) above the threshold value and in which the detection of a photon by the detector (8) causes at least some of the charge on the capacitance (2) to discharge through the detector (8). The apparatus also comprises control circuitry (12) for controlling the switching circuitry (6) to initiate refresh and sensing periods in alternating sequence. In one aspect, the control circuitry (12) is adapted to control the switching circuitry (6) to initiate successive refresh periods at predetermined intervals.