An energy sensor comprises a radiation-sensitive detector, a circuit, and an analog-to-digital converter. The radiation-sensitive detector is arranged to receive a pulsed radiation beam and to generate a current in response thereto. The circuit is equivalent to an RC network and is electrically connected across the radiation-sensitive detector. The analog-to-digital converter is electrically connected across a resistive component of the circuit and is arranged to output digital samples measuring the voltage across the resistive component at a sampling rate that is greater than the pulse repetition rate of the pulsed radiation beam. The energy sensor may be provided as part of a transmission image sensor.