A photodetector including an amorphous silicon photodiode having its anode connected to a reference voltage, an initialization MOS transistor connected between the cathode of the photodiode and a first supply voltage to set the cathode to the first supply voltage during an initialization phase, and means for measuring the voltage of the photodiode cathode, including saturation means for bringing the photodiode cathode to a saturation voltage close to the reference voltage immediately before the initialization phase.