Method of monitoring a sequence of documents

A method of monitoring a sequence of documents passing along a transport path, the method comprising:

• operating a radiation transmitter with a control signal at a first, working, level to cause radiation at a first intensity to impinge on one side of a document as it passes an inspection position in the transport path;
• receiving at a radiation receiver, radiation from the transmitter that has passed through the document, the radiation receiver generating an output signal with a level related to the intensity of the received radiation;
• and
• monitoring the output signal to detect the presence and/or a characteristic of the document;
• carrying out a calibration process during the passage of documents along the transport path and when a gap between documents exists at the inspection position, the calibration process comprising
• operating the radiation transmitter with a control signal at a second, calibration, level to cause radiation at a second intensity less than the first intensity to be transmitted towards the receiver whereby the receiver generates a resultant output signal, and adjusting the level of the resultant output signal from the receiver to a predetermined value, the level of the resultant output signal from the receiver being adjusted by adjusting the amplification applied by an amplifier coupled to the output of the receiver,
• wherein the radiation comprises ultrasound.