Patent ID: 8570622

Claim:
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.