METHOD OF MONITORING A SEQUENCE OF DOCUMENTS

摘要

A method of monitoring a sequence of documents passing along a transport path is described. The method comprises: operating (44) 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 (46) the output signal to detect the presence and/or a characteristic of the document. A calibration process (50) is carried out between successive documents, the calibration process comprising operating (54) 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, and adjusting (56) the level of the resultant output signal from the receiver to a predetermined value.