Patent ID: 8177046

Claim:
A method of examining a banknote, the method comprising performing a validation operation including: using an emitter and a sensor both located on one side of a banknote path to take reflection measurements, and using an optical device on the other side of the banknote path together with either the emitter or the sensor to take transmission measurements, thereby to determine the optical characteristics of the banknote at respective different positions on the banknote; and applying acceptance criteria to the measurements to determine whether they represent a target banknote class; the method further comprising performing a calibration operation to obtain a calibration measurement which is independent of the banknote and to influence the validation operation in accordance with the calibration measurement so as to compensate for component variations; wherein the calibration operation comprises: after insertion of a banknote, but prior to the banknote validation operation, operating said emitter; and detecting light via said sensor which has been emitted by the emitter and reflected from a window on the other side of said banknote path and overlying said optical device.