The evaluation of banknotes to determine their authenticity in theory is relatively straightforward, however, in practice, it is quite difficult to carry out in a cost effective manner. Banknotes are evaluated by scanning stripped regions of the banknote or security paper as it is moved past a sensor. The banknote is normally evaluated with respect to optical characteristics, magnetic characteristics and/or with respect to capacitance.
The optical characteristics of a banknote are evaluated by measuring the amount of radiation reflected from the banknote. The optical sensors include emitters which produce radiation of different wave lengths, and focus the radiation on a particular target location of the banknote. The reflected radiation is measured and compared with reference signals to determine whether to accept or reject the banknote.
This optical evaluation is difficult in that the exact spacing of the banknote from the optical sensor varies as the banknote is, essentially, floating within an oversized pathway along which the banknote moves. In addition, the banknote can be angled in the pathway longitudinally and laterally, even though the banknote is generally centered. Thus, the spacing and the angle change, which influences the measured signal. Furthermore, creases in the banknote also cause angle variations which in turn impact the amount of radiation that will be reflected from the banknote back to a sensor. Other factors which affect the measured signal include the amount of radiation reflected back to a receiver by the optical sensor which radiation has not been reflected by the banknote. This portion of the signal typically produces what is referred to as cross-talk and it is desirable to keep this to as small a level as possible.
The pathway typically includes additional elements or surfaces between the optical sensor and the banknote and these elements or surfaces can cause reflected radiation, which again is not dependent upon the banknote. For example, there could be a window member which forms part of the pathway with the optical sensor directly behind the window. The window provides the desired smooth pathway, but increases cross-talk.
Other optical banknote scanning arrangements have positioned the emitter at a first acute angle, relative to the banknote, and appropriately positioned the receiver at a different angle for receiving the reflected radiation. Two distinct optical arrangements are provided to focus the emitted and received radiation. Unfortunately, these systems produce significant variations with respect to variations in the position of the banknote in the pathway as well as variations due to creases in the banknote.
The present invention provides an optical sensor with improved accuracy in the measurement of the optical reflecting properties of a banknote as the banknote is moved past the sensor.