Method and apparatus for validating bank notes

1. 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.

2. A method as claimed in claim 1 , including using a plurality of optical devices arranged along a line transverse to a direction in which the banknote is scanned to take measurements corresponding to optical characteristics of the banknote, the devices having a common window.

3. A method as claimed in claim 1 , wherein the window has a planar surface facing the emitter and sensor.

4. A method as claimed in claim 1 , wherein the adjusting comprises one or more of the following:

(a) controlling the intensity with which the emitter illuminates the banknote during the validation operation in accordance with the calibration measurement;

(b) controlling the gain applied to an output of the sensor during the validation operation in accordance with the calibration measurement; and

(c) digitally adjusting the measurements obtained during the validation operation in accordance with the calibration measurement.

5. A method as claimed in claim 1 , wherein influencing the validation operation comprises controlling the intensity with which the emitter illuminates the banknote during the validation operation in accordance with the calibration measurement.

6. A method as claimed in claim 1 , wherein influencing the validation operation comprises controlling the gain applied to an output of the sensor during the validation operation in accordance with the calibration measurement.

7. A method as claimed in claim 1 , wherein influencing the validation operation comprises digitally adjusting the measurements obtained during the validation operation in accordance with the calibration measurement.

8. The method of claim 1 , wherein said emitter is a multi-color emitter and said operating emitter further includes driving said emitter to emit light of each wavelength;

said detecting step further comprises detecting each said wavelength; and

adjusting either or both of the emitter or sensor with respect to one or more wavelengths.

9. A method as claimed in claim 1 or 4 , for use in a validation apparatus which comprises a plurality of emitters and a plurality or sensors arranged so as to enable, in the validation operation, a plurality of measurements to be made of banknote characteristics in different wavelengths at positions distributed along a line transverse to a scanning direction of the banknote, and wherein the calibration operation includes using said emitters and sensors to take a calibration measurement corresponding to each position and wavelength measured in the validation operation.

10. A method as claimed in claim 1 or 4 , the validation operation including taking reflection measurements on each side of the banknote path using a respective emitter/sensor pair, the emitter of one such pair and the sensor of the other such pair also being used to take transmission measurements.

11. An apparatus for examining a banknote comprising:

an emitter and a sensor both located on one side of a banknote path to take reflection measurements from a banknote;

an optical device on the other side of the banknote path operable together with either the emitter or the sensor to take transmission measurements to determine optical characteristics of the banknote at respective different positions on the banknote;

a decision unit operable to apply acceptance criteria to the measurements to determine whether they represent a target banknote class;

a measuring unit operable to perform a calibration operation to obtain a calibration measurement which is independent of the banknote and to influence a banknote validation operation in accordance with the calibration measurement so as to compensate for component variations, wherein the calibration operation comprises operating the emitter and using the sensor to detect light that has been emitted by the emitter and reflected from a window on the other side of the banknote path and overlying the optical device.

12. The apparatus of claim 11 wherein the measuring unit is operable to influence the validation operation by performing one or more of the following:

(a) controlling the intensity with which the emitter illuminates the banknote during the validation operation in accordance with the calibration measurement;

(b) controlling the gain applied to an output of the sensor during the validation operation in accordance with the calibration measurement; and

(c) digitally adjusting the measurements obtained during the validation operation in accordance with the calibration measurement.