Method and apparatus for validating bank notes

1. A method of examining a banknote, the method comprising performing a validation operation comprising:

taking measurements along a plurality of scan lines to determine optical characteristics of the banknote at respective different positions on the banknote and in respective different wavelengths;

normalising the measurements; and

applying acceptance criteria to the normalised measurements to determine whether they represent a target banknote class;

wherein each particular measurement is normalised with respect to a measurement group comprising a plurality of sets of measurements, each set comprising measurements of a respective different wavelength at positions distributed over a particular one of the scan lines in which the particular measurement was taken, and

wherein the method is performed by a banknote validator.

2. A method as claimed in claim 1 , wherein the measurements of each set are distributed along at least substantially the entire length of a scanned line of the banknote.

3. A method as claimed in claim 2 , wherein each measurement group is formed by a plurality of sets of measurements all derived by scanning a common line.

4. A method as claimed in claim 1 , wherein the measurements of each group are made using a respective different sensor from the measurements of the other groups.

5. A method of examining a banknote, the method comprising performing a validation operation including:

taking measurements to determine optical characteristics of the banknote at respective different positions on the banknote and in respective different wavelengths;

deriving normalised measurement values from the measurements; and

applying acceptance criteria to the normalised measurement values to determine whether they represent a target banknote class;

wherein the normalised measurement values are derived by (i) classifying first measurement values obtained from the measurements into measurement groups each comprising measurement values for different wavelengths at least one position on the banknote, and (ii) deriving normalised measurement values each representing a relationship between a respective first measurement value and a dispersion value representing the dispersion of first measurement values in the group to which that first measurement value belongs, and

wherein the method is performed by a banknote validator.

6. A method as claimed in claim 5 wherein each normalised measurement value represents the ratio of (i) the difference between the corresponding first measurement value and the average of the measurement values of the measurement group, and (ii) said dispersion value.

7. A method as claimed in claim 5 , wherein each first measurement value is derived by averaging measurements of a respective wavelength at selected positions distributed over the banknote.

8. A method as claimed in claim 5 , wherein each first measurement value is derived by a normalisation operation in which a quantity representing the measurement of a wavelength at least one position on the banknote is normalised with respect to a measurement group derived from measurements of plural different wavelengths at positions distributed over a substantial area of the banknote.

9. A method as claimed in claim 1 or 5 , including performing a calibration operation to obtain a calibration measurement which is independent of the banknote and influencing the validation operation in accordance with the calibration measurement so as to compensate for component variations.

10. A method as claimed in claim 9 , wherein the calibration operation comprises:

operating an emitter on one side of a banknote path; and

using a sensor located on said one side of the banknote path to detect light which has been emitted by the emitter and reflected from a window on the other side of said banknote path.

11. A method as claimed in claim 10 , wherein the measurements taken in said validation operation comprise reflection measurements made using said emitter and said sensor, and transmission measurements using light travelling between (i) an optical device which is overlain by said window and (ii) one of said emitter and sensor.

12. An apparatus for examining a banknote comprising:

a measuring unit operable to take measurements along a plurality of scan lines to determine optical characteristics of the banknote at respective different positions on the banknote and in respective different wavelengths;

a decision unit operable to normalize the measurements and operable to apply acceptance criteria to the normalized measurements to determine whether they represent a target banknote class, wherein each particular measurement is normalized with respect to a measurement group comprising a plurality of sets of measurements, each set comprising measurements of a respective different wavelength at positions distributed over a particular one of the scan lines in which the particular measurement was taken.

13. An apparatus for examining a banknote comprising:

a measuring unit operable to take measurements to determine optical characteristics of the banknote at respective different positions on the banknote and in respective different wavelengths;

a decision unit operable to deriving normalized measurement values from the measurement and operable to apply acceptance criteria to the normalized measurement values to determine whether they represent a target banknote class, wherein the decision unit is operable to derive the normalized measurement values by (i) classifying first measurement values obtained from the measurements into measurement groups each comprising measurement values for different wavelengths at least one position on the banknote, and (ii) deriving normalised measurement values each representing a relationship between a respective first measurement value and a dispersion value representing the dispersion of first measurement values in the group to which that first measurement value belongs.