Patent ID: 7198157

Claim:
A method of determining whether or not an article of currency belongs to any one of a plurality of target classes, the method comprising: (i) deriving a plurality of measurements of the article; and (ii) using the measurements in a plurality of correlation calculations, each of said correlation calculations being associated with a respective one of said plurality of target classes, to determine the extent to which a relationship between the measurements conforms to a correlation between the measurements in a population of a respective one of said target classes, and hence whether or not said article of currency belongs to said respective target class, wherein each of said plurality of correlation calculations is an n-parameter Mahalanobis distance calculation; wherein, for each of said plurality of correlation calculations and respective target classes: (iii) said correlation calculation comprises a plurality of successive classification stages, each successive classification stage performing a part only of said n-parameter Mahalanobis distance calculation, at least one of said successive classification stages using a subset of n said measurements corresponding to said n parameters, and the successive classification stages being such that the sum of successive partial correlation calculations is either equal to the full n-parameter Mahalanobis distance calculation or a part of the n-parameter Mahalanobis distance calculation; and (iv) at least one said successive classification stage is used to determine whether the article does not belong to said respective target class.