1. A method of deriving a classification for classifying items of currency into two or more classes comprising measuring known samples for each class and deriving feature vectors from the measured samples, selecting a function corresponding to a mapping of the feature vector space to a second space, mapping feature vectors to image vectors, and deriving coefficients representing N−1 axes, where N is the number of classes, in the second space, obtaining values representing the projections of the image vectors for the measured samples onto the N−1 axes, and using those values to derive a separating function for separating the classes equivalent to a separating function in the second space.