1. A method of deriving a function for classifying items of currency, the method comprising: processing training data vectors corresponding to features of a plurality of currency items, forming a subset of training data vectors used as a basis to reconstruct input space vectors in a feature space, subsequently deriving a support vector machine classification function involving a plurality of support vectors from reconstructed training data vectors and storing the support vector machine classification function in a currency validator processing system, wherein the support vector machine classification function is applied by the currency validator processing system to received items of currency.