1. A method of performing a biometric function on a purported skin site, the method comprising: illuminating the purported skin site under a plurality of distinct optical conditions; receiving spatially distributed multispectral light reflected from the purported skin site; deriving a feature value for each of a plurality of features from the received light; performing a comparison of the derived feature value for each of the plurality of features with reference feature values; and determining whether the purported skin site is a true skin site from the comparison.