1. A method of identifying a document ( 55 ), the method comprising exposing the document to infrared radiation; detecting infrared radiation reflected from or transmitted through a plurality of regions of the document ( 55 ) to generate at least one test pattern; determining if the or each test pattern satisfies a predetermined relationship with a predetermined pattern or patterns corresponding to a known document; and, if the predetermined relationship is satisfied, identifying the document ( 55 ) as being the same as the known document.