1. Apparatus for checking the validity of a banknote, said apparatus including: a light source for illuminating the banknote with a plurality of wavelengths of light; an aperture through which the illuminating light passes to reach a portion of the banknote; imaging means for detecting light reflected or transmitted by the banknote at least one wavelength; said imaging means generating at least one image representing said portion from said reflected or, transmitted light; characterised in that a note path is provided along which the banknote to be validated is passed to expose a plurality of portions of the banknote to the illuminating light via said aperture when each of said portions is presented to the aperture.