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; and wherein the imaging means generate an image for each of the portions and processing means are provided to combine the images to create a representation of the banknote, and the representation is a three dimensional matrix in which the coordinates of the matrix relate to a longitudinal coordinate of the banknote, the wavelength range, and the intensity of the light detected.