A method of generating a non-reproducible printed image, for example on a check, involves printing a first, background layer of ink on the document, and then printing data on top of the background layer. The ink used to print the background layer is reflective of light of a predetermined wavelength, typically in the infra red spectrum. The ink used to print the data is visually indistinguishable from the first ink, but absorbs light in the infra red spectrum, so that when the printed image is illuminated with infra red light, the image can be discerned. The image typically comprises an encrypted symbolic image and is used for security purposes.