Patent ID: 7883762

Claim:
A security document comprising a security element against two-sided copying, said security element comprising: printed indicia present on the front side and on the reverse side, wherein said printed indicia comprise a first set of lines printed on the front side and a second set of lines printed on the reverse side, said first set of lines and said second set of lines being in accurate registration with each other, wherein the first set of lines is visible and the second set of lines is not visible observed in reflected light from the front side, and the second set of lines is visible and the first set of lines not visible observed in reflected light from the reverse side, wherein both the first set of lines and the second set of lines are visible observed in transmitted light from any of the front side and the reverse side, the first and second sets of lines being arranged to form an image observed in transmitted light, wherein the image includes a 3D effect that is a relief or volume effect created by the planar arrangement of the first and second sets of lines observed in accurate registration in transmitted light, whereas the 3D effect would not be present if the first and second sets of lines were not in accurate registration, wherein the first set of lines and the second set of lines complement each other to constitute the image so that the image with a 3D effect is visible observed in transmitted light but not visible observed in reflected light, so as to identify an authentic security document by the accurate registration.