Patent ID: 7197161

Claim:
A process for generating an authenticable document, wherein an authentication image inserted therein is not readily visually perceptible, including: generating a first halftone screen; generating a second halftone screen, related and distinct from the first halftone screen, wherein the macro structure of the second halftone screen is identical to that of the first halftone screen and the micro structure of the second halftone screen is conjugate to that of the first halftone screen generating a first user image using the first halftone screen and the second halftone screen, wherein a region is generated using the first halftone screen and a remaining region is generated using the second halftone screen; generating a second user image using the first halftone screen whereby upon placing the first and second user images in a superposed relationship to allow viewing of both user images together, a positive correlation occurs between the two user images within the regions in which the first halftone screen is used in the first user image, and a negative correlation occurs between the two user images within the regions in which the second halftone screen is used in the first user image, so that the authentication image becomes visible in contrast between the positive correlation of the corresponding regions and the negative correlation of the remaining regions enabling authentication of the authenticable document; and a negative correlation occurs between the two halftone screens in the first and second user images everywhere within the region in which the second halftone screen is a conjugate of the corresponding region of the first halftone screen, and where a positive correlation occurs where the two screens are identical, so that the authentication image becomes visible in contrast between the positive correlation of the corresponding regions and the negative correlation of the remaining regions enabling authentication of the authenticable document.