EMBEDDED PRINTED DATA - METHOD AND APPARATUS

摘要

An article is provided comprising data embedded within the screen (1) of a halftone printing process. The data (2-5) is encoded by the positioning of the centroids of individual elements or dots comprising one or more screens (1) used for printing a document, as distinct from the normal variation of the weight of these dots relative to their spacing to produce the variations in density of conventional halftone printed matter. The embedded data is readily made visible by apparatus which provides interference between the screen (1) comprising the data (2-5) and a reference pattern having a similar spatial frequency. Alternatively the data (2-5) may be detected by an electronic imaging process in which controlled movement of the reference pattern relative to the printed screen provides a series of images which are processed together to extract the embedded data (2-5). The use of a physical reference pattern may be replaced by scanning a document and referencing the positions of screen dots in one location to those in other locations. Inter alia, the invention has application in the field of security printing by allowing the incorporation of security features and other embedded data in halftone printed documents.