This article describes a novel visual information concealment technique, referred to as optical watermarking, for the authentication of original printed documents. An optical watermark is a two-dimensional binary image. It can be of any shape and can be printed on any part of a document. The optical watermark is constructed by the superposition of multiple two-dimensional binary images (referred to as layers), each with different carrier structural patterns embedding various hidden information. The hidden information is embedded into each layer using phase modulation. Based on properties of the human visual system and modulation principle, the hidden information becomes visible to the human eyes only when a right "key" is positioned on top of the optical watermark with the right alignment. Here, "keys" play the similar role as keys in encryption, that is, to decode hidden information. Thus, with such a "lock and key" approach, it greatly improves the security level of the optical watermark. In addition, the multiple layer structure of the optical watermark makes it extremely robust against reverse engineering attacks. Due to its high security and tight link with electronic document systems, which requires documents to be finally printed on paper, the optical watermark has been applied to various electronic document systems. These are online ticketing, online bill of lading, and remote signing and printing of documents, where critical and unique information are embedded in watermarks and printed together with individual documents for future authentication. It has also been used in offline and traditional antiforgery applications, such as brand protection, preprinted high-value tickets, and identification documents

Recognition is a major capability of human being and animals. In the discipline of pattern recognition, research has been carried out extensively on recognition by classification. As a matter of fact, most of the time, human perform recognition tasks by recall. This paper describes the framework of recognition by recall. The definitions are given from the perspective of content-based retrieval. The approach and preliminary results are also presented.