Digital Watermarking Using Clear Toner

摘要

It is often desirable to protect a document against unauthorized copying. This could be easily achieved by printing part of the image such that it can be readily observed by a human reader but not by a copier or scanner. An image can be printed using clear toner or ink so as to create a difference in reflected and diffused light that can be distinguished by a human reader who looks at the paper at an angle; however, this feature cannot be detected by copiers or scanners because they are restricted to reading at right angles to the page. In the printing industry, clear or white toners are often used to produce digital watermarks that cannot be copied or scanned but can be observed by a human reader. These methods need specifically matched paper and toners that have a considerable difference in gloss. It works with the combination of glossy toner on matte paper or matte toner on glossy paper. However, this approach does not work when glossy paper is used with a toner that provides images in the same gloss range. Such a combination is very common in the field of digital commercial printing. Several digital watermarking processes have been developed that embed a visually striking gloss image into one to four color images. In most cases, controlled angular dependence of image gloss is used to create visual differential gloss in the form of a digital watermark. Printing equipment with fifth imaging station allows for the production of digital watermarks with clear or low-pigmented toner. It provides a method to produce digital watermarks on paper without the limitations described above. This approach is not limited by the color or gloss properties of the toner, the substrate, and/or the selected fusing technology. This technology allows for the production of clear and visible watermarks using toners that produce an image of about the same gloss as the paper and image with the appropriate fusing technology. The additional clear toning station allows, as well, the addition of clear toner to areas of the digital watermarking process that alone cannot provide a sufficient digital watermarking effect in low-density and high-density areas.