Watermark

A watermark is an identifying image or pattern in paper that appears as varying shades of light and dark when viewed by either transmitted light or reflected light. These patterns are created from variations in the thickness or density of the paper. Watermarks have historically been used on postage stamps, currency, and other official documents to discourage counterfeiting. There are two primary methods of producing watermarks in paper: the dandy roll process and the more complex cylinder mould process. Watermarks vary widely in their visibility. While some are easy to see, other watermarks can be covert. Aids to spot watermarks have been developed, such as watermark fluid that wets the paper without damaging it. A watermark is particularly valuable in the examination of paper goods because it can be used to date documents and artworks. Watermarks can identify sizes, mill trademarks and locations, and the quality of a sheet of paper. The term watermarking is also used in digital contexts that share similarities with physical watermarks. A digital watermark may be used to identify output from an unlicensed trial version of a program. In another instance, identifying codes can be embedded as a digital watermark for a music, video, picture, or other file. An artist may add their identifying digital signature, graphic, or logo to their digital artworks as an identifier or anti-counterfeiting measure.