The image above is from Wikipedia. It’s an illustration of something called “printer steganography.” Try not to focus on the ruler marks — they are included to give a sense of scale — but instead on the barely visible yellow dots. Those yellow dots are akin to a fingerprint, made by your color printer, identifying the document’s source.
In 2004, Purdue University announced this innovation which allows printers to embed a secret, almost invisible code into printed documents. A person who knows of the code’s existence and, of course, how to decode it, can use this added information to verify the authenticity of the document itself. At the time, PC Magazine waxed poetic about the advancement, wondering aloud if this innovation could be used, for example, to print your own driver’s license at home. But the more practical value? As of 2005, the Secret Service (which was originally charged with stopping counterfeit money) used the technology as a tool in tracking down alleged counterfeiters.
What information is in there? Also in 2005, a watchdog group, the Electronic Frontier Foundation (EFF) reversed the code insofar as a line of Xerox printers was concerned (in-detail explanation here). EFF was able to determine the time and date the print was made, as well as the serial number of the printer which created the document.
Does your printer add these dots? EFF has taken the initiative in building a list of printers which employs this fingerprinting technology; a list (which appears out of date) is available here.
Bonus fact: Many forms of currency have similar technology, known as the EURion constellation, which is used to help software such as Adobe Photoshop detect a scanned image of paper currency. Understandibly, very little is known about the design (for example, its Wikipedia entry is sparce and poorly sourced). How does Photoshop react when it comes across an image with the constellation? It prevents editing, as seen here.
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Related: “The Economics of Counterfeit Trade: Governments, Consumers, Pirates and Intellectual Property Rights” by Peggy Chaudhry.