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printer codeSleuths from the Electronic Frontier Foundation have analyzed a tracking code that's printed on every page produced by a Xerox Docucolor 12 laser printer. Many printer manufacturers embed code to give the Secret Service a tool to track counterfeiters, although it can easily be used for other purposes.
The tracking code is printed in tiny yellow dots which are generally not visibile by the naked eye under white light. Under blue light and magnified, the yellow dots appear black.
This is what the dots look like under 60x magnification photographed by a Digital Blue QX5 computer microscope.