The Electronic Frontier Foundation (EFF), a consumer privacy and digital rights organization, has analyzed codes embedded in printouts in color laser printers. The code cracked by the EFF was an invisible bar code that contained the serial number of the printer for tracking users as well as the date and time a page was printed.
In Xerox printers the code appears in a pattern of yellow dots visible only with a magnifying glass and a blue light. The codes are supposed to be for government agencies looking out for counterfeit currency printers.
The EFF said it has identified similar coding on pages printed from virtually every major printer manufacturer though it has so far managed to crack the codes for only one Xerox model.
Seth David Schoen, staff technologist, EFF, said that the dots are yellow, less than one millimeter in diameter, and are typically repeated over each page of a document. The pattern is visible under blue light with the help of a magnifying glass or a microscope.
A complete list of the printers that have the yellow dots is given here. www.eff.org/Privacy/printers/list.php. Though some printers do not produce the yellow dots, some of them leave other watermarks and may have other ways of identifying them.