Tuesday, January 18, 2011

Speckles in presswork - secret of the yellow dots

Most manufacturers of laser toner printers have embedded within them a technology that leaves microscopic yellow dots on each printed page. The dots are intended to identify the date and time of the printed sheet (if known by the printer itself) as well as the printer's serial number in order to identify the owner and location of the printer.
On the left, a close up of the "secret" yellow laser dots. On the right the same dots viewed under a blue light to enhance contrast and visibility.

The dots, which are normally invisible to the naked eye, form a code which is used by authorities such as the U.S. Secret Service to investigate the printing of counterfeit money made with laser printers.
The secret yellow dots are typically too small to be seen at normal viewing distances.

The yellow dots are a bit larger than the halftone dots used to create the actual image.

Unfortunately, if the laser printer is not calibrated properly, or depending on the design of the graphic being printed, the yellow dots may be dark enough to be visible. Also, the dots may affect the reporting integrity of color measurement instruments – e.g. a cyan patch intended to be 100% cyan only may contain yellow security dots and cause a slight green shift.

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