Secret printer dots

Since the 1980s most colour printers and photocopiers add a set of secret near-invisible dots to every page they print. The dots uniquely identify the origin and timestamp of that printout.

Printer dots
Florian Heise, / Public domain

When the very first colour photocopier came out of the very first colour photocopier factory and the very first user walked up to that very first photocopier, I bet that they tried to copy some money. Or at least that’s what Fuji Xerox probably thought when they patented a technique to identify and block the photocopying of banknotes. But hidden inside that patent was the following technique:

According to a further aspect of the present invention, in the image processing system for printing pattern information, such as character information and coded information, on a copy sheet, information peculiar to the image processing system may be contained, and the system may be readily identified by decoding the information.

When the pattern data is printed using specific color information, e.g., yellow to which the eyes of a human are sensitive to the smallest degree, or below the threshold level of visual perception, the printed pattern information is not or scarcely seen.


What does that mean? It means that photocopiers and printers can leave a series of nearly-invisible yellow dots on every printout – a hidden code. And for several decades, this is precisely what they did. Every printed page contained yellow dots that included the serial number of the printer / copier and a date stamp. It’s an anti-counterfeit trick, essentially. If someone prints out fake money, all that law enforcement needs to do is find the yellow dots with an ultraviolet lamp and decode them. Once decoded, they’ll know just where it came from (assuming, of course, that they can track the unique serial number) and when it was printed.

Feels a bit dodgy, no? Two things make it even dodgier. First, the general public didn’t find out about this until an exposé in PC World magazine in 2004. Second, these hidden marks were printed on everything… which is a problem if you are (for example) a whistle-blower in a totalitarian government and don’t want your leaked documents to connect back to you.

As far as I can tell these yellow dots are still used in modern printers, but seeing as no-one prints out anything important anymore they’re less of a threat. (I kid, I kid.)


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