Banknotes carry the EURion pattern that makes colour photocopiers refuse to copy them

The BBC reported in 2015 how Cambridge computer scientist Markus Kuhn, then a PhD student, put a British £20 note on a new Xerox colour copier in the early 2000s and got no copy, only a message that copying banknotes is illegal. On a 10-euro note he spotted a recurring pattern of five small circles, which he found on sterling and other currencies too, hidden on the £20 as the heads of printed musical notes. He named it the EURion Constellation, after Orion.

Tests suggested how it works. A black-and-white drawing of the circles copied fine, but colouring them in triggered the anti-copying message, so Kuhn thinks the circuitry needs the circles in a colour channel. Some suggest copiers also check the distances between circles; Kuhn could not verify that. A 2005 Reserve Bank of India press release linked the Japanese firm Omron to the pattern, as did patents filed in 1995, and a blog post by a retired Indian official says it has been used on banknotes since 1996. The industry calls the rings 'doughnuts'.

Nobody wants to talk about it. The Bank of England has never commented on the pattern; Xerox said only that it consults with the Central Bank Counterfeit Deterrence Group and would not say how it is deployed. That group's director, Pierre Laprise, confirmed it developed technologies to prevent photocopying and image editing, and says they have been effective and counterfeiting 'pretty well contained'. Another expert noted that photo-editing software may rely on a separate digital watermark.

A pattern small enough to hide in a note's design can still be recognised automatically by a machine.

Putting detection inside copiers and scanners stops the copying at the moment of use.

The pattern is reused across currencies, so manufacturers need one detector for many notes.

Secrecy about the details makes it harder for forgers to defeat.

Put the defense in the machines everyone uses, and keep the details secret so forgers can't engineer around them.

By 2015, according to the BBC, the circle pattern still appeared on banknotes worldwide and the CBCDG described its deterrence work as effective, while officials still declined to detail how it works. The article could not independently confirm that copiers rely on the pattern as researchers believe.