The small circles or dots constituting the EURion constellation are clearly visible on the centre-left of 10 euro banknotes.

A number of recent banknote designs contain a pattern now known as the EURion constellation. It is added to help software detect the presence of a banknote in a digital image. Such software can then block the user from reproducing banknotes to prevent counterfeiting using colour photocopiers. The name was coined by Markus Kuhn, who discovered it in early 2002 while experimenting with a Xerox colour photocopier that refuses to reproduce banknotes. It is a portmanteau of Orion, a constellation of similar shape, and EUR, the euro's ISO 4217 designation.

The EURion constellation first described by Kuhn consists of a pattern of five small yellow, green or orange circles, which is repeated across areas of the banknote at different orientations. Andrew Steer later noted simple integer ratios between the squared distances of nearby circles, which gives further clues as to how the pattern is meant to be detected efficiently by image-processing software.

The EURion constellation is most prominent and was therefore first recognized on the 10 Euro banknotes. It was then also found on (now obsolete) Deutschmark notes, recent British banknotes, the backs of the new U.S. $20 and $50 bill, the current (post ca 1998) Danish banknotes, and all of the 2001 series of Canadian bills.

Users of recent versions of image editors, such as Adobe Photoshop or Paint Shop Pro, discovered that these also refuse to process banknotes. According to an article in Wired magazine, the banknote detection code in these applications, called the Counterfeit Deterrence System (CDS), was designed by the Central Bank Counterfeit Deterrence Group and supplied to companies such as Adobe as a binary module. However, experiments by Steven J. Murdoch and others showed that this banknote detection code does not rely on the EURion pattern. It detects other features of banknote designs that have yet to be described in public.

External links

• http://www.cl.cam.ac.uk/~mgk25/eurion.pdf (in Adobe Acrobat format)
• http://www.freedom-to-tinker.com/archives/000497.html
• http://www.wildspark.com/eurionize/ - Add the EURion Constellation to a PostScript document
• http://www.wired.com/news/print/0,1294,61890,00.html
• http://www.cl.cam.ac.uk/users/sjm217/projects/currency/ - Demonstration that the CDS does not use the EURion constellation and also other related information
• http://www.nationalbanken.dk/DNDK/money.nsf/side/200-kr!OpenDocument - current danish currency