The EURion constellation (also known as Omron rings^[1] or doughnuts^[2]) is a pattern of symbols incorporated into a number of secure documents such as banknotes, checks, and ownership title certificates designs worldwide since about 1996. It is added to help imaging software detect the presence of such a document in a digital image. Such software can then block the user from reproducing banknotes to prevent counterfeiting using colour photocopiers.

The name "EURion constellation" was coined by security researcher Markus Kuhn, who uncovered the pattern on the 10 Euro (€10) banknote in early 2002 while experimenting with a Xerox colour photocopier that refused to reproduce banknotes.^[3] The pattern has never been mentioned officially; Kuhn named it the EURion constellation as it resembled the astronomical Orion constellation, and EUR is the ISO 4217 designation of the euro currency.^[4]

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. The mere presence of five of these circles on a page is sufficient for some colour photocopiers to refuse processing.

Some banks integrate the constellation tightly with the remaining design of the note. On 50 DM German banknotes, the EURion circles formed the innermost circles in a background pattern of fine concentric circles. On the front of former Bank of England Elgar £20 notes, they appear as green heads of musical notes; however, on the Smith £20 notes of 2007 the circles merely cluster around the "£20" text. On some U.S. bills, they appear as the digit zero in small, yellow numbers matching the value of the note. On Japanese yen, these circles sometimes appear as flowers.

Technical details regarding the EURion constellation are kept secret by its inventors and users.^[2] A 1995 patent application^[5] suggests that the pattern and detection algorithm were designed at Omron Corporation, a Japanese electronics company. It is also not clear whether the feature has any official name. The term "Omron anti-photocopying feature" appeared in an August 2005 press release by the Reserve Bank of India.^[6] In 2007 the term "Omron rings" was used in an award announcement by a banknote collectors society.^[7]

The following table lists some banknotes on which the EURion constellation or Omron rings have been found. Current currencies for which all recent banknotes use the constellation are in bold and whose central banks are members of the Central Bank Counterfeit Deterrence Group are italicised.

• Printer steganography, used by some colour laser printers to add hidden encoded information to printouts
• Coded anti-piracy, an anti–copyright-infringement technology which marks each film print of a motion picture with distinguishing patterns of dots, used as a forensic identifier to identify the source of illegal copies

1. ^ Some currencies (marked ^[€]) were replaced by the euro before the complete adoption of the EURion constellation.