Recommended reading: The secret codes of British banknotes
The BBC has a very interesting article dated 25 June 2015 by Chris Baraniuk entitled "The secret codes of British banknotes," which discusses the so-called EURion Constellation (aka Omron rings) patterns' role in deterring digital counterfeiting.
Unmentioned in this article is that the simplest way to defeat this anti-counterfeiting feature is to use older scanners and photo editing software which do not interfere with the manipulation of banknote imagery. For example, Adobe Photoshop 3 will detect if you attempt to open a banknote image, but it will allow you to open, edit, and save same, but you can't print. With Photoshop 4, you're blocked from even viewing any image determined by the software to be a banknote.
Courtesy of Anthony Rodov and Christof Zellweger.