Wikipedia on the EURion Constellation.

Markus Kuhn, "The EURion Constellation," Computer Laboratory, University of Cambridge, 2002-02-08.

Steven J. Murdoch, "Software Detection of Currency".

The "constellation" consists of five circles arranged like the starry constellation of Orion.

It is found in different orientations on the paper money of many countries. The EURion Constellation is intended to prevent color copiers from reproducing images of contemporary currency. This constellation is printed in yellow (or variants of green and orange) is most visible in the blue-channel of any of the color images. Currency designers have made special efforts to "hide" the pattern in the "look" of the banknote. Adobe PhotoShop and other software also detects images of currency and will not open them. They key in on other patterns more subtle than the constellation. The algorithms for these other "tests" for currency are not publically known, but are the subject of research by computer scientists. Questions? Comments? Please email me... nick.gessler(at)duke.edu