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Some currency contains a pattern which instructs color copiers not to copy
it. Markus Kuhn documented the pattern in this PDF and named it the EURion Constellation.
Eurionize is a program that marks a postscript document with the EURion Constellation, thereby preventing it from being copied on most color copiers. Eurionize reads from standard input and writes to standard output. It can be invoked as:
eurionize < document.ps > protected.ps
Here is a postscript document before and after eurionization. And here are PDF conversions of the before and after.
Eurionize is written in Perl. It should work on all operating systems that support Perl.
Adding the EURion Constellation to a document doesn't add much to the difficulty of copying it. Black and white copiers do not appear to recognize the mark, so it could only protect documents that must be in color. There are numerous ways for the holder of the document to remove or mask the mark if he's only interested in copying the informational content.
Still, the technique might provide a small increase in security of documents like coupons and entertainment admission tickets.