On the back of the US $20 bill is a strange pattern of “20” numbers. They are yellow, which is odd for the overwhelmingly green bill, and they seem to be randomly placed. Unlike the rest of the bill, which uses randomness as a way to tight pack an area, the back seems sparsely dotted with these yellow numbers. It has the whiff of having some other purpose.
Turns out these numbers are part of the EURion constellation on the bill. The zeroes are circles in a specific geometric arrangement that allows for detection of moneyness (to coin a term, har!) by copiers and software like Photoshop, which refuses to open scanned images of money.
Except it seems that software doesn’t use the EURion constellation. The linked page demonstrates all sorts of clipping and manipulation of money images, some of which are detected as money, and some of which are not. The EURion constellation being part of the image seems not to be correlated to detection. Interesting.
There must be all sorts of fascinating algorithms at work to detect bills from many countries in many denominations...