...The algorithm looks in the blue channel of a color image for little circles and most likely examines the distance distribution encountered. I have discovered a small constellation of just five circles (a bit like Orion with the belt starts merged) that will be rejected by a Xerox color photocopier installed next door from here as a banknote. Black on white circles do not work.

These little yellow, green or orange 1 mm large circles have been on European banknotes for many years. I found them on German marks, British pounds and the euro notes. In the US, they showed up only very recently on the new 20$ bill. On some notes like the euro, the circles are blatantly obvious, whereas on others the artists carefully integrated them into their design. On the 20 pound note, they appear as "notes" in an unlikely short music score, in the old German 50 mark note, they are neatly embedded into the background pattern, and in the new 20 dollar bill, they are used as the 0 of all the yellow 20 number printed across the note. The constellation are probably detected by the fact that the squares of the distances of the circles are integer multiples of the smallest one.

I have later been told that this scheme was invented by Omron and that the circle patter also encodes the issuing bank.

I like the comment that follows that one, about currency watermarking underwear to prevent upskirt photos. (As opposed to legislation requiring digital cameras to make a loud noise every time a picture is taken.)
- tom moody 1-11-2004 9:02 pm