i mostly use credit cards

january 09, 2004 @ 23:59

I thought this PDF that shows how some copy machines and scanners know how prevent people from illegally copying money was neat. Turns out that it's just simple geometry involving a bunch of little 1 mm circles arranged in a pattern. The same pattern is used on all kinds of money all over the world.

Here in the US, for example, it's built into the new $20 bill. On the backside, there are a bunch of little yellow '20' symbols that seem randomly placed, but taken by themselves, the '0's create the special pattern that prevents them from being scanned/copied.

Cool, eh?

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