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As we do with many of our technology-related questions, we consulted a fantastic resource called How Stuff Works. After some hunting and pecking, we found a relevant question about dollar bill changers that discusses the mechanics of automated currency identification. While vending machines use a variety of methods to read bills, they check four different characteristics used to verify paper money: - Bills are printed with magnetic ink, and different denominations generate distinctive magnetic fields.
- Bills have different conductivities (the rate at which they can conduct low levels of electricity).
- Ultraviolet scanning technology can determine the different fluorescent properties of bills.
- Digital cameras can reference the optical patterns in bills with a database of authentic patterns.
A vending machine has to verify whether a bill is authentic before it determines its value. The U.S. Bureau of Engraving and Printing hosts this cool fact sheet about the security features used on its currency: color-shifting inks, watermarks, and microprinting. Those nifty currency counters that you see operated by shady types in gangster movies use a similar array of technology: magnetic sensors, ultraviolet readers, fluorescent bulbs,
and optical magnifiers. After all, criminals need to watch out for counterfeit bills, too.
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