How does a vending machine read dollar bills?

HOW MACHINE READ MONEY??

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 the security features used on its currency: color-shifting inks, watermarks, and micro printing.

Those nifty currency counter 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.

source: Ask Yahoo!