Comment by minikites

"The primary method vending machines use to recognize the denomination of paper money is through a magnetic scan; paper currency is printed with magnetic ink, similar to the ink on the MICR line of a check, that makes it easily identifiable to machines with magnetic scanners. In addition, each denomination is marked with different fluorescent properties. Many vending machines and other machines that read paper currency use an ultraviolet light to scan the bill, read the fluorescent response and issue the appropriate credit."