system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do machines that accept cash payment determine that a five dollar bill is a five dollar bill, etc?
gpt
Optical scanning.  An optical scanner looks at the denomination Portrait, and  other parts of the bill to reduce fraud.  Optical scanning is a follow-on from magnetic scanning developed and commercialized by Stanford Research Institute (now SRI International) for bank check clearing automation.  Look at the font at the bottom of a bank check (or cheque).  It was originally developed to be printed with a high iron content and read by magnetic arrays (early 1950s).  As computer processing and digital imaging progressed, better optical recognition was developed.  Some currency scanners also read the metal strips put into US currency starting in the 1990's.