Whether altering the design of ancient coins or making
photocopies of bills with high-tech machines, counterfeiters
have made illegal money throughout history.
Merchants on the other hand try
to protect themselves by testing and identify counterfeit bills in
any possible way.
I have seen two different
methods of identifying counterfeit bills. In a bank I noticed that
the cashier holds the bill under a purple light. In a supermarket I
saw the cashier puts a check mark on the bill using a special
marker or pen.
But, how do these methods work? How
can a bank clerk pass the bill under a purple light and quickly
determine if it is a valid bill. What does the marker do?