Counterfeit banknote detection pen

A counterfeit pen is a pen that uses a patented method for the detection of counterfeit United States banknotes. When a marking is made on a suspect bill, if the marking stays light yellow, it indicates that the bill is probably authentic. If the marking turns dark brown, it indicates that the bill is suspect. It claims over 98% effectiveness.[1]

Reference: http://www.securityworld.com/surveillance/NS91417.html