Counterfeit detector pen with UV light cap is an inexpensive tool for detecting bad bills with two effective tests in one convenient, pocket-size unit. Patented ink tests the paper fibers for authenticity, and the reaction helps identify computer-generated counterfeit bills. Powerful, UV light can also authenticate other official documentation such as drivers' licenses, passport, credit cards and most foreign currency. Cap includes three button-cell batteries that are easily replaceable. Pen is not guaranteed to detect all counterfeit bills.