How to Use a Black Light to Identify Reprints and Fakes of Antique Paper Collectibles
My favorite is the little LED flashlight shown in the article. But there are different styles that give off the same blacklight, so the style is just a personal preference. As long as they give off blacklight, they all work the same.
For more esoteric reading, shining a blacklight on a baseball card is an elemental form of spectroscopy, which is a highly advanced nuclear physics area.
Spectroscopy in artifacts examination