Dri Mark UVL800B Dri Mark Security Marking System w/Light Sneaky Security Finally, a way to find out who took your stuff. Drimark's UV Security Marking System has a pen full of invisible reactive ink and a 2-3/8" long UV LED keychain light. Write on your stapler, MP-3 player, or lunch and a flash from your little light will expose your writing. Equally useful for validating re-admission tickets or leaving secret messages. You get a spare set of button-cell batteries for the light.