Inks, completely invisible, reveal colours only after lighting IR light on them. Mainly used for protection of securities against counterfeit. They are a high quality protection against forgery, often used for printing bank notes. To detect prints secured with IR inks, a special device which emits infrared light is needed.
Another type of IR inks is used in commerce and various other activities which use IR readers (scanners). Black IR inks come in two types: the first one is visible for the scanners, while the secont type is not. These properties allow to hide bar codes and other elements detectible by scanners by covering them with a black ink, invisible for scanners.