Luminescence (lat. lumen = light) denotes the optical radiation of a physical system. This optical radiation occurs during transition from an excited state back to the ground state (radiative deactivation). Luminescent inks utilise the principle of photoluminescence. Here, light occurs not only as the product, but also as the reactant. The special ink is invisible under normal lighting conditions (daylight, artificial light) because only irradiation with UV or infrared light triggers the light emission.
Luminescent materials are already being used on credit cards, packaging, documents and brand products in order to distinguish originals from counterfeits. Using luminescent inks, highly individual and seemingly invisible security features can be applied to any printable surface.