Optical variable ink (OVI) is a type of ink that is used in security printing, especially on banknotes and other valuable documents, to prevent counterfeiting. OVI contains special pigments that change color or appearance when viewed from different angles or under different lighting conditions. OVI works by using a combination of thin-film interference and diffraction. Thin-film interference occurs when light passes through a thin film and some of it reflects off the surface while some of it passes through and reflects off the bottom of the film. This causes the light waves to interfere with each other, creating a pattern of light and dark bands. When combined with diffraction, which causes light waves to spread out and bend when passing through narrow slits or gratings, the result is a unique, multi-colored, and dynamic effect. The OVI pigment is usually made up of a mixture of materials, such as metal oxides or nanoparticles, that have different refractive indices and shapes.
