Diffractive optically variable devices (DOVDs) – or the bright split-colour images on the surface of various banknotes, documents and duty stamps – are popular security features. Complex DOVDs are difficult to counterfeit and often embrace an extensive range of hidden security elements. Moreover, they are easily recognised by the layman. But how are they produced and how do the different manufacturing techniques compare? Tomas Tethal is Managing Director (and joint owner) of Optaglio, a company established in the Czech Republic. He graduated from the Czech Technical University in Prague (Faculty of Nuclear Sciences and Physical Engineering). In 2000, he became Managing Director of Metallic Security, one of the top-ten manufacturers of security holograms in the world. Tomas has published numerous articles on subjects such as nuclear chemistry, nuclear physics and the use of holography.