5€, 10€ and 20€ notes have a vertical band printed on their reverse in iridescent ink which changes colour depending on the viewing angle and the angle of illumination. The iridescent stripe makes it easier for the general public to authenticate their banknotes.
Ink with fluctuating shine and colour depending on the viewing angle and lighting. In this respect, iridescent inks differ from typical printing inks, which have a shine and colour that is the same regardless of the viewing angle or lighting. Iridescent inks are useful for banknote security because their optical effect cannot be imitated by colour copiers. There are various types of iridescent ink: Metallic ink, pearlescent ink and optically variable ink.