What is an Optically Variable Device (OVD)?
An optically variable device (often abbreviated as OVD) is a security feature that changes how it appears to the viewer. The banknote or feature itself does not change, but how it interacts with light and viewing conditions makes it seem to vary, to an observer.
Many different security features can be considered OVDs, from color-shifting inks to holograms. There are also many ways to unite an OVD with a banknote, from inserting the feature into the note substrate (in the case of security threads) to printing it onto the surface of the note (in the case of color-shifting inks).
Multiple factors must be considered when determining the suitability of a particular OVD for use on a banknote. Resistance to physical wear, ease of use, the visual significance of the change and resistance to counterfeiting are some (but not all) important factors.