What is optically variable ink (OVI)?

Optically variable ink (OVI) refers to a specialized type of ink that exhibits color-shifting properties when viewed from different angles or under varying lighting conditions. This unique characteristic makes it an effective feature in security printing, particularly in the production of banknotes and other important documents. The ability to change color enhances the difficulty of counterfeiting, as this feature is not easily replicable by standard printing methods.

In terms of the other options, while ink that appears like glitter or that is invisible under standard light conditions may have its own applications, these traits do not define OVI. Similarly, while there may be inks used in military applications that feature various technological advancements, OVI specifically pertains to ink that demonstrates optical variability, which is not solely limited to military use. The defining feature of OVI is its ability to create distinct visual effects that enhance security and authenticity in printed materials.