In modern bank notes they widely use dyes which are perceived at sight as dyes of the same colour but have different reflection in infra-red spectrum range. Such an effect is called as IR-metamerism of dyes. If a drawing is made with two dyes identical in hue but different in properties, than in IR-range of spectrum one will see only a part of the whole drawing corresponding to the zone coated with the dye reflecting IR-rays.

Zones IR on the euro banknote