Fluorescent Dyes Used on Banknotes for Security

The hidden world of many world banknotes is seen when the notes are displayed under ultraviolet (UV) light, often called Black Lights. The same effect seen on posters occurs on the banknotes. The areas of the note with fluorescent security dyes fluoresce, often quite brightly. Most notes do not require the natural light to be removed to see these hidden images, since the inks were added for anti-counterfeiting purposes. The inks must fluoresce under all lighting conditions to be effective.
Afghanistan 64a, 1 Afghans'

Azerbaijan 19b, 500 Manat

Here's a group of 225 notes and growing of banknotes exposed to UV light.