Europium(III) oxide, alongside other chemicals, is phosphorescent and is used in the anti-counterfeiting details in Euro banknotes.

Europium (symbol Eu) is one of the rare earth elements and belongs to the class of  “lanthanides”.

Most of the trivalent rare earth elements are luminescent. This means that they can be excited by shining a light of a particular wavelength at them. When the ions relax again, they emit light – of a different wavelength. That is luminescence.

Euro notes luminesce in the red, green and blue (excited by 254 nm). The red light is clearly linked to europium and most likely to a Eu³⁺-β-diketone complex.