Phosphorescent inks are not only fluorescent under UV lights, but after extinction of the UV lights, they still continue to emit lights for a certain time, this is called after-glow.

They are mainly used in the postal sorting-out, because they are read by a phophoroscope and in that case can be printed on any paper, with or without optical brighteners. Inks are whitish and, when printed on white paper, are invisible.

Phosphorescent inks exist for flexography and  silk-screen, but offset printing process is only available for printing postal stamps.