In the back of the banknotes there are designs printed with fluorescent ink, which glow when placed against ultraviolet light (also known as “black light”). In particular, two fluorescent inks are used in the new design of the 50-peso banknote.
In regard to banknotes made with paper they have color fibers on both sides—that is, tiny fibers embedded in the paper and distributed at random. These fibers are fluorescent and can only be seen under black light. In addition, in the case of the 100-, 200-, 500-, and 1000-peso banknotes, the 3D-thread also shines when a banknote is exposed to black light.