What makes highlighter pens so bright?

highlighter pens

Fluorescent ink, such as that made famous by DayGlo, is known for being brighter than normal types of ink, especially under ultraviolet light. DayGlo ink contains fluorescent pigments, which absorb and reflect more light than conventional inks. Normal pigments cause materials to be a certain colour because they absorb some parts of the visible spectrum, while reflecting the wavelengths of light corresponding to the colour you see.

Fluorescent pigments absorb visible light too, but they are also very sensitive to the higher energy of ultraviolet light. Ultraviolet light absorbed by these pigments excite electrons surrounding the atoms of the pigments, giving them energy. This energy is then released in the form of a photon which happens to be of a lower energy corresponding to visible light, meaning the ink emits more light in total making it brighter. Fluorescent pigments are often combined with conventional pigments to develop a wider range of colours that fluoresce.

Image credit: BrokenSphere