Highlighter ink is designed to sit brightly on paper while leaving printed text legible underneath.

The longer version

Highlighter ink is formulated as a transparent fluorescent colour rather than an opaque paint. It stains the paper so the letters underneath still read, while the pigment converts some incoming light into a brighter glow, so the marked band looks vivid without hiding the words it sits on top of.

Fluorescence means the ink gives out light at a longer wavelength than it absorbs, which is why marked areas look almost luminous under bright light. Ordinary ink would simply cover the text. Because the dye is transparent it works best on paper that resists bleeding, and on thin pages the colour often soaks through to the far side.

Where this entry sits

Odd Little Things looks at the ordinary objects people touch every day and the small mechanisms and histories hiding inside them. Buttons, zips, pencils, mirrors, stairs, stamps, packaging, and road signs all get their turn. The full list sits on the Odd Little Things page, where every entry is listed in order.