How handy would it be if you could make your tattoos invisible for a job interview in the morning, and reveal them again for a party in the evening? This sounds like science fiction, but it may soon be reality.

Researchers at the University of Colorado Boulder have developed invisible tattoo ink than can be revealed with a UV torch and stays visible until switched off with white light. They have published their research in the journal Matter & Light.

They created different motifs with this ink ranging from a lantern that reveals its light when activated, to multicoloured scales like those in the much-loved children’s book The Rainbow Fish.

The pigments used in these inks are so-called photochromic dyes (from the Greek “photos”, which means “light” and “chroma”, which means “colour”). In other words, they are compounds that change colour when exposed to light. These dyes have been used for years in colour-changing sunglasses.

Light carries energy, and photochromic compounds are designed in such a way that this energy is enough to cause a chemical change that alters the shape and therefore the properties of the........

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