PARIS: The next generation of banknotes may incorporate an iridescent shimmer inspired by the opal that changes colour when you turn or twist the note, New Scientist reports in next Saturday's issue.

The iridescence comes from polyester film with photonic crystals, whose layered structure blocks certain wavelengths of light at some viewing angles, but lets other wavelengths through.Until now, iridescence in plastic sheets comes through etching patterns into the film, but the result is often poor, with dull colours and a “milky” effect if viewed from the wrong angle.

The new film, devised by the University of Southampton in Britain, the German Institute for Polymers in Darmstadt and the pharmaceutical giant Merck, “shimmers from every angle” and should be cheap to mass-produce, the British weekly says.

Used in banknotes, the shimmer will be a major headache for forgers because it raises the technological barrier for fakery.

The opalescent effect could also be used in food safety. Nanoparticles incorporated in the film could be engineered to change shape in response to certain toxins, so that if food spoils, the packaging would change colour as a warning to the consumer.—AFP