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A new kind of pixel will have you seeing double. Researchers have printed two entirely different images in the same space using special nanomaterials instead of regular inks.

Traditionally, a pixel is the smallest controllable piece of a printed picture or an image on a screen. Each is usually a tiny square or dot of a particular colour and brightness.

The new nanopixels, created by at the University of Glasgow, UK, and his colleagues are each less than a micrometre across. And instead of being a dot of dye or light, each pixel is made of a hole in a thin sheet of aluminium.

The punctures are shaped like minuscule crosses, with a long arm and a short one. The length of each corresponds to the wavelength of one colour of light. When white light is shone on the aluminium sheet, each hole can let through light of one of these colours at a time. The effect requires the aluminium sheet to be between the viewer and the light source.

Which colour is seen at each pixel depends on how the light coming through the pinhole is polarised – that is, whether the waves of light oscillate vertically, horizontally, or some angle in between – and so whether it lines up with the long or short arm.

Changing colours

Clark and his team can change the colour of the pixels by altering the lengths of their arms or the distance between them.

“Think about it like painting on a canvas: you dip your brush in yellow paint and paint something yellow,” says at the University of Central Florida. “Here, instead of dipping your brush, you change the pattern.”

With the right arrangement of crosses, two different pictures can be stored on the same surface at the same time, each visible under a different type of polarised light.

Unlike regular inks and paints, these pixels won’t become washed out from long-term exposure to light, heat and humidity because the colours come from the structure of the material itself.

“These colours will never, ever fade over time, whereas dyes will fade when they’re exposed to sunlight,” says Clark. “This will be as bright and as lovely in 500 years as it is now.”

Anti-counterfeiting and data storage

The pixels’ tiny size and ability to store two pictures without fading make them ideal for anti-counterfeit measures, particularly since they would be tricky to forge.

“This team has a $40 million university facility to generate the structure, but a common man sitting in a garage trying to make counterfeit currency will not have that,” says Chanda.

However, while the pixels won’t fade, the aluminium sheets are much thinner and more fragile than standard aluminium foil. The sheets would still work for long-term information storage provided they were protected, but it could prove tricky to put them anywhere where they would face daily use.

“If you’re putting this on currency, it’s going through thousands and thousands of human hands, purses, ATMs. It needs to be very robust,” Chanda says. But, Clark says, it wouldn’t be too difficult to embed the thin foil in a transparent plastic to keep it protected.