The bigger picture

Scientists have unveiled a new technology that could lead to video displays that faithfully reproduce a fuller range of colours than current models.

The invention, based on fine-tuning light using microscopic artificial muscles, could turn into competitively priced consumer products in eight years, the scientists claim.

In ordinary displays such as TV tubes, flat-screen LCDs, or plasma screens, each pixel is composed of three light-emitting elements, one for each of the fundamental colours red, green, and blue. Shades of orange and yellow are displayed by mixing different amounts of red and green. The colour elements in a pixel are indistinguishable as the eye sees a single, composite colour.

The fundamental colours in each pixel are fixed, and only their amounts can change -- by adjusting the brightness of the colour elements -- to create different composite colours. That way, existing displays can reproduce most visible colours, but not all. For example, current displays do not faithfully reproduce the hues of blue one can see in the sky or in the sea.

‘State-of-the-art displays such as LCD displays can only reproduce a limited range of colours because the three mixing colours red, green and blue are determined during the time of production,’ said Manuel Aschwanden, a nanotechnology expert at the Swiss Federal Institute of Technology (ETH) in Zurich, Switzerland.

Aschwanden and his colleague Andreas Stemmer deduced that these limitations can be overcome by changing the fundamental colours themselves, not just their brightness. To obtain different colours, they used diffraction.

In their setup, white light hits a so-called diffraction grating, a pattern of equally spaced grooves on a surface. Their grating is a rubbery, one-tenth of a millimetre wide membrane, with one side moulded into a shape that resembles microscopic pleated window shades. The membrane consists of an ‘artificial muscle,’ a polymer that contracts when voltage is applied.

White light contains the full spectrum of colours of the rainbow, which correspond to all wavelengths of light. But when white light hits a diffraction grating, different wavelengths fan out at different angles.

‘It's like when you hold a CD in direct sunlight, and you rotate it,’ Aschwanden said. Like the microscopic tracks on a CD surface, the grooves on the artificial muscle split white light into a rainbow of colours. But instead of rotating the surface to obtain different colours, the ETH team adjusts the light's angle by applying different voltages to the artificial muscle. As the membrane stretches or relaxes, the incoming light ‘sees’ the grooves spaced closer or tighter. All the angles of reflection change, so the entire fan of wavelengths turns as a whole. The desired colour can then be isolated by passing the light through a hole: As the hole stays fixed, different parts of the spectrum will hit it and go through it.

To obtain composite colours, every pixel would use two or more diffraction gratings. By this method, a display could produce the full range of colours that the human eye can perceive, Aschwanden said.

Tuneable diffraction gratings are routinely used in applications such as fibre optic telecommunications and video projectors, but existing technologies are based on hard materials rather than artificial muscles, limiting their stretchability to less than a percentage point.

By contrast, artificial muscles can change their length by large amounts. Correspondingly, the fan of reflected light will move enough for the part of the beam going through a hole to change from one end of the spectrum to the other.

Getting a full range of colours requires a source of ‘true’ white light to begin with -- rather than a combination of red, green and blue that looks like white light to the human eye. For that purpose, the technology could exploit a new generation of white LED lights that have recently been developed, Aschwanden said.

Though Aschwanden and Stemmer have so far just a proof of concept, it demonstrates the feasibility of the technology, Aschwanden said. With enough investment, it could turn into consumer products, perhaps in less than eight years, he said. ‘Once you have one pixel, it doesn't take too long to develop a new product.’

The team is now improving the technology to bring it closer to industrial application. In particular, the artificial muscles operated at several thousand volts, while in a consumer product that would have to come closer to the 120 volts of household AC. The team has already reduced the voltage to 300 volts, and new materials currently being developed could allow voltage to drop further, Aschwanden said.

An abstract of the study can be found here.