By Jonathan Fildes
Science and technology reporter, BBC News
Arrays of thousands of tiny "super prisms" controlled by robotic muscles could bring real colour to TV screens for the first time, scientists say.
Screens have difficulty representing the true blue of the sky
The devices, known as electrically tunable diffraction gratings, have been built by researchers in Switzerland.
They manipulate light to reproduce the full spectrum of colours on screen, impossible using existing technology.
The team says the devices could also be used to make computer displays with the same resolution as high-end LCDs.
"Today's displays can only reproduce a limited range of colours," said Manuel Aschwanden, of the Swiss Federal Institute of Technology in Zurich and one of the team behind the work.
"The main advantage of this technology is that it can display all colours."
Blue sky thinking
Existing screen technology, such as TV cathode ray tubes, LCDs and plasma screens, reproduce colours using three lighting elements coloured red, green and blue.
Other colours are created by combining the primary colours. For example, yellow is created by mixing red and green.
To show complex pictures, a display must combine the colours at thousands of individual points across the screen.
Different types of screen do this in different ways. For example, an LCD is divided into thousands of individual pixels, further divided into three subpixels coloured red, green, and blue by filters.
Altering the brightness of each coloured subpixel creates a palette of millions of different shades that can be used to represent most pictures.
Methods like this are unable to reproduce every colour we see in the real world. This is particularly evident when reproducing images of the sky.
"When you take a picture and download it to your laptop the blues are never the same as the real sky," said Mr Aschwanden.
Problems like this occur because the three primary colours current displays use to reproduce on-screen colour are fixed. The green, blue and red a manufacturer chooses to use in a display determine all the other colours that can be reproduced.
The new system is not limited to the three-colour system.
Instead, the researchers have developed a flexible approach that uses the full spectrum of colours visible to the naked eye.
Even the latest television screens cannot reproduce all colours
To do this, the team have built a diffraction grating, a slotted grate like a miniature Venetian blind.
Diffraction grates are nothing new. They are already used in projector systems and fibre optic telecommunications.
However, unlike existing solid grates the new one is made of a flexible polymer.
The rubbery material is normally used to build artificial muscles for robots as it contracts if a voltage is applied.
When pure white light from a light emitting diode (LED) hits the grate it is split into the full spectrum of colours like a rainbow produced by a prism.
By applying different voltages to the artificial muscle, the grate expands and contracts, causing the fan of split light to shift from side to side.
Different colours can then be isolated from the spectrum using a tiny hole fixed in front of the grate. Adjusting the voltage across the muscle allows different parts of the colour spectrum to be lined up with the hole.
In a working screen, multiple grates behind each pixel would also allow composite colours to be mixed, reproducing the full range of colours the human eye can perceive.
At the moment, the team has built a proof-of-concept array of 400 gratings side by side. Although too small to be useful, the miniature display has a high resolution.
"It is the same density as a high-quality LCD display," Mr Aschwanden told the BBC News website.
The team is now working on refining their experimental setup and in particular trying to lower the voltage required to make the system work.
The device uses the full spectrum of colours visible to the naked eye
Initial experiments required thousands of volts to flex the muscle, but the team has now reduced that to 300, making the technology more attractive to electronics firms.
With more refinement, Mr Aschwanden says that the devices could have multiple uses in microscopes, fibre optic communications as well as high-end colour screens.
"Tuning or steering light is at the core of all optical systems," he said. "This offers a cheap, accurate way to do it."
The work was carried out with Professor Andreas Stemmer at the Swiss Federal Institute of Technology and is published in the US journal Optics Letters.