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“Artificial muscles” to liven TV color

Aug. 17, 2006
Courtesy Optical Society of America
and World Science staff

Scientists are exploring a technology that they say could produce fuller, more lifelike colors on TV through the use of tiny “artificial muscles.”

The oval-shaped region represents all the colors human eyes can perceive. The "pure," or spectral, colors are on its boundary. Inside, the triangle represents the color space that can be reproduced by mixing red, green, and blue, with the three fundamental colors at its vertices, or corners. Since this image is itself encoded in the RGB channels, the colors outside the triangle are not faithfully reproduced. Displays based on diffraction gratings could instead faithfully reproduce the entire gamut of visible colors. (Courtesy ETH Zurich)

The technology could hit store shelves within eight years, they predict. The research is published online in the research journal Optics Letters, and is scheduled to appear in the journal’s Sept. 1 print issue.

In ordinary displays such as TVs, flat-screen LCDs, or plasma screens, each pixel—or dot of light onscreen—consists of three light-emitting devices. Each gives off one of three “fundamental” colors: red, green, and blue. 

Different colors are produced by combining those three. For example, shades of orange and yellow mixtures of different amounts of red and green. 

Unless you look closely, the colors in a pixel are indistinguishable: the eye sees a single combined, or composite, color. 

The fundamental colors in each pixel are fixed. Only their amounts can change, by adjusting the brightness of the color elements. 

That way, existing displays can reproduce most visible colors, but not all. For example, they don’t faithfully reproduce the blue hues of the sky or sea.

The new technology’s developers, Manuel Aschwanden of the Swiss Federal Institute of Technology in Zurich, Switzerland, and colleague Andreas Stemmer, plan to overcome such limitations by changing the fundamental colors themselves, not just their brightness. 

To get different colors, they used an optical trick called diffraction. When white light hits a pattern of equally spaced grooves on a surface, called a diffraction grating, the light spreads out into its component colors. The effect is similar to the way light spreads out when it goes through a prism, or reflects off a CD.

The colors coming out of a diffraction grating fan out at different angles. In Aschwanden’s setup, the grating is a rubbery, ultra-thin membrane, with one side molded into a shape resembling microscopic pleated window shades. 

It also has the feature of doubling as an “artificial muscle,” a substance that contracts in the presence of a voltage. As the membrane stretches or relaxes, the incoming light “sees” the grooves spaced closer or tighter. This changes the angles of the colors. 

A desired color can then be isolated by passing its light through a hole. The color can be changed by altering the voltage so that different parts of the spectrum pass through the hole. The artificial muscles enable the “fan” of colors to move enough that the isolated light beam can change from one end of the spectrum to the other, Aschwanden said.

To create composite colors, each pixel would use two or more diffraction gratings. This way, a display could produce the full range of visible colors, he claimed. 

Getting the full range requires a source of “true” white light to begin with, he added—rather than a mere combination of red, green and blue that looks white to human eyes, and that is typically used for white. For this purpose, he said, the technology could exploit a new generation of white lights recently developed using devices known as light-emitting diodes.

Though Aschwanden and Stemmer have so far only what they call a proof of concept, it demonstrates the technology’s feasibility, Aschwanden claimed; with enough investment, it could turn into consumer products. 

“Once you have one pixel, it doesn’t take too long to develop a new product,” he said. 

A problem so far is that it only works with voltages of 300 volts or higher, he continued, and this number must reach the 120 volts used in households. But new materials under development have been allowing the technique to function at lower and lower voltages, he added, so if this continues, the technology could become practical.

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