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Adjustable
diffraction gratings made of tiny artificial muscles could bring
more lifelike colors to TVs and computer displays, physicists at
the Swiss Federal Institute of Technology (Eidgenössische Technische Hochschule, or ETH) in Zurich, Switzerland
show in the September 1 issue of Optics Letters.
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 colors red, green, and blue. The fundamental colors
in each pixel are fixed, and only their amounts can change -- by
adjusting the brightness of the color elements -- to create different
composite colors. That way, existing displays can reproduce most
visible colors, 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, says Manuel Aschwanden.
Aschwanden and his colleague Andreas Stemmer
figured that one can overcome such limitations by changing the
fundamental colors themselves, not just their brightness, using a
tunable diffraction grating.
 A tunable diffraction grating. The vertical membrane is made of artificial muscle, and has carbon electrodes attached to its sides. The membrane has one side molded into a diffraction grating and coated with gold to increase reflectivity. As the applied voltage varies, so does the periodicity of the diffraction grating, changing the angle of the diffracted light. Picture: www.aip.org
"It's like when you hold a CD in direct
sunlight, and you rotate it," Aschwanden says. Like the microscopic
tracks on a CD surface, the grooves on the artificial muscle split
white light into a rainbow of colors. But instead of rotating the
surface to obtain different colors, the ETH team adjusts the
diffraction 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 color 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 colors, every pixel would use two or more
diffraction gratings. By this method, a display could produce the
full range of colors that the human eye can perceive, Aschwanden
says.
Tunable diffraction gratings are routinely used in
applications such as fiberoptic telecommunications and video
projectors, but existing technologies are based on hard,
piezoelectric 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.
Getting a full range of colors requires a source of "true" white
light to begin with -- rather than a mere 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 says.
Aschwanden and Stemmer, Optics Letters, 1 September 2006
Credits: AIP
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