Muscling up colors for electronic displays

Colorful as they are, today’s television and computer screens generate only about half the hues of the visible-light spectrum. Now, experimenters in Switzerland have found a way to provide the complete color palette. To do so, they use a material called artificial muscle because it changes length in response to electricity (SN: 7/1/06, p. 8: Pumping Alloy).

GRATE EXERCISE. A prototype color source for television and computer screens that’s controlled by a plastic artificial muscle transforms from green (left) to red (right) in response to electricity. Aschwanden and Stemmer/ETHZ

In conventional displays, three light emitters—one red, one green, and one blue—make up each pixel. Varying their relative intensities yields composite colors such as orange. The three fixed hues can’t combine to yield every color.

However, three light sources of variable hues could, says electrical engineer Manuel Aschwanden. To make such adjustable color sources, he and Andreas Stemmer, both of the Swiss Federal Institute of Technology in Zurich, have invented a novel diffraction grating—a rippled surface that separates white light into component colors, each of which travels away from the grating at a slightly different angle.

Increasing the spacing between a grating’s ripples shrinks the angular spread of colors, and decreasing the spacing increases the spread. This can control which color heads in a given direction, says Aschwanden. To achieve such control, the researchers first fashioned a simple grating out of stretchy plastic. Then, they adhered that grating to a base of rubbery acrylic polymer that acts as an artificial muscle.

Applying a voltage to the acrylic polymer caused it to stretch, increasing the ripple-to-ripple distance of the attached grating, the team reports in the Sept. 1 Optics Letters. At a fixed angle from the grating, a camera recorded a wide range of colors corresponding to various voltages. Aschwanden says that to make television and computer screens using such devices, a manufacturer might place pinholes in front of multiple gratings. Each pinhole would restrict the light coming through to just one color, which would change as its grating stretched or shrank.

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