More-Efficient Phone Displays

Innovative displays that let more light reach the viewer could double the battery life of handheld devices.

The popularity of Apple’s iPhone illustrates that large, good-quality screens matter on mobile devices. But the larger the display, the more battery power it uses. Now, a startup called Unipixel, based in Woodlands, TX, claims to have a design for a handheld display that is 60 percent more efficient than traditional displays. The net result, says Tod Cox, vice president of engineering at Unipixel, is that the displays can double the battery life of a cell phone. The company is partnering with a major display manufacturer, says Cox, and expects to have a prototype ready by the end of the year.

Dynamic display: A Unipixel display consists of a glass or plastic back panel into which red, green, and blue LEDs shine from the edge. A thin membrane is placed on top of the panel, with an air gap between the two. To turn the pixels on, and thereby emit light from the surface, the membrane contacts the back panel (top). The membrane consists of microscopic structures that direct light from the back panel to the viewer (bottom).

Unipixel’s design uses many of the components found in existing displays, such as low-power light-emitting diodes (LEDs), but it puts them together in a novel way that provides a high-contrast picture that consumes less power and costs less to manufacture. “Other folks have tried to invent new technologies to bring to market, and they’ve required new materials and new processes, and that’s been problematic,” says Cox. “What we’re trying to do is learn from the past and use what’s available today.”

Liquid-crystal displays, the technology with which Unipixel is competing, use LEDs to backlight a display, explains Cox, but in order to produce an actual image, this light must pass through a series of filters and light polarizers. As little as 5 percent of the original light can make it from the LEDs to the viewer, depending on the manufacturer. By contrast, says Cox, Unipixel’s displays allow 61 percent of the light through. This light can be used to make the screens brighter and more easily readable in the daylight, but the brightness can also be dialed back to allow for power savings and extended battery life. The trick to Unipixel’s display design is to remove the backlight and all the filters that are used in a liquid-crystal display and illuminate the screen using LEDs along its edge.

Light that is shined into the edge of a material, such as glass or plastic, can be trapped inside it, depending on the optical properties of the material and the surrounding medium. The same concept explains why bits of data can travel along miles of fiber-optic cable without much loss. In the case of Unipixel’s displays, LEDs shine into the edge of a glass or plastic screen. Light stays trapped inside the screen when the surrounding medium is air, but when another material comes in contact with the screen, the trapped light scatters and shines out of the surface. To create an image employing this process, which is known as frustrated total internal reflection (FTIR), Unipixel uses a membrane with microscopic structures on it that bend light toward a viewer. This membrane is separated from the screen by a thin air gap. When the membrane comes in contact with the screen, a pixel turns on, letting light out.

Multimedia

Each pixel in a liquid-crystal display consists of three subpixels–one red, one green, and one blue–that work together to produce a gamut of colors. A Unipixel display doesn’t have subpixels; the color comes from red, green, and blue LEDs located at the edge of the screen. Cox explains that the LEDs flash so quickly, and the membrane opens and closes the pixels at such a rapid rate, that the eye perceives a gamut of colors that is just as wide as that of a liquid-crystal display. (Texas Instruments takes this approach, called sequential color, for its digital light projectors, which are used for business presentations and theatrical films.)

FTIR is already used in large-scale touch-screen displays, such as those made by Jeff Han, a researcher at New York University and founder of the startup Perceptive Pixel. (See video and “Touch Screen for Many Fingers.”) Han places infrared LEDs along the edges of his displays, and when a person’s finger touches the screen, it scatters the light. This scatter is detected by cameras and used to determine the position of a person’s finger on the screen. Han says that Unipixel’s displays show “another clever use of an elegant phenomenon.”

And the market is always looking for more-power-efficient displays. Today’s displays “throw away so much light,” says Han. “People are very gung ho about lower power in portable devices, so if you can get a substantial improvement in power efficiency, it’s valuable.” Unipixel’s theory is sound, he says, “but like a lot of technologies, the devil’s in the details.” Han adds that it’s hard to predict how well the company will be able to compete with existing technologies in an industry that moves so fast that displays drop 30 percent in price every year.

Unipixel is a few years away from an actual product, Cox says, but by leveraging preexisting display technologies and new materials that come along, his company can keep up with the industry. At this point, Unipixel is focusing on displays for mobile devices, but Cox expects that the technology will work for large displays as well.

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