December 2004

Energy-Saving Screens

By Wade Roush

Processors and memory chips keep growing in capacity, but batteries don't improve fast enough to keep up. So the only way to increase the battery life of mobile devices such as PDAs and smart phones is to reduce the amount of power they consume. Working with a new generation of displays based on organic light-emitting diodes (OLEDs), researchers at Hewlett-Packard have found a way to do that: dimming the parts of the screen that aren't in use.

"We have energy-aware central processors; why don't we have energy-aware interfaces?" asks Parthasarathy Ranganathan, a senior research scientist at HP Labs. The prevailing approach to energy-saving displays -- leaving the entire screen illuminated while a device is active but turning it off after a minute or two of inactivity -- is less than ideal, since it uses a lot of energy when the screen's on and, when it's off, forces the user to push a button to return to his or her task. Instead, Ranganathan's team developed special software that monitors a PDA's screen when it's in use and automatically dims the unimportant pixels -- for example, everything in the background behind an active pop-up menu or dialogue box.

In studies with human volunteers, Ranganathan's team found that Pocket PC devices equipped with the energy-saving software could last 1.3 to 8 times longer on a single charge than those without the software. Not only that, but "95 percent of our users preferred the new interfaces, even without the energy advantages," Ranganathan says. "Deemphasizing low-interest areas means you're emphasizing high-interest areas" -- which seemed to help users focus on their immediate tasks.

The method is not effective with most of today's standard liquid-crystal displays, which are illuminated by fluorescent bulbs that remain on even if a particular group of pixels is dark. But in OLED screens, each pixel emits its own light, so "if you turn off a pixel, you don't have to spend power on it," explains Ranganathan. Since phones and PDAs with OLED screens are expected to become commonplace within two years, the new software could soon be a standard feature of the operating systems of mobile devices.