Screen saver

Smaller and lighter handheld electronic devices could soon be available using display technology that uses significantly less power than existing screens.

The passive matrix LCD display technology, developed by ZBD Displays, allows high-quality images to be kept on the screen indefinitely without power, enabling device manufacturers to use much smaller batteries.

Existing LCD screens require each image to be rewritten at least 30 times every second, whether the image is changed or not. But ZBD’s technology only uses power when the image is changed, and the information stays visible even when the device itself is switched off, said director of research Dr Cliff Jones, the co-inventor of the device.

The technology can be used for a wide range of electronic devices, including mobile phones, electronic books and PDAs. It could also be used for electronic labels within supermarkets, and smartcards for shopping and travel.

When typing on to a PDA screen, the display allows the image to be partially updated, said Jones. ‘When you are typing, only the line you are typing needs to be updated, the rest remains unchanged, so the device can offer big power savings.’

Even a PDA screen changing each of its lines once a second would use less than a quarter of the power needed by existing LCDs.

The display can be made in glass or flexible plastic substrates, and has three times the contrast and one and a half times the brightness of an existing LCD.

It can also operate in transflective mode, making it possible to view information in all types of lighting, from dimly lit rooms to bright sunshine, impossible for emerging display technologies such as organic light emitting diodes.

The display relies on liquid crystal molecules that have two stable energy states in which they reflect either black or white. The molecules are sandwiched between two surfaces, which have been stamped to form a series of tiny grooves and ridges.

This grating surface aligns the molecules into their stable positions. Each molecule can be switched between the two states using a very low amount of electricity.

As the grating stores the image at the surface of the display, it remains on the screen even after the power is turned off.

The device can also write extremely complex images one line at a time – without the extra cost of using thin film transistors behind each pixel, as in laptop computers.

As well as black and white images, the company expects to produce both greyscale, where multiple grey levels are contained within a single pixel, and full colour demonstrator displays later this year, by varying the grating shape within each pixel.The system would allow you to carry smartcards for shopping and travel, said Jones.

‘Why not have an air miles card with a display that tells you how many miles you have left? The card would have no batteries, but when swiped the machine would update the display, using just a 9V battery inside the writer. You could also have a railcard that displays when you have paid for your journey.’

The company recently reached an agreement with LCD manufacturer Varitronix to produce the devices.

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