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HP Labs demos plastic LCDs

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HP Labs, HP's research arm, demonstrated a prototype plastic colour LCD screen in London yesterday. Although the prototype is small - just 3cm by 4cm - the team says the technology could be scaled up to extremely large displays, with resolution comparable to that of printed material.

The researchers have developed a way to eliminate the LCD's active matrix, the silicon layer that controls the polarity, and hence the opacity, of the crystals. Instead, they use a layer of imprinted plastic which makes the liquid crystal bistable - that is, able to stay in either orientation without a continuous power supply.

HP's prototype plastic LCD

In essence, a normal LCD is a sandwich of liquid crystal between two sheets of polarising glass. The liquid crystal changes orientation when an electric field is applied: in one orientation it transmits light; and in the other, it does not. By controlling where and when that field is applied, the screen can display an image.

The field is controlled by the active matrix layer, a silicon sheet that, in effect, can switch individual pixels on or off. However, there are several drawbacks to this technology, not least in terms of cost: silicon is not cheap, after all. Using thin film transistor (TFT) technology also means that the image only lasts as long as there is a power supply.

The researchers at HP wanted to replace this layer with something more physically flexible, and less expensive. They came up with the idea of printing a template on to plastic that the liquid crystal would sit on.

"If you put LCD crystals on this surface, they have to arrange themselves around the posts," Adrian Geisow, a researcher at HP Labs explained.

Imprinted posts makes the crystals bistable

The team discovered that when the posts of this template are at exactly the right height, both polarisations of the crystal are stable, meaning that an image can be retained indefinitely without being powered.

Within each cell, there is also scope for finer control of opacity, meaning that each pixel can take on five shades in greyscale, the researchers said. This could be refined further still, and eventually as many as 33 shades could be displayed per pixel.

However, the technology has been explicitly developed for static displays, and does not handle video well at the moment. It takes approximately half a millisecond to switch a cell, so there is some shimmer in moving images.

The researchers stress that the technology is still in the early stages of development, and has some way to go before commercialisation.

"This is a research prototype, and there is plenty of commercial R&D to be done," Geisow told reporters. Some areas still be researched include the limits of the flexibility of the material and back lighting for very large displays, he added. ®

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