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Technology for reflective full-colour display

SCIENTISTS OF Philips present a novel technology for reflective, paper-like displays, says a report in the journal Nature. The technology is fast enough to display video content and can be used to build a reflective full-colour display that is at least two times brighter than what is possible with any other technology. The display is based on electro-wetting — a technology that allows rapid control and manipulation of fluid motion on a micrometer scale.

The technology is based on controlling the shape of a confined water/oil interface by an applied voltage. With no voltage applied, the (coloured) oil forms a flat film between the water and a hydrophobic (water-repellent), insulating coating of an electrode, resulting in a coloured pixel.

When a voltage is applied between the electrode and the water, the interfacial tension between the water and the coating changes. As a result the stacked state is no longer stable, causing the water to move the oil aside.

This results in a partly transparent pixel, or, in case a reflective white surface is used under the switchable element, a white pixel. Because of the small size of the pixel, the user only experiences the average reflection, which means that a high-brightness, high-contrast switchable element is obtained, which forms the basis of the reflective display.

Displays based on electro-wetting have several attractive features. The switching between white and coloured reflection is fast enough to display video content.

Furthermore, it is a low-power and low-voltage technology, and displays based on the effect can be made flat and thin. The reflectivity and contrast are better or equal to those of other reflective display types and are approaching those of paper.

In addition, the technology offers a unique path towards high-brightness full-colour displays, leading to displays that are four times brighter than reflective LCDs and twice as bright as other emerging technologies.

Instead of using red, green and blue (RGB) filters or alternating segments of the three primary colours, which effectively result in only one third of the display reflecting light in the desired colour, electro-wetting allows for a system in which one sub-pixel is able to switch two different colours independently.

This results in the availability of two thirds of the display area to reflect light in any desired colour. This is achieved by building up a pixel with a stack of two independently controllable coloured oil films plus a colour filter.

The colours used are cyan, magenta and yellow, which is a so-called subtractive system, comparable to the principle used in inkjet printing for example. Compared to LCD another factor two in brightness is gained because no polarisers are required.

The technology is particularly appealing for electronic-paper like applications, for which high-brightness and contrast-rich reflective displays are needed, and opens up new application areas that make use of the full colour and video speed capability.

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