Monday, May 19, 2008

Field emission boosts Samsung LCD contrast to 300,000

Samsung has demonstrated an LCD with a field emission backlight (FEB) - achieving a claimed 300,000:1 contrast ratio.

Key to this astonishingly high contrast - using a LCD panel, where 1,500:1 is generally the limit - is to make a backlight of a course array of pixels, and to turn off the pixels behind dark areas of the screen.

In Samsung's case, the backlight is broken into 2,800 (70x40) 1cm squares behind the 32in. LCD, and each of these pixels can be brightened as well as extinguished.

Between LCD and backlight, a translucent sheet blurs the squares preventing their sharp edges being seen through the LCD.

Field emission devices use sharp points in a vacuum as electron sources, and a high voltage to accelerate these electrons in to a phosphor screen.

In this case, the FEB uses carbon nanotubes as emissive tips and a triode structure including a grid to allow each 1cm square to be modulated.

The cathode is a tangle of nanotubes deposited on a flat surface, treated with an elastomer which encourages the free ends of exposed nanotubes stand up straight. This creates a random forest of emissive tips.

Above this sits the control grid for each square which has 120x120 40[micro]m holes on a 70[micro]m pitch for electrons to pass through.

Above this is the transparent anode, biased to 15kV, coated with a mixed red-green-blue phosphor.

The backlight produces 6,000cd/m2, reduced by 95 per cent by the inherent absorption of the LCD panel.

Current density in the emissive tips, said Samsung, is low enough to expect long life from the backlight.


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