REPORTS

Panel technologies – there’s one for everyone

By Sandra Wojciech
14.03.2008, 09:53 Uhr

Translation by Siobhán Hayes

Today’s screens cannot be imagined without panel technologies. They are found in all kinds of areas of everyday life in which LCD monitors (Liquid Cristal Display), or the much-loved flat screens, are applied: mobile phone displays, digital watch displays, TV sets und PC monitors.

The decisive difference between LCDs and other well-known flat screen types such as plasma displays is the way in which the screen works with light. Here, we differentiate between light-creating and light-transparent display technologies. LCDs belong in the latter category, unlike tube and plasma monitors; they need an additional light source in the form of lamps located behind the display surface in order to create an image.

LCDs are based on liquid crystals and their property of changing position when subjected to electric voltage, thereby blocking light or letting it through. There are different options regarding the construction LCD, with which liquid crystals it works and how it is controlled.

LCDs are based on different panel types

The core of an LCD monitor is known as the panel; here, the image is created. TN panels represent a comparatively simple method for creating an image. If no power is available, the respective liquid crystal cell, which generally represents a sub-pixel, allows light from the backlight to pass through.

The light is first polarised through a filter. This acts as a valve and directs the light so that it is going in a "straight" line, to put it simply. The liquid crystals, which are twisted by 90 degrees in their basic positions – hence the name Twisted Nomadic - turn the polarised light by 90 degrees so that it cam pass through the second polarization filter as well: an image appears on the screen.

This is given colour by colour filters in the liquid crystal cell which only allow certain wavelengths of light to pass through: Red, green or blue. Since every pixel on the screen is made up of three sub-pixels, all other colours can be created from the three basic RGB colours.

If, on the other hand, an electric voltage is applied to the cell, the liquid crystals take on a vertical position. This means that the light can no longer pass through to the screen and is largely absorbed: the image then remains black.

Left: the monitor is switched off, the vertically positioned molecules are twisted by 90 degrees and the light can pass through the two filters (polariser). The black lines on the yellow plates mark the filter openings. Rights: if voltage is present, the liquid crystals take on a vertical position and the light is blocked by the second polarisation filter (upper yellow layer). (Picture: CMO)

Since the first TN displays did not yet deliver ideal results and had very poor contrast ratios, the technology was quickly developed further. The problem arising from the fact that the liquid crystals never become 100 percent vertical when the screen was switched on could be solved by the application of a thin film: the technology, dubbed TN+Film, improved the contras-reducing incorrect angles of the liquid crystals.

TN+Film is now the standard, although it is still commonly referred to as TN panel technology. Especially for gamers, LCD models with TN panels are popular, since they offer an extremely fast image construction thanks to fast response times for the liquid crystals used. TN screens are also reasonably priced and do not consume much electricity.

They are therefore recommended for everyday use and for companies if you want to surf the Internet or work in Word. TN panels today are able to display up to 16.7 million colours thanks to Dithering and Frame Rate Control (technologies used to create mixed colours where colour depth is limited by using differently coloured pixels which are not side by side.

In this regard, they can almost match the performance of VA and IPS panels, which are also able to display 256 colour gradients per basic colour and therefore 16.7 million colours (256 x 256 x 256), but without the need for technical tricks.

In Plane Switching – first choice for graphics workers

Construction of an IPS panel from leading LCD research company Merck. The liquid crystals are arranged horizontally to the image surface and parallel to each other (Picture: Merck)

IPS technology (In Plane Switching) differs to TN in that the liquid crystals are arranged parallel and not twisted by 90 degrees. When voltage is applied, they arrange themselves in such a way that the light can make its way out and the pixel is illuminated.

Since the electrodes for the creation of the voltage and the turning of the liquid crystals can only be located on the back of an IPS panel due to the way in which it is constructed and the amount of light allowed through is reduced as a result, a stronger backlight is needed. As a result, IPS screens consume more energy than TN models. There is also some loss in terms of contrast.

The different arrangement of the liquid crystals on TN and IPS panels affects the extent of the viewing angles. (Picture: CMO)

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