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Liquid crystal display

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A liquid crystal display is a thin, lightweight display device with no moving parts. It consists of an electrically-controlled light-polarising liquid trapped in cells between two transparent polarising sheets. The polarising axes of the two sheets are aligned in the same direction. Each cell is suplied with electrical contacts that allow an electric field to be applied to the liquid inside.

Before an electric field is applied, the molecules in the liquid are in a relaxed state. Light can pass through the liquid without any effect on its direction of polarisation, so the entire assembly appears transparent. When an electric field is applied, the molecules in the liquid twist into an excited state, causing the axis of polarisation of the light to rotate. Thus the light that has been polarised by passing through the first sheet, then rotated by passing through the liquid, now has the wrong polarisation to penetrate the second sheet. The result is that the activated part of the display appears dark.

A group at RCA, headed by George Heilmeier[?], demonstrated the first operational LCD based on the dynamic scattering mode (DSM) in 1968. Heilmeier's company Optel produced a number of LCDs based on this principle. In 1969 James Fergason[?] at Kent State University in Ohio discovered the twisted nematic field effect in liquid crystals, and in 1971 his company ILIXCO produced the first LCD based on this effect. These displays superseded the poor-quality DSM types.

A nit is a unit of luminance which is often used to quote the brightness of displays, which typically have luminances of 200 to 300 nits.

Table of contents
1 Zero Power Displays

Transmissive and Reflective Displays

LCDs can be used in transmissive or reflective modes. A transmissive LCD is illuminated from one side and viewed from the opposite side. Activated cells therefore appear dark while inactive cells appear bright. This type is used is high-brightness applications such as pocket television receivers. The lamp used to illuminate the LCD in such a product usually consumes more battery power than the LCD itself.

A reflective LCD, as used in pocket calculators and digital watches, is viewed by ambient light reflected in a mirror behind the display. This type has lower contrast than the transmissive type, because the ambient light passes twice through the display before reaching the viewer. The advantage of this type is that there is no lamp to consume power, so the battery life is long. A small LCD consumes so little power that it can run from a photovoltaic cell[?].

Transflective LCDs use a combination of transmissive and reflective modes.

Colour Displays

The liquid crystal used in LCDs rotates all visible wavelengths equally, but additional refinements have been added to the basic LCD to produce a colour display.

In a colour LCD each pixel is divided into three sections, one with a red filter, one with a green filter and the other with a blue filter. The pixel can be made to appear an arbitrary colour by varying the relative brightnesses of its three coloured sections.

Active and Passive Displays

LCDs with a small number of segments, such as those used in digital watches[?] and pocket calculators, are supplied with one electrical contact for each segment. The electrical signal to drive each segment is supplied from an external circuit. This passive display structure becomes unwieldy when the number of elements increases.

Medium-sized displays, such as those in monochrome personal organisers and pocket television sets, have a passive matrix structure. This type has one set of contacts for each row and column of the display, rather than one for each pixel. However, the disadvantage is that that only one pixel can be addressed at any instant. The other pixels have to remember their last state until the control circuit has time to revisit them. This results in reduced contrast and a poor response to fast-moving images. As the number of pixels increases, this type of display becomes less and less attractive. The technology used in these displays is typically supertwist nematic (STN), or a double-layer version DSTN[?] that corrects the colour-shifting problem of STN.

For high-resolution colour displays such as large LCD monitors for computer display, an active-matrix system is used. The LCD panel contains, besides the polarising sheets and cells of liquid crystal, a matrix of thin-film transistors (TFTs). These devices store the electrical state of each pixel on the display while all the other pixels are being updated. This method provides a much brighter, sharper display than a passive matrix of the same size.

Zero Power Displays The zenithal bistable device, developed in 2000 by ZBD Displays Limited, can retain an image without power. This technology is intended for use in low-power mobile applications such as e-books and wearable computers.



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