- LCD redirects here. For other meanings of LCD, see LCD (disambiguation).
Reflective twisted nematic liquid crystal display.
- Vertical filter film to polarize the light as it enters.
- Glass substrate with ITO electrodes. The shapes of these electrodes will determine the dark shapes that will appear when the LCD is turned on. Vertical ridges are etched on the surface so the liquid crystals are in line with the polarized light.
- Twisted nematic liquid crystals.
- Glass substrate with common electrode film (ITO) with horizontal ridges to line up with the horizontal filter.
- Horizontal filter film to block/allow through light.
- Reflective surface to send light back to viewer.
A liquid crystal display, or LCD, is a thin, lightweight display device with no moving parts. It consists of an electrically-controlled light-polarising liquid trapped in cell between two transparent polarising sheets. The polarising axes of the two sheets are aligned perpendicular to each other. Each cell is supplied with electrical contacts that allow an electric field to be applied to the liquid inside.
Before an electric field is applied, the long, thin molecules in the liquid are in a relaxed state. Ridges in the top and bottom sheet encourage polarisation of the molecules parallel to the light polarisation direction of the sheets. Between the sheets, the polarisation of the molecules twists naturally between the two perpendicular extremes. Light is polarised by one sheet, rotated through the smooth twisting of the crystal molecules, then passes through the second sheet. The whole assembly looks nearly transparent. A slight darkening will be evident because of light losses in the original polarising sheet.
When an electric field is applied, the molecules in the liquid align themselves with the field, inhibiting rotation of the polarised light. As the light hits the polarising sheet perpendicular to the direction of polarisation, all the light is absorbed and the cell 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 These displays superseded the poor-quality DSM types.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. 1 nit is equal to 1 candela per square metre (cd/m2), and this latter unit is the correct SI method of describing luminance.
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 in 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 diffuse (light-scattering) reflector 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. These displays usually use reflective mode when the ambient illuminance level is high, and a low-power backlight to provide transmissive illumination in darker situations.
The liquid crystal used in LCDs rotates all visible wavelengths equally, but additional refinements have been added to the basic LCD to produce a color display.
In a color 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 color by varying the relative brightnesses of its three colored sections.
The color components are arranged in different ways, forming a kind of pixel geometry depending on the monitor's usage.
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 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. An important specification for these displays is their viewing-angle.
The zenithal bistable device, developed in 2000 by ZBD Displays Limited, can retain an image without power, but this technology is not yet mass-manufactured.
A French company, Nemoptic, has developed another zero-power, paper-like LCD technology which has been mass-produced in Taiwan since July 2003.
This technology is intended for use in low-power mobile applications such as e-books and wearable computers.
Zero-power LCD displays are in competition with electronic paper.
Some LCD panels have defective transistors. This causes dark and bright pixels which are always off or always on. Unlike integrated circuits, LCD panels with a few defective pixels are usually still usable. It is also economically prohibitive to discard a panel with just a few bad pixels because LCD panels are much larger than ICs. Every company has its own policy to reject LCD panels that are too defective. For example, here is the quality standard for IBM's ThinkPad laptop computers. If a panel's defective pixels are fewer than the numbers, it will be deemed marketable.
|Resolution ||Bright Dots ||Dark dots ||Total |
|QXGA ||15 ||16 ||16 |
|UXGA ||11 ||16 ||16 |
|SXGA+ ||11 ||13 ||16 |
|XGA ||8 ||8 ||9 |
|SVGA ||5 ||5 ||9 |
The LCD panels, due to their large sizes, are more likely to have defects than most ICs. In this example, a 12" SVGA LCD has 8 defects and a 6" wafer has 3 defects. That 6" wafer contains 137 dies and only 3 of them will be rejected. But this piece of LCD panel is only marginally acceptable by IBM (4 bright and 4 dark dots). One more bad dot, it will be rejected.
(It should be noted that the standard is much higher now due to fierce competition between manufacturers and better quality control. A LCD panel with 4 defective pixels is usually considered defective and customers can demand an exchanged for a new one.) The location of a defective pixel also matters. Often manufacturers will still replace an LCD panel when the number of deffective pixels is less than their quality standard, but they are located in the center of the panel instead of to the side. Some manufacturers started to offer zero dead pixel policy.
- How LCDs Work (http://electronics.howstuffworks.com/lcd.htm)
- Liquid Crystal Institute (Kent State University) (http://www.lci.kent.edu)