Liquid crystal display
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A Liquid crystal display is a special thin flat panel with a number of pixels in front of a light source. Each pixel is filled with liquid crystals that are capable of showing many colors or one color only. The technology they use is called Electro-optical modulation. Liquid crystal displays are often abbreviated LCDs. Liquid crystal displays are often used in battery-powered devices, because they use very little electricity.
[change] Overview
Each pixel of a LCD is typically made of a layer of molecules which are aligned between two transparent electrodes. There are also two polarizing filters, one on each side. These two filters are perpendicular to one another. This means that without the liquid crystals between them, they would block all light from passing through - the light passing through the first filter is polarized in such a way that the second filter blocks it.
The surface of the electrodes that are in contact with the liquid crystal materials are treated so that they align the liquid crystal molecules in a particular direction. This treatment typically consists of a thin polymer layer that is unidirectionally rubbed using, for example, a cloth. The direction of the liquid crystal alignment is then defined by the direction of rubbing. Electrodes are made of a transparent conductor called Indium Tin Oxide (ITO).
Before applying an electric field, the orientation of the liquid crystal molecules is determined by the alignment at the surfaces. In a twisted nematic device (still the most common liquid crystal device), the surface alignment directions at the two electrodes are perpendicular to each other. The molecules therefore arrange themselves in a helical structure, or twist. This reduces the rotation of the polarization of the incident light, and the device appears grey. If the applied voltage is large enough, the liquid crystal molecules in the center of the layer are almost completely untwisted and the polarization of the incident light is not rotated as it passes through the liquid crystal layer. This light will then be mainly polarized perpendicular to the second filter, and thus be blocked and the pixel will appear black. By controlling the voltage applied across the liquid crystal layer in each pixel, light can be allowed to pass through in varying amounts - this will yield different levels of gray.
The optical effect of a twisted nematic device in the voltage-on state is far less dependent on variations in the device thickness than that in the voltage-off state. Because of this, these devices are usually operated between crossed polarizers such that they appear bright with no voltage (the eye is much more sensitive to variations in the dark state than the bright state). These devices can also be operated between parallel polarizers, in which case the bright and dark states are reversed. The voltage-off dark state in this configuration appears blotchy, however, because of small variations of thickness across the device.
Both the liquid crystal material and the alignment layer material contain ionic compounds. If an electric field of one particular polarity is applied for a long period of time, this ionic material is attracted to the surfaces and degrades the device performance. This is avoided either by applying an alternating current or by reversing the polarity of the electric field as the device is addressed (the response of the liquid crystal layer is identical, regardless of the polarity of the applied field).
When a large number of pixels are needed in a display, it is not technically possible to drive each directly since then each pixel would require independent electrodes. Instead, the display is multiplexed. In a multiplexed display, electrodes on one side of the display are grouped and wired together (typically in columns), and each group gets its own voltage source. On the other side, the electrodes are also grouped (typically in rows), with each group getting a voltage sink. The groups are designed so each pixel has a unique, unshared combination of source and sink. The electronics, or the software driving the electronics then turns on sinks in sequence, and drives sources for the pixels of each sink.
[change] History
The researches on liquid crystals began at the end of the 19th century but the first publication on this topic was made in 1904 by Otto Lehman and after this first publication there followed a series of other related articles, which described the same topic.
LCD displays use two pieces of glass, which contain a solution of liquid crystals between them. When an electric current passes through the solution the crystals align in such a way that light cannot come through them.
LCD displays have matrixes, which build up a colored picture. There are two types of matrixes: "active" and "passive". A passive matrix uses a simpler concept, which can be met at electronic wrist watches and pocket calculators. An active matrix, on the other hand, uses the technology, known as thin film transistor (TFT). Images produced by displays using this concept are as sharp as the images from the older CRT displays. They are, on the other hand, more expensive, but are extensively used in producing LCD computer monitors and TV panels.
Main features of an LCD monitor are:
- resolution: unlike CRT monitors, LCDs have one resolution, at which the display has the best image quality
- response time: how much it takes the crystals to change from one color to another.
- viewable size: diagonal length of the display
- matrix type: passive or active
- viewing angle: shows maximum angle at which the image on the display stays unchanged
- brightness
- contrast ratio
- input ports: an LCD can have both analog and digital input ports, as well as some additional ports.
Addional types are: Reflective: often referred to as day light viewable, can be viewed in direct sunlight.
[change] Other websites
- Buying an LCD monitor... Things to know - Some additional useful tips about LCD monitors
- Cleaning an LCD Display - Taking care of any LCD screens
