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Each pixel consists of a column of liquid crystal molecules suspended between two transparent electrodes, and two polarizing filters, the axes of polarity of which are perpendicular to each other. Without the liquid crystals between them, light passing through one would be blocked by the other. The liquid crystal twists the polarization of light entering one filter to allow it to pass through the other. In color LCDs each individual pixel is divided into three cells, or subpixels, which are colored red, green, and blue, respectively, by additional filters. Each subpixel can be controlled independently to yield thousands or millions of possible colors for each pixel. Older CRT monitors employ a similar method for displaying color. Color components may be arrayed in various pixel geometries, depending on the monitor's usage. LCDs can be either transmissive or reflective, depending on the location of the light source. A transmissive LCD is illuminated from the back by a backlight and viewed from the opposite side (front). This type of LCD is used in applications requiring high luminance levels such as computer displays, televisions, personal digital assistants, and mobile phones. The illumination device used to illuminate the LCD in such a product usually consumes much more power than the LCD itself. Enlarge Enlarge
Before applying an electrical charge, the liquid crystal molecules are in a relaxed state. Charges on the molecules cause these molecules to align themselves in a helical structure, or twist (the "crystal"). In some LCDs, the electrode may have a chemical surface that seeds the crystal, so it crystallizes at the needed angle. Light passing through one filter is rotated as it passes through the liquid crystal, allowing it to pass through the second polarized filter. A small amount of light is absorbed by the polarizing filters, but otherwise the entire assembly is transparent. The Plasma display panel was invented at the University of Illinois at Urbana-Champaign by Donald L. Bitzer and H. Gene Slottow in 1964 for the PLATO Computer System. The original monochrome (usually orange or green) panels enjoyed a surge of popularity in the early 1970s because the displays were rugged and needed neither memory nor refresh circuitry. There followed a long period of sales decline in the late 1970s as semiconductor memory made CRT displays incredibly cheap. Nonetheless, plasma's relatively large screen size and thin profile made the displays attractive for high-profile placement such as lobbies and stock exchanges. In 1983, IBM introduced a 19" orange on black monochrome display (model 3290 'information panel') which was able to show four simultaneous 3270 virtual machine (VM) terminal sessions. In 1992, Fujitsu introduced the world's first 21-inch full color display. It was a hybrid based on the plasma display created at the University of Illinois at Urbana-Champaign and NHK STRL, achieving superior brightness. For high-resolution color displays such as modern LCD computer monitors and televisions, an active matrix structure is used. A matrix of thin-film transistors (TFTs) is added to the polarizing and color filters. Each pixel has its own dedicated transistor, which allows each column line to access one pixel. When a row line is activated, all of the column lines are connected to a row of pixels and the correct voltage is driven onto all of the column lines. The row line is then deactivated and the next row line is activated. All of the row lines are activated in sequence during a refresh operation. Active-matrix displays are much brighter and sharper than passive-matrix displays of the same size, and generally have quicker response times. Many LCDs are driven to darkness by an alternating current, which disrupts the twisting effect, and become faint or transparent when no current is applied. During the 1970s and early 1980s, LCD technology was not yet mature. However, during the early 80's timeframe, a tabletop video game called Popeye was made with a color LCD, a device with technology ahead of it's time. Technologies used for portable devices made prior to the 1990s to use color graphics include tabletop video games that use Vacuum fluorescent displays and also, before modern laptop computers that used color graphics, the so-called luggable computer, Commodore SX-64 used color graphics on a mini-CRT. The Commodore SX-64 was however bulky hence the aforementioned term luggable. LCDs with a small number of segments, such as those used in digital watches and pocket calculators, have a single electrical contact for each segment. An external dedicated circuit supplies an electric charge to control each segment. This display structure is unwieldy for more than a few display elements.
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* LCD displays generally have a lower contrast ratio than that on a plasma display or CRT. This is due to their "light valve" nature: some light always leaks out making black grey. Twisted Nematic displays contain liquid crystal elements which twist and untwist at varying degrees to allow light to pass through. When no voltage is applied to a TN liquid crystal cell, the light is polarized to pass through the cell. In proportion to the voltage applied, the LC cells twist up to 90 degrees changing the polarization and blocking the light's path. By properly adjusting the level of the voltage most any grey level or transmission can be achieved. Twisted Nematic displays contain liquid crystal elements which twist and untwist at varying degrees to allow light to pass through. When no voltage is applied to a TN liquid crystal cell, the light is polarized to pass through the cell. In proportion to the voltage applied, the LC cells twist up to 90 degrees changing the polarization and blocking the light's path. By properly adjusting the level of the voltage most any grey level or transmission can be achieved. Vertical Alignment displays are a form of LC displays in which the liquid crystal material naturally exists in a horizontal state removing the need for extra transistors (as in IPS). When no voltage is applied the liquid crystal cell, it remains perpendicular to the substrate creating a black display. When voltage is applied, the liquid crystal cells shift to a horizontal position, parallel to the substrate, allowing light to pass through and create a white display. VA liquid crystal displays provide some of the same advantages as IPS panels, particularly an improved viewing angle and improved black level. For high-resolution color displays such as modern LCD computer monitors and televisions, an active matrix structure is used. A matrix of thin-film transistors (TFTs) is added to the polarizing and color filters. Each pixel has its own dedicated transistor, which allows each column line to access one pixel. When a row line is activated, all of the column lines are connected to a row of pixels and the correct voltage is driven onto all of the column lines. The row line is then deactivated and the next row line is activated. All of the row lines are activated in sequence during a refresh operation. Active-matrix displays are much brighter and sharper than passive-matrix displays of the same size, and generally have quicker response times.
LCD technology still has a few drawbacks in comparison to some other display technologies: Brief history Vertical Alignment (VA) A diagram of the Pixel layout Until quite recently (c. 2004) the superior brightness and viewing angle of color plasma panels, when compared to LCD, made them one of the most popular forms of display for HDTV. However since that time improvements in LCD technology have closed the gap dramatically. The much lower weight, price and power consumption of LCDs have seen them make large inroads into the former plasma market. Sony now only sell a very limited range of Plasma screens and appear set to quit this market altogether.