Liquid-crystal display
A liquid-crystal display is a flat-panel display or other electronically modulated optical device that uses the light-modulating properties of liquid crystals combined with polarizers to display information. Liquid crystals do not emit light directly but instead use a backlight or reflector to produce images in color or monochrome.
LCDs are available to display arbitrary images or fixed images with low information content, which can be displayed or hidden: preset words, digits, and seven-segment displays are all examples of devices with these displays. They use the same basic technology, except that arbitrary images are made from a matrix of small pixels, while other displays have larger elements.
LCDs are used in a wide range of applications, including LCD televisions, computer monitors, instrument panels, aircraft cockpit displays, and indoor and outdoor signage. Small LCD screens are common in LCD projectors and portable consumer devices such as digital cameras, watches, calculators, and mobile telephones, including smartphones. LCD screens have replaced heavy, bulky and less energy-efficient cathode-ray tube displays in nearly all applications since the late 2000s to the early 2010s.
LCDs can either be normally on or off, depending on the polarizer arrangement. For example, a character positive LCD with a backlight has black lettering on a background that is the color of the backlight, and a character negative LCD has a black background with the letters being of the same color as the backlight.
LCDs are not subject to screen burn-in like on CRTs. However, LCDs are still susceptible to image persistence.
General characteristics
Each pixel of an LCD typically consists of a layer of molecules aligned between two transparent electrodes, often made of indium tin oxide, and two polarizing filters, the axes of transmission of which are perpendicular to each other. Without the liquid crystal between the polarizing filters, light passing through the first filter would be blocked by the second polarizer. Before an electric field is applied, the orientation of the liquid-crystal molecules is determined by the alignment at the surfaces of electrodes. In a twisted nematic device, the surface alignment directions at the two electrodes are perpendicular to each other, and so the molecules arrange themselves in a helical structure, or twist. This induces the rotation of the polarization of the incident light, and the device appears gray. 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 thus constituting different levels of gray.The chemical formula of the liquid crystals used in LCDs may vary. Formulas may be patented. An example is a mixture of 2--5-alkylpyrimidine with cyanobiphenyl, patented by Merck and Sharp Corporation. The patent that covered that specific mixture has expired.
Most color LCD systems use the same technique, with color filters used to generate red, green, and blue subpixels. The LCD color filters are made with a photolithography process on large glass sheets that are later glued with other glass sheets containing a thin-film transistor array, spacers and liquid crystal, creating several color LCDs that are then cut from one another and laminated with polarizer sheets. Red, green, blue and black colored photoresists are used to create color filters. All resists contain a finely ground powdered pigment, with particles being just 40 nanometers across. The black resist is the first to be applied; this will create a black grid that will separate red, green and blue subpixels from one another, increasing contrast ratios and preventing light from leaking from one subpixel onto other surrounding subpixels. After the black resist has been dried in an oven and exposed to UV light through a photomask, the unexposed areas are washed away, creating a black grid. Then the same process is repeated with the remaining resists. This fills the holes in the black grid with their corresponding colored resists. Black matrices made in the 1980s and 1990s when most color LCD production was for laptop computers, are made of Chromium due to its high opacity, but due to environmental concerns, manufacturers shifted to black colored photoresist with carbon pigment as the black matrix material. Another color-generation method used in early color PDAs and some calculators was done by varying the voltage in a super-twisted nematic LCD, where the variable twist between tighter-spaced plates causes a varying double refraction birefringence, thus changing the hue. They were typically restricted to 3 colors per pixel: orange, green, and blue.
File:LCDneg.jpg|thumb|left|LCD in a Texas Instruments calculator with top polarizer removed from device and placed on top, such that the top and bottom polarizers are perpendicular. As a result, the colors are inverted.
The optical effect of a TN 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, TN displays with low information content and no backlighting are usually operated between crossed polarizers such that they appear bright with no voltage. As most of 2010-era LCDs are used in television sets, monitors and smartphones, they have high-resolution matrix arrays of pixels to display arbitrary images using backlighting with a dark background. When no image is displayed, different arrangements are used. For this purpose, TN LCDs are operated between parallel polarizers, whereas IPS LCDs feature crossed polarizers. In many applications IPS LCDs have replaced TN LCDs, particularly in smartphones. 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.
File:Casio W-59 digital watch.jpg|thumb|A Casio Alarm Chrono digital watch with LCD
Displays for a small number of individual digits or fixed symbols can be implemented with independent electrodes for each segment. In contrast, full alphanumeric or variable graphics displays are usually implemented with pixels arranged as a matrix consisting of electrically connected rows on one side of the liquid crystal layer and columns on the other side, which makes it possible to address each pixel at the intersections. The general method of matrix addressing consists of sequentially addressing one side of the matrix, for example by selecting the rows one-by-one and applying the picture information on the other side at the columns row-by-row.
Manufacturing
History
The origin and the complex history of liquid-crystal displays from the perspective of an insider during the early days were described by Joseph A. Castellano in Liquid Gold: The Story of Liquid Crystal Displays and the Creation of an Industry.Another report on the origins and history of LCD from a different perspective until 1991 has been published by Hiroshi Kawamoto, available at the IEEE History Center.
A description of Swiss contributions to LCD developments, written by Peter J. Wild, can be found at the Engineering and Technology History Wiki.
Background
In 1888, Friedrich Reinitzer discovered the liquid crystalline nature of cholesterol extracted from carrots and published his findings. In 1904, Otto Lehmann published his work "Flüssige Kristalle". In 1911, Charles Mauguin first experimented with liquid crystals confined between plates in thin layers.In 1922, Georges Friedel described the structure and properties of liquid crystals and classified them in three types. In 1927, Vsevolod Frederiks devised the electrically switched light valve, called the Fréedericksz transition, the essential effect of all LCD technology. In 1936, the Marconi Wireless Telegraph company patented the first practical application of the technology, "The Liquid Crystal Light Valve". In 1962, the first major English language publication Molecular Structure and Properties of Liquid Crystals was published by Dr. George W. Gray. In 1962, Richard Williams of RCA found that liquid crystals had some interesting electro-optic characteristics and he realized an electro-optical effect by generating stripe patterns in a thin layer of liquid crystal material by the application of a voltage. This effect is based on an electro-hydrodynamic instability forming what are now called "Williams domains" inside the liquid crystal.
Building on early MOSFETs, Paul K. Weimer at RCA developed the thin-film transistor in 1962. It was a type of MOSFET distinct from the standard bulk MOSFET.
1960s
In 1964, George H. Heilmeier, who was working at the RCA laboratories on the effect discovered by Richard Williams, achieved the switching of colors by field-induced realignment of dichroic dyes in a homeotropically oriented liquid crystal. Practical problems with this new electro-optical effect made Heilmeier continue to work on scattering effects in liquid crystals and finally the achievement of the first operational liquid-crystal display based on what he called the dynamic scattering mode. Application of a voltage to a DSM display switches the initially clear transparent liquid crystal layer into a milky turbid state. DSM displays could be operated in transmissive and in reflective mode but they required a considerable current to flow for their operation. George H. Heilmeier was inducted in the National Inventors Hall of Fame and credited with the invention of LCDs. Heilmeier's work is an IEEE Milestone.In the late 1960s, pioneering work on liquid crystals was undertaken by the UK's Royal Radar Establishment at Malvern, England. The team at RRE supported ongoing work by George William Gray and his team at the University of Hull who ultimately discovered the cyanobiphenyl liquid crystals, which had correct stability and temperature properties for application in LCDs.
The idea of a TFT-based liquid-crystal display was conceived by Bernard Lechner of RCA Laboratories in 1968. Lechner, F.J. Marlowe, E.O. Nester and J. Tults demonstrated the concept in 1968 with an 18x2 matrix dynamic scattering mode LCD that used standard discrete MOSFETs.