The world of PC peripherals. TFT display: description, operating principle

Now the technology of flat panel monitors, including liquid crystal monitors, is the most promising. Although LCD monitors currently account for only about 10% of sales worldwide, they are the fastest growing market sector (65% per year).

Operating principle

LCD monitor screens (Liquid Crystal Display) are made of a substance (cyanophenyl) that is in a liquid state, but at the same time has some properties inherent in crystalline bodies. In fact, these are liquids that have anisotropy of properties (in particular optical ones) associated with order in the orientation of molecules.
Oddly enough, liquid crystals are almost ten years older than CRTs; the first description of these substances was made back in 1888. However, for a long time no one knew how to use them in practice: there are such substances and everyone, and no one except physicists and chemists, they were not interesting. So, liquid crystal materials were discovered back in 1888 by the Austrian scientist F. Renitzer, but only in 1930 did researchers from the British Marconi corporation receive a patent for their industrial use. However, things did not go further than this, since the technological base at that time was still too weak. The first real breakthrough was made by scientists Fergason and Williams from RCA (Radio Corporation of America). One of them created a thermal sensor based on liquid crystals, using their selective reflective effect, the other studied the effect of an electric field on nematic crystals. And at the end of 1966, the RCA Corporation demonstrated a prototype LCD monitor - a digital clock. Sharp Corporation played a significant role in the development of LCD technology. It is still among the technology leaders. The world's first calculator CS10A was produced in 1964 by this corporation. In October 1975, the first compact digital watch was produced using TN LCD technology. In the second half of the 70s, the transition began from eight-segment liquid crystal displays to the production of matrices with addressing of each point. So, in 1976, Sharp released a black-and-white TV with a 5.5-inch screen diagonal, based on an LCD matrix with a resolution of 160x120 pixels.
The operation of LCD is based on the phenomenon of polarization of the light flux. It is known that the so-called polaroid crystals are capable of transmitting only that component of light whose electromagnetic induction vector lies in a plane parallel to the optical plane of the polaroid. For the remainder of the light output, the Polaroid will be opaque. Thus, the polaroid “sifts” the light, this effect is called polarization of light. When liquid substances were studied, the long molecules of which are sensitive to electrostatic and electromagnetic fields and are capable of polarizing light, it became possible to control polarization. These amorphous substances, due to their similarity to crystalline substances in electro-optical properties, as well as their ability to take the shape of a vessel, were called liquid crystals.
Based on this discovery and through further research, it was possible to discover a connection between increasing the electrical voltage and changing the orientation of the crystal molecules to enable image creation. Liquid crystals were first used in displays for calculators and in electronic watches, and then they began to be used in monitors for laptop computers. Today, as a result of progress in this area, LCD displays for desktop computers are becoming increasingly common.

An LCD monitor screen is an array of small segments (called pixels) that can be manipulated to display information. An LCD monitor has several layers, where the key role is played by two panels made of sodium-free and very pure glass material called a substrate or substrate, which actually contain a thin layer of liquid crystals between them [see. rice. 2.1]. The panels have grooves that guide the crystals into specific orientations. The grooves are positioned so that they are parallel on each panel but perpendicular between two panels. Longitudinal grooves are obtained by placing thin films of transparent plastic on the glass surface, which is then specially processed. In contact with the grooves, the molecules in liquid crystals are oriented identically in all cells. Molecules of one of the varieties of liquid crystals (nematics), in the absence of voltage, rotate the vector of the electric (and magnetic) field in the light wave by a certain angle in the plane perpendicular to the axis of beam propagation. Applying grooves to the surface of the glass makes it possible to ensure the same angle of rotation of the plane of polarization for all cells. The two panels are located very close to each other. The liquid crystal panel is illuminated by a light source (depending on where it is located, liquid crystal panels work by reflecting or transmitting light).

The plane of polarization of the light beam rotates 90° when passing through one panel [see. rice. 2.2].
When an electric field appears, the molecules of liquid crystals partially line up vertically along the field, the angle of rotation of the plane of polarization of light becomes different from 90 degrees, and light passes through the liquid crystals unhindered [see Fig. rice. 2.3].
The rotation of the plane of polarization of the light beam is invisible to the eye, so it became necessary to add two more layers to the glass panels, which are polarizing filters. These filters transmit only that component of the light beam whose polarization axis corresponds to a given one. Therefore, when passing through a polarizer, the light beam will be weakened depending on the angle between its plane of polarization and the axis of the polarizer. In the absence of voltage, the cell is transparent, since the first polarizer transmits only light with the corresponding polarization vector. Thanks to liquid crystals, the polarization vector of the light is rotated, and by the time the beam passes to the second polarizer, it has already been rotated so that it passes through the second polarizer without problems [see. Fig. 2.4a].

In the presence of an electric field, the rotation of the polarization vector occurs at a smaller angle, thereby the second polarizer becomes only partially transparent to radiation. If the potential difference is such that the rotation of the plane of polarization in liquid crystals does not occur at all, then the light beam will be completely absorbed by the second polarizer, and the screen, when illuminated from behind, will appear black from the front (the backlight rays are completely absorbed in the screen) [see. Fig. 2.4b]. If you place a large number of electrodes that create different electric fields in separate places on the screen (cell), then it will be possible, with proper control of the potentials of these electrodes, to display letters and other image elements on the screen. The electrodes are placed in transparent plastic and can be of any shape. Technological innovations have made it possible to limit their size to the size of a small dot; accordingly, a larger number of electrodes can be placed on the same screen area, which increases the resolution of the LCD monitor and allows us to display even complex images in color. To display a color image, the monitor needs to be backlit so that the light comes from the back of the LCD display. This is necessary so that the image can be viewed in good quality even if the surrounding environment is not bright. Color is produced by using three filters that separate three main components from the emission of a white light source. By combining the three primary colors for each point or pixel on the screen, it is possible to reproduce any color.
In fact, in the case of color, there are several possibilities: you can make several filters one after another (leading to a small fraction of transmitted radiation), you can take advantage of the property of a liquid crystal cell - when the electric field strength changes, the angle of rotation of the polarization plane of the radiation changes differently for components of light with different wavelengths. This feature can be used to reflect (or absorb) radiation of a given wavelength (the problem is the need to accurately and quickly change the voltage). Which mechanism is used depends on the specific manufacturer. The first method is simpler, the second is more effective.
The first LCD displays were very small, around 8 inches, while today they have reached 15" sizes for use in laptops, and 20" or larger LCD monitors are being produced for desktop computers. An increase in size is followed by an increase in resolution, which results in the emergence of new problems that were solved with the help of emerging special technologies; we will describe all of this below. One of the first problems was the need for a standard to define display quality at high resolutions. The first step towards the goal was to increase the rotation angle of the plane of polarization of light in crystals from 90° to 270° using STN technology.

Advantages and disadvantages of LCD monitors

The advantages of TFT include excellent focusing, absence of geometric distortion and color registration errors. Plus, their screen never flickers. Why? The answer is simple - these displays do not use an electron beam to draw each line on the screen from left to right. When in a CRT this beam is transferred from the lower right to the upper left corner, the image goes out for a moment (beam reversal). On the contrary, the pixels of a TFT display never go out, they simply continuously change the intensity of their glow.
Table 1.1 shows all the main differences in performance characteristics for different types of displays:

Table 1.1. Comparative characteristics of CRT and LCD monitors.

Legend: ( + ) dignity, ( ~ ) is acceptable, ( - ) flaw

From Table 1.1 it follows that the further development of LCD monitors will be associated with an increase in image clarity and brightness, an increase in viewing angle and a decrease in screen thickness. For example, there are already promising developments of LCD monitors made using technology using polycrystalline silicon. This makes it possible, in particular, to create very thin devices, since the control chips are then placed directly on the glass substrate of the display. In addition, the new technology provides high resolution on a relatively small screen (1024x768 pixels on a 10.4-inch screen).

STN, DSTN, TFT, S-TFT

STN is an abbreviation for "Super Twisted Nematic". STN technology allows the torsion angle (angle of twist) of crystal orientation inside the LCD display to be increased from 90° to 270°, which provides better image contrast as the monitor size increases.
STN cells are often used in pairs. This design is called DSTN (Double Super Twisted Nematic), in which one double-layer DSTN cell consists of 2 STN cells, the molecules of which rotate in opposite directions during operation. Light passing through such a structure in a “locked” state loses most of its energy. The contrast and resolution of DSTN are quite high, so it became possible to produce a color display in which there are three LCD cells and three optical filters of primary colors for each pixel. Color displays are not capable of operating from reflected light, so a backlight lamp is a mandatory attribute. To reduce dimensions, the lamp is located on the side, and opposite it is a mirror [see. rice. 2.5], so most LCD matrices in the center have higher brightness than at the edges (this does not apply to desktop LCD monitors).

STN cells are also used in TSTN (Triple Super Twisted Nematic) mode, where two thin layers of polymer film are added to improve the color rendition of color displays or to ensure good quality of monochrome monitors.
The term passive matrix comes from dividing the monitor into points, each of which, thanks to electrodes, can set the orientation of the plane of polarization of the beam, independently of the others, so that as a result, each such element can be individually illuminated to create an image. The matrix is ​​called passive because the technology for creating LCD displays, which was described above, cannot provide a quick change of information on the screen. The image is formed line by line by sequentially applying control voltage to individual cells, making them transparent. Due to the rather large electrical capacitance of the cells, the voltage on them cannot change quickly enough, so the picture is updated slowly. This type of display has many disadvantages in terms of quality because the image does not appear smoothly and appears shaky on the screen. The low rate of change in crystal transparency does not allow moving images to be displayed correctly.
To solve some of the problems described above, special technologies are used. To improve the quality of the dynamic image, it was proposed to increase the number of control electrodes. That is, the entire matrix is ​​divided into several independent submatrices (Dual Scan DSTN - two independent image scanning fields), each of which contains a smaller number of pixels, so alternating management of them takes less time. As a result, the inertia time of the LCD can be reduced.
Also, better results in terms of stability, quality, resolution, smoothness and brightness of the image can be achieved using active matrix screens, which, however, are more expensive.
The active matrix uses separate amplification elements for each screen cell to compensate for the effect of cell capacitance and significantly reduce the time it takes to change their transparency. The active matrix has many advantages over the passive matrix. For example, better brightness and the ability to look at the screen even with a deviation of up to 45° or more (i.e. at a viewing angle of 120°-140°) without compromising image quality, which is impossible in the case of a passive matrix, which allows you to see a high-quality image only from a frontal position relative to the screen. Note that expensive models of LCD monitors with an active matrix provide a viewing angle of 160° [see fig. 2.6], and there is every reason to assume that the technology will continue to improve in the future. Active matrix can display moving images without visible judder as the response time of an active matrix display is around 50 ms versus 300 ms for a passive matrix, in addition, the contrast of active matrix monitors is higher than that of CRT monitors. It should be noted that the brightness of an individual screen element remains unchanged throughout the entire time interval between picture updates, and does not represent a short pulse of light emitted by the phosphor element of the CRT monitor immediately after the electron beam passes over this element. That is why for LCD monitors a vertical scanning frequency of 60 Hz is sufficient.

The functionality of active matrix LCD monitors is almost the same as that of passive matrix displays. The difference lies in the matrix of electrodes that controls the display's liquid crystal cells. In the case of a passive matrix, different electrodes receive an electrical charge in a cyclic manner when the display is updated line by line, and as a result of the discharge of the capacitances of the elements, the image disappears as the crystals return to their original configuration. In the case of active matrix, a memory transistor is added to each electrode, which can store digital information (binary values ​​0 or 1) and as a result, the image is stored until another signal is received. Part of the problem of delayed image attenuation in passive matrices is solved by using more liquid crystal layers to increase passivity and reduce movement, but now, with the use of active matrices, it is possible to reduce the number of liquid crystal layers. Memory transistors must be made from transparent materials that will allow light to pass through them, which means that the transistors can be placed on the back of the display, on a glass panel that contains liquid crystals. For these purposes, plastic films called "Thin Film Transistor" (or simply TFT) are used.
Thin Film Transistor (TFT), i.e. thin film transistor - these are the control elements with which each pixel on the screen is controlled. A thin film transistor is really very thin, its thickness is 0.1 - 0.01 microns.
The first TFT displays, introduced in 1972, used cadmium selenide, which has high electron mobility and supports high current densities, but over time there was a transition to amorphous silicon (a-Si), and high-resolution matrices use polycrystalline silicon ( p-Si).
The technology for creating TFTs is very complex, and there are difficulties in achieving an acceptable percentage of suitable products due to the fact that the number of transistors used is very large. Note that a monitor that can display an image with a resolution of 800x600 pixels in SVGA mode and with only three colors has 1,440,000 individual transistors. Manufacturers set standards for the maximum number of transistors that may not work in an LCD display. True, each manufacturer has its own opinion about how many transistors may not work.
The TFT-based pixel is designed as follows: three color filters (red, green and blue) are integrated one behind the other in a glass plate. Each pixel is a combination of three colored cells or subpixel elements [see rice. 2.7]. This means, for example, that a display with a resolution of 1280x1024 has exactly 3840x1024 transistors and subpixel elements. The dot (pixel) size for a 15.1" TFT display (1024x768) is approximately 0.0188 inches (or 0.30 mm), and for an 18.1" TFT display it is approximately 0.011 inches (or 0.28 mm).

TFTs have a number of advantages over CRT monitors, including reduced energy consumption and heat dissipation, a flat screen and the absence of traces from moving objects. Recent developments provide higher quality images than conventional TFTs.

More recently, Hitachi specialists have created a new technology of multilayer Super TFT LCD panels, which has significantly increased the confident viewing angle of the LCD panel. Super TFT technology uses simple metal electrodes mounted on a bottom glass plate and causes the molecules to rotate, constantly being in a plane parallel to the plane of the screen [see rice. 2.8]. Since the crystals of a conventional LCD panel are turned towards the screen surface with their ends, such LCDs are more dependent on the viewing angle than Hitachi LCD panels with Super TFT technology. As a result, the image on the display remains bright and clear even at large viewing angles, achieving quality, comparable to the image on a CRT screen.

The Japanese company NEC recently announced that its LCD displays will soon reach the level of laser printers in image quality, crossing the threshold of 200 ppi, which corresponds to 31 dots per mm 2 or a dot pitch of 0.18 mm. As NEC reported, TN (twisted nematic) liquid crystals used today by many manufacturers make it possible to build displays with a resolution of up to 400 dpi. However, the main limiting factor in increasing resolution is the need to create appropriate filters. In the new "color filter on TFT" technology, the light filters covering the thin-film transistors are formed using photolithography on the underlying glass substrate. In conventional displays, filters are applied to a second, top substrate, which requires very precise alignment of the two plates.

At the Society for Information Display conference held in the United States in 1999, several reports were made indicating success in the creation of liquid crystal displays on a plastic substrate. Samsung has presented a prototype of a monochrome display on a polymer substrate with a diagonal of 5.9 inches and a thickness of 0.5 mm. The thickness of the substrate itself is about 0.12 mm. The display has a resolution of 480x320 pixels and a contrast ratio of 4:1. Weight - only 10 grams.

Engineers from the Film Technology Laboratory of the University of Stuttgart used not thin film transistors (TFTs), but MIM (metal-insulator-metal) diodes. The latest achievement of this team is a two-inch color display with a resolution of 96x128 pixels and a contrast ratio of 10:1.

A team of IBM specialists has developed a technology for the production of thin-film transistors using organic materials, which makes it possible to produce flexible screens for e-readers and other devices. The elements of transistors developed by IBM are sprayed onto a plastic substrate at room temperature (traditional LCD displays are manufactured at high temperatures, which excludes the use of organic materials). Instead of conventional silica, barium zirconate titonate (BZT) is used to make the gate. An organic substance called pentacene, which is a compound of phenylethylammonium with tin iodide, is used as a semiconductor.

To increase the resolution of LCD screens, Displaytech proposed not to create an image on the surface of a large LCD screen, but to display the image on a small high-resolution display, and then use an optical projection system to enlarge it to the required size. At the same time, Displaytech used the original Ferroelectric LCD (FLCD) technology. It is based on the so-called chiral-smectic liquid crystals, proposed for use back in 1980. A layer of material with ferroelectric properties and capable of reflecting polarized light with rotation of the polarization plane is deposited on a CMOS substrate that supplies control signals. When the reflected light flux passes through the second polarizer, a picture of dark and light pixels appears. A color image is obtained by quickly alternating illumination of the matrix with red, green and blue light. Based on FLCD matrices, it is possible to produce large screens with high contrast and color rendering quality, wide viewing angles and short response times. In 1999, an alliance between Hewlett-Packard and DisplayTech announced the creation of a full-color microdisplay based on FLCD technology. The matrix resolution is 320x240 pixels. Distinctive features of the device are low power consumption and the ability to play full-color “live” video. The new display is designed for use in digital cameras, camcorders, handheld communicators and wearable computer monitors.

Toshiba is developing low-temperature technology using polycrystalline silicon LTPS. According to representatives of this corporation, they are positioning new devices so far only as intended for the mobile device market, not including laptops, where a-Si TFT technology dominates. 4-inch VGA displays are already being produced, and 5.8-inch matrices are on the way. Experts believe that 2 million pixels on the screen is far from the limit. One of the distinctive features of this technology is its high resolution.

According to experts from DisplaySearch Corporation, which researches the flat-panel display market, technologies are currently being replaced in the manufacture of almost any liquid crystal matrices: TN LCD (Twisted Nematic Liquid Crystal Display) with STN (Super TN LCD) and especially with a-Si TFT LCD ( amorphous-Silicon Thin Film Transistor LCD). In the next 5-7 years, in many applications, conventional LCD screens will be replaced or supplemented by the following devices:

• microdisplays;
• light-emitting displays based on organic LEP materials;
• displays based on field emission FED (Field Emisson Display);
• displays using low-temperature polycrystalline silicon LTPS (Low Temperature PolySilicon);
• plasma displays PDP (Plasma Display Panel).

Taken from http://monitors.narod.ru

The screen is one of the most important components of a smartphone; it occupies almost its entire front surface and should be liked by the user. Everyone has different tastes: some people like the natural colors of LCD screens, others the poisonous and bright colors of AMOLED screens. Let's figure out what the difference is between them and where it comes from.

In LCD screens, the pixels are made of liquid crystals, and each pixel has three subpixels: red, green and blue. Liquid crystals themselves do not glow, so they need a light-emitting substrate. AMOLED screens use LEDs, and, as their name implies, they can glow themselves; they do not need additional backlighting. The black color of AMOLED is almost perfect: the pixels do not glow, there is no backlight. For LCD screens, black may turn out to be gray or purple, and a small manufacturing defect will affect the unevenness of the backlight: cheap devices may have white luminous stripes along the edges.

The most important difference between LCD and AMOLED is the colors displayed, they are different. AMOLED screens span the entire sRGB color spectrum and beyond, resulting in some colors being unnaturally oversaturated.

On the spectrogram it looks like this:

A triangle with black edges is the sRGB color gamut, and a triangle with white edges is the coverage of the AMOLED screen of the Samsung Galaxy S4. You may notice that the Galaxy S4 has an unnaturally high amount of blue and green. The dots show how uniformly the color changes occur. Ideally, the distance between the points should be the same.

A high-quality LCD screen fits almost perfectly into the sRGB gamut. True, recently some manufacturers of LCD screens have been trying to bring their saturation closer to AMOLED standards, and as a result they get not only unnatural color, but also uneven shade transitions. This is what the LG G2 spectrogram looks like with oversaturated and uneven green:

And this is HTC One with slightly more natural colors:

Recently, manufacturers of smartphones with AMOLED screens have been fighting for naturalness: recent flagships from Nokia and Samsung now have settings where you can specify the desired color temperature of the screen and correct color saturation.

The viewing angles of high-quality screens are close to the ideal 180 degrees, but at a greater angle the colors are still distorted: on LCDs they become even paler, and on AMOLED they shimmer either red, then green, or blue. Some AMOLED screens use a PenTile structure with a reduced number of subpixels (for example, the Galaxy S4 has five subpixels by two pixels). Most often, the pixels on such screens are visible to the naked eye, although on LCD screens with the same resolution they are invisible.

Since an AMOLED screen does not require a backlight, energy consumption depends on how bright its pixels are: in a dark picture, energy consumption decreases, in a light one, energy consumption increases. An LCD screen consumes energy almost linearly, regardless of who shows what colors. Different colored pixels in AMOLED consume different amounts of power. Blue pixels require the most electricity, so they burn out faster, after which the image becomes faded and unnatural.

Which screen is better depends primarily on the manufacturer. A high-quality FullHD LCD screen will certainly outperform an AMOLED matrix with a low resolution and PenTile structure. If we talk about the screens of modern flagships, the choice depends only on the user’s tastes, what he prefers: pale but natural colors, bright, oversaturated, but with real black, or no difference at all.

LCD TVs appeared on the market quite a long time ago and everyone has already gotten used to them. However, every year more and more new models appear, differing in appearance, screen size, interface and more. In addition, there are also models of liquid crystal displays that differ in their special update speed, types of LEDs and backlighting. However, let's talk about everything one by one. To begin with, I propose to understand what it is – LCD monitors.

The technology for displaying pictures is based on the use of crystals in liquid form and their amazing properties. Such panels have a huge number of positive qualities thanks to the use of this technology. So let's figure out how it works.

How does an LCD monitor work?

The crystals used to create these monitors are called cyanophenyls. When they are in a liquid state, they develop unique optical and other properties, including the ability to position themselves correctly in space.

Such a screen consists of a pair of transparent polished plates, onto which transparent electrodes are applied. Between these two plates the cyanophenyls are located in a certain order. Voltage is supplied through the electrodes on the plates, which is supplied to sections of the screen matrix. There are also two filters located parallel to each other near the plates.

The resulting matrix can be manipulated, causing the crystals to transmit a beam of light or not. In order to obtain different colors, filters of three basic colors are installed in front of the crystals: green, blue and red. Light from the crystal passes through one of these filters and produces the corresponding pixel color. A certain combination of colors allows you to create other shades that will match the moving picture.

Types of matrices

LCD monitors can use several types of matrices, which differ from each other in their technology.

TN+film. This is one of the simplest standard technologies, which is distinguished by its popularity and low cost. This type of module has low power consumption and a relatively low update frequency. You can especially often find a similar module in older panel models. The “+film” in the name means that another layer of film was used, which should make the viewing angle larger. However, since today it is used everywhere, the name of the matrix can be shortened to TN.

Such an LCD monitor has a large number of disadvantages. Firstly, they have poor color reproduction due to the use of only 6 bits for each color channel. Most shades are obtained by mixing primary colors. Secondly, the contrast of LCD monitors and viewing angle also leaves much to be desired. And if some subpixels or pixels stop working for you, then most likely they will constantly glow, which will make few people happy.

IPS. Such matrices differ from other types in that they have the best color reproduction and a wide viewing angle. The contrast in such matrices is also not the best, and the refresh rate is lower than even that of a TN matrix. This means that if you move quickly, a noticeable trail may appear behind the picture, which will interfere with watching TV. However, if a pixel burns out on such a matrix, it will not glow, but, on the contrary, will remain black forever.

Based on this technology, there are other types of matrix, which are also often used in monitors, displays, TV screens, etc.

• S-IPS. Such a module appeared in 1998 and differed only in its lower response update frequency.
• AS-IPS. The next type of matrix, in which, in addition to the update speed, the contrast has also been improved.
• A-TW-IPS. This is essentially the same S-IPS matrix, to which a color filter called “True White” has been added. Most often, such a module was used in monitors intended for publishing houses or darkrooms, as it made the white color more realistic and increased the range of its shades. The disadvantage of such a matrix was that the black color had a purple tint.
• H-IPS. This module appeared in 2006 and was distinguished by screen uniformity and improved contrast. It does not have such an unpleasant black light, although the viewing angle has become smaller.
• E-IPS. Appeared in 2009. This technology has helped improve the viewing angle, brightness and contrast of LCD monitors. In addition, the screen refresh time has been reduced to 5 milliseconds and the amount of energy consumed has been reduced.
• P-IPS. This type of module appeared relatively recently, in 2010. This is the most advanced matrix. It has 1024 gradations for each subpixel, resulting in 30-bit color, which no other matrix could achieve.

V.A.. This is the very first type of matrix for LCD displays, which is a compromise solution between the previous two types of modules. Such matrices best convey image contrast and color, but at a certain viewing angle some details may disappear and the white color balance may change.

This module also has several derivative versions that differ from each other in their characteristics.

• MVA is one of the first and most popular matrices.
• PVA – this module was released by Samsung and features improved video contrast.
• S-PVA was also manufactured by Samsung for LCD panels.
• S-MVA
• P-MVA, A-MVA - manufactured by AU Optronics. All further matrices differ only in the manufacturing companies. All improvements are based only on the reduction in response speed, which is achieved by applying higher voltage at the very beginning of the change in the position of subpixels and using a full 8-bit system that encodes color on each channel.

There are also several other types of LCD matrices, which are also used in some panel models.

• IPS Pro - they are used in Panasonic TVs.
• AFFS – matrices from Samsung. Used only in some specialized devices.
• ASV - matrices from Sharp Corporation for LCD TVs.

Types of backlight

Liquid crystal displays also differ in the types of backlighting.

• Plasma or gas discharge lamps. Initially, all LSD monitors were backlit by one or more lamps. Basically, such lamps had a cold cathode and were called CCFL. Later, EEFL lamps began to be used. The light source in such lamps is plasma, which appears as a result of an electrical discharge passing through a gas. At the same time, you should not confuse LCD TV with plasma TV, in which each of the pixels is an independent light source.
• LED backlight or LED. Such TVs appeared relatively recently. Such displays have one or more LEDs. However, it is worth noting that this is only the type of backlight, and not the display itself, which consists of these miniature diodes.

Fast response time and the required value for watching 3D video

Response speed is how many frames per second the TV can display. This setting affects the image quality and smoothness. In order for this quality to be achieved, the refresh rate must be 120 Hz. In order to achieve this frequency, TVs use a video card. In addition, this frame rate does not create screen flickering, which in turn is better for the eyes.

To watch movies in 3D format, this refresh rate will be quite enough. At the same time, many TVs have a backlight that has a refresh rate of 480 Hz. This is achieved by using special TFT transistors.

Other characteristics of LCD TVs

And also all laptop displays use matrices with 18-bit color (6 bits for each RGB channel), 24-bit is emulated by flickering with dithering.

Initially, small LCD displays (with short service life) found application in wristwatches, calculators, indicators, etc.

Large screens have become widely used with the proliferation of laptops and notebooks, which are increasingly in demand.

Specifications

The most important characteristics of LCD displays:

• Matrix type - the technology by which the LCD display is made.
• Matrix class - according to ISO 13406-2, they are divided into four classes.
• Resolution - horizontal and vertical dimensions, expressed in pixels. Unlike CRT monitors, LCDs have one fixed resolution, the rest are achieved by interpolation. (CRT monitors also have a fixed number of pixels, which also consist of red, green and blue dots. However, due to the nature of the technology, interpolation is not necessary when displaying non-standard resolutions.)
• Point size (pixel size) is the distance between the centers of adjacent pixels. Directly related to physical resolution.
• Screen aspect ratio (proportional format) - width to height ratio (5:4, 4:3, 3:2 (15÷10), 8:5 (16÷10), 5:3 (15÷9), 16: 9, etc.)
• The apparent diagonal is the size of the panel itself, measured diagonally. The area of ​​displays also depends on the format: a monitor with a 4:3 format has a larger area than one with a 16:9 format with the same diagonal.
• Contrast is the ratio of the brightness of the lightest and darkest points at a given backlight brightness. Some monitors use an adaptive backlight level using additional lamps; the contrast figure given for them (the so-called dynamic) does not apply to a static image.
• Brightness is the amount of light emitted by a display, usually measured in candelas per square meter.
• Response time is the minimum time required for a pixel to change its brightness. Composed of two quantities:
  • Buffering time ( input lag). A high value interferes with dynamic games; usually kept silent; measured by comparison with a kinescope in high-speed photography. Now (2011) within 20-50 ms; in some early models it reached 200 ms.
  • The switching time is what is indicated in the monitor's specifications. A high value degrades video quality; measurement methods are ambiguous. Now in almost all monitors the stated switching time is 2-6 ms.
• Viewing angle - the angle at which the drop in contrast reaches a given value is calculated differently for different types of matrices and by different manufacturers, and often cannot be compared. Some manufacturers indicate in those. in the parameters of their monitors, viewing angles such as: CR 5:1 - 176/176°, CR 10:1 - 170/160°. The abbreviation CR (contrast ratio) denotes the contrast level at specified viewing angles relative to the perpendicular to the screen. At viewing angles of 170°/160°, the contrast in the center of the screen is reduced to a value of no lower than 10:1, at viewing angles of 176°/176° - to no lower than 5:1.

Device

Subpixel of color LCD display

Structurally, the display consists of an LCD matrix (a glass plate, between the layers of which liquid crystals are located), light sources for illumination, a contact harness and a frame (case), often plastic, with a metal frame of rigidity.

Each pixel of an LCD matrix consists of a layer of molecules between two transparent electrodes, and two polarizing filters, the planes of polarization of which are (usually) perpendicular. If there were no liquid crystals, then the light transmitted by the first filter would be almost completely blocked by the second filter.

The surface of the electrodes in contact with the liquid crystals is specially treated to initially orient the molecules in one direction. In a TN matrix, these directions are mutually perpendicular, so the molecules, in the absence of tension, line up in a helical structure. This structure refracts light in such a way that the plane of its polarization rotates before the second filter and light passes through it without loss. Apart from the absorption of half of the unpolarized light by the first filter, the cell can be considered transparent.

If voltage is applied to the electrodes, then the molecules tend to line up in the direction of the electric field, which distorts the screw structure. In this case, elastic forces counteract this, and when the voltage is turned off, the molecules return to their original position. With a sufficient field strength, almost all molecules become parallel, which leads to an opaque structure. By varying the voltage, you can control the degree of transparency.

If a constant voltage is applied for a long time, the liquid crystal structure may degrade due to ion migration. To solve this problem, alternating current or changing the polarity of the field is used each time the cell is addressed (since the change in transparency occurs when the current is turned on, regardless of its polarity).

In the entire matrix, it is possible to control each of the cells individually, but as their number increases, this becomes difficult to achieve, as the number of required electrodes increases. Therefore, row and column addressing is used almost everywhere.

The light passing through the cells can be natural - reflected from the substrate (in LCD displays without backlight). But it is more often used; in addition to being independent of external lighting, it also stabilizes the properties of the resulting image.

On the other hand, LCD monitors also have some disadvantages, which are often fundamentally difficult to eliminate, for example:

OLED displays (organic light-emitting diode matrix) are often considered a promising technology that can replace LCD monitors, but it has encountered difficulties in mass production, especially for large-diagonal matrices.

Technologies

The main technologies in the manufacture of LCD displays: TN+film, IPS (SFT, PLS) and MVA. These technologies differ in the geometry of surfaces, polymer, control plate and front electrode. The purity and type of polymer with liquid crystal properties used in specific designs are of great importance.

Response time of LCD monitors designed using SXRD technology. Silicon X-tal Reflective Display - silicon reflective liquid crystal matrix), reduced to 5 ms.

TN+film

TN + film (Twisted Nematic + film) is the simplest technology. Word film in the name of the technology means an additional layer used to increase the viewing angle (approximately from 90 to 150°). Currently, the prefix film is often omitted, calling such matrices simply TN. A way to improve contrast and viewing angles for TN panels has not yet been found, and the response time of this type of matrix is ​​currently one of the best, but the contrast level is not.

The TN+ film array works like this: When no voltage is applied to the subpixels, the liquid crystals (and the polarized light they transmit) rotate 90° relative to each other in a horizontal plane in the space between the two plates. And since the polarization direction of the filter on the second plate is exactly 90° with the polarization direction of the filter on the first plate, light passes through it. If the red, green and blue sub-pixels are fully illuminated, a white dot will appear on the screen.

The advantages of the technology include the shortest response time among modern matrices, as well as low cost. Disadvantages: worse color rendition, smallest viewing angles.

IPS (SFT)

AS-IPS (Advanced Super IPS- extended super-IPS) - was also developed by Hitachi Corporation in 2002. The improvements mainly concerned the contrast level of conventional S-IPS panels, bringing it closer to the contrast of S-PVA panels. AS-IPS is also used as the name for NEC monitors (eg NEC LCD20WGX2) based on S-IPS technology developed by the LG.Philips consortium.

H-IPS A-TW (Horizontal IPS with Advanced True Wide Polarizer ) - developed by LG.Philips for NEC Corporation. It is an H-IPS panel with a TW (True White) color filter to make the white color more realistic and increase viewing angles without distorting the image (the effect of glowing LCD panels at an angle is eliminated - the so-called “glow effect”) . This type of panel is used to create high quality professional monitors.

AFFS (Advanced Fringe Field Switching , unofficial name - S-IPS Pro) is a further improvement of IPS, developed by BOE Hydis in 2003. The increased power of the electric field made it possible to achieve even greater viewing angles and brightness, as well as reduce the interpixel distance. AFFS-based displays are mainly used in tablet PCs, on matrices manufactured by Hitachi Displays.

MVA/PVA

MVA/PVA matrices (VA is short for vertical alignment- vertical alignment) are considered a compromise between TN and IPS, both in cost and in consumer properties.

MVA technology ( Multi-domain Vertical Alignment ) was developed by Fujitsu as a compromise between TN and IPS technologies. Horizontal and vertical viewing angles for MVA matrices are 160° (on modern monitor models up to 176-178°), and thanks to the use of acceleration technologies (RTC), these matrices are not far behind TN+Film in response time. They significantly exceed the characteristics of the latter in terms of color depth and accuracy of their reproduction.

MVA is the successor to VA technology introduced in 1996 by Fujitsu. When the voltage is turned off, the liquid crystals of the VA matrix are aligned perpendicular to the second filter, that is, they do not transmit light. When voltage is applied, the crystals rotate 90° and a light dot appears on the screen. As in IPS matrices, pixels do not transmit light when there is no voltage, so when they fail they are visible as black dots.

The advantages of MVA technology are the deep black color (when viewed perpendicularly) and the absence of both a helical crystal structure and a double magnetic field. Disadvantages of MVA compared to S-IPS: loss of details in shadows when viewed perpendicularly, dependence of the color balance of the image on the viewing angle.

Analogues of MVA are technologies:

• PVA ( Patterned Vertical Alignment) from Samsung.
• Super PVA from Sony-Samsung (S-LCD).
• Super MVA from CMO.

PLS

PLS matrix ( Plane-to-Line Switching) was developed by Samsung as an alternative to IPS and was first demonstrated in December 2010. This matrix is ​​expected to be 15% cheaper than IPS.

Advantages:

• pixel density is higher compared to IPS (and similar to *VA/TN);
• high brightness and good color rendition;
• large viewing angles;
• full sRGB coverage;
• low power consumption comparable to TN.

Flaws:

• response time (5-10 ms) comparable to S-IPS, better than *VA, but worse than TN;
• lower contrast (600:1) than all other types of matrices;
• uneven illumination.

Backlight

Liquid crystals themselves do not glow. In order for the image on the liquid crystal display to be visible, you need. The source can be external (for example, the Sun) or built-in (backlight). Typically, built-in backlight lamps are located behind the layer of liquid crystals and shine through it (although side lighting is also found, for example, in watches).

External lighting

Monochrome displays on wristwatches and mobile phones use external lighting most of the time (from the Sun, room lighting, etc.). Typically behind the liquid crystal pixel layer is a mirror or matte reflective layer. For use in the dark, such displays are equipped with side lighting. There are also transflective displays, in which the reflective (mirror) layer is translucent and the backlight lamps are located behind it.

Incandescent lighting

In the past, some monochrome LCD wristwatches used a subminiature incandescent lamp. But due to high energy consumption, incandescent lamps are unprofitable. In addition, they are not suitable for use, for example, in televisions, as they generate a lot of heat (overheating is harmful to liquid crystals) and often burn out.

Electroluminescent panel

The monochrome LCD displays of some clocks and instrument displays use an electroluminescent panel for backlighting. This panel is a thin layer of crystalline phosphorus (for example, zinc sulfide), in which electroluminescence occurs - glow under the influence of current. Typically glows greenish-blue or yellow-orange.

Illumination with gas-discharge (“plasma”) lamps

During the first decade of the 21st century, the vast majority of LCD displays were backlit by one or more gas-discharge lamps (most often cold cathode lamps - CCFL, although EEFLs have recently come into use). In these lamps, the light source is plasma produced by an electrical discharge through a gas. Such displays should not be confused with plasma displays, in which each pixel itself glows and is a miniature discharge lamp.

Light-emitting diode (LED) backlight

In the early 2010s, LCD displays backlit by one or a small number of light-emitting diodes (LEDs) became widespread. Such LCD displays (often called LED TV or LED displays in the trade) should not be confused with true LED displays, in which each pixel itself lights up and is a miniature LED.

Manufacturers

See also

• Industrial LCD Display

Notes

Literature

• S. P. Miroshnichenko, P. V. Serba. LCD device. Lecture 1
• Mukhin I. A. How to choose an LCD monitor? Computer business market No. 4(292), January 2005. pp. 284-291.
• Mukhin I. A. Development of liquid crystal monitors BROADCASTING Television and radio broadcasting: part 1 - No. 2(46) March 2005. P. 55-56; Part 2 - No. 4(48) June-July 2005. pp. 71-73.
• Mukhin I. A.

Describing the differences between IPS and TN matrices as part of advice when buying a monitor or laptop. It's time to talk about all the modern display production technologies that we may encounter and have an idea about types of matrices in devices of our generation. Do not confuse with LED, EDGE LED, Direct LED - these are types of screen backlighting and display technologies are indirectly related.

Probably everyone can remember the monitor with a cathode ray tube that they used before. True, there are still users and fans of CRT technology. Currently, screens have increased in diagonal size, display manufacturing technologies have changed, and there are more and more varieties in the characteristics of matrices, denoted by the abbreviations TN, TN-Film, IPS, Amoled, etc.

The information in this article will help you choose a monitor, smartphone, tablet and other various types of equipment. In addition, it will highlight the technologies for creating displays, as well as the types and features of their matrices.

A few words about liquid crystal displays

LCD (Liquid Crystal Display) is a display made from liquid crystals that change their location when voltage is applied to them. If you come close to such a display and look closely at it, you will notice that it consists of small dots - pixels (liquid crystals). In turn, each pixel consists of red, blue and green subpixels. When voltage is applied, the subpixels are arranged in a certain order and transmit light through them, thus forming a pixel of a certain color. Many such pixels form an image on the screen of a monitor or other device.

The first mass-produced monitors were equipped matrices TN- having the simplest design, but which cannot be called the highest quality type of matrix. Although among this type of matrices there are very high-quality specimens. This technology is based on the fact that in the absence of voltage, subpixels transmit light through themselves, forming a white dot on the screen. When voltage is applied to the subpixels, they are arranged in a certain order, forming a pixel of a given color.

Disadvantages of TN matrix

• Due to the fact that the standard pixel color, in the absence of voltage, is white, this type of matrix does not have the best color rendering. Colors appear duller and faded, and blacks appear more of a dark gray.
• Another main disadvantage of a TN matrix is ​​small viewing angles. Partially they tried to cope with this problem by improving TN technology to TN+Film, using an additional layer applied to the screen. Viewing angles became larger, but still remained far from ideal.

At the moment, TN+Film matrices have completely replaced TN.

Advantages of TN matrix

• fast response time
• relatively inexpensive cost.

Drawing conclusions, we can say that if you need an inexpensive monitor for office work or surfing the Internet, monitors with TN+Film matrices are best suited.

The main difference between IPS matrix technology and TN— perpendicular arrangement of subpixels in the absence of voltage, which form a black point. That is, in a state of calm the screen remains black.

Advantages of IPS matrices

• better color reproduction compared to screens with TN matrices: you have bright and rich colors on the screen, and black remains truly black. Accordingly, when voltage is applied, the pixels change color. Considering this feature, owners of smartphones and tablets with IPS screens can be advised to use dark color schemes and wallpapers on the desktop, then the smartphone’s battery life will last a little longer.
• large viewing angles. On most screens they are 178°. For monitors, and especially for mobile devices (smartphones and tablets), this feature is important when the user chooses a gadget.

Disadvantages of IPS matrices

• long screen response time. This affects the display in dynamic pictures such as games and movies. In modern IPS panels, things are better with response time.
• higher cost compared to TN.

To summarize, it is better to choose phones and tablets with IPS matrices, and then the user will receive great aesthetic pleasure from using the device. The matrix for a monitor is not so critical, modern ones.

AMOLED screens

The latest smartphone models are equipped with AMOLED displays. This technology for creating matrices is based on active LEDs, which begin to glow and display color when voltage is applied to them.

Let's take a look features of Amoled matrices:

• Color rendition. The saturation and contrast of such screens are higher than required. The colors are displayed so brightly that some users may experience eye strain when using their smartphone for long periods of time. But the black color is displayed even blacker than even in IPS matrices.
• Display power consumption. Just like IPS, displaying black requires less power than displaying a specific color, much less white. But the difference in power consumption between displaying black and white in AMOLED screens is much greater. Displaying white requires several times more energy than displaying black.
• "Picture Memory". If a static image is displayed for a long time, marks may remain on the screen, and this in turn affects the quality of the information displayed.

Also, due to their rather high cost, AMOLED screens are currently only used in smartphones. Monitors built on this technology are unreasonably expensive.

VA (Vertical Alignment)- this technology, developed by Fujitsu, can be considered as a compromise between TN and IPS matrices. In VA matrices, the crystals in the off state are located perpendicular to the screen plane. Accordingly, the black color is ensured as pure and deep as possible, but when the matrix is ​​rotated relative to the direction of view, the crystals will not be visible equally. To solve the problem, a multi-domain structure is used. Technology Multi-Domain Vertical Alignment (MVA) provides protrusions on the plates that determine the direction of rotation of the crystals. If two subdomains rotate in opposite directions, then when viewed from the side, one of them will be darker and the other lighter, so for the human eye the deviations cancel out. There are no protrusions in PVA dies developed by Samsung, and the crystals are strictly vertical when turned off. In order for the crystals of neighboring subdomains to rotate in opposite directions, the lower electrodes are shifted relative to the upper ones.

To reduce response time, Premium MVA and S-PVA matrices use a dynamic voltage increase system for individual sections of the matrix, which is usually called Overdrive. Color rendition of PMVA and SPVA matrices is almost as good as that of IPS, response time is slightly inferior to TN, viewing angles are as wide as possible, black color is the best, brightness and contrast are the highest possible among all existing technologies. However, even with a slight deviation of the direction of view from the perpendicular, even by 5–10 degrees, distortions in halftones can be noticed. This will go unnoticed by most, but professional photographers continue to dislike VA technology for this.

MVA and PVA matrices have excellent contrast and viewing angles, but the situation with response time is worse - it grows as the difference between the final and initial states of the pixel decreases. Early models of such monitors were almost unsuitable for dynamic games, but now they show results close to TN matrices. Color rendering *VA matrices, of course, is inferior to IPS matrices, but remains at a high level. However, due to their high contrast, these monitors are an excellent choice for working with text and photography, with drawing graphics, and also as home monitors.

In conclusion, I can say that the choice is always yours...