Creating a liquid crystal display

The first working liquid crystal display was created by Fergason in 1970. Prior to this, liquid crystal devices consumed too much power, their life was limited, and the image contrast was deplorable. The new LCD was presented to the public in 1971 and then it received enthusiastic approval. Liquid crystals (Liquid Crystal) are organic substances that can change the amount of transmitted light under voltage. The liquid crystal monitor consists of two glass or plastic plates, between which there is a suspension. The crystals in this suspension are arranged parallel to each other, thereby allowing light to pass through the panel. When applying electric current the arrangement of the crystals changes, and they begin to obstruct the passage of light. LCD technology has become widespread in computers and projection equipment. The first liquid crystals were distinguished by their instability and were of little use for mass production. The real development of LCD technology began with the invention by English scientists of a stable liquid crystal - biphenyl (Biphenyl). First generation liquid crystal displays can be seen in calculators, electronic games and watches. Modern LCD monitors are also called flat panels, dual scan active matrix, thin film transistors. The idea of ​​LCD monitors has been in the air for more than 30 years, but the research has not led to an acceptable result, so LCD monitors have not gained a reputation for good image quality. Now they are becoming popular - everyone likes their elegant appearance, thin body, compactness, economy (15-30 watts), in addition, it is believed that only wealthy and serious people can afford such a luxury.

Characteristics of LCD monitors

Types of LCD monitors

Monitor group layers

There are two types of LCD monitors: DSTN (dual-scan twisted nematic - crystal screens with double scanning) and TFT (thin film transistor - on thin film transistors), they are also called passive and active matrices, respectively. Such monitors consist of the following layers: a polarizing filter, a glass layer, an electrode, a control layer, liquid crystals, another control layer, an electrode, a glass layer, and a polarizing filter. Early computers used eight-inch (diagonal) passive black and white matrices. With the transition to active matrix technology, the screen size has grown. Virtually all modern LCD monitors use thin-film-transistor panels, which provide a bright, clear image of a much larger size.

Monitor resolution

The size of the monitor determines the workspace it occupies, and, importantly, its price. Despite the well-established classification of LCD monitors depending on the diagonal screen size (15-, 17-, 19-inch), the classification by working resolution is more correct. The fact is that, unlike CRT-based monitors, the resolution of which can be changed quite flexibly, LCD displays have a fixed set of physical pixels. That is why they are designed to work with only one permission, called working. Indirectly, this resolution also determines the size of the diagonal of the matrix, however, monitors with the same working resolution may have a matrix of different sizes. For example, monitors with a diagonal of 15 to 16 inches generally have an operating resolution of 1024X768, which means that this monitor actually has 1024 pixels horizontally and 768 pixels vertically. The working resolution of the monitor determines the size of the icons and fonts that will be displayed on the screen. For example, a 15-inch monitor can have an operating resolution of both 1024X768 and 1400X1050 pixels. In the latter case, the physical dimensions of the pixels themselves will be smaller, and since when forming standard icon in both cases, the same number of pixels is used, then at a resolution of 1400x1050 pixels, the icon will be smaller in physical size. For some users, too small icon sizes at a high monitor resolution may be unacceptable, so when buying a monitor, you should immediately pay attention to the working resolution. Of course, the monitor is capable of displaying an image in a different resolution than the working one. This mode of operation of the monitor is called interpolation. In the case of interpolation, the image quality leaves much to be desired. The interpolation mode significantly affects the quality of the display of screen fonts.

Monitor interface

LCD monitors are inherently digital devices, therefore, the "native" interface for them is the DVI digital interface, which can have two types of convectors: DVI-I, combining digital and analog signal s, and DVI-D, which transmit only a digital signal. It is believed that the DVI interface is more preferable for connecting an LCD monitor to a computer, although it is also possible to connect via a standard D-Sub connector. The DVI interface is also supported by the fact that in the case of an analog interface, a double conversion of the video signal occurs: first, the digital signal is converted to analog in the video card (DAC conversion), which is then transformed into a digital electronic unit of the LCD monitor itself (ADC conversion), as a result, the risk of various signal distortions increases. Many modern LCD monitors have both D-Sub and DVI connectors, which allows you to connect two monitors to the monitor at the same time. system block. You can also find models with two digital connectors. In inexpensive office models, there is basically only a standard D-Sub connector.

LCD matrix type

The basic component of the LCD matrix are liquid crystals. There are three main types of liquid crystals: smectic, nematic, and cholesteric. According to the electrical properties, all liquid crystals are divided into two main groups: the first group includes liquid crystals with positive dielectric anisotropy, the second - with negative dielectric anisotropy. The difference lies in how these molecules respond to an external electric field. Molecules with positive dielectric anisotropy are oriented along the field lines, and molecules with negative dielectric anisotropy are perpendicular to the field lines. Nematic liquid crystals have a positive dielectric anisotropy, while smectic liquid crystals, on the contrary, have a negative one. Another remarkable property of LC molecules is their optical anisotropy. In particular, if the orientation of the molecules coincides with the direction of propagation of plane polarized light, then the molecules have no effect on the plane of polarization of the light. If the orientation of the molecules is perpendicular to the direction of light propagation, then the plane of polarization is rotated so as to be parallel to the direction of orientation of the molecules. The dielectric and optical anisotropy of LC molecules makes it possible to use them as a kind of light modulators, which make it possible to form the required image on the screen. The principle of operation of such a modulator is quite simple and is based on changing the plane of polarization of the light passing through the LC cell. The LC cell is located between two polarizers, the polarization axes of which are mutually perpendicular. The first polarizer cuts plane polarized radiation from the light passing from the backlight. If there were no LC cell, then such plane polarized light would be completely absorbed by the second polarizer. An LC cell placed in the path of the transmitted plane polarized light can rotate the plane of polarization of the transmitted light. In this case, part of the light passes through the second polarizer, that is, the cell becomes transparent (fully or partially). Depending on how the rotation of the polarization plane in an LC cell is controlled, several types of LC matrices are distinguished. So, an LC cell placed between two crossed polarizers makes it possible to modulate the transmitted radiation, creating black-and-white color gradations. To obtain a color image, it is necessary to use three color filters: red (R), green (G) and blue (B), which, when installed in the path of white propagation, will allow you to get three basic colors in the right proportions. So, each LCD pixel consists of three separate sub-pixels: red, green and blue, which are controllable LCD cells and differ only in the filters used, installed between the top glass plate and the output polarizing filter

Classification of TFT-LCD displays

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

TN matrix

TN cell structure

A TN-type liquid crystal matrix (Twisted Nematic) is a multilayer structure consisting of two polarizing filters, two transparent electrodes, and two glass plates, between which there is a nematic type liquid crystal substance with positive dielectric anisotropy. Special grooves are applied to the surface of glass plates, which makes it possible to initially create the same orientation of all liquid crystal molecules along the plate. The grooves on both plates are mutually perpendicular, so the layer of liquid crystal molecules between the plates changes its orientation by 90°. It turns out that LC molecules form a structure twisted in a spiral (Fig. 3), which is why such matrices are called Twisted Nematic. Glass plates with grooves are located between two polarizing filters, and the polarization axis in each filter coincides with the direction of the grooves on the plate. In the normal state, the LC cell is open, since liquid crystals rotate the plane of polarization of the light passing through them. Therefore, plane-polarized radiation formed after passing through the first polarizer will also pass through the second polarizer, since its polarization axis will be parallel to the polarization direction of the incident radiation. Under the influence of an electric field created by transparent electrodes, the molecules of the liquid crystal layer change their spatial orientation, lining up along the direction of the field lines of force. In this case, the liquid crystal layer loses the ability to rotate the plane of polarization of the incident light, and the system becomes optically opaque, since all light is absorbed by the output polarizing filter. Depending on the applied voltage between the control electrodes, it is possible to change the orientation of the molecules along the field not completely, but only partially, that is, to control the degree of twisting of the LC molecules. This, in turn, allows you to change the intensity of light passing through the LCD cell. Thus, by installing a backlight behind the LCD matrix and changing the voltage between the electrodes, it is possible to vary the degree of transparency of one LCD cell. TN matrices are the most common and cheapest. They have certain drawbacks: not very large viewing angles, low contrast and the inability to get perfect black. The point is that even when the maximum voltage is applied to the cell, it is impossible to completely unwind the LC molecules and orient them along the field lines of force. Therefore, such matrices remain slightly transparent even when the pixel is completely turned off. The second drawback is associated with small viewing angles. To partially eliminate it, a special diffusing film is applied to the surface of the monitor, which allows you to increase the viewing angle. This technology was named TN+Film, indicating the presence of this film. Finding out exactly what type of matrix is ​​​​used in the monitor is not so easy. However, if there is a “broken” pixel on the monitor, which has arisen due to the failure of the transistor controlling the LCD cell, then in TN matrices it will always burn brightly (red, green or blue), since for a TN matrix an open pixel corresponds to the absence of voltage on the cell. You can also recognize the TN matrix by looking at the black color at maximum brightness - if it is more gray than black, then this is probably the TN matrix.

IPS matrices

IPS cell structure

IPS monitors are also called Super TFT monitors. A distinctive feature of IPS matrices is that the control electrodes are located in them in the same plane on the underside of the LCD cell. In the absence of voltage between the electrodes, the LC molecules are parallel to each other, to the electrodes, and to the direction of polarization of the lower polarizing filter. In this state, they do not affect the polarization angle of the transmitted light, and the light is completely absorbed by the output polarizing filter, since the polarization directions of the filters are perpendicular to each other. When voltage is applied to the control electrodes, the generated electric field rotates the LC molecules by 90° so that they are oriented along the field lines of force. If light is passed through such a cell, then due to the rotation of the polarization plane, the upper polarizing filter will pass light without interference, that is, the cell will be in the open state (Fig. 4). By varying the voltage between the electrodes, the LC molecules can be forced to rotate through any angle, thereby changing the transparency of the cell. In all other respects, IPS cells are similar to TN matrices: a color image is also formed by using three color filters. IPS matrices have both advantages and disadvantages compared to TN matrices. The advantage is the fact that in this case it turns out perfectly black, not gray, as in TN-matrices. Other indisputable advantage given technology are large viewing angles. The disadvantages of IPS matrices include a longer pixel response time than for TN matrices. However, we will return to the question of the reaction time of a pixel. In conclusion, we note that there are various modifications of IPS matrices (Super IPS, Dual Domain IPS) that improve their performance.

MVA matrices

Domain structure of an MVA cell

MVA is an evolution of VA technology, i.e. vertical molecular alignment technology. Unlike TN and IPS matrices, in this case, liquid crystals with negative dielectric anisotropy are used, which are oriented perpendicular to the direction of electric field lines. In the absence of voltage between the plates of the LC cell, all liquid crystal molecules are oriented vertically and have no effect on the plane of polarization of the transmitted light. Since light passes through two crossed polarizers, it is completely absorbed by the second polarizer and the cell is in a closed state, while, unlike a TN matrix, it is possible to obtain a perfect black color. If a voltage is applied to the electrodes located above and below, the molecules rotate 90°, orienting themselves perpendicular to the electric field lines. When plane polarized light passes through such a structure, the polarization plane rotates by 90° and the light freely passes through the output polarizer, i.e., the LC cell is in the open state. The advantages of systems with vertical ordering of molecules are the possibility of obtaining perfect black color (which, in turn, affects the possibility of obtaining high-contrast images) and a short pixel response time. In order to increase viewing angles in systems with vertical ordering of molecules, a multidomain structure is used, which leads to the creation of MVA-type matrices. The meaning of this technology lies in the fact that each subpixel is divided into several zones (domains) using special ledges that slightly change the orientation of the molecules, forcing them to align with the surface of the ledge. This leads to the fact that each such domain shines in its own direction (within a certain solid angle), and the combination of all directions expands the viewing angle of the monitor. The advantages of MVA matrices include high contrast (due to the possibility of obtaining perfect black) and large viewing angles (up to 170°). Currently, there are several varieties of MVA technology, such as PVA (Patterned Vertical Alignment) by Samsung, MVA-Premium, etc., which further enhance the performance of MVA matrices.

Brightness

Today, in LCD monitors, the maximum brightness declared in the technical documentation is from 250 to 500 cd / m2. And if the brightness of the monitor is high enough, then this is necessarily indicated in advertising booklets and presented as one of the main advantages of the monitor. However, this is precisely one of the pitfalls. The paradox lies in the fact that it is impossible to focus on the numbers indicated in the technical documentation. This applies not only to brightness, but also to contrast, viewing angles and pixel response time. Not only can they not correspond to the actually observed values ​​at all, it is sometimes difficult to understand what these numbers mean at all. First of all, there are different measurement techniques described in different standards; accordingly, measurements carried out by different methods give different results, and you are unlikely to be able to find out by which method and how the measurements were carried out. Here is one simple example. The measured brightness depends on the color temperature, but when they say that the brightness of the monitor is 300 cd / m2, the question arises: at what color temperature is this very maximum brightness achieved? Moreover, manufacturers indicate the brightness not for the monitor, but for the LCD matrix, which is not at all the same thing. To measure brightness, special reference signals from generators with a precisely set color temperature are used, so the characteristics of the monitor itself as a final product may differ significantly from those stated in the technical documentation. But for the user, the characteristics of the monitor itself, and not the matrix, are of paramount importance. Brightness is a really important characteristic for an LCD monitor. For example, with insufficient brightness, you are unlikely to be able to play various games or watch DVD movies. In addition, it will be uncomfortable to work behind the monitor in daylight conditions (external illumination). However, it would be premature to conclude on this basis that a monitor with a declared brightness of 450 cd/m2 is somehow better than a monitor with a brightness of 350 cd/m2. Firstly, as already noted, the declared and actual brightness are not the same thing, and secondly, it is quite enough for the LCD monitor to have a brightness of 200-250 cd / m2 (but not declared, but actually observed). In addition, the fact how the brightness of the monitor is adjusted is of no small importance. From the point of view of physics, brightness adjustment can be done by changing the brightness of the backlight lamps. This is achieved either by adjusting the discharge current in the lamp (in monitors, fluorescent lamps with a cold cathode Fluorescent Lamp, CCFL are used as backlight lamps), or by the so-called pulse-width modulation of the lamp power. With pulse-width modulation, the voltage to the backlight is supplied by pulses of a certain duration. As a result, the illumination lamp does not glow constantly, but only at periodically repeating time intervals, but due to the inertia of vision, it seems that the lamp is constantly on (the pulse repetition rate is more than 200 Hz). Obviously, by changing the width of the applied voltage pulses, it is possible to adjust the average brightness of the glow of the backlight lamp. In addition to adjusting the brightness of the monitor due to the backlight, sometimes this adjustment is carried out by the matrix itself. In fact, a constant component is added to the control voltage at the electrodes of the LC cell. This allows the LCD cell to be fully opened, but does not allow it to be completely closed. In this case, when the brightness is increased, the black color ceases to be black (the matrix becomes partially transparent even when the LCD cell is closed).

Contrast

An equally important characteristic of an LCD monitor is its contrast ratio, which is defined as the ratio of the brightness of a white background to the brightness of a black background. Theoretically, the contrast of a monitor should be independent of the brightness level set on the monitor, that is, at any brightness level, the measured contrast should have the same value. Indeed, the brightness of the white background is proportional to the brightness of the backlight. Ideally, the ratio of light transmittance of an LCD cell in the open and closed state is a characteristic of the LCD cell itself, however, in practice, this ratio may depend on both the set color temperature and the set brightness level of the monitor. In recent years, the image contrast on digital monitors has increased markedly, and now this figure often reaches a value of 500:1. But even here everything is not so simple. The fact is that the contrast can be specified not for the monitor, but for the matrix. However, as experience shows, if a contrast ratio of more than 350:1 is indicated in the passport, then this is quite enough for normal operation.

Viewing angle

The maximum viewing angle (both vertically and horizontally) is defined as the viewing angle from which the image contrast at the center is at least 10:1. Some manufacturers of matrices, when determining viewing angles, use a contrast ratio of not 10:1, but 5:1, which also introduces some confusion into the technical specifications. The formal definition of viewing angles is rather vague and, most importantly, is not directly related to the correct color reproduction when viewing an image at an angle. In fact, for users, a much more important circumstance is the fact that when viewing an image at an angle to the monitor surface, there is not a drop in contrast, but color distortions. For example, red turns to yellow and green turns to blue. Moreover, such distortions different models manifest themselves in different ways: for some, they become noticeable already at a slight angle, much smaller than the viewing angle. Therefore, comparing monitors in terms of viewing angles is basically wrong. It is possible to compare something, but such a comparison has no practical value.

Pixel response time

Typical pixel turn-on timing diagram for a TN+Film matrix

Typical pixel turn-off timing diagram for TN+Film-matrix

Response time, or pixel response time, is usually specified in the technical documentation for the monitor and is considered one of the most important characteristics of the monitor (which is not entirely true). In LCD monitors, the pixel response time, which depends on the type of matrix, is measured in tens of milliseconds (in the new TN + Film matrices, the pixel response time is 12 ms), and this leads to blurring of the changing picture and can be noticeable to the eye. Distinguish between on time and off time of a pixel. The pixel on time refers to the amount of time required to open the LCD cell, and the off time refers to the amount of time required to close it. When they talk about the reaction time of a pixel, they understand the total time of turning on and off the pixel. The time a pixel is turned on and the time it is turned off can vary significantly. When they talk about the pixel response time indicated in the technical documentation for the monitor, they mean the response time of the matrix, not the monitor. In addition, the pixel response time indicated in the technical documentation is interpreted differently by different manufacturers of matrices. For example, one of the options for interpreting the on (off) time of a pixel is that this is the time for changing the brightness of a pixel from 10 to 90% (from 90 to 10%). Until now, when talking about measuring the reaction time of a pixel, it is understood that we are talking about switching between black and white colors. If there are no questions with the black color (the pixel is simply closed), then the choice of white color is not obvious. How will the reaction time of a pixel change if you measure it when switching between different halftones? This question is of great practical importance. The fact is that switching from a black background to a white one or vice versa is relatively rare in real applications. In most applications, as a rule, transitions between semitones are implemented. And if the switching time between black and white colors turns out to be less than the switching time between grayscale, then the pixel response time will not have any practical value and it is impossible to focus on this monitor characteristic. What conclusion can be drawn from the above? Everything is very simple: the pixel response time declared by the manufacturer does not allow one to unambiguously judge the dynamic characteristics of the monitor. It is more correct in this sense to speak not about the time of switching a pixel between white and black colors, but about the average time of switching a pixel between halftones.

Number of displayed colors

All monitors are RGB devices by nature, that is, their color is obtained by mixing in various proportions the three basic colors: red, green and blue. Thus, each LCD pixel consists of three colored sub-pixels. In addition to the fully closed or fully open state of the LC cell, intermediate states are also possible when the LC cell is partially open. This allows you to form a color shade and mix the color shades of the base colors in the right proportions. At the same time, the number of colors reproduced by the monitor theoretically depends on how many color shades can be formed in each color channel. Partial opening of the LC cell is achieved by applying the required voltage level to the control electrodes. Therefore, the number of reproducible color shades in each color channel depends on how many different voltage levels can be applied to the LCD cell. To form an arbitrary voltage level, it will be necessary to use DAC circuits with a large capacity, which is extremely expensive. Therefore, in modern LCD monitors, 18-bit DACs are most often used and less often 24-bit ones. When using an 18-bit DAC, each color channel has 6 bits. This allows you to form 64 (26=64) different voltage levels and, accordingly, get 64 color shades in one color channel. In total, by mixing the color shades of different channels, it is possible to create 262,144 color shades. When using a 24-bit matrix (24-bit DAC circuit), each channel has 8 bits, which makes it possible to form 256 (28 = 256) color shades in each channel, and in total such a matrix reproduces 16,777,216 color shades. At the same time, for many 18-bit matrices, the passport states that they reproduce 16.2 million colors. What is the matter here and is it possible? It turns out that in 18-bit matrices, due to all sorts of tricks, you can bring the number of color shades closer to what is reproduced by real 24-bit matrices. To extrapolate color shades in 18-bit matrices, two technologies (and their combinations) are used: dithering (dithering) and FRC (Frame Rate Control). The essence of dithering technology is that the missing color shades are obtained by mixing the nearest color shades of neighboring pixels. Let's consider a simple example. Suppose that a pixel can only be in two states: open and closed, and the closed state of the pixel forms black, and the open state - red. If, instead of one pixel, we consider a group of two pixels, then, in addition to black and red, we can also obtain an intermediate color, thereby extrapolating from a two-color mode to a three-color one. As a result, if initially such a monitor could generate six colors (two for each channel), then after such dithering it will reproduce 27 colors already. The dithering scheme has one significant drawback: an increase in color shades is achieved at the expense of a decrease in resolution. In fact, this increases the pixel size, which can adversely affect the rendering of image details. The essence of FRC technology is to manipulate the brightness of individual subpixels by turning them on/off. As in the previous example, a pixel is considered to be either black (off) or red (on). Each sub-pixel is commanded to turn on at a frame rate, that is, at a frame rate of 60 Hz, each sub-pixel is commanded to turn on 60 times per second. This allows the color red to be generated. If we force the pixel to turn on not 60 times per second, but only 50 (on every 12th cycle, turn off the pixel, not turn it on), then as a result the pixel brightness will be 83% of the maximum, which will allow to form an intermediate color shade of red. Both considered methods of color extrapolation have their drawbacks. In the first case, this is a possible flickering of the screen and a slight increase in reaction time, and in the second, the probability of losing image details. It is quite difficult to distinguish by eye an 18-bit matrix with color extrapolation from a true 24-bit one. At the same time, the cost of a 24-bit matrix is ​​much higher.

The principle of operation of TFT-LCD displays

The general principle of image formation on the screen is well illustrated in Fig. 1. But how to control the brightness of individual subpixels? Beginners are usually explained this way: behind each subpixel there is a liquid crystal shutter. Depending on the voltage applied to it, it transmits more or less light from the backlight. And everyone immediately imagines some flaps on small loops that turn to the desired angle ... something like this:

In fact, of course, everything is much more complicated. There are no material flaps on the hinges. In a real liquid crystal matrix, the luminous flux is controlled something like this:

The light from the backlight (we go through the picture from bottom to top) first of all passes through the lower polarizing filter (white shaded plate). Now this is no longer an ordinary stream of light, but polarized. Further, the light passes through translucent control electrodes (yellow plates) and encounters a layer of liquid crystals on its way. By changing the control voltage polarization luminous flux can be changed by up to 90 degrees (in the picture on the left), or left unchanged (in the same place on the right). Attention, the fun begins! After the layer of liquid crystals, light filters are located and here each subpixel is painted in the desired color - red, green or blue. If you look at the screen with the upper polarizing filter removed, you will see millions of luminous sub-pixels - and each glows with maximum brightness, because our eyes cannot distinguish the polarization of light. In other words, without the top polarizer, we will see just a uniform white glow over the entire surface of the screen. But it is worth putting the upper polarizing filter back in place - and it will "show" all the changes that liquid crystals have made with the polarization of light. Some subpixels will remain brightly glowing, like the left one in the figure, whose polarization has been changed by 90 degrees, and some will go out, because the upper polarizer is in antiphase to the lower one and does not transmit light with the default (the one that is by default) polarization. There are also subpixels with intermediate brightness - the polarization of the light stream that passed through them was rotated not by 90, but by a smaller number of degrees, for example, by 30 or 55 degrees.

Advantages and disadvantages

Conventions: (+) dignity, (~) acceptable, (-) disadvantage
	LCD monitors	CRT monitors
Brightness	(+) from 170 to 250 cd/m2	(~) 80 to 120 cd/m2
Contrast	(~) 200:1 to 400:1	(+) 350:1 to 700:1
Viewing angle (by contrast)	(~) 110 to 170 degrees	(+) over 150 degrees
Viewing angle (by color)	(-) 50 to 125 degrees	(~) over 120 degrees
Permission	(-) Single resolution with fixed pixel size. Optimally can only be used in this resolution; higher or lower resolutions may be used depending on the supported expansion or compression functions, but these are not optimal.	(+) Various resolutions are supported. At all supported resolutions, the monitor can be used optimally. The limitation is imposed only by the acceptability of the refresh rate.
Vertical frequency	(+) Optimum frequency 60 Hz, which is sufficient for no flicker	(~) Only at frequencies above 75 Hz there is no clearly noticeable flicker
Color matching errors	(+) no	(~) 0.0079 to 0.0118 inch (0.20 - 0.30 mm)
Focusing	(+) very good	(~) fair to very good>
Geometric/linear distortion	(+) no	(~) possible
Pixels that don't work	(-) up to 8	(+) no
Input signal	(+) analog or digital	(~) analog only
Scaling at different resolutions	(-) absent or interpolation methods are used that do not require large overheads	(+) very good
Color display accuracy	(~) True Color is supported and the required color temperature is simulated	(+) True Color is supported and at the same time there are a lot of color calibration devices on the market, which is a definite plus
Gamma correction (color adjustment to the characteristics of human vision)	(~) satisfactory	(+) photorealistic
Uniformity	(~) often the image is brighter at the edges	(~) often the image is brighter in the center
Color Purity/Color Quality	(~) good	(+) high
flicker	(+) no	(~) imperceptibly above 85 Hz
Inertia time	(-) 20 to 30 ms.	(+) disparagingly small
Imaging	(+) The image is formed by pixels, the number of which depends only on the specific resolution of the LCD panel. The pixel pitch depends only on the size of the pixels themselves, but not on the distance between them. Each pixel is individually shaped for superb focus, clarity and definition. The image is more coherent and smooth	(~) Pixels are formed by a group of dots (triads) or stripes. The pitch of a point or line depends on the distance between points or lines of the same color. As a result, the sharpness and clarity of the image is highly dependent on the size of the dot or line pitch and on the quality of the CRT.
Power consumption and emissions	(+) Virtually no hazardous electromagnetic radiation. Power consumption is about 70% lower than standard CRT monitors (25W to 40W).	(-) Electromagnetic emissions are always present, however their level depends on whether the CRT complies with any safety standard. Energy consumption in working condition at the level of 60 - 150 watts.
Dimensions/weight	(+) flat design, light weight	(-) heavy construction, takes up a lot of space
Monitor interface	(+) Digital interface, however, most LCD monitors have a built-in analog interface for connecting to the most common analog outputs of video adapters	(-) Analog interface

Literature

• A.V. Petrochenkov “Hardware-computer and peripherals“, -106str.ill.
• V.E. Figurnov “IBM PC for the user”, -67p.
• “HARD "n" SOFT “ (computer magazine for a wide range of users) No. 6 2003
• N.I. Gurin “Work on personal computer“,-128p.

Main parameters of LCD monitors

So what do we know about liquid crystal monitors? First, they differ in size and color. Secondly - the price. Thirdly, they are produced by more than a dozen different companies. This, perhaps, the knowledge of an ordinary computer user is limited. We will try to expand them.

The most important consumer characteristics of an LCD monitor (or LCD monitor) are as follows: price, screen aspect ratio, resolution, diagonal, contrast, brightness, response time, viewing angle, availability defective pixels, interfaces, matrix type, dimensions, power consumption.

Price
Regarding pricing: in general, the more expensive the monitor, the better it is. However, there are nuances. Two manufacturers can create their models based on the same matrix, but the difference in price can reach more than a thousand rubles. All because of the design, marketing policy of the company and other factors.
In addition, each additional function or the possibility of increasing the final cost of the monitor. Moreover, these improvements are not always necessary for the user. Many of them have enough picture quality and functionality of cheap models based on a TN-matrix. But some require accurate color reproduction, which can only be provided by more expensive models based on IPS- or *VA-matrix.
Prices for the cheapest 18.5-inch and 19-inch monitors start at $100.

Screen Format
The now obsolete CRT monitors had a standard aspect ratio of 4:3 (width to height). The first LCD monitors were also produced like this (plus a 5:4 format was produced). Now it is already difficult to find them on sale: widescreen models are on store shelves - models with an aspect ratio of 16:10, 16:9, 15:9, which is associated with the active introduction of video in HD format (16:9).
Monitors 4:3 are more preferable for web surfing, work in text, publishing and other programs where work is carried out mainly on vertical objects (pages). But as a home monitor and a means of entertainment (viewing a variety of video content, three-dimensional games), a widescreen monitor will be the best choice.

Screen resolution
This parameter indicates how many dots (pixels) are placed on the visible part of the monitor. For example: 1680x1050 (1680 dots horizontally and 1050 dots vertically). This parameter is determined based on the frame format (the number of dots is a multiple of the aspect ratio). In this case it is 16:10. There are a finite number of such pairs of numbers (a permission table can be found online).
In CRT monitors, you could set any resolution that is supported by the monitor or video card. In LCD monitors, there is only one fixed resolution, the rest are achieved by interpolation. This degrades the picture quality. Therefore, when choosing between monitors with the same resolution, it is better to choose a larger diagonal. Especially if you have impaired vision, which is not uncommon in our time. Also, the resolution of the LCD monitor must be supported by your graphics card. Problems can arise with outdated video cards. Otherwise, you will have to set a non-native resolution. And this is an unnecessary distortion of the picture.
Buying a monitor with a resolution of 1920x1080 (Full HD) or 2560x1600 is not necessary at all. Because your computer can run 3D games at this resolution, and Full HD videos are still not very common.

Screen diagonal
This value is traditionally measured in inches and shows the distance between two opposite corners. The optimal diagonal for today in terms of size and price is 20-22 inches. By the way, with the same diagonal size, a 4:3 monitor will have a larger surface area.

Contrast
This value indicates the maximum brightness ratio between the lightest and darkest points. Usually specified as a pair of numbers like 1000:1. The more static contrast, the better, as it will allow you to see more shades (for example, instead of black areas - shades of black in photos, games or movies). Please note that the manufacturer may replace static contrast information with dynamic contrast information, which is calculated differently and should not be relied upon when choosing a monitor.

Brightness
This parameter shows the amount of light emitted by the display. It is measured in candela per square meter. A high brightness value won't hurt. In which case, you can always reduce the brightness depending on your own preferences and the illumination of the workplace.

Response time
Response time is the minimum time it takes for a pixel to change its brightness from active (white) to inactive (black) and back to active. The response time is the sum of the buffering time and the switching time. The last parameter is indicated in the characteristics. Measured in milliseconds (ms). Less is better. Long response times result in blurry images in fast scenes in movies and games. In most inexpensive models based on a TN-matrix, the response time does not exceed 10 ms and is quite enough for comfortable work. By the way, some manufacturers are cunning, measuring the transition time from one shade of gray to another and giving this value as the response time.

Viewing angle
This parameter indicates at what viewing angle the contrast falls to the specified value. In this case, the distortion becomes unacceptable for viewing. Alas, each company calculates the viewing angle differently, so the best thing to do is to take a closer look at the monitor before buying.

Defective pixels
After the production of the LCD matrix, it may contain image defects, which are divided into dead and “hot” (dependent) pixels. The appearance of the latter depends on some factors: for example, they can appear when the temperature rises. You can try to remove “hot” pixels using the “remap” procedure (damaged pixels will be turned off). Getting rid of pixels is unlikely to succeed.
Agree, it is unpleasant to work on a monitor with a constantly burning green or red dot. Therefore, when inspecting the monitor in a store, run some test program to determine the presence or absence of defective pixels. Or alternately fill the screen with black, white, red, green and blue and take a closer look. If there are no dead pixels, feel free to take it. Unfortunately, they may appear later, but the likelihood of this is low.
One more thing to be aware of: the ISO 13406-2 standard establishes four quality classes for monitors according to the allowable number of dead pixels. Therefore, the seller may refuse to exchange the model if the number of dead pixels does not exceed the quality class determined by the manufacturer.

Matrix type
Three main technologies are used in the production of displays: TN, IPS and MVA/PVA. There are others, but they do not have such a distribution. We are not interested in technological differences, let's move on to consumer properties.
TN+film. The most massive and cheap panels. They have a good response time, but a poor level of contrast and a small viewing angle. Also the color rendition is lame. Therefore, they are not used in areas where accurate work with color is necessary. For home use - the best option.
IPS (SFT). Dear panels. Good viewing angle, high contrast, good color reproduction, but long response time. The only ones that can render the full gamut of RGB colors. Developments are currently underway to improve response times, extend the color range even further, and improve other parameters.
MVA/PVA. Something between TN and IPS, both in terms of cost and performance. The response time is not much worse than TN, and the contrast, color reproduction and viewing angle are better.

Interfaces
Modern monitors can be connected to a computer using analog and digital interfaces. Analog VGA (D-Sub) is obsolete, but most likely will be used for a long time. Gradually replaced by digital DVI. HDMI and DisplayPort digital interfaces can also be found.
You basically need to know one thing: whether your video card has the appropriate interface. For example, you bought a new monitor with digital DVI, but the video card only has analog. In this case, you will have to use an adapter.

Dimensions, design, power consumption
The monitor must be selected not only based on consumer characteristics, but also appearance. But this is an individual setting. As we already wrote, a beautiful design increases the cost of the monitor. You can ignore the power consumption. In almost all modern models, it is quite small. The device passport indicates the power consumption: active (in operating mode) and passive (when the monitor is turned off, but not disconnected from the network).
One more question: to take a monitor with a glossy or matte finish? Gloss gives greater contrast, but more glare and gets dirty faster.

Cons of LCD monitors
Despite the fact that LCD monitors have several advantages over CRT monitors, there are a number of disadvantages that should be noted:
1) only one “regular” resolution, the rest are obtained by interpolation with loss of clarity;
2) color gamut and color accuracy are worse;
3) a relatively low level of contrast and black depth;
4) the response time to image changes is longer than that of CRT monitors;
5) the problem of the dependence of contrast on the viewing angle has not yet been solved;
6) the possible presence of unrecoverable defective pixels.

The Future of LCD Monitors
LCD monitors are currently in their heyday. But a few years ago, experts started talking about a technology that could someday replace them. The most promising are OLED displays (matrix with organic light emitting diodes). However, their mass production is still fraught with difficulties and is limited by a rather high price. In addition, LCD monitor technology is constantly improving, so the announcement of their imminent demise is premature.

The type of matrix used in an LCD monitor is, of course, one of the most important characteristics of monitors, but not the only one. In addition to the type of matrix, monitors are characterized by working resolution, maximum brightness and contrast, viewing angles, pixel switching time, as well as other, less significant parameters. Let's consider these characteristics in more detail.

If traditional CRT monitors are usually characterized by the diagonal screen size, then for LCD monitors such a classification is not entirely correct. It is more correct to classify LCD monitors by working resolution. The fact is that, unlike CRT-based monitors, the resolution of which can be changed quite flexibly, LCD displays have a fixed set of physical pixels. That is why they are designed to work with only one permission, called working. Indirectly, this resolution also determines the size of the matrix diagonal, however, monitors with the same working resolution may have a matrix of different sizes. For example, monitors with a diagonal of 15 to 16 inches generally have an operating resolution of 1024x768, which, in turn, means that this monitor actually has 1024 pixels horizontally and 768 pixels vertically.

The working resolution of the monitor determines the size of the icons and fonts that will be displayed on the screen. For example, a 15-inch monitor may have a working resolution of 1024x768 pixels, or maybe 1400x1050 pixels. In the latter case, the physical dimensions of the pixels themselves will be smaller, and since the same number of pixels is used in the formation of a standard icon in the first and second cases, then at a resolution of 1400x1050 pixels, the icon will be smaller in physical dimensions. Too small icon sizes at a high monitor resolution may be unacceptable for some users, so you should immediately pay attention to the working resolution when buying a monitor.

Of course, the monitor is capable of displaying an image in a resolution other than the working one. This mode of operation of the monitor is called interpolation. Note that in the case of interpolation, the image quality leaves much to be desired: the image is hacked and rough, and in addition, scaling artifacts such as bumps on circles can occur. The interpolation mode has a particularly strong effect on the display quality of screen fonts. Hence the conclusion: if you, when purchasing a monitor, plan to use it to work at a non-standard resolution, then in a simple way checking the monitor operation mode during interpolation is to view some text document in small print. It will be easy to notice interpolation artifacts along the contours of the letters, and if a better interpolation algorithm is used in the monitor, the letters will be more even, but still blurry. The speed at which the LCD monitor scales a single frame is also an important parameter to pay attention to, because the monitor electronics take time to interpolate.

One of the strengths of an LCD monitor is its brightness. This indicator in liquid crystal displays sometimes exceeds that in CRT-based monitors by more than twice. To adjust the brightness of the monitor, change the intensity of the backlight. Today, in LCD monitors, the maximum brightness declared in the technical documentation is from 250 to 300 cd / m2. And if the brightness of the monitor is high enough, then this is necessarily indicated in advertising booklets and presented as one of the main advantages of the monitor.

Brightness is indeed an important characteristic for an LCD monitor. For example, if the brightness is insufficient, it will be uncomfortable to work behind the monitor in daylight conditions (external illumination). As experience shows, it is quite enough for an LCD monitor to have a brightness of 200-250 cd / m2 - but not declared, but actually observed.

In recent years, the image contrast on digital panels has increased markedly, and now often this figure reaches a value of 1000:1. This parameter is defined as the ratio between the maximum and minimum brightness on a white and black background, respectively. But not everything is so simple here either. The fact is that the contrast can be indicated not for the monitor, but for the matrix, and in addition, there are several alternative methods for measuring contrast. However, as experience shows, if a contrast ratio of more than 350:1 is indicated in the passport, then this is quite enough for normal operation.

Due to the rotation of the LC molecules in each of the color subpixels through a certain angle, it is possible to obtain not only the open and closed states of the LC cell, but also intermediate states that form the color shade. Theoretically, the angle of rotation of LC molecules can be made any in the range from minimum to maximum. However, in practice there are temperature fluctuations that prevent the exact setting of the angle of rotation. In addition, to form an arbitrary voltage level, it will be necessary to use DAC circuits with a large capacity, which is extremely expensive. Therefore, in modern LCD monitors, 18-bit DACs are most often used and less often 24-bit ones. When using an 18-bit DAC, each color channel has 6 bits. This makes it possible to form 64 (26 = 64) different voltage levels and, accordingly, set 64 different orientations of LC molecules, which, in turn, leads to the formation of 64 color shades in one color channel. In total, by mixing the color shades of different channels, it is possible to obtain 262 K color shades.

When using a 24-bit matrix (24-bit DAC circuit), each channel has 8 bits, which makes it possible to form 256 (28 = 256) color shades in each channel, and in total such a matrix reproduces 16,777,216 color shades.

At the same time, for many 18-bit matrices, the passport states that they reproduce 16.2 million colors. What is the matter here and is it possible? It turns out that in 18-bit matrices, due to various tricks, you can increase the number of color shades so that this number approaches the number of colors reproduced by real 24-bit matrices. For extrapolation of color shades in 18-bit matrices, two technologies (and their combinations) are used: Dithering (dithering) and FRC (Frame Rate Control).

The essence of the Dithering technology lies in the fact that the missing color shades are obtained by mixing the nearest color shades of adjacent subpixels. Let's consider a simple example. Suppose that a subpixel can only be in two states: open and closed, and the closed state of the subpixel forms black, and the open state - red. If, instead of one pixel, we consider a group of two subpixels, then, in addition to black and red colors, we can also obtain an intermediate color and thereby extrapolate from a two-color mode to a three-color one (Fig. 1). As a result, if initially such a monitor could generate six colors (two for each channel), then after such dithering, the monitor will reproduce 27 colors already.

Figure 1 - Dithering scheme for obtaining color shades

If we consider a group of not two, but four subpixels (2x2), then the use of dithering will allow us to obtain an additional three color shades in each channel and the monitor will turn from 8-color to 125-color. Accordingly, a group of 9 subpixels (3x3) will allow you to get an additional seven color shades, and the monitor will already be 729-color.

The dithering scheme has one significant drawback: an increase in color shades is achieved at the expense of a decrease in resolution. In fact, this increases the pixel size, which can adversely affect the rendering of image details.

In addition to dithering technology, FRC technology is also used, which is a way to manipulate the brightness of individual subpixels by turning them on / off. As in the previous example, we will assume that the subpixel can be either black (off) or red (on). Recall that each sub-pixel is commanded to turn on at a frame rate, that is, at a frame rate of 60 Hz, each sub-pixel is commanded to turn on 60 times per second, which allows red to be generated. If, however, the subpixel is forced to turn on not 60 times per second, but only 50 (on each 12th cycle, do not turn on, but turn off the subpixel), then as a result, the brightness of the subpixel will be 83% of the maximum, which will allow to form an intermediate color shade of red.

Both considered methods of color extrapolation have their drawbacks. In the first case, this is the possibility of loss of image details, and in the second, a possible flickering of the screen and a slight increase in reaction time.

However, it should be noted that it is not always possible to distinguish by eye an 18-bit matrix with color extrapolation from a true 24-bit one. In this case, a 24-bit matrix will cost significantly more.

The traditional problem of LCD monitors is viewing angles - if the image on a CRT practically does not suffer even when viewed almost parallel to the plane of the screen, then on many LCD matrices even a slight deviation from the perpendicular leads to a noticeable drop in contrast and color distortion. According to current standards, sensor manufacturers define the viewing angle as the angle relative to the perpendicular to the center of the sensor, when viewed under which the image contrast in the center of the sensor drops to 10:1 (Fig. 2).

Figure 2 - Scheme for determining the viewing angles of the LCD matrix

Despite the apparent unambiguity of this term, it is necessary to clearly understand what exactly the manufacturer of the matrix (and not the monitor) understands at the viewing angle. The maximum viewing angle both vertically and horizontally is defined as the viewing angle from which the image contrast is at least 10:1. At the same time, remember that image contrast is the ratio of the maximum brightness on a white background to the minimum brightness on a black background. Thus, by definition, viewing angles are not directly related to color accuracy when viewed from an angle.

The reaction time, or response time, of a subpixel is also one of the most important indicators of a monitor. It is often this characteristic that is called the weakest point of LCD monitors, because, unlike CRT monitors, where the pixel response time is measured in microseconds, in LCD monitors this time is tens of milliseconds, which ultimately leads to blurring of the changing picture and can be noticeable to the eye. From a physical point of view, the reaction time of a pixel is determined by the time interval during which the spatial orientation of liquid crystal molecules changes, and the shorter this time, the better.

In this case, it is necessary to distinguish between the turn-on and turn-off times of a pixel. The pixel on time refers to the time required for the LC cell to fully open, and the pixel off time refers to the time required to fully close the LC cell. When talking about the reaction time of a pixel, then this is understood as the total time of turning on and off the pixel.

The time a pixel is turned on and the time it is turned off can differ significantly from each other. For example, if we consider common TN + Film matrices, then the process of turning off a pixel consists in the reorientation of molecules perpendicular to the directions of polarization under the influence of an applied voltage, and the process of turning on a pixel is a kind of relaxation of LC molecules, that is, the process of transition to their natural state. In this case, it is obvious that the turn-off time of a pixel will be less than the turn-on time.

Figure 3 shows typical timing diagrams for switching on (Fig. 3a) and switching off (Fig. 3b) a TN+Film-matrix pixel. In the example shown, the turn-on time for a pixel is 20ms and the turn-off time is 6ms. The total reaction time of a pixel is 26 ms.

When they talk about the pixel response time indicated in the technical documentation for the monitor, they mean the response time of the matrix, not the monitor. Oddly enough, but this is not the same thing, since the first case does not take into account all the electronics required to control the pixels of the matrix. In fact, the reaction time of the matrix pixel is the time required for the reorientation of molecules, and the reaction time of the monitor pixel is the time between the signal to turn on / off and the very fact of turning on / off. In addition, speaking of the pixel response time indicated in the technical documentation, it must be taken into account that matrix manufacturers can interpret this time in different ways.

Figure 3 - Typical time diagrams for turning on (a) and turning off (b) a pixel for a TN matrix

So, one of the options for interpreting the on/off time of a pixel is that this means the time for changing the brightness of the pixel glow from 10 to 90% or from 90 to 10%. At the same time, it is quite possible that for a monitor with a good pixel response time, when the brightness changes from 10 to 90%, the total pixel response time (when the brightness changes from 0 to 100%) will be quite large.

So, maybe it is more correct to make measurements within the range of brightness change from 0 to 100%? However, brightness from 0 to 10% is perceived by the human eye as absolutely black, and in this sense, it is the measurement from the brightness level of 10% that is of practical importance. Similarly, it does not make sense to measure a change in brightness level up to 100%, since brightness from 90 to 100% is perceived as white, and therefore it is precisely the measurement of brightness up to 90% that is of practical importance.

Until now, speaking about measuring the reaction time of a pixel, we meant that we are talking about switching between black and white colors. If there are no questions with the black color (the pixel is simply closed), then the choice of white color is not obvious. How will the reaction time of a pixel change if you measure it when switching between different halftones? This question is of great practical importance. The fact is that switching from a black background to a white background or vice versa, which determines the reaction time of a pixel, is used relatively rarely in real applications - an example would be scrolling black text on a white background. In most applications, as a rule, transitions between semitones are implemented. And if it turns out that the switching time between gray and white colors will be less than the switching time between grayscale, then the pixel response time simply has no practical value, so you can’t rely on this monitor characteristic. Indeed, what is the point in determining the reaction time of a pixel, if the actual time of switching between halftones can be longer and if the image will blur when the image changes dynamically?

The answer to this question is quite complicated and depends on the type of monitor matrix. For the widely used and cheapest TN + Film matrices, everything is quite simple: the pixel response time, that is, the time required to completely open or close the LCD cell, turns out to be the maximum time. If the color is described by gradations of R-, G- and B-channels (R-G-B), then the transition time from black (0-0-0) to white (255-255-255) color is longer than the transition time from black to gray gradation. Similarly, the turn-off time of a pixel (transition from white to black) is longer than the transition time from white to any grayscale.

On fig. 4 shows a graphical representation of the switching time between black and grayscale and vice versa between grayscale and black. As you can see from the graph, it is the time of switching between black and white and vice versa that determines the reaction time of a pixel. That is why for TN+Film matrices the pixel response time is fully characterized by the dynamic properties of the monitor.

Figure 4 - Graph of switching time between black and grayscale

For IPS and MVA matrices, everything is not so obvious. For these types of sensors, the transition time between color shades (grayscale) may be longer than the transition time between white and black. In such matrices, knowledge of the pixel response time (even if you are assured that this is a record low time) is of no practical importance and cannot be considered as a dynamic characteristic of the monitor. As a result, for these matrices, a much more important parameter is the maximum transition time between grayscale levels, but this time is not indicated in the documentation for the monitor. Therefore, if you do not know the maximum pixel switching time for a given type of matrix, then the best way to evaluate the dynamic characteristics of the monitor is to run some dynamic game application and determine the picture blur by eye.

All LCD monitors are digital by nature, so DVI digital interface is considered to be their native interface. The interface can have two types of connectors: DVI-I, which combines digital and analog signals, and DVI-D, which transmits only a digital signal. It is believed that the DVI interface is preferable for connecting an LCD monitor to a computer, although connection via a standard D-Sub connector is also possible. In favor of the DVI interface is the fact that in the case of an analog interface, a double conversion of the video signal is performed: initially, the digital signal is converted to analog in the video card (DAC conversion), and then the analog signal is transformed into a digital electronic unit of the LCD monitor itself (ADC conversion) , and as a result of such transformations, the risk of various signal distortions increases. In fairness, we note that in practice, signal distortions introduced by double conversion do not occur, and you can connect a monitor via any interface. In this sense, the monitor interface is the last thing worth paying attention to. The main thing is that the corresponding connector is on the video card itself.

Many modern LCD monitors have both D-Sub and DVI connectors, which often allows you to connect two system units to the monitor at the same time. There are also models that have two digital connectors.

Structural diagram of the LCD view monitor in Fig. 5

Figure 5 - Structural diagram of the LCD monitor

The signal from the video adapter is fed to the display input via analog RGB VGA D-sub or digital DVI interface. In the case of using an analog interface, the video adapter converts the frame buffer data from digital to analog, and the LCD monitor electronics, for its part, is forced to perform the reverse, analog-to-digital conversion. Obviously, such redundant operations at least do not improve image quality, moreover but they require additional costs for their implementation. Therefore, with the ubiquity of LCD displays VGA interface D-sub is being replaced by digital DVI. In some monitors, manufacturers deliberately do not support the DVI interface, limiting themselves only to VGA D-sub, since this requires the use of a special TMDS receiver on the monitor side, and the cost of a device that supports both analog and digital interfaces compared to the option with the only analog input would be higher.

From RGB A/D conversion, scaling, processing, and LVDS output signal processing, the LCD image processing circuitry is based on a single, highly integrated IC called the Display Engine.

The LCD matrix block contains a control circuit, the so-called matrix driver, in which the LVDS control output receiver and source and gate drivers are integrated, converting the video signal into addressing specific pixels in columns and rows.

The LCD matrix block also includes its illumination system, which, with rare exceptions, is made on cold cathode discharge lamps (Cold Cathode Fluorescent Lamp, CCFL). The high voltage for them is provided by an inverter located in the monitor's power supply. Lamps are usually located above and below, their radiation is directed to the end of a translucent panel located behind the matrix and acting as a light guide. The quality of matting and the homogeneity of the material of this panel depends on such important characteristic, as the uniformity of the matrix illumination

Addressing LCD displays with a passive matrix, in principle, can be implemented in the same way as for gas discharge panels. The front electrode, common to the entire column, conducts voltage. The rear electrode, common to the entire row, serves as the "ground".

There are drawbacks to such passive matrices and they are known: the panels are very slow, and the picture is not sharp. And there are two reasons for that. The first is that after we address a pixel and rotate the crystal, the latter will slowly return to its original state, blurring the picture. The second reason lies in the capacitive coupling between the control lines. This coupling results in inaccurate voltage propagation and slightly "spoils" neighboring pixels.

The noted shortcomings led to the development of active matrix technology (Fig. 6).

Figure 6 - Scheme of switching on the subpixel of the active LCD matrix

LCD monitor resolution matrix

Here, a transistor is added to each pixel, acting as a switch. If it is open (on), then data can be written to the storage capacitor. If the transistor is closed (off), then the data remains in the capacitor, which acts as an analog memory. The technology has many benefits. When the transistor is closed, the data is still in the capacitor, so the voltage supply to the liquid crystal will not stop while the control lines will address another pixel. That is, the pixel will not return to its original state, as happened in the case of a passive matrix. In addition, the write time to the capacitor is much less than the die turn time, which means we can poll the panel pixels faster and transfer data to them.

This technology is also known as "TFT" (thin film transistors, thin film transistors). But today it has become so popular that the name "LCD" has long become synonymous with it. That is, by LCD we mean a display that uses TFT technology.

Moscow State Institute of Electronics and Mathematics

(Technical University)

Department:

"Information and Communication Technologies"

Course work

"LCD Monitors: Internal Organization, Technologies, Perspectives".

Performed:

Starukhina E.V.

Group: S-35

Moscow 2008
Content

1. Introduction............................................... ................................................. ......................................... 3

2.Liquid crystals ............................................... ................................................. ......................... 3

2.1.Physical properties of liquid crystals .............................................................. ............................... 3

2.2.History of the development of liquid crystals .............................................. ..................................... 4

3.Structure of the LCD monitor............................................... ................................................. ................. 4

3.1.Sub-pixel of the LCD color display .............................................. ............................................. five

3.2. Matrix illumination methods .............................................................. ................................................. . five

4.Specifications LCD Monitor ............................................... ................................ five

5. Current technologies for the manufacture of LCD matrices .................................................... ......................... 7

5.1.TN+film (Twisted Nematic + film).................................................. ................................................. .7

5.2.IPS (In-Plane Switching).................................................. ................................................. ............... 8

5.3.MVA (Multi-Domain Vertical Alignment) ........................................................ ....................................... nine

6.Advantages and disadvantages ............................................... ................................................. .......... nine

7.Promising technologies for the manufacture of flat-panel monitors .............................................. 10

8. Market Overview and Selection Criteria for LCD Monitor ........................................................ ............................... 12

9.Conclusion................................................... ................................................. .................................. 13

10. List of references ............................................... ................................................. .................... fourteen

Introduction.

At present, most of the monitor market is occupied by LCD monitors, represented by brands such as Samsung, ASUS, NEC, Acer, Philips, etc. LCD technologies are also used in the manufacture of television panels, laptop displays, mobile phones, players, cameras, etc. Due to their physical properties (we will consider them below), liquid crystals allow you to create screens that combine such qualities as high image clarity, economical power consumption, small display thickness, high resolution, but at the same time wide range of diagonals: from 0.44 inches / 11 millimeters (January 2008, the smallest screen from microdisplay manufacturer Kopin), to 108 inches / 2.74 meters (largest LCD panel, introduced June 29, 2008 by Sharp Microelectronics Europe) . Also, the advantage of LCD monitors is the absence of harmful radiation and flicker, which was a problem with CRT monitors.

But still, LCD monitors have a number of disadvantages: the presence of such characteristics as response time, not always a satisfactory viewing angle, insufficiently deep blacks and the possibility of matrix defects (broken pixels). Are LCD panels worthy successors to CRT monitors, and do they have a future in view of the rapidly developing plasma technology? We will have to understand this issue by studying the physical structure of LCD monitors, their characteristics and comparing them with those of competing technologies.

1. Liquid crystals.

1.1. Physical properties of liquid crystals.

Liquid crystals are substances that have properties inherent in both liquids and crystals: fluidity and anisotropy. Structurally, liquid crystals are jelly-like liquids. Molecules have an elongated shape and are ordered throughout their volume. The most characteristic property of LCs is their ability to change the orientation of molecules under the influence of electric fields, which opens up wide opportunities for their application in industry. According to the type of LC, they are usually divided into two large groups: nematics and smectics. In turn, nematics are subdivided into proper nematic and cholesteric liquid crystals.

Cholesteric liquid crystals - are formed mainly by compounds of cholesterol and other steroids. These are nematic LCs, but their long axes are rotated relative to each other so that they form spirals that are very sensitive to temperature changes due to the extremely low formation energy of this structure (about 0.01 J/mol). Cholesterics are brightly colored and the slightest change in temperature (up to thousandths of a degree) leads to a change in the pitch of the helix and, accordingly, a change in the color of the LC.

LCDs have unusual optical properties. Nematics and smectics are optically uniaxial crystals. Cholesterics, due to their periodic structure, strongly reflect light in the visible region of the spectrum. Since the liquid phase is the carrier of properties in nematics and cholesterics, it is easily deformed under the influence of external influences, and since the helix pitch in cholesterics is very sensitive to temperature, therefore, the reflection of light changes sharply with temperature, leading to a change in the color of the substance.

These phenomena are widely used in various applications, such as finding hot spots in microcircuits, localizing fractures and tumors in humans, imaging in infrared rays, etc.

1.2. The history of the development of liquid crystals.

Liquid crystals were discovered by the Austrian botanist F. Reinitzer in 1888. Investigating the crystals of cholesteryl benzoate and cholesteryl acetate, he found that the substances have 2 melting points and 2 different liquid states - transparent and cloudy. However, the properties of these substances, at first, did not attract the attention of scientists. Moreover, liquid crystals destroyed the theory of three aggregate states of matter, so physicists and chemists long time did not recognize liquid crystals in principle. Strasbourg University professor Otto Lehmann, as a result of many years of research, provided proof, but even after that, liquid crystals did not find application.

In 1963, the American J. Ferguson used the most important property of liquid crystals - to change color under the influence of temperature - to detect thermal fields that are not visible to the naked eye. After he was granted a patent for an invention, interest in liquid crystals increased dramatically.

In 1965, the First International Conference devoted to liquid crystals met in the USA. In 1968, American scientists created fundamentally new indicators for information display systems. The principle of their operation is based on the fact that the molecules of liquid crystals, turning in an electric field, reflect and transmit light in different ways. Under the influence of voltage, which was applied to the conductors soldered into the screen, an image appeared on it, consisting of microscopic dots. And yet, only after 1973, when a group of British chemists led by George Gray synthesized liquid crystals from relatively cheap and accessible raw materials, these substances became widespread in various devices.

For the first time, liquid crystal displays began to be used in the manufacture of laptops due to their compact size. In the early stages, the final products were very expensive, and their quality was very low. However, a few years ago, the first full-fledged LCD monitors appeared, the cost of which also remained quite high, but their quality improved markedly. And finally, now the market for LCD monitors is developing rapidly. This is due to the fact that technologies are developing very actively and, in addition, competition among manufacturers has led to a noticeable decrease in prices for this species products.

2. The structure of the LCD monitor.

A liquid crystal monitor is a device designed to display graphic information from a computer, camera, etc.

A feature of liquid crystal displays is that liquid crystals themselves do not emit light. Each pixel of an LCD monitor is made up of three primary color sub-pixels (red, green, blue). The light passing through the cells can be natural - reflected from the substrate (in LCD displays without backlight). But more often an artificial light source is used, in addition to independence from external lighting, this also stabilizes the properties of the resulting image. The image is formed using individual elements, usually through a sweep system. Thus, a full-fledged LCD monitor consists of electronics that processes the input video signal, an LCD matrix, a backlight module, a power supply, and a housing. It is the combination of these components that determines the properties of the monitor as a whole, although some characteristics are more important than others.

2.1. Sub-pixel color LCD.

Each pixel of an LCD display consists of a layer of molecules between two transparent electrodes, and two polarizing filters whose planes of polarization are (usually) perpendicular. In the absence of liquid crystals, the light transmitted by the first filter is almost completely blocked by the second.

The surface of the electrodes in contact with liquid crystals is specially treated for the initial orientation of the molecules in one direction. In a TN matrix, these directions are mutually perpendicular, so the molecules line up in a helical structure in the absence of stress. This structure refracts light in such a way that before the second filter its plane of polarization rotates, and light passes through it without loss. Except for the absorption of half of the unpolarized light by the first filter, the cell can be considered transparent. If a voltage is applied to the electrodes, the molecules tend to line up in the direction of the field, which distorts the helical structure. In this case, the elastic forces counteract this, and when the voltage is turned off, the molecules return to their original position. At a sufficient field strength, almost all molecules become parallel, which leads to the opacity of the 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, an alternating current is applied, or a change in the polarity of the field with each addressing of the cell (the opacity of the structure does not depend on the polarity of the field). In the entire matrix, it is possible to control each of the cells individually, but as their number increases, this becomes difficult, as the number of required electrodes increases. Therefore, addressing by rows and columns is used almost everywhere.

Liquid crystal monitor (also liquid crystal display, LCD, LCD monitor, English liquid crystal display, LCD, flat indicator) - a flat monitor based on liquid crystals. LCD monitors were developed in 1963.

LCD TFT (English TFT - thin film transistor - thin film transistor) is one of the names for a liquid crystal display that uses an active matrix that is driven by thin film transistors. Amplifier TFT for each subpixel is used to improve the speed, contrast and clarity of the display image.

LCD monitor device

The image is formed using individual elements, usually through a scanning system. Simple devices (electronic clocks, phones, players, thermometers, etc.) can have a monochrome or 2-5 color display. A multicolor image is formed using RGB triads. Most desktop monitors based on TN - (and some *VA ) matrices, and all laptop displays use matrices with 18-bit color (6 bits per channel), 24-bit is emulated with dithered flicker.

Sub-pixel color LCD

Each pixel of an LCD display consists of a layer of molecules between two transparent electrodes, and two polarizing filters whose planes of polarization are (usually) perpendicular. In the absence of liquid crystals, the light transmitted by the first filter is almost completely blocked by the second.

The surface of the electrodes in contact with liquid crystals is specially treated for the initial orientation of the molecules in one direction. In the TN matrix, these directions are mutually perpendicular, so the molecules line up in a helical structure in the absence of stress. This structure refracts light in such a way that before the second filter its plane of polarization rotates, and light passes through it without loss. Except for the absorption of half of the unpolarized light by the first filter, the cell can be considered transparent. If a voltage is applied to the electrodes, the molecules tend to line up in the direction of the field, which distorts the helical structure. In this case, the elastic forces counteract this, and when the voltage is turned off, the molecules return to their original position. At a sufficient field strength, almost all molecules become parallel, which leads to the opacity of the 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, an alternating current is applied, or a change in the polarity of the field with each addressing of the cell (the opacity of the structure does not depend on the polarity of the field). In the entire matrix, it is possible to control each of the cells individually, but with an increase in their number, this becomes difficult, since the number of required electrodes increases. Therefore, addressing by rows and columns is used almost everywhere. The light passing through the cells can be natural - reflected from the substrate (in LCD displays without backlight). But more often an artificial light source is used, in addition to independence from external lighting, this also stabilizes the properties of the resulting image. Thus, a full-fledged LCD monitor consists of electronics that processes the input video signal, an LCD matrix, a backlight module, a power supply, and a housing. It is the combination of these components that determines the properties of the monitor as a whole, although some characteristics are more important than others.

LCD Monitor Specifications

Permission: Horizontal and vertical dimensions expressed in pixels. Unlike CRT monitors, LCDs have one, "native", physical resolution, the rest are achieved by interpolation.

Dot size: The distance between the centers of adjacent pixels. Directly related to physical resolution.

Screen aspect ratio (format): The ratio of width to height, for example: 5:4, 4:3, 5:3, 8:5, 16:9, 16:10.

Visible Diagonal: the size of the panel itself, measured diagonally. The display area also depends on the format: a 4:3 monitor has a larger area than a 16:9 monitor with the same diagonal.

Contrast: The ratio of the brightness of the lightest point to the darkest point. Some monitors use an adaptive backlight level using additional lamps, the contrast figure given for them (so-called dynamic) does not apply to a static image.

Brightness: The amount of light emitted by the display, usually measured in candela per square meter.

Response time: The minimum time it takes for a pixel to change its brightness. Measurement methods are ambiguous.

Viewing angle: the angle at which the drop in contrast reaches the specified value, for different types matrices and different manufacturers is considered differently, and often cannot be compared.

Matrix type: the technology by which the LCD is made

Inputs: (ex. DVI, D-SUB, HDMI etc.).

Technology

The main technologies in the manufacture of LCD displays: TN + film, IPS And MVA. These technologies differ in the geometry of surfaces, polymer, control plate and front electrode. Of great importance are the purity and type of polymer with liquid crystal properties used in specific developments. Response time of LCD monitors built with technology SXRD (Silicon X-tal Reflective Display)- silicon reflective liquid crystal matrix), reduced to 5 ms. Sony companies, Sharp and Philips jointly developed PALC technology (Eng. Plasma Addressed Liquid Crystal- plasma control of liquid crystals), which combines the advantages LCD(brightness and richness of colors, contrast) and plasma panels (large viewing angles on the horizon, H, and vertical, V , high refresh rate). These displays use gas-discharge plasma cells as a brightness control, and an LCD matrix is ​​used for color filtering. PALC technology allows you to address each display pixel individually, which means unsurpassed controllability and image quality.

TN+ film (Twisted Nematic + film)

Closeup of TN+ film monitor matrix NEC LCD1770NX. On a white background - a standard Windows cursor.

Part " 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"often omitted, calling such matrices simply TN. Unfortunately, a way to improve the contrast and response time for TN panels has not yet been found, and the response time for this type of matrix is ​​currently one of the best, but the contrast level is not.

Matrix TN+ film works like this: if no voltage is applied to the sub-pixels, 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 direction of polarization of the filter on the second plate makes an angle of 90° with the direction of polarization 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 form on the screen.

IPS (In-Plane Switching)

Technology In- Plane Switching was developed by Hitachi and NEC and was intended to get rid of the shortcomings of TN + film. However, while IPS was able to achieve a 170° viewing angle, as well as high contrast and color reproduction, the response time remained low.

If no voltage is applied to the IPS, the liquid crystal molecules do not rotate. The second filter is always rotated perpendicular to the first, and no light passes through it. Therefore, the display of black color is close to ideal. If the transistor fails, the "broken" pixel for the IPS panel will not be white, as for the TN matrix, but black.

When a voltage is applied, the liquid crystal molecules rotate perpendicular to their initial position and allow light to pass through. AS-IPS - Advanced Super IPS technology (Advanced Super-IPS), was also developed by Hitachi Corporation in 2002. The main improvements were in the contrast level of conventional S-IPS panels, bringing it closer to that 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.

A-TW-IPS - Advanced True White IPS (Advanced True White IPS), developed by LG.Philips for NEC Corporation. It is an S-IPS panel with a TW (True White) color filter to make whites more realistic and expand the color range. This type of panel is used to create professional monitors for use in photo labs and/or publishing houses.

AFFS- Advanced Fringe Field Switching(unofficial name S-IPS Pro). The technology 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 to reduce the interpixel distance. AFFS-based displays are mainly used in tablet PCs, on matrices manufactured by Hitachi Displays.