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Basic parameters of LCD monitors. LCD Monitor Specifications LCD Monitor Physical Specifications

Introduction

1. Creating a liquid crystal display

2. Characteristics of LCD monitors

2.1 Types of LCD monitors

2.2 Monitor resolution

2.3 Monitor Interface

2.4 LCD type

2.5 Classification of TFT-LCD displays

2.5.1 TN matrix

2.5.2 IPS panels

2.5.3 MVA matrices

2.5.4 Features of various LCD matrices

2.6 Brightness

2.7 Contrast

2.8 Viewing angle

2.9 Pixel response time

2.10 Number of displayed colors

Conclusion

Bibliography

Introduction

The fact that LCD models dominate the consumer monitor segment today is beyond doubt. What is hidden in the mysterious, fantastic name LCD? Until relatively recently, few people knew anything, except for the name Liquid Crystal Monitor, surrounded by secrets, accidentally heard! However, progress does not stand still, and the situation in this area has changed quite significantly.

Even 4 years ago, PC users did not even think about such a chic acquisition. And no matter how much they argue about which monitors are better - LCD or CRT (cathode beam), - the user has practically no choice left. Manufacturers have shifted to the production of LCD monitors and offer users a wide range of products. As a rule, in order to attract consumers to their products, monitor manufacturers pay a lot of attention to the design of monitors.

However, the technical characteristics of monitors are constantly improving. But the cost of these devices has been steadily falling, and in a fairly short period of time, LCD monitors have become available to a wide range of buyers. But still, many still take a very irresponsible approach to choosing such a “miracle”, or rather, they do not attach much importance to its parameters. After that, as a rule, they suffer greatly, because in practice the characteristics indicated in the passport and colorfully touted by sellers do not satisfy the buyer's requirement. And the point is how these characteristics are determined by certain individuals. Some parameters are generally recommended to be personally checked visually, not content with the faceless numbers of the data sheet.

Thus, in order to purchase a more or less high-quality LCD monitor (LiquidCrystalDisplay for the especially curious), it is advisable to first, at least in general terms, study its device and, accordingly, know how to check one or another parameter in accordance with its physical properties.

1. Creating a liquid crystal display

liquid crystal display monitor matrix

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 (LiquidCrystal) 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 an electric current is applied, the arrangement of the crystals changes, and they begin to interfere with 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 British 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, slim figure, compactness, efficiency (15-30 watts), in addition, it is believed that only wealthy and serious people can afford such a luxury.

2.1 Types of LCD monitors

There are two types of LCD monitors: DSTN (dual-scantwistednematic - dual-scan crystal screens) and TFT (thinfilmtransistor - 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 (Fig. 1).

Rice. 1. − Monitor composite layers

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.

2.2 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 the same number of pixels is used in the formation of a standard icon in both cases, the physical dimensions of the icon will be smaller at a resolution of 1400x1050 pixels. 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.

2.3 Monitor Interface

LCD monitors are digital devices by their nature, therefore, the “native” interface for them is the DVI digital interface, which can have two types of convectors: 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 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 system units to the monitor at the same time. You can also find models with two digital connectors. In inexpensive office models, there is basically only a standard D-Sub connector.

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).

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 (Fig. 1).

Rice. one.

Monitor resolution

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 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 a working 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 the easiest way to check the monitor's operation mode during interpolation is to view a 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 figure 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, there are 6 bits per color channel. 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 color extrapolation methods have their drawbacks. In the first case, this is the possibility of losing 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 completely 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 real 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, the VGA D-sub interface 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 power supply of the monitor. The 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. Such an important characteristic as the uniformity of the matrix illumination depends on the quality of the matting and the uniformity of the material of this panel.

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 shorter than the die turn time, which means we can poll the panel pixels and transfer data to them faster.

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 for the 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 a 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 crystals of cholesteryl benzoate and cholesteryl acetate, he found that 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 did not recognize liquid crystals in principle for a long time. 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 reduction in prices for this type of product.

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 with the help of individual elements, as a rule, through a scanning 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 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 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, 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

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 is considered differently for different types of matrices and by different manufacturers, 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 poor.

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.

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