In the domestic computer market, there is strong competition among flat-panel monitors of various brands, and all businesses want to share the largest share in the large flat-screen cake. And when people bought the flat screen as they did when they moved the 15-inch monitor. We not only have to ask: what is the hotspot of next-generation displays? The spear is pointed directly at the LCD. The liquid crystal display has the advantages of clear and accurate images, flat display, thin thickness, light weight, no radiation, low energy consumption, and low working voltage.
According to different control methods, liquid crystal displays can be divided into two types: passive matrix LCD and active matrix LCD.
1. The passive matrix LCD is greatly restricted in brightness and viewing angle, and the response speed is also slow. Due to the problem of picture quality, this display device is not conducive to the development of a desktop display, but due to the low cost factor, some displays on the market still use passive matrix LCD. Passive matrix LCD can be divided into TN-LCD (Twisted NemaTIc-LCD, twisted nematic LCD), STN-LCD (Super TN-LCD, super twisted nematic LCD) and DSTN-LCD (Double layer STN-LCD, double Layer super twisted nematic LCD).
2. Currently widely used active matrix LCD, also known as TFT-LCD (Thin Film Transistor-LCD, thin film transistor LCD). TFT LCD monitors have built-in transistors in each pixel of the screen, which can make the brightness brighter, richer colors and wider viewing area. Compared with CRT monitors, the flat display technology of LCD monitors embodies fewer parts, occupies less desktop and consumes less power, but CRT technology is more stable and mature.
The working principle of LCD
We have long known that matter has three types: solid, liquid, and gas. Although the arrangement of liquid molecular centroids does not have any regularity, if these molecules are elongated (or flat), their molecular orientation may be regular. So we can subdivide the liquid state into many types. Liquids with no regular molecular orientation are directly called liquids, while liquids with molecular orientations are called "liquid crystals", or "liquid crystals" for short. LCD products are actually not new to us. Mobile phones and calculators that we commonly see belong to LCD products. The liquid crystal was discovered by the Austrian botanist Reinitzer in 1888. It is an organic compound with a regular molecular arrangement between solid and liquid. The most commonly used liquid crystal type is nematic liquid crystal. The molecular shape is a slender rod shape with a length and width of about 1nm to 10nm. Under the action of different electric current fields, the liquid crystal molecules will be regularly rotated by 90 degrees to produce transparency. The difference, so that the difference between light and dark under the power ON / OFF, each pixel is controlled according to this principle, and the desired image can be formed.
1. Working principle of passive matrix LCD
The display principle between TN-LCD, STN-LCD and DSTN-LCD is basically the same, the difference is that the twist angle of liquid crystal molecules is somewhat different. The following takes a typical TN-LCD as an example to introduce its structure and working principle.
In a TN-LCD panel with a thickness of less than 1 cm, it is usually a plywood made of two large glass substrates with color filters, alignment films, etc.? Two polarizers are wrapped on the outside, They can determine the maximum value of luminous flux and the generation of color. The color filter is a filter composed of three colors of red, green, and blue, and is regularly made on a large glass substrate. Each pixel is composed of three color units (or sub-pixels). If a panel has a resolution of 1280 × 1024, it actually has 3840 × 1024 transistors and sub-pixels. The upper left corner (gray rectangle) of each sub-pixel is an opaque thin-film transistor, and the color filter can produce RGB primary colors. Each interlayer contains electrodes and grooves formed on the alignment film, and the upper and lower interlayers are filled with multiple layers of liquid crystal molecules (liquid crystal space is less than 5 × 10-6m). In the same layer, although the position of the liquid crystal molecules is irregular, the long axis orientation is parallel to the polarizing plate. On the other hand, between different layers, the long axis of the liquid crystal molecules is continuously twisted 90 degrees along the parallel plane of the polarizing plate. Among them, the orientation of the long axis of the two layers of liquid crystal molecules adjacent to the polarizing plate is consistent with the polarization direction of the adjacent polarizing plate. The liquid crystal molecules near the upper interlayer are arranged in the direction of the upper trench, and the liquid crystal molecules in the lower interlayer are arranged in the direction of the lower trench. Finally, it is packaged into a liquid crystal cell and connected with the driver IC, the control IC and the printed circuit board.
Under normal circumstances, when the light is irradiated from the top to the bottom, usually only one angle of light can penetrate through the upper polarizer into the groove of the upper interlayer, and then pass through the twisted arrangement of liquid crystal molecules from the lower polarizer Form a complete light penetration path. Two polarizers are attached to the interlayer of the liquid crystal display, and the arrangement and transmission angle of the two polarizers are the same as the groove arrangement of the upper and lower interlayers. When a certain voltage is applied to the liquid crystal layer, due to the influence of the external voltage, the liquid crystal will change its initial state, and will no longer be arranged in a normal manner, but will become an upright state. Therefore, the light passing through the liquid crystal is absorbed by the second polarizer and the entire structure is opaque. As a result, black appears on the display screen. When no voltage is applied to the liquid crystal layer, the liquid crystal is in its initial state, and the direction of the incident light is twisted 90 degrees, so that the incident light of the backlight can pass through the entire structure, and the result appears white on the display screen. In order to achieve that each individual pixel on the panel can produce the color you want, multiple cold cathode tubes must be used as the backlight of the display.
2. Working principle of active matrix LCD
The structure of the TFT-LCD liquid crystal display is basically the same as that of the TN-LCD liquid crystal display, except that the electrode on the upper layer of the TN-LCD is changed to an FET transistor, and the lower layer is changed to a common electrode.
The working principle of TFT-LCD liquid crystal display is different from that of TN-LCD. The developing principle of TFT-LCD liquid crystal display is to adopt "back-illuminated" illumination. When the light source is irradiated, it first penetrates upward through the lower polarizer and transmits light by means of liquid crystal molecules. Because the electrodes on the upper and lower interlayers are changed to FET electrodes and common electrodes, the arrangement state of liquid crystal molecules will also change when the FET electrodes are turned on, and the purpose of display is also achieved by shading and light transmission. But the difference is that due to the capacitive effect of the FET transistor, it can maintain the potential state. The liquid crystal molecules that have previously been transmitted will remain in this state until the FET electrode is powered on again to change its arrangement.
Technical parameters of LCD
1. The size of the visible area LCD is the same as the actual screen range. For example, a 15.1-inch LCD display is approximately equal to the viewing range of a 17-inch CRT screen.
2. Viewing angle The viewing angle of the LCD monitor is symmetrical from left to right, but not necessarily from top to bottom. For example, when the incident light of the backlight passes through the polarizing plate, liquid crystal and alignment film, the output light has specific directional characteristics, that is, most of the light emitted from the screen has a vertical direction. If we look at a completely white screen from a very oblique angle, we may see black or distorted colors. Generally speaking, the up-down angle should be less than or equal to the left-right angle. If the viewing angle is 80 degrees to the left and right, it means that the screen image can be clearly seen when the position starts at 80 degrees from the screen normal. However, due to different human vision ranges, if you do not stand within the best viewing angle, the color and brightness you see will have errors. Now some manufacturers have developed various wide viewing angle technologies in an attempt to improve the viewing angle characteristics of liquid crystal displays, such as: IPS (In Plane Switching), MVA (MulTIdomain VerTIcal Alignment), TN + FILM. These technologies can increase the viewing angle of the LCD display to 160 degrees or more.
3. Dot pitch We often ask how big is the dot pitch of the LCD display, but most people don't know how to get this value. Now let's understand how it is obtained. For example, the general viewable area of ​​a 14-inch LCD is 285.7mm × 214.3mm, and its maximum resolution is 1024 × 768, then the dot pitch is equal to: visible width / horizontal pixels (or visible height / vertical pixels), That is 285.7mm / 1024 = 0.279mm (or 214.3mm / 768 = 0.279mm).
4. Color
The most important thing about LCD is color expression. We know that any color in nature is composed of three basic colors of red, green and blue. The LCD panel is composed of 1024 × 768 pixels, and the color of each independent pixel is controlled by three basic colors of red, green, and blue (R, G, B). Most liquid crystal displays produced by most manufacturers have each basic color (R, G, B) reaching 6 bits, that is, 64 expression levels, then each individual pixel has 64 × 64 × 64 = 262144 colors. Many manufacturers also use the so-called FRC (Frame Rate Control) technology to simulate the full-color display, that is, each basic color (R, G, B) can reach 8 bits, that is, 256 kinds of expression , Then each individual pixel has up to 256 × 256 × 256 = 16777216 colors.
5. Contrast value Contrast value is the ratio that defines the maximum brightness value (full white) divided by the minimum brightness value (full black). The contrast value of a CRT monitor is usually as high as 500: 1, so that it is easy to present a truly black screen on the CRT monitor. However, it is not easy for LCD. The backlight composed of cold cathode ray tube is difficult to switch quickly, so the backlight is always on. In order to get a completely black screen, the liquid crystal module must completely block the light from the backlight, but in terms of physical characteristics, these components cannot fully meet such requirements, and some light leakage will always occur. In general, the acceptable contrast value for the human eye is about 250: 1.
6. Brightness value The maximum brightness of a liquid crystal display is usually determined by a cold cathode ray tube (backlight). The brightness value is generally between 200 and 250 cd / m2. The brightness of the LCD monitor is slightly lower, and the screen will feel dark. Although it is technically possible to achieve higher brightness, this does not mean that the higher the brightness value, the better, because a display with too high brightness may damage the viewer's eyes.
7. Response time Response time refers to the speed at which each pixel of the liquid crystal display reacts to the input signal. Of course, the smaller the value, the better. If the response time is too long, it may make the LCD display have the feeling of trailing tail when displaying dynamic images. The response time of a general LCD is between 20 and 30ms.
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