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Liquid crystal display screen

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A type of flat screen display

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Liquid Crystal Display, English abbreviation LCD full name is liquid crystal display, is a flat display. For TV sets and computer The screen display. The advantages of the display are low power consumption, small size and low radiation.

The LCD uses a liquid crystal solution in two polarized materials, so that when an electric current passes through the liquid, the crystals are rearranged for the purpose of imaging.

Chinese name: Liquid crystal display screen
Foreign name: LCD

use: Screen display for televisions and computers
Driving mode: Static drive, simple matrix, active matrix

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Brief introduction

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EDITOR

Liquid crystal Display (LCD) A type of display used in digital clocks and many portable computers. The LCD display uses two polarized materials with a liquid crystal solution between them. When an electric current passes through the liquid, it rearranges the crystals so that light cannot pass through them. Thus, each crystal acts like a shutter, both allowing light to pass through and blocking it. Liquid crystal display (LCD) At present, the scientific and technological information products are developing towards the goal of light, thin, short and small, and the display products with a long history in the computer periphery are of course no exception. Under the premise of ease of carrying and handling, traditional display methods such as CRT image tube displays and LED display boards, etc., are subject to factors such as large volume or huge power consumption, and can not meet the actual needs of users. The development of liquid crystal display technology is just in line with the current trend of information products, whether it is right-angle display, low power consumption, small size, or zero radiation advantages, can allow users to enjoy the best visual environment.

The birth of liquid crystals

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Liquid crystal display screen

To trace the origin of liquid crystal displays, we must first start from the birth of "liquid crystal". In 1888 AD, a Austria A botanist, Friedrich Reinitzer, discovered a special substance. He extracted a compound called helical toluate from plants, and when doing heating experiments for this compound, he accidentally found that this compound had two melting points at different temperatures. And its state is between the Liquid and solid substances that we are generally familiar with, somewhat similar to the gelatinous solution of soapy water, but it has the properties of liquid and crystalline substances in a certain temperature range, and because of its unique state, it was later named "Liquid Crystal", that is, the meaning of liquid crystalline substances. However, although liquid crystals were discovered as early as 1888, it was not until 80 years later that they were really used in everyday objects.

Liquid crystal display screen

In 1968, at the Sarnoff Research and Development Center of RCA (the invention company of radio and television) in the United States, engineers discovered that liquid crystal molecules can be affected by voltage, change the arrangement of their molecules, and can deflect incoming light. Using this principle, RCA invented the world's first screen using liquid crystal displays. After that, liquid crystal display technology is widely used in general electronic products, such as calculators, electronic watches, Mobile phone screen Instruments used in hospitals (because of radiometric considerations) or screens on digital cameras. It is interesting to note that liquid crystals were discovered more than vacuum tubes or The cathode ray tube It's still early, but the world didn't know much about this phenomenon until 1962, when the first one was published, by Joe, a chemist in the RCA research group. In the book published by Joe Castellano. As with the image tube, these two technologies were invented by RCA in the United States, but were respectively developed by Japan New power (Sony) and Sharp (Sharp) The two companies have thrived.

Main classification

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Liquid crystal display screen

Liquid crystal display, according to the drive mode can be divided into Static drive (Static), Simple matrix Drive (Simple Matrix) and Active Matrix drive (Active Matrix) three. Among them, the passive matrix type can be divided into Twisted Nematic type; TN), Super Twisted Nematic (Super Twisted Nematic; STN) and other passive matrix driven liquid crystal displays; Active matrix type can be roughly divided into Thin Film Transistor type (Thin Film Transistor; TFT) and two-terminal diode type (Metal/Insulator/Metal; MIM) two ways.

TN, STN and TFT type liquid crystal displays differ in viewing Angle, color, contrast and animation display quality due to the difference in the use of liquid crystal molecular torsion principle, so that they are also significantly different in the application range of products. In terms of the scope and level of application of liquid crystal display technology at present, the active matrix drive technology is based on thin film transistor type (TFT) as the mainstream, which is mostly used in notebook computers and animation and image processing products. The simple matrix drive technology is mainly torsional nematic (TN) and super torsional nematic (STN), and the current application is mainly in the word processor and consumer products. In the midst of this, TFT liquid crystal display The required capital investment and technical requirements are high, while the technical and financial requirements required by TN and STN are relatively low.

Operating principle

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Figure 1 LCD screen

At present, liquid crystal display technology mostly uses TN, STN, TFT three technologies as the main axis, so we will discuss their operating principles from these three technologies. TN type liquid crystal display technology can be said to be the most basic in liquid crystal displays, and then other types of liquid crystal displays can be said to be improved with TN type as the origin. Similarly, its operation principle is also simpler than other technologies, please refer to the picture below. FIG. 1 shows a simple construction diagram of a TN-type liquid crystal display, including vertical and horizontal polarizers, orientation films with fine grooves, liquid crystal materials, and conductive glass substrates. The development principle is to place the liquid crystal material in the transparency of two vertical polarizing plates attached to the optical axis Conductive glass Once in a while, the liquid crystal molecules will rotate in the direction of the thin groove of the film, and if the electric field is not formed, the light will smoothly beam through the polarizer, rotate its direction according to the liquid crystal molecules, and then shoot out from the other side. If after the two conductive glasses are energized, an electric field will be caused between the two glasses, which will affect the arrangement of liquid crystal molecules, so that the molecular rod is twisted, and the light can not penetrate, and then cover the light source. The phenomenon of light and dark contrast obtained in this way is called the twisted nematic field effect (TNFE). Liquid crystal displays used in electronic products are almost all made of the principle of torsional nematic field effects. The display principle of STN type is similar, except that the liquid crystal molecule of TN torsional nematic field effect rotates the incident light by 90 degrees, while the STN supertorsional nematic field effect rotates the incident light by 180~270 degrees. To explain here is that the simple TN LCD itself only light and dark two situations (or black and white), and there is no way to do color change. The STN LCD involves the relationship between liquid crystal materials and the interference of light, so the colors displayed are mainly light green and orange. But if you add one to a traditional monochrome STN LCD display Color filter (color filter), and the monochrome display matrix of any pixel (pixel) is divided into three sub-pixels (sub-pixel), respectively through the color filter to display red, green, blue three primary colors, and then through the three primary color ratio harmony, can also show the color of the full color mode. In addition, if the TN type LCD display screen is larger, its screen contrast will appear poor, but by STN's improved technology, you can make up for the lack of contrast.

Liquid crystal display (3 photos)

Liquid crystal display screen

TFT type liquid crystal display is more complex, the main composition includes, fluorescent tube, light guide plate, polarizer, filter plate, glass substrate, direction film, liquid crystal material, thin mode transistor and so on. First of all, the liquid crystal display must first use the backlight, that is, the fluorescent tube to project the light source, these light sources will first pass through a polarizer and then through the liquid crystal, then the arrangement of liquid crystal molecules and then change the Angle of light through the liquid crystal. The light then has to pass through a colored filter film in front of it and another polarizer. Therefore, as long as we change the voltage value of the stimulated liquid crystal, we can control the light intensity and color of the final appearance, and then can Liquid crystal panel There are different shades of color combinations.

Driving mode

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Figure 2 LCD screen

In the liquid crystal display of TN and STN type, the way of simply driving the electrode is driven by the cross mode of X and Y axis, as shown in Figure 2, so if the display part is bigger and bigger, the reaction time of the electrode in the central part may be longer. In order to make the screen display consistent, the overall speed will be slower. To make it simple, it's like it's CRT display The screen update frequency is not fast enough, then the user will feel the screen flicker and beat; Or when you need a fast 3D animation display, but the display speed can not keep up, the display may have a delay phenomenon. As a result, early LCD monitors were limited in size and were not suitable for watching movies or playing 3D games.

In order to improve this situation, liquid crystal display technology was later driven by active-matrix addressing, which is currently the ideal device for achieving high data density liquid crystal display results with extremely high resolution. The method is to use a silicon transistor electrode made of thin film technology, and use the scanning method to select the on and off of any display point (pixel). This is actually the use of thin film transistor nonlinear function to replace the difficult to control the nonlinear function of the liquid crystal. As shown in Figure 2, in the TFT LCD, the conductive glass is painted with a network of small lines, and the electrodes are matrix switches arranged by thin film transistors. At the intersection of each line, there is a control box. Although the drive signal is quickly scanned at each display point, However, only the selected display point in the transistor matrix on the electrode gets enough voltage to drive the liquid crystal molecule, so that the liquid crystal molecule axis turns into a "bright" contrast, and the unselected display point is naturally the "dark" contrast, and therefore avoids the display function's reliance on the electric field effect ability of the liquid crystal.

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