The liquid crystal market has been in full swing since the beginning of 200 1. First, Samsung launched a 24-inch large-screen LCD, then Philips launched a streamlined LCD, and then EMC launched six LCD monitors at one time, which set off a whirlwind of LCD in the display market. Generally speaking, due to the innate technical advantages, liquid crystal displays are quietly heating up, which is likely to replace CRT displays. So, what are the technical advantages of LCD? Let's have a look.
Technical advantages of liquid crystal display
Smaller and lighter traditional CRT displays need to use CRT technology to build a vacuum CRT for imaging, and then an electron gun is equipped at the end, so the length is generally more than 30 cm, and the volume of the whole display is of course larger. Liquid crystal display uses liquid crystal materials, and then uses the corresponding imaging technology to achieve the display purpose. There is no need to install a picture tube inside the display, and the volume is of course small.
Due to the limitation of display technology, the mark size of traditional CRT display with large display area is smaller than that of fluorescent screen. General 15 inch CRT display is marked with the size of 15 inch, but the actual visual range may only be about 14. 1 inch, while 17 inch display may only have 17 inch left. However, due to the different imaging principles of LCD, its marked size is the actual display area. For example, the display area of Samsung's 15 inch LCD is completely 15 inch, which is equivalent to the display area of a 17 inch CRT display. If the prices of the two are similar, it is certainly more cost-effective to buy an LCD.
A zero-radiation, flicker-free CRT display is imaged by a cathode ray tube, and the electron beam contained in it will generate a lot of static electricity and radiation when it works. The faster the electron beam runs, the greater its radiation, which will cause harm to eyes and skin for a long time, causing myopia, skin allergy and other problems. The liquid crystal display is made of liquid crystal material, so there is no need to use electron beam when working, so there is no static electricity and radiation problem that affects vision. In addition, a picture of CRT display is formed by scanning, and flicker will not appear only when the scanning frequency reaches a certain value, while liquid crystal display does not need scanning process, and forms a picture almost at the same time, even if the refresh frequency is very low, flicker will not appear.
Low power consumption, strong anti-interference ability CRT display has the power consumption of display besides circuit and picture tube, while LCD display mainly consumes the power consumption of backlight and circuit, and the power consumption of its display can be ignored. In addition, because the liquid crystal display does not use a picture tube and an electron gun for imaging like a CRT display, there is no need to consider the high radiation effect brought by the electron beam emitted by the electron gun, but only the backlight emitted by the cathode fluorescent lamp is used to obtain brightness, so the anti-interference ability is stronger, and even in the environment with concentrated light, it will receive good display effect.
Traditional CRT monitors mostly adopt analog display mode, and the displayed signal output adopts analog output mode, which may cause image loss and image quality degradation during transmission, while the signal transmission of LCD monitors adopts digital mode, and the digital signal is directly output by the graphics card, which will not cause signal loss. But at present, most liquid crystal displays still use VGA interface for analog display, and only a few manufacturers such as Acer, EMC and Samsung have set up digital video signal interfaces.
Use the function more intelligently. Due to the different materials and processes used in LCD, some of its parameters are generally fixed, which requires more intelligent performance adjustment of the display. In this regard, each manufacturer has its own mature technology.
A leap in applied materials
One of the reasons why LCD has so many advantages is that it uses liquid crystal as the main imaging material. Traditional CRT monitors use ultra-thick glass display screens. Although the outer surface is as flat as the LCD screen, the inner surface is somewhat curved, which seems to have a concave phenomenon, and the image will be slightly distorted. The basic material of LCD is liquid crystal, which has both the fluidity of liquid and the regular arrangement of crystals. When heated to a certain extent, the liquid crystal will become a transparent liquid, and after cooling, it will show the characteristics of crystals. Because the characteristics of liquid crystal are between solid and liquid, it has both the optical characteristics of solid crystal and the flow characteristics of liquid. Liquid crystal displays use this feature to achieve the purpose of imaging.
Because liquid crystal has both solid optical characteristics and liquid flow characteristics, when light enters the liquid crystal substance, it will inevitably follow the arrangement of liquid crystal molecules and produce natural deflection. The electronic structure in liquid crystal molecules has strong electron yoke mobility, so when liquid crystal molecules are subjected to an external electric field, it is easy to change the arrangement mode, thus changing the light propagation mode accordingly. Acer, EMC, Samsung and other manufacturers of liquid crystal display products, is to use the photoelectric effect of liquid crystal, through the external voltage control, and then through the refractive characteristics of liquid crystal molecules and the ability to rotate light to control the light and dark state (or visual optical contrast), so as to achieve the purpose of imaging.
Imaging principle of liquid crystal display
At present, most liquid crystal technologies are TN, STN and TFT. This paper discusses the imaging principle of LCD from these three technologies.
TN LCD technology is the most basic LCD, and other types of LCD are improved based on TN, so its working principle is simpler than other technologies. It mainly includes vertical and horizontal polarizers, orientation films, liquid crystal materials and conductive glass substrates. The imaging principle is that the liquid crystal material is placed between two transparent conductive glasses, and the liquid crystal molecules will be arranged in turn according to the direction of the thin grooves of the alignment film. If no electric field is formed, light will enter smoothly from the polarizer, rotate in the direction of liquid crystal molecules, and then exit from the other side. If two pieces of conductive glass are electrified, an electric field will be formed between the two pieces of glass, which will further affect the arrangement of liquid crystal molecules between them and distort their molecular rods, so that light cannot penetrate and cover the light source. The contrast between light and dark obtained in this way is called TNFE (Twisted Nematic Field Effect). Almost all liquid crystal displays used in electronic products are made by using the twisted nematic field effect principle. However, because the simple TN liquid crystal display itself only has light and dark conditions, it can only form black and white colors, and there is no way to change the color.
The display principle of STN type is similar to TN, except that TN twisted nematic field effect liquid crystal molecules rotate incident light by 90 degrees, while STN super twisted nematic field effect rotates incident light by 180 ~ 270 degrees. This difference leads to the interference of light, which realizes a certain degree of color change and makes STN LCD have some light green and orange tones. If a color filter is added, and any pixel of the monochrome display matrix is divided into three sub-pixels, the three primary colors of red, green and blue are displayed by the color filter respectively, and then the color of the full-color mode can be displayed by coordinating the proportions of the three primary colors.
The main components of TFT LCD include cathode fluorescent lamp, light guide plate, polarizer, filter plate, glass substrate, orientation film, liquid crystal material, thin transistor and so on. This kind of liquid crystal must first project a light source with a fluorescent tube, and the light source will first pass through a polarizer and then through the liquid crystal. At this time, the arrangement of liquid crystal molecules will change the angle of light passing through the liquid crystal, and then these light must pass through the front color filter film and another polarizer. We can control the intensity and color of the final light by changing the voltage value of stimulating the liquid crystal, and then we can change the color combination of different shades on the liquid crystal panel. The difference between TN and the former two is that the upper interlayer electrode of TN is changed to FET transistor, and the lower layer is changed to * * * common electrode.
Liquid crystal molecular driving technology
The driving modes adopted by the three liquid crystal displays are also different. Generally, the first two adopt simple matrix driving mode, while the latter adopts active driving mode.
The simple matrix driving mode consists of vertical electrode and horizontal electrode, the driven part is controlled by horizontal voltage, and the vertical electrode is responsible for driving liquid crystal molecules. In TN and STN LCD, an intermediate layer made of a glass substrate, an ITO film, an orientation film, a polarizer, etc. is adopted, which has two layers. Each interlayer includes electrodes and grooves formed on the orientation film. In the upper interlayer and the lower interlayer, the liquid crystal molecules near the upper interlayer are arranged in the direction of the upper groove, while the liquid crystal molecules in the lower interlayer are arranged in the direction of the lower groove. The upper and lower grooves are criss-crossed, that is, the arrangement of liquid crystal molecules in the upper layer is horizontal and the arrangement of liquid crystal molecules in the lower layer is vertical, while the arrangement of liquid crystal molecules between the upper and lower layers is horizontally close to the upper layer and vertically close to the lower layer. On the whole, the arrangement of liquid crystal molecules is like a spiral twisted row. However, the disadvantage of this technology is that the display part cannot be too large. If the display part is too large, the electrode reaction time in the middle part may be longer, and the overall speed will slow down in order to make the screen display consistent. Simply put, it seems that the screen update frequency of CRT display is not fast enough, so users will feel the screen flicker and jump; Or when fast 3D animation display is needed, but the display speed of the monitor can't keep up, the display may be delayed. Therefore, the early liquid crystal displays were limited in size and were not suitable for watching movies or playing 3D games.
Active matrix driving mode is to make each pixel correspond to a group of electrodes, which is a bit like the cyclic mode of DRAM. The voltage changes the state of each pixel by scanning (or charging for a certain time). This method is a silicon transistor electrode made of thin film technology, and the on-off of any display point is selected by scanning method. In fact, the nonlinear effect of thin film transistor is used to control the uncontrollable nonlinear effect of liquid crystal. In EMC's BM-568, the conductive glass is painted with fine mesh lines, and the electrodes and thin film transistors are arranged in a matrix switch. A control box is formed at the intersection of each line. Although the driving signal is rapidly scanned at each display point, only the selected display point in the transistor matrix on the electrode gets enough voltage to drive the liquid crystal molecules, so that the axis of the liquid crystal molecules turns into a bright state, and the unselected display points are naturally in a dark state, thus avoiding the electric field effect of the display function on the liquid crystal.
TN, STN and TFT liquid crystal displays are different in viewing angle, color, contrast and animation display quality due to the different twisting principles of liquid crystal molecules. Among them, TFT liquid crystal display requires high capital investment and technology, and high requirements for manufacturers, while TN and STN require relatively low technology and capital.