LCD technology details
There are three forms of liquid crystal substances: solid, liquid and gas.
In 1888, when the Australian botanist Reinitzer studied the role of cholesterol in plants, he experimented with cholesterylbenzene and accidentally discovered liquid crystals, but the actual application of liquid crystals was not until the 1950s. Start. As the name suggests, liquid crystal is an intermediate state between solid and liquid. Liquid crystal is an organic compound. Within a certain temperature range, it has not only mechanical properties such as fluidity, viscosity, and deformation of the liquid, but also heat (thermal effect), light (optical anisotropy), and electricity (electro-optic effect) of the crystal. ), Magnetism (magneto-optical effect) and other physical properties. The path of light penetrating the liquid crystal is determined by the arrangement of the molecules that constitute it. It has been found that charging the liquid crystal changes its molecular arrangement, which in turn causes distortion or refraction of light.
Liquid crystals are divided into three types according to the different molecular structures: the crystal particles are clay-like called smectic phase (SmecTIc) liquid crystals, the thin matchsticks are called nematic phase (NemaTIc) liquid crystals, and the cholesterol-like ones are called bile Steroid (CholesTIc) liquid crystal. The physical characteristics of these three kinds of liquid crystals are not the same. The second type of nematic (NemaTIc) liquid crystal is used for liquid crystal displays.
The principle of LCD can only be targeted when purchasing, and more scientific and reasonable when it is applied and maintained only by first understanding its structure and principle, and its technical and technological characteristics. Liquid crystal is an organic compound composed of long rod-shaped molecules. In a natural state, the long axes of these rod-shaped molecules are roughly parallel. The first characteristic of LCD is that liquid crystal must be poured between two planes with narrow grooves to work properly. The grooves on these two planes are perpendicular to each other (90 degrees intersecting), that is to say, if the molecules on one plane are arranged from north to south, the molecules on the other plane are arranged from east to west, and the molecules between the two planes Forced into a 90-degree twisted state. Because the light travels in the direction of the molecular arrangement, the light is also twisted 90 degrees when passing through the liquid crystal. But when a voltage is applied to the liquid crystal, the molecules will be rearranged vertically, so that the light can be emitted directly without any twisting. The second characteristic of LCD is that it relies on the polarization filter and the light itself. Natural light is randomly scattered in all directions. The polarization filter is actually a series of increasingly thin parallel lines. These lines form a net, blocking all light that is not parallel to these lines. The line of the polarizing filter is just perpendicular to the first one, so it can completely block the light that has been polarized. Only when the lines of the two filters are completely parallel, or the light itself has been twisted to match the second polarization filter, can the light penetrate. The LCD is composed of two polarizing filters that are perpendicular to each other, so under normal circumstances, all light trying to penetrate should be blocked. However, since the two filters are filled with twisted liquid crystal, after the light passes through the first filter, it will be twisted 90 degrees by the liquid crystal molecules, and finally pass through the second filter. On the other hand, if you apply a voltage to the liquid crystal, the molecules will be rearranged and completely parallel, so that the light is no longer twisted, so it is just blocked by the second filter. In short, the light is blocked by power, and the light is emitted without power. Of course, the arrangement of the liquid crystals in the LCD can also be changed so that the light is emitted when powered on and not blocked when not powered. However, since the LCD screen is almost always on, only the "power off light" solution can achieve the most power-saving purpose.
The classification of LCD can divide LCD into passive technology and active technology. The representative products are DSTN (double-layer supertwist nematic) and TFT (thin film transistor). DSTN has always been the standard for passive notebook displays, and HPA and CSTN are the latest improvements in passive technology. HPA is also known as high-performance addressing or fast DSTN. Both HPA and CSTN provide better contrast and brightness than DSTN. CSTN's response time has now dropped to 100ms, and provides a 140-degree viewing angle.
DSTN is developed from Super Twisted Nematic Display (STN). Because DSTN uses dual scanning technology, the display effect is greatly improved compared with STN. When the notebook computer first appeared, it mainly used STN. STN's response time is slow, generally around 300ms, and users can feel the tailing (afterglow). Because the DSTN is scanned simultaneously on both the top and bottom screens, a bright line may appear in the center of the display during use.
Active matrix display screens are directly addressed by thin film transistors, which is also the origin of the technical name, that is, TFT (thin film transistor). TFT is one of the active matrix liquid crystal displays, and the response time is greatly improved, reaching 25ms. It has higher contrast and richer colors. Compared with DSTN, the main feature of TFT is that each pixel is equipped with a semiconductor switching device, and its processing technology is similar to large-scale integrated circuits. Since each pixel can be directly controlled by the dot pulse, each node is relatively independent and can be continuously controlled, which not only improves the reaction time, but also can be very accurate in grayscale control, which is that the TFT color is more realistic than DSTN s reason. At present, most mainstream products of notebook computer manufacturers use TFT display.
LCD and CRT (traditional display) comparison and precautions when buying
We have introduced the working principle of LCD, then introduce CRT again, then we compare. The working principle of CRT is that an electron gun is composed of a filament, a cathode, and a control grid. After being energized, the filament heats up, the cathode is excited, and an electron flow is emitted. The electron flow is accelerated by an internal metal layer with a high voltage, and is focused by a lens to form a very fine The electron beam hits the fluorescent screen to make the phosphor glow. Under the action of the magnetic field generated by the deflection yoke, the electron beam can be directed to the designated position of the fluorescent screen. When the electron beam hits the fluorescent screen, it will form a light-emitting point, and several light-emitting points can form an image. When the RGB tri-color fluorescent dots are hit by electron beams of different intensities, various colors will be produced. By controlling the strength and on and off of the electron beams, various colorful pictures can be formed. Generally, a shadow mask-like picture tube has a mesh-like screen inside. The electron beam hits the fluorescent dots arranged in a triangle through the mesh. The three electron guns correspond to the RGB three colors, so it is called the "three-gun three-beam" picture tube. The principle of the shaded picture tube (such as Trilon and Diamond Dragon) is the same, except that the screen of this kind of picture tube is formed by fixing many gratings in the frame longitudinally.
The following is a detailed introduction to their differences:
Resolution Resolution is a very important performance indicator. It refers to the number of points that can be displayed on the screen in the horizontal and vertical directions (the lines and areas displayed on the screen are composed of points). The higher the resolution, the more information can be contained in the same screen. For a CRT that can support 1280x1024 resolution, whether it is 320x240 or 1280x1024 resolution, it can be expressed perfectly (because the electron beam can be elastically adjusted). But its maximum resolution may not be the most suitable resolution, because if the resolution of 1280x1024 is reached on a 17-inch display, the font of WINDOWS will be very small, and the eyes will be tired easily after a long time, so the best resolution of the 17-inch display Should be 1024x768.
This is not the case for LCDs. The maximum resolution of the LCD is its true resolution, which is the best resolution. Once the set resolution is less than the real resolution (for example, 15-inch LCD, the real resolution is 1024x768, and the resolution set in WINDOWS is 800x600), there will be two display methods. The first is centered display. Only the 800x600 points in the middle of the LCD will display the image, and the other unused points will not emit light, keeping a dark background. It seems that the picture is centered and reduced. The other is the extended display. This method uses every pixel on the screen, but because the pixels are easily distorted, it will have a certain impact on the display effect. So anyway, when choosing LCD, it should be noted that the resolution is not as large as possible but appropriate and easy to use.
Refresh rate For CRT, the graphic image on the screen is composed of fluorescent dots that emit light due to the impact of the electron beam. Since the phosphor in the kinescope is hit by the electron beam for a short time, the electron beam must be constantly Hit the phosphor to make it glow continuously. The electron gun starts from the first line in the upper left corner of the screen (the number of lines is determined by the resolution of the display at the time, for example, at 800X600 resolution, the electron gun will scan 600 lines), scanning line by line from left to right, the first line scans After that, start from the leftmost end of the second line to the rightmost end of the second line, and then scan the entire screen and then start from the upper left corner of the screen. At this time, the screen is refreshed once and again. This way we can understand why the higher the resolution of the display, the lower the maximum refresh rate it can achieve. Generally speaking, the refresh rate of the screen must be above 75HZ, and it is not easy for human eyes to feel the flicker of the screen. The refresh rate of the CRT monitor is determined by its line frequency and the resolution at the time. The higher the refresh rate is; while the line frequency is fixed, the higher the resolution, the lower the refresh rate it can achieve. For LCD, there is no refresh rate problem, it does not need to refresh at all. Because each pixel in the LCD continues to emit light until the voltage that does not emit light changes and is sent to the controller, the LCD will not have the flicker phenomenon caused by "continuous charging and discharging."
Viewing angle The viewing angle of most flat screen monitors can reach 180 degrees, that is to say, the displayed content can be clearly seen from any direction in front of the screen. The LCD is different. The viewing angle is different depending on whether the process is advanced or not. Some new products have a viewing angle of about 160, which is very close to the 180 degree of CRT. Some LCDs have a nominal viewing angle of 160 degrees, but in fact they do not meet this standard. Once the user's viewing angle exceeds its actual viewing range during use, the color of the screen will fade, darken, and even the positive image becomes a negative image. It is very likely that everyone is confused by Philips advertisements. In fact, the viewing angle of LCD is not very large, but it is much smaller than CRT. It is a place that is obviously weaker than CRT. Of course, the situation will be better if the manufacturer adds the technology of increasing the angle of view to the product. Introduce below.
TN + Film (TN + Viewing Angle Expanding Film) technology is structurally speaking, the liquid crystal display uses "liquid crystal" as the display material. Liquid crystal is a substance between solid and liquid. It will show a transparent liquid state at a certain temperature, and after cooling, it will become a turbid solid state with crystalline particles. The liquid crystals are divided into three types according to the molecular structure: Smectic liquid crystals similar to clay, Nematic liquid crystals similar to thin matchsticks, and Cholestic liquid crystals similar to cholesterol. The physical characteristics of these three kinds of liquid crystals are not the same. Usually, the second type of Nematic liquid crystal is used for liquid crystal displays. The liquid crystal display manufactured by this type of liquid crystal is also called LCD (Liquid Crystal Display). The arrangement of liquid crystal molecules on the upper layer of an ordinary liquid crystal screen is horizontal, the arrangement of liquid crystal molecules on the lower layer is vertical, and the liquid crystal molecules between the upper and lower layers are arranged horizontally close to the upper layer, and those arranged close to the lower layer are arranged vertically. Overall, the arrangement of liquid crystal molecules is like a spiral rotation arrangement, but the liquid crystal molecules of liquid crystal displays based on TN + viewing angle expansion film technology are arranged perpendicular to the display screen, so a special layer is added on the surface of the upper layer. The film can increase the viewing angle. Technically, the technology is based on the more mature standard TFT-Twisted Nematic (twisted nematic) liquid crystal technology. As long as a special film (turning film) is added to the upper surface of the substrate, the horizontal viewing angle can be increased from 90 degrees to 140 degrees. The advantages of this technology are self-evident, that is, relatively cheap and more mature technology, high yield. But the shortcomings of this technology are also obvious, that is, the inherent shortcomings of lower contrast and slower response speed have not changed qualitatively.
IPS (In-board switching or Super-TFT) technology
IPS or "in-board switching" technology was first developed by Hitachi (Hitachi), and now NEC and Nokia (Nokia) also use this technology to produce TFTs.
principle:
The biggest difference between IPS and TN + Film (twisted nematic liquid crystal + viewing angle expansion film combination) technology is that the direction of liquid crystal molecules is parallel to the substrate rather than perpendicular to the substrate. This is achieved by applying voltage.
Using IPS or Super TFT technology can expand the viewing angle to 170 degrees, basically the same viewing angle as CRT monitors. But this technology also has shortcomings, because the arrangement direction of the liquid crystal molecules makes the electrodes must be comb-shaped and placed on the underlying glass substrate, not like the TN mode (the molded TN liquid crystal display usually includes a glass substrate, ITO film, Plywood made of alignment films, polarizing plates, etc., has two layers, called upper and lower interlayers. Each interlayer contains electrodes and grooves formed on the alignment film. The upper and lower interlayers contain liquid crystal molecules) and are placed on two layers of glass substrates. on. Doing so will reduce the contrast, so you must increase the backlight to achieve the required brightness. Compared with the TN + Film (TN + Viewing Angle Expanding Film) technology, the contrast and response time in the same IPS mode are not much improved compared with the traditional TFT-TN.
3 MVA (Multi-Domain Vertical Alignment) technology
MVA technology was developed by Fujitsu. From a technical point of view, MVA should be the best solution for LCD monitors with wide viewing angle and short response time. The MVA technology enables a viewing angle of 160 degrees and a response time of 20ms. In MVA technology, M stands for "multi-domain", which refers to the use of convex objects in a single color unit to form multiple regions. VA stands for "Vertical Alignment". Due to the protrusions, the liquid crystal molecules are not aligned vertically at rest. When a voltage is applied to generate an electric field, the liquid crystal molecules become aligned horizontally, so that the light emitted by the backlight can pass through the various layers. MVA technology can provide a shorter response time than TN + viewing angle expansion film technology and IPS technology, which is very important for the performance of video and games. Contrast has also been improved, but it will change with the angle of view.
TN + Film (TN + Viewing Angle Expansion Film) technology has low cost, high yield, and a viewing angle of 140 degrees, without much improvement in contrast and response time. IPS (Internal Switching or Super-TFT) technology: the viewing angle is 170 degrees, and the contrast and response time are not greatly improved. MVA (Multi-Domain Vertical Alignment) technology has a viewing angle of 160 degrees, a large improvement in contrast and response time, and is suitable for the playback of videos and games.
Visible area Visible area refers to the area of ​​the part of the screen that can be used to display images in practical applications. Because the size of the CRT display is actually the size of its picture tube, the part that can be used to display the image cannot reach this size at all, because the frame of the picture tube takes up part of the space. Generally speaking, the viewing area of ​​a 17-inch CRT monitor is about 15.8-16 inches, while the viewing area of ​​a 15-inch CRT monitor is only about 13.8 inches. But for LCD, the nominal size is basically the size of the visible area, the space occupied by the border is very small, the visible area of ​​the 15-inch LCD is about 14.5 inches, which is why the LCD looks better than the same The reason for the larger CRT size. So when buying LCD, 15 villages is basically enough.
The display function of the brightness and contrast LCD is mainly to have a backlight light source. The brightness of this light source determines the brightness and color saturation of the entire LCD screen. In theory, the higher the brightness of the LCD display, the better. The unit of measurement for brightness is cd / m2 (candle per square meter), also called NIT lumens. At present, most of the brightness of the TFT screen starts from 150Nits, and usually 200Nits can show a better picture. Contrast is the contrast measurement of different levels of black and white colors. When the contrast ratio is 120: 1, vivid and rich colors can be displayed (because the human-resolvable contrast ratio is about 100: 1), and the contrast ratio of up to 300: 1 can support colors of all levels. At present, most LCD monitors have a contrast ratio of about 100: 1 to 300: 1. At present, there is no set of fair standard values ​​to measure the contrast between brightness and contrast, so the purchase of LCD all depends on a pair of sharp eyes. Therefore, pay attention to this index when purchasing LCD. It is also the biggest part of the difference in performance of LCD products. It is estimated that it is difficult to purchase.
Response Speed ​​The unit of time for measuring the response speed is milliseconds (ms), which refers to the time required for a pixel to change from light to dark and from dark to two. The smaller the value, the better, and the smaller the value, the faster the response. At present, the response speed of mainstream LCDs is all above 25ms. In general commercial applications (such as word processing or text processing), it does not matter much, because such applications do not have to care too much about the reaction time of the LCD. When it is used to play games and watch full-screen high-speed dynamic images such as VCD / DVD, the response time is particularly important. If the response time is longer, the screen will appear smearing and afterimages. As a simple example, most LCD monitors on the market now have varying degrees of tailing when playing QUAKE3, especially when the screen is updated at high speed. CRT does not have this problem at all, because the response time of CRT is only 1ms, and there will be no tailing.
When it comes to colors, LCDs are not as good as CRTs. In theory, the colors that CRTs can display are as infinite as TVs. The LCD can only display about 260,000 colors, most products claim to be able to display 16.77 million colors (16777216 colors, 32 bits), but in fact all are realized by dithering algorithm, and the real 32 bits There is still a big gap between colors, so the color expression and transition are still not as good as traditional CRT. For the same reason, LCD's ability to express grayscale is not as good as CRT. If you have the conditions, you can compare it yourself: find a 17-inch Trinitron CRT monitor, and then set up a 15-inch LCD to display an image of 16.77 million colors at the same time. The screen displayed by the CRT is very bright, and the LCD is a bit "false". Although it is not correct, it looks comfortable without the CRT.
Let's talk about the CRT first. The most domestic CRTs currently have problems with focusing, convergence, and breathing effects to varying degrees. This is inseparable from the manufacturer's technology. If the relevant control circuit designed by the manufacturer is not advanced enough, the problems mentioned above can easily occur. This is also the reason why Trillon CRTs, SONY original monitors and other manufacturers' monitors have very different performances. The LCD does not have any problems such as focusing, because it does not need to focus at all. However, LCD and linear distortion problems may occur, but CRT is more likely to appear.
Radiation Because the light of the CRT display will be emitted through the cathode tube and also emit radiation, it is very unfavorable to the human body, but then the TCO9X requirement is that the CRT has been greatly improved in this regard. However, due to its working principle, LCD will not emit a little radiation when working, which is much stronger than CRT. Therefore, it is considered that it is more appropriate for the general family to use CRT. The multimedia effect will be better and the price is relatively cheaper. The LCD liquid crystal display is more suitable for commercial use, and the stock trading and media editing are more suitable for LCD.
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