LCD screen, types of LCD screen, principle of LCD screen
The quality of a liquid crystal display first depends on its panel, because the quality of the panel directly affects the viewing effect of the picture, and the LCD TV panel accounts for more than half of the cost of the whole machine, which is the main factor affecting the cost of the LCD TV, so to choose a good LCD display, we must first choose its panel. The LCD panel can largely determine the brightness, contrast, color, viewing Angle and other very important parameters of the LCD display. The speed of development of LCD panels is very fast, from the three generations of previous years, rapid development to four generations, five generations, and then skip six generations to reach seven generations, and the updated eighth generation panel is also being planned. At present, the manufacturers of LCD panels are mainly Samsung, LG-Philips, AU, etc., due to the differences in technical levels, the LCD panels produced are roughly divided into different types of machines. Common VA-type panels include TN panel, MVA panel, and PVA panel, IPS panel, and CPA panel.
1. TN panel
TN full name TwistedNematic(twisted nematic) panel, low production costs make TN become the most widely used entry-level LCD panel, in the current market mainstream in the low-end LCD is widely used. At present, most of the TN panels we see are improved TN+film, film is a compensation film, used to make up for the lack of TN panel viewing Angle, the current improved TN panel viewing Angle is up to 160°, of course, this is the manufacturer in the contrast of 10∶1 in the case of the limit value measured. In fact, when the contrast is reduced to 100:1, the image has been distorted and even biased.
As a 6Bit panel, TN panel can only display red/green/blue 64 colors, the maximum actual color is only 262.144, through the “dithering” technology can make it obtain more than 16 million color performance ability, can only display 0 to 252 gray level of the three primary colors, so the final color display information is 16.2M color. Instead of what we usually call true color 16.7M color; Coupled with the difficulty of improving the contrast of the TN panel, the directly exposed problem is that the color is thin, the reduction ability is poor, and the transition is not natural.
The advantage of TN panel is that due to the small number of output gray class, the liquid crystal molecule deflection speed is fast, and the response time is easy to improve, and the liquid crystal products below 8ms on the market are basically used by TN panel. In addition, Samsung has developed a B-TN(Best-TN) panel, which is actually an improved version of the TN panel, mainly in order to balance the high-speed response of the TN panel must sacrifice the contradiction of picture quality. At the same time, the contrast ratio can reach 700:1, which is already close to the MVA or early PVA panels. Many panel manufacturers in Taiwan produce TN panel, TN panel is a soft screen, gently paddling with your hand will appear similar water patterns, in addition to carefully look at the screen is roughly like this:
2. VA panel
The VA panel is now the most used panel type of high-end liquid crystal, which belongs to the wide Angle panel. Compared with the TN panel, the 8bit panel can provide 16.7M color and large viewing Angle is the capital of the high-end positioning of this type of panel, but the price is also more expensive than the TN panel. The VA panel can be divided into the MVA panel led by Fujitsu and the PVA panel developed by Samsung, of which the latter is the inheritance and improvement of the former. The front (face) contrast of the VA class panel is the highest, but the uniformity of the screen is not good enough, and color drift often occurs. Sharp text is its killer, black and white contrast is quite high.
Fujitsu’s MVA technology (Multi-domain Alignment, multi-quadrant vertical alignment technology) can be said to be the earliest wide-angle LCD panel technology. This type of panel can provide a greater viewing Angle, usually up to 170°. Through technology authorization, panel companies such as Chi Mei Electronics (Chi Jing Optoelectronics) and AU Optronics in Taiwan Province have adopted this panel technology. The improved P-MVA class panel can be viewed at a near-horizontal 178° Angle and has a gray response time of less than 8ms
Samsung Samsung electronics technology of PVA (PatternedVerticalAlignment) also belongs to the category of VA technology, it is the successor of the MVA technology and development. Its overall quality has been comprehensively surpassed by the latter, and the improved S-PVA has been able to keep pace with the P-MVA, obtaining extremely wide viewing angles and faster and faster response times. PVA uses transparent ITO electrode to replace the liquid crystal layer bulges in MVA. Transparent electrode can obtain better opening rate and minimize the waste of backlight. This mode greatly reduces the possibility of “bright spots” in the LCD panel, and its status in the LCD TV era is equivalent to the “long tube” in the picture tube TV era. Samsung’s PVA mode wide perspective technology, due to its strong production capacity and stable quality control system, is widely used by Japanese and American manufacturers. At present, PVA technology is widely used in middle and high-end LCD display or LCD TV. VA panel is also a soft screen, with the hand gently row will appear similar water lines, carefully look at the screen is roughly like this:
3. IPS Panel
IPS (In-planeswitching, flat conversion) technology is a liquid crystal panel technology launched by Hitachi In 2001, commonly known as “SuperTFT”. The IPS camp is led by Hitachi and gathers a group of manufacturers such as LG-Philips, Hanyu Color Crystal, IDTech(a joint venture between Chi Mei Electronics and IBM of Japan), but not many models can be seen in the market. The biggest feature of the IPS panel is that its two poles are on the same surface, unlike other liquid crystal mode electrodes are on both sides, three-dimensional arrangement. Since the electrodes are on the same plane, the liquid crystal molecules are always parallel to the screen in any state, which will reduce the opening rate and reduce the light transmittance, so the IPS application in LCDTV will need more backlight. There is also an S-IPS panel that is a modified version of IPS
The advantages of IPS panels are high viewing Angle, fast response speed, accurate color reproduction, and cheap price. However, the disadvantage is that the light leakage problem is more serious, the black purity is not enough, and it is slightly worse than PVA, so it is necessary to rely on the compensation of the optical film to achieve better black. Currently, IPS panels are mainly produced by LG-Philips. Compared with other types of panels, the screen of the IPS panel is relatively “hard”, and it is not easy to deform the water pattern when you gently scratch it with your hand, so it is also known as a hard screen. When you look at the screen carefully, if you see a fish scale pixel in the direction of the left, plus a hard screen, then you can be sure that it is the IPS panel.
4. CPA Panel (ASV Panel)
CPA (ContinuousPinwheelAlignment, continuous fireworks pattern) model wide-field technology (soft screen), CPA model wide-field technology strictly also belong to the VA a member of the camp, the liquid crystal molecules toward the center electrode are arranged in a fireworks form of radiation. Since the electric field on the pixel electrode is continuously changing, this wide-angle mode is called the “continuous fireworks arrangement” mode. CPA is mainly promoted by Sharp, the “father of liquid crystal”, it should be noted here that the ASV that Sharp has always advertised does not refer to a specific wide perspective technology, it uses TN+Film, VA, CPA wide perspective technology products collectively referred to as ASV. In fact, only CPA model is Sharp’s own creation of a broad perspective technology, the model’s products are basically comparable to MVA and PVA. In other words, Sharp brand LCD TV may not be the use of Sharp’s own production of CPA mode LCD panel, it may use Taiwan manufacturers VA mode panel or other manufacturers LCD panel. Sharp’s CPA panel color is true, the viewing Angle is excellent, the image is delicate, the price is relatively expensive, and Sharp rarely sells CPA panels to other manufacturers. CPA panel is also a soft screen, and a similar water pattern will appear when you gently scratch it with your hand. Look at the screen carefully.
In addition, there are some other manufacturers also have their own LCD panel technology, such as NEC’s ExtraView technology, Panasonic’s OCB technology, modern FFS technology, etc., these technologies are improvements to the old TFT panel, providing viewing Angle and response time, usually only used in their own brand LCD monitors or LCD TVS. In fact, these panels all belong to the TFT class panel, but now various panels have their own technology and name, so the name TFT is not often used.
What it looks like LCD (liquid crystal display)
Speaking of liquid crystal screens, we have to talk about the concept of liquid crystal
The birth of liquid crystals
(1) Discovery of liquid crystal
To trace the origin of liquid crystal displays, we must first start from the birth of “liquid crystal”. In 1888 AD, an Austrian botanist, Frederick. 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.
(3) The first use of liquid crystal
Although liquid crystals were discovered as early as 1888, it was not until 80 years later that they were really used in daily life. 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. Then, liquid crystal display technology is widely used in general electronic products, such as calculators, electronic watches, mobile phone screens, instruments used in hospitals (because of radiation metering considerations) or digital camera screens and so on. Interestingly, the discovery of liquid crystals predated the vacuum tube or the cathode ray tube, but the world did not know much about this phenomenon until 1962, when the first one was made by Joe, a chemist at RCA Research Group. In the book published by Joe Castellano. The same as the image tube, although these two technologies were invented by the United States RCA company, they were carried forward by two companies in Japan, Sony and Sharp.
Physical properties of liquid crystals
Liquid crystal display is based on liquid crystal material as the basic component, because liquid crystal is between solid and liquid, not only has solid crystal optical characteristics, but also has liquid flow characteristics, so it can be said to be an intermediate phase. To understand the photoelectric effect of liquid crystals, we must explain the physical properties of liquid crystals, including its viscosity (visco-sity) and elasticity (elasticity) and its polarization (polarizalility). The viscosity and elasticity of liquid crystal from the point of view of fluid mechanics, it can be said that a liquid with the nature of arrangement, according to the direction of the force of action, should have different effects. It is like throwing a short wooden stick into a flowing river. The short wooden stick flows with the river and appears messy at first. After a while, the long axis of all the short wooden sticks naturally becomes consistent with the direction of the river flow, which indicates the flow mode with the lowest sub-viscosity and is also a physical model with the lowest free energy of flow. In addition, in addition to the viscous reaction, the liquid crystal also has an elastic reaction, and they are all for the external force, showing a directional effect. Therefore, the light emitted into the liquid crystal substance will inevitably travel in accordance with the arrangement of liquid crystal molecules, resulting in a natural deflection image. As for the electronic structure of liquid crystal molecules, they all have a strong ability of electron conjugation movement, so when the liquid crystal molecules are subjected to the action of external electric fields, they can easily be polarized to produce induced dipolar, which is also the source of the interaction force between liquid crystal molecules. The liquid crystal display used in general electronic products is to use the photoelectric effect of the liquid crystal, through the external voltage control, and then through the refraction characteristics of the liquid crystal molecule, and the rotation ability of the light to obtain the light and dark situation (or known as the contrast of visual optics), and then achieve the purpose of imaging.
LCD screen working principle
In simple terms, the basic principle of the screen can be displayed is to fill the liquid crystal material between the two parallel plates, through the voltage to change the internal molecules of the liquid crystal material in the column, in order to achieve the purpose of shading and light transmission to display different depths, scattered images, and as long as the two plates plus a ternary color filter layer, you can achieve the display of color images.
Understand its structure and principle, understand its technology and process characteristics, in order to be targeted in the purchase, in the application and maintenance of more scientific and reasonable. Liquid crystals are organic compounds made of long rod-like molecules. In the natural state, the long axes of these rod-like molecules are roughly parallel. The first feature of the LCD is that the liquid crystal must be injected between two planes with thin grooves to work properly. The slots in the two planes are perpendicular to each other (90 degrees of intersection), that is, if the molecules in one plane are aligned north-south, the molecules in the other plane are aligned east-west, and the molecules between the two planes are forced into a 90-degree twist. Because light travels in the direction of the molecules, it is also twisted 90 degrees as it passes through the liquid crystal. But when a voltage is applied to the liquid crystal, the molecules rearrange themselves vertically, allowing light to shine straight out without any twisting. The second feature of LCD is that it relies on the polarized filter and the light itself, the natural light is randomly divergent in all directions, the polarized filter is actually a series of thinner and thinner parallel lines. These lines form a net that blocks all light rays that are not parallel to these lines, and the lines of the polarized filter are exactly perpendicular to the first one, so that those rays that have been polarized are completely blocked. Only if the lines of the two filters are perfectly parallel, or if the light itself has been twisted to match the second polarized filter, can the light penetrate. LCD is made up of such two mutually perpendicular polarized filters, so under normal circumstances should block all the light trying to penetrate. However, because the two filters are filled with twisted liquid crystals, after the light passes through the first filter, it is twisted 90 degrees by the liquid crystal molecules and finally exits the second filter. On the other hand, if a voltage is applied to the liquid crystal, the molecules will be rearranged and completely parallel, so that the light is no longer twisted, so that it is just blocked by the second filter. In short, the power will block the light, and the light will be emitted without power. Of course, it is also possible to change the liquid crystal arrangement in the LCD, so that the light is emitted when the power is added, and is blocked when the power is not added. However, since the LCD screen is almost always on, only the “power will block the light” scheme can achieve the most power-saving purpose.
LCD screen common categories
1, STN LCD screen
STN, short for “Super Teisted Nematic,” is a passive passive matrix LCD, the material used in almost all black and white mobile phone LCDS. Color STN LCD screen is to add a color filter on the basis of the monochrome STN LCD screen, and each pixel in the monochrome display matrix is divided into three sub-pixels, respectively through the color filter to display red, green, blue three colors, so as to achieve a color picture. Due to technical limitations, the current STN LCD screen is only 65536 colors, most of the STN products seen on the market are 4096 colors, so STN is also known as “false color”.
2, GF LCD screen
GF is the abbreviation of “Glass Fine Color”, perhaps we are unfamiliar with GF LCD screen, because there are very few digital products using GF LCD screen on the market now, in fact, GF belongs to a kind of STN, GF’s main feature is: the brightness has been improved under the premise of ensuring small power consumption, but GF LCD screen has some color bias.
3, TFT LCD screen
TFT is an abbreviation of Thin Film Transistor, also known as “true color”, it belongs to the active matrix LCD screen, it is composed of thin film transistor screen, each LCD pixel is driven by thin film transistor, behind each pixel there are four independent thin film transistor to drive the pixel to emit color light. Display 24-bit depth of true color. In terms of resolution, the maximum TFT LCD screen can reach UXGA (1600×1200).
The arrangement of TFT is memorized, so the current will not return to the original state immediately after the current disappears, thus improving the shortcomings of STN LCD screen flicker and blur, effectively improving the effect of LCD screen display dynamic picture, and more prominent in the display of static picture. The advantages of TFT LCD screen are short response time and beautiful color. So it is widely used in notebook computers and DV, DC. The disadvantage of TFT LCD screen is that it consumes more power, and the cost is relatively high.
4, TFD LCD screen
TFD is the abbreviation of “Thin Film Diode”. Because TFT LCD screen has high power consumption and high cost, which greatly increases the cost of products, EPSON developed TFD technology specifically for mobile phone screens. It also belongs to active matrix LCD screen, each pixel on the LCD is equipped with a separate diode. Each pixel can be controlled individually, so that each pixel does not affect each other, which can significantly improve the resolution, and can display dynamic images and gorgeous colors without tailing.
In terms of performance, TFD LCD screen takes into account the advantages of TFT LCD screen and STN LCD screen, TFD LCD screen is brighter than STN LCD screen, and the color is more vivid, while more power saving than TFT LCD screen, but in color and brightness is still inferior to TFT LCD.
5, OLED LCD screen
OLED is the abbreviation of “Organic Light Emitting Display”, also known as organic light-emitting display, it uses organic light-emitting technology, which is the latest display technology, OLED display technology is different from the traditional LCD display, it does not need backlight. But the use of very thin organic material coating and glass substrate, when there is a current through, these organic materials will light up, so it has a large Angle of view, from all directions can see the content of the screen, and can be done very thin, and OLED display can significantly save energy, known as “dream display”.
But OLED is not without shortcomings, because it is still an immature technology, so at this stage its service life is still relatively short, the screen area is also relatively small.
Correlation parameter
1. Resolution
Resolution is a very important performance indicator. It refers to the number of points that can be displayed in the horizontal and vertical directions on the screen (the lines and planes displayed on the screen are made up of points), the higher the resolution, the more information that can be contained in the same screen. For a CRT that can support 1280×1024 resolution, whether it is 320×240 or 1280×1024 resolution, it can be more perfect (because the electron beam can be adjusted flexibly). But its maximum resolution may not be the most appropriate resolution, because if the 17-inch display to 1280×1024 resolution, WINDOWS word will be very small, a long time the eyes are easy to fatigue, so the best resolution of the 17-inch display should be 1024×768.
But not for LCDS. The maximum resolution of the LCD is its true resolution, that is, the best resolution. Once the set resolution is less than the real resolution (for example, a 15-inch LCD, its real resolution is 1024×768, while the WINDOWS set resolution is 800×600), there will be two ways to display. One is the center display, only the 800×600 points in the middle of the LCD will display the image, and the other points that are not used will not glow, keep the dark background, and it looks like the picture is centered and narrowed. The other is to expand the display, which will use every pixel on the screen, but because the pixel is easy to distort, it will have a certain impact on the display effect. So in any case, when choosing LCD, pay attention to the resolution not the larger the better, but the appropriate use.
2. Perspective
At present, most flat-screen displays can reach 180 degrees of viewing Angle, that is, the content displayed can be clearly seen from any direction in front of the screen. LCD is different, its viewing Angle varies according to whether the process is advanced or not, and the viewing Angle of some new products has been able to reach about 160, which is very close to the 180 degrees of CRT. There are also some LCDS that have a nominal viewing Angle of 160 degrees, but actually do not meet this standard. Once the user’s Angle of view exceeds its actual visual range in the process of use, the color of the screen will fade, darken, and even turn positive into negative. It is likely that everyone is confused by the advertisement of Philips. In fact, the Angle of view of LCD is not very large, but much smaller than the CRT, which is obviously weaker than the CRT, so do not worry about being seen by colleagues. Of course, the situation would be better if manufacturers added technology to their products to increase the perspective. The following is an introduction.
3. Visual area
Visual area refers to the area of the screen that can be used to display an image in a practical application. 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 simply cannot reach this size, because the frame of the picture tube takes up a part of the space. Generally speaking, the viewable area of a 17-inch CRT display is about 15.8-16 inches, while the viewable area of a 15-inch display 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 15-inch LCD is about 14.5 inches, which is why the LCD looks larger than the same size CRT. So when buying LCD, 15 villages is basically enough.
4. Brightness and contrast
The display function of the LCD is mainly to have a backlit light source, and the brightness of the light source determines the brightness and color saturation of the entire LCD screen. In theory, the brightness of the liquid crystal display is the higher the better, the measurement unit of brightness is cd/m2 (candlelight per meter square), also known as 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 measurement of the contrast between different levels of two colors, black and white. When the contrast is 120:1, vivid and rich colors can be displayed (because the human eye can distinguish the contrast is about 100:1), and when the ratio is as high as 300:1, various levels of color can be supported. At present, most LCD displays 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 value of brightness and contrast, so the purchase of LCD depends on a pair of sharp eyes. So in the purchase of LCD should pay attention to this indicator, it is also the largest performance difference on the LCD product estimate on the purchase of some difficulty.
5. Reaction speed
The time unit for measuring the reaction speed is the millisecond (ms), which refers to the time required for the pixel to change from light to dark and from dark to two. The smaller the number, the better. The smaller the number, the faster the reaction. At present, the reaction speed of the mainstream LCD is more than 25ms, which does not matter much in general commercial uses (such as word processing or text processing), because such uses do not have to care too much about the reaction time of the LCD. And if it is used to play games, watch VCD/DVD and other full-screen high-speed dynamic images, the reaction time is especially important, if the reaction time is long, the screen will appear tailing, residual shadow and other phenomena. For a simple example, the vast majority of 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 reaction time of CRT is only 1ms, and there is absolutely no tailing phenomenon.
6. Color
When it comes to color, LCD is not as good as CRT, in theory, CRT can display the same color as the TV is unlimited. LCD can only display about 260,000 colors, most of the products are claimed to be able to display 16.77 million colors (16777216 colors, 32-bit), but in fact are achieved through dithering algorithm (dithering), compared with the real 32-bit color is still a big gap. Therefore, it is still not as good as the traditional CRT in terms of color expression and transition. In the same way, LCD’s ability to represent gray is not as good as CRT. If you have conditions, you can compare it yourself: find a 17-inch Trinitron tube display, and then place a 15-inch LCD, and display a 16.77 million color image. CRT shows a very bright picture, while LCD appears to be a little “fake”, although I can not say where the wrong, but look is not that long tube of the CRT comfortable.
LCD screen protection
When we talk about the protective film of the screen, we recommend that you paste it when you do not use the LCD screen, and then remove it when you use the LCD screen, which can effectively protect the chemical coating of the outer layer of the screen, so that the outermost coating will not be oxidized prematurely. In the process of using a laptop, do not easily use your hand to point/press the LCD screen, or use hard objects to contact the screen, if you often do not pay attention to the LCD screen for a long time will appear such as white printing and other scars that will never be erased, then it will be too late to regret. New machine packaging usually comes with a layer of cotton paper, you can also put this layer of cotton paper between the laptop screen and the keyboard, thereby reducing wear between the screen and the health cap. If your laptop uses the pointer, we also recommend that you take the pointer cap off and store it separately when taking the laptop out long distances to avoid the screen being jacked
Water can be described as the “natural enemy” of the LCD screen, in addition to trying to avoid drinking drinks and eating fruits in the LCD screen, you should also pay attention not to keep the machine in a wet place, serious moisture will damage the components inside the LCD screen. It is particularly noteworthy that in winter and summer, when entering and leaving a heated or air-conditioned room, the large temperature difference will also lead to “condensation phenomenon”, and the user may also lead to corrosion of the LCD electrode at this time, causing permanent damage. Therefore, we also recommend that your ambient temperature change should not be greater than 10℃/10min. Once the screen water situation occurs, if it is only found that there is fog on the surface of the screen before starting, it can be gently wiped off with a soft cloth and then started. If the water has entered the LCD, the LCD should be placed in a warmer place, such as under the lamp, and the water inside will gradually evaporate. In the plum rain season, we should also pay attention to the regular operation of the LCD screen for a period of time, so that the heating components can disperse the moisture, and it is best to put a small package of moisture-proof agents in the bag of the LCD screen to create a good home for the love machine.
For screen maintenance, in addition to paying attention to the above problems, you can also manually or with soft cooperation, because the life of the LCD screen is still much shorter than the CRT, its aging speed is much faster, so we need to pay special attention when we usually use. For example, set a time interval for automatically closing the screen when the computer does not respond in the power management interface, or you simply develop a habit of closing the screen when you do not use the laptop for a long time to reduce unnecessary screen loss. In addition, to delay the aging of the LCD screen, attention should also be paid to avoid the long-term direct exposure of strong sunlight to the screen, try to use moderate brightness/contrast, and reduce the long-term display of fixed patterns (to avoid excessive local aging). Finally, there is a point, that is, we should often use a special soft brush, glasses cloth, ear ball, etc. to wipe the screen, if necessary, you can use a neutral cleaning agent or a little water to clean the surface stains. These tips are very good for LCD screens.