Types of LCD screen
The quality of a liquid crystal display depends on its panel first, 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 total cost of the whole machine, which is the main factor affecting the cost of the LCD TV. Therefore, to select a good liquid crystal display, it is necessary to select its panel first. LCD panel can largely determine the brightness, contrast, color, visual angle and other very important parameters of LCD display. The development speed of LCD panel is very fast, from three generations in previous years to four and five generations, and then skip six generations to seven generations, and the updated eighth generation panel is also under planning. At present, the manufacturers of LCD panels are mainly Samsung, LG-Philips, Yoda, etc. Due to the differences in the technical level of each manufacturer, the LCD panels produced are also roughly divided into different types of models. The common ones are TN panel, MVA, PVA and other VA panel, IPS panel and CPA panel.
1. TN panel
TN is known as Twisted Nematic (twisted nematic) panel. The low production cost makes TN the most widely used entry-level LCD panel, which is widely used in the mainstream medium and low-end LCD displays on the market at present. At present, most of the TN panels we see are improved TN+film, which is a compensation film, used to make up for the lack of TN panel's visual angle. At present, the improved TN panel's visual angle reaches 160 °. Of course, this is the limit value measured by the manufacturer when the contrast ratio is 10 ∶ 1. In fact, when the contrast ratio drops to 100:1, the image has been distorted or even distorted.
As a 6Bit panel, TN panel can only display 64 colors of red, green and blue, and the maximum actual color is only 262.144. Through "dithering" technology, it can obtain the performance ability of more than 16 million colors, and can only display three primary colors of 0 to 252 gray scale, so the final color display number information is 16.2M colors, rather than the real color 16.7M colors we usually say; In addition, it is difficult to improve the contrast of TN panel. The problems directly exposed are the thin color, poor restoration ability and unnatural transition.
The advantages of TN panel are that the output gray class is small, the liquid crystal molecule deflection speed is fast, and the response time is easy to improve. At present, the liquid crystal products below 8ms on the market basically use TN panel. In addition, Samsung has also developed a B-TN (Best-TN) panel, which is actually an improved version of TN panel. It is mainly to balance the contradiction between the high speed response of TN panel and the sacrifice of image quality. At the same time, the contrast ratio can reach 700 ∶ 1, which is close to the panel of MVA or early PVA. Many panel manufacturers in Taiwan produce TN panel. TN panel is a soft screen, and similar watermarks will appear when you gently scratch it with your hand. In addition, the screen is roughly as follows:
2. Class VA panel
VA panel is a panel type with more high-end LCD applications, belonging to wide-angle panel. Compared with TN panel, 8bit panel can provide 16.7M color and large visual angle, which is the capital of high-end positioning of such panel, but the price is also higher than TN panel. VA panel can be divided into MVA panel led by Fujitsu and PVA panel developed by Samsung. The latter is the inheritance and improvement of the former. The front (front) contrast of the VA panel is the highest, but the uniformity of the screen is not good enough, and color drift often occurs. Sharp text is its killer, and the black and white contrast is quite high.
Fujitsu's MVA technology (Multi-domain Vertical Alignment) can be said to be the earliest wide-view LCD panel technology. This type of panel can provide a larger visual angle, usually up to 170 °. Through technical authorization, Qimei Electronics (Qijing Optoelectronics), Youda Optoelectronics and other panel enterprises in Taiwan Province of China have adopted this panel technology. The visual angle of the improved P-MVA panel can reach 178 ° close to the horizontal, and the gray-scale response time can reach less than 8 ms
The PVA (Pattern Vertical Alignment) technology of Samsung Electronics also belongs to the category of VA technology, which is the successor and developer of MVA technology. Its comprehensive quality has surpassed the latter in an all-round way, and the improved S-PVA has been able to keep pace with P-MVA to obtain a wide visual angle and faster response time. PVA uses transparent ITO electrode to replace the liquid crystal layer protrusion in MVA. The transparent electrode can obtain better opening rate and minimize the waste of backlight. This mode greatly reduces the possibility of "bright spots" on the LCD panel, and its position in the era of LCD TV is equivalent to the "long tube" in the era of CRT TV. Samsung's PVA model wide-angle technology is widely adopted by Japanese and American manufacturers due to its strong capacity and stable quality control system. At present, PVA technology is widely used in high-end LCD or LCD TV. The VA type panel is also a soft screen, and similar watermarks will appear when you gently scratch it with your hand. Look at the screen carefully, and it looks like this:
3. IPS panel
IPS (In-Plane Switching) technology is a liquid crystal panel technology launched by Hitachi in 2001, commonly known as "SuperTFT". The IPS camp, led by Hitachi, gathered a number of manufacturers such as LG Philips, Hanyu Caijing, IDTech (a joint venture between Qimei Electronics and IBM in Japan), but there are not many models available in the market. The biggest feature of IPS panel is that its two poles are on the same plane, unlike other liquid crystal mode electrodes, which are arranged on the upper and lower sides in a three-dimensional manner. Because 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 the light transmittance, so IPS application on LCDTV will require more backlight. In addition, there is an S-IPS panel which is an improved version of IPS
IPS panel has the advantages of high visual angle, fast response, accurate color restoration and low price. However, the disadvantage is that the problem of light leakage is serious, and the black purity is not enough, which is slightly worse than PVA. Therefore, it needs to rely on the compensation of optical film to achieve better black. At present, IPS panels are mainly produced by LG Philips. Compared with other types of panels, the screen of IPS panel is more "hard", and it is not easy to deform the water pattern with a slight stroke by hand, so it is also known as hard screen. When you look at the screen carefully, if you see the scaly pixels facing left, plus the hard screen, you can determine that it is the IPS panel.
4. CPA panel (ASV panel)
CPA (Continuous Pinwheel Alignment) mode wide view technology (soft screen), CPA mode wide view technology is also a member of VA camp strictly speaking, and liquid crystal molecules are arranged in the form of fireworks towards the central electrode. Because the electric field on the pixel electrode is continuously changing, this wide-angle mode is called "continuous fireworks arrangement" mode. CPA is mainly promoted by Sharp, the "father of LCD". It should be noted here that the ASV that Sharp has always promoted does not actually refer to a specific wide-angle technology. It refers to the products that have used TN+Film, VA, and CPA wide-angle technology as ASV. In fact, only the CPA model is the broad perspective technology created by Sharp itself, and the products of this model are basically equivalent to MVA and PVA. That is to say, Sharp's LCD TV is not necessarily the CPA mode LCD panel produced by Sharp itself. It may use the VA mode panel from Taiwan or the LCD panel from other manufacturers. Sharp's CPA panel color restoration is true, the visual angle is excellent, the image is exquisite, and the price is relatively expensive, and Sharp rarely sells CPA panels to other manufacturers. CPA panel is also a soft screen, and similar water marks will appear when you gently scratch it with your hand. Look at the screen carefully, and it is roughly as follows:
In addition, some other manufacturers also have their own LCD panel technology, such as NEC's ExtraView technology, Panasonic's OCB technology, and modern FFS technology. These technologies are improvements to the old TFT panel, providing visual angle and response time, and are usually only used on proprietary LCD monitors or LCD TVs. In fact, these panels are all TFT panels, but now various panels have their own technologies and names, so the name TFT is not often used.
It looks like liquid crystal display (LCD)
Speaking of LCD screen, we have to talk about the concept of LCD and the birth of LCD
(1) Discovery of liquid crystal
To trace the origin of LCD, we must start with the birth of "LCD". In 1888, an Austrian botanist, Friedrich Reinitzer, discovered a special substance. He extracted a compound called helical toluenate from plants. When doing heating experiments for this compound, he unexpectedly found that this compound has two melting points at different temperatures. Its state is between the liquid and solid substances we generally know. It is a little like the colloidal solution of soapy water, but it has the properties of both liquid and crystalline substances within a certain temperature range. Because of its unique state, it was later named "Liquid Crystal", which means liquid crystalline substances.
(3) Initial application of liquid crystal
Although the liquid crystal was discovered as early as 1888, it was 80 years later when it was really used in daily life. In 1968, at the Sanov R&D Center of RCA (the invention company of radio and television), engineers found that liquid crystal molecules would be affected by voltage, change their molecular arrangement, and deflect the incoming light. Using this principle, RCA invented the world's first LCD screen. Later, LCD technology is widely used in general electronic products, such as calculators, electronic watches, mobile phone screens, instruments used in hospitals (due to radiation measurement considerations) or screens on digital cameras. Interestingly, the discovery of liquid crystal was earlier than that of vacuum tube or cathode ray tube, but the world did not know much about this phenomenon. It was not until 1962 that the first time it was described in a book published by Joe Castellano, a chemist of the RCA research team. Like the image tube, although both of these technologies were invented by RCA in the United States, they were carried forward by Sony and Sharp in Japan respectively.
Physical properties of liquid crystal.
Liquid crystal display is based on liquid crystal materials. Because liquid crystal is between solid and liquid, it not only has optical properties of solid crystal, but also has liquid flow characteristics, so it can be said to be a mesophase. To understand the photoelectric effect of liquid crystal, we must explain the physical properties of liquid crystal, including its viscosity, elasticity and polarization. From the viewpoint of hydrodynamics, the viscosity and elasticity of liquid crystal can be said to be a liquid with arrangement properties, which should have different effects according to different directions of action force. It is like throwing a short wooden stick into the flowing river. The short wooden stick flows along with the river, and it looks messy at first. After a while, the long axis of all the short wooden sticks naturally becomes consistent with the flow direction of the river, which indicates the flow mode with the lowest viscosity and is also a physical model with the lowest free energy of flow. In addition, liquid crystals have not only viscous reactions, but also elastic reactions. They are all directional effects on external forces. Therefore, when the light enters the liquid crystal material, it will inevitably follow 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. Therefore, when liquid crystal molecules are subjected to an external electric field, they can easily be polarized to produce induced dipole, which is also the source of interaction force between liquid crystal molecules. The liquid crystal display used in general electronic products is to use the photoelectric effect of liquid crystal, through the external voltage control, and through the refractive properties of liquid crystal molecules, as well as the ability to rotate light to obtain the light and dark conditions (or known as visual optical contrast), to achieve the purpose of imaging.