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Detailed analysis of the types and principles of LCD screens

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Detailed analysis of the types and principles of LCD screens

LCD screen

The liquid crystal screen uses liquid crystal material as the basic component. The liquid crystal material is filled between two parallel plates. The voltage is used to change the alignment of molecules inside the liquid crystal material. Consistent images, and as long as a ternary color filter layer is added between the two flat plates, a color image can be displayed. LCD screens have very low power consumption, so they are popular with engineers and are suitable for battery-powered electronic devices.

Which LCD screen is good

From a technical perspective, LCD panels can be divided into two camps: VA soft screen and IPS hard screen. The VA soft screen camp includes Samsung, Taiwan AUO, Chi Mei and Sharp. The IPS hard-screen camp includes two manufacturers, IPS, a joint venture of LG and Philips, and IPS Alpha, a joint venture of Hitachi, Panasonic and Toshiba. In fact, the above boards belong to the TFT category.

VA soft screen

The full name of VA is Vertical alignment panel, which can provide 16.7M colors and large viewing angle. Of course, the price is also more expensive. It is the most commonly used soft screen panel for LCD TVs.

The front (front view) contrast of the VA panel is the highest, but the uniformity of the screen is not good enough, and color drift often occurs. The sharp text is its killer, and the black and white contrast performance is also very good. VA panel can be divided into MVA panel led by Fujitsu and PVA panel developed by Samsung, the latter is the successor and improvement of the former.


Fujitsu MVA technology can be said to be the earliest wide-view LCD panel technology. This type of panel can provide a larger viewing angle, usually up to 170 °. Panel manufacturers such as Taiwan Chi Mei Electronics and AU Optronics have adopted this technology. The improved P-MVA panel has a viewing angle of approximately 178 °, and the gray-scale response time can reach below 8ms.

Samsung PVA technology also belongs to the category of VA technology, it is the successor and developer of MVA technology. Its comprehensive quality has surpassed the latter comprehensively. At present, the S-PVA panel has been developed on the basis of the PVA panel. The characteristics of the S-PVA screen are relatively obvious. It will be found that the shape of the pixel is "", but there are also some models of pixel opening direction is opposite. And the improved S-PVA can already go hand in hand with P-MVA, get extremely wide viewing angle and faster and faster response time.

Many domestic 42 and 47-inch FULL HDs currently use this type of panel.

IPS hard screen

IPS panel technology is a liquid crystal panel technology launched by Hitachi in 2001, commonly known as "Super TFT". It is currently the only hard-screen panel of TFT. It has developed a wide range of applications so far. Products using IPS hard-screen are generally positioned relatively high.

The biggest feature of this type of IPS board is that its two poles are on the same side, while the electrodes of other liquid crystal modes are arranged on the upper and lower sides in a three-dimensional arrangement. Because the electrodes are on the same plane, the liquid crystal molecules are always parallel to the screen regardless of the state, which will reduce the aperture ratio and reduce the light transmittance.


Then, the advantages and disadvantages of the IPS panel are obvious: higher viewing angles, faster response speeds, and accurate color reproduction; there are light leakage problems, and black purity does not perform well without VA, and it is necessary to rely on optical films to compensate.

In addition to LG TVs using IPS hard screens, domestic color TV manufacturers Skyworth, Konka, Hisense, and Changhong also use IPS hard screen panels.

LCD panel classification
The LCD panel can be divided into three levels of A, B, and C according to quality, and the distinction is based on the number of dead pixels. However, there are no relevant international regulations, so the grade standards of different countries and regions are not the same. In general:


Note: According to different panel production areas, the delineation of A-class panels is also different: South Korean A-class boards require less than 3 dead pixels, Japan has 5 or less, and Taiwan requires 8 or less.

Which is better, VA soft screen or IPS hard anchor screen

At present, the above two types of LCD screen panels are more commonly used in the market, and IPS hard screens have more advantages in the performance of multiple advantages (the price is also more expensive than VA soft-screen TVs of the same size). Therefore, the author believes that with the continuous improvement of IPS hard screen technology, in the future IPS hard screen panel will inevitably become the core panel of LCD TVs.

What is the LCD display principle
The principle of liquid crystal display is simply to add some liquid crystal material between two parallel panels, and then change the molecular arrangement inside the liquid crystal material by voltage, resulting in changes in the display of light-shielding and light-transmitting, which will form those staggered images Then, as long as some ternary color filter layers are added in the two parallel panels, a colored pattern will be displayed.

working principle

In simple terms, the basic principle of screen display is to fill liquid crystal material between two parallel plates, and change the arrangement of molecules inside the liquid crystal material by voltage to achieve the purpose of light shielding and light transmission to display different shades and staggered. Consistent images, and as long as a ternary color filter layer is added between the two flat plates, a color image can be displayed.

After knowing its structure and principle, and understanding its technical and technological characteristics, can it be targeted at the time of purchase and be more scientific and reasonable in application and maintenance. Liquid crystal is an organic complex composed of long rod-shaped molecules. In the natural state, the long axes of these rod-shaped molecules are approximately parallel.

The first feature of LCDs is that liquid crystal must be filled between two planes with small slots to work properly. The grooves on these two planes are perpendicular to each other (intersecting at 90 degrees). Forced into a 90-degree twist. Since the light propagates along the direction of the molecules, the light is also twisted 90 degrees as it passes through the liquid crystal. However, when a voltage is applied to the liquid crystal, the molecules will re-align vertically, so that the light can be directly emitted without any twist.

The second characteristic of LCD is that it relies on polarizing filters and the light itself. Natural light is scattered randomly in all directions. The polarizing filters are actually a series of increasingly thin parallel lines. These lines form a net that blocks all light that is not parallel to these lines. The lines of the polarization filter are exactly perpendicular to the first one, so they can completely block those polarized light. 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 pass through. On the one hand, 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 distorted liquid crystals, 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 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 it is just blocked by the second filter. In short, the power will block the light, and the power will make the light out. Of course, you can also change the arrangement of liquid crystals in the LCD, so that light is emitted when power is applied, but not blocked when power is applied. However, since the LCD screen is almost always on, only the "power-on and light-blocking" solution can achieve the most power-saving purpose.

Active matrix LCD

The structure of a TFT-LCD liquid crystal display is basically the same as that of a TN-LCD liquid crystal display, except that the electrodes on the upper layer of the TN-LCD are changed to FET transistors, and the lower layer is changed to a common electrode.

The working principle of TFT-LCD is different from that of TN-LCD. The development principle of the TFT-LCD liquid crystal display is "back-through" illumination. When the light source is irradiated, it first penetrates upward through the lower polarizing plate, and conducts light with the help of liquid crystal molecules. Because the electrodes of the upper and lower sandwiches are changed to FET electrodes and common electrodes, when the FET electrodes are turned on, the alignment state of the liquid crystal molecules will also change, and the purpose of display is also achieved by shading and transmitting light. But the difference is that because the FET transistor has a capacitive effect and can maintain a potential state, the previously transmitted liquid crystal molecules will remain in this state until the next time the FET electrode is powered on to change its arrangement.

Passive matrix LCD

The display principles between TN-LCD, STN-LCD and DSTN-LCD are basically the same, the difference is that the twist angles of liquid crystal molecules are somewhat different. The following takes a typical TN-LCD as an example to introduce its structure and working principle. In a TN-LCD liquid crystal display panel with a thickness of less than 1 cm, it is usually a plywood made of two large glass substrates with color filters, alignment films, etc. inside, and two polarizing plates around it. They determine the maximum luminous flux and the generation of color. The color filter is a filter composed of three colors of red, green, and blue, and is regularly produced on a large glass substrate. Each pixel is composed of three color units (also called sub-pixels). If a panel has a resolution of 1280 × 1024, it actually has 3840 × 1024 transistors and sub-pixels. The upper left corner (gray rectangle) of each sub-pixel is an opaque thin film transistor, and the color filter can produce three primary colors of RGB. Each interlayer contains grooves formed on the electrodes and the alignment film, and the upper and lower interlayers are filled with multiple layers of liquid crystal molecules (the liquid crystal space is less than 5 × 10-6m). Although the positions of the liquid crystal molecules in the same layer are irregular, the long axis orientation is parallel to the polarizing plate. On the other hand, between different layers, the long axis of the liquid crystal molecules is continuously twisted 90 degrees along the parallel plane of the polarizer.

Among them, the orientation of the long axes of the two liquid crystal molecules adjacent to the polarizing plate is consistent with the polarization direction of the adjacent polarizing plate. The liquid crystal molecules near the upper interlayer are aligned in the direction of the upper trench, while the liquid crystal molecules in the lower interlayer are aligned in the direction of the lower trench. Finally, it is packaged into a liquid crystal box and connected with the driver IC, the control IC and the printed circuit board.

Under normal circumstances, when the light is radiated from the top to the bottom, usually only one angle of light can penetrate it. It is introduced into the groove of the upper sandwich through the upper polarizing plate, and then exits from the lower polarizing plate through the path twisted by liquid crystal molecules. Form a complete light penetration path. The interlayer of the liquid crystal display is attached with two polarizing plates, and the arrangement and transmission angle of the two polarizing plates are the same as the groove arrangement of the upper and lower interlayers. When a certain voltage is applied to the liquid crystal layer, the liquid crystal will change its initial state due to the influence of the external voltage, and it will no longer be arranged in a normal manner, but will become an upright state. Therefore, the light passing through the liquid crystal will be absorbed by the second-layer polarizing plate and the entire structure will appear opaque. As a result, black appears on the display screen. When no voltage is applied to the liquid crystal layer, the liquid crystal is in its initial state, which will twist the direction of incident light by 90 degrees, so that the incident light of the backlight can pass through the entire structure, and as a result, white appears on the display screen. In order to achieve the desired color for each individual pixel on the panel, multiple cold cathode lamps must be used as the backlight of the display.

What are the main types of LCD screen technology
1 IPS (In-Plane Switching)

IPS technology is commonly known as "super TFT". The liquid crystal molecules of traditional LCD displays are generally switched between vertical and parallel states. MVA and PVA have improved it to a vertical-dual switching method. The biggest difference between IPS technology and the above technology is that No matter in any state, the liquid crystal molecules are always parallel to the screen, but the direction of rotation of the molecules is different in the home appliance / conventional state-note that the rotation of MVA and PVA liquid crystal molecules belongs to spatial rotation, and the rotation of liquid crystal molecules Belongs to rotation in the plane.

In order to match this structure, IPS requires improvements to the electrodes. The electrodes are on the same side to form a planar electric field. The impact of this design is twofold. On the one hand, the viewing angle problem is solved, and on the other hand, the liquid crystal molecules Large rotation angle and low panel opening (light transmittance), so IPS also has the disadvantages of slow response time and difficult to improve contrast.

However, there are not many models that can be seen on the market.16.7M, 170-degree viewing angle and 16MS response time represent the highest level of current IPS LCD displays!

2 VA type panel:

VA type panel is a type of panel that is currently used in high-end LCD. panel

MVA technology can be said to be the earliest wide-view LCD panel technology

MVA technology uses a clever approach to improve this model. The liquid crystal layer of the MVA liquid crystal panel contains a protrusion for the liquid crystal molecules to attach. In the state where no voltage is applied, the MVA panel looks no different from the traditional technology. The liquid crystal molecules are perpendicular to the screen. Once the voltage is applied, The liquid crystal molecules will deflect on the protrusions, forming a state perpendicular to the surface of the protrusions. At this time, it will also have a deflection effect with the screen surface, improve the transmittance, and form a picture output.

3 TN panel

TN panels are widely used in entry-level and terminal LCD monitor panels.They do not perform well in performance indicators, cannot display 16.7m colors, and have poor viewing angles, so the TN panels we see on the market are all improved TN + film, film is a compensation film, which is used to make up for the lack of viewing angle of the TN panel. At the same time, the trial of the color dithering technology also made the TN panel that could only display 260,000 colors obtain a 16.2m display capability.

It is said that the only advantage of the TN panel over the previous two panels is that because of its lower number of gray connections and the rapid deflection speed of the liquid crystal molecules, its response time is easily improved.Currently, LCD products below 8ms are used in the market. It is a TN panel.In general, the TN panel is a product with obvious advantages and disadvantages.It is cheap and the response time can meet the requirements of high-speed sports games. Its advantages are that the viewing angle is not ideal and the color performance is unreal. It is a clear disadvantage.

Pub Time : 2020-04-01 14:22:47 >> News list
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