This article is mainly about the introduction of LCD screens, and focuses on the types and principles of LCD screens.
LCD screenThe LCD screen is based on the liquid crystal material as the basic component. The liquid crystal material is filled between two parallel plates, and the arrangement of the molecules inside the liquid crystal material is changed by voltage to achieve the purpose of shading and transmitting light to display different shades, staggered A consistent image, and as long as a ternary color filter layer is added between the two plates, a color image can be displayed. LCD screens have very low power consumption, so they are highly favored by engineers and are suitable for electronic devices that use batteries.
The LCD screen is based on liquid crystal materials. Since the liquid crystal is between solid and liquid,
It not only has the optical properties of solid crystals, but also has liquid flow properties, so it can be said to be an intermediate phase. To understand the photoelectric effect produced by liquid crystals, we must explain the physical properties of liquid crystals, including its viscosity (visco-sity) and elasticity (elasticity) and its polarization (polarizalility).
From the point of view of fluid mechanics, the viscosity and elasticity of liquid crystals can be said to be a liquid with alignment properties. Depending on the direction of the acting force, it should have different effects. It's like throwing a short wooden stick into the flowing river water. The short wooden stick flows with the river and looks messy at first. After a while, the long axis of all the short wooden sticks naturally changes to the direction of the river flowing. Consistent, this represents the sub-viscous flow mode, and it is also a physical model with the lowest free energy of flow. In addition, liquid crystals not only have viscous responses, but also have elastic responses. They all have a directional effect on applied forces. Therefore, when light enters the liquid crystal material, it will inevitably proceed in accordance with the arrangement of the liquid crystal molecules, resulting in a natural deflection phenomenon. As for the electronic structure of liquid crystal molecules, they all have strong electronic conjugate movement ability. Therefore, when liquid crystal molecules are subjected to an external electric field, they are easily polarized to produce induced dipolar. This is also a liquid crystal. The source of the interaction forces between molecules. The liquid crystal display used in general electronic products uses the photoelectric effect of liquid crystals, through external voltage control, and then through the refraction characteristics of liquid crystal molecules and the ability to rotate light to obtain bright and dark conditions (or known as the Optometry contrast), and then achieve the purpose of imaging.
How the LCD screen worksSimply put, the basic principle of screen display is to fill liquid crystal material between two parallel plates.
The arrangement of molecules inside the liquid crystal material is changed by voltage to achieve the purpose of shading and transmitting light to display images with different depths and colors. And as long as the three-color filter layer is added between the two plates, It can display color images.
Knowing its structure and principles, and understanding its technical and technological characteristics, can you have a targeted when purchasing, and be more scientific and reasonable in application and maintenance. Liquid crystal is an organic compound composed of long rod-shaped molecules. In the natural state, the long axes of these rod-shaped molecules are roughly parallel.
The first feature of LCD is that the liquid crystal must be filled between two planes with fine grooves to work normally. The grooves on these two planes are perpendicular to each other (intersect at 90 degrees). That is to say, if the molecules on one plane are arranged in the north-south direction, the molecules on the other plane are arranged in the east-west direction, and the molecules between the two planes are arranged in the east-west direction. Forced into a state of 90-degree twist. Since the light travels along the direction of the arrangement of the molecules, 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 re-arranged vertically, so that the light can be directed out without any twisting.
The second characteristic of LCD is that it relies on polarization filters and the light itself. Natural light diverges randomly in all directions.
The polarization filter is actually a series of increasingly thinner parallel lines. These lines form a net to block all the light that is not parallel to these lines. The line of the polarization filter is exactly 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 polarized filter, the light can penetrate. On the one hand, LCD is composed of two polarized filters that are perpendicular to each other, so under normal circumstances, all light that attempts to penetrate should be blocked. However, because the two filters are filled with twisted 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 rearrange and be completely parallel, so that the light will no longer be twisted, so it will be blocked by the second filter. In short, power is applied to block the light, and no power is applied to cause the light to be emitted. Of course, it is also possible to change the arrangement of the liquid crystals in the LCD so that light is emitted when power is applied, but is blocked when power is not applied. However, since the LCD screen is almost always on, only the solution of "powering on and blocking the light" can achieve the most power saving purpose.
Active matrix LCD screen
The structure of the TFT-LCD liquid crystal display is basically the same as that of the TN-LCD liquid crystal display, except that the TN-LCD is sandwiched
The electrode of is changed to a FET transistor, and the lower interlayer is changed to a common electrode.
There are many differences between the working principle of TFT-LCD and TN-LCD. The imaging principle of the TFT-LCD liquid crystal display is to use the "back-through" illumination method. When the light source is irradiated, it first penetrates upward through the lower polarizing plate, and transmits light with the help of liquid crystal molecules. Since the upper and lower interlayer electrodes are changed to FET electrodes and common electrodes, when the FET electrodes are turned on, the arrangement of the liquid crystal molecules will also change, and the purpose of display is achieved by shielding and transmitting light. But the difference is that because the FET transistor has a capacitance effect and can maintain a potential state, the previously transparent liquid crystal molecules will remain in this state until the FET electrode is energized next time to change its arrangement.
Passive matrix LCD screen
The display principle between TN-LCD, STN-LCD and DSTN-LCD is basically the same,
The difference is that the twist angle of the liquid crystal molecules is somewhat different. Let's take 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 a color filter, an alignment film, etc., and two polarizing plates on the outside. They can determine the maximum luminous flux and color production. The color filter is a filter composed of three colors of red, green, and blue, which are regularly fabricated on a large glass substrate. Each pixel is composed of three color units (or 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 the three primary colors of RGB. Each interlayer contains electrodes and grooves formed on 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). In the same layer, although the position of the liquid crystal molecules is irregular, the long axis orientation is parallel to the polarizer. On the other hand, between different layers, the long axis of the liquid crystal molecules is continuously twisted 90 degrees along the plane parallel to the polarizer.
Among them, the orientation of the long axis of the two layers of 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 arranged in the direction of the upper groove, and the liquid crystal molecules in the lower interlayer are arranged in the direction of the lower groove. 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 light is irradiated from top to bottom, usually only one angle of light can penetrate, through the upper polarizing plate into the groove of the upper interlayer, and then passing through the lower polarizing plate through the passage of the twisted arrangement of liquid crystal molecules. Form a complete path of light penetration. The interlayer of the liquid crystal display is attached with two polarizing plates, and the arrangement and light 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, due to the influence of the external voltage, the liquid crystal will change its initial state, and will no longer be arranged in a normal way, but will become an upright state. Therefore, the light passing through the liquid crystal will be absorbed by the second layer of polarizing plate and the entire structure will appear opaque, resulting in a black color on the display screen. When no voltage is applied to the liquid crystal layer, the liquid crystal is in its initial state and will twist the direction of the incident light by 90 degrees, so that the incident light from the backlight can pass through the entire structure, resulting in white on the display. In order to achieve the color you want for each individual pixel on the panel, multiple cold cathode lamps must be used as the backlight of the display.
Introduction to common LCD screen types and differencesGF LCD screen
GF is the abbreviation of "Glass Fine Color". Perhaps everyone is relatively unfamiliar with GF LCD screens, because there are very few digital products using GF LCD screens on the market. In fact, GF is also a kind of STN. The main features of GF are: Under the premise of ensuring low power consumption, the brightness is increased, but some color casts will appear on the GF's LCD screen.
TFT LCD screen
TFT is the abbreviation of "Thin Film Transistor". It belongs to the active matrix LCD screen. It is a screen composed of thin film transistors. Each of its liquid crystal pixels is basically driven by thin film transistors. Each pixel is driven by thin film transistors. There are four independent thin film transistors on the back of each other. They drive pixels and emit colored light, which can display true colors with 24bit color depth. In terms of resolution, the TFT LCD screen can reach UXGA (1600×1200) to the greatest extent.
TFD LCD screen
TFD is the abbreviation of "Thin Film Diode". Because the power consumption of TFT LCD screen is relatively high, and its cost is also high, which greatly increases the cost of the product, EPSON has developed TFD technology specifically for mobile phone screens. It is also an active matrix liquid crystal screen. Each pixel on the display has a separate diode, which can control each pixel individually, so that each pixel will not affect each other, so that the resolution can be significantly improved. Rate, can display dynamic pictures and brilliant colors without tailing.
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. This is currently the latest display technology. OLED display technology is different from traditional liquid crystal display. The thing is, it doesn’t need a backlight. Instead, it uses a very thin coating of organic materials and a glass substrate. When it has a current passing through, these organic materials will emit light by themselves, so its viewing angle will change very much. Large, the content on the screen can be seen clearly from all directions, and it can also be made very thin, and the OLED display can significantly save power, and is known as the "dream display."
Active
Super AMOLED panel is called Super AcTIve Matrix Organic Light EmitTIng Diode (Super AcTIve Matrix Organic Light EmitTIng Diode).
Since LCD display technology is inherently supported (supported by backlight), there is always a loss of brightness no matter what, and the light has to pass through two layers of glass and various films to produce polarized light, which will bring color loss, and pixels It is also more difficult to increase the density, and the cost will be higher, so people need a screen that can be close to lossless, so the offensive display technology that can self-illuminate has been developed. This is what we call AMOLED.
Since it does not require thick glass and a backlight, the light emitted by this screen can be directly accepted by the human eye, so that this screen is an ideal screen regardless of color loss or viewing angle. But God is often fair, OLED also has its insurmountable shortcomings, that is, the three-color luminescence loss is inconsistent.
We know that the current white light is actually composed of three primary colors, namely red, green, and blue. If we want to emit these three kinds of light, the energy we have to give is not consistent, which reflects that the applied current is not consistent (E =hv, the energy required for different frequencies is also different), this is like you hit something, the greater the force you use, the easier the tool is to damage, so the red-emitting electrode in AMOLED is damaged than blue-green The electrode should be slow, which means that the more the screen is used, the more red it will be. Therefore, in order to slow down this effect, some manufacturers will adjust the screen to be blue when it leaves the factory, so that the color of the screen will be normal after a period of use.
passive
Passive panels need backlight support, and there are mainly the following types.
The TN panel is called Twisted Nematic Panel (Twisted NemaTIc), and its low cost is destined to be the most widely used one. TN is sometimes called TFT (well, this is a popular and unscientific term for folks) , The disadvantages of TN panels are small viewing angles and limited color reproduction capabilities.
The full name of VA panel is Vertical Alignment, and there are two types: Fujitsu's MVA and Samsung's PVA. Compared with TN panels, VA panels can provide wider viewing angles and better color reproduction capabilities. Samsung's PVA (Patterned Vertical Alignment) panel technology is developed and inherited from Fujitsu's MVA. The disadvantages of VA panels are higher power consumption and higher prices.
The full name of IPS panel is In-Plane Switching. It is a panel technology developed by Hitachi in 1996. It is improved from TFT panel, so it is also called "Super-TFT" panel. IPS panels are divided into S-IPS, AS-IPS, H-IPS, S-IPS and E-IPS. They also have the advantages of large viewing angle and strong color reproduction ability, but their power consumption is higher than that of Super AMOLED screens. high.
CPA is a wide viewing angle panel (Continuous Pinwheel Alignment) with a continuous firework-like arrangement mode. This type of panel also belongs to Sharp. Sharp CPA panel has excellent color reproduction and viewing angle, but it is expensive. It should be noted that Sharp collectively refers to the wide-view technology products such as TN+Film, VA, and CPA that it has used as ASV.
Many high-end mobile phones use IPS panels as a selling point
Introduction of several display material technologies
a-Si is an amorphous silicon technology that is currently the most widely used. It has simple technology and low cost. However, the area of ​​the pixel itself occupied by the switch is so large that the brightness cannot be made very high (that is, the aperture ratio is low). In addition, PPI can only achieve a level of about 200 PPI.
IGZO is the abbreviation of Indium Gallium Zinc Oxide. It is a thin-film transistor technology that achieves better electronic performance by adding an IGZO metal oxide layer on the active layer of TFT-LCD. Compared with a-Si, its switching transistor is smaller in size and can achieve a higher pixel aperture ratio, and its PPI is generally below 300. The advantages of IGZO are high precision, low power consumption and high touch performance. Apple's iPad uses this technical panel.
LTPS (Low Temperature Poly-silicon) low temperature polysilicon technology was developed to solve the shortcomings of monocrystalline silicon. Compared with a-Si, LTPS integrates peripheral circuits into the panel substrate and has stronger operability and faster carrier movement. , The design of the panel is simpler, and the PPI can reach up to 500+. Generally, those above 300PPI use this technology. The representative products are HTC One X and iPhone 4/4S/5.
CGS (CG-silicon) continuous granular crystalline silicon screen technology is a variant of LTPS (Sharp’s official original "CG-silicon is a variant of the LTPS process using laser annealing to get larger domains"), its carrier moving speed It is 3 times that of LTPS (Low Temperature Poly-silicon) technology and 600 times that of ordinary A-si (amorphous silicon) technology. A higher aperture ratio can be achieved. Under the same backlight brightness condition, the screen brightness is higher, and when the screen brightness remains unchanged, a lower brightness backlight can be used to save power. In addition, it is lighter and thinner, resistant to impact and distortion.
CGS screen technology
About glass bonding and touch screen integration process
We have talked about single glass bonding technology in our previous reviews of multiple mobile phones. These technologies all integrate the touch control part into the inner glass or the display screen to achieve the purpose of reducing the thickness, simplifying the process, increasing the transparency of the screen, reducing reflection, and preventing dust. At present, this type of technology mainly includes One Glass/Touch on Lens solutions led by touch screen manufacturers, and On-Cell and In-Cell solutions led by panel manufacturers.
One Glass/Touch on Lens integrates the touch screen and the protective glass by plating an ITO conductive layer on the inside of the protective glass. The representative products are Meizu MX2 and Xiaomi mobile phone 2. If the mobile phone screen using this solution is broken, The touch will also fail;
ConclusionThis is the end of the related introduction about the LCD screen. If there are any deficiencies, please correct me.
Related reading recommendations: Analysis of various display panels, and recognize their advantages and disadvantages. Related reading recommendations: Understanding the difference between touch screens and display screens in one article
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