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Liquid crystal display screen

Liquid Crystal Display, commonly known in English as LCD (Liquid Crystal Display), belongs to a flat display, is a liquid crystal material as the basic component, between two parallel plates filled with liquid crystal material, 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, The image is scattered, and the color image can be displayed as long as the ternary color filter layer is added between the two plates.

Working principle

The principle of liquid crystal display is that liquid crystal will show different light characteristics under the action of different voltages. Liquid crystal is physically divided into two categories, one is Passive passive (also known as passive), this kind of liquid crystal itself does not emit light, the need to provide external light source, according to the position of the light source, and can be divided into reflection and transmission two types. The cost of Passive liquid crystal display is lower, but the brightness and contrast are not large, and the effective Angle of view is small, and the color saturation of color passive liquid crystal display is small, so the color is not bright enough. The other type is powered, mainly TFT (Thin Film Transitor). Each liquid crystal is actually a transistor that can emit light, so strictly speaking it is not a liquid crystal. Liquid crystal display is composed of many liquid crystal array, in the monochrome liquid crystal display, a liquid crystal is a pixel, and in the color liquid crystal display, each pixel is composed of red, green and blue three liquid crystal. At the same time, it can be considered that there is an 8-bit register behind each liquid crystal, and the value of the register determines the respective brightness of the three liquid crystal units, but the value of the register does not directly drive the brightness of the three liquid crystal units, but is accessed through a “palette”. It is not practical to have a physical register for each pixel, in fact there are only one line of registers, which in turn connect to each pixel row and load the contents of that row, driving all the pixel rows once to display a complete Frame.
Liquid crystal is a liquid in shape and appearance, but its crystalline molecular structure also shows the form of a solid. Like a metal in a magnetic field, when affected by an external electric field, its molecules will produce a precise ordered arrangement; If the arrangement of molecules is properly controlled, the liquid crystal molecules will allow light to penetrate; The path of light through a liquid crystal can be determined by the arrangement of the molecules that make it up, another characteristic of solids. 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 structure of the LCD is to place the liquid crystal box in two parallel glass substrates, the TFT (thin film transistor) is set on the lower substrate glass, the color filter is set on the upper substrate glass, and the rotation direction of the liquid crystal molecules is controlled by the signal and voltage change on the TFT, so as to control the polarized light emitted by each pixel and achieve the purpose of display.

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.
Liquid crystal display with its high definition, high brightness, high contrast, high color saturation and a series of high quality display advantages that other displays do not have are highly favored by users, and are widely used in many industries such as security, entertainment, advertising and so on.

Liquid crystal display can be divided into three categories according to the drive mode: Static drive (Static), Simple Matrix drive (Simple Matrix) and Active matrix drive (Active Matrix). Among them, the passive matrix type can be divided into Twisted Nematic type; TN), Super Twisted Nematic (Super Twisted Nematic; STN) and other passive matrix driven liquid crystal displays; Active matrix type can be roughly divided into Thin Film Transistor type (Thin Film Transistor; TFT) and two-terminal diode type (Metal/Insulator/Metal; MIM) two ways.
TN, STN and TFT type liquid crystal displays differ in viewing Angle, color, contrast and animation display quality due to the difference in the use of liquid crystal molecular torsion principle, so that they are also significantly different in the application range of products.
TN (Twisted Nematic) panel is a type of display screen. Due to its low production cost, TN has become the most widely used entry-level LCD panel, TN LCD is usually used in low-channel products, generally a field or pattern display with black characters on a white background. TN LCD has better contrast, and the driving conditions are not complicated. Common are watches, counters and other simple instruments.
STN (Super Twisted Nematic super twisted nematic nematic) has CSTN and DSTN points, belongs to the reflection LCD device, the power consumption is small, compared to the TFT-LCD screen in the product standby time is long, but this is at the expense of other aspects of performance. STN screen screen display is much less than TFT screen, the difference between color brightness and screen brightness is particularly obvious, so STN screen is difficult to use in outdoor daylight environment. The STN screen displays a slow response time of about 200 milliseconds, with severe tailing when playing animations. STN-LCD screen usually appears in the early launch of low-end color screen MP3, MP4 products, many thousand yuan below 4096 color screen MP3 often use this LCD panel, in the new product has been less seen, I believe in the future will be gradually replaced by other technologies.
TFT (Thin Film Transistor) is one of the active matrix liquid crystal display screen (AM-LCD), TFT-LCD screen on the back of the substrate set up a special light source, can “actively” on the screen of each independent pixel control. This is the origin of the so-called active matrix TFT (active matrix TFT), which can greatly improve the display response time, the general TFT response time is relatively fast, less than 80ms. And because TFT is an active matrix LCD, the arrangement of the liquid crystal can be memorized, and will not be restored immediately after the current disappears. TFT also improves the STN-LCD fuzzy flicker (water ripple) phenomenon, effectively improving the ability to play dynamic images. Compared with STN, TFT has excellent color saturation, color reproduction ability and higher contrast. The disadvantage is that it consumes more power and the production cost is relatively high.
TFT LCD LCD screen is the most mainstream type of LCD display at present, and it is widely used in desktop LCD monitors, notebook computers, TVS, mobile phones and other products.

LTPS (Low Temperature Polycrystalline Silicon, low temperature polycrystalline silicon) TFT-LCD technology is a new generation of technical products derived from TFT LCD. LTPS screens are manufactured by adding a laser processing process to conventional amorphous silicon (A-SI) TFT-LCD panels. The laser processing process of the LTPS screen, which uses a laser to melt liquid crystal materials and recrystallize them, can achieve much faster (about 99 times) electron flow speed in the transistor compared to the uncrystallized silicon of A-SI TFT-LCD, resulting in higher resolution and richer colors. LTPS LCDS offer greater design flexibility than other LCD technologies, allowing smaller transistors to be placed on the LCD panel. Therefore, it minimizes the space occupied by the peripheral circuits of the screen.
The number of components in the LTPS-LCD module can be reduced by 40%, and the connection part can be reduced by 95%, greatly reducing the chance of product failure. In addition, LTPS display screens offer significant improvements in energy consumption and durability. In terms of display parameters, LTPS low temperature polysilicon has made a great breakthrough, both horizontal and vertical viewing angles can reach 170 degrees, display response time up to 12ms, display brightness up to 500 nits, contrast up to 500:1.
LTPS-LCD technology has been around for many years and has many advantages in performance. Because the traditional a-SI TFT technology has been very mature in recent years, and the manufacturing of LTPS TFT LCD screen needs to be higher than the technical level of manufacturing traditional TFT-LCD, the number of manufacturers capable of manufacturing LTPS TFT LCD screen is relatively small, making the production and procurement cost of LTPS TFT LCD screen is relatively high. Therefore, there are not many products using LTPS LCD screens on the market.

Selection guide

1. Use environment

Consider the use environment: temperature, light, humidity, mechanical strength and other factors.
Operating temperature: LCD screen operating temperature range generally refers to normal temperature (0℃ ~ 50℃) or wide temperature (outside the range of -20℃ ~ 70℃), industrial LCD screen in line with special environmental applications, its operating temperature can reach -30℃-80℃.
Brightness: The higher the brightness value, the more beautiful the picture naturally, and the fog will not be hazy. The brightness unit of the LCD screen is cd/m2 or Nit(nIT), which has higher requirements for the brightness of the LCD screen in the face of different application environments, such as outdoor, ship and some special places.
According to the applicable ambient lighting:
a), reflective (no backlight, lighting or daytime use), suitable for outdoor with strong light.
b), transparent reflection type (with backlight, light or dark), can be opened and closed according to the light, suitable for outdoor.
c), transmission type (with backlight, light or darkness), the backlight must be on, not suitable for outdoor.
For high humidity and high mechanical vibration intensity: TAB or COG products are recommended, and the connection method is more reliable and not easy to mold. Because COB is connected by conductive tape, the seismic ability is weak, and the PCB gold finger part is prone to mildew.

2, LCD screen backlight selection

LCD screen backlight selection: LCD on the other hand can be divided into transmission type, reflection type, semi-inverse semi-transparent three types, because the liquid crystal is passive luminous display, so there must be an external light source will be displayed, transmitted liquid crystal must be coupled with background light, reflective liquid crystal needs strong ambient light, semi-inverse semi-transparent liquid crystal requires strong ambient light or backlight. There are usually three types of background light for liquid crystals: LED, EL, and CCFL(cold cathode luminescence).
Cold cathode fluorescent lamp CCFL is a low pressure mercury and rare gas discharge lamp using a cold cathode instead of a hot cathode such as tungsten wire. Under the action of a strong electric field, it relies on ion bombardment of electrodes composed of metals such as nickel, tantalum and zirconium to emit electrons, so that mercury atoms are excited and ionized to form a rich 253.7nm ultraviolet light and enough current. Ultraviolet radiation re-excites the phosphor coating on the tube wall to emit light. Because the composition of the fluorescent material is different, it can emit different colors of light. It has the advantages of high brightness, high efficiency, low energy consumption, long life, and long time effective light. Good shock-proof performance, repeatable light out, can be freely started at low temperatures. Compared with the hot cathode fluorescent lamp, it has low calorific value and small diameter, and can be used as a light source for various liquid crystal display.
Comparison of several backlights:
CCFL is suitable for large and medium-sized transmission type liquid crystal, requiring light guide plate, high brightness, long life, and inverter.
Leds are suitable for small and medium-sized liquid crystals, require light guides, high brightness, life of up to 50,000 hours, and usually do not require a dedicated power supply.
EL is suitable for small and medium-sized liquid crystals, ultra-thin (<0.4MM), low brightness, low power consumption, and short life (2000 hours).
3, interface mode selection
Based on CPU resources, select the interface mode and whether the LCD module has a built-in controller.
Now many ARM chips have LCD controllers inside, you can consider the cost, choose the right LCD module.
Usually, the liquid crystal module below 128 lines does not require a special liquid crystal controller, and this kind of liquid crystal driver IC is to integrate the drive and control together. Lines above 128 to 240 usually use the driver IC+ control IC.
Comparison of STN and FSTN: FSTN is faster, better contrast and viewing Angle, suitable for high line number, the cost is 10~20% higher than STN, and the color and background color are slightly different.
TAB and COB comparison: TAB connection is reliable, ultra-thin structure, low power consumption, low structural strength, weak impact resistance. COB has high structural strength, heavy structure, poor connection mode and large power consumption, which is suitable for desktop computers.


When cleaning the LCD screen, do not wipe the surface of the screen with any alkaline solution or chemical solution. The stains of the LCD panel are generally divided into two kinds, one is because of the dust in the air stuck over time, and the other is the fingerprints and oil left by the user inadvertently. Many people have misunderstandings when cleaning the LCD screen, home appliance experts remind that if you do not pay attention, these details have some damage to the LCD.
1. Wipe the LCD screen with a soft cloth or paper towel. Do not use general soft cloth (such as glasses cloth) or paper towels to wipe the LCD screen, for the soft LCD screen, their surface is still too rough, it is easy to scratch the delicate LCD screen.
2. Clean the LCD screen with water. When clean with water, liquid is easy to drip into the inside of the LCD screen, which will cause a short circuit of the equipment, thus burning out expensive electronic equipment.
3. Clean the LCD screen with alcohol and other chemical solvents. LCD screens are coated with a special coating on the screen, once the use of alcohol to wipe the display screen, it will dissolve this special coating, causing adverse effects on the display effect.


The LCD is abnormal or not displayed

The first trick: check whether the connection between the monitor and the graphics card is loose. Poor contact will lead to the appearance of “clutter”, “clutter” shaped flower screen is the most common phenomenon.
The second trick: Check whether the graphics card is overclocked. If the graphics card is overclocked, it will generally appear irregular and discontinuous stripes. At this time, the overclocking amplitude should be appropriately reduced. Note that the first thing to reduce the memory frequency.
The third trick: Check the quality of the video card. If there is a problem with the screen after replacing the video card, and after the use of the first and second tricks fails to work, the anti-electromagnetic interference and electromagnetic shielding quality of the video card should be checked. The specific method is: some parts that may produce electromagnetic interference are as far away from the graphics card installation as possible (such as hard disk), and then see whether the screen disappears. If it is determined that the electromagnetic shielding function of the video card is not enough, the video card should be replaced, or the homemade shield.
Fourth tip: Check whether the resolution or refresh rate of the display is set too high. The resolution of liquid crystal display is generally lower than that of CRT display, if it exceeds the best resolution recommended by the manufacturer, it may appear the phenomenon of flower screen.
Tip 5: Check if you have an incompatible video card driver installed. This situation is generally easy to overlook, because the graphics driver update rate is getting faster and faster (especially NVIDIA graphics cards), some users always can’t wait to install the latest version of the driver. In fact, some of the latest drivers are either beta versions or versions optimized for a specific graphics card or game, and using such drivers can sometimes result in a splitscreen. Therefore, it is recommended that you try to use drivers certified by Microsoft, and it is best to use drivers provided by graphics card manufacturers.
Sixth trick: If the use of the above five tricks, still can not solve the problem, it may be the quality of the display. In this case, replace the monitor with another monitor.

Liquid crystal display screen