LCD is the full name of Liquid Crystal Display, mainly including TFT, UFB, TFD, STN and other types of LCD. TFT is commonly used in notebook LCD screens. TFT (Thin Film Transistor) refers to thin film transistors. Each liquid crystal pixel is driven by a thin film transistor integrated behind the pixel, which can display screen information at high speed, high brightness, and high contrast, which is currently the best One of the LCD color display devices is now the mainstream display device on notebook computers and desktop computers. Compared with STN, TFT has excellent color saturation, reduction ability and higher contrast. It can still be seen very clearly under the sun, but the disadvantage is that it consumes more power and costs more.

The working principle of LCD display

A liquid crystal display (LCD) has liquid crystal material sandwiched between two sheets of glass. Without any voltage applied between transparent electrodes, liquid crystal molecules are aligned in parallel with the glass surface. When voltage is applied, they change their direction and they turn vertical to the glass surface. They vary in optical characteristics, depending on their orientation. Therefore, the quantity of light transmission can be controlled by combining the motion of liquid crystal molecules and the direction of polarization of two polarizing plates attached to the both outer sides of the glass sheets. LCDs utilize these characteristics to display images.

You may have such a question—“what is the liquid crystal？”  well, it is a substance which flows like a liquid but has some degree of ordering in the arrangement of its molecules.

In order to better understand the working principle of LCD display, I specially made a schematic diagram of the working principle of LCD display for you, as shown below

Working characteristics of LCD screen

A: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, if the molecules on one plane are arranged north-south, the molecules on the other plane are arranged east-west, and the molecules between the two planes are 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 rearrange vertically, so that the light can be directed out without any twisting.

B:The second characteristic of LCD is that it relies on polarization filters and the light itself. Natural light diverges randomly in all directions. Polarization filters are actually a series of increasingly thin 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, can the light be penetrated.

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 by the liquid crystal molecules 90 degrees, 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, the light is blocked when power is applied, and the light is emitted when the power is not applied.

Types of LCD display

A: TN(Twisted Nematic)

This is the most common type of LCD screen, it is the abbreviation of baiTwisted Nematic panel.This is a low-cost entry-level LCD panel with the most widely used. It is widely used in the mainstream low-end LCD displays on the market. At present, most of the TN panels we see are modified TN+films. Film is the compensation film, which is used to make up for the lack of viewing angles of TN panels. The current viewing angles of improved TN panels reach 160°

 

The advantage of TN panel is that the output gray class number is less, the deflection speed of liquid crystal molecules is fast, and the response time is easy to improve. At present, the TN panel is basically used in LCD products below 8ms in the market. In addition, Samsung has developed a b-TN (best-tn) panel, which is actually an improved type of TN panel. In order to balance the high-speed response of the TN panel, the image quality must be sacrificed. At the same time, the contrast can reach 700 ∶ 1, which is close to the MVA or early PVA panel.

How to quickly distinguish the TN screen from a lot of LCD screens？—Touch the display screen with your finger, when the water ripple appears, you can conclude that it is a TN screen

B: Super-twisted nematic display

STN liquid crystal (STN) is the abbreviation of super twisted nematic liquid crystal. After the invention of TN liquid crystal, people naturally think of using TN liquid crystal matrix to display complex graphics. Compared with TN liquid crystal, the torsion of STN liquid crystal is 180 to 270 degrees. In the early 1990s, color STN liquid crystal came into being. A pixel of this liquid crystal is composed of three liquid crystal units, covered with a layer of color filter board, and the brightness of the liquid crystal unit is controlled by voltage to generate color.

Although STN liquid crystal is composed of TN liquid crystal unit with relatively fast reaction speed, its particle size, scanning speed and brightness are not as good as those of TFT liquid crystal. Due to its low price, STN liquid crystal is widely used as the main display in palm mobile phones, video game machines and notebook computers produced before 2000. Now it has basically withdrawn from the consumer market.

STN liquid crystal has black characters on yellow green background, and white characters on blue background. The former is used in the case of pursuing contrast, and the latter is used in occasions emphasizing brightness

C: In-Plane Switching

The biggest feature of IPS panel is that its two poles are on the same surface, unlike other liquid crystal mode, the electrodes are on the upper and lower sides, three-dimensional arrangement. This technology optimizes the arrangement of liquid crystal molecules, adopts horizontal arrangement. When encountering external pressure, the molecular structure will sink down slightly, but the whole molecule is still horizontal. When encountering external force, the rigidity and stability of hard screen liquid crystal molecular structure is far better than that of soft screen! Therefore, it will not produce distortion and affect the color of the picture, which can protect the image effect from being damaged to the greatest extent

The IPS hard screen technology changes the arrangement of liquid crystal molecules and adopts horizontal conversion technology to make the response speed of the LCD screen faster and more stable. When dealing with continuous dynamic images, a major advantage of horizontal conversion is that it accelerates the deflection speed of liquid crystal molecules, which is reflected in the advantage of the fast response speed of the IPS hard screen, which makes the image movement trajectory more delicate and clear, and solves the troubles. The problem of image smear and jitter.
When consumers enjoy high-speed motion pictures for a long time, the IPS hard screen can ensure clear, smooth and real effects, and the stimulation to the eyes is significantly reduced, which fully meets the health requirements.

D: OCB(Optical Compensated Birefringence)

The crystal molecules are always perpendicular to the screen, while the liquid crystal molecules before and after have a symmetrical deflection angle—the farther from the center, the greater the deflection angle, and the liquid crystal molecules in contact with the front and rear glass panels are parallel to the screen. If a voltage is applied to it, these liquid crystal molecules begin to deflect and eventually appear symmetrical superposition. Since the liquid crystal layer is symmetrical at any time, the phase deviation caused by the birefringence of the lower liquid crystal molecules can just be offset by the upper liquid crystal molecules, and a very outstanding display effect can be obtained and increased to a certain extent. Perspective

The biggest advantage of OCB is its super fast response speed. As you can see in the figure, the deflection angle of the liquid crystal molecules is extremely small under the power-on state, and the conversion speed is very fast, reaching below 10 milliseconds easily. At present, there are even products of 1 millisecond to 5 milliseconds, which are very suitable for use. For displaying dynamic images. There is no doubt that OCB is by far the fastest LCD display technology-at least in this index, OCB is far ahead of other competitors. The second advantage of OCB that other technologies cannot catch up with is the bright display effect. The unique structure allows the liquid crystal molecules to have the characteristics of light-compensating birefringence, allowing it to achieve high color purity more than three times the traditional TFT-LCD, and the output is rich and colorful Color, which is unmatched by current LCD display technologies

E:ASV(advance super view)

From a certain point of view, this is not a type of panel technology, but a technology used to improve image quality, mainly by reducing the spacing between the particles on the liquid crystal panel, increasing the aperture on the liquid crystal particles, and Adjust the overall arrangement of the liquid crystal particles to reduce the reflection of the LCD TV and increase the brightness, viewing angle and contrast. The liquid crystal panel using ASV technology is designed for the reflection and transmission of light. It treats the external light by adding several layers of film optical substances with special chemical coatings on the surface of the screen. On the one hand, it refracts into different proportions. The reflected light can change direction and cancel each other. On the other hand, it can absorb external light to the maximum extent and change the wavelength and reflection of light propagation.

For panels with ASV technology, when the product is tested at the factory, the contrast is tested in full black, because this is the way to measure the best value. But in this way, the contrast value under normal light is ignored. In an environment with light reflection, both the brightness and contrast of the screen will decrease to a certain extent. In order to overcome this problem, Sharp used a low-reflective coating on the ASV panel, which can maximize the black purity of the TV under the condition of light reflection and improve the overall contrast. According to Sharp’s claim, under normal indoor light, ASV panels can have a contrast ratio of 650:1, and in AQUOS home environment, the contrast ratio is as high as 1200:1.