Working principle and characteristics of liquid Crystal display (lcd display)

- Oct 08, 2019-

1:Liquid crystal display (lcd) is now a very common display. It has the advantages of small volume, light weight, power saving, low radiation, easy to carry and so on. The principle of liquid crystal display (lcd) is very different from that of cathode ray tube display (crt). LCD is a display device based on liquid crystal electro-optic effect. It includes the character segment display device of the segment display mode, the character, graph and image display device of the matrix display mode, the large screen liquid crystal projection TV LCD screen of the matrix display mode, and so on. The working principle of liquid crystal display is to make use of the physical characteristics of liquid crystal to turn on when electrified, so that the arrangement of liquid crystal becomes orderly and the light is easy to pass through; when the power is not turned on, the arrangement becomes chaotic and prevents the light from passing through. The working principle of three kinds of liquid crystal displays is introduced below. 1. "twisted nematic liquid crystal display" (twisted nematic liquid crystal display), abbreviated as "t type liquid crystal display". The liquid crystal assembly structure of this display is shown in figure 11. Nematic liquid crystals are sandwiched between two pieces of glass. The glass is first coated with a transparent and conductive film as an electrode. This kind of thin film is usually a kind of indium (indium) and tin (tin) oxide (oxide), abbreviated as ito.. The surface dispensing agent is then coated on the glass with ito so that the liquid crystal is arranged in a specific direction parallel to the glass surface. (Fig. 11 a) the left glass makes the liquid crystal line up and down, while the right glass rows the liquid crystal perpendicular to the surface. The natural state of liquid crystal in this component is distorted from left to right, which is why it is called twisted liquid crystal display. According to the principle that the liquid crystal can be rotated by the electric field, adding voltage to the two electrodes will make the polarization direction of the liquid crystal turn parallel to the direction of the electric field. Because the refractive index of liquid crystal changes with the direction of liquid crystal, the result is that the polarization of light will change after passing through t n liquid crystal cell. We can choose the appropriate thickness to make the polarization direction of light just change. Then we can use two parallel polarizers to make light completely impossible (figure 12). If the voltage v is large enough to make the direction of liquid crystal turn parallel to the direction of electric field, the polarization of light will not change. Therefore, light can pass through the second polarizer smoothly. As a result, we can use the electric switch to control the light and darkness. In this way, the characters can be displayed on the screen when they are white when they are transparent and black when they are not transparent. 

3-2-inch-tft-lcd

 2. The principle of TFT liquid crystal display also adopts the design of filling liquid crystal molecules between two clips. The electrode of the left interlayer is changed to fet transistor, and the electrode of the right interlayer is changed to common electrode. In the light source design, the tft display adopts the "back through" irradiation mode, that is, the hypothetical light source path is not from left to right like tn liquid crystal, but from right to left. This method is to set up a fluorescent lamp light tube on the back of the liquid crystal. When the light source is irradiated, the light is transmitted to the left through the right polarizer, and the light is transmitted by liquid crystal molecules. Because the electrode of the left and right interlayer is changed to fet electrode and common electrode, the performance of liquid crystal molecules, such as the arrangement state of tn liquid crystal, will change when the fet electrode is turned on, and the display will also be achieved by shading and transmittance. But the difference is that because the fet transistor has capacitance effect and can maintain the potential state, the previous transparent liquid crystal molecules will remain in this state until the fet electrode is next electrified to change its arrangement. Relatively speaking, tn does not have this property, and liquid crystal molecules return to their original state as soon as they are not pressured, which is the biggest difference between tft liquid crystal and tn liquid crystal display principle. 


H320_01

3. Polymer scattered liquid crystal display (polymer dispersed liquid crystal display), abbreviated as PDLC liquid crystal display. The liquid crystal assembly structure of this display is shown in figure 13. The polymer monomer (monomer) is mixed with liquid crystal and sandwiched between two pieces of glass to make a liquid crystal box. The glass, which is the same as the one used above, is coated with a transparent and conductive film as electrode. However, there is no need to coat the glass with a surface dispensing agent. At this time, the liquid crystal cell is irradiated under UV lamp to connect a monomer into a polymer. At the same time, liquid crystal is separated from polymer to form many small liquid crystal particles. These small particles are fixed by polymer. When the light is irradiated on the liquid crystal cell, the refractive index is different, and the refraction and reflection are produced at the surface of the particle. After multiple reflection and refraction, scattering (scattering). Is produced. This liquid crystal box is like milk with an opacified milky white.