LCD structure, working principle and main technical indicators (With TN LCD as an example).
LCD structure
① Polarizer; ② Base plate glass; ③ SiO2 barrier layer; ④ Electrodes; ⑤ Directional membrane; ⑥ Sealing frame; ⑦ Transition electrode; ⑧ Liquid crystal; ⑨ Reflector; ⑩ Sealing glue
Working principle of LCD:
To understand the working principle of LCD, we must first understand the light. This is an electromagnetic wave. That is, the electromagnetic field propagates in the form of wave. The wavelength of light visible to the human eye ranges from 380 nm to 780 nm. Generally, light propagates along a straight line, and the vibration direction of light wave is perpendicular to the propagation direction of light. For natural light (such as sunlight), the vibration direction of light wave is randomly and uniformly distributed in each plane perpendicular to the light propagation direction. If the direction of light wave vibration is along one direction, such light is called polarized light, and this direction of vibration is called polarization direction. The plane formed by the polarization direction and the propagation direction of light wave is called the vibration plane.
The polarizer has a fixed polarizing axis. The function of the polarizer is to allow only the light whose vibration direction is consistent with the direction of its polarization axis to pass through. The light whose vibration direction is perpendicular to the polarization axis will be absorbed by it. In this way, when the natural light passes through the incident polarizer (called its Polarizer) of the liquid crystal cell, only the light whose vibration direction is the same as the polarizing axis of the polarizer is left. It becomes linearly polarized light. The polarized light passes through the liquid crystal cell and then passes through the polarizer (called the polarizer). In this way, whether the light passes through the polarizer depends on the polarization state of the linearly polarized light after passing through the liquid crystal cell. Thus, the display can be realized by controlling the light state of the polarizer.
Specifically, the liquid crystal molecules in the TN type liquid crystal cell form a twisted structure. Under certain conditions, the polarization of incident light will rotate along the twisted direction of liquid crystal molecules. As shown in figure 4-2-1, the long axis of the liquid crystal molecule is twisted by 900, resulting in the optical rotation of 900. When a certain voltage is applied to the electrodes on the two glass sheets, the liquid crystal molecules will be arranged perpendicular to the upper and lower glass sheets, and the twisted structure will disappear, resulting in the disappearance of the optical rotation. This electro-optic effect is called the twisted electric field effect.
For the black type LCD with white background, the upper and lower polarizers are placed orthogonally. That is, the polarization axes are perpendicular to each other, and the incident natural light becomes plane polarized light after passing through the polarizer. When the liquid crystal cell is not applied with an electric field, the polarized light will twist 900 along the twisted structure of the molecules, and the vibration direction will become consistent with the polarization axis of the polarizer. Therefore, it can pass through the polarizer smoothly. At this time, the display is transparent and in a non display state. At the same time, when the driving circuit adds the driving signal voltage to the relevant electrodes to be displayed, the twisted structure of the liquid crystal molecules will disappear. Loss of optical rotation. The polarized light from the polarizer goes directly to the polarizer without change. Because its polarization direction is perpendicular to the axis direction of the polarizer. The polarized light will not pass through the polarizer, so some electrodes of the on signal voltage will be black and display.