LCD display principle

Polarizer 

Everyone knows that light is also a kind of wave, and the traveling direction of light waves is perpendicular to the electric and magnetic fields. At the same time, the electric and magnetic field components of the light waves are also perpendicular to each other. That is to say, the direction of travel and the electric and magnetic field components are parallel to each other. The polarizer acts like a fence, blocking the component perpendicular to the fence and only allowing the component parallel to the fence to pass. When rotating the relative angle of the two polarizing plates, you will find that with the relative angle, the brightness of the light will become darker and darker. When the fence angles of the two polarizing plates are perpendicular to each other, the light cannot pass through at all. The liquid crystal display is done by using this feature. Use the upper and lower fences between the vertical polarizers to fill the liquid crystal, and then use the electric field to control the rotation of the liquid crystal to change the direction of light. In this way, different electric field sizes will result in different grayscale brightness.

Two layers of glass and alignment film

The upper and lower layers of glass are mainly used to clamp the liquid crystal, and there are thin film transistors on the lower layer of glass. (Thin film transistor, TFT), and the upper layer of glass is pasted with a color filter. The two pieces of glass are not smooth on the side that touches the liquid crystal, but have jagged grooves. The main purpose of this groove is to hope that the long rod-shaped liquid crystal molecules will be arranged along the groove. In this way, the arrangement of the liquid crystal molecules will be neat. Because if it is a smooth plane, the arrangement of liquid crystal molecules will be irregular, causing light scattering and light leakage. In fact, this is just a theoretical explanation, telling us that we need to deal with the contact surface of glass and liquid crystal so that the arrangement of liquid crystal has a certain order. However, in the actual manufacturing process, it is impossible to make the glass have such a groove-like distribution. Generally, a layer of PI (polyimide) is coated on the surface of the glass, and then a cloth is used for rubbing. So that the surface molecules of PI are no longer strayly distributed, and will be arranged in a fixed and uniform direction. And this layer of PI is called the alignment film, and its function is like a groove of glass, providing the interface conditions for the uniform arrangement of liquid crystal molecules, so that the liquid crystals are arranged in a predetermined order.

TN LCD liquid crystal display principle

When no voltage is applied between the upper and lower glasses, the arrangement of the liquid crystal will be determined by the alignment films of the two liquid crystal glasses. For TN-type liquid crystals, the angle difference between the upper and lower alignment films is exactly 90 degrees. So the liquid crystal molecules The arrangement will automatically rotate 90 degrees from top to bottom. When the incident light passes through the upper polarizer, only unidirectional polarized light waves will remain. When passing through the liquid crystal molecules, because the liquid crystal molecules rotate 90 degrees in total, when the light wave reaches the lower polarizer, the polarization direction of the light wave is exactly 90 degrees. The angle between the lower polarizer and the upper polarizer is exactly 90 degrees. Therefore, the light can pass smoothly, but if we apply voltage between the upper and lower glass, because TN-type liquid crystals are mostly liquid crystals with positive dielectric anisotropy (ε//>ε⊥, which represents parallel The dielectric constant of the direction is larger than the dielectric constant of the vertical direction. Therefore, when the liquid crystal molecules are affected by the electric field, their arrangement direction tends to be parallel to the direction of the electric field. The arrangement of the liquid crystal molecules becomes standing and passes through the upper polarizing plate. The unidirectional polarized light wave will not change the polarization direction when passing through the liquid crystal molecules, so it cannot pass through the lower polarizer.

TFT LCD liquid crystal screen display principle

Thin film transistor liquid crystal display. We mentioned that liquid crystal display needs voltage control to generate gray scale. The display that uses thin film transistors to generate voltage to control the rotation of liquid crystals is called TFT LCD. From the perspective of the cross-sectional structure, the liquid crystal is sandwiched between the upper and lower layers of glass to form a parallel plate capacitor, which we call CLC (capacitor of liquid crystal). Its size is about 0.1pF, but in practical applications, this capacitor cannot maintain the voltage until the next time the screen data is updated. That is to say, when the TFT charges the capacitor, it cannot hold the voltage until the next time the TFT recharges this point. (At a general image update frequency of 60Hz, it needs to be maintained for about 16ms.) In this way, if the voltage changes, the displayed gray scale will be incorrect. Therefore, generally in the panel design, a storage capacitor CS (storage capacitor is about 0.5pF) is added so that the charged voltage can be maintained until the next time the screen is updated. But to be correct, the TFT itself growing on glass is just a switch made of transistors. Its main job is to determine whether the voltage on the LCD source driver should be charged to this point. As for how high voltage should be charged at this point, in order to display what kind of grayscale. It is determined by the external LCD source driver.