Basic knowledge of LCD

Basic knowledge of LCD

Polarizer 

Light is also a kind of fluctuation. 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 components of the electric field and magnetic field are parallel to each other. The polarizer acts like a fence, blocking the components perpendicular to the fence and only allowing the components parallel to the fence to pass. So if we pick up a polarizer and look at the light source, it will feel like wearing sunglasses, and the light will become darker. But if you stack two polarizers together, it will be different. When you rotate 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, light cannot pass through at all. And the liquid crystal display is completed 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.

Upper and lower two layers of glass and alignment film 

The upper and lower layers of glass are mainly used to clamp the liquid crystal. There are thin film transistors (TFT) on the bottom layer of glass, and the color filter (Color filter) is attached to the top layer of glass. If you notice, 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, the glass cannot be made into 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 an alignment film, and its function is like a groove in 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(Twisted Nematic) LCD

When there is no voltage applied between the upper and lower glass, the arrangement of the liquid crystal will be determined by the alignment film of the upper and lower glass. For TN-type liquid crystal, the angle difference between the upper and lower alignment films is exactly 90 degrees. So the arrangement of the liquid crystal molecules will automatically rotate 90 degrees from top to bottom. When the incident light passes through the upper polarizer, only Light waves polarized in one direction. When passing through the liquid crystal molecules, since the liquid crystal molecules have rotated a total of 90 degrees, when the light wave reaches the lower polarizer, the polarization direction of the light wave turns by exactly 90 degrees. The angle between the lower polarizer and the upper polarizer is exactly 90 degrees. So the light can pass smoothly, but if we apply a voltage between the upper and lower glass, because the TN type liquid crystal is mostly different in dielectric coefficient Positive liquid crystals (ε// >ε⊥, which means that the permittivity in the parallel direction is greater than that in the vertical direction, so 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. ), so we can see that the arrangement of liquid crystal molecules becomes standing. At this time, the unidirectional polarized light wave passing through the upper polarizer will not change the polarization direction when passing through the liquid crystal molecules, so it cannot pass through the lower polarizer.