Black-and-white liquid crystal display devices generally use TN (Twist Nematic) to twist nematic liquid crystals. Its working principle is that a nematic liquid crystal is sandwiched between two glass plates, and a transparent conductive film ITO (indium tin oxide) is first plated on the surface of the glass. For use as an electrode, an alignment layer PI (polyimide) is then coated on a glass with a thin film electrode so that the liquid crystal is aligned in a direction that is specific and parallel to the glass surface. The natural state of the liquid crystal has a twist of 90 degrees, and the liquid crystal molecules are rotated by the electric field, and the birefringence of the liquid crystal changes with the direction of the liquid crystal. As a result, the polarized light rotates after passing through the TN liquid crystal. As long as the proper thickness is selected so that the polarization direction of polarized light is just changed by 90 degrees, two parallel polarizers can be used so that the light cannot pass through at all. The sufficiently large voltage can make the direction of the liquid crystal parallel to the direction of the electric field, so that the polarization direction of the light will not change and the light can pass through the second polarizer. As a result, light and darkness can be controlled. A color liquid crystal display generally uses a thin film transistor (TFT) thin film transistor liquid crystal display. The principle is to deposit a layer of silicon on a glass substrate, and to fabricate a transistor array by printing photolithography and the like. Pixels are equipped with a semiconductor switch whose processing process is similar to large-scale integrated circuits. The liquid crystal is then poured between the two glass plates. Since each pixel can be directly controlled by a point pulse, each node is relatively independent and can be continuously controlled. This design not only improves the response speed of the display screen, At the same time can accurately control the display gray, so the TFT LCD color more realistic, called true color. TFT is divided into polysilicon, amorphous silicon and monocrystalline silicon.