LCD (Liquid Crystal Display)

LCD

LCD (Liquid Crystal Display)

Liquid crystalline substance is a compound called helical toluate extracted from plants. It has two melting points at different temperatures when heated, but it has both liquid and crystalline properties within a certain temperature range. It is named "Liquid Crystal"

Liquid crystal displays can be classified into static drive (Static), simple matrix drive (Simple Matrix) and active matrix drive (Active Matrix) according to the driving mode. Among them, the passive matrix type can be divided into twisted nematic (Twisted Nematic; TN), super twisted nematic (Super Twisted Nematic; STN) and other passive matrix-driven liquid crystal displays; and the active matrix type can be roughly divided into Thin Film Transistor (TFT) and two-terminal diode (Metal/Insulator/Metal; MIM) two methods. TN, STN and TFT type liquid crystal displays are different in Viewing Angle, color, contrast, and animation display quality due to the different principle of twisting liquid crystal molecules, which makes them clearly differentiated in the application range of products. In terms of the current scope and level of application of liquid crystal display technology, the active matrix drive technology is based on thin film transistor type (TFT) as the mainstream, and is mostly used in notebook computers and animation and image processing products. The pure matrix drive technology is currently based on twisted nematic (TN) and super twisted nematic (STN), and the current applications are mostly word processors and consumer products.

The TN type is the most basic of liquid crystal displays, and the principle is simple. The TN type liquid crystal display includes vertical and horizontal polarizing plates, an alignment film with fine grooves, liquid crystal materials and a conductive glass substrate. The imaging principle is to place the liquid crystal material between two pieces of transparent conductive glass attached to the vertical polarizing plate of the optical axis. The liquid crystal molecules will rotate in order according to the direction of the fine grooves of the alignment film. If the electric field is not formed, the light will be smooth Inject from the polarizer, rotate the direction of travel 90 degrees according to the liquid crystal molecules, and then exit from the other side. If the two pieces of conductive glass are energized, an electric field will be created between the two pieces of glass, which will affect the arrangement of the liquid crystal molecules between them, so that the molecular rods will be twisted, and the light will not be able to penetrate, thus covering the light source. The phenomenon of light-dark contrast obtained in this way is called twisted nematic field effect, or TNFE (twisted nematic field effect) for short. The liquid crystal displays used in electronic products are almost all made by the principle of twisted nematic field effect. The pure TN liquid crystal display itself has only two situations (or black and white), and there is no way to change the color.

The display principle of STN type is similar. The difference is that the liquid crystal molecules of the TN twisted nematic field effect rotate the incident light by 90 degrees, while the STN super twisted nematic field effect rotates the incident light by 180 to 270 degrees. What I want to explain here is that a pure TN liquid crystal display itself has only two situations (or black and white), and there is no way to achieve color changes. The STN liquid crystal display involves the relationship between liquid crystal materials and the interference of light, so the color tones displayed are mainly light green and orange. However, if a color filter is added to the traditional monochrome STN liquid crystal display, any pixel (pixel) of the monochrome display matrix is divided into three sub-pixels (sub-pixel), which pass through the color filter. The film displays the three primary colors of red, green, and blue, and through the reconciliation of the ratios of the three primary colors, the colors of the full-color mode can also be displayed. In addition, if the TN-type liquid crystal display has a larger display screen, its screen contrast will appear poorer, but the improved technology of STN can make up for the lack of contrast.

TFT-type liquid crystal displays are more complicated, and the main components include fluorescent tubes, light guide plates, polarizing plates, filter plates, glass substrates, alignment films, liquid crystal materials, thin-mode transistors, and so on. First of all, the liquid crystal display must first use a backlight, that is, a fluorescent tube to project a light source. These light sources pass through a polarizing plate and then through the liquid crystal. At this time, the arrangement of the liquid crystal molecules changes the angle of light penetrating the liquid crystal. Then the light must pass through the color filter film and another polarizing plate in the front. Therefore, we can control the final light intensity and color by changing the voltage value that stimulates the liquid crystal, and then can change the color combination of different shades on the liquid crystal panel.