The principle of Liquid crystal display

1. TN type liquid crystal display principle

The TN-type liquid crystal display technology can be said to be the most basic in the liquid crystal display, and other types of liquid crystal displays can also be said to be improved with the TN-type as the origin. Likewise, its working principle is simpler than other technologies. Shown in the figure is a simple structure diagram of a TN-type liquid crystal display, including polarizers in vertical and horizontal directions, an alignment film with fine-grained grooves, a liquid crystal material, and a conductive glass substrate. (1) In the case of no electric field, the incident light passes through the liquid crystal layer after passing through the polarizer, and the polarized light is rotated 90 degrees by the liquid crystal layer whose molecules are twisted and arranged. Therefore, the light can pass through smoothly, and the entire electrode surface is bright. (2) When the electric field is added, the optical axis of each liquid crystal molecule turns in the same direction as the electric field, and the liquid crystal layer loses the ability to rotate light. As a result, the polarized light from the incident polarizer has the same polarization direction as the polarized light of another polarizer. The direction is in a vertical relationship and cannot pass through, so the electrode surface is in a dark state. The imaging principle is to place the liquid crystal material between two transparent conductive glass attached with a polarizer perpendicular to the optical axis, and the liquid crystal molecules will be rotated and arranged in sequence according to the direction of the fine grooves of the alignment film. If the electric field is not formed, the light will be smooth. Entered from the polarizer, the liquid crystal molecules rotate in their traveling direction, 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 liquid crystal molecules between them, causing the molecular rods to twist, so that the light cannot penetrate, thereby blocking 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. Almost all liquid crystal displays used in electronic products are made using the principle of twisted nematic field effect.

2. STN liquid crystal display principle

The display principle of STN type is similar to that of TN, 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.

It should be noted here that a simple TN liquid crystal display itself has only two situations of light and dark (or black and white), and there is no way to change the color. The STN liquid crystal display involves the relationship between liquid crystal materials and the interference phenomenon of light, so the displayed hues are mainly light green and orange. But if a color filter (color filter) is added to the traditional monochrome STN liquid crystal display, and any pixel (pixel) of the monochrome display matrix is divided into three sub-pixels (sub-pixel), which pass through the color filter respectively. Display the three primary colors of red, green and blue, and then through the reconciliation of the ratio of the three primary colors, it can also display the colors of the full color mode. In addition, if the display screen of TN type liquid crystal display is bigger, the contrast ratio of the screen will appear poor, but with the improved technology of STN, it can make up for the lack of contrast ratio.

3. TFT liquid crystal display principle

TFT-type liquid crystal displays are more complex, and the main components include fluorescent tubes, light guide plates, polarizers, 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 the backlight, that is, the fluorescent tube to project the light source. These light sources will first pass through a polarizer 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 then pass through the color filter in front and another polarizer. Therefore, as long as we change the voltage value that stimulates the liquid crystal, we can control the final light intensity and color, and then we can change the color combination of different shades on the liquid crystal panel.