Mobile Phone LCD Screen Quality Control Expert

The working principle of matrix LCD module

by:Kimeery     2021-08-23
The matrix liquid crystal module (LCM) is one of three types of liquid crystal modules, which are divided into passive matrix liquid crystal modules and active matrix liquid crystal modules according to whether switching devices are used. The passive matrix liquid crystal module is driven by applying a voltage to the pixels at the intersection of the upper and lower substrate electrodes, while the active matrix liquid crystal module is driven by switching devices on each pixel. Passive matrix liquid crystal modules are greatly restricted in terms of brightness and contrast characteristics. The response speed is also slow. Due to defects in display characteristics, this type of display is not conducive to the development of a desktop display. Active matrix liquid crystal modules are divided into two-terminal type and three-terminal type according to the number of terminals of the switching device. Two-terminal switching devices use diodes and metal-insulator-metal devices, and three-terminal switching devices use TFTs. According to the different types of TFT active layers, three-terminal switches are classified into amorphous silicon TFTs, polysilicon TFTs, and CdSe TFTs. At present, most of the switching devices of TFT-LCD products use amorphous silicon TFTs, and some small-size products have turned to polysilicon TFTs, and CdSe TFTs were only produced in 1998 for a period of time. 1. The working principle of passive matrix LCD The display principle of TN-LCD, STN-LCD and DSTN-LCD is basically the same, the difference is that the twist angle of the liquid crystal molecules is somewhat different. Let's take a typical TN-LCD as an example to introduce its structure and working principle. In the TN-LCD LCD screenpanel with a thickness of less than 1 cm, it is usually a plywood made of two large glass substrates with a color filter, an alignment film, etc. inside? Two polarizing plates are wrapped on the outside. They can determine the maximum luminous flux and color production. The color filter is a filter composed of three colors of red, green, and blue, which are regularly fabricated on a large glass substrate. Each pixel is composed of three color units (or called sub-pixels). If a panel has a resolution of 1280×1024, it actually has 3840×1024 transistors and sub-pixels. The upper left corner (gray rectangle) of each sub-pixel is an opaque thin film transistor, and the color filter can produce the three primary colors of RGB. Each interlayer contains electrodes and grooves formed on the alignment film, and the upper and lower interlayers are filled with multiple layers of liquid crystal molecules (the liquid crystal space is less than 5×10-6m). In the same layer, although the position of the liquid crystal molecules is irregular, the long axis orientation is parallel to the polarizer. On the other hand, between different layers, the long axis of the liquid crystal molecules is continuously twisted 90 degrees along the plane parallel to the polarizer. Among them, the orientation of the long axis of the two layers of liquid crystal molecules adjacent to the polarizing plate is consistent with the polarization direction of the adjacent polarizing plate. The liquid crystal molecules near the upper interlayer are arranged in the direction of the upper groove, and the liquid crystal molecules in the lower interlayer are arranged in the direction of the lower groove. Finally, it is packaged into a liquid crystal box and connected with the driving IC, the control IC and the printed circuit board. Under normal circumstances, when light is irradiated from top to bottom, usually only one angle of light can penetrate, through the upper polarizing plate into the groove of the upper interlayer, and then passing through the lower polarizing plate through the passage of the twisted arrangement of liquid crystal molecules. Form a complete path of light penetration. The interlayer of the liquid crystal display is attached with two polarizing plates, and the arrangement and light transmission angle of the two polarizing plates are the same as the groove arrangement of the upper and lower interlayers. When a certain voltage is applied to the liquid crystal layer, due to the influence of the external voltage, the liquid crystal will change its initial state, and will no longer be arranged in a normal way, but will become an upright state. Therefore, the light passing through the liquid crystal will be absorbed by the second layer of polarizing plate and the entire structure will appear opaque, resulting in a black color on the display screen. When no voltage is applied to the liquid crystal layer, the liquid crystal is in its initial state and will twist the direction of the incident light by 90 degrees, so that the incident light from the backlight can pass through the entire structure, resulting in white on the display. In order to achieve each individual pixel on the panel can produce the color you want, multiple cold cathode lamps must be used as the backlight source of the display. 2. The working principle of active matrix LCD TFT-LCD liquid crystal display The structure is basically the same as that of the TN-LCD liquid crystal display, except that the electrodes of the upper interlayer of TN-LCD are changed to FET transistors, and the lower interlayer is changed to a common electrode. The working principle of TFT-LCD LCD screenis different from that of TN-LCD. The imaging principle of the TFT-LCD LCD screenis to use the 'back-through' illumination method. When the light source illuminates, it first penetrates upward through the lower polarizing plate, and transmits light with the help of liquid crystal molecules. Since the upper and lower interlayer electrodes are changed to FET electrodes and common electrodes, when the FET electrodes are turned on, the arrangement of the liquid crystal molecules will also change, and the purpose of display is achieved by shielding and transmitting light. But the difference is that because the FET transistor has a capacitance effect and can maintain a potential state, the previously transparent liquid crystal molecules will remain in this state until the FET electrode is energized next time to change its arrangement.
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