STN-LCD display principle

by:Kimeery     2021-08-26
STN (SuperTwistedNematic) uses an electric field to change the arrangement of liquid crystal molecules that were twisted above 180 degrees to change the optical rotation state. The external electric field changes the electric field by scanning line by line. When the electric field repeatedly changes the voltage, each point is restored. It is slower, thus producing afterglow. Its advantage is low power consumption and the biggest advantage of power saving. The display principle of color STN is to add a color filter to the traditional monochrome STN liquid crystal display, and divide each pixel in the monochrome display matrix into three sub-pixels, and display the three primary colors of red, green and blue through the color filter. , The color screen can be displayed. Unlike TFT, STN is a passive type LCD, which can generally display up to 65536 colors. The STN type display principle is similar to 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. What I want to explain here is that the pure TN LCD screenitself has only two situations (or black and white), and there is no way to change the color. But if you add a color filter to the traditional monochrome STN liquid crystal display, and divide any pixel (pixel) of the monochrome display matrix into three sub-pixels (sub-pixel), they are displayed through the color filter. The three primary colors of red, green, and blue can also be displayed in full-color mode by reconciling the ratio of the three primary colors. In addition, if the TN LCD display screen is made larger, its screen contrast will appear worse, but with the improved technology of STN, it can make up for the lack of contrast. The traditional TN-LCD (twisted nematic liquid crystal display device) has the weakness of slow electro-optical response speed and very inconspicuous threshold characteristics, which causes difficulties in multi-channel driving and limits its use in video display with large amounts of information. STN-LCD (Super Twisted Nematic Device) can be made by increasing the twist angle of TN-LCD liquid crystal molecules from 90° to between 180° and 360°. STN-LCD has greatly improved the display characteristics. At present, almost all dot-matrix graphics and most dot-matrix character LCDs have adopted STN mode. STN-LCD technology is in the stage of gradual maturity and perfection in the liquid crystal industry. The glass coated with a transparent conductive layer is lithographically formed to form specific transparent electrodes, a layer of STN-LCD material is sandwiched between two such glass plates, and the sides are sealed to form a flat liquid crystal cell with a thickness of only micrometers. . Since the inner surface of the glass is coated with an alignment layer film and has been oriented, the liquid crystal molecules in the cell are arranged in parallel along the glass surface. If the direction of the alignment layer treatment on the inner surface of the two glass sheets is at a certain angle α, the liquid crystal molecules are in the two sheets. The glass is twisted at an angle of α. Since the twisting pitch of the STN-LCD liquid crystal molecules in the cell is much larger than the wavelength of visible light, when the linearly polarized light on the side perpendicular to the glass surface is incident, its polarization direction passes through the entire liquid crystal layer. Then it will be distorted at an angle of α and emitted from the other side, so this liquid crystal cell has the function and function of transmitting light between polarizers at an angle of α. If a voltage is applied to the liquid crystal cell and reaches a certain value, the long axis of the liquid crystal molecules will start to tilt along the direction of the electric field. When the voltage reaches twice the threshold voltage, except for the molecules on the electrode surface, all the liquid crystal cells will All of the liquid crystal molecules become rearranged in the direction of the electric field. At this time, the α-angle optical rotation function disappears, and the optical rotation function between the α-angle polarizers is lost, and the device can no longer transmit light. Therefore, by placing the STN-LCD between the polarizing plates at an angle of α, the liquid crystal cell can be energized to change the light transmission and shielding state to realize the display function. The threshold voltage Vth of the STN-LCD can be expressed as where ΔX is the dielectric constant of the STN-LCD, and k11, k22, and k33 are the elastic coefficients. Figure 1 and Figure 2 are the electro-optical response characteristic curves of TN-LCD and STN-LCD (twist angle is 270°). Widely used in capacity and multi-line dot matrix display.
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