TN type liquid crystal display is also called "twisted nematic (TN) display", it is the most common type of liquid crystal display device. The LCD screens used in some common watches, digital meters, electronic clocks, and most calculators are actually TN-type devices. Generally, most of the liquid crystal display devices used for pen-segment digital display are TN-type devices. Therefore, this kind of liquid crystal device should be the most well-known liquid crystal display.
The basic structural principle of the TN-type liquid crystal display is: lithographically etch a certain transparent electric plate pattern on the glass coated with ITO transparent conductive layer, and then sandwich the front and back two glass substrates with transparent conductive electrode patterns. Hold a layer of nematic liquid crystal material with positive dielectric anisotropy, and then seal around it to form a flat liquid crystal box with a thickness of only a few microns. Because it is coated with an alignment layer on the inner surface of the glass and undergoes an alignment treatment, the liquid crystal molecules in the cell are aligned parallel to the surface of the glass. However, because the orientation processing directions of the surface alignment layers in the two pieces of glass are perpendicular to each other, the liquid crystal molecules are twisted at 90 degrees between the two pieces of glass. This is the origin of the name of the twisted nematic liquid crystal screen. Figure 1 is TN Figure 2 shows the schematic diagram of the structure of the LCD screen.
Since the twisted screw moment of the liquid crystal molecules in the cell of the TN-type liquid crystal display device is much greater than the wavelength of visible light, when the liquid crystal molecules on the side of the glass surface are aligned in the same or orthogonal linearly polarized light incident After that, the direction of its polarized light will be twisted by 90° after passing through the entire liquid crystal layer and then emitted from the other side. Therefore, this liquid crystal cell has the function of shielding light between the parallel polarizers, and the surface is between the orthogonal polarizers. Light can be shielded between the crossed polarizers, and light can be transmitted between the orthogonal polarizers.
If a voltage is applied to the liquid crystal box and reaches a certain value at this time, the long axis of the liquid crystal molecules will start to tilt along the direction of the electric field. When the voltage reaches about twice the threshold voltage, except for the liquid crystal molecules on the electrode surface, All the liquid crystal molecules in the liquid crystal cell and between the two electrodes will become rearranged along the direction of the electric field. At this time, the function of 90° optical rotation will disappear, and the function of optical rotation between the orthogonal polarizers is lost. Makes the device can not transmit light. The surface between the parallel polarizers loses its optical rotation effect, so that the device can no longer block light.
Therefore, if we place the liquid crystal box between orthogonal or parallel polarizers, we can use the method of energizing the liquid crystal box to make the light change its transmission-shielding state, so as to realize its display. Usually, when we look at the LCD screen, black characters appear faintly, not because the liquid crystal is changing color, but because the liquid crystal display device allows light to pass through or be absorbed.
