The Working Principle Of Common Liquid Crystal Display Devices (mainly Passive Devices).

The Working Principle Of Common Liquid Crystal Display Devices (mainly Passive Devices).

Dec 20, 2017

  The organic electrolyte plasma type conductive material into the liquid crystal material, liquid crystal liquid crystal box clamp between two pieces of glass, in the liquid crystal box with a small amount of additives, the main purpose is to increase the conductivity of liquid crystal, and then made the tomb tablets pretreatment, the liquid crystal molecules arranged along the surface, as shown in Fig. 9 shows.


In case of power failure, the liquid crystal box is transparent; when the voltage with appropriate high, due to anisotropy of conductivity and permittivity of the liquid crystal molecules, will appear in turbulent liquid crystal, additive strengthened this kind of turbulent motion, which caused by the strong scattering of incident light,

So the liquid crystal cell becomes turbid. This state is called dynamic scattering, so the liquid crystal cell produces turbid image corresponding to the electrode pattern on the transparent background. The main disadvantage of the dynamic scattering liquid crystal device is that the contrast is low, and the edge of the image is not clear because of the turbulence in the scattering. Due to the different gray image of different time delay, the image gray scale variation, also due to the incorporation of conductive additives in liquid crystal materials of poor quality, high power consumption, to show the application of the general device, the working life is not long, so the DS type liquid crystal devices now have very little use, but it is the current type the only device, and liquid crystal display device is the first practical, a liquid crystal display time.

Twisted nematic liquid crystal display (TN - LCD). In the two block of conductive glass substrate between the filling thickness of approximately 10 mu m nematic liquid crystal (NP crystal), liquid crystal molecules arranged along the surface, but the molecular long axis between the upper and lower substrate for twisted 90 degrees, arrangement of the liquid crystal box is twisted, orthogonal set from the polarization axis of polarizer and analyzer. As shown in Figure 10.


When no voltage, the linearly polarized light through the liquid crystal cell, due to the arrangement of liquid crystal molecules of the liquid crystal with a 90 degree rotation, so that the polarization direction of incident polarized light (light direction) rotate 90 degrees, while the polarizer and analyzer is orthogonal set, so the light refraction polarizer through polarization the exit, which can be transparent. If the liquid crystal box with voltage, NP LC will begin to tilt along the direction of the electric field, when the voltage is high enough, the liquid crystal molecules in the liquid crystal box are re arranged along the direction of the electric field, then the TN liquid crystal box 90 degree optical properties disappear, linearly polarized light through the liquid crystal box, the same polarization direction due to polarization device and a polarizer transmission axis orthogonal polarized light is thus a polarizer the barrier, no light injection, thereby shading. Because the gray scale images show amplitude signal strength (Note: in the liquid crystal cell is pulse train, different width than analog DC voltage), electric field strength and thus to the liquid crystal box, liquid crystal molecules along the electric field direction of the inclined degree is different, this is a polarizer blocked by light the intensity is different, and can be converted to visual changes, so as to achieve the purpose of image display, this is the most basic principle of liquid crystal display work. The TN liquid crystal has the following shortcomings:

(1) the electro-optic properties of TN liquid crystal are not steep, so the crossover effect is serious when working in dot matrix display. Generally, it is only suitable for static driving or dynamic driving segment display under four roads. At present, the best TN devices can only achieve 8~16 drive.

(2) the electro-optic response speed is slow. The response speed of the TN type liquid crystal display is about 100ms, so it is only suitable for the display of static or slow change, which is incompetent for the video display TN liquid crystal.

(3) light transmission and closure are not thorough. Only the effect of black paper is achieved, and the effect of black word is not reached. So the contrast is not ideal. Although the TN liquid crystal is widely used, it is limited to the low-grade products in the liquid crystal. Such as liquid crystal watch, liquid crystal digital instrument, electronic clock, calculator and black-and-white mobile phone screen, etc.

(4) super twisted nematic liquid crystal display unit (STN).

TN electro-optical characteristics of liquid crystal devices is not steep, so its application has been limited, but the experiment found that as long as the liquid crystal molecules will distort the traditional N LCD device can obviously improve the angle increase, the electro-optic properties of this kind of gradient twist angle greater than 90 degrees, usually in the liquid crystal device of 180 ~360 degrees called Supertwist nematic liquid crystal. In order to maintain the twist angle greater than 90 degrees, the nematic liquid crystal has the intrinsic or distorted structure of nature, called chiral nematic phase, the chiral nematic phase is doped with a few percent of ordinary optical materials of nematic liquid crystal molecules doped, left or right to give the entire nematic structure with the macro structural distortions inherent, as shown in figure 11.


Experiments show that when the twist angle is 270 degrees. The electro-optical characteristic curve steepness, have been enough for general computer terminals and office equipment.