LCD working principle

LCD working principle

Liquid Crystal Display ( referred to as LCD) is a display that uses "Liquid Crystal" as a material. Liquid crystal is a substance between solid and liquid. When heated, it will appear transparent and liquid. When cooled, it will crystallize into a chaotic solid state. Liquid crystal is an organic compound with regular molecular arrangement. Moreover, when the liquid crystal is energized, the liquid crystal molecules are arranged in an orderly manner, allowing light to pass through easily; when not energized, the liquid crystal molecules are arranged chaotically, preventing light from passing through. With and without electricity, the liquid crystal can be blocked like a gate or light can pass through. The two states that can control the light are the prerequisites for the formation of images on the LCD. The liquid crystal panel of the TN (twisted nematic) monochrome liquid crystal display is composed of two pieces of special glass sandwiched between a layer of liquid crystal, and the structure is like a "sandwich". Liquid crystal is not simply poured into it, but into two interlayers with grooves inside. The two interlayers with grooves mainly allow the liquid crystal molecules to be arranged neatly. In order to achieve the effect of neat arrangement, these grooves are made very fine, the liquid crystal molecules will be arranged along the grooves, the grooves are very parallel, so the molecules are also completely parallel. The two interlayers are usually called upper and lower interlayers. The upper and lower interlayers are all neatly arranged liquid crystal molecules. The upper and lower grooves are cross-staggered (90 degrees perpendicular), that is, the arrangement of the upper layer of liquid crystal molecules is horizontal, and the arrangement of the lower layer of liquid crystal molecules is Vertically, the liquid crystal molecules located between the upper and lower interlayers are arranged horizontally when they are close to the upper layer, and vertically arranged when they are close to the lower layer. There is also a key device in the interlayer, called a polarization filter. The arrangement and light transmission angle of the two filters are the same as the groove arrangement of the upper and lower interlayer. Assuming that the light is irradiated from top to bottom under normal conditions, Only one angle of light can penetrate through the upper filter into the groove of the upper interlayer, and then pass through the lower filter through the twisted arrangement of liquid crystal molecules to form a complete light penetration path. Once the liquid crystal molecules are energized through the electrodes, the liquid crystal molecules are no longer arranged in the normal way due to the influence of the external voltage, and the sample light cannot pass through, resulting in black on the display screen. In this way, it will be white when light is transmitted (that is, when power is not applied), and black when light is not transmitted (when power is applied), and characters can be displayed on the screen. This is the simplest display principle.

1. The working principle of ordinary LCD

   Because the liquid crystal material itself does not emit light, there are lamp tubes as light sources on both sides of the display screen. At the same time, there is a backlight plate and a reflective film on the back of the liquid crystal display screen, whose main function is to provide a uniform background light source. Here, the light emitted by the backlight panel enters the liquid crystal layer after passing through the filter. The crystal droplets in the liquid crystal layer are all contained in a small cell structure. One or more cells constitute a pixel on the screen. And these pixels can be bright or not bright. A large number of neatly arranged pixels are bright and not bright to form a monochromatic image.

Control the lighting in a large number of elements or not,

2. Principle of TFT LCD

      The working principle of the new TFT liquid crystal display is also based on the principle of the TN liquid crystal display. The structure of the two is also basically the same. The design of filling liquid crystal molecules between the two interlayers is also adopted, except that the electrode of the upper interlayer of TN is changed to FET transistor, and the lower layer is changed to a common electrode. In terms of light source design, the TFT display adopts a "back-through" illumination method, and a fluorescent tube (LED lamp) similar to a fluorescent lamp is set on the back of the liquid crystal. When the light source is irradiated, it first penetrates upward through the lower polarizing plate. When the FET electrode is turned on, the behavior of the liquid crystal molecules will change like the arrangement state of the TN liquid crystal, and the purpose of display is achieved by shading and transmitting light. 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 powered up next time to change its arrangement. Relatively speaking, TN does not have this characteristic. Once the liquid crystal molecules are not pressed, they immediately return to the original state. This is the biggest difference between TFT liquid crystal and TN liquid crystal display, and it is also the superiority of TFT liquid crystal. This is mainly controlled by the control circuit. There are transparent electrodes between the glass plate and the liquid crystal material. The electrodes are divided into rows and columns. At the intersection of rows and columns, the light-emitting state of the liquid crystal is changed by changing the voltage. . There are a control circuit part and a drive circuit part around the liquid crystal material, so that signals can be used to control the generation of monochromatic images.