How does the LCD work?

- Mar 16, 2018-

              We have known for a long time that there are three types of solid, liquid, and gaseous materials. Although the arrangement of the centroids of liquid molecules does not have any regularity, if these molecules are elongated (or flat), their molecular orientation may be regular. So we can subdivide the liquid into many types. The liquid with no regular molecular orientation is called liquid directly, and the liquid with molecular directionality is called "liquid crystal", or "liquid crystal" for short. In fact, LCD products are not unfamiliar to us. The mobile phones and calculators we commonly find are LCD products. The liquid crystal was discovered in 1888 by Austrian botanist Reinitzer, an organic compound with a regular molecular arrangement between solid and liquid. The most commonly used liquid crystal type is a nematic liquid crystal, and its molecular shape is an elongated rod shape with a length and width of about 1 nm to 10 nm. Under the action of different current electric fields, the liquid crystal molecules will be regularly rotated by 90 degrees to produce a light transmittance. The difference is that the difference between light and dark is generated when the power is turned on/off. According to this principle, each pixel is controlled and a desired image can be formed.

              The principle of liquid crystal display is that liquid crystals will exhibit different optical characteristics under the effect of different voltages. Liquid crystals are physically divided into two major categories. One is passive Passive (also known as passive). Such liquid crystals do not emit light by themselves and require external light. Provide light source, according to the light source position, but also can be divided into reflective and transmissive two. Passive liquid crystal display, the lower the cost, but the brightness and contrast is not, but the effective viewing angle is small, color passive liquid crystal display color saturation Small, so the color is not bright enough. The other is a power supply, mainly TFT (Thin Film Transitor). Each liquid crystal is actually a transistor that can emit light, so strictly speaking it is not liquid crystal. The liquid crystal display is composed of many liquid crystals. Arranged in an array, in a monochrome liquid crystal display, one liquid crystal is a pixel, and in a color liquid crystal display, each pixel is formed by three liquid crystals of red, green, and blue, and each liquid crystal can be considered There is an 8-bit register behind it. The value of the register determines the brightness of each of the three liquid crystal cells, but the value of the register does not directly drive the brightness of the three liquid crystal cells, but through a "Palette" to access. It is not practical to equip each pixel with a physical register. In fact, only one line of registers is used. These registers are connected to each line of pixels in turn and the contents of the line are loaded. The prime line is driven once to display a complete frame.

              The liquid crystal looks like a liquid from its shape and appearance, but its crystalline molecular structure shows a solid form. Like a metal in a magnetic field, when subjected to an external electric field, its molecules produce a precisely ordered arrangement; if the arrangement of the molecules is properly controlled, the liquid crystal molecules will allow light to penetrate; the path through which light passes through the liquid crystal can be It is determined by the arrangement of its molecules, which is again a feature of solids. Liquid crystal is an organic compound composed of long rod-shaped molecules. In the natural state, the major axes of these rod-shaped molecules are approximately parallel. The first characteristic of the Liquid Crystal Display (LCD) is that the liquid crystal must be poured between two thin-slotted planes to work normally. The grooves in these two planes are perpendicular to each other (90 degree intersections). That is, if the molecules in one plane are aligned in the north-south direction, the molecules in the other plane are arranged in the east-west direction, and the molecules located between the two planes are Forced into a 90-degree twisted state. Because the light travels in the direction of the molecules, the light is also twisted 90 degrees through the liquid crystal. However, when a voltage is applied to the liquid crystal, the molecules will be re-arranged vertically so that the light can be emitted directly without any twisting. The second characteristic of the LCD is that it relies on polarization filters and the light itself, natural light is randomly diverging in all directions, and the polarizing filter is actually a series of increasingly fine parallel lines. These lines form a net that blocks all light that is not parallel to these lines. The lines of the polarizing filter are exactly perpendicular to the first, so they can completely block those rays that have been polarized. Only if the lines of the two filters are completely parallel, or the light itself has been twisted to match the second polarizing filter, the light can penetrate. 

              The LCD is composed of such two polarizing filters that are perpendicular to each other, so under normal circumstances, all light that tries to penetrate should be blocked. However, because the two filters are filled with twisted liquid crystals, after passing through the first filter, the liquid crystal molecules are twisted by 90 degrees, and finally pass through the second filter. On the other hand, if a voltage is applied to the liquid crystal, the molecules will be rearranged and completely parallel, so that the light will not be twisted anymore, so it is blocked by the second filter. Taking Synaptics TDDI technology as an example, the touch controller and display driver are integrated into a single chip, which reduces the number of components and simplifies the design. The ClearPad 4291 supports a hybrid multi-point embedded design, which eliminates the need for discrete touch sensors due to the use of existing layers in liquid crystal displays (LCDs). The ClearPad 4191 takes another step forward, utilizing the existing electrodes in the LCD, thus achieving a more concise system architecture. Both solutions make the touch screen thinner and the display brighter, helping to improve the overall aesthetics of smartphone and tablet designs. For a reflective TN (Twisted Nematic) liquid crystal display, the construction consists of the following layers: polarizing filters, glass, both vertical and horizontal electrodes that are insulated and transparent, liquid crystal, electrodes, glass, polarized Filters, reflectors.