TFT LCD screen display principle

Liquid crystal formation and types

We generally think that objects have three states: solid, liquid, and gaseous. In fact, this is only for water. There are some organic compounds, and the state between solid and liquid is the liquid crystal state.

From the perspective of composition and physical conditions for the appearance of liquid crystals, liquid crystals can be divided into two categories: thermotropic liquid crystals and lyotropic liquid crystals.

Thermotropic liquid crystal: The liquid crystal formed by heating and dissolving certain organic substances, which destroys the liquid crystal lattice.

Lyotropic liquid crystals: liquid crystals formed by putting certain organics in a certain solvent, and the solvent destroys the liquid crystal lattice.

The liquid crystal materials used for display are all thermotropic liquid crystals. There are a large number of lyotropic liquid crystals in biological systems. There are nearly 10,000 kinds of liquid crystal substances found at present, which constitute the molecules of the liquid crystal substance, and show different arrangements in each liquid crystal phase. Smectic liquid crystal molecules are two-dimensionally ordered, and the molecules are arranged in layers. This kind of liquid crystal has relatively high viscosity and surface tension, and is insensitive to external changes in electricity, magnetism, and temperature.

Nematic liquid crystal molecules are one-dimensionally ordered. They can slide up and down, left and right, and back and forth. The arrangement and movement of the molecules are relatively free, and they are more sensitive to external magnetic fields, temperature, and stress. They are currently the main materials for displays.

The structure of the LCD screen

When the angles of the two polarized light barriers are perpendicular to each other, light cannot pass through at all. The liquid crystal display is completed by using the characteristics of polarizing plates. The upper and lower polarizing plates are filled with liquid crystals, and the electric field is used to control the branches of the liquid crystal. Rotate to change the direction of light travel. In this way, different electric fields will form different degrees of color, so when the light wave reaches the upper polarizer, the polarization direction of the light wave turns by exactly 90 degrees. The angle between the lower polarizer and the upper polarizer is exactly 90 degrees different. Therefore, the light can pass smoothly, and if the light hits the red filter, it will appear red. If the liquid crystal molecules are all standing and the light path is not changed, the light cannot pass through the upper polarizer and cannot be displayed.

The so-called NW (Normally white) means that when we do not apply voltage to the LCD panel, the panel we see is a bright picture, so it is called normally white. The other is that when no voltage is applied to the LCD panel, the panel cannot transmit light and looks black, which is called NB (Normally black). These two structures are mainly for different application environments. Generally, desktop computers or notebook computers are mostly NW configurations. That is because of the use environment of general computer software. You will find that the entire screen is mostly bright, which means that the computer software is mostly applications with black letters on a white background. Since the bright spots account for the majority, it is of course more convenient to use NW, and because the bright spots of NW do not require voltage, it will save electricity on average.

Opening rate:

   One of the most important parameters in liquid crystal displays is brightness, and the most important factor that determines brightness is the aperture ratio. The aperture ratio is the ratio of the effective area through which light can pass.

   When light is emitted through the backlight board, not all the light can pass through the panel, such as the signal traces for the LCD driver chip, the TFT itself, and the storage capacitor. These places are not completely transparent. Because the light passing through these places is not controlled by the voltage and cannot display the correct gray scale, it needs to be shielded so as not to interfere with the correct brightness of other light-transmitting areas.