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What is TFT LCD Screen

TFT LCD (Thin Film Transistor Liquid Crystal Display) is the abbreviation of Thin Film Transistor Liquid Crystal Display. The cross section of the TFT panel is shown in Figure 1.

The structure of LCD is introduced. Polarizer.

The top polarizer can polarize stray light from random polarization in a given polarization direction. Before applying an electric field to the electrodes, the liquid crystals are arranged in a twisted structure. The light in this case changes according to the twisted structure of the liquid crystal. The lower polarizer is oriented perpendicular to the upper polarizer. When the light reaches the bottom polarizer, the two polarizers are aligned with each other. Light can pass through them unhindered, as shown in Figure 2

LCD structure introduction

Glass substrate, TFT substrate, color film substrate

Using high-precision photolithography, the pattern is applied to the glass substrate to gradually transfer the images of multiple LCD electrodes in sequence (Figure 3). TFT glass has as many transistors as there are pixels on the display, and color generation is provided by color filter glass with color filters. The movement of the liquid crystal is caused by the potential difference between the electrodes on the TFT glass and the color filter glass. It is this movement of the liquid crystal that produces the color and determines the brightness of the LCD.

LCD liquid crystal

Liquid crystals are almost transparent substances and exhibit the characteristics of crystals and liquids at the same time. Two glass plates sealed with epoxy resin and with a groove in the left corner allow the introduction of liquid crystals (under vacuum) before the final sealing of the glass plates. The potential difference determines the orientation of the liquid crystal. When polarizers and color filters are used, the difference in the orientation of the liquid crystal causes the difference in transmittance (or reflectance) and the resulting color. Liquid crystals are substances that present different phases (solid, liquid crystal or liquid) at different temperatures

Leveling film

The film is deposited on two glass plates (upper and lower), with a series of parallel grooves, so that the liquid crystal molecules are aligned in the appropriate direction (Figure 5 has a series of parallel grooves, so that the liquid crystal molecules are aligned in the corresponding direction)

LCD development

Liquid crystal was discovered more than 100 years ago. When heated, their external state can change from solid to liquid crystal, and even completely transform into liquid form as the temperature rises further. Over the years, people have made great efforts to improve liquid crystals, and as a result they have been widely used in electronic calculators and digital clocks. At present, the application range of color liquid crystal is wider: mobile phones, personal computers and TVs, which have the characteristics of low thickness, low power consumption, high resolution and brightness. In addition, in the foreseeable future, driven by the rapid spread of flat panel displays, the demand for LCD panels is expected to grow substantially.

How does LCD work

When a voltage is applied to the two LCD electrodes, the stronger the "unfolding" of the liquid crystal molecules, the higher the applied potential (Figure 6). Voltage sensitivity is one of the main characteristics of liquid crystals. Figure 7 shows the normal "white" mode of the LCD. As long as no potential difference is applied, light can pass through the liquid crystal layer, and the liquid crystal molecules will change the orientation of the light plane according to their own angle. However, when a voltage is applied, the liquid crystal molecules will "unfold" and "straighten" the light directed to the upper polarizing filter. Therefore, light will not be able to pass through the active area of the LCD, and this area will be darker than the surrounding area.

LCD control mode

Figure 8 shows the LCD control circuit. In a selected period of time, the switch is closed and an input voltage is applied to the liquid crystal, which will cause the orientation of the liquid crystal molecules to change. When the switch is closed, a certain charge is stored in Clc, and the voltage across Clc decreases with time. Consider adding a storage capacitor Cst in parallel with Clc to expand the storage capacity of the charge.

Energy storage capacitor

In fact, the control of the liquid crystal must be carried out by AC voltage. To activate the LCD, voltage is applied only when the switch is turned on, and then the switch is turned off immediately. In some cases, the voltage across the liquid crystal drops due to leakage. To prevent this, we can use a parallel capacitor to compensate for the leakage voltage. As the capacitance Cst increases, the shape of the voltage across it is close to a zigzag

The working principle of TFT LCD

The TFT acts as a switch. The gate of the TFT is connected to the scan line, the source is connected to the data line, and the drain is connected to Clc and Cst. When the shutter is activated (selected on the scan line), the TFT channel is opened, and the image data will be written into Clc and Cst. When the shutter is not selected, the TFT channel is closed

The basic structure of TFT LCD

The core of TFT-LCD structure includes liquid crystal, two polarizers and glass plate: upper color film substrate and lower TFT array substrate. Inject liquid crystal material between the two glass plates

Luminous flux adjustment

By controlling the magnitude of the input voltage applied to the liquid crystal, the arrangement, orientation and direction of the molecules can be changed, which will cause the volume of the luminous flux through the liquid crystal to change accordingly

Color formation

When the light flux passes through the color filter integrated in the top colored glass, each pixel of the image is formed by mixing basic RGB color elements (R-red, G-green, and B-cyan). If the red, green, and blue pixel elements are selected in equal proportions, white light will be produced. By adjusting the ratio of these three elements, the required number of different colors is obtained


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