1. Introduction to TFT.

TFT (Thin Film Transistor) screens are liquid crystal displays where each pixel is driven by an integrated thin film transistor located behind it. This not only improves the response speed of the display but also allows for precise control over color gradation, resulting in more realistic colors in TFT LCDs.

2. Main Components of a TFT Screen.

The main components of a TFT screen include: backlight source, light guide plate, diffusion film, prism film, alignment film, liquid crystal material, and thin film transistors.

The backlight source is the light that comes from the light source located at the bottom of the LCD screen. The LCD light source can be a long fluorescent lamp (CCFL) used in teacher’s offices, or a light-emitting diode (LED). Typically, LCD screens use LEDs, which allow for thinner screens. The light source can be placed in two ways: one is to place the LED flat on the bottom reflective plate, like in computer monitors; the other is to place the LED on the side of the screen, such as in mobile phone screens.

• The light guide plate is used when the LED is placed on the side of the screen, which can cause some areas to be very bright and others very dark. Therefore, a light guide plate is needed to convert the line light source into a surface light source to make the light more uniform.

  The diffusion film enlarges the area of the light diffusion and makes the light softer, just like looking at a lamp through frosted glass, which is much easier on the eyes.

The prism film is used to focus scattered light into a specific angle. Only light that meets a certain angle will pass through the prism film, while light that does not meet the angle will be reflected back to the reflective plate below. This allows some of the light that does not meet the angle to be reused.

The use of prism film is because light is scattered in all directions, but the mobile phone displays we use are facing directly at the screen, so we want to direct the light towards the viewer as much as possible, which can also increase the brightness of the screen.

The polarizing film (two layers) acts as a filter. The polarizing film is made from stretched plastic, resembling parallel grid lines like a Venetian blind. Only light in a specific direction can pass through this layer, while light in other directions will be absorbed.

The second layer of polarizing film is perpendicular to the first layer, so light cannot pass through the second layer. If light can pass through the second layer, the screen will naturally display black. If light can pass through the second layer of polarizing film, the screen will display white.

The liquid crystal is added between the two layers of polarizing film to allow light to pass through the second layer. The molecules in the liquid crystal layer are arranged in a relatively orderly manner in a natural environment.

The alignment film holds the liquid crystal in place.

• The liquid crystal

The surface of the alignment film has fine grooves, which help to align the liquid crystal molecules in the grooves. At this point, the liquid crystal molecules will align along the direction of the grooves.

Light passing through the first layer of polarizing film will change direction as it goes through the liquid crystal layer, becoming horizontally polarized light.

Horizontally polarized light can pass through the second layer of polarizing film, allowing the screen to display white.

The electrode layer

How do we make the screen respond, displaying white when needed and black when necessary? In this case, we add an electrode layer. When the electrode layer is energized, the electric field affects the liquid crystal molecules. To display black, we simply increase the voltage of the electrode layer; under the influence of the electric field, the liquid crystal molecules will twist. At this point, light passing through the liquid crystal layer will not change direction and thus cannot pass through the second layer of polarizing film, causing the screen to display black.

The color filter film is added if we want the screen to display colors.

Pixel: A pixel is the smallest unit of light emitted by a display, composed of three sub-pixels: red, green, and blue.

If the brightness of the three sub-pixels passing through the pixel is the same, it will display white light.

• To display other colors with the pixel, we only need to adjust the brightness of the light passing through the three sub-pixels to mix different colors. The question then arises: how do we adjust the brightness of these three sub-pixels?

• We add a glass substrate on top of the TFT thin film transistor electrode layer, which has another layer of thin film transistors.

  

Each sub-pixel has a corresponding transistor that acts as a brightness adjustment switch for each sub-pixel. It can adjust the voltage within the sub-pixel range, which will affect the twisting of the liquid crystal molecules in that sub-pixel range. Thus, each sub-pixel’s transistor can adjust the voltage as needed. Each sub-pixel can have 256 different brightness levels, meaning the three sub-pixels can mix to create over 16 million colors.

3. Summary of TFT Working Principles.

Based on the structural components of TFT, we summarize the working principle as follows: when the screen is powered, the backlight source emits white light, which is illuminated throughout the screen by the light guide plate, the diffusion film makes the light softer, and the prism film ensures that the light travels in the correct direction, while the polarizing film filters out light from other directions, allowing only vertically polarized light to pass through. TFT controls the voltage of the sub-pixels as needed, affecting the twisting degree of the liquid crystal molecules and controlling the brightness of the light. The light passes through the liquid crystal and the color filter film, resulting in red, green, and blue light. Finally, the last polarizing film allows only horizontally polarized light to pass through, and the LCD displays the image.

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