TFT-LCD basic knowledge

TFT-LCD basic knowledge

Back light (BL)

In a general CRT screen, a high-speed electron gun is used to emit electrons to hit the phosphors on the silver screen to produce bright light to display the screen. However, the liquid crystal display itself can only control the brightness of light passing through, and has no light-emitting function. Therefore, a backlight must be added to the LCD to provide a light source with high brightness and uniform brightness distribution. As we can see, the main parts that make up the backlight panel are lamp tubes (cold cathode tubes), reflector plates, light guide plates, prism sheets, diffuser plates and so on. The lamp tube is the main light-emitting part, and the light is distributed everywhere by the light guide plate. The reflective plate restricts the light to the direction of the TFT LCD. Finally, with the help of the prism sheet and the diffuser, the light is evenly distributed to each area to provide a bright light source for the TFT LCD. The TFT LCD controls the rotation of the liquid crystal by voltage, and controls the brightness of the passing light to form different gray levels.

Principle of Monochrome Liquid Crystal Display(LCD) 

LCD technology is to pour liquid crystal between two planes lined with fine grooves. The grooves on these two planes are perpendicular to each other (intersect at 90 degrees). That is to say, if the molecules on one plane are arranged north-south, the molecules on the other plane are arranged east-west, and the molecules between the two planes are forced into a 90-degree twisted state. Since the light travels along the direction of the molecular arrangement, the light is also twisted 90 degrees when passing through the liquid crystal. But when a voltage is applied to the liquid crystal, the molecules will re-arrange vertically so that the light can be directed out without any twisting.

LCD relies on polarization filters (plates) and light itself. Natural light diverges randomly in all directions. The polarization filter is actually a series of increasingly thinner parallel lines. These lines form a net, blocking all light that is not parallel to these lines. The line of the polarization filter is exactly perpendicular to the first one, so it can completely block those polarized light. Only when the lines of the two filters are completely parallel, or the light itself has been twisted to match the second polarized filter, the light can penetrate. LCD is composed of two polarized filters that are perpendicular to each other, so under normal circumstances, all light that attempts to penetrate should be blocked. However, because the two filters are filled with twisted liquid crystals, after the light passes through the first filter, it will be twisted 90 degrees by the liquid crystal molecules, and finally pass through the second filter. On the other hand, if a voltage is applied to the liquid crystal, the molecules will rearrange and be completely parallel, so that the light will no longer be twisted, so it will be blocked by the second filter. In short, the light is blocked when power is applied, and the light is emitted when the power is not applied.