LCD principle

The working principle of LCD

When the liquid crystal is energized, the liquid crystal molecules are arranged in an orderly manner, allowing light to pass through easily; when not energized, the liquid crystal molecules are arranged chaotically, preventing light from passing through. With or without electricity, the liquid crystal can be blocked like a gate or light can pass through. The two states that can control the light are the prerequisites for the LCD to form an image. Of course, a certain structure is needed to realize the conversion of light to image.

Liquid crystal light principle

The liquid crystal panel of the TN (twisted nematic) monochromatic liquid crystal display contains two very delicate soda-free glass materials, called Substrates, with a layer of liquid crystal in between. The structure is like a "sandwich". In the middle layer, the liquid crystal layer, the liquid crystal is not simply poured into it, but into two interlayers with grooves inside. The two interlayers with grooves mainly allow the liquid crystal molecules to be arranged neatly, because If the arrangement is not neat, the light cannot pass through the liquid crystal part smoothly. In order to achieve the effect of neat arrangement, these grooves are made very fine, the liquid crystal molecules will be arranged along the grooves, the grooves are very parallel, so the molecules are also completely parallel. These two interlayers are usually called upper and lower interlayers. The upper and lower interlayers are all neatly arranged liquid crystal molecules, but the arrangement direction is different. The liquid crystal molecules of the upper interlayer are arranged in the direction of the upper groove, and the liquid crystal molecules of the lower interlayer are arranged according to the direction of the upper groove. The direction of the lower groove is aligned. In the production process, the upper and lower grooves are cross-staggered (90 degrees perpendicular), that is, the arrangement of the liquid crystal molecules in the upper layer is horizontal, and the arrangement of the liquid crystal molecules in the lower layer is vertical, which causes the liquid crystal molecules located between the upper and lower interlayers to approach The upper layer is arranged horizontally, and the one near the lower layer is arranged vertically.

If viewed as a whole, the arrangement of liquid crystal molecules is like a spiral twisted arrangement, and the key to the twist is the molecules between the sandwich layers. A key device is set in the interlayer, called a polarization filter. The arrangement and light transmission angle of the two filters are the same as the groove arrangement of the upper and lower interlayer. It is assumed that the light is irradiated from top to bottom under normal conditions. Usually, only one angle of light can penetrate, through the upper filter into the groove of the upper interlayer, and then through the lower filter through the twisted arrangement of liquid crystal molecules, forming a complete light penetration path. Once the liquid crystal molecules are energized through the electrodes, due to the influence of external voltage, the liquid crystal molecules are no longer arranged in the normal way, but become an upright state, so that light cannot pass through, resulting in black on the display.

Principle of TFT LCD

When the light source is irradiated, it first penetrates upward through the lower polarizing plate. It also uses the principle of liquid crystal to transmit light, because the upper and lower interlayer electrodes are changed to FET electrodes and common electrodes. When the FET electrode is turned on, the behavior of the liquid crystal molecules will change like the arrangement state of the TN liquid crystal, and the purpose of display is also achieved by shading and transmitting light. But the difference is that because the FET transistor has a capacitance effect and can maintain a potential state, the liquid crystal molecules that were previously light-transmissive will remain in this state until the FET electrode is energized next time to change its arrangement. Relatively speaking, TN does not have this characteristic. Once the liquid crystal molecules are not pressed, they immediately return to the original state. This is the biggest difference between TFT liquid crystal and TN liquid crystal display, and it is also the advantage of TFT liquid crystal. In summary, the liquid crystal display receives digital signals from the computer display card through the DVI interface. These signals are transmitted to the control circuit through the data line. The control circuit adjusts the thin film transistors and transparent display panels of the liquid crystal display to realize the light and No light characteristics.

When driving the LCD, an AC voltage needs to be applied to the segment electrode and the common electrode. If only a DC voltage is applied to the electrodes, the liquid crystal itself deteriorates. Liquid crystal driving methods include static driving, dynamic driving, and multiple driving methods.

The multi-channel driving method can be divided into dot inversion driving and frame inversion driving. The dot inversion drive is suitable for low duty cycle applications. It inverts the data when each segment of data is output. The frame inversion drive is suitable for high duty cycle applications. It inverts the data when each frame is output. For multi-grayscale and color display control methods, frame rate control (FRC) and pulse width modulation (PWM) methods are usually used. Frame rate control is to achieve multi-gray and color control by reducing the number of frame output and controlling the effective value of the output signal. The pulse width modulation is to achieve multi-gray and color control by changing the pulse width of the segment output signal and controlling the effective value of the output signal.