What are the three types of LCD?

The screen types are related to important factors such as the response time, color, viewing angle, and contrast ratio of the liquid crystal display. There are three types of LCD screens: TN (Twisted Nematic), IPS (In-Plane-Switching), and VA (Vertical Alignment).

1. TN mode (twisted nematic)

The TN display mode is inject nematic liquid crystal into the display screen and forms a 90-degree twisted alignment structure in the display screen. This twisted alignment structure will cause the nematic liquid crystal to polarize the incoming linear light under the action of an external voltage. output after twisting. This unique phenomenon of the TN display mode is called the optical rotation feature. The optical rotatory properties and birefringence of TN coexist, and the optical rotatory properties show that when the refractive index anisotropy of the TN liquid crystal changes slowly, the polarization axis of the liquid crystal rotates accordingly.

In the TN display mode, under the condition of no applied voltage, after the natural light enters the polarizing plate on the TFT side, linearly polarized light parallel to the long axis of the liquid crystal molecules is formed. After  90 degree twist alignment of the liquid crystal layer, a 90 degree deflection linearly polarized light is formed. Because the polarization axis of the CF side polarizer and the polarization axis of the TFT side polarizer are vertically distributed at 90 degrees, the linearly polarized light deflected by 90 degrees will pass through the CF side polarizer and then exit. Therefore, in the TN display mode, in the normal case of no applied voltage, the backlight is transmitted through and appears white, which is called “normally white mode.”

If a vertical voltage is applied on the upper and lower sides of the liquid crystal, most of the liquid crystals in the liquid crystal layer are vertically distributed except for the liquid crystal near the alignment films on the upper and lower sides, and the linearly polarized light passing through the TFT side polarizer passes through the liquid crystal layer without deflection, parallel to the CF Polarization axis of the side polarizer. Linearly polarized light is absorbed, no light is emitted, and it is in a black state.

2. IPS mode

The characteristic of the In-Plane switching (IPS) display mode is that the liquid crystal molecules of Homogenize alignment are in the plane parallel to the glass substrate. After a voltage is applied between the comb-shaped electrodes, the liquid crystal molecules are rotated in the plane, causing birefringence to control the amount of light transmission. The pixel electrode and the common electrode of the IPS display mode are designed on the same side to form a planar electric field. The rotation of the IPS liquid crystal molecules is always in the plane, and the liquid crystal rotation only needs to overcome the elastic coefficient K22.

Under the condition of no applied voltage, after the natural light enters the polarizing plate on the TFT side, it forms linearly polarized light parallel to the short axis of the liquid crystal molecules, and the polarization direction cannot be rotated and enters the polarizing plate on the CF side in the same state. Because the linearly polarized light is perpendicular to the polarization axis of the CF side polarizer, the linearly polarized light here is absorbed by the CF side polarizer, and the display is in a black state. This mode in which the backlight cannot pass through and displays black under normal conditions without an applied voltage is called “normally black mode”.

If a lateral voltage is applied to the left and right sides of the liquid crystal on the horizontal plane, the liquid crystal molecules are aligned along the direction of the electric field, so that the linearly polarized light passing through the TFP side polarizer passes through the liquid crystal layer. It is in the state of elliptical polarization and finally passes through the CF side polarizing plate, showing a white state.

The IPS mode is a natural display mode with a wide viewing angle. However, the electrodes in the IPS mode are designed on the same side, and the distance between the electrodes cannot be too close. On the one hand, such a structure will reduce the aperture ratio of the pixel, and on the other hand, the IPS liquid crystal needs to have a high dielectric constant anisotropy and a large driving voltage.

3. VA mode

The vertical alignment (VA) display mode is characterized by the alignment of the liquid crystal molecules perpendicular to the glass substrate. In the two states of no power and power on, the direction of the liquid crystal molecules is switched between vertical and tilt, and the rotation of the liquid crystal molecules belongs to the spatial rotation. The polarization axes of the TFT side polarizer and the CF side polarizer used in the VA display mode are arranged orthogonally.

Under the condition of no applied voltage, after the natural light enters the polarizing plate on the TFT side, it forms linearly polarized light parallel to the short axis of the liquid crystal molecule, and the polarization direction cannot be rotated. The polarizing axis of the plate is vertical, so the linearly polarized light here is absorbed by the CF side polarizing plate, and the display is in a black state. This black is formed orthogonally by the upper and lower polarizers, which excludes the influence of the liquid crystal layer and has the best black effect.

If a vertical voltage is applied to the upper and lower sides of the liquid crystal, the liquid crystal molecules lie down along the direction of the electric field, and the linearly polarized light passing through the TFT side polarizer passes through the liquid crystal layer and becomes an elliptically polarized state, and finally passes through the CF side polarizer, showing a white state

In TN mode and IPS mode, rubbing alignment is required on the alignment film to handle the pretilt angle required to form the horizontal alignment. According to the characteristics of the birefringence of the liquid crystal, a part of the elliptically polarized light will be formed after the linearly polarized light passes through the liquid crystal layer. So it can’t be true black. The VA mode has the best black state, the denominator of the contrast ratio is very small, and the best contrast ratio can be achieved, so the VA mode is a display mode with a high contrast ratio by nature. Like the IPS mode, the VA mode is also known as the “normally black mode”

4. TN vs VA vs IPS Comparison Table

TN panels respond fast and at a low cost. The IPS panel has better visual fidelity, but the cost is higher. The VA panel is located between the TN and IPS panels. The VA panel has excellent contrast