Complete Guide to TFT-LCD's Wide Viewing Angle Technology
TN-type display, IPS-type display, and VA-type display, these three types of displays cover almost all tft displays we can access, and each of them has its own wide viewing angle technology. In addition to wide viewing angle technology, there is also knowledge about display power consumption and sunlight readable display.
If we view a TFT-LCD from a wide angle, you will notice a rapid loss of brightness and discoloration of the display. Older flat panel displays usually only have a 90 degree viewing angle, which is 45 degrees on each side of the left/right side.
When the incident light from the backlight passes through the polarizer, liquid crystal and orientation film, the output light has a specific directional characteristic, which means that most of the light coming from the screen has a vertical direction. If we view an all-white screen from a very oblique angle, we may see black or color distortion. This effect is useful in some situations, but in most applications we don’t want it. Manufacturers have spent a lot of time trying to improve the viewing angle characteristics of LCDs, and several wide viewing angle technologies have been proposed: IPS (IN-PLANE-SWITCHING), MVA (MULTI-DOMAIN VERTICAL ALIGNMENT), TN+FILM, all of which can increase the viewing angle of LCDs to 160 degrees, or even more. or even more, just like the viewing angle characteristics of CRT screens. Maximum viewing angle is defined as a viewing angle with a contrast ratio of at least 10:1 (usually in four directions, up/down/left/right). LCD manufacturers have not stopped in their tracks, but have recently introduced the first new technologies that can improve viewing angle characteristics. The most important wide viewing angle technologies include TN+Film, IPS (also known as Super LCD) and MVA.
IPS (Planar Control Mode) wide viewing angle technology
One of the biggest features of IPS is that its electrodes are all on the same side, unlike other liquid crystal mode electrodes that are on the upper and lower sides. Because only in this way can a plane electric field be created to drive the liquid crystal molecules to move laterally. This kind of electrode has a negative impact on the display effect: when a voltage is applied to the electrode, the liquid crystal molecules close to the electrode will gain greater power, and it is no problem to quickly twist 90 degrees. However, the upper liquid crystal molecules far away from the electrodes cannot obtain the same power and move more slowly. Only by increasing the driving voltage can the liquid crystal molecules far away from the electrodes also get a lot of power. Therefore, the driving voltage of IPS will be higher, generally 15 volts. Since the electrodes are on the same plane, the aperture ratio will be reduced and the light transmittance will be reduced, so the IPS display needs more backlights.
Pixel two domains technology
When a voltage is applied to the electrodes, the liquid crystal molecules that were originally parallel to the electrodes will rotate to the direction perpendicular to the electrodes, but the long axis of the liquid crystal molecules is still parallel to the substrate. Controlling the magnitude of the voltage will rotate the liquid crystal molecules to the required angle. The film can modulate the transmittance of polarized light to display different color levels. The working principle of IPS is somewhat similar to that of TN mode liquid crystal, the difference is that the arrangement of liquid crystal molecules in IPS mode is not twisted nematic and its long axis direction is always parallel to the substrate.
For the gray scale reversal phenomenon of IPS mode in the oblique 45° direction, in addition to using optical films to compensate, IPS can also be “optimized” according to the characteristics of MVA. As shown in the figure, the original straight electrode of IPS is changed to a zigzag electrode “human-shaped electrode” like MVA mode. This improved IPS absorbs the advantages of IPS and MVA, and can be called “double-domain IPS”. It is a new generation of Super-IPS.
FFS (Fringe Field Switching) Wide Viewing Angle Technology
The FFS structure is similar to the IPS mode, and the positive and negative electrodes are no longer arranged at intervals. Transparent electrodes do not block light, so a higher aperture ratio can be achieved. The new generation of FFS technology also has revolutionary improvements in other aspects. For example, the negative-type liquid crystal has high optical efficiency, but the torsional viscosity is large and the response time is slow; while the positive-type liquid crystal has a faster response time, but the optical efficiency is very low. By optimizing the liquid crystal, FFS technology also obtains about 90% of the light efficiency of negative liquid crystal on positive liquid crystal, so that both light transmittance and response time can be achieved. The optimized wedge-shaped electrode can automatically suppress light leakage, so that even the inherent black matrix of the color filter can be discarded, which greatly increases the light transmittance
MVA (Multi-Domain Vertical Alignment) Wide Viewing Angle Technology
MVA (Multi-domain Vertical Alignment) mode liquid crystal display, the long axis of the liquid crystal molecules is not parallel to the screen like the TN mode when not powered, but perpendicular to the screen, and each pixel is composed of multiple such vertical Aligned liquid crystal molecular domains. When a voltage is applied to the liquid crystal, the liquid crystal molecules flip in different directions. In this way, the compensation in the corresponding direction can be obtained when viewing the screen from different angles, which improves the viewing angle. MVA (Multi-Domain Vertical Alignment) From a technical point of view, MVA is the best solution for viewing angle and response time. MVA can obtain a 160-degree viewing angle, and can also provide excellent performance with high contrast and fast response.
What is the principle of MVA? In MVA, M stands for “multi-domain” (multi-quadrant), which is a sub-pixel (cell). VA stands for “Vertical Alignment”. Because this mode of LCD display is black when it is not powered, it is also called NB (Normal Black, normally black) mode LCD.
In the MVA mode, because the movement amplitude of the liquid crystal molecules is not as large as that in the TN mode, the liquid crystal molecules will rotate to a predetermined position relatively faster after power-on, and the liquid crystal molecules close to the electrode slope will rotate rapidly when receiving electricity, driving the Liquid crystal molecules further away from the electrodes move. Therefore, the MVA wide viewing angle technology after changing the arrangement of the liquid crystal molecules is beneficial to improve the response speed of the liquid crystal.
The vertical alignment of liquid crystal molecules means that the liquid crystal molecules at both ends of the Panel do not need to be aligned parallel to the Panel, which means that MVA no longer needs rubbing treatment in manufacturing, which improves production efficiency. The front contrast ratio of the MVA mode liquid crystal display with the optical compensation film can be done very well, even if it is not difficult to reach 1000:1. Unfortunately, the color of MVA liquid crystal will become lighter as the viewing angle increases. If the viewing angle is defined by the color difference change, the MVA mode will suffer, but in general, it is still a big improvement over the traditional TN mode. .
MVA mode isn’t a perfect wide-angle technology. Its special electrode arrangement makes the electric field intensity uneven. If the electric field intensity is not enough, the grayscale display will be incorrect. Therefore, it is necessary to increase the driving voltage to 13.5V in order to precisely control the rotation of the liquid crystal molecules. In addition, because its liquid crystal molecular arrangement is completely different from the traditional TN mode, if the traditional process is used when filling the liquid crystal, the time required will be greatly increased. Therefore, the cost of MVA has increased compared with the traditional TN mode liquid crystal.
When the liquid crystal of the dual-domain VA mode is not powered, the long axis of the liquid crystal molecules is perpendicular to the screen, and only the liquid crystal molecules close to the bump electrodes are slightly inclined, and the light cannot pass through the upper and lower polarizers at this time. When power is applied, the liquid crystal molecules near the protrusions quickly drive other liquid crystals to rotate to a state perpendicular to the surface of the protrusions, that is, the long axis of the molecules is inclined to the screen, and the transmittance increases to achieve modulated light.
In this dual-domain mode, the states of adjacent domain molecules are just symmetrical, and the long axes point in different directions. The VA mode uses this different molecular long axis pointing to achieve optical compensation.
When the straight triangular prism-shaped protrusions in the dual-domain mode liquid crystal are changed into triangular prism-shaped protrusions bent back and forth at 90°, the liquid crystal molecules can be subtly divided into four domains, that is, the multi-domain mode. After the four-domain mode liquid crystal is powered, the liquid crystal molecules of the A, B, C, and D domains rotate in four directions, which compensates the upper, lower, left, and right viewing angles of the liquid crystal display at the same time. Therefore, the MVA mode liquid crystal display is in these four directions. There are good viewing angles in all directions. Based on this compensation principle, the shape of the protrusions can be changed, and more liquid crystal domains in different directions can be used to compensate any viewing angle to achieve good results.
TN+Film (TN+ viewing angle expansion film) wide viewing angle technology
TN+Film wide viewing angle technology is based on the improved technology of TN liquid crystal display. The arrangement of liquid crystal molecules is still in TN mode, and the motion state is still twisted from the parallel direction of the panel to the vertical direction after power-on. It uses a transparent film with birefringence △n<0 to compensate for the phase retardation caused by the TN liquid crystal cell (△n>0) to achieve a wide viewing angle, so this Film is also called a phase difference film or compensation film (also has a widening viewing angle). film). The phase difference film is a component made by stretching a transparent film into a predetermined deformation.
The compensation film is not only attached to the surface side of the liquid crystal panel, but on both sides of the liquid crystal cell. When the light passes through the compensation film from below, there will be a negative phase retardation (because the compensation film △n<0), after entering the liquid crystal cell due to The effect of liquid crystal molecules, when reaching the middle of the liquid crystal cell, the negative phase retardation offsets the positive retardation to 0. When the light continues to move upward, it has a positive phase retardation when it passes through the liquid crystal cell due to the action of the upper part of the liquid crystal molecules. When the light passes through the upper compensation film, the phase retardation is just offset to 0 again. In this way, a good viewing angle improvement effect can be achieved by using a precise compensation film with a TN mode liquid crystal.
TN-type displays, IPS-type displays, and VA-type displays all have their own wide viewing angle technology. TN type display can improve viewing angle through compensation film, IPS can achieve large viewing angle through advanced design such as FFS, VA type display and IPS type display can also achieve large viewing angle through special pixel design, using these technologies can achieve better display effect .