Liquid crystal display technology is developed based on the birefringence effect of liquid crystal molecules, and it is divided into four types according to the motion state of liquid crystal molecules, namely TN, IPS, VA and FFS.
This article mainly discusses the display principles, advantages and disadvantages of products with the four modes respectively:
01 TN Mode
TN (Twist Nematic) mode, commonly known as the “soft screen” in the industry, is an early-used and technically mature panel type in liquid crystal display products. Its display principle is as follows:
① Without voltage applied: Natural light from the backlight passes through the lower polarizer to form linearly polarized light that enters the liquid crystal cell. Under the action of the alignment layers (PI) on the inner surfaces of the upper and lower glass substrates, liquid crystal molecules twist 90 degrees from bottom to top, making the polarization direction of light rotate synchronously during propagation.
When the polarized light reaches the upper polarizer, it has rotated 90 degrees and is parallel to the optical axis of the upper polarizer, so the light can pass through, and the LCD is in a bright state at this time.
② With voltage applied (vertical electric field): The interaction between the dielectric anisotropy of liquid crystals and the electric field causes the twisted structure of liquid crystal molecules to disintegrate, and the long axes of liquid crystal molecules are parallel to the direction of the electric field. The polarization direction of linearly polarized light no longer rotates. When the polarized light reaches the upper polarizer, it is perpendicular to the optical axis of the polarizer, resulting in no light transmission, and the LCD is in a dark state at this time.
In summary, the TN mode panel shows a bright state when not powered on and a dark state when powered on. Since the TN display mode is bright when unpowered, it is called a normally white mode.
The TN mode panel adopts a vertical electric field, and the long axes of liquid crystal molecules rotate along the vertical direction. The optical path difference varies greatly when viewed from different angles, so the display effect will differ significantly when people look at the display screen from different viewing angles, and the picture is the most realistic only when viewed from the front angle.
Of course, the display difference from different viewing angles can be solved by using polarizers with viewing angle compensation. However, there is always an optimal viewing angle for TN mode panels, such as the 6 o’clock or 12 o’clock direction.
Main Advantages:
Fast phase response time: TN mode panels are generally 6-bit panels with a small number of gray levels, so liquid crystal molecules deflect quickly and the response speed is fast.
Low cost: Its production process is relatively simple and mature, resulting in low production costs, which is suitable for users who have low requirements for display image quality and focus on cost.
Low power consumption: TN panels consume relatively low power at the same brightness, making them suitable for long-term use.
Main Disadvantages:
Small viewing angle: TN mode panels have a narrow viewing angle, especially in the vertical direction, where obvious color and contrast shifts occur. When observing the screen from the side, the brightness and color saturation of the image will decrease, and blanching or distortion may occur.
Poor color richness: TN mode panels are generally 6-bit, and each primary color (RGB) can only display 2 to the 6th power (i.e., 64) gray levels, with a total of 64×64×64 = 262,144 colors available. Therefore, the color reproduction and layering are relatively poor, and the image colors may appear less bright and vivid.
Low contrast: The contrast is generally only about 800:1. When displaying black and white, the black is not deep enough and the white is not bright enough, resulting in insufficient layering and three-dimensionality of the picture, which leads to poor detail expression of the image.
Due to the obvious shortcomings of TN mode panels, which cannot meet consumers’ requirements for display effects, their market share is getting smaller and smaller. At present, they are mainly used in low-end products and even face the risk of being eliminated.
02 IPS Mode
IPS (In-plane Switching) mode, commonly known as the “hard screen” in the industry, is an advanced liquid crystal display technology. It was developed by Hitachi in the early 2000s and further promoted and improved by LG and other companies.
IPS technology is popular in the market for its excellent display performance and wide application fields, becoming one of the mainstream display modes at present. Its display principle is as follows:
① Without voltage applied: When no voltage is applied, natural light from the backlight passes through the lower polarizer to form short-axis linearly polarized light. Since the liquid crystal molecules do not rotate, the polarization direction cannot be changed, and the light is perpendicular to the upper polarizer and blocked by it, with no light emitted. At this time, the LCD is in a dark state.
② With voltage applied (horizontal electric field): When voltage is applied, liquid crystal molecules rotate parallel to the direction of the electric field, changing the polarization direction of the light passing through the lower polarizer by 90 degrees to make it parallel to the light transmission direction of the upper polarizer, so the light can pass through the upper polarizer, and the LCD is in a bright state at this time.
By controlling the magnitude of the applied voltage, IPS panels can adjust the rotation angle of liquid crystal molecules, thereby controlling the light transmittance and achieving the display of different grayscales and colors.
In summary, the IPS panel shows a dark state when not powered on and a bright state when powered on. Since the IPS display mode is dark when unpowered, it is called a normally black mode.
IPS LCD adopts a horizontal electric field, and the long axes of liquid crystal molecules rotate along the horizontal direction. The optical path difference is very small when viewed from different angles, and there is no significant difference in viewing angle and display effect when observing from all directions.
Main Advantages:
Large viewing angle: Clear images and accurate color performance can be maintained when viewing the product from different angles.
High color reproduction: Ordinary IPS panels are basically 8-bit, and each primary color (RGB) can display 2 to the 8th power (i.e., 256) gray levels, with a total of 256×256×256 = 16,777,216 colors available. Therefore, it can control the changes of color and brightness more finely and present richer and more delicate color layers and light-dark transition effects.
Durability and long service life: IPS panels are more durable than OLED screens and are less susceptible to screen burn-in and high protection requirements. Even if the LCD screen is slightly damaged, it can still be used normally; if an OLED screen is damaged, water vapor and oxygen will seep into the panel and make it unusable.
Main Disadvantages:
Relatively low contrast: Compared with VA panels, IPS panels generally have lower static contrast, usually around 1500:1.
Light leakage in black screen: When displaying a black screen, light may overflow at the edges of the screen, resulting in impure black and affecting the contrast and visual effect of the picture.
Relatively high cost: Due to the high manufacturing cost, IPS panels are usually more expensive than TN panels.
03 VA Mode
VA (Vertical Alignment) mode inherently has a higher contrast than TN and IPS display modes.
This is because VA mode panels do not require alignment treatment of the alignment film, ensuring a very high contrast of VA panels (at rest: liquid crystal molecules of VA panels are arranged vertically, while those of IPS panels are pre-tilted along the alignment direction of the PI film).
VA mode panels are also one of the mainstream display modes at present, mainly used in large-size TVs and curved screens. They can be further divided into MVA, PVA, PSVA and UV2A. This article mainly focuses on MVA, and its display principle is as follows:
① Without voltage applied: Liquid crystal molecules are arranged vertically at rest. After natural light from the backlight passes through the lower polarizer, its polarization direction cannot be changed due to the vertical alignment characteristic of liquid crystal molecules, and the light is blocked by the upper polarizer. At this time, the LCD is in a dark state.
② With voltage applied (vertical electric field): When voltage is applied, liquid crystal molecules tilt and become horizontally arranged, changing the polarization direction of light so that it can pass through the upper polarizer, and the LCD is in a bright state at this time.
Both VA and IPS adjust the rotation angle of liquid crystal molecules by controlling the magnitude of the applied voltage, thereby controlling the light transmittance and achieving the display of different grayscales and colors.
In summary, the VA panel shows a dark state when not powered on and a bright state when powered on. Since the VA display mode is dark when unpowered, it is called a normally black mode.
Like TN panels, VA panels also adopt a vertical electric field, and the long axes of liquid crystal molecules rotate along the vertical direction. However, due to the Two Domain design of VA panels, which compensates for the optical path difference of different viewing angles, there is no significant difference in viewing angle and display effect when observing from all directions.
Main Advantages:
High contrast: The contrast of VA mode panels can usually reach 3000:1 or even higher, which makes the black in the picture darker and the white brighter, with stronger layering and three-dimensionality of the image. It has good performance in displaying dark details and bright scenes.
Large viewing angle: Although its viewing angle is slightly inferior to that of IPS screens, it still has a wide viewing angle.
High color reproduction: Like IPS panels, VA panels are also 8-bit, so they can present relatively rich and accurate colors and provide a good color performance.
Main Disadvantages:
Slow response time: Compared with TN screens and some fast-response IPS screens, VA screens have a longer response time, especially when displaying dynamic images. The speed of switching from one color to another is slow, which may cause motion blur and ghosting.
High power consumption: VA products require a higher driving voltage, so they consume relatively high power during operation.
Average uniformity: The uniformity of the product is not good, and color shift is prone to occur. There may be differences in color and brightness in different display areas of the product.
04 FFS Mode
FFS (Fringe Field Switching) mode is a patented technology owned by Hyundai Hydis of South Korea, and it is an advanced extension of the IPS wide viewing angle technology.
FFS mode generates a fringe electric field between pixel electrodes on the same plane, making liquid crystal molecules directly above the electrodes and between the electrodes rotate in the plane direction parallel to the substrate, thereby significantly improving the brightness and image quality of the product.
FFS technology overcomes the problem of low transmittance of conventional IPS technology, achieves high transmittance on the premise of a wide viewing angle, and avoids the water ripple phenomenon caused by extrusion, providing a more realistic image effect and display close to real colors.
Its display principle is similar to that of IPS, both adopting a horizontal electric field: the LCD is in a dark state when no voltage is applied, and in a bright state when voltage is applied.
The difference is that after voltage is applied to the IPS panel, only the liquid crystal molecules directly above the electrodes rotate; while after voltage is applied to the FFS panel, the liquid crystal molecules both directly above the electrodes and between the electrodes can rotate.
Main Advantages:
High transmittance: FFS mode adopts transparent electrodes to improve the aperture ratio of pixels, thereby increasing the transmittance of the panel.
Wide viewing angle: FFS mode enlarges the display viewing angle through the optimization of the fringe electric field.
Low power consumption: Due to its high transmittance, a lower backlight brightness is required to display images of the same brightness, thus reducing power consumption.
Main Disadvantages:
High production cost: The manufacturing process of FFS technology is relatively complex, requiring high-precision equipment and strict production process control, resulting in high production costs.
Response time: The improvement of transmittance and viewing angle by FFS technology may affect its response time, thus impacting the display effect of dynamic images.
That’s all for today’s sharing. We’ll post more knowledge about LCD digital signage at irregular intervals.
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