Understanding the Structure and Characteristics of LCD Panel Driving Methods
In the previous three installments of the “ITmedia-style LCD Display Course,” we briefly touched on LCD panel driving methods.
Although they are not usually listed first in specifications, they are important elements that determine image quality and response speed characteristics.
This time, we will explain the basic mechanisms and features of the main driving methods, as well as how to identify them using specification sheets and in-store observation.
The article below was originally published on October 7, 2005, as “ITmedia-style LCD Display Course I, Part 4.”
Viewing Angle and Response Speed Characteristics That Differ by Driving Method
LCD panel driving methods can be broadly classified into three types:
- TN (Twisted Nematic), VA (Vertical Alignment), and IPS (In-Plane Switching).
- Among PC LCD displays, TN is the most widely used method, followed by VA and then IPS.
- While it is not possible to generalize completely, in terms of cost, the order is typically TN → VA → IPS.
- For still image display on PCs, image quality generally follows the order IPS → VA → TN.
Basic Structure of LCD Panels
Let us briefly explain the basic structure of LCD panels. LCD panels control light by applying voltage to rice-grain-shaped liquid crystal molecules and changing their orientation. The “amount of light” here refers to the amount of backlight light that passes through the liquid crystal molecules and reaches the screen. If the backlight is completely blocked by the liquid crystal molecules, the screen becomes black. If all light passes through, the screen becomes white.
In reality, complete blocking or transmission does not occur due to light leakage and diffusion. This basic principle is common to all driving methods.
Differences Between TN, VA, and IPS Driving Methods
What differs between TN, VA, and IPS driving methods is how the liquid crystal molecules are arranged and how they are moved by voltage. The differences in driving methods greatly affect viewing angle characteristics and response speed characteristics. Focusing on these two points, we will first describe the TN method, which is currently the mainstream, and then explain VA and IPS methods in comparison with TN.
Structure and Characteristics of the TN Method
Figure 1 shows a simplified schematic of the liquid crystal molecule arrangement in the TN method. When voltage is OFF [(C)], liquid crystal molecules are aligned horizontally, allowing backlight light to pass through and making the screen white. As voltage is gradually applied, the liquid crystal molecules rise vertically. At maximum voltage [(A)], the backlight light is blocked and the screen becomes black.
Intuitively, it may seem that liquid crystal molecules block light when they are horizontal, but due to the polarizers surrounding the liquid crystal layer and the 90-degree twist of the molecules, the light path behaves as described above.
Advantages and Disadvantages of the TN Method
The advantages of the TN method are low driving voltage and low cost. The disadvantages are large changes in color and brightness depending on viewing angle.
Viewing angle behavior can be easily understood by looking at position (B) in Figure 1. Because backlight light is controlled by the angle of the liquid crystal molecules, the amount of transmitted light changes depending on the viewing angle. In other words, TN panels are not suitable for applications where color accuracy is important.
Response Speed Characteristics of TN Panels
In general, TN panels have slow rise times (black → white) and fast fall times (white → black). In addition, response speed in mid-tones drops sharply compared to rise and fall transitions. For this reason, many recent products include overdrive technology to improve mid-tone response speed and enhance video display quality.
Structure and Characteristics of the VA Method
In the VA method, liquid crystal molecule alignment is as shown in Figure 2. When voltage is OFF [(A)], liquid crystal molecules are vertical. When voltage reaches its maximum [(C)], the molecules align horizontally. With VA panels, voltage OFF corresponds to black, and maximum voltage corresponds to white.
Key Feature of the VA Method
The major feature of the VA method is that when voltage is OFF, backlight light is almost completely blocked by the polarizers without being affected by the liquid crystal molecules. This allows VA panels to produce very pure blacks and achieve high contrast ratios.
Viewing Angle and Response Speed of VA Panels
On the other hand, VA Panel share the same weakness as TN panels regarding viewing angle-related brightness and color changes. Because backlight light is controlled by the angle of the liquid crystal molecules, the transmitted light amount changes depending on viewing angle. Response speed characteristics are also similar to TN panels. Rise time (black → white) is slow, fall time (white → black) is fast, and mid-tone response is even slower. As with TN panels, VA displays with overdrive technology have appeared to improve mid-tone response speed.
Multi-Domain Technology in VA Panels
Many VA panels incorporate liquid crystal orientation division technology (multi-domain) to improve viewing angle characteristics. In multi-domain structures, the direction in which liquid crystal molecules tilt during vertical-to-horizontal movement is reversed depending on the area. For example, in area A molecules tilt to the right, while in area B they tilt to the left.
While TN-like viewing angle characteristics exist within each area, averaging light output across the entire screen significantly reduces color changes caused by viewing angle.
Structure and Characteristics of the IPS Method
In the IPS method shown in Figure 3, backlight light is controlled by rotating horizontally aligned liquid crystal molecules sideways. Because there is no vertical tilt of the liquid crystal molecules, IPS panels exhibit minimal brightness and color changes caused by viewing angle.
Weak Points of the IPS Method
From a structural standpoint, IPS panels have difficulty achieving high contrast ratio, high brightness, and fast response speed. Regarding contrast ratio, even when the screen displays black, backlight light leakage is significant, making it difficult to achieve deep blacks. As a result, black luminance remains high, lowering the contrast ratio.
High brightness is difficult due to aperture efficiency, and fast response is difficult due to the rotation-based movement of liquid crystal molecules. However, unlike TN and VA panels, IPS panels have relatively consistent response speed across all gray levels.
Typical Use of IPS Panels
IPS LCD displays maintain a high market share in professional graphics and medical applications. In these environments, where high-speed video performance is not required, IPS panels are highly valued for their excellent color reproduction and viewing angle characteristics.
How to Identify LCD Panel Driving Methods
Most PC LCD displays currently sold as standalone products use the TN method. VA panels follow, while IPS panels are now a minority.
Identifying Panels Using Specification Sheets
Key indicators in specification sheets are viewing angle, maximum color count, and response speed. However, these cannot be applied as strict rules and should only be used as rough guidelines. Ideally, vendors should clearly disclose the panel driving method. Even if the information is inconvenient, disclosing it properly benefits users.
Viewing Angle Specifications
If vertical viewing angle values differ between upper and lower directions, the panel is almost certainly TN. However, there is a pitfall. Viewing angle values indicate the range in which a certain minimum contrast ratio is maintained. Previously, a contrast ratio of 10:1 was standard. Many vendors do not disclose which contrast ratio standard they use, adding to ambiguity. A 5:1 contrast ratio only indicates that the screen is visible, without considering brightness or color changes compared to front viewing.
Maximum Color Count
The standard maximum color count is 16.77 million colors. If a display lists 16.19 million colors or includes notes such as “dithering used,” it is almost certainly a TN panel. However, some TN panels do support 16.77 million colors, so this alone does not confirm VA or IPS.
The value 16.77 million comes from 8-bit gradation per RGB channel: 256 × 256 × 256 = 16,777,216 colors.
Displays with 16.19 million colors use 6-bit gradation plus FRC (Frame Rate Control) to simulate full color.
Response Speed
Response speed evaluation has become more complex with the introduction of overdrive technology. Some products list both traditional response speed (black → white → black) and mid-tone response speed.
Others list only traditional response speed, even if overdrive is supported.
As a rough guideline (as of September 2005), displays with black → white → black or white → black → white response times of 12 ms or less can be considered TN panels.
Identifying Panels in Stores
Those who can visit stores with many displays should observe screens while keeping driving methods in mind. The most noticeable difference appears in vertical viewing angles.
TN panels show dramatic color changes when viewed from above or below, while VA and IPS panels change less. This allows identification between TN and non-TN panels.
IPS panels, however, are becoming increasingly rare, especially in popular 17-inch and 19-inch classes, which are dominated by TN and VA panels.