Analysis of Contrast and Brightness Reduction in IPS Panels at Wide Viewing Angles

 

IPS (In-Plane Switching) panels are well-known for their superior wide viewing angles. However, when observed from large angles, reductions in contrast and brightness are commonly observed. These phenomena are closely related to the arrangement of liquid crystal molecules, light propagation characteristics, and panel optical design. Below is a detailed analysis of the reasons behind this issue.

1. Key Factors Affecting Contrast Reduction

(1) Polarization of Light

The function of polarizers is to polarize incoming light into a specific direction. At wide viewing angles, changes in the light’s propagation path cause the polarization direction to deviate from the absorption axis of the polarizer, impacting contrast levels.

(2) Limitations in Liquid Crystal Alignment

The effective alignment range of liquid crystal molecules is limited. At wide viewing angles, light traverses through liquid crystal layers and interacts with molecules oriented in various directions, altering the polarization state and thereby affecting contrast.

(3) Influence of Optical Films

High-end IPS panels often use specialized optical films, such as wide-viewing-angle enhancement films, to address viewing angle issues. While these films mitigate some problems, they may adversely affect contrast in certain scenarios.

(4) Supporting Data from Testing

According to Rtings' testing data, with an 80% acceptable contrast reduction threshold, IPS panels exhibit approximately a 30-degree viewing angle. Beyond this angle, contrast reduction becomes significantly noticeable.

 

2. Reasons Behind Brightness Reduction

(1) Limitations of Liquid Crystal Alignment and Light Transmittance

Restricted Liquid Crystal Deflection Angles:
IPS panels feature liquid crystal molecules aligned parallel to the glass substrate when no voltage is applied. When voltage is applied, the molecules tilt horizontally to modulate light transmission. At large viewing angles (e.g., beyond 60° from the screen's normal), light traverses the liquid crystal layer at oblique paths. This reduces the molecules’ ability to modulate light, causing part of the light to refract or scatter, resulting in reduced transmittance and brightness.

 

Impact of Alignment Uniformity:
Although IPS panels exhibit relatively uniform molecular alignment, light traverses longer paths through the liquid crystal layer at wide angles. Even minor alignment inconsistencies accumulate, further diminishing light transmission consistency and reducing brightness.

 

(2) Angle Dependency of Polarizers and Optical Films

Misalignment of Polarizer Transmission Axes:
IPS panels typically utilize two orthogonal polarizers (with transmission axes at a 90° angle). At perpendicular viewing angles, the liquid crystals precisely adjust light polarization to align with the polarizer axes. However, at wide viewing angles, the light’s entry path deviates from the polarizer’s axis, partially blocking polarized light and reducing brightness.

 

Limitations of Optical Compensation Films:
Optical compensation films (e.g., phase retardation films) are used to improve wide-angle performance by compensating for light refraction at varying angles. These films are optimized for specific angular ranges, and their effectiveness diminishes outside these ranges. This leads to increased light scattering and reduced brightness.

 

(3) Optical Coupling Efficiency Between Backlight and Liquid Crystal Layer

Backlight Emission Angle Limitation:
The backlight (e.g., LED) of a liquid crystal panel primarily emits light vertically. At wide viewing angles, light must enter the liquid crystal layer at steeper angles from the backlight edges. This reduces the optical coupling efficiency between the backlight and the liquid crystal layer, as some light undergoes total internal reflection or edge absorption, leading to brightness attenuation.

 

Absorption Effects of Color Filters:
Color filters (CF) contain pigment layers that exhibit higher absorption rates for obliquely incident light. At wide viewing angles, the light path through the filter becomes longer, resulting in increased absorption of the red, green, and blue light components. This not only reduces brightness but may also cause color shifts (e.g., reduced saturation).

 

3. Conclusion

Contrast reduction in IPS panels at wide viewing angles arises primarily from the misalignment of the light’s polarization direction with the polarizer’s absorption axis and the limited alignment range of liquid crystal molecules. Brightness reduction is attributed to factors including liquid crystal molecular arrangement, the angle dependency of polarizers and optical films, and the optical coupling efficiency between the backlight and the liquid crystal layer. Although IPS panels perform relatively well at wide angles, these factors inevitably cause reductions in contrast and brightness under extreme viewing conditions.