In the commercial display field, TFT-LCD technology is the most common display technology, but the resulting image quality issues often cause headaches for manufacturers and users.
Rainbow patterns on LCD screens are a common visual defect, especially noticeable under pure white or grayscale images. They not only affect the aesthetics of the product but can also degrade the user experience.
This article will begin by defining and explaining the phenomenon of rainbow patterns, delve into its optical mechanisms, and provide practical and effective improvement strategies, hoping to help you when selecting or optimizing products.
What is a rainbow effect in TFT-LCD displays?
A rainbow effect is an optical interference phenomenon that manifests as rainbow-like colored stripes or patches on the screen of a TFT-LCD display module. These stripes typically alternate between red, green, and blue colors, similar to the glossy effect of soap bubbles or oil films.
Reflective rainbow effects: Primarily caused by the optical films (such as diffusion films and brightness enhancement films) of the PET substrate and reflective polarizers (APF).
Transmissive rainbow effects: Related to the interference of transmitted light between multilayer film structures.
An important characteristic of rainbow effects is their viewing angle dependence: They may be inconspicuous at normal viewing angles, appearing only when viewed from specific angles, and changing or disappearing as the viewing angle shifts.
In TFT-LCD modules, many optical components, such as diffuser films, brightness enhancement films, and the ITO touch layer, use PET substrates. This material is manufactured through a biaxial stretching process, resulting in an ordered arrangement of molecular chains along the stretching direction, thus creating anisotropy-that is, differences in the material's physical and optical properties in different directions.
The most direct manifestation of this anisotropy is birefringence. When light enters the PET film, it splits into two beams of polarized light: ordinary light (o-ray) and extraordinary light (e-ray).
These two beams have different refractive indices (no ≠ ne), and their propagation speeds and paths also differ, resulting in a phase difference or optical path difference. If these beams interact with other layers (such as polarizers or LCD panels), the number of reflections and refractions increases, and the accumulated optical path difference, when it reaches a certain level, triggers light interference, ultimately appearing as a rainbow effect on the screen.
In simple terms, the chain of events is as follows: The PET stretching process causes the molecular arrangement to orient itself, forming the basis of birefringence.
When light passes through multiple layers of film, the uneven path creates an accumulated optical path difference.
Interference occurs, visually appearing as colored fringes.
1. Optimize Material Selection And Reduce Sources Of Birefringence.
For touch solutions, prioritize In-cell or On-cell technologies to avoid the additional PET layer required by external touch films (such as GFF).
If an external solution is necessary, a GF structure is recommended to reduce interlayer gaps and lower the risk of interference.
In backlight modules, evaluate whether the optical film on the PET substrate can be replaced or reduced.
2. Adjusting The Optical Path And Matching The Optical Path Difference
By changing the cutting angle of the brightness enhancement film and adjusting its angle with the incident light and the panel, the optical path is reconstructed, reducing interference.
Testing brightness enhancement films with different stretching positions is crucial, as differences in stretching℃affect the birefringence direction. Materials with lower interference levels can be selected.
3. Increase Material Haze To Disrupt Interference Conditions
Use high-haze upper brightness enhancement film back coatings or high-haze lower diffuser films in the backlight system, or even add an upper diffuser film (for medium to large sizes).
Perform AG (anti-glare) treatment on the surface of the lower polarizer to increase haze (e.g., 25% haze), disrupting phase consistency through light scattering.
Note: Increasing haze will sacrifice some brightness; a trade-off must be made.
4. Use Reflective Polarizers With Caution.
Reflective down-polarizers (APF brightening filters) achieve brightness through multiple reflections, but they also significantly increase optical path difference and the risk of interference.
When brightness allows, prioritize ordinary down-polarizers or down-polarizers, and avoid using APFs.
Although rainbow patterns are a common visual defect in TFT-LCDs, their frequency can be controlled through material innovation and process adjustments.
Addressing rainbow patterns is not only a technical challenge but also a crucial step in improving product visual uniformity and customer satisfaction.
