In the field of TFT-LCD displays, composite optical films are a crucial component, especially in product designs that prioritize high brightness, thinness, and reliability. Today, we'll focus on two common composite optical films: DOP (Diffuser on Prism) and POP (Prism on Prism), exploring their basic structure, application scenarios, and key factors affecting brightness.
We hope this blog post will provide designers and engineers in the TFT-LCD display field with some helpful and practical guidance.
Regarding DOP composite optical film
DOP composite optical film is essentially a combination of optical diffuser film and brightness enhancement film. The diffuser layer is usually placed on the upper surface of the brightness enhancement film. This composite method is commonly used in display products such as notebooks, mini-phones, and TVs.
Why Choose DOP Composite Optical Film?
DOP composite optical film effectively reduces the risk of film warping and surface scratches, while also reducing the overall thickness of the display module. From a production perspective, this not only simplifies the assembly process but also improves overall yield.
In practical applications, DOP can also adjust the layer sequence according to requirements, such as placing the optical diffuser film behind the brightness enhancement film. However, this often involves three-layer composites (such as DOPP) and is suitable for more complex structures.
About POP Composite Optical Film
Compared to DOP composite optical film, POP composite optical film is composed of two layers of brightness enhancement film, so it is also called "composite brightness enhancement film". It has a wider range of applications, covering a full range of product sizes including mobile phones, tablets, laptops, monitors and TVs.
Why Choose POP Composite Optical Film?
If the display device being manufactured places great emphasis on ultra-thin and narrow bezel designs, the advantages of POP composite optical film become very apparent.
It can significantly reduce module thickness and minimize backlight edge issues that appear after reliability testing. The flexibility of POP optical film makes it the first choice for high-resolution displays, but this requires accurate control of the composite film parameters to balance brightness and durability.
Factors Affecting the Brightness of DOP and POP Composite Films
The brightness of composite optical films is typically slightly lower than that of single-material films due to interlayer interactions. However, this doesn't mean it's unoptimizable-performance can be significantly improved through targeted adjustments.
Below are several key factors for enhancing the brightness of DOP/POP optical composite films, but their application requires comprehensive evaluation in conjunction with the product's performance requirements.
Prism bonding adhesive layer thickness
The prism bonding adhesive layer thickness refers to the depth to which the prism peak of the brightness enhancement film penetrates into the bonding adhesive layer. The adhesive layer thickness determines the control of the penetration depth of the prism peak.
With a fixed adhesive layer thickness, increasing the penetration depth will lead to a greater decrease in brightness, resulting in lower brightness of the composite optical film because it interferes with the forward propagation of light. Similarly, when the adhesive layer and penetration depth are fixed, reducing the prism spacing will also amplify the attenuation effect.
Optical Film Prism Structure Design
The prism structure of a brightness enhancement film is not uniform and integral. The height and oscillation shape of the prisms vary on the same film, primarily to avoid the risk of adsorption.
In composite optical films, the prism bonding structure we are discussing here mainly refers to the "prism height." Common prism structures include: equal-height structures, 1-high-1-low structures, 1-high-2-low structures, and 1-high-multiple-low structures.
Tests show that the brightness ranking is typically 1 high 2 low > 1 high 1 low > equal height. This is because more low-prisms introduce more air layers, promoting light refraction.
However, too many low-prisms weaken the peel strength, thus requiring a trade-off between brightness and mechanical strength. Therefore, composite brightness enhancement films need to find a balance between brightness and peel strength.
Prism Peak Angle Optimization
The prism peak angle refers to the angle of the prism's apex, which is the angle of the prism at the contact point with the material above it. In the same brightness enhancement film, the prism peak angle is always the same, and the typical range of prism peak angle is 70°~110°.
For composite films, it is recommended to set the peak angle to 90° to maximize the center brightness, but in wide-viewing-angle products, it can be slightly adjusted to extend the angle.
Prism Adhesive Refractive Index
The refractive index of the prism adhesive is directly related to the UV adhesive used in the prism. The higher the refractive index of the UV adhesive, the higher the brightness of the corresponding composite optical film. Prism adhesive refractive indices are generally around 1.4 to 1.7.
While some manufacturers use adhesives with even higher refractive indices, a higher refractive index is not always better. Exceeding a certain threshold will sacrifice viewing angle and mechanical properties, and also increase manufacturing costs.
Increasing the refractive index can improve brightness when viewed directly, but angular attenuation needs to be monitored. In mobile phone applications, a medium refractive index is often more practical.
Particle density at the light-emitting surface of the diffuser film
For DOP (Diffusing Optical Film), the particle density at the light-emitting surface of the diffuser film affects the haze of the film. When other conditions remain constant, increasing the particle density at the light-emitting surface of the diffuser film can improve the overall haze of the diffuser film.
As the haze of the diffusion film increases, the brightness of the composite DOP optical film will first decrease and then increase. After testing, the brightness of the composite optical film is the highest when the haze of the diffusion film is 90°.
| Sample 1 | Sample 2 | Sample 3 | Sample 4 | Sample 5 | Sample 6 | |
| Diffusion Haze | 25% | 35% | 60% | 75% | 83% | 90% |
| Brightness | 913.5nit | 896.8nit | 893.9nit | 902.4nit | 908.6nit | 914.1nit |
Uniformity of light-emitting surface particle size in the diffuser film
The uniformity of light-emitting surface particle size in the diffuser film only affects the DOP optical film. When the uniformity of light-emitting surface particle size in the diffuser film is better, the brightness loss caused by diffuse reflection of light by the diffused particles will be smaller, the proportion of light emitted from the front will increase, the utilization rate of light will be improved, and thus the brightness of the composite optical film will increase.
Conclusion
In practical applications of TFT-LCDs, the factors affecting the brightness of DOP and POP optical composite films are not limited to these. Systematic debugging and verification are needed, addressing the six factors mentioned above, considering product positioning, brightness, viewing angle, thickness, and reliability requirements. If you are interested in commercial displays, please contact us to discuss customized solutions.
