Optical Film Stack Retardance Layer Brightness Gain
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Solution Overview
Problem
Optical film stacks in display devices, such as LCDs, face challenges in achieving optimal brightness gain and contrast ratio due to inefficiencies in light polarization and reflection between reflective polarizers, leading to decreased system efficiency and noticeable oscillations in brightness.
Innovation Solution
Incorporating a retardance layer with specific in-plane and out-of-plane index of refraction ratios and thickness between two reflective polarizers, and optionally a brightness enhancement film with prismatic structures, to optimize light phase shifting and polarization conversion, thereby enhancing brightness gain and contrast ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If a retardance layer is added between reflective polarizers to improve brightness gain and contrast ratio, then optical performance is improved, but device complexity increases
Solution Approach 1:
A retardance layer is introduced as an intermediary component between the first and second reflective polarizers. This layer mediates the optical interaction by introducing a phase shift to reflected light, enabling polarization conversion that enhances brightness gain and contrast ratio without requiring fundamental changes to the polarizer structure.
Solution Approach 2:
The optical film stack employs a composite structure combining reflective polarizers with a retardance layer having specific birefringent properties. The composite arrangement leverages the complementary optical functions of each layer - the polarizers for polarization selection and the retardance layer for phase modulation - to achieve superior optical performance.
2Illumination intensity
If the retardance layer has high in-plane birefringence to enhance brightness, then brightness gain improves, but oscillations in brightness increase
Solution Approach 1:
The patent optimizes the retardance layer parameters by controlling the in-plane birefringence to fall within a specific range (0.05 to 0.20) and setting the layer thickness between 1-25 micrometers. These parameter adjustments ensure sufficient phase shift for brightness enhancement while preventing excessive oscillations that would compromise brightness uniformity.
Solution Approach 2:
The retardance layer is positioned specifically between the reflective polarizers where the optical path requires phase modulation. This localized placement ensures that the birefringent effect is applied precisely where needed to enhance brightness gain, while other regions of the optical stack maintain their original optical characteristics to preserve overall brightness uniformity.
3Illumination intensity
If the retardance layer thickness is increased to enhance polarization conversion, then contrast ratio improves, but detectable oscillations in brightness increase
Solution Approach 1:
The patent establishes an optimized thickness range for the retardance layer (1-25 micrometers, with preferred sub-range of 3-15 micrometers) that balances two competing requirements: sufficient thickness to provide the necessary optical path difference for effective polarization conversion and contrast ratio enhancement, while limiting thickness to prevent excessive brightness oscillations that would reduce uniformity and become visually detectable.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The described optical film stack achieves high brightness and contrast ratios with minimal detectable oscillations, improving overall display efficiency and image quality by aligning the retardance layer's refractive indices and thickness within specific ranges and using a structured brightness enhancement film.
Implementation Method 1
The retardance layer has an optimal in-plane to out-of-plane ratio which is defined as the difference in the in-plane indices of refraction (x vs. y) divided by the difference between in-plane index of refraction and out-of-plane index of refraction (x and y vs. z)
Implementation Method 2
Some display devices also include one or more optical film stacks placed between the backlight and the LC panel. Optical film stacks may be designed to improve output luminance, illumination uniformity, viewing angle, pixel integrity, contrast ratio and overall system efficiency
Data Source
AI summary
Optical film stacks are disclosed. The optical film stacks can include a first reflective polarizer, a second reflective polarizer, and a retardance layer disposed between the first reflective polarizer and the second reflective polarizer.