UV-Stable Optical Films for Windshield Applications
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Solution Overview
Problem
Traditional multilayer optical films containing polymers like PEN 'yellow' in high UV environments, such as sunlight, making them unsuitable for applications like windshields and heads-up displays, where UV protection is needed.
Innovation Solution
An optical stack comprising a mirror with a plurality of first polymeric layers on a reflective polarizer with second polymeric layers, each with an average thickness of less than 500 nm, designed to block UV wavelengths while allowing visible light to pass, using a reflection band edge that separates UV from visible wavelengths, ensuring high reflectance for UV and low reflectance for visible light.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional multilayer optical films containing PEN polymer are used, then optical performance is achieved, but the films yellow in high UV environments
Solution Approach 1:
The optical film is segmented into multiple thin polymeric layers (at least 50 layers, each less than 500 nm thick) with alternating refractive indices. This segmentation creates a distributed Bragg reflector structure that provides UV protection through controlled reflection rather than absorption, preventing the yellowing that occurs in traditional single-layer or fewer-layer films.
Solution Approach 2:
The invention uses composite polymeric materials with different refractive indices arranged in alternating layers. The first polymeric layers and second polymeric layers have different optical properties, creating a composite structure that reflects UV wavelengths while maintaining visibility in the visible range, thereby preventing yellowing while preserving optical performance.
2Object-affected harmful factors
If a reflection band edge is designed to block UV wavelengths, then UV protection is achieved, but visible light transmission may be affected
Solution Approach 1:
The invention carefully controls the thickness parameters of each polymeric layer (less than 500 nm) and the refractive index differences between alternating layers to position the reflection band edge specifically in the UV wavelength range. By adjusting these parameters, the structure reflects UV wavelengths (providing protection) while allowing visible light to pass through with minimal interference.
Solution Approach 2:
The optical film structure provides different optical properties at different wavelength ranges. In the UV range, the film exhibits high reflectance due to the Bragg reflector effect, while in the visible range, it maintains high transmittance. This local differentiation of optical quality allows UV blocking without compromising visible light transmission.
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 optical stack effectively protects high contrast ratio reflective polarizers from blue light sources by blocking damaging UV wavelengths, maintaining optical performance and preventing yellowing in UV environments.
Implementation Method 1
an optical reflectance of the plurality of first polymeric layers versus wavelength has a reflection band edge separating a shorter wavelength range, where the plurality of first polymeric layers reflects greater than about 70% of the incident light, from a longer wavelength range, where the plurality of first polymeric layers reflects less than about 30% of the incident light
Implementation Method 2
In the shorter wavelength range, the pluralities of first and second polymeric layers absorb respective A1% and A2% of the incident light, such that A2/A1 is greater than or equal to about 50
Data Source
AI summary
An optical stack includes a first optical film with a plurality of first polymeric layers disposed on a second optical film with a plurality of second polymeric layers, such that for an incident light and for a first polarization state: a reflectance of the plurality of first polymeric layers versus wavelength has a reflection band edge separating a shorter wavelength range with higher reflectance a longer wavelength range with lower reflectance; for at least a first wavelength in the shorter wavelength range, the plurality of second polymeric layers reflects less than about 70% of the incident light, and for at least a second wavelength in the longer wavelength range, the plurality of second polymeric layers reflects greater than about 80% of the incident light; and in the shorter wavelength range, the pluralities of first and second polymeric layers absorbs respective A1% and A2% of the incident light, A2/A1≥50.