HUD Windscreen Coating for Ghost-Free Color-Neutral Projection
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Solution Overview
Problem
Existing head-up display (HUD) projection systems face issues with ghost images due to windshield reflections, which are mitigated using wedge-shaped interlayers, but these are costly. Additionally, existing reflective coatings for p-polarized radiation exhibit wavelength-dependent reflectance, leading to color imbalances in HUD projections.
Innovation Solution
A reflective coating for HUDs using a single silver layer with asymmetrical dielectric layers above and below, optimized for p-polarized radiation, ensuring high reflectivity and color neutrality by smoothing the reflection spectrum.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a reflective coating is used for p-polarized radiation, then ghost images are eliminated, but the reflection spectrum shows wavelength dependence causing color imbalance
Solution Approach 1:
The patent applies asymmetry by using dielectric layers with different optical thicknesses above and below the silver layer. The first dielectric layer has a different optical thickness than the second dielectric layer, creating an asymmetric structure that smooths the reflection spectrum across different wavelengths while maintaining high reflectivity for p-polarized radiation. This asymmetric design eliminates the wavelength-dependent color imbalance that occurs with symmetric coatings.
Solution Approach 2:
The patent changes the optical parameters of the dielectric layers by specifically controlling their refractive indices and optical thicknesses. The first dielectric layer has a refractive index of at least 1.9, and the second dielectric layer has a refractive index of at least 1.6, with their optical thicknesses carefully adjusted to achieve a flattened reflection spectrum. This parameter optimization ensures color-neutral HUD projections while maintaining high reflectivity.
2Ease of manufacture
If a single silver layer is used, then production costs are reduced compared to multiple metallic layers, but reflectivity may be insufficient
Solution Approach 1:
The patent creates a composite reflective coating structure by combining a single silver layer with multiple dielectric layers having different refractive indices and optical thicknesses. This composite structure leverages the high reflectivity of silver while using the dielectric layers to enhance and flatten the spectral response. The combination achieves superior optical performance compared to simple metallic layers, maintaining high reflectivity across the visible spectrum without requiring multiple metallic layers.
Solution Approach 2:
The dielectric layers serve as intermediaries between the silver layer and the surrounding media (air and glass). These intermediary layers with specific refractive indices (≥1.9 and ≥1.6) optimize the optical coupling and enhance the reflectivity of the silver layer. They mediate the optical interaction to achieve high and wavelength-independent reflectivity, compensating for the limitations of using a single thin silver layer.
3Object-affected harmful factors
If the angle of incidence is set to 65° (close to Brewster angle), then p-polarized radiation is minimally reflected by the windshield surface, but a reflective coating is required to achieve sufficient reflection for HUD
Solution Approach 1:
The patent changes the optical parameters of the reflective coating by optimizing the refractive indices and optical thicknesses of the dielectric layers. The first dielectric layer has a refractive index of at least 1.9 and the second has at least 1.6, with their optical thicknesses carefully controlled. These parameter changes enable the coating to achieve high reflectivity for p-polarized radiation at 65° incidence angle, compensating for the minimal surface reflection and eliminating the need for complex wedge-shaped interlayers.
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 solution eliminates ghost images and allows color-neutral HUD projections without wedge films, maintaining high transmission and visibility, compatible with polarization-selective sunglasses, and reducing production costs.
Implementation Method 1
Since the typical angle of incidence for HUD projection arrangements, approximately 65°, is relatively close to the Brewster angle for an air-to-glass interface (56.5°, soda-lime glass), p-polarized radiation is hardly reflected by the lens surfaces, but primarily by the conductive coating.
Implementation Method 2
The ratio of the optical thicknesses of the upper and lower dielectric layer sequences according to the invention smooths the reflection spectrum, thus ensuring a color-neutral display.
Data Source
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AI summary
The present invention relates to a projection assembly for a head-up-display (HUD), comprising at least - a windscreen (10), comprising an outer pane (1) and an inner pane (2), which are connected to one another by way of a thermoplastic interlayer (3), having an HUD area (B); and - a projector (4), which is aimed at the HUD area (B); wherein - the beam from the projector (4) is predominantly p-polarized and - the windscreen (10) is provided with a reflection coating (20), which is suitable for reflecting p-polarized radiation; and wherein - the reflection coating (20) has precisely one electrically conducting layer (21) based on silver, - a lower dielectric layer (22a) or layer sequence (22a, 22b, 22c), with a refractive index of at least 1.9, is arranged beneath the electrically conducting layer (21), - an upper dielectric layer (23a) or layer sequence (23a, 23b, 23c) with a refractive index of at least 1.9 is arranged above the electrically conducting layer (21) - the ratio of the optical thickness of the upper dielectric layer (23a) or layer sequence (23a, 23b, 23c) to the optical thickness of the lower dielectric layer (22a) or layer sequence (22a, 22b, 22c) is at least 1.7.