Waveguide AR Coating Reduces Light Leakage
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Solution Overview
Problem
Near eye and heads up display systems using optical waveguides face issues with light leakage, leading to overall loss in intensity, dark spots, and ghost images due to undesirable light transmission and reflection, which affect the clarity and completeness of the virtual image displayed.
Innovation Solution
A coating with specific reflectance properties is applied to the waveguides, having low reflectance (<2%) for light at angles below 25 degrees and high reflectance (≥50%) for light at angles above 30 degrees, reducing light leakage and enhancing image clarity by minimizing ghost images and intensity loss.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a standard waveguide is used without specialized coatings, then the structure is simple and manufacturing is easier, but light leakage occurs causing loss of image intensity, dark spots, and ghost images
Solution Approach 1:
The patent applies anti-reflective coatings with specific refractive index parameters and thicknesses (e.g., quarter-wave coatings) to the waveguide surfaces. By changing the optical parameters of the coating layers, the reflectance is reduced to minimize light leakage and maintain image intensity while preventing ghost images and dark spots.
Solution Approach 2:
The patent uses multi-layer composite coatings on the waveguide surfaces, combining materials with different refractive indices (e.g., high-index TiO2 and low-index SiO2 layers). This composite structure optimally manages light reflection and transmission, reducing light leakage and improving image quality without requiring fundamentally new waveguide materials.
2Manufacturing precision
If the waveguide surface is left uncoated or with simple coating, then manufacturing is easier and cost is lower, but ghost images and dark spots appear reducing image clarity
Solution Approach 1:
The patent specifies precise coating thickness parameters (e.g., quarter-wave thickness of approximately 100nm for visible light) and refractive index requirements for the anti-reflective coating layers. These controlled parameter changes reduce ghost images and dark spots by minimizing unwanted reflections, achieving high image clarity through predictable optical behavior.
Solution Approach 2:
The patent applies different coating configurations to different surfaces of the waveguide - for example, applying anti-reflective coatings specifically to the exit surface where light emerges, while the input surface may have different or no coating. This localized approach optimizes image clarity at critical locations without unnecessarily complicating the entire manufacturing process.
3Illumination intensity
If anti-reflective coating is applied to reduce light leakage, then image intensity is maintained, but the coating adds manufacturing steps and potential defects
Solution Approach 1:
The patent specifies optimal coating thickness parameters (quarter-wave thickness) and refractive index ranges that maximize light transmission and minimize reflection. By carefully controlling these parameters, the coating maintains virtual image intensity while reducing the sensitivity to minor manufacturing variations, thereby improving reliability.
Solution Approach 2:
The anti-reflective coating structure is designed to be self-compensating within certain tolerances. The quarter-wave thickness design provides a wavelength-centered optimization that maintains performance even with minor thickness variations, reducing the need for extremely tight manufacturing controls and minimizing defects.
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 coating significantly reduces light leakage and ghost images, maintaining image intensity and clarity by optimizing light transmission through the waveguides, allowing users to view augmented reality images effectively.
Implementation Method 1
the at least one such coating has a high reflectance (e.g., of at least 50 percent) for light within the specific wavelength range for the waveguide that is incident on the major surface on which the coating is located at an angle above a high threshold angle (e.g., that is at least 30 degrees) relative to the normal of the major surface having the coating
Implementation Method 2
Such a coating can beneficially permit external light corresponding to a real world image to pass through the waveguide
Implementation Method 3
reduce the loss of some of the full image content (associated with a virtual image) that may otherwise occur if light undesirably leaks out of a waveguide while traveling from an input-coupler to an output-coupler of the waveguide by way of TIR
Data Source
Figure 1A~1C
Figure 2
Figure 3
AI summary
A near eye or heads up display system includes a display engine, at least two optical waveguides, and a respective coating on at least one of the major surfaces of at least one of the waveguides. At least one such coating has a low reflectance for light within a specific wavelength range for the waveguide and incident on a major surface of the waveguide on which the coating is located at an angle below a low threshold angle relative to a normal, and has a high reflectance for light within the specific wavelength range for the waveguide that is incident on the major surface on which the coating is located at an angle above a high threshold angle relative to the normal.