Transparent Lightguide Facets With Lower Reflectance for AR Viewing
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Solution Overview
Problem
Conventional lightguides in head-mounted displays (HMDs) suffer from undesirable visibility effects for external observers due to light reflection from the viewer's face and eye, causing darkening and glare, while also compromising the transparency needed for viewing the natural scene.
Innovation Solution
The design of lightguide optical elements (LOEs) with reduced facet reflectance and increased transmittance, optimized for both S- and P-polarized light, and incorporating a light-absorbing layer at the end surface to minimize visibility and glare.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If partially reflecting surfaces are used to couple out light in a lightguide, then the projected image can be delivered to the viewer's eye, but the facets become visible to external observers causing darkening and glare effects
Solution Approach 1:
The patent applies different optical properties to different regions of the lightguide. The partially reflecting surfaces (facets) are designed with specific reflectance values (5-20%) optimized for coupling out projected image light to the viewer's eye, while the end surfaces are treated with light-absorbing coatings to prevent harmful reflections back toward external observers. This local differentiation of optical properties allows the system to maintain reliable image delivery while minimizing unwanted visibility effects.
Solution Approach 2:
The patent converts the potentially harmful effect of light reflections into a beneficial outcome. By carefully controlling the reflectance of facets and applying light-absorbing coatings at strategic locations, the system manages reflection paths so that reflections beneficial for image delivery are maintained while harmful reflections toward external observers are absorbed. This transforms what would be unwanted glare into controlled optical pathways that serve the display function.
2Object-generated harmful factors
If light-absorbing coating is applied to end surfaces, then glare and visibility to external observers is reduced, but some light that could reach the viewer is lost
Solution Approach 1:
The light-absorbing coatings are applied to end surfaces in advance to prevent harmful reflections before they can propagate back through the lightguide toward external observers. This preliminary action at the boundaries ensures that any light reaching the end surfaces is absorbed rather than reflected, eliminating glare sources before they can affect external visibility. The coating application is a pre-configured solution that prevents problematic optical pathways from forming.
3Object-generated harmful factors
If facet reflectance is reduced to improve transparency, then visibility of facets is decreased, but the optical throughput to the viewer is reduced
Solution Approach 1:
The patent optimizes the reflectance parameter of the partially reflecting surfaces to a specific range (5-20%). This parameter change balances two competing requirements: sufficiently low reflectance to minimize facet visibility and glare to external observers, while maintaining sufficiently high reflectance to ensure adequate optical throughput to the viewer's eye. The end surface coatings further refine this balance by absorbing stray light that would otherwise create glare, allowing the facet reflectance to be optimized for transparency without excessive energy loss.
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
Significantly reduces the visibility of the LOE facets to external observers and enhances the transparency for viewing the natural scene, while maintaining sufficient optical throughput for the viewer.
Implementation Method 1
light conveying a projected image introduced into the LOE propagates within the LOE by internal reflection at the first and second major surfaces
Implementation Method 2
a plurality of mutually-parallel partially reflecting surfaces internal to the block and obliquely oriented relative to the first major surface, the partially reflecting surfaces being configured so as to couple-out a part of the light through the second major surface
Data Source
Figure 1A~1B
Figure 2
Figure 3A~3B
AI summary
A light-guide optical element (LOE) for simultaneous viewing, of a real scene and of a projected image introduced into the LOE, having a transparent block along which light conveying a projected image propagates by internal reflection, and a plurality of internal partially reflecting surfaces obliquely oriented and configured so as to couple-out a part of said light, wherein the reflectance of each of the partially reflecting surfaces is such that the total power of the light that is coupled out is less than one third of the total power of the light that is introduced into the LOE. In some embodiments, the light of the projected image is polarized and the reflectance of the partially reflecting surfaces for light polarized in an orthogonal orientation is substantially reduced. In some embodiments, the reflectance of the partially reflecting surfaces for light not reaching the viewer is substantially reduced.