Compact HMD Light Guide with Slanted Edge Prism
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Solution Overview
Problem
Conventional compact optical modules for head-mounted displays (HMDs) become bulky and impractical as the desired field-of-view increases, suffering from limited eye-motion-box and manufacturability issues, leading to sensitive performance and inadequate pupil motion for comfortable viewing.
Innovation Solution
A compact light-guide optical element with a light-transmitting substrate featuring a prism with specific surface orientations and a slanted edge for total internal reflection, allowing wide field-of-view and large eye-motion-box while minimizing dimensions and chromatic aberrations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of moving object
If a conventional free-space optical module is used to increase field-of-view, then the field-of-view increases, but the system becomes larger, heavier and bulkier
Solution Approach 1:
The patent transitions from conventional free-space optical paths to a planar light-guide substrate architecture. By guiding light through the thickness and surface of a flat substrate rather than through free space, the system achieves wide field-of-view in a two-dimensional plane, eliminating the need for bulky three-dimensional optical components while maintaining or expanding the viewing angle.
Solution Approach 2:
The patent extracts and eliminates intermediate optical components such as relay lenses, beam splitters, and complex mirror arrangements from the conventional optical path. By directly coupling the display source to the light-guide substrate and using total internal reflection at the substrate surfaces, the system achieves compactness while preserving wide field-of-view performance.
2Volume of moving object
If compact optical solutions are implemented to reduce system size, then the system becomes more compact, but the eye-motion-box becomes very small (less than 8 mm)
Solution Approach 1:
The patent implements different optical properties at different locations within the light-guide substrate. The substrate features spatially varying refractive index distributions, localized coupling regions, and position-dependent reflection characteristics that enable the entire substrate area to function as a large eye-motion-box, rather than concentrating the optical function in a small region.
Solution Approach 2:
By utilizing the thickness dimension of the substrate and creating three-dimensional light propagation paths within the planar structure, the patent expands the effective eye-motion-box area. Light can enter and exit the substrate at multiple heights and positions, effectively creating a volumetric eye-motion region that projects a large two-dimensional viewing area.
3Volume of moving object
If compact optical solutions are implemented, then the system size is reduced, but manufacturability suffers and performance becomes very sensitive to small movements
Solution Approach 1:
The patent merges multiple optical functions (collimation, reflection, beam combining, and image projection) into a single integrated light-guide substrate structure. This monolithic design eliminates the need for precise alignment and assembly of multiple separate components, dramatically improving manufacturability while maintaining compact size and reducing sensitivity to positional variations.
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 provides a high-quality, large image with wide field-of-view and accommodates large eye movements, achieving a more compact and robust optical system suitable for HMDs and other display applications.
Implementation Method 1
an optical prism having at least a first, a second and a third surface, for coupling light waves having a given field-of-view into the substrate by total internal reflection
Implementation Method 2
at least one partially reflecting surface located in the substrate, the partially reflecting surface being orientated non-parallelly with respect to the major surfaces of said substrate, for coupling light waves out of the substrate
Data Source
AI summary
There is provided an optical system, including a light-transmitting substrate (20) having at least two major surfaces (26) and edges, all optical prism (54) having at least a first (58), a second (56) and a third (60) surface, for coupling light waves having a given field-of-view into the substrate by total internal reflection, at least one partially reflecting surface located in the substrate, the partially reflecting surface being orientated non-parallelly with respect to the major surfaces of the substrate, for coupling light waves out of the substrate, at least one of the edges (50) of the substrate is slanted at an oblique angle with respect to the major surfaces, the second surface of the prism is located adjacent to the slanted edge of the substrate, and a part of the substrate located next to the slanted edge is substantially transparent, wherein the light waves enter the prism through the first surface of the prism, traverse the prism without any reflection and enter the substrate through the slanted edge.