Compact HMD Optical Substrates Using Total Internal Reflection
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Solution Overview
Problem
Conventional head-mounted displays (HMDs) and compact displays face challenges with large field-of-view (FOV) requirements, leading to bulkier and less practical designs, and suffer from eye-motion-box limitations that affect image quality and pupil motion, especially in mobile applications like cellular phones, where display quality is restricted by physical size.
Innovation Solution
An optical device comprising at least two light-transmitting substrates optically cemented with an adhesive having a lower refractive index than the substrates, allowing light waves to impinge on an interface plane at specific angles for wide FOVs and large eye-motion-boxes, with transparent and reflective properties optimized for efficient light propagation and image projection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If conventional free-space optical modules are used to increase field-of-view, then the FOV is improved, but the device becomes larger, heavier and bulkier
Solution Approach 1:
The patent merges the imaging lens and combiner functions into a single optical module. The light-transmitting substrate serves dual purposes: focusing light from the display source (imaging lens function) and reflecting light into the viewer's eye (combiner function). This integration eliminates the need for separate conventional optical components, achieving a compact design that provides wide field-of-view without increasing device size.
Solution Approach 2:
The patent utilizes total internal reflection at the interface between the light-transmitting substrate and the adhesive layer. By controlling the incident angles of light waves and exploiting the refractive index difference, the system achieves wide field-of-view in a planar, two-dimensional configuration rather than requiring bulky three-dimensional optical paths.
2Reliability
If conventional optical modules are used to achieve moderate performance, then the field-of-view is improved, but the device becomes impractical due to size and weight
Solution Approach 1:
The patent combines multiple optical functions into a single lightweight substrate, eliminating the need for heavy conventional optical modules. The light-transmitting substrate with integrated reflecting surface provides both imaging and combining functions, achieving moderate to high performance while maintaining a lightweight design suitable for head-mounted displays.
Solution Approach 2:
The patent changes the optical parameters by utilizing total internal reflection at the substrate-adhesive interface. By controlling the refractive index difference and incident angles, the system achieves efficient light guidance and reflection without requiring heavy conventional optical components, thus maintaining high performance with reduced weight.
3Volume of moving object
If compact optical solutions are used to reduce device size, then the device becomes more compact, but the eye-motion-box becomes very small (less than 8 mm)
Solution Approach 1:
The patent achieves a large eye-motion-box by utilizing angular control of light propagation. The light-transmitting substrate guides light at specific incident angles that correspond to different eye positions. This angular dimensionality allows the system to accommodate large eye movements (eye-motion-box greater than 8 mm) while maintaining a compact physical form factor.
Solution Approach 2:
The patent creates a dynamic optical system where the effective viewing area adapts to eye movements. The light guidance structure maintains optimal optical paths across a range of incident angles, allowing the eye to move freely within a large eye-motion-box while the physical device remains compact. The system dynamically adjusts light propagation angles based on eye position.
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 compact, high-quality image with wide FOV and large eye-motion-box, suitable for HMDs and mobile devices, enhancing image quality and accommodating eye movements, while maintaining a small form factor.
Implementation Method 1
an external surface of the first light-transmitting substrate is optically cemented to an external surface of the second light-transmitting substrate by means of an optical adhesive defining an interface plane, the substrates and the optical adhesive having a refractive index, the refractive index of the optical adhesive is substantially lower than the refractive index of the first substrate defining a critical angle of the interface plane, part of the light waves entering the device through the input aperture and exiting the device through the output aperture, impinge on the interface plane of the first substrate having incidence angles smaller than the critical angle, another part of the light waves impinging on the interface plane, having incidence angles higher than the critical angle
Data Source
AI summary
An optical device, having at least first and second light-transmitting substrates, each having at least two external surfaces and an input aperture and an output aperture. The external surface of the first light-transmitting substrate is optically cemented to an external surface of the second light-transmitting substrate by an optical adhesive defining an interface plane. The refractive index of the optical adhesive is substantially lower than the refractive index of the first substrate. Part of the light waves entering the device through the input aperture and exiting the device through the output aperture impinge on the interface plane of the first substrate having incidence angles smaller than the critical angle. Another part of the light waves impinging on the interface plane have incidence angles higher than the critical angle. The interface plane is substantially transparent for the light waves impinging on interface plane having incidence angles smaller than the critical angle.