Light Guide Element Thermal Stability via Diffraction Layer Positioning
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Solution Overview
Problem
Existing light guide elements in head-mounted display devices suffer from image deterioration due to temperature-induced changes in diffraction angles, which are not effectively addressed by current techniques, leading to shifts in the optical axis and incidence angle, particularly in devices using diffractive optical elements and light-guiding substrates.
Innovation Solution
A light guide element with a diffraction part formed on a substrate, where the distance from the substrate to the optical layer is less than 10% of the substrate thickness, and specific refractive index and expansion coefficient conditions are met to minimize temperature-induced changes in diffraction angles, ensuring |ΔD|=|DΔT/cos2θ×(α+β/n)|≦0.15 mm, thereby stabilizing the optical path.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a diffractive optical element is used to guide light in a head-mounted display, then the device can achieve compact design and optical enlargement, but the diffraction angle changes with temperature causing image deterioration
Solution Approach 1:
The patent applies parameter changes by selecting specific material parameters (refractive index temperature coefficient and linear expansion coefficient) that satisfy a mathematical relationship. The light-guiding substrate is chosen to have parameters within specific ranges: refractive index temperature coefficient β between -1.0×10^-6/℃ to +1.0×10^-6/℃, and linear expansion coefficient α between 0.5×10^-6/℃ to 7.0×10^-6/℃. This ensures the diffraction angle remains stable across temperature changes while maintaining the optical enlargement function.
Solution Approach 2:
The patent uses composite material selection by combining specific glass materials with diffractive optical elements. The light-guiding substrate is made of glass materials (such as optical glasses from OHARA Inc.) with specifically controlled thermal and optical properties. This composite approach allows the system to maintain both the diffractive optical element's enlargement capability and the substrate's thermal stability.
2Stability of the object's composition
If the light-guiding substrate material is changed to reduce thermal expansion, then temperature-induced diffraction angle changes are reduced, but the refractive index and other optical properties must be carefully controlled
Solution Approach 1:
The patent systematically controls multiple material parameters simultaneously. The light-guiding substrate must satisfy: linear expansion coefficient α between 0.5×10^-6/℃ to 7.0×10^-6/℃, refractive index temperature coefficient β between -1.0×10^-6/℃ to +1.0×10^-6/℃, and refractive index n between 1.4 to 2.2. This multi-parameter specification ensures both thermal stability and optical performance without requiring extreme precision in any single parameter.
3Device complexity
If conventional diffractive optical elements are used without considering thermal expansion, then device complexity is reduced, but diffraction angle fluctuations occur with temperature changes
Solution Approach 1:
The patent implements self-service by making the light-guiding substrate itself provide thermal compensation. The substrate's intrinsic thermal and optical properties (carefully selected α and β values) automatically compensate for temperature-induced diffraction angle changes. No additional active control mechanisms or complex compensation structures are needed - the substrate material properties themselves perform the compensation function, maintaining both simplicity and reliability.
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
This configuration effectively inhibits quality deterioration due to temperature changes, maintaining image stability by controlling thermal expansion and refractive index changes within permissible limits, ensuring minimal shifts in the optical axis.
Implementation Method 1
light that is deflected by the diffractive action of a diffraction part formed on a substrate and is then emitted into the substrate
Implementation Method 2
in the case of a device including a light guide element which propagates light while utilizing total reflection within the substrate
Data Source
AI summary
A light guide element includes a light-guiding substrate and a diffraction part formed on a surface of the light-guiding substrate. The diffraction part includes an optical layer capable of exerting a diffractive action. The distance from the light-guiding substrate to a surface of the optical layer is equal to or less than 10% of a thickness of the light-guiding substrate. The light guide element satisfies |ΔD|≦0.15 mm, where ΔD is an amount of a change in D with the temperature change ΔT, D [mm] is a distance, at the temperature T (° C.), between the center of the diffraction part and a position predetermined as a position where the ray of light comes out of the light-guiding substrate in a direction horizontal with respect to the light-guiding substrate, and ΔT is a temperature change.