HMD Optical Path Bending and Hologram Diffraction for Compact Design
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Solution Overview
Problem
Conventional image display apparatuses for head-mounted displays (HMDs) face challenges in downsizing due to the use of polarized beam splitters (PBS) and inefficient optical path layouts, which hinder the reduction of overall size and weight, and also struggle to widen the color reproduction area for improved image quality.
Innovation Solution
The introduction of an optical-path bending member, such as a concave mirror or prism, that bends the optical path from the light source to the reflective display device, allowing the light source to be positioned closer to the eyepiece optical system without increasing optical power, combined with a hologram optical element that diffracts light to enhance color reproduction by adjusting the diffraction wavelength range relative to the light source's wavelength range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a polarized beam splitter (PBS) is used to separate illumination and view optical paths, then the optical path separation is achieved, but the overall size and weight of the HMD increase
Solution Approach 1:
The patent extracts and eliminates the polarized beam splitter (PBS) from the optical system. Instead of using a PBS to separate illumination and view optical paths, the invention uses a reflective display device that reflects view light back through the illumination optical system, removing the need for the PBS and thereby reducing weight and size.
Solution Approach 2:
The patent inverts the conventional optical path arrangement. Rather than having separate illumination and view paths that require a PBS for separation, the view optical path is arranged to pass through the illumination optical system in reverse, using the reflective display device to redirect light back through the same path, thereby eliminating the need for path separation components.
2Ease of manufacture
If the optical path layout is conventional and straight, then the optical components are easy to align, but the overall size of the HMD increases
Solution Approach 1:
The patent folds the optical path into a compact three-dimensional arrangement using reflective surfaces. The illumination optical system and view optical path are arranged in a folded configuration where light paths bend and intersect in three-dimensional space, allowing compact packaging while maintaining proper optical alignment through careful design of the folded paths.
3Productivity
If the aperture ratio is reduced to increase pixel integration density, then more pixels can be integrated, but the illumination efficiency and image quality decline
Solution Approach 1:
The patent replaces the conventional transmissive LCD mechanism with a reflective display device that uses micro-mirrors or reflective pixels. This substitution allows the display pixels to efficiently reflect illumination light back through the optical system, maintaining high illumination efficiency even with high pixel density, because each pixel acts as a reflective element rather than requiring light transmission through multiple substrates.
4Duration of action of moving object
If the HMD is worn for a considerable length of time, then the viewer can use it extensively, but the apparatus should be as small and light as possible to ensure comfort
Solution Approach 1:
The patent merges the illumination optical system and the view optical path into a single integrated optical train. By combining these functions and using the reflective display device to reflect view light back through the illumination path, the overall system size and weight are reduced, making the HMD more suitable for extended wear while maintaining functionality.
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 enables a more compact and lightweight HMD design while significantly improving the color reproduction area and image quality by optimizing the optical path layout and utilizing the hologram optical element to enhance color purity and brightness.
Implementation Method 1
an optical-path bending member for bending the optical path leading from the light source to the reflective type display device
Implementation Method 2
a hologram optical element that diffracts light to enhance color reproduction by adjusting the diffraction wavelength range relative to the light source's wavelength range
Data Source
AI summary
The light directed from a light source to a concave mirror and the light directed from a display device to an eyepiece optical system are intersected by each other in a layout. This allows the light source, the concave mirror, and the display device to be arranged in compactness adjacent to the eyepiece optical system without increasing the optical power of an illumination optical system. As the result, the apparatus can easily be minimized in the thickness or the overall size. A hologram optical element is provided where the relationship between the wavelength range Δλ1 at half of the diffraction efficiency of each of the three primary colors of the light in the hologram optical element and the wavelength range Δλ2 at half of the intensity of each of the three primary colors of the light emitted from the light source is defined by Δλ1<Δλ2. Accordingly, a component at desired wavelengths of each of the R, G, and B colors of the light emitted from the light source can be diffracted by the action of the hologram optical element and then directed to the pupil of the viewer. This allows the image to be increased in the color reproduction area and improved in the quality regardless of the display device actuated in a time-division mode.