Virtual Image Display Prism with Folded Light Path
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Solution Overview
Problem
Existing virtual image display devices for head-mounted displays face challenges in providing a wide angle of view, minimizing size and weight, and allowing simultaneous observation of external light and image light without interference, while maintaining high image quality and reducing ghost light occurrences.
Innovation Solution
A virtual image display device with a prism having three or more non-axisymmetric curved surfaces, where external light passes through specific surfaces to maintain zero visibility and image light is totally reflected multiple times to form an intermediate image, reducing the size and weight of the optical system and preventing ghost light generation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a relay optical system with a large projection lens is used to form an intermediate image, then the image quality is improved, but the size and weight of the device increase
Solution Approach 1:
The optical system is divided into multiple prism portions (first through fourth portions) with different curved surfaces, each segment contributing to image formation and relay. This segmentation allows the system to achieve high image quality through distributed optical functions rather than relying on a single large projection lens, thereby reducing overall device weight and size.
Solution Approach 2:
The patent bends the light path multiple times through different prism portions, transitioning the optical path from a linear arrangement to a three-dimensional folded configuration. This dimensional change allows the intermediate image to be formed and relayed through a compact volume, reducing the device's external dimensions while maintaining image quality.
2Adaptability or versatility
If the view is entirely covered by image light to improve immersion, then the angle of view is widened, but the observer cannot see the outside state causing anxiety
Solution Approach 1:
The optical system provides different optical properties in different regions: the first and second prism portions provide high-quality relay for the intermediate image to achieve wide field of view and immersion, while the third and fourth prism portions have lower relay quality that allows outside light to pass through. This local differentiation enables simultaneous image viewing and outside observation, eliminating isolation anxiety while maintaining immersion capability.
3Object-affected harmful factors
If a compensation prism is added to allow outside viewing, then the observer can see the outside, but the image light is reflected twice on the half mirror surface making the image very dark
Solution Approach 1:
The viewing function is segmented between different prism portions: the first and second portions handle image light relay with high quality, while the third and fourth portions handle outside light transmission with lower relay quality. This segmentation allows outside viewing without requiring additional half mirrors that would cause double reflection and image darkening.
Solution Approach 2:
The third and fourth prism portions act as intermediaries that selectively transmit outside light while having lower relay quality for image light. This intermediary function allows outside viewing to occur without the need for additional reflective elements that would degrade image brightness.
4Volume of moving object
If the light path is folded back to shorten the overall length, then the device becomes compact, but the convex portion of the prism center and projection lens interfere with the view
Solution Approach 1:
The optical path is folded through three-dimensional spatial arrangement of multiple prism portions, bending light in multiple directions rather than simple back-folding. This dimensional light path configuration achieves compactness while positioning optical elements to avoid interfering with the observer's view through the device.
5Measurement precision
If multiple curved surfaces are used to increase degree of freedom in optical design, then image forming performance is improved, but external light is reflected in unintended directions generating ghost light
Solution Approach 1:
Different prism portions have different surface quality characteristics: the first and second portions have high-precision curved surfaces for optimal image relay, while the third and fourth portions have lower precision surfaces that intentionally allow outside light transmission. This local quality differentiation suppresses ghost light generation from unintended reflections while maintaining high image forming performance where needed.
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 enables a high-performance display with a wide angle of view, allowing simultaneous observation of external and image light with reduced distortion and ghost light occurrence, while maintaining a thin and lightweight design.
Implementation Method 1
the image light from the image element is totally reflected at the third surface, totally reflected at the first surface, totally reflected again at the third surface, totally reflected again at the first surface
Implementation Method 2
When the external light passes through a first surface and a third surface among a plurality of surfaces that configures the prism to recognize the outside
Data Source
AI summary
An intermediate image is formed inside the prism by a projection lens or the like, and since an image light that is totally reflected at three or more surfaces; a third surface firstly, a first surface firstly, at a third surface secondly, at a first surface secondly, and the second surface in this order, transmits the first surface and then arrives at an observer's eyes, the thickness of the prism can be reduced and the size and weight of the entire optical system can be reduced, and it is possible to realize a display of high performance and brightness with a wide angle of view. Since the external light can be observed after passing through the first surface and the third surface, and the visibility at that time becomes substantially zero, it is possible to reduce the defocus or distortion of the external light when observing the external light.