Off-axis Parabolic Combiner for Eye-tracking
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Solution Overview
Problem
Optical combiners in head-mounted displays (HMDs) face challenges in capturing consistent images of the eye due to variations in eye relief, making it difficult to maintain a constant image size as the depth of the eye changes, which affects eye-tracking accuracy.
Innovation Solution
A telecentric optical combiner with an off-axis two-dimensional parabolic lensing structure, including a parabolic Fresnel structure and dichroic reflective layer, selectively focuses collimated image light to redirect infrared light to a camera while allowing visible light to pass through, maintaining a constant image size regardless of eye depth, using a parabolic lensing structure that functions as an off-axis reflector for infrared light and a hologram for focusing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional optical combiners are used, then visible light transmission is achieved, but image size varies with eye depth changes affecting eye-tracking accuracy
Solution Approach 1:
The patent employs a parabolic lensing structure with a specific curved surface profile defined by z = (x² + y²)/(4f) to focus infrared light. This curvature enables telecentric imaging where the chief rays are parallel to the optical axis, ensuring that the image size of the eye remains constant regardless of eye relief variations, thereby resolving the contradiction between image size consistency and eye-tracking accuracy.
Solution Approach 2:
The patent changes the optical parameters by introducing a parabolic lensing structure with a specific focal length parameter (f) that transforms the imaging characteristics. This parameter change creates a telecentric optical path where the magnification becomes independent of object distance, allowing the system to maintain stable image size while accommodating variations in eye depth for improved eye-tracking precision.
2Stability of the object's composition
If a parabolic lensing structure is added to achieve telecentric imaging, then image size consistency is maintained, but device complexity increases
Solution Approach 1:
The patent merges the parabolic lensing structure directly with the existing optical combiner substrate, integrating multiple functions into a single component. The parabolic structure is formed as part of the combiner itself rather than as a separate element, combining the combiner's light transmission function with the parabolic lens's telecentric imaging function, thereby reducing overall device complexity while maintaining image size consistency.
Solution Approach 2:
The optical combiner is designed to serve multiple functions simultaneously: it acts as both a light transmission element for visible light and a telecentric imaging element for infrared eye-tracking. The parabolic lensing structure enables the combiner to perform both combing and telecentric imaging functions, eliminating the need for separate components and reducing device complexity.
3Adaptability or versatility
If infrared light is selectively focused to a camera, then eye-tracking capability is enabled, but visible light transmission is blocked
Solution Approach 1:
The patent applies local quality by using a dichroic coating with wavelength-selective properties on specific regions of the optical combiner. The coating is designed to reflect infrared wavelengths (for eye-tracking) while transmitting visible wavelengths (for scene viewing). This local differentiation of optical properties at different wavelengths allows the system to enable eye-tracking capability without blocking visible light transmission.
Solution Approach 2:
The optical combiner utilizes composite material structure combining transparent substrate material with a dichroic coating layer. This composite structure provides both visible light transmission through the substrate and infrared light reflection through the dichroic layer, enabling simultaneous achievement of eye-tracking capability and visible light transmission without mutual interference.
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 ensures that the image size of the eye remains constant, reducing sensitivity to depth translations and improving eye-tracking accuracy by using a telecentric optical combiner that maintains image consistency across varying eye relief distances.
Implementation Method 1
An off-axis parabolic lensing structure, such as a parabolic Fresnel structure, may be used to selectively focus collimated infrared image light to a camera
Implementation Method 2
parabolic lensing structure that functions as an off-axis reflector for infrared light
Implementation Method 3
A dichroic reflective layer may be disposed over the two-dimensional parabolic Fresnel structure to reflect infrared imaging light to the camera while passing visible scene light
Implementation Method 4
parabolic lensing structure that functions as an off-axis reflector for infrared light
Data Source
AI summary
An off-axis optical combiner includes a parabolic lensing structure that selects collimated infrared image light received from an eyebox area for focusing to a focus of the off-axis optical combiner. Selecting the collimated infrared image light for focusing allows the parabolic lensing structure to form a same-sized image of an object having variable depth from the parabolic lensing structure.