Waveguide Eye Tracker Using Switchable Bragg Gratings
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Solution Overview
Problem
Current eye trackers for Head-Mounted Displays (HMDs) face challenges such as high latency due to complex image processing, limited field of view, and the need for large optics that obscure the line of sight, while also being bulky and heavy, which hinders their use in augmented and virtual reality applications.
Innovation Solution
The development of a compact, transparent eye tracker using switchable Bragg Gratings (SBGs) that employ a waveguide structure with separate illumination and imaging gratings, allowing for efficient infrared illumination and image capture without obstructing the user's view, and utilizing low-resolution high-speed image sensors to track speckle patterns for reduced latency and increased field of view.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If flat beam splitters and large optics are used to image eye reflections onto a sensor, then the tracker can function, but the exit pupil is limited and the line of sight is obscured
Solution Approach 1:
The patent extracts the imaging function from traditional large optics and beam splitters, replacing them with a compact waveguide-based optical system. The waveguide extracts and guides infrared light from the eye at a small angle, eliminating the need for large obscuring optics while maintaining tracking accuracy.
Solution Approach 2:
The patent replaces the mechanical optical system (flat beam splitters and large lenses) with a waveguide-based optical system that uses total internal reflection and diffraction gratings. This substitution enables compact integration without compromising measurement precision.
2Measurement precision
If traditional eye tracking optics are used, then the tracker can capture eye reflections, but the device becomes bulky and heavy
Solution Approach 1:
The patent embeds the imaging optics within a waveguide structure, nesting multiple optical functions (illumination, imaging, beam steering) within a single integrated component. This nesting eliminates the need for separate bulky optical components while maintaining detection capability.
Solution Approach 2:
The patent uses thin waveguide films to replace thick traditional optical components. The waveguide is a thin transparent structure that can guide and manipulate infrared light without adding significant weight or bulk to the device.
3Measurement precision
If high-resolution image sensors are used to track eye features, then tracking accuracy is improved, but processing latency increases
Solution Approach 1:
The patent uses low-resolution sensors that capture only the essential speckle pattern information needed for eye tracking, rather than full high-resolution eye images. This partial action approach reduces processing requirements and latency while maintaining sufficient tracking accuracy for AR/VR applications.
Solution Approach 2:
The patent captures speckle patterns which are optical copies or fingerprints of the eye's optical properties. These speckle patterns contain sufficient information for tracking without requiring detailed high-resolution imaging, thus reducing processing time while maintaining measurement precision.
4Measurement precision
If the angular separation between illumination and detection optical axes is increased, then corneal reflections can be tracked, but the field of view is limited
Solution Approach 1:
The patent uses a switchable Bragg grating that can dynamically change its diffraction angle by applying different voltages. This dynamic adjustment allows the system to track corneal reflections across a wide field of view by adapting the illumination angle to match the detection angle for different gaze directions.
Solution Approach 2:
The patent changes the optical parameters of the Bragg grating by applying voltage to alter the refractive index and diffraction angle. This parameter change enables the system to maintain optimal illumination-detection geometry across a wide field of view without physical movement of components.
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 solution provides a low-latency, compact, and lightweight eye tracker with a wide field of view, enhancing user experience in AR and VR by minimizing processing overhead and maintaining eye safety standards, while allowing for efficient illumination and image capture without obstructing the user's view.
Implementation Method 1
The waveguide propagates illumination light towards an eye and propagates image light reflected from at least one surface of the eye
Implementation Method 2
switchable Bragg Gratings (SBGs) that employ a waveguide structure with separate illumination and imaging gratings
Implementation Method 3
operating in the infrared which offers the benefit of invisibility and can be made eye safe by operating at wavelengths around 1550 nm
Data Source
Figure 1(A)~1(C)
Figure 2
Figure 3~4
AI summary
An object tracker comprises: a source of light; at least one illumination waveguide optically coupled to said source containing at least one grating lamina for diffracting said light towards an external object; at least one detector waveguide containing a grating lamina for in-coupling and deflecting a first polarization of light reflected from said object into a first waveguide direction and deflecting a second polarization of light reflected from said object into a second waveguide direction; at least one detector optically coupled to said detector waveguide operative to receive light propagating in said first waveguide direction; and at least one detector optically coupled to said detector waveguide operative to receive light propagating in said second waveguide direction, wherein said first and second waveguide directions correspond to first and second directions of relative object motion. The optical tracker may be implemented in an eye tracker.