Switchable Grating Waveguide for Eye Tracking
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Solution Overview
Problem
Existing see-through mixed reality display devices face challenges in implementing eye tracking without impairing their see-through properties, particularly when used with prescription eyewear, and struggle to cover the entire eye movement range and inter-pupillary distance range effectively.
Innovation Solution
A waveguide system with an input-coupler comprising a stack of electronically switchable diffractive gratings, which are aligned parallel to each other and have different focal lengths, is used to couple infrared light into the waveguide for eye tracking, allowing for total internal reflections and precise imaging of the eye, while a controller selects the best-focused image for tracking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a simple camera is directly focused on the user's eye, then the eye tracking system is simple and inexpensive, but the camera needs to be positioned close to the eye level which causes at least partial obstruction of the see-through properties of the mixed reality display device system
Solution Approach 1:
The patent uses a waveguide to redirect infrared light from the eye along a different spatial path (through the waveguide substrate) rather than requiring a direct line-of-sight path. This dimensional redirection allows the camera to be positioned away from the eye level while still capturing eye reflections, eliminating the obstruction problem while maintaining tracking functionality.
Solution Approach 2:
The waveguide acts as an intermediary optical element that couples infrared light from the eye to the camera sensor. Instead of requiring direct camera-to-eye alignment, the waveguide mediates the light path, enabling the camera to be positioned in a non-obstructive location while maintaining effective eye tracking.
2Ease of manufacture
If a partial reflector is used to fold the camera view path to the user's temple, then the camera can be positioned outside the see-through field, but implementation is problematic if the eye tracking needs to work with prescription eyewear
Solution Approach 1:
The waveguide is designed to handle multiple optical paths simultaneously - it guides both the display optics and the eye tracking infrared light through the same substrate. This multi-functionality allows the system to work with or without prescription eyewear, as the waveguide's internal light guiding is not affected by external glasses.
3Device complexity
If reverse optical path imaging in a free form prism or other eyepiece based mixed reality display device system is used, then the actual display optics provide the imaging functionality for eye tracking, but components of a free form prism or eyepiece tend to be rather large in size making this approach not always practical
Solution Approach 1:
The patent employs a thin waveguide substrate (typically 0.5-2mm thick) instead of bulky free-form prisms or eyepieces. The waveguide's thin film structure provides the necessary optical guidance functionality while maintaining a compact, lightweight form factor suitable for head-mounted displays.
4Adaptability or versatility
If a free form optical element for eye tracking only is added, then eye tracking functionality is provided, but this would be expensive and would add significant weight and size to the system
Solution Approach 1:
The waveguide integrates multiple functions into a single component - it serves as both the display optical element and the eye tracking light guide. By merging these functions into one substrate rather than adding a separate eye tracking element, the system avoids the additional weight and cost of separate components while maintaining full 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 solution enables effective eye tracking without obstructing the see-through display, supports use with prescription eyewear, and covers the necessary eye movement and inter-pupillary distance ranges, providing accurate gaze direction detection.
Implementation Method 1
the input-coupler comprises a stack of two or more electronically switchable diffractive gratings arranged parallel to one another... each of the electronically switchable diffractive gratings, when turned on, is adapted to receive infrared light having the infrared wavelength and couple the received infrared light into the waveguide
Implementation Method 2
propagates within the waveguide from the input-coupler to the output-coupler by way of total internal reflections
Data Source
Figure 1
Figure 2A~2B
Figure 3A
AI summary
A transparent waveguide, for use in tracking an eye illuminated by infrared light, includes an input-coupler and an output-coupler. The input-coupler includes a stack of electronically switchable diffractive gratings arranged parallel to one another, each of which has a respective lens power that causes each of the gratings in the stack to have a different focal length. Each grating, when turned on, couples received infrared light into the waveguide. A sensor images an eye in dependence on infrared light beams that exit the waveguide at the output-coupler. Images of an eye, obtained using the sensor, are analyzed to determine which one of the electronically switchable diffractive gratings, when turned on, provides a best focused image of the eye or portion thereof. The one of the electronically switchable diffractive gratings, which provides the best focused image of the eye, is used for imaging the eye during eye tracking.