Diffractive Optics for Eye Imaging

Waveguide displays use diffractive optical elements (DOEs) to guide light to the eye. Can we use similar structures for eye tracking? This dual-use approach could simplify the overall system.

The Waveguide Display Primer

AR displays need to project images while the user sees through to the real world. Waveguide approach:

1. Light enters waveguide at input coupler (DOE)
2. Light propagates via total internal reflection
3. Light exits at output coupler (DOE) toward eye
4. User sees virtual image overlaid on real world

The Eye Tracking Challenge

Standard eye tracking adds cameras alongside the display. Problems:

• Additional hardware cost and complexity
• Optical paths must not interfere
• Form factor impact

The Diffractive Solution

What if the same waveguide used for display could also image the eye?

Concept:

• IR illumination through the waveguide (different layer or wavelength)
• Eye reflection couples back into waveguide
• Exits waveguide at sensor location

         ┌─────────────────────────┐
    Eye →│ Waveguide with DOEs     │← Display light in
         │                         │
         └────────────┬────────────┘
                      │
                      ▼
               Eye camera
         (captures light from eye via waveguide)

DOE Design Considerations

Wavelength Separation

Display uses visible light (450-640nm). Eye tracking uses NIR (850nm).

DOE response is wavelength-dependent. Design DOEs that:

• Efficiently couple display wavelengths for image delivery
• Efficiently couple NIR for eye imaging
• Minimize crosstalk between paths

Angular Bandwidth

Eye imaging needs to capture across the eye box (~20mm).

DOE must have sufficient angular bandwidth to collect light from different eye positions.

Efficiency Trade-offs

Every DOE has efficiency losses. Dual-use means:

• Some display light leaks into eye-track path (background noise)
• Some eye-track light leaks into display (ghost images)

Managing these trade-offs is the design challenge.

Prototype Results

Built a bench prototype with:

• Commercial waveguide (single layer)
• Custom NIR DOE overlay
• CMOS sensor at edge

Results:

• Pupil clearly visible
• Glints from co-propagated IR LEDs detectable
• Image quality sufficient for basic gaze estimation

Challenges:

• Efficiency lower than dedicated camera path
• Crosstalk creates background haze
• Manufacturing the custom DOE is expensive

Path to Product

For V1: separate eye tracking cameras (proven, lower risk).

For V2+: integrated diffractive eye imaging could:

• Reduce component count
• Enable thinner form factor
• Share manufacturing steps with display waveguide

We're continuing R&D and building patent portfolio.

[Patent granted 2021: US11237631 "Eye-Imaging Apparatus Using Diffractive Optical Elements"]