Terms to know first
A short warm-up before the technical explanation. These are the ideas the rest of the guide will build on.
- Waveguide
- A thin, transparent slab of glass or plastic that uses total internal reflection to guide light horizontally from a projector to your eye, extracting copies gradually to expand the exit pupil.
- Exit-pupil expansion or pupil replication
- The process of creating many copies of a small projector beam spread across the lens, forming a large eyebox so you see the full image as your pupil moves naturally.
- Total internal reflection
- The phenomenon where light hitting the boundary between a dense material like glass and air at a steep angle is completely reflected back rather than escaping, which occurs beyond the critical angle determined by the refractive index.
- Field of view (FOV)
- The angular width of the virtual image you see through the AR display, typically measured in degrees diagonal; current waveguide designs range from twenty to seventy degrees, while birdbath optics can exceed fifty degrees.
- Optical combiner
- Any optical element that merges real-world light entering your eye with virtual imagery from a display, whether a waveguide, a beam-splitter, or a partially reflective surface.
The question
How does an AR lens combine real-world light entering your pupil with digital imagery generated by a projector located at the temple, and why do competing optical designs produce such different tradeoffs in weight.