(Diffractive and reflective) optical technologies used in AR glasses to project virtual images in front of the eyes while still allowing the real world to be seen. The principle: a micro-projector embedded in the frame sends light into an optical lens (the “waveguide”), which directs it to the retina. Diffractive waveguides use diffraction gratings etched into the glass to redirect light. Reflective waveguides use semi-reflective layers. Each technology involves different trade-offs in terms of field of view, brightness, lens thickness and manufacturing cost.