A Head-Up Display (HUD) in smart glasses functions like a digital overlay, projecting "glanceable" information—such as text messages, navigation arrows, or live subtitles—directly into your line of sight without blocking your view of the real world.

True HUD (Diffractive Waveguide): Uses microscopic gratings in the lens to reflect light from a tiny projector into your eyes. It is highly transparent (85%+), allowing you to walk or drive safely while viewing data. Virtual Screen (Birdbath/OLED): Projects a larger, opaque image that feels like sitting in front of a 200-inch cinema screen. These are primarily for media consumption and typically block more of your environment. MicroLED vs. Micro-OLED: MicroLED (used in the RayNeo X3 Pro) is much brighter and ideal for outdoor sunlight, while Micro-OLED (used in XREAL) offers better contrast for movies.