HUDs present the vital parameters of aircraft position, velocity, demand information and trend cues on a screen suspended from the flight-deck roof between the pilot's eyes and the aircraft windscreen. The pilot looks through this screen (the combiner) to the real world beyond and, as the combiner is collimated (focused at, or near, infinity), the pilot can accommodate simultaneously the outside picture and the HUD symbology. The major symbology is conformal - it overlies the real world. For example, the HUD horizon will coincide with the visual horizon. This requires an overhead electro-optical unit, or symbol generator, which projects the symbols on to the combiner. The combiner must focus these symbols crisply and yet allow the world outside to be seen without distortion. It is, therefore, optically sophisticated - a curved lens, with a multi-layer coating (a synthetic hologram), which reflects a narrow band of green light (the symbol colour) to the eye position, while allowing the pilot to see the forward view with the symbology imposed upon it.

Source: Flight International