Head-up Display - Design Factors

Design Factors

There are several factors that interplay in the design of a HUD:

• Field of View — also "FOV", indicates the angle(s), vertically as well as horizontally, subtended at the pilot's eye, that the combiner displays symbology in relation to the outside view. A narrow FOV means that the view (of a runway, for example) through the combiner might include little additional information beyond the perimeters of the runway environment; whereas a wide FOV would allow a 'broader' view. For aviation applications, the major benefit of a wide FOV is that an aircraft approaching the runway in a crosswind might still have the runway in view through the combiner, even though the aircraft is pointed well away from the runway threshold; where a narrow FOV the runway would be 'off the edge' of the combiner, out of the HUD's view. Because the human eyes are separated, each eye receives a different image. The HUD image is viewable by one or both eyes, depending on technical and budget limitations in the design process. Modern expectations are that both eyes view the same image, in other words a "binocular Field of View (FOV)".
• Collimation - The projected image is collimated which makes the light rays parallel. Because the light rays are parallel the lens of the human eye focusses on infinity to get a clear image. Collimated images on the HUD combiner are perceived as existing at or near optical infinity. This means that the pilot's eyes do not need to refocus to view the outside world and the HUD display...the image appears to be "out there", overlaying the outside world.
• Eyebox — The optical collimator produces a cylinder of parallel light so the display can only be viewed while the viewer's eyes are somewhere within that cylinder, a three-dimensional area called the head motion box or eyebox. Modern HUD eyeboxes are usually about 5 lateral by 3 vertical by 6 longitudinal inches. This allows the viewer some freedom of head movement but movement too far up/down left/right will cause the display to vanish off the edge of the collimator and movement too far back will cause it to crop off around the edge (vignette). The pilot is able to view the entire display as long as one of the eyes is inside the eyebox.
• Luminance/contrast — Displays have adjustments in luminance and contrast to account for ambient lighting, which can vary widely (e.g., from the glare of bright clouds to moonless night approaches to minimally lit fields).
• Boresight — Aircraft HUD components are very accurately aligned with the aircraft's three axes – a process called boresighting – so that displayed data conforms to reality typically with an accuracy of ±7.0 milliradians. In this case the word "conform" means, "when an object is projected on the combiner and the actual object is visible, they will be aligned". This allows the display to show the pilot exactly where the artificial horizon is, as well as the aircraft's projected path with great accuracy. When Enhanced Vision is used, for example, the display of runway lights are aligned with the actual runway lights when the real lights become visible. Boresighting is done during the aircraft's building process and can also be performed in the field on many aircraft.
• Scaling - The displayed image (flight path, pitch and yaw scaling, etc.), are scaled to present to the pilot a picture that overlays the outside world in an exact 1:1 relationship. For example, objects (such as a runway threshold) that are 3 degrees below the horizon as viewed from the cockpit must appear at the -3 degree index on the HUD display.
• Compatibility — HUD components are designed to be compatible with other avionics, displays, etc.