Head-Up Display

For other meanings of Hud, see this article

A Head-Up Display, also known as a Heads-Up Display or simply HUD, is a means of projecting information directly into a human's visual field. This technique was pioneered for military aviation, but has been used experimentally in other applications.

HUDs have in common the following characteristics:

• Information is projected onto a person's visual field, and follows either their gaze or the direction in which their head points.
• The display is largely transparent, meaning the information is displayed in contrasting superposition over the wearer's normal environment.
• The information is projected with its focus at infinity. Doing this means that a pilot doesn't need to refocus their eyes (which takes several tenths of a second) when changing their attention between the instrument and the outside world.

The most common means by which current HUDs are implemented is to project the image either onto a pilot's visor or onto a clear optical glass element that is located in front of the eye (much like a spectacle glass). This projection is done by means of a tiny head-mounted projector and lens arrangement.

Some experimental HUD systems work instead by directly writing information onto the wearer's retina using a low-powered LASER.

Head-Up displays were pioneered for jets and later for low-flying military helicopter pilots, for whom information overload was a significant issue, and for whom changing their view to look at the aircraft's instruments could prove to be a fatal distraction.

HUDs have been proposed or experimentally developed for a number of other applications, including:

• overlaying tactical information onto the vision of an infantryman (such as the output of a laser rangefinder or the relative location of the solder's squadmates)
• providing basic information for car drivers, by projecting an image (again, at infinity) onto the inner surface of the car's windscreen. This has been released as a product by a few manufacturers (usually showing a speedometer) but is presently illegal in several jurisdictions (where laws prohibiting driver-viewable TV sets currently include HUDs). HUDs are likely to become more common in future vehicles.
• providing surgeons with an enhanced view, showing the results of x-rays or scans overlayed over their normal view of the patient, and thus allowing them to "see" structures normally invisible.

Many computer and video games also overlay information (ammo-counters, maps, scores, etc.) over the game's normal display, and the term HUD is informally used for such displays (although, by virtue of being displayed on an ordinary computer monitor, such displays don't meet the formal definition above).