Head-up display
A Head-Up Display, also known as a Heads-Up Display or simply HUD, is any type of display that presents data without blocking the user's view. This technique was pioneered for military aviation and is now used in commercial aviation, motor vehicle and other applications.
There are two types of HUD:
- Fixed, in which the user looks through a display element attached to the airframe or vehicle chassis. Commercial aircraft and motor vehicle HUDs are of this type. The system determines the image to be presented depending on the orientation of the vehicle. The size and weight of the display system can be much greater than in the other type which is:
- Helmet-mounted, or head-mounted, in which the display element moves with the user's head. This requires a system to precisely monitor the user's direction of gaze and determine the appropriate image to be presented. The user must wear a helmet or other headgear which is securely fixed to the user's head so that the display element does not move with respect to the user's eye. Such systems are often monocular.
HUDs have in common the following characteristics:
- The display element is largely transparent, meaning the information is displayed in contrasting superposition over the user's normal environment.
- The information is projected with its focus at infinity. Doing this means that a user doesn't need to refocus his eyes (which takes several tenths of a second) when changing his attention between the instrument and the outside world.
The most common means by which current HUDs are implemented is to project the image onto a clear glass optical element ('combiner'). Traditionally, the source for the projected image has been a Cathode Ray Tube (CRT), however newer image sources based on micro-display technologies are now being introduced. Micro-display technologies that have been demonstrated include Liquid Crystal Display (LCD), Liquid Crystal On Silicon (LCOS), Digital Micro Mirrors (DMDs), Organic Light-Emitting Diode (OLED) and Laser.
Some experimental HUD systems work instead by directly writing information onto the wearer's retina using a low-powered Laser (see Virtual retinal display).
Head-Up displays were pioneered for fighter 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 in use in commercial aviation since the 1970s, and are now in regular use, notably with Alaska Airlines.
As of 2002, the Chevrolet Corvette uses a HUD to display navigation, vehicle speed, engine RPM, and more.
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[1] (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.
- In the James Bond story Licence Renewed, Bond's car, a Saab 900 turbo, was fitted with a HUD.
- 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.
- providing an interface for access to a universal network. Cory Doctorow elaborated on this concept in his book Down and Out in the Magic Kingdom.
- A motorcycle helmet HUD system has also been produced and is commercially available.