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.
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.
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.
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. By virtue of
being displayed on an ordinary
computer monitor, such displays do not meet the formal definition
above; however, in some games the HUD is displayed as being projected onto a virtual helmet, or
used in a virtual air/spacecraft in a similar manner as it would be used in real-life. 
