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Head-up display

A head-up display, or heads-up display,[1] also known as a HUD (/hÊŒd/) or head-up guidance system (HGS), is any transparent display that presents data without requiring users to look away from their usual viewpoints. The origin of the name stems from a pilot being able to view information with the head positioned "up" and looking forward, instead of angled down looking at lower instruments. A HUD also has the advantage that the pilot's eyes do not need to refocus to view the outside after looking at the optically nearer instruments.

This article is about the military and vehicle technology. For its use in gaming, see HUD (video gaming).

Although they were initially developed for military aviation, HUDs are now used in commercial aircraft, automobiles, and other (mostly professional) applications.


Head-up displays were a precursor technology to augmented reality (AR), incorporating a subset of the features needed for the full AR experience, but lacking the necessary registration and tracking between the virtual content and the user's real-world environment.[2]

First Generation—Use a to generate an image on a phosphor screen, having the disadvantage of the phosphor screen coating degrading over time. The majority of HUDs in operation today are of this type.

CRT

Second Generation—Use a solid state light source, for example , which is modulated by an LCD screen to display an image. These systems do not fade or require the high voltages of first generation systems. These systems are on commercial aircraft.

LED

Third Generation—Use to produce images directly in the combiner rather than use a projection system.

optical waveguides

Fourth Generation—Use a scanning laser to display images and even video imagery on a clear transparent medium.

Field of View – also "FOV", indicates the angle(s), vertically as well as horizontally, subtended at the pilot's eye, at which the combiner displays 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; whereas with a narrow FOV the runway would be 'off the edge' of the combiner, out of the HUD's view. Because 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)".

symbology

Collimation – The projected image is which makes the light rays parallel. Because the light rays are parallel the lens of the human eye focuses 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. This feature is critical for effective HUDs: not having to refocus between HUD-displayed symbolic information and the outside world onto which that information is overlaid is one of the main advantages of collimated HUDs. It gives HUDs special consideration in safety-critical and time-critical manoeuvres, when the few seconds a pilot needs in order to re-focus inside the cockpit, and then back outside, are very critical: for example, in the final stages of landing. Collimation is therefore a primary distinguishing feature of high-performance HUDs and differentiates them from consumer-quality systems that, for example, simply reflect uncollimated information off a car's windshield (causing drivers to refocus and shift attention from the road ahead).

collimated

Eyebox – The 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 (13x8x15 cm). This allows the viewer some freedom of head movement but movement too far up/down or 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 eye is inside the eyebox.[12]

optical collimator

Luminance/contrast – Displays have adjustments in 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).

luminance

Boresight – Aircraft HUD components are very accurately aligned with the aircraft's three axes â€“ a process called  â€“ so that displayed data conforms to reality typically with an accuracy of ±7.0 milliradians (±24 minutes of arc), and may vary across the HUD's FOV. 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 is 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.[9]

boresighting

Scaling – The displayed image (flight path, pitch and yaw scaling, etc.), is 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.

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

boresight or waterline symbol — is fixed on the display and shows where the nose of the aircraft is actually pointing.

flight path vector (FPV) or velocity vector symbol — shows where the aircraft is actually going, as opposed to merely where it is pointed as with the boresight. For example, if the aircraft is up but descending as may occur in high angle of attack flight or in flight through descending air, then the FPV symbol will be below the horizon even though the boresight symbol is above the horizon. During approach and landing, a pilot can fly the approach by keeping the FPV symbol at the desired descent angle and touchdown point on the runway.

pitched

acceleration indicator or energy cue — typically to the left of the FPV symbol, it is above it if the aircraft is accelerating, and below the FPV symbol if decelerating.

indicator — shows the wing's angle relative to the airflow, often displayed as "α".

angle of attack

navigation data and symbols — for approaches and landings, the flight guidance systems can provide visual cues based on navigation aids such as an or augmented Global Positioning System such as the Wide Area Augmentation System. Typically this is a circle which fits inside the flight path vector symbol. Pilots can fly along the correct flight path by "flying to" the guidance cue.

Instrument Landing System

Further development and experimental uses[edit]

HUDs have been proposed or are being experimentally developed for a number of other applications. In military settings, a HUD can be used to overlay tactical information such as the output of a laser rangefinder or squadmate locations to infantrymen. A prototype HUD has also been developed that displays information on the inside of a swimmer's goggles or of a scuba diver's mask.[36] HUD systems that project information directly onto the wearer's retina with a low-powered laser (virtual retinal display) are also being tested.[37][38]


Some head-up displays can perform real-time language translation.[39]

Index of aviation articles

Acronyms and abbreviations in avionics

Augmented reality

Eyes-on-the-Road-Benefit

EyeTap

HUD (video gaming)

Optical head-mounted display

- A stage play version of a similar reflection effect.

Pepper's ghost

Smartglasses

Virtual retinal display

VR positional tracking

Wearable computer

Rochester Archives Article—'Buccaneer HUD PDU'

BBC Article—'Pacman comes to life virtually'

'Clinical evaluation of the 'head-up' display of anesthesia data'

'When will the Head-up go Civil' – Flight 1968 archive

'Elliott Brothers to BAE SYSTEMS' – a short history of Elliott Brothers

– a 1964 Flight International article on flying using an early Specto head-up display

Head-up Over the Hills

Jaguar Unveils 'Virtual Widescreen' Technology to Assist Drivers – Latin Post

The story of how all Miramar Tomcat squadrons got the funds to purchase riflescopes to attach to the HUD of their F-14 fighter jets