Head-up display


A head-up display or heads-up display, also known as a HUD or head-up guidance system, 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.
Although they were initially developed for military aviation, HUDs are now used in commercial aircraft, automobiles, and other applications.
Head-up displays were a precursor technology to augmented reality, 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.

Overview

A typical HUD contains three primary components: a projector unit, a combiner, and a video generation computer.
The projection unit in a typical HUD is an optical collimator setup: a convex lens or concave mirror with a cathode-ray tube, light emitting diode display, or liquid crystal display at its focus. This setup produces an image where the light is collimated, i.e., the focal point is perceived to be at infinity.
The combiner is typically an angled flat piece of glass located directly in front of the viewer, that redirects the projected image from projector in such a way as to see the field of view and the projected infinity image at the same time. Combiners may have special coatings that reflect the monochromatic light projected onto it from the projector unit while allowing all other wavelengths of light to pass through. In some optical layouts combiners may also have a curved surface to refocus the image from the projector.
The computer provides the interface between the HUD and the systems/data to be displayed and generates the imagery and symbology to be displayed by the projection unit.

Types

Other than fixed mounted HUD, there are also head-mounted displays. These include helmet-mounted displays, forms of HUD that feature a display element that moves with the orientation of the user's head.
Many modern fighters use both a HUD and HMD concurrently. The F-35 Lightning II was designed without a HUD, relying solely on the HMD, making it the first modern military fighter not to have a fixed HUD.

Generations

HUDs are split into four generations reflecting the technology used to generate the images:
  • First Generation—Use a CRT 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.
  • Second Generation—Use a solid-state light source, for example LED, 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.
  • Third Generation—Use optical waveguides to produce images directly in the combiner rather than use a projection system.
  • Fourth Generation—Use a scanning laser to display images and even video imagery on a clear transparent medium.
Newer micro-display imaging technologies are being introduced, including liquid-crystal display, liquid crystal on silicon, digital micro-mirrors, and organic light-emitting diode.

History

HUDs evolved from the reflector sight, a pre-World War II parallax-free optical sight technology for military fighter aircraft. The gyro gunsight added a reticle that moved based on the speed and turn rate to solve for the amount of lead needed to hit a target while maneuvering.
During the early 1940s, the Telecommunications Research Establishment, in charge of UK radar development, found that Royal Air Force night fighter pilots were having a hard time reacting to the verbal instruction of the radar operator as they approached their targets. They experimented with the addition of a second radar display for the pilot, but found they had trouble looking up from the lit screen into the dark sky in order to find the target. In October 1942 they had successfully combined the image from the radar tube with a projection from their standard GGS Mk. II gyro gunsight on a flat area of the windscreen, and later in the gunsight itself. A key upgrade was the move from the original AI Mk. IV radar to the microwave-frequency AI Mk. VIII radar found on the de Havilland Mosquito night fighter. This set produced an artificial horizon that further eased head-up flying.
In 1955 the US Navy's Office of Naval Research and Development did some research with a mockup HUD concept unit along with a sidestick controller in an attempt to ease the pilot's burden flying modern jet aircraft and make the instrumentation less complicated during flight. While their research was never incorporated in any aircraft of that time, the crude HUD mockup they built had all the features of today's modern HUD units.
HUD technology was next advanced by the Royal Navy in the Buccaneer, the prototype of which first flew on 1958. The aircraft was designed to fly at very low altitudes at very high speeds and drop bombs in engagements lasting seconds. As such, there was no time for the pilot to look up from the instruments to a bombsight. This led to the concept of a "Strike Sight" that would combine altitude, airspeed and the gun/bombsight into a single gunsight-like display. There was fierce competition between supporters of the new HUD design and supporters of the old electro-mechanical gunsight, with the HUD being described as a radical, even foolhardy option.
The Air Arm branch of the UK Ministry of Defence sponsored the development of a Strike Sight. The Royal Aircraft Establishment designed the equipment and the earliest usage of the term "head-up-display" can be traced to this time. Production units were built by Rank Cintel, and the system was first integrated in 1958. The Cintel HUD business was taken over by Elliott Flight Automation and the Buccaneer HUD was manufactured and further developed, continuing up to a Mark III version with a total of 375 systems made; it was given a 'fit and forget' title by the Royal Navy and it was still in service nearly 25 years later. BAE Systems, as the successor to Elliotts via GEC-Marconi Avionics, thus has a claim to the world's first head-up display in operational service. A similar version that replaced the bombing modes with missile-attack modes was part of the AIRPASS HUD fitted to the English Electric Lightning from 1959.
In the United Kingdom, it was soon noted that pilots flying with the new gunsights were becoming better at piloting their aircraft. At this point, the HUD expanded its purpose beyond weapon aiming to general piloting. In the 1960s, French test-pilot Gilbert Klopfstein created the first modern HUD and a standardized system of HUD symbols so that pilots would only have to learn one system and could more easily transition between aircraft. The modern HUD used in instrument flight rules approaches to landing was developed in 1975. Klopfstein pioneered HUD technology in military fighter jets and helicopters, aiming to centralize critical flight data within the pilot's field of vision. This approach sought to increase the pilot's scan efficiency and reduce "task saturation" and information overload.
Use of HUDs then expanded beyond military aircraft. In the 1970s, the HUD was introduced to commercial aviation, and in 1988, the Oldsmobile Cutlass Supreme became the first production car with a head-up display.
Until a few years ago, the Embraer 190, Saab 2000, Boeing 727, and Boeing 737 Classic and Next Generation aircraft were the only commercial passenger aircraft available with HUDs. However, the technology is becoming more common with aircraft such as the Canadair RJ, Airbus A318 and several business jets featuring the displays. HUDs have become standard equipment on the Boeing 787. Furthermore, the Airbus A320, A330, A340 and A380 families are currently undergoing the certification process for a HUD. HUDs were also added to the Space Shuttle orbiter.

Design factors

There are several factors that interplay in the design of a HUD:
  • Field of View – also "FOV", indicates the angle, vertically as well as horizontally, subtended at the pilot's eye, at which the combiner displays symbology in relation to the outside view. A narrow FOV means that the view 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 ".
  • Collimation – The projected image is collimated 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
  • 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 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 The pilot is able to view the entire display as long as one eye is inside the eyebox.
  • Luminance/contrast – Displays have adjustments in luminance and contrast to account for ambient lighting, which can vary widely
  • 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, 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.
  • Scaling – The displayed image, is scaled to present to the pilot a picture that overlays the outside world in an exact 1:1 relationship. For example, objects 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.