A head-up display (HUD) is a transparent display that displays information without requiring users to shift their eyes away from their normal viewing positions. The name comes from a pilot’s ability to examine the information with his head “up” and facing forward, rather than tilted down and staring at lower instruments. A HUD also eliminates the need for the pilot’s eyes to refocus after looking at the optically closer instruments to observe the outside. HUDs are now widely used in commercial aircraft and automobiles, despite their origins in military aviation.
It uses decent text and symbols on the HUD to show the pilot important flying information, such as airspeed, altitude, and the horizon line, as well as the flight path vector, turn/bank indicators, angle of attack, and more. The HUD in military aircraft often displays a variety of targeting, weapon sensor, firing status, and other vital data.
The goal of the head-up display is to make it as simple as possible for pilots to observe and absorb critical flight or mission information while remaining “head-up and eyes-out” rather than looking down or away from what’s going on in the sky ahead of them. This not only makes pilots and their crews safer but also improves situational awareness and lowers tiredness.
HUDs were first proposed during World War II as a remedy for pilots who were having trouble locating their targets in challenging skies while depending entirely on verbal directions. Pilots weren’t able to access information hands-free until HUDs were invented. Because it is possible to maintain an external lookout without losing access to key aircraft instrumentation, the ‘applied’ benefits of a HUD to transport aircraft flight safety have primarily been seen as an enhancement of situational awareness for flight in limited (or night) visibility in the vicinity of visible terrain, water, ground-based obstacles, or other aircraft. This is true for the initial climb after takeoff, but it is especially true for the approach and landing phases of flight, which account for the vast majority of all aircraft accidents – and the vast majority of fatal Controlled Flight Into Terrain (CFIT) accidents involving public transport aircraft. By displaying the expected touchdown position, a HUD can represent for the pilot any ‘gap’ that may exist between the required aircraft track to a safe landing and a projection of the consequences of present aircraft condition.
Main three components of a standard HUD
A projector unit, a combiner, and a video generation computer.
In a conventional HUD, the projection unit is an optical collimator arrangement, which consists of a convex lens or concave mirror with a cathode ray tube, light-emitting diode display, or liquid crystal display at its focal point. This setup (which dates back to the creation of the reflector sight in 1900) creates a collimated image, in which the focal point appears to be infinity.
The combiner is often an angled flat piece of glass (a beam splitter) placed directly in front of the viewer that redirects the projected image from the projector in such a way that the spectator may see both the field of view and the projected infinity picture.
The computer serves as an interface between the HUD and the systems/data to be displayed, as well as generating the visuals and symbols that the projection unit will display.
• First Generation – Use a CRT to generate an image on a phosphor screen, but be aware that the phosphor screen covering will deteriorate with time. This is the type of HUD that is currently in use.
• Second Generation – Display an image using a solid-state light source, such as an LED, that is modified by an LCD screen. These systems do not fade and do not require the high voltages required by first-generation systems. Commercial aircraft have these systems.
• Third Generation – Instead of using a projection system, optical waveguides are used to produce images directly in the combiner.
• Fourth Generation – Display visuals and even video footage on clear transparent media using a scanning laser.
Liquid crystal display (LCD), liquid crystal on silicon (LCoS), digital micromirrors (DMD), an organic light-emitting diode (OLED) are some of the newer micro-display imaging technologies being introduced.
Researchers have developed a head-up display that employs holographic technology to improve visibility for pilots while also freeing up cockpit space.
University of Arizona researchers employed holographic optical elements to build a much larger 3D eye box. Rather than using traditional optics, the use of holography produces a small optical element that can be directly placed on a windscreen.
Holographic optical elements in the new head-up display route light from a small image into a piece of glass, where it is confined until it reaches another holographic optical element, which extracts it. After, the extraction hologram displays a viewable image with a larger eye box than the original image.
The same laser light interactions that are used to generate holograms that safeguard credit cards from forgery can also be utilized to construct optical elements in light-sensitive materials, such as lenses and filters. Not only are these holographic elements smaller than typical optical components, but they can also be mass-produced due to their ease of fabrication.
Holographic optical elements transport light from a small image into a piece of glass, where it is kept until it reaches another holographic optical element, which extracts the light for the next head-up display. The extraction hologram displays a viewable image with a larger eye box than the original.
Two major issues with the use of HUD
• Attention Capture, also known as tunneling, is a condition in which pilots become fixated on the HUD display to the exclusion of external events or information.
• Display graphics obscure important data in the outside-aircraft sight; the design approach is to keep the number of symbols low enough to avoid confusion. Reduced distraction can also aid in the capture of attention.
To sum up, that image on the screen appears to be far out in front of the aircraft, thanks to collimators and now holographic technology, so the pilot doesn’t have to adjust eye focus to watch a screen that’s only 20km away. The main advantage has been that it makes the transition between controlling the aircraft using the instrument panel and using exterior indicators easier in both directions. It also makes it simple to combine these sources for a single pilot operation.
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