View source for Head-up display

{{short description|Transparent display presenting data within normal sight lines of the user}}
{{About|the military and vehicle technology|its use in gaming|HUD (video gaming)}}
[[File:Hud on the cat.jpg|thumb|HUD of an [[McDonnell Douglas F/A-18 Hornet|F/A-18 Hornet]]]]

A '''head-up display''', or '''heads-up display''',<ref>Oxford Dictionary of English, Angus Stevenson,  Oxford University Press – 2010, page 809 (''head-up display (N.Amer. also heads-up display)'')</ref> also known as a '''HUD''' ({{IPAc-en|h|ʌ|d}}) or '''head-up guidance system''' ('''HGS'''), is any [[see-through display|transparent display]] that presents data without requiring users to look away from their usual viewpoints. The origin of the name stems from a [[Aircraft pilot|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 [[Accommodation (eye)|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 (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.<ref>{{Cite web |date=2019-06-06 |title=Augmented reality brings VR to the real world in all sorts of exciting ways |url=https://www.digitaltrends.com/computing/what-is-augmented-reality/ |access-date=2022-10-10 |website=Digital Trends |language=en}}</ref>

== Overview ==
[[File:PZL TS-11F Iskra (HUD).jpg|thumb|HUD mounted in a [[PZL TS-11 Iskra]] jet trainer aircraft with a glass plate combiner and a convex collimating lens just below it]]
A typical HUD contains three primary components: a ''projector unit'', a ''combiner'',  and a ''video generation computer''.<ref>{{cite book|author1=Fred H. Previc|author2=William R. Ercoline|title=Spatial Disorientation in Aviation|url=https://books.google.com/books?id=oYP7m9m2RocC&pg=PA452|year=2004|publisher=AIAA|isbn=978-1-60086-451-3|page=452}}</ref>

The projection unit in a typical HUD is an [[Collimator#Optical collimators|optical collimator]] setup: a [[convex lens]] or [[concave mirror]] with a [[cathode-ray tube]], [[LED display|light emitting diode display]], or [[liquid crystal display]] at its focus. This setup (a design that has been around since the invention of the [[reflector sight]] in 1900) 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 (a [[beam splitter]]) 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 [[wavelength]]s 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 (i.e. the projection unit) 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 display]]s (HMDs.) These include [[helmet-mounted display]]s (both abbreviated HMD), forms of HUD that feature a display element that moves with the orientation of the user's head.

Many modern fighters (such as the [[F/A-18]], [[F-16]], and [[Eurofighter]]) use both a HUD and HMD concurrently. The [[F-35 Lightning&nbsp;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 [[Cathode-ray tube|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 waveguide]]s 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 displays|liquid crystal display]] (LCD), [[liquid crystal on silicon]] (LCoS), [[digital light processing|digital micro-mirrors]] (DMD), and [[organic light-emitting diode]] (OLED).

== History ==
[[File:Revi C12-A.jpg|thumb|right|Longitudinal cross-section of a basic ''reflector sight'' (1937 German Revi C12/A)]]
[[File:C-130J Co Pilot's Head-up display.jpg|thumb|right|Copilot's HUD of a [[C-130J]]]]

HUDs evolved from the [[reflector sight]], a pre-World War II [[parallax]]-free optical sight technology for [[military]] [[fighter aircraft]].<ref>{{cite book|url=https://books.google.com/books?id=ESR4-pns2JMC&pg=PA189 |author=D. N. Jarrett |title=Cockpit engineering |page=189 |publisher=Ashgate Pub. |year=2005
|isbn=0-7546-1751-3 |access-date=2012-07-14}}</ref> The [[gyro gunsight]] added a [[reticle]] that moved based on the speed and turn rate to solve for the amount of [[Deflection (ballistics)|lead]] needed to hit a target while maneuvering.

During the early 1940s, the [[Telecommunications Research Establishment]] (TRE), in charge of UK [[radar]] development, found that [[Royal Air Force]] (RAF) [[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.<ref>Ian White, [https://books.google.com/books?id=SMMMAwAACAAJ "The History of Air Intercept Radar & the British Nightfigher"], Pen & Sword, 2007, p. 207</ref> 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.{{citation needed|date=February 2019}}

In 1955 the [[US Navy]]'s Office of Naval Research and Development did some research with a mockup HUD concept unit along with a [[Side-stick|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.<ref>[https://books.google.com/books?id=fNwDAAAAMBAJ&dq=1954+Popular+Mechanics+January&pg=PA101 "Windshield TV Screen To Aid Blind Flying."] ''Popular Mechanics'', March 1955, p. 101.</ref>

HUD technology was next advanced by the [[Royal Navy]] in the [[Blackburn Buccaneer|Buccaneer]], the prototype of which first flew on {{Nowrap|30 April}} 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]] (RAE) designed the equipment and the earliest usage of the term "head-up-display" can be traced to this time.<ref>John Kim, [http://digitalcommons.macalester.edu/books/1/ Rupture of the Virtual], Digital Commons Macalester College, 2016, p. 54</ref> Production units were built by [[Cintel|Rank Cintel]], and the system was first integrated in 1958. The Cintel HUD business was taken over by [[Elliott Brothers (computer company)|Elliott Flight Automation]] and the Buccaneer HUD was manufactured and further developed, continuing up to a Mark&nbsp;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.<ref>Rochester Avionics Archives</ref> 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.{{Citation needed|date=September 2008}} 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.<ref name=autogenerated1>Spitzer, Cary R., ed. "Digital Avionics Handbook". Head-Up Displays. Boca Raton, FL: CRC Press, 2001</ref> Klopfstein pioneered HUD technology in military [[fighter jet]]s and [[helicopter]]s, 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]] [[Boeing 737 Classic|Classic]] (737-300/400/500) and [[Boeing 737 Next Generation|Next Generation]] aircraft (737-600/700/800/900 series) 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&nbsp;787]].<ref>{{cite book|author=Norris, G.|author2=Thomas, G.|author3=Wagner, M.|author4=Forbes Smith, C.|name-list-style=amp|title=Boeing 787 Dreamliner—Flying Redefined|publisher=Aerospace Technical Publications International|date=2005|isbn=0-9752341-2-9}}</ref> Furthermore, the Airbus A320, A330, A340 and A380 families are currently undergoing the certification process for a HUD.<ref>{{cite web|url=http://www.airbus.com/en/presscentre/pressreleases/pressreleases_items/07_12_03_a318_hud.html |title=Airbus A318 approved for Head Up Display |publisher=Airbus.com |date=2007-12-03 |access-date=2009-10-02 |archive-url = https://web.archive.org/web/20071207164904/http://www.airbus.com/en/presscentre/pressreleases/pressreleases_items/07_12_03_a318_hud.html |archive-date = December 7, 2007}}</ref> HUDs were also added to the [[Space Shuttle]] orbiter.

== Design factors ==
[[File:Headset computer.png|thumb|alt= Photograph of a Headset computer |Headset computer]]
There are several factors that interplay in the design of a HUD:
* '''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 [[symbology (avionics)|symbology]] 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)".
* '''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 (causing drivers to refocus and shift attention from the road ahead.)
* '''Eyebox''' – The [[Collimator#Optical collimators|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 (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 ([[Vignetting|vignette]].) The pilot is able to view the entire display as long as one eye is inside the eyebox.<ref>{{cite book|author1=Cary R. Spitzer|title=Digital Avionics Handbook|url=https://books.google.com/books?id=A69m3ADmHGkC&pg=SA4-PA7|year=2000|publisher=CRC Press|isbn=978-1-4200-3687-9|page=4}}</ref>
* '''Luminance/contrast''' – Displays have adjustments in [[luminance]] 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.)
* '''Boresight''' – Aircraft HUD components are very accurately aligned with the aircraft's three axes&nbsp;– a process called ''[[gun harmonisation|boresighting]]''&nbsp;– so that displayed data conforms to reality typically with an accuracy of ±7.0&nbsp;[[milliradian]]s (±24&nbsp;[[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 flight vision systems|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.<ref name=autogenerated1 />
* '''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.

== Aircraft ==
[[File:F-14A VF-33 Cockpitview.JPEG|thumb|right|Head-up display of an F-14A Tomcat]]
On aircraft avionics systems, HUDs typically operate from dual independent redundant computer systems. They receive input directly from the sensors ([[pitot-static]], [[gyroscopic]], navigation, etc.) aboard the aircraft and perform their own computations rather than receiving previously computed data from the flight computers. On other aircraft (the Boeing 787, for example) the HUD guidance computation for Low Visibility Take-off (LVTO) and low visibility approach comes from the same flight guidance computer that drives the autopilot. Computers are integrated with the aircraft's systems and allow connectivity onto several different data buses such as the [[ARINC 429]], ARINC 629, and [[MIL-STD-1553]].<ref name=autogenerated1 />

=== Displayed data ===
[[File:Heads up display symbology.png|thumb|right|Displayed data symbology of a head-up display]]
Typical aircraft HUDs display [[airspeed]], [[altitude]], a [[horizon]] line, [[Aircraft heading|heading]], turn/bank and [[Slip indicator#Turn coordinator|slip/skid]] indicators. These instruments are the minimum required by 14 CFR Part 91.<ref>{{cite web|url=http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/MainFrame?OpenFrameSet |title=14 CFR Part 91 |publisher=Airweb.faa.gov |access-date=2009-10-02}}</ref>

Other [[symbology (avionics)|symbols]] and data are also available in some HUDs:
* ''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 [[Flight dynamics|pitched]] 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.
* ''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.
* ''[[angle of attack]] indicator'' — shows the wing's angle relative to the airflow, often displayed as ''"α"''.
* navigation data and symbols — for approaches and landings, the flight guidance systems can provide visual cues based on navigation aids such as an [[Instrument Landing System]] or augmented [[Global Positioning System]] such as the [[WAAS|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.

Since being introduced on HUDs, both the FPV and acceleration symbols are becoming standard on head-down displays (HDD.) The actual form of the FPV symbol on an HDD is not standardized but is usually a simple aircraft drawing, such as a circle with two short angled lines, (180 ± 30 degrees) and "wings" on the ends of the descending line. Keeping the FPV on the horizon allows the pilot to fly level turns in various angles of bank.

=== Military aircraft specific applications ===
[[File:F-18 HUD gun symbology.jpeg|thumb|FA-18 HUD while engaged in a mock [[dogfight]]]]

In addition to the generic information described above, military applications include weapons system and sensor data such as:
* ''target designation (TD)'' indicator — places a cue over an air or ground target (which is typically derived from [[radar]] or [[inertial navigation system]] data.)
* ''V<sub>c</sub>'' — closing velocity with target.
* ''Range'' — to target, waypoint, etc.
* ''weapon seeker'' or sensor line of sight — shows where a seeker or sensor is pointing.
* ''weapon status'' — includes type and number of weapons selected, available, arming, etc.

=== VTOL/STOL approaches and landings ===
During the 1980s, the United States military tested the use of HUDs in vertical take off and landing (VTOL) and short take off and landing (STOL) aircraft. A HUD format was developed at [[NASA]] [[Ames Research Center]] to provide pilots of VTOL and STOL aircraft with complete flight guidance and control information for [[Instrument Landing System#ILS categories|Category III C]] terminal-area flight operations. This includes a large variety of flight operations, from STOL flights on land-based runways to VTOL operations on [[aircraft carrier]]s. The principal features of this display format are the integration of the flightpath and pursuit guidance information into a narrow field of view, easily assimilated by the pilot with a single glance, and the superposition of vertical and horizontal situation information. The display is a derivative of a successful design developed for conventional transport aircraft.<ref>Vernon K. Merrick, Glenn G. Farris, and Andrejs A. Vanags. "A Head Up Display for Application to V/STOL Aircraft Approach and Landing". NASA Ames Research Center 1990.</ref>

=== Civil aircraft specific applications ===
[[File:SVS Cockpit Gulfstream GV.jpg|thumb|The cockpit of [[NASA]]'s [[Gulfstream V|Gulfstream GV]] with a synthetic vision system display. The HUD combiner is in front of the pilot (with a projector mounted above it). This combiner uses a curved surface to focus the image.]]
The use of head-up displays allows commercial aircraft substantial flexibility in their operations. Systems have been approved which allow reduced-visibility takeoffs, and landings, as well as full manual [[Instrument Landing System#ILS categories|Category III A]] landings and roll-outs.<ref>{{Cite web|url=http://www.faa.gov/library/manuals/examiners_inspectors/8700/hbga/media/hbga9916.doc|archiveurl=https://web.archive.org/web/20061001014951/http://www.faa.gov/library/manuals/examiners_inspectors/8700/hbga/media/hbga9916.doc|url-status=dead|title=Order: 8700.1 Appendix: 3 Bulletin Type: Flight Standards Handbook Bulletin for General Aviation (HBGA) Bulletin Number: HBGA 99-16 Bulletin Title: Category&nbsp;III Authorization for Parts 91 and 125 Operators with Head-Up Guidance Systems (HGS); LOA and Operations Effective Date: 8-31-99|archivedate=October 1, 2006}}</ref><ref>Falcon 2000 Becomes First Business Jet Certified Category III A by JAA and FAA; Aviation Weeks Show News Online September 7, 1998</ref><ref>{{cite web|url=http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgDAC.nsf/0/650d5669656fd15086256eeb0066a6cf?OpenDocument |title=Design Guidance for a HUD System is contained in Draft Advisory Circular AC 25.1329-1X, "Approval of Flight Guidance Systems" dated 10/12/2004 |publisher=Airweb.faa.gov |access-date=2009-10-02}}</ref> Initially expensive and physically large, these systems were only installed on larger aircraft able to support them. These tended to be the same aircraft that as standard supported autoland (with the exception of certain turbo-prop types{{clarify|date=September 2023}} that had HUD as an option) making the head-up display unnecessary for Cat III landings. This delayed the adoption of HUD in commercial aircraft. At the same time, studies have shown that the use of a HUD during landings decreases the lateral deviation from centerline in all landing conditions, although the touchdown point along the centerline is not changed.<ref>{{cite journal|title = HUD With a Velocity (Flight Path) Vector Reduces Lateral Error During Landing in Restricted Visibility|journal = [[International Journal of Aviation Psychology]]|date = 2007|volume = 17|issue = 1|pages = 91–108|author1 = Goteman, Ö.|author2 = Smith, K.|author3 = Dekker, S.|doi = 10.1080/10508410709336939|s2cid = 219641008}}</ref>

For [[general aviation]], MyGoFlight expects to receive a [[supplemental type certificate|STC]] and to retail its SkyDisplay HUD for $25,000 without installation for a single piston-engine as the [[Cirrus SR22]]s and more for [[Cessna Caravan]]s or [[Pilatus PC-12]]s single-engine turboprops: 5 to 10% of a traditional HUD cost albeit it is non-[[conformal projection|conformal]], not matching exactly the outside terrain.<ref>{{cite news |url= https://www.ainonline.com/aviation-news/general-aviation/2018-08-24/hud-rest-us |title= A HUD For the Rest of Us by |author= Matt Thurber |date= August 24, 2018 |work= AIN online}}</ref>
Flight data from a [[tablet computer]] can be projected on the $1,800 Epic Optix Eagle 1 HUD.<ref>{{cite news |url= https://www.ainonline.com/aviation-news/business-aviation/2018-12-26/huds-you |title= This HUD's For You |author= Matt Thurber |date= December 26, 2018 |work= AIN online}}</ref>

=== Enhanced flight vision systems ===
{{main|Enhanced flight vision system}}
{{overly detailed|section|date=August 2018}}
[[File:F-15E LANTIRN IR HUD image.jpg|thumb|[[Thermal image]] viewed through a head-up display]]
In more advanced systems, such as the US [[Federal Aviation Administration]] (FAA)-labeled 'Enhanced Flight Vision System',<ref name=14449-45>[http://www.regulations.gov/#!documentDetail;D=FAA-2003-14449-0045 U.S. DOT/FAA – Final Rule: Enhanced Flight Vision Systems] www.regulations.gov</ref> a real-world visual image can be overlaid onto the combiner. Typically an [[infrared]] camera (either single or multi-band) is installed in the nose of the aircraft to display a conformed image to the pilot. "EVS Enhanced Vision System" is an industry-accepted term which the FAA decided not to use because "the FAA believes [it] could be confused with the system definition and operational concept found in 91.175(l) and (m)"<ref name="14449-45" /> In one EVS installation, the camera is actually installed at the top of the vertical stabilizer rather than "as close as practical to the pilots eye position". When used with a HUD however, the camera must be mounted as close as possible to the pilots eye point as the image is expected to "overlay" the real world as the pilot looks through the combiner.

"Registration", or the accurate overlay of the EVS image with the real world image, is one feature closely examined by authorities prior to approval of a HUD based EVS. This is because of the importance of the HUD matching the real world and therefore being able to provide accurate data rather than misleading information.

While the EVS display can greatly help, the FAA has only relaxed operating regulations<ref>14 CFR Part 91.175 change 281 "Takeoff and Landing under IFR"</ref> so an aircraft with EVS can perform a [[Instrument Landing System#ILS categories|CATEGORY I approach to CATEGORY II minimums]]. In all other cases the flight crew must comply with all "unaided" visual restrictions. (For example, if the runway visibility is restricted because of fog, even though EVS may provide a clear visual image it is not appropriate (or legal) to maneuver the aircraft using only the EVS below 100 feet above ground level.)

=== Synthetic vision systems ===
{{main|Synthetic vision system}}
{{overly detailed|section|date=August 2018}}
[[File:Synthetic Vision.JPG|thumb|A synthetic vision system display (Honeywell)]]

HUD systems are also being designed to display a synthetic vision system (SVS) graphic image, which uses high precision navigation, attitude, altitude and terrain databases to create realistic and intuitive views of the outside world.<ref>{{cite web|url=http://www.faa.gov/aircraft/air_cert/design_approvals/transport/media/Pt23ApproachSlides.pdf |title=Slide 1 |access-date=2009-10-02 |archive-url = https://web.archive.org/web/20080309093806/http://www.faa.gov/aircraft/air_cert/design_approvals/transport/media/Pt23ApproachSlides.pdf |archive-date = March 9, 2008}}</ref><ref>[http://techreports.larc.nasa.gov/ltrs/PDF/2003/tp/NASA-2003-tp212643.pdf For additional information see Evaluation of Alternate Concepts for Synthetic Vision Flight Displays with Weather-Penetrating Sensor Image Inserts During Simulated Landing Approaches, NASA/TP-2003-212643] {{webarchive|url=https://web.archive.org/web/20041101134641/http://techreports.larc.nasa.gov/ltrs/PDF/2003/tp/NASA-2003-tp212643.pdf |date=2004-11-01 }}</ref><ref>{{cite web|url=http://www.nasa.gov/vision/earth/improvingflight/svs_reno.html |title=No More Flying Blind, NASA |publisher=Nasa.gov |date=2007-11-30 |access-date=2009-10-02}}</ref>

In the 1st SVS head down image shown on the right, immediately visible indicators include the airspeed tape on the left, altitude tape on the right, and turn/bank/slip/skid displays at the top center. The boresight symbol (-v-) is in the center and directly below that is the flight path vector (FPV) symbol (the circle with short wings and a vertical stabilizer.) The horizon line is visible running across the display with a break at the center, and directly to the left are numbers at ±10 degrees with a short line at ±5 degrees (the +5 degree line is easier to see) which, along with the horizon line, show the pitch of the aircraft. Unlike this color depiction of SVS on a head down primary flight display, the SVS displayed on a HUD is monochrome – that is, typically, in shades of green.

The image indicates a wings level aircraft (i.e. the flight path vector symbol is flat relative to the horizon line and there is zero roll on the turn/bank indicator.) Airspeed is 140 knots, altitude is 9,450 feet, heading is 343 degrees (the number below the turn/bank indicator.) Close inspection of the image shows a small purple circle which is displaced from the flight path vector slightly to the lower right. This is the guidance cue coming from the Flight Guidance System. When stabilized on the approach, this purple symbol should be centered ''within'' the FPV.

The terrain is entirely computer generated from a high resolution terrain database.

In some systems, the SVS will calculate the aircraft's current flight path, or possible flight path (based on an aircraft performance model, the aircraft's current energy, and surrounding terrain) and then turn any obstructions red to alert the flight crew. Such a system might have helped prevent the crash of [[American Airlines Flight 965]] into a mountain in December 1995.{{Citation needed|date=November 2008}}

On the left side of the display is an SVS-unique symbol, with the appearance of a purple, diminishing sideways ladder, and which continues on the right of the display. The two lines define a "tunnel in the sky". This symbol defines the desired trajectory of the aircraft in three dimensions. For example, if the pilot had selected an airport to the left, then this symbol would curve off to the left and down. If the pilot keeps the flight path vector alongside the trajectory symbol, the craft will fly the optimum path. This path would be based on information stored in the Flight Management System's database and would show the FAA-approved approach for that airport.

The tunnel in the sky can also greatly assist the pilot when more precise four-dimensional flying is required, such as the decreased vertical or horizontal clearance requirements of [[Required Navigation Performance]] (RNP.) Under such conditions the pilot is given a graphical depiction of where the aircraft should be and where it should be going rather than the pilot having to mentally integrate altitude, airspeed, heading, energy and longitude and latitude to correctly fly the aircraft.<ref>{{cite web|url=http://www.faa.gov/aircraft/air_cert/design_approvals/transport/media/PathwayintheSky-RLN.pdf |title=PowerPoint Presentation |access-date=2009-10-02 |archive-url = https://web.archive.org/web/20080309093816/http://www.faa.gov/aircraft/air_cert/design_approvals/transport/media/PathwayintheSky-RLN.pdf |archive-date = March 9, 2008}}</ref>

== Tanks ==
{{Update|section|date=December 2023}}
In mid-2017, the [[Israel Defense Forces]] will begin trials of [[Elbit]]'s Iron Vision, the world's first helmet-mounted head-up display for tanks. Israel's Elbit, which developed the helmet-mounted display system for the [[F-35]], plans Iron Vision to use a number of externally mounted cameras to project the 360° view of a tank's surroundings onto the helmet-mounted visors of its crew members. This allows the crew members to stay inside the tank, without having to open the hatches to see outside.<ref name="janes.com">[http://www.janes.com/article/69059/idf-to-trial-elbit-s-ironvision-in-merkava-mbt IDF to trial Elbit's IronVision in Merkava MBT] Peter Felstead, Tel Aviv - IHS Jane's Defence Weekly, 27 March 2017</ref>

A program announced in 2025 by a collaboration of Patria Technologies and Distance Technologies aims to place the head-up display on the windshield of vehicles, so as to not require a helmet. The program also intends on using AI to aid in data display and processing.<ref>{{Cite web |title=Patria, Distance Partner on Mixed Reality Heads-Up Display for 6x6 Vehicles |url=https://www.defensemirror.com/news/39073/Patria__Distance_Partner_on_Mixed_Reality_Heads_Up_Display_for_6x6_Vehicles |access-date=2025-04-11 |website=www.defensemirror.com}}</ref>

== Automobiles ==
{{Main|Automotive head-up display}}
[[File:E60hud.JPG|thumb|HUD in a [[BMW E60]]]]
[[File:Head Up Display of a Toyota Prius.JPG|thumb|The green arrow on the windshield near the top of this picture is a Head-Up Display on a 2013 [[Toyota Prius]]. It toggles between the [[Global Positioning System|GPS]] navigation instruction arrow and the speedometer. The arrow is animated to appear scrolling forward as the car approaches the turn. The image is projected without any kind of glass combiner.]]

These displays are becoming increasingly available in production cars, and usually offer [[speedometer]], [[tachometer]], and [[Global Positioning System#Navigation equations|navigation system]] displays. [[Automotive night vision|Night vision]] information is also displayed via HUD on certain automobiles. In contrast to most HUDs found in aircraft, automotive head-up displays are not parallax-free. The display may not be visible to a driver wearing sunglasses with polarised lenses.

Add-on HUD systems also exist, projecting the display onto a glass combiner mounted above or below the windshield, or using the windshield itself as the combiner.

The first in-car HUD was developed by General Motors Corporation in 1999 with the function of displaying the navigation service in front of the driver's line of sight. Moving into 2010, AR technology was introduced and combined with the existing in-car HUD. Based on this technology, the navigation service began to be displayed on the windshield of the vehicle.<ref>{{Cite journal |last1=Liang |first1=Yongshi |last2=Zheng |first2=Pai |last3=Xia |first3=Liqiao |date=January 2023 |title=A visual reasoning-based approach for driving experience improvement in the AR-assisted head-up displays |url=https://doi.org/10.1016/j.aei.2023.101888 |journal=Advanced Engineering Informatics |volume=55 |pages=101888 |doi=10.1016/j.aei.2023.101888 |issn=1474-0346|url-access=subscription }}</ref>

In 2012, [[Pioneer Corporation]] introduced a HUD navigation system that replaces the driver-side sun visor and visually overlays animations of conditions ahead, a form of [[augmented reality|augmented reality (AR.)]]<ref>Alabaster, Jay (June 28, 2013). [http://www.computerworld.com/s/article/9240445/Pioneer_launches_car_navigation_with_augmented_reality_heads_up_displays/ "Pioneer launches car navigation with augmented reality, heads-up displays".] ''[[Computerworld]]''.</ref><ref>Ulanoff, Lance (January 11, 2012). [http://mashable.com/2012/01/11/pioneer-ar-heads-up-display/ "Pioneer AR Heads Up Display Augments Your Driving Reality".] ''[[Mashable]]''.</ref> Developed by Pioneer Corporation, AR-HUD became the first aftermarket automotive Head-Up Display to use a direct-to-eye laser beam scanning method, also known as [[virtual retinal display]] (VRD.) AR-HUD's core technology involves a miniature laser beam scanning display developed by MicroVision, Inc.<ref>Freeman, Champion (2014). [http://www.microvision.com/wp-content/uploads/2014/07/OPN_Article.pdf "Madhaven—Scanned Laser Pico-Projectors: Seeing the Big Picture (with a Small Device)". ]</ref>

[[Motorcycle helmet]] HUDs are also commercially available.<ref>{{cite web |url=http://news.motorbiker.org/blogs.nsf/dx/SportVue.htm |title=Mike, Werner. "Test Driving the SportVue Motorcycle HUD". Motorcycles in the Fast Lane. 8 November 2005. Accessed 14 February 2007 |publisher=News.motorbiker.org |access-date=2009-10-02 |url-status=dead |archive-url=https://web.archive.org/web/20100330072505/http://news.motorbiker.org/blogs.nsf/dx/SportVue.htm |archive-date=30 March 2010 }}</ref>

In recent years, it has been argued that conventional HUDs will be replaced by holographic [[Augmented reality|AR]] technologies, such as the ones developed by [[WayRay]] that use [[holographic optical element]]s (HOE.) The HOE allows for a wider field of view while reducing the size of the device and making the solution customizable for any car model.<ref>{{Cite news|url=https://techcrunch.com/2018/01/09/wayrays-ar-in-car-hud-convinced-me-huds-can-be-better/|title=WayRay's AR in-car HUD convinced me HUDs can be better|work=TechCrunch|access-date=2018-10-03|language=en-US}}</ref><ref>{{Cite news|url=https://atelier.bnpparibas/en/smart-city/article/ar-smart-driving-tool-set-replace-gps|title=AR Smart Driving Tool Set to Replace GPS? - L'Atelier BNP Paribas|work=L'Atelier BNP Paribas|access-date=2018-10-03|language=en-US}}</ref> Mercedes Benz introduced an Augmented Reality-based Head Up Display<ref>{{Cite web|url=https://arstechnica.com/cars/2020/07/augmented-reality-heads-up-displays-for-cars-are-finally-a-real-thing/|title = Augmented reality heads-up displays for cars are finally a real thing|date = 10 July 2020}}</ref> while Faurecia invested in an eye gaze and finger controlled head up display.<ref>{{Cite journal|last1=Prabhakar|first1=Gowdham|last2=Ramakrishnan|first2=Aparna|last3=Madan|first3=Modiksha|last4=Murthy|first4=L. R. D.|last5=Sharma|first5=Vinay Krishna|last6=Deshmukh|first6=Sachin|last7=Biswas|first7=Pradipta|title=Interactive gaze and finger controlled HUD for cars|journal=Journal on Multimodal User Interfaces|volume=14|pages=101–121|language=en|doi=10.1007/s12193-019-00316-9|issn=1783-8738|year=2020|s2cid=208261516}}</ref>

== Further development and experimental uses ==
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 [[Rangefinding telemeter|rangefinder]] or squadmate locations to [[infantry]]men. A prototype HUD has also been developed that displays information on the inside of a swimmer's goggles or [[Diving mask#Types of diving mask|of a scuba diver's mask]].<ref>{{cite web|author= Clothier, Julie |url=http://edition.cnn.com/2005/TECH/06/23/spark.goggle/index.html?section=cnn_tech |title=Clothier, Julie. "Smart Goggles Easy on the Eyes". CNN.Com. 27 June 2005. CNN. Accessed 22 February 2007 |publisher=CNN |access-date=2009-10-02}}</ref> HUD systems that project information directly onto the wearer's [[retina]] with a low-powered [[laser]] ([[virtual retinal display]]) are also being tested.<ref>{{cite web|author=Panagiotis Fiambolis |url=http://www.cs.nps.navy.mil/people/faculty/capps/4473/projects/fiambolis/vrd/vrd_full.html |title="Virtual Retinal Display (VRD) Technology". Virtual Retinal Display Technology. Naval Postgraduate School. 13 February 2007 |publisher=Cs.nps.navy.mil |access-date=2009-10-02 |archive-url = https://web.archive.org/web/20080413063727/http://www.cs.nps.navy.mil/people/faculty/capps/4473/projects/fiambolis/vrd/vrd_full.html |archive-date = April 13, 2008}}</ref><ref>{{cite news|last=Lake |first=Matt |url=https://www.nytimes.com/2001/04/26/technology/26HOWW.html |title=Lake, Matt (26 April 2001). "How It Works: Retinal Displays Add a Second Data Layer"|work=[[The New York Times]] |date=2001-04-26 |access-date=2009-10-02}}</ref>

A HUD product developed in 2012 could perform real-time language translation.<ref>Borghino, Dario (29 July 2012). [http://www.gizmag.com/language-translating-glasses/23494/ Augmented reality glasses perform real-time language translation]. ''gizmag''.</ref> In an implementation of an [[Optical head-mounted display]], the [[EyeTap]] product allows superimposed computer-generated graphic files to be displayed on a lens. The [[Google Glass]] was another early product.

== See also ==
* [[Index of aviation articles]]
* [[Acronyms and abbreviations in avionics]]
* [[Augmented reality]]
* [[Eyes-on-the-Road-Benefit]]
* [[HUD (video gaming)]]
* [[Pepper's ghost]] - A stage play version of a similar reflection effect.
* [[Smartglasses]]
* [[Virtual retinal display]]
* [[VR positional tracking]]
* [[Wearable computer]]

== References ==
{{Reflist|30em}}

== External links ==
{{Commons category|Head-up displays}}
* [http://rochesteravionicarchives.co.uk/collection/head-up-display-hud/buccaneer-hud-pilot-s-display-unit Rochester Archives Article—'Buccaneer HUD PDU']
* [http://news.bbc.co.uk/2/hi/technology/4607449.stm BBC Article—'Pacman comes to life virtually']
* [https://doi.org/10.1007%2FBF01142442 'Clinical evaluation of the 'head-up' display of anesthesia data']
* [http://www.flightglobal.com/pdfarchive/view/1968/1968-1%20-%200233.html 'When will the Head-up go Civil' – ''Flight'' 1968 archive]
* [http://rochesteravionicarchives.co.uk/about-us/history-elliott-brothers/ 'Elliott Brothers to BAE SYSTEMS' – a short history of Elliott Brothers]
* [https://web.archive.org/web/20120207155448/http://www.flightglobal.com/pdfarchive/view/1964/1964%20-%202714.html ''Head-up Over the Hills''] – a 1964 ''[[Flight International]]'' article on flying using an early Specto head-up display
* [http://www.latinpost.com/articles/17338/20140717/jaguar-unveils-virtual-widescreen-technology-assist-drivers.htm Jaguar Unveils 'Virtual Widescreen' Technology to Assist Drivers –  Latin Post]
* [https://theaviationgeekclub.com/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/ 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]

{{CarDesign nav}}
{{Flight instruments}}
{{Mixed reality}}
{{emerging technologies|displays=yes}}

[[Category:Vehicle technology]]
[[Category:Avionics]]
[[Category:Aircraft instruments]]
[[Category:Automotive technologies]]
[[Category:Optical devices]]
[[Category:Multimodal interaction]]
[[Category:Mixed reality]]
[[Category:British inventions]]