Editing Head-up display (section)

== 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).