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A head-up display or heads-up display,[1] also known as a HUD (/hʌd/), 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 (mostly professional) applications.
A typical HUD contains three primary components: a projector unit, a combiner, and a video generation computer.[2]

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 (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 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 (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 .

HUDs evolved from the reflector sight, a pre-World War II parallax-free optical sight technology for military fighter aircraft.[3] 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 (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.[4] 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]

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