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I have seen a couple videos of fighter jets and commercial planes with HUD systems and was wondering how do the heads up displays work?

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HUD

The most common form of head-Up Display (HUD) is simply an out-of-sight display screen reflected in a transparent flat sheet of glass in the viewers line of sight out the cockpit front window.

The glass sheet is called a combiner. It combines the HUD information with the view out of the aircraft.

There are coatings for glass (E.g.) which allow for reflecting a monochromatic image while allowing transmission of other colors.

The display is obviously tailored for that specific use. It must avoid obscuring the view. Therefore it mostly consists of minimal light-colored lines and numerals/letters against a dark background. The dark background allows the pilot to see through the reflection to brighter objects outside - it's a bit like the matt black upper surface typically found on many car dashboards which prevents distracting unwanted images of the top of the dash being reflected into the driver's view out of the windscreen.

Here's an example from Boeing enter image description here

A HUD display is visible on the left in the pilot's line of sight. Above it, mounted on the ceiling is the projector unit.

The Projector contains optics to make the pilot's view of the display appear to be at infinity (rather than the few feet away it is in reality). This allows the pilot to read the data while their eyes are focused on far-away real-world objects outside the aircraft (runways, weather, other aircraft etc).

EFVS

Beyond the projection of basic flight instrument data is the use of HUDs for Enhanced Flight Vision Systems (EFVS) which provide a kind of augmented reality display that typically uses external infra-red camera systems to identify landmarks (e.g. runway outlines) that may be obscured to the pilot due to fog etc.

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  • $\begingroup$ Very nice improvement, thanks! $\endgroup$ – TypeIA Jul 7 '14 at 21:11
  • $\begingroup$ How do these systems control for parallax error? Or I assume the ones on commercial flights don't display any info that relies on precise head position? $\endgroup$ – RoboKaren Jul 9 '14 at 3:20
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    $\begingroup$ @RoboKaren: The lens that makes the display appear at infinity happens to correct for parallax error as well. The optical element creates virtual image at infinity and in behaves as if it was really there in all regards. $\endgroup$ – Jan Hudec Jul 13 '14 at 21:02
  • $\begingroup$ You actually don't need a special coating, though it helps; the angle of the glass itself, coupled with a well-lit projection, is enough. The concept and implementation are similar to the "Pepper's Ghost" illusion. $\endgroup$ – KeithS Aug 21 '15 at 19:51
  • $\begingroup$ Thanks for the simple explanation. But I have seen in some cars a glass that reflects all the colors, yet it's transparent. Can you explain why? $\endgroup$ – zagoku Apr 2 '16 at 17:15

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