Head Up Display (HUD)
Head Up Display (HUD)
Head Up Display (HUD)
DSL737
Head-Up Display (HUD)
• A transparent display that presents the data without blocking the users view.
• Overlays information onto a pilot’s field of view through a combination of special projection
technology, optics and displays.
• The HUDs are transparent and are thus able to provide user with a view of both the real
world and overlaid information projected onto the display surface.
• Pilot's eyes do not need to refocus to view outside after looking at the optically nearer
instruments.
• Although, initially developed for military aviation, HUDs are now used in commercial aircraft,
automobiles and other professional applications.
Operational Benefits
Projection Unit
This unit produces an image where the light is collimated, i.e. the focal point is perceived to be
at infinity. It consists of
• Optical collimator
• convex lens or concave mirror
• Screen (CRT, LED or LCD display at its focus).
Basic Elements of HUD
Monocular FOV
• The solid angle subtended at the eye by the clear apertures of the HUD optics from a fixed
eye position.
• The monocular FOV size and shape may change as a function of eye position within the
HUD eyebox.
Optical Configurations in HUD
• The most significant change in the optical
configurations of HUD has been the method
Window/canopy
of collimation. Systems that collimate by:
Refraction Combining
glass
Reflection
Diffraction.
Collimator
• The move towards more complex (and
expensive) reflective collimation systems has CRT
resulted in larger display FOV which expand
the usefulness of HUDs as full-time primary
flight references.
Refractive Optical Systems
• The paralleled rays from CRT are projected onto a combiner and directed to the pilot’s eyes.
• Advantage: Pilot is able to move his head while still being able to see the displayed image
on the combining glass.
• Most useful in air combat
Refractive Optical Systems
• In this optical configuration, the CRT image is collimated by a combination of refractive lens
elements designed to provide a highly accurate display over a moderate display field of view.
• The vertical instantaneous FOV can be increased by adding a second flat combiner
glass displaced vertically above and parallel with the first.
Reflective Optical Systems
• The technique is based on reflection of the CRT-rays in the combiner where collimating
lens is integrated in a curved, transparent combiner.
• The curved combiner (off-axis mirror) is bracket mounted to fold down directly in the pilot’s
field of view and it displays a 30-degree wide view.
• Compared to refractive HUD, this requires pilot to maintain his head in a fairly fixed position.
• Fabricating an off-axis optical component requires grinding a larger surface than usually
needed.
Reflective Optical Systems
• Use of Reflective Optical Systems increases the total and instantaneous FOVs.
• As in the classical refractive optical system, the displayed image is generated on a
small CRT, about 3 in. in diameter.
• The reflective optics can be thought of as two distinct optical subsystems.
1. Relay lens assembly designed to re-image and pre-aberrate the CRT image source to
an intermediate aerial image, located at one focal length from the optically powered
combiner/collimator element.
Reflective Optical Systems: Raytrace
• The Combiner/collimator element re-
images and collimates the intermediate aerial
image for viewing by the pilot.
• As in the refractive systems, the pilot’s eyes
focus at optical infinity, looking through the
combiner to see the virtual image.
• To prevent the pilot’s head from blocking
rays from the relay lens to the combiner,
the combiner is tilted off-axis with respect
to the axial chief ray from the relay lens
assembly.
• The combiner off-axis angle, although
required for image viewing reasons,
significantly increases the optical aberrations
within the system, which must be
compensated in the relay lens to have a well-
correlated, accurate virtual display.
Refractive vs. Reflective HUD
• The disadvantage of the reflective HUD is consequently the great level of complexity
involved in producing the curved combiner in terms of material and engineering.
• The great advantage is the capability of enhanced symbol brightness, minimized light
attenuation from the external visual scene and the possibility to save room in the
cockpit, as the collimating lens is unnecessary.
• These are the main reasons why the reflective HUD is the most common type in civil
aviation industry.
• The reflective HUD is also controlled via a control panel.
Typical HUD Fields of View
Type of HUD based upon mounting
1. Fixed HUDs
2. Helmet Mounted HUDs