Aircraft head-up display

A head-up instrument display for a vehicle including a light emitting image source for producing imaging illumination and a combiner including a spherical beam-splitter coating for partially reflecting the imaging illumination to produce a virtual image viewable by the vehicle operator without diverting attention from the outside scene. In one embodiment, the head-up instrument display is self-contained with the combiner being rotatable into a stowed position when not in use.

BACKGROUND OF THE INVENTION 
The disclosed invention is generally directed to a head-up virtual image 
display for aircraft avionic instrumentation, and is directed more 
particularly to an aircraft instrument display that includes a combiner 
that is separate from the windshield. 
Instrument displays for the primary aircraft avionic instruments are 
commonly located on an instrument panel located in front of the pilot and 
co-pilot seats and below the windshield. Secondary instrument displays and 
annunciators are sometimes located in an overhead panel above the 
windshield. 
Depending on flying conditions, an aircraft pilot can be continually 
alternating between scanning the outside scene and scanning the aircraft 
avionic instrument displays, which requires the pilot to divert his or her 
eyes from the outside to the instruments and back to the outside. Each 
diversion also requires refocusing of the eyes. 
In recognition of the desirability of head-up viewing of avionics 
instrumentation displays, head-up displays have been utilized in military 
aircraft to (1) provide the pilot with target information from radar, (2) 
provide flight instrumentation information such as an artificial horizon 
or glideslope indicators, and/or (3) produce synthetic land maps for 
flight in low visibility weather conditions or flight at low altitude 
above ground level. However, military head-up displays are complex and 
expensive, typically achieving their intended functions with large fields 
of view, complex optics, and cathode ray tubes. Moreover, aircraft for 
commercial and general aviation do not require the capabilities of 
military head-up displays, which would be particularly expensive for 
smaller general aviation aircraft. 
Head-up displays have also been implemented for automobiles. However, such 
head-up displays generally utilize the windshield or apparatus attached to 
the windshield as the combiner element. The use of the windshield as part 
of the combiner element presents difficulties in the use of coatings for 
improving the reflected image and preventing ghost images. 
SUMMARY OF THE INVENTION 
It would therefore be an advantage to provide an aircraft head-up display 
that is inexpensive and not complex. 
A further advantage would be to provide an aircraft head-up display that 
does not utilize the windshield as the combiner element. 
The foregoing and other advantages are provide by the invention in a 
head-up display for a vehicle that includes an image source for producing 
imaging illumination, and a combiner independent of a vehicle windshield 
for partially reflecting the imaging illumination to produce a virtual 
image of the image source that is viewable by the operator of the vehicle. 
More particularly, the combiner comprises a spherical beamsplitter for 
providing a magnified virtual image.

DETAILED DESCRIPTION 
In the following detailed description and in the several figures of the 
drawing, like elements are identified with like reference numerals. 
The subject disclosure is directed to a head-up display system for a 
vehicle that is advantageously utilized in commercial and general aviation 
aircraft, and will be discussed primarily in that context. However, it 
should be appreciated from the following discussion that the head-up 
display system of the invention can be utilized in other vehicles. 
Referring now to FIGS. 1 and 2, shown therein is a head-up display system 
10 that includes a light emitting image source 11 for providing imaging 
illumination to a planar fold mirror 13, which reflects the imaging 
illumination to a combiner 15. 
By way of example, the image source 11 comprises a high intensity vacuum 
fluorescent display (VFD), for example, such as those commercially 
available from Futaba Corporation of America, Plymouth, MI. Also, the 
image source 11 can be a segmented or matrix addressable liquid crystal 
display (LCD), which for example can be obtained from Seiko of Japan. 
More particularly as to an image source 11 comprising an LCD, it would 
present negative graphics if directly viewed, wherein most of the display 
would dark and only the segments or pixels comprising the desired display 
indicia would "on" or transparent. The LCD would include a backlighting 
scheme, an incandescent bulb and collimating elements for example, whereby 
light would be transmitted substantially only by the segments or pixels 
that are on or transparent. Preferably, the illumination elements would be 
designed so that in combination with the combiner 15 the image of the bulb 
filament would nominally re-image at the center of the eye box (which is 
the region in space where the pilot's eyes would located to view the 
virtual image). Depending on the desired eye box size, the LCD could 
include a diffuser, for example a thin ground glass layer, on the image 
side of the LCD for providing a slight amount of diffusion. 
Also as to an LCD image source, such displays emit polarized light and 
preferably would be configured to emit P-polarized light so that polarized 
sunglasses, which are designed to block S-polarized light, can be used by 
the pilot without substantially blocking the imaging illumination 
reflected from the combiner 15. 
Since the head-up display system 11 includes an even number of reflective 
elements in the projection path, the image source 11 is configured to be 
oriented the same as the virtual image when viewed directly; i.e., the 
image source is not a mirror image of the virtual image. 
The image source 11 can be configured to display one or more instrument 
readouts, for example selectively pursuant to appropriate control inputs 
by the pilot. FIG. 3 illustrates an example of a graphical layout for an 
image source 11 that includes segmented elements for the instrument 
readout, and fully formed characters and symbols for readout units and 
annunciators. 
The combiner 15 is an off-axis, spherical beam splitter that partially 
reflects the incident imaging illumination from the fold mirror 13. The 
combiner includes an appropriate substrate, such as optical grade glass, 
which includes a concave surface 15a and a convex surface 15b having 
substantially the same radius. The concave side 15a of the combiner 15 
receives the incident imaging illumination, at an incidence angle of about 
12 degrees relative to normal for example, and comprises a beam splitter 
coating that provides about 70% transmission, for example. By way of 
specific examples, the beam splitter coating on the concave incident side 
15a can be a multi-layer metallic coating or a multi-layer dielectric 
coating. The convex side 15b of the combiner includes an anti-reflection 
coating to reduce ghost imaging. 
The beamsplitter coating on the concave incident side 15a is preferably 
tailored to P-polarization for a randomly polarized image source as well 
as for a P-polarized LCD, so that the use of polarizing sunglasses does 
not substantially block the reflected imaging illumination. 
The concave incident side 15a and the convex side 15b of the combiner 15 
have substantially the same radius so that the combiner introduces very 
little distortion in the illumination transmitted therethrough, while 
magnifying the illumination reflected by the concave incident side 15a. 
Thus, the combiner 15 produces a magnified virtual image of the image 
source 11 without substantially distorting the portion of the outside 
scene viewed through the combiner 15. 
Referring now to FIG. 4, shown therein is a further aircraft head-up 
display system 20 which is similar to the head-up display system 10 of 
FIG. 1, except that the imaging illumination provided by an image source 
11' is relayed to the fold mirror 13 by a fold mirror 12 that is close to 
the image source 11'. The system of FIG. 4 includes an odd number of 
reflection elements in the projection path, and would be utilized with an 
image source 111 that is the mirror image of the intended virtual image 
when viewed directly. 
Referring now to FIGS. 5-7, shown therein is a self-contained head-up up 
display 30 that utilizes the head-up display system 10 schematically shown 
in FIG. 1. The display system elements are mounted in a housing 111 which 
is covered by a fixed top cover 113 at the front end 111a thereof, and a 
slidable top cover 115 that is adapted to cover the remaining portion of 
the housing 111 and is slidable over the fixed top cover 113. 
The image source 11 is mounted at the front end 111a beneath the fixed top 
cover 113, and can be electrically coupled to circuitry on a circuit board 
117 that includes appropriate driver circuitry, for example. Electrical 
connections are made via a connector 123 which is accessible on the 
outside of the housing 111. 
The fold mirror 13 is mounted at the back end 111b of the housing 111, and 
an optical baffle 121 is interposed in the optical path between the image 
source 11 and the fold mirror 13 to prevent ambiant stray illumination 
(e.g., sunlight) from reaching the image source 11 and also for preventing 
stray image source illumination from reaching the fold mirror 13. 
While not shown, it should be appreciated that dust protection can be 
provided for the image source 11, for example, an off-axis window that is 
tilted so as to direct reflections to the top or bottom portions of the 
housing 111. Also for protection of the mirror surface of the fold mirror 
13, a second surface mirror could be utilized instead of the first surface 
mirror depicted in FIG. 5. 
The combiner 15 is secured to a bracket 119 that is pivotally mounted in 
the housing for rotation about a laterally oriented axis. A rotation lever 
121 on the outside of the housing is connected to the bracket 119 for 
rotation of the bracket and combiner about a laterally extending axis. A 
locking mechanism including a locking knob 122 selectively retains the 
bracket and combiner 15 in a fixed position. 
The bracket 119, the housing 111, and the slidable top cover 115 are 
configured to allow the combiner 15 to be rotated into a generally upright 
position for use, and to allow the combiner to be rotated into the housing 
111 and covered by the slidable top cover 115 when not in use. 
The self-contained head-up display 30 advantageously permits the combiner 
15 to be folded and enclosed when not in use, for example during certain 
flying conditions or when the aircraft is not being used. Folding and 
enclosure of the combiner 15 provides for protection of the delicate 
coatings thereon, and moreover stows it out of the way when use of the 
head-up display is not desired. Further, the self-contained configuration 
allows for easy installation of the head-up display in many types of 
aircraft upon original manufacture or by retrofit. 
The foregoing has been a disclosure of a head-up display for aircraft that 
is simple, relatively inexpensive, and provides the capability of viewing 
aircraft instrument readouts while viewing the outside scene. 
Although the foregoing discussion has been in the context of aircraft, it 
should be appreciated that the disclosed head-up display system can be 
utilized in other vehicles wherein a windshield or similar protective 
device is not utilized as the combiner. 
Although the foregoing has been a description and illustration of specific 
embodiments of the invention, various modifications and changes thereto 
can be made by persons skilled in the art without departing from the scope 
and spirit of the invention as defined by the following claims.