Holographic optical element for instrument panel displays

A holographic optical system includes a hologram which is located at an instrument panel so as to reflect or transmit an image from a source located off the panel. The image is redirected only to an area at which the viewer may observe the image and is not directed to other areas so as to prevent unwanted reflections and glare such as where the instrument panel is within an aircraft cockpit.

BACKGROUND OF THE INVENTION 
This invention relates to a display system. In a situation such as in an 
aircraft, where space is limited, there is a restriction on the number of 
instruments and displays that will fit in a given amount of panel space. 
For example, it would be desirable to locate an artificial horizon display 
near other navigational and control displays in fighter aircraft at the 
top of the instrument panel. However, in most modern fighters, the 
artificial horizon is placed near the bottom of the instrument panel, and 
the pilot, with his oxygen mask on, must alter his direction of gaze by 
more than 30.degree. to check the instrument. Problems of size and space 
restrictions for instruments are heightened by the need for redundancy of 
critical instruments. Due to the tight space restrictions, the panel 
layouts are not easily modified when an aircraft is reconfigured. 
Conventional cockpit displays are typically of two 
types--electro-mechanical and cathode ray tube (CRT)--although liquid 
crystal displays, light emitting diode displays, and electroluminescent 
displays are also used. Beyond the space restrictions, other problems of 
conventional instrument displays include the production of canopy glare 
from instrument lighting which is distracting to the pilot. Conventional 
electro-mechanical multi-instrument panels are complex and the servicing 
of such panels is time-consuming and expensive. Electro-mechanical 
instruments provide a limited symbology, i.e. they are limited to 
alphanumeric characters or mechanical analog dials and read-outs. 
To increase the amount of information displayed on the instrument panel 
without increasing the space requirements of the instruments, it may be 
desirable to selectively project an image, such as an image of an 
artificial horizon, onto a portion of the instrument panel which is 
occupied by existing switches, key pads, controls, and other non-display 
components. However, such light projection can be impractical because of 
the power required to compete with direct sunlight, and since normal 
reflection would introduce light into the cockpit area as unwanted 
reflections and glare, including reflections off of the cockpit canopy, a 
particularly significant problem during night-time flying. 
SUMMARY OF THE INVENTION 
In the present invention, information display on the instrument panel is 
obtained by reflecting from or transmitting through a display screen 
symbolic information projected from a source, such as a cathode ray tube 
(CRT), into the pilot's eyes. 
For a reflection type display screen, the pilot sees the additional 
information as being overlaid on the existing instruments, controls and 
panel. The reflected symbols are designed to be sufficiently distinct from 
those already in use on the panel that they would not be confused with 
each other. In addition, the medium of the display is located in such a 
way as not to interfere with the normal operation of the instruments or 
controls over which it is superimposed. 
The display screen can be in the form of a medium which can be applied over 
existing controls or instruments on an instrument panel, and cut out, if 
necessary, so as to permit knobs or touch keys to protrude through the 
screen. The superior surfaces of the knobs or keys can be covered in the 
same medium, so as to substantially fill the area over which it has been 
applied. The medium is optically clear, so as not to interfere with the 
viewing of the controls or instruments over which it has been applied. 
The display screen medium behaves as a reflector or transmitter of high 
efficiency for a specific, narrow bandwidth of light and may be optically 
clear to all other visible light. The diffuse reflecting or transmitting 
properties of the medium are optimized such that an observer at a specific 
distance from the medium is able to see the display image only if his eyes 
are within a predetermined area. This property of the display prevents its 
luminence from being directly radiated into areas where it is not desired. 
The medium is referred to herein as the projection or display screen. 
Further objects, features, and advantages of the invention will be apparent 
from the following detailed description when taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The typical layout of the display system of the invention is shown in FIG. 
1. The display system consists of a projector 1, projection screen 2, and 
the necessary electronics 3 to support the projector 1. The projection 
screen 2 is mounted in a position which is overlaying a section of the 
instrument panel 4, and which is shown magnified in FIG. 3. It can be seen 
that the projection screen 2 is not perpendicular to the projector 1 nor 
to the observer 5. 
The instrument panel 4 is a device that depicts a large amount of symbolic 
information about the status and situation of the aircraft to the pilot. 
The instrument panel 4 also contains a number of devices that allow the 
pilot to present information to the aircraft, such as display selection, 
altimeter settings, weapons set-up, engine controls, etc. Input to the 
aircraft is accomplished by means of various knobs, switches, and buttons. 
The instruments are arranged so that they are functionally grouped, and 
all of the instruments must be visible from the "design eye," i.e., the 
area of the normal head position and the normal range of head movements of 
all pilots who will fly the aircraft. The knobs, switches, and buttons 
must all be reachable by the pilot without moving from his normal flying 
position, with the exception of those items used only infrequently, or 
during start-up or shut-down. 
As shown in FIG. 2, the projector 1 consists of a 1-1.5 inch diameter 
cathode ray tube (CRT) 50 with a phosphor coating 52 of narrow spectral 
emission such as P-43, and a relay lens assembly 29. The relay lens 
assembly 29 consists of conventional optical elements but may be 
implemented using holographic optical elements the properties of which are 
well known to those skilled in the art of holography. The optical elements 
may be tilted and decentered to accommodate for the aberations that arise 
from the fact that the projector is not perpendicular to the projection 
screen. The aperture of the relay lens assembly 29 is preferably two 
inches in diameter, and the focal length is preferably such as to yield a 
5X magnification of the CRT image when focused on the projection screen 2. 
The projection screen 2 is implemented as a holographic optical element 
sandwiched between two layers of fine Lexan, polymethyl methacrylate 
(PMMA), Plexiglas, or glass, or, conversely, embossed on a suitable 
plastic such as Mylar polyester, or made from a holographic material such 
as Polaroid DMP 128 (T.M. Reg'd), all of which are techniques well known 
in the art. The media for making the master holographs may be dichromated 
gelatin, while the techniques for making the holographic optical element 
(HOE) are well-known two-step processes for generating image-plane 
holograms. The source of light for generating the holograms is a green 
laser with its spectral output in the vicinity of the spectral emission of 
P-43 phosphor. Any small difference in matching the wavelengths can be 
compensated for by adjusting the geometry of the exposures, a technique 
which is well known in the art. 
FIG. 3 shows more clearly how the resulting HOE, which makes up the 
projection screen, is superimposed over the existing features of the 
instrument panel, such as gauges 26, switches 25, and the panel itself 24. 
Since the switches 25 extend outwardly from the panel, the HOE is cut or 
punched out to permit said extension through the HOE, and the cut out 
portions can be affixed to the surface of said switches, so as to provide 
a more or less continuous surface when seen from the optimal viewing area. 
Knobs, or other such narrow features 27 would simply protrude through the 
HOE and would not be covered with a holographic "cap". 
FIG. 4 illustrates the geometry required to construct the transmission 
master hologram. The object to be holographed is an optical diffuser plate 
6 which is back-lit with a wave front 7 propagating in a direction 
indicated by the arrow 8. This will cause the diffuse light 9 to fall 
primarily on the film plate 10, which is dichromated gelatin or other such 
photographic material as is usual for holography. The plate 10 is also 
illuminated by a reference wave front 11 propagating in the direction 
indicated by the arrow 12. After suitable exposure, the plate 10 is 
appropriately processed to yield the transmission master hologram 13. 
FIG. 5 illustrates the geometry required to construct the reflection copy 
hologram which will comprise the projection screen. The transmission 
master hologram 13 is illuminated with wave front 14 which is the 
conjugate of wave front 11. Wave front 14 is propagating in the direction 
indicated by the arrow 15. This conjugate illumination accomplishes the 
spatially undistorted projection of a real image of the diffuser 16. Film 
plate 17 is placed adjacent to the real image of the diffuser 16 and is 
illuminated by a reference wave front 18 which propagates in the direction 
of arrow 19. The reference wave front 18 is converged to the point 20 by a 
lens 21. Point 20 relative to the film plate 17 represents the relative 
location of the projector 1 with respect to the projection screen 2 in 
FIG. 1. 
The transmission master hologram is masked by opaque material 22, 23 in a 
way such that the unmasked area corresponds to the observer's optimal 
viewing area(s). After suitable exposure and subsequent processing, the 
film plate 17 becomes a reflection copy hologram which comprises the 
projection screen of the display system. In commercial production, the 
plate 17 may be produced by holographic, printing or embossing techniques 
which are well-known in the art. 
When the reflection copy hologram is used as the projection screen 2, in 
the geometry illustrated in FIG. 1, it will have the following properties: 
An image comprised of symbols generated on the face of the CRT will be 
projected onto the projection screen; an observer placing his eyes in the 
viewing area will see the image, magnified 5X, coplanar or not with the 
projection screen; the optimal viewing area will be uniformly illuminated 
with the light reflecting from the projection screen, by virtue of the 
diffusing element; no light will be reflected into areas other than the 
optimum viewing area; and the projection screen will reflect only the 
light from the CRT, and will appear otherwise substantially transparent, 
so that symbols and features beneath the projection screen will be clearly 
visible through the projection screen. 
The instrument panel 4 may alternately employ a transmission hologram 
rather than the above-described reflection hologram. The transmission 
holographic instrument panel is analogous to the hologram of the 
reflection variety, except for the location of the CRT projector. As shown 
in FIG. 6, the projector 101 is located behind the instrument panel 104, 
facing the "design eye" of the observer 105. The projector 101 is 
supported by electronics 103 FIG. 6 illustrates the use of a transmission 
hologram 102 where the projection and viewing axes are not coaxial. The 
light projected from the CRT projector 101 is focused onto the hologram 
102 of the holographic instrument panel 104 by means of the relay lens 
assembly 129. The hologram 102 is unimpeded by other features of the 
instrument panel and causes all of the light emanating from the relay lens 
129 to be directed toward the design eye of the observer 115. This is a 
task that requires, among other things, that the HOE act as a 
semi-diffuser, since the design eye is a virtual aperture rather than a 
point in space. 
Achromatization of the HOE display screens may be accomplished by varying 
the thickness of dichromated gelatin emulsion and using other well known 
practices. This may be done if it is necessary to display full color 
images from a full color projector or from a plurality of different 
monochrome projectors of different colors. 
FIG. 7 shows a holographic display for an instrument panel which is 
illuminated by three projectors 101a, 101b and 101c, each presenting a 
different primary color although it is apparent that a single multicolor 
CRT projector may also be used. In practice the projectors 101a, 101b, and 
101c should be positioned out of the line of sight of the observer 105, 
otherwise the images on the projectors may be seen directly through the 
holograms. This happens because the holograms, as a practical matter, are 
not 100% efficient at redirecting the light focused upon them. As a 
consequence, some of the light from the projectors will pass through the 
holographic instrument panel as though the holograms were not there, and 
interfere with the viewing of the intended image. The positioning of the 
projectors and relay lenses out of the line of sight can be done in many 
ways, including the use of mirrors. FIGS. 7-9 show only that the 
projectors are located to one side of the line of sight. 
If a hologram is made from three beams, a reference and two object beams, 
then upon viewing, both objects will be visible. In like manner, the light 
from two separately located projectors could be directed toward the design 
eye by a single hologram. FIG. 8 shows how such an arrangement can be used 
to satisfy the requirement for redundancy. Two projectors 101 of identical 
colors simultaneously have their images focused onto the holographic 
instrument panel 104 by their respective relay lenses. With suitable care, 
the images will overlap and appear as a single image, if both projectors 
are displaying identical images. If one of the projectors should fail, 
then the image from the other one would remain, and the only effect 
noticed by the viewer is that the brightness is diminished to half of the 
original. The above principle could be used to provide redundancy for each 
of the three primary colors, such as are projected by the projectors 101a, 
101b, and 101c of FIG. 7. It should be apparent that the use of redundant 
projectors would apply equally to both transmission and reflection 
holograms. 
It would be possible to have the holographic instrument panel present 
images which appear to be three-dimensional. This can be done in several 
ways. As shown in FIG. 9, the most effective is to have a liquid crystal 
lightgate or shutter 154 positioned in front of each eye 105, by means of 
a pair of goggles. Signals from a computer 156 alternately render each 
crystal opaque, then clear. The two gates or shutters alternate exactly 
out of phase with each other, so that when one eye can see, the other can 
not, and vice versa. This alternation is synchronized with the times at 
which the computer 156 generates the images on the CRT projector, so that 
while the left eye was viewing the CRTs would be showing the image 
appropriate for the left eye and so forth. The system could be made to run 
at 60 Hz or faster, so that the viewer would not be aware of the flicker. 
Rather the observer would simply see a different image in each eye, which 
would be interpreted by the brain as normal stereopsis. Suitable systems 
for carrying out this function are commercially available, as described in 
the article entitled "3-D TV", Popular Science, June 1988, pp. 58-63, 110. 
In the event that the system should fail, it can be so designed that the 
liquid crystal shutters fail in the clear mode, and if that should happen, 
the computer can be instructed to present only one image, as with all the 
other examples described above. 
FIG. 10 illustrates the geometry required to construct the holographic 
optical element in a transmission mode where the projection and viewing 
axes are not coaxial. The source of light for generating the holograms is 
a green laser with its spectral output in the near vicinity of the 
spectral emission of P-43 phosphor. Any small differences in spectra can 
be compensated for by a small adjustment in the geometry of the exposures. 
All other practices that are well-known to those skilled in the art of 
holography such as vibration isolation, path length matching, liquid 
gates, etc. are assumed. 
The object to be holographed is an optical diffuser plate 159 which is 
back-lit with a wave front 160 propagating in a direction indicated by an 
arrow 161. When the wave front 160 passes through the diffuser 159, the 
light appears to emanate from many point sources, as depicted by 162. The 
now diffuse wave front 162 is windowed and/or apodized by a film 
transparency 163 and propagated towards a lens 164 as indicated by an 
arrow 165. The lens 164 relays the image of the apodized and windowed 
diffuser via an object wave front 166 through a film plate (for example, 
of dichromated gelatin) 167 and an observer's viewing window 168. The 
focal length of the lens 164 and the location of the diffuser 159 and the 
film transparency 163 are dependent on the required shape and location of 
the observer's viewing window 168 and may be calculated using well-known 
optics formulae that relate the relaying of images through lenses. The 
film plate 167 must be entirely located in a triangular area bounded by a 
line 169, a line 170, and the lens 164. The lines 169 and 170 are lines of 
convergence from the perimeter of the lens 164. 
A reference wave front 171 is made to appear to emanate from a point 172 by 
the focusing properties of a lens 173 which is illuminated by a wave front 
174 propagating in a direction indicated by an arrow 175. The reference 
wave front 171 propagates in the direction indicated by an arrow 176 and 
passes through the film plate 167, totally illuminating the film plate 
167. The reference wave front 171 must not pass through the lens 164 
before it illuminates the film plate 167. The interference pattern formed 
by the reference wave front 171 and the object wave front 166 exposes the 
film plate 167. After suitable exposure time and subsequent processing, 
the film plate 167 becomes a transmission hologram which comprises the 
projection screen 102 of the display system of FIG. 6. The wave front 174 
and the wave front 160 are derived from a common green laser source and 
thus maintain a common phase relationship to each other. 
Multiple viewing windows may be generated by having a plurality of 
transparent areas on the film transparency 163 or a multiplicity of 
diffuser/transparency/lens arrangements. Multiple exposures of the film 
plate 167 using different diffuser/transparency/lens arrangements while 
retaining the same reference beam would also yield multiple viewing 
windows. 
The film transparency 163 is made of a transparent substrate such as 
plastic or glass and is covered with a material that blocks the passage of 
light in varying amounts. The pattern of the light blocking material on 
the film transparency 163 is distributed in accordance with the 
requirements of the observer viewing window 68 as seen through the lens 
164. The relationship of the pattern on the film transparency 163 to the 
shape and intensity profile requirements of the observer viewing window 
168 as seen through the lens 164 is related using well-known formulae for 
the relaying of images through lenses. 
It should be noted that in using the recording method shown in FIG. 10, the 
geometry may be modified such that the observer viewing window 168 and the 
reference wave front 171 may have angles of orientation with respect to 
the film plate 167 which can be varied to allow for off axis projection 
angles or on axis observer viewing windows. The limitation of this 
recording geometry is that having an on axis observer viewing window and 
on axis projection is not possible simultaneously. 
A typical layout of the display system for a transmission hologram where 
the projection and viewing axes are coaxial is shown in FIG. 11. The 
display system comprises a projector 177, a projection screen 178, and the 
necessary electronics 179, to support the projector 177. It can be seen 
that the projection screen 178 is perpendicular to the observer 180 as 
well as to the projector 177. 
FIG. 12 illustrates the geometry required to construct the holographic 
optical element in a transmission hologram where the projection and 
viewing axes are coaxial. The source of light for generating the holograms 
is a green laser with its spectral output in the near vicinity of the 
spectral emission of P-43 phosphor. Any small difference in spectra can be 
compensated for by a small adjustment in the geometry of the exposures. 
All other practices that are well-known to those skilled in the art of 
holography, such as vibration isolation, path length matching, etc. are 
assumed. 
The object to be holographed is an optical diffuser plate 181 which is 
back-lit with a wave front 182 propagated in a direction indicated by an 
arrow 183. When the wave front 182 passes through the diffuser plate 181, 
the light appears to emanate from many point sources as depicted by 184. 
The now diffuse wave front 184 is windowed and apodized by a film 
transparency 185 and propagates towards a beam splitter 186 as indicated 
by an arrow 187. The diffuse wave front 184 passes through the beam 
splitter 186 and a lens 202 relays the image of the apodized and windowed 
diffuser via an object wave front 188 through a film plate (dichromated 
gelatin) 189 to an observer's viewing window 190. The focal length of the 
lens 202 and the location of the diffuser 181 and film transparency 185 
are dependent on the required shape and location of observer's viewing 
window 190 and may be calculated using well-known optics formulae that 
relate the relaying of images through lenses. The film plate 189 must be 
located entirely within a triangular area bounded by a line 191, a line 
192, and the lens 202. The lines 191 and 192 are lines of convergence from 
the perimeter of the lens 202. 
A reference wave front 193 is made to appear to emanate from the point 194 
by the focusing properties of a lens 195 which is illuminated by a wave 
front 196 propagating in a direction indicated by an arrow 197. The 
reference wave front 193 propagates in the direction indicated by an arrow 
198, substantially reflects from the beam splitter 186, is redirected in a 
direction indicated by an arrow 199 and passes through the lens 202. By 
the optical combining properties of the beam splitter 186, the reference 
beam 193 is made to appear to be emanating from a direction which is to 
the left of perpendicular to the lens 202. When viewed from the location 
of the film plate 189 due to optical refracting properties of the lens 
202, the distance from which the reference beam appears to be coming is 
longer than the sum of the distances from the film plate 189 to a point 
200 and from the point 200 to the point 194. The distance from which the 
reference beam appears to be coming must match the distance between the 
projector 177 and the display screen 178 in FIG. 10. Knowing the distance 
between the projector 177 and the display screen 178 and knowing the focal 
length of the lens 202 allows the required location of point 194 to be 
easily calculated using well-known formulas that relate conjugate points 
of refracting lenses. 
The reference beam 193 which is reflected towards the lens 202 by the beam 
splitter 186 is refracted through the lens 202 and passes through the film 
plate 189 totally illuminating the film plate 189 as a refracted reference 
wave front 201. The refracted reference wave front 201 must totally 
illuminate the film plate 189. The interference pattern formed by the 
refracted reference wave front 201 and the object wave front 188 exposes 
the film plate 189. After suitable exposure time and subsequent processing 
the film plate 189 becomes a transmission hologram which comprises the 
projection screen 178 of the display system of FIG. 11. The wave front 196 
and the wave front 182 are derived from a common green laser source and 
thus maintain a common phase relationship to each other. 
Multiple viewing windows may be generated by having a plurality of 
transparent areas on the film transparency 185 or a multiplicity of 
diffuser/transparency/lens arrangements. Multiple exposures of the film 
plate 189 using different diffusers/transparency/lens arrangements while 
retaining the same reference beam would also yield multiple viewing 
windows. 
The film transparency 185 is made of a transparent substrate such as 
plastic or glass and is covered with a material that blocks the passage of 
light in varying amounts. The pattern of the light blocking material on 
the film transparency 185 is distributed in accordance with the 
requirements of the observer viewing window 190 as seen through the lens 
202. The relationship of the pattern on the film transparency 185 to the 
shape and intensity profile requirements of the observer viewing window 
190 as seen through the lens 202 is related using well-known formulae for 
the relaying of images through lenses. 
It is understood that the invention is not limited to the particular 
embodiments set forth herein, but embraces such modified forms thereof as 
come within the scope of the following claims.