Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1~19854
Improvemen-ts in or rela-tin~ to Visual Displa"v Apparatus
~
Description
This invention relates to visual display apparatus,
particularly for ground-based flight simulators and
particularly for providing a display covering a wide-angle
field of view. The invention may be used in apparatus
capable of providing either pseudo-collimated or s-tereo-
scopic viewing for a sole pilot or simultaneously for
two pilots.
The apparatus is of the head-coupled area-of-interest
-type, wherein an image is projected upon a screen and is
appropriately changed both according to the simulated craft
orientation
position and angular/and according to the viewer's
instantaneous line of view and is also moved on
the screen to occupy the viewer's field of view.
Apparatus of this type was described in prior patent
-specification Number 1,489,758. Such apparatus provided
an area-of~interest display for a sole observer which was
pseudo-collimated, that is, the same image was projected
for left and right eyes, so as to appear at infinity.
A highly retro-reflective screen material, that is
one by which incident light is reflected within a very
small angle cone with its axis directed towards the incident
light source, is necessary to provide high luminosity of
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the image. This, however, requires that the image is projected along the viewer's
line of view. If this requirement is not observed, incident light is reflected
to the source and not the the viewer's eyes. The present invention provides a
highly retro-reflective screenJ by which incident light is reflected within a
very small angle cone, but the reflection characteristic is modified in the ver-tical planes or in other planes of re1ection so that the reflected light cone
axis is directed downwardly with respect to the line of incident light. This
modified characteristic permits of the projection source being offset above the
viewer's eye positions. The invention permits of offset in arbitrary direction,
but the following discussion is limited to the case of vertical offset.
Accordingly, the invention provides or projectlon apparatus in which
an image is projected upon a retro-reflective screen along a line of incidence
which diverges by a small angle from a viewer's line of view, a screen having a
retro-reflective characteristic such that a reflected beam of light has a cone
axis which diverges by a small angle from the line of incidence or the light
beam, whereby the reflected beam of light substantially coincides with the view-er's line of view, the screen comprising a rear retro-reflective surface of char-
acteristic such that the cone axis of a reflected beam of light is colinear withor along the line of incidence of the light beam and a forward transparent screen
of solid material having a predetermined refractive index and a front or rear
surface having grooves, the depth of which grooves is determined by the predeter-
mined refractive index.
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Short Description of Drawin~s
In order that the invention may readily be carried into
practice, one embodiment will now be described in detail,
by way of example, with reference to the accompanying
drawings, in which:-
Fig. 1 is a diagrammatic perspective view showing apilot seated in relation to a part-spherical screen for
pseudo-collimated viewing of a head-coupled area-of-interest
display;
Fig. 2 is a partial, vertical cross-section view showing
the construction of a retro-reflective screen and diffraction
grating combination having a modified reflection
characteristic; and
Fig. 3 is a partial, vertical cross-section showing a
diffraction grating having a different contour from that in
Fig. 2.
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Description of the ExamFle
The apparatus of Fig. 1 will be described first in
order to illustrate the form of apparatus in which the
presen-t invention may be used with advantage.
Fig. 1 shows in diagrammatic form the apparatus
according to the invention for generating and displaying a
pseudo-collimated area-of~interest view. A pilot 10 wearing
a helmet 12 is seated wi-thin a part-spherical shell having a
retro-reflective inner surface partially represented in Fig. 1
by the concave retro-reflective screen 14. The pilot's line
of vision, for righ-t and left
eyes and for distant viewing, in-tersects the screen at
points 16 and 18, respectively. The field of view for each
eye is centred on the respective one of these two points.
The views displayed are identical for right eye and left eye
but are displaced laterally by the dis-tance between the
points 16 and 18 so that the pilot 10 sees a pseudo-collimated
view, that is to say, the displayed view appears to be at
infinity and not at the distance of the screen 14. The
combined left eye and right eye views will be referred to as
the displayed scene.
The displayedscene depends, in this example, upon the
simulated position of an aircraft during an exercise flight,
the attitude of the aircraft, the pilot's seating position in
the aircraft and the pilot's instantaneous line of view as
determined by the instantaneous orientation of the pilot's
head and helmet. The position of points 16 and 18 on the
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screen 14 and hence the posi-tion o~ the displayed view on
the screen depends only on the pilot's head and helmet
orientation.
The image required is generated by an image generator 20
OI the computer-generated image type and which includes a
frame buffer store 20'. The pilot's head orientation is
sensed by a head orientation sensor 22, which is fixedly
mounted within the simulated aircraft cockpit in a mounting 24.
The displayed view is projected onto the screen 14, centred
in the appropriate locations as two raster-scanned images,
the line scan apparatus being cockpit-mounted and the fra~e
scan apparatus being mounted on the helmet 12. Line scan may
be either across the screen 14 or up or down. In the present
example, the projected scan line upon th6 screen and the line
between the pilot's two eyes lie in the same plane. The frame
scan is orthogonal thereto. Thus, when the pilot's head is
upright line scan is horizontal and frame scan vertical.
Referring still to Fig. 1, a laser source 30 provides an
output laser beam 31 which is directed through a full colour
modulator 38 to provide a modulated laser beam 31'. The
modulated beam 31' is directed through beam-splitter and
reflector elements 32, 33 to provide two beams 34 and 36 of
equal intensity. The modulator 38 is controlled from the
image generator 20 according to the view to be projected.
Both modulated beams 34 and 36 pass to a double lirie
scanner 42 fixedly mounted in the simulated aircraft cockpit.
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The two scanners, described in detail later herein, provide
t;wo respective scanned beams 44 and 46 which are respectively
scanned over the input ends 48 and 50 of two fibre optic
l.ight guide ribbons 52 and 54.
The two fibre optic light guides provide a flexible
linkage between the fixed line scanner 42 and the movable
helmet 12. The emergent scanned light beams from the
respective ends 56 and 58 of the light guides 52 and 54 are
focussed by spherical lenses 62 and 64 onto the screen 14
and directed onto a plane mirror 60. The right eye beams
are re~lected by the mirror 60 along divergent paths to
form a scan line the centre of which is shown at 66.
Similarly, the left eye beams are reflected by -the mirror 60
along divergent paths to form a scan line the centre of which
is shown at 68. The centre line of the respective right eye
and left eye vlews is thereby formed on the screen 14, each
line having its respective mid point at 16 and 18 and
being viewed by the pilot 10 in the respective line of view
70 and 72.
The mirror 60 is long in relation to its width and is
carried in bearings at its end which are mounted on the
helmet 12. These bearings are provided by motors 74 and 76
at the two ends which move the mirror 60 to provide the
required frame scan.
The mirror 60 may be a single plane mirror which is
either oscillated or rotated by the motors 74, 76 on its axis
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parallel to the plane in.which the line scan is pro~ected
or the mirror 60 may be a multi-faceted polygon mirror
rod of, for example, octagonal cross-section which is
continuously rotated by the motors 74, 76. In tlle present
example, the mirror 60 is a single plane mirror and is
rotationally oscillated for frame scan.
As the pilot's head moves, so does the displayed view
move over the screen, so as to be in the pilot's new line
of view and the view itself is changed according to the
simulated real world view in the direction of the line of
view.
To this end, the visual system receives data from the
host flight computer on lines 80 and 81. Position data
defining the simulated aircraft position throughout a
simulated flight exercise is supplied to the image generator
20 on line 80~ Attitude data, defining the simulated aircraft
instantaneous attitude, is supplied on line 81 to a vector
summing unit 82 together with head orien-tation data, defining
the pilot's actual instantaneous line of view, on line 84.
The summed output is supplied to the image generator 20 on
line 86. A throughpu-t delay error signal obtained by
subtracting the head attitude input to the image generator
one throughput delay period ago from the current head attitude
position, is supplied to the throughput delay error control
unit 100 on line 119.
The duplicated image, respectively for the right eye and
left eye views, in accordance with the inputted data, and
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allowing for the known seating posi-tion of` the pilot in the
simulated alrcraft type, are supplied to the respective
modulators 38 and L~0 on lines 88 and ~0.
It will be noted that the projection middle lines 66
and 68 do not coincide with the lines of view 70 and 72
for the reason that projection is effected from above the
pilot's eyes. Projected onto any horizontal plane, the
respective lines are coincident but, projec-ted onto any
vertical plane, the respective lines diverge away from the
screen. The angle of divergence is small but is nevertheless
great enough, compared with the apex angle of the half-
brilliance cone of reflection of a retro-reflective screen
material to result in a viewed scene of much reduced
brilliance. It is preferred there~ore to use a screen of
modified retro-reflective material for which the axis
of the half-brilliance cone of reflection is depressed
downwardly by the angle between the projection lines 66, 68
and the line of view lines 70, 72.
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Modified Re_ro-Reflective Screen
Retro-reflective projection screen rnaterial such as
that sold under the name SCOTCHLITE (Registered Trade Mark)
has a reflection characteristic such that what light is reflected
by the screen is reflected back closely along the line of
incidence. That is to say, reflected light is brightest
on the line of incidence, falling in intensi-ty rapidly as
he eye is displaced from the line of incidence in any
sample of
direction. With one/retro-reflec-tive material, observed
brightness falls to one-half in-tensity at an angle of
0.8 displace~ent from the line of incidence. Stated in
ther words, the area of half-brightness is the base area of
has
a cone that l its axis on the line of incidence and having
a half-angle of 0.8 at its apex.
In the projection apparatus described with reference
to Fig. 1, the line of incidence 66, between the frame
scanner 60 and the screen 14, makes an angle which is also
approximately 0.8 with the line of view 70, between the
screen 14 and the eye of pilot 10. Thus, with an unmodified
retro-reflective screen, the projected image would be
seen at half-brightness by the pilot.
In the apparatus of the invention, i-t is preferred to
modify the reflection characteristic of the screen in order
o increase the brightness of the projected image on the
while decreasing it elsewhere.
pilot's line of view,/ This modification is effected by
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placing a diffraction grating in front of the screen surface.
Fig. 2 shows an example of one suitable construction.
In Fig. 2, which is a cross-section view in the
vertical plane including both the line of incidence 66 and
the line of view 70, the surface o~ the retro-re~lective
screen is shown at 14. Placed in front of the screen is a
diffracting layer 140 of material having a refractive index
of 1.5. The layer 140 may be separated from the screen 14
by a layer of air 142. The depth of the layer of air 142
and that of the refracting layer 140 should be kept small
to reduce multiple internal reflections.
The front face of the refracting layer 140 is of the
form of a diffraction grating of horizontal grooves 144,
leaving lands 146 be-tween. In this simple example, the
width of the grooves 144 and lands 146 is approximately
equal. Calculated f`or light of 550 nm., and a refractive
index of 1.5, the depth of the grooves 144 is 0.3 Jum., and
the spacing of the grooves is 36 ~m., in the vertical
direction, as shown in the drawing. In practice, groove
profile is selected for maximum light in the desired
direction. A material is chosen, the refractive index of
which varies with wavelength in such a way as to minimise
variation of this direction with colour (wavelength) of
the light.
The reflection characteristic of the composite retro-
reflective surface and diffraction layer, in the plane of
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t;he drawing, is that the light reflection along the line
of incidence is reduced, while light reflected at a desired
angle to the line of incidence is enhanced. The projected
image brightness is thus improved along the line of view,
as desired, instead of being concentrated along the line
of incidence, as is the case with known reflecting screens.
Fig. 3 shows in cross-section a composite retro-
reflective screen and diffraction grating wherein the cross-
section of the diffraction grating is serrated, the grooves
of the forward surface running horizontally, which is normal
to the plane of the drawing. In this modified form of the
diffraction grating of Fig. 2, the angle at the apex of the
serrations is simply related to the angle of depression of
the emergent light beam. Again, for the example of an angle
of 0.8 divergence between the line of incidence and the line
of view, the apex angle is 0.4.
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