Note: Descriptions are shown in the official language in which they were submitted.
7~373
Imp ovements in or relating to Visual Display Apparatus
escri~tlon
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 provides suc~ apparatus capable of
providing a display for a sole pilot or s~ultaneously for two pilots.
The apparatus is of the head-coupled area-of-interest type, wherein an
image is projected upo~ a screen and is appropriately changed both according to
the simulated craft position and angular orientation and according to the
viewer's instantaneous line of view and is simultaneously m~ved on the screen to
occupy the viewer's field of view.
Apparatus of this type was described in prior British patent specifica-
tion 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 pro-
jected for left and right eyes, so as to appear at infinity.
The present invention provides improved apparatus of this type which
provides a display having tw~ zc)nes, a larger area zone of lc~er definition and
an inset smaller area zone of higher definition.
Accondingly, the invention provides head-coupled area-
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of-interes-t, vlsual display appara-tus providing a
displayedscene having two zones -the first zone being of
greater area and having a lower definition image relatively
to the second zone, the second zone being of smaller area,
being inset within the first zone and having a higher
efinition image rela~tively to -the first zone, comprising
concave
a part-spherical retro-reflective/screen of area greater
than a viewer's instantaneous field of view, a helmet,
sensing means for sensing the orientation of the viewer's
head and helmet, visual image generating means for generating
a simulated scenein -the direction of the viewer's
instantaneous line of view according to the viewer's simulated
position and orientation and under control of the said
sensing means, the said image genera-tor being adapted for
providing two visual images corresponding respectively to
the two said zones of the displayed scene, a laser light
source, separate laser beam modulators for each zone of the
displayed scene, separate line scanners for each zone of the
said scene for scanning the modulated laser beam over the
input ends of respective fibre optic light guides, the
said fibre optic light guides having their output ends at
spaced-apart positions on the viewer's helmet, and frame
scanning means mounted on the said helmet for receiving
light from the light guide outputs and projecting the light
as simultaneous scan lines of the two said zones to form
a composite two-zone displayed scene on the screen.
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Short Description of Drawlngs
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 diagramma-tic,simplified perspective view
showing a pilot seated in relation to a part-spherical screen
for viewing a display comprising a lower-definition zone
and an inset zone of higher definition;
Fig. 2 is a diagrammatic, simplified representation of
one laser source, laser beam modulator, line scanner, fibre
optic light guide ribbon and helmet-mounted frame scanner
combination used in the apparatus of Fig. 1;
Fig. 3 is a diagrammatic side view of the frame scanner
of Fig. 2;
Fig. 4 is a diagrammatic represen-tation of the projection
screen of Fig. 1 with a two-zone display corresponding to
a particular line of view shown thereon;
Fig. 5 is a diagrammatic representation of a preferred
arrangement of laser source, two-zone laser beam modulators,
fibre optic guides and helmet mounted frame scanner used in
the invention;
Fig. 6 shows diagramma-tically the me-thod of s-toring an
image in a buffer store so that it may be displayed as an
image with low and high resolution par-ts;
Fig. 7 is a diagramma-tic perspec-tive view showing
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apparatus for a preferred form of the invention providing
a pseudo-co:llimated display of large area with lower
definition and inse-t smaller area of high definition; and
Fig. 8 is a detail view showing an alternative line
scanner of those of Fig. 2 and Fig. 5.
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Description of the Example
In the acco~panying drawings, -the same elements are indicated by the
same reference numerals throughout.
Fig. 1 shcws in simplified form the app æ atus according to the inven-
tion for generating and displaying a two-zone area-of-interest view. A pilot lO
ing a helmet 12 is seated within a p æt-spherical shell having a retro-
reflective interior surface partially represented in Fig. l by the concave retro-
reflective screen 14. me pilot's line of vision 67 intersects the screen at
point 17. me field of view for each eye is centred on the point 17. me view
displayed comprises two zones, each zone covering at least half of the field of
view. For simplicity, the combined zones will be referred to as the displayed
scene.
m e displayed scene depends, in this example, upon the simulated posi-
tion of an aircr æ t 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 hel-met. I'he position of point 17 on the screen 14, and hence the position of the
displayed view on the screen, depends only on the pilot's head and helmet orien-tation.
m e two zone images are generated by an image generator 20 of the cam-
puter-generated image type which includes a frame buffer store 20'. me pilot's
head orientation is sensed by a head orientation
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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-moun-ted and the frame 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,
line scan is such that the projected scan line on the screen
and the line between the pilot's eyes lie in the same plane.
The frame scan is orthogonal thereto. Thus 9 when the pilo~t's
head is uprigh-t, 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 beam-
splitter and reflector elements 32, 33 to provide two beams
34 and 36 of equal intensi-ty. Laser beam 34 passes through
a full-colour modulator 38 controlled from the image
generator 20 according to the first zone image. Laser beam 36
passes through a full-colour modulator 40 controlled from
the image generator 20 according to the second zone image.
Both modulated beams 34' and 36' pass to a double line scanner
42 fixedly mounted in the simula-ted aircraft cockpit. The
two scanners, described in detail later herein, provide two
respective scanned beams 44 and 46 which are respectively
scanned over the input ends 48 and 50 of two fibre optic
light guide ribbons 52 and 54.
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In Fig. 1, the output ends of the two light guides
52 and 54 are shown spaced apart on the helmet 12, for
clarity, and the emergent light beams are separately
focussed by spherical lenses 62 and 64 respectively onto
the mirror 60 of a common frame scanner. A practical form
of the apparatus uses a common spherical lens 62, as is
shown in Fig. 5, which is described later herein.
The two fibre optic light guides provide a flexible
linkage between the fixed line scanner 42 and the movable
helmet 12. In Fig. 1, however, 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 direc-ted onto a plane mirror 60. The
first zone beams are reflected by -the mirror 60 along
divergent paths -to form a scan line of the first zone image.
Similarly, the second zone beams are reflected by the mirror
60 along divergent paths to form a scan line of the second
zone image. The centre line of the displayed scene is
thereby formed on the screen 14 at point 17.
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 projected
or the mirror 60 may be a multi-faceted polygon mirror rod
of, for example, octagonal cross-section which is
con-tinuously rotated by the motors 7L~, 76. In the 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 ~rom 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 genera-tor 20
on line 80. ~t-titude data, de~ining -the simula-ted aircraft
ins-tantaneous attitude, is supplied on line 81 to a vector
summing unit 82 together with head orientation 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 throughput delay error signal obtained by
subtracting the head atti-tude input to the image generator
one throughput delay period ago from the current head attitude
posi-tion is supplied to the throughput delay error control
unit 100 on line 119.
The two images, respectively for the first and second
zone views, in accordance with the inputted data, and al]owing
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for the known seating position of the pilot in the simulated
aircraf-t type, are supplied to the respec-tive modulators 38
and 40 on lines 88 and 90.
It will be appreciated that the change of the displayed
image with simulated aircraf~t position is relatively slow.
However, the change of the displayed image with head
orienta-tion may be complete and is relatively very rapid.
The image generator is unable to compute an entirely new
image immediately a new line of view is established due -to
the throughput delay of the image generator computer. To
overcome this limitation the residual old displayed view is
derotated to its former screen position until the computed
new displayed view is available.
The required image dero-tation can be effected by
controlling the relationship between the video signal and
the line scan and frame scan posi-tions. This control can be
produced in a number of ways.
The line scanner is typically a continuously rotating
polygon mirror which sweeps the input laser beam or beams
through an arc to produce a line scan, as in the example of
Fig. 2. Three alternatives are available:
(i) If the video signal is produced at a constant rate
then the line scan drive may be phase modulated to maintain
the correct line in space to produce an image with the
correct spatial orientation. If the line projection system
is capable of transmitting only the displayed field of view,
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then the image size will only be that part which is common
to both the computed and projected images. If the fibre
optic ribbon and the projection system is capable of projecting
more than the required field of view in the line scan direction
then the field of view obtained may be held constant.
(ii) The video signal may be produced at a cons-tant
rate and the line scanner rotated at a constant ra-te. The
required angular shif-t may -then be introduced with a
supplementary mirror. Line scanning apparatus, al-ternative
to that of Fig. 2 and including such a supplementary mirror,
is described later herein with reference to Fig. 8.
(iii) The polygon mirror may be run at a constant
angular velocity and the video signal timing adjusted by
altering the time at which the video signal is read out of
the frame store 20' of the image generator 20. This
ensures that the video signal corresponding to a point in
space is produced at -the predetermined time tha-t -the scanner
points the light beam at that part of the screen representing
the required point in space.
Of these three methods described above, method (i)
involves the phase modulation of a mechanical system rotating
at high speed and has the disadvantages associated with the
inertia and response times of such a system. Method (ii)
overcomes some of these problems by using a supplementary
mirror. This mirror does not rotate at high speed but
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nevertheless has iner-tia inherent in any mechanical system
and so it will have some response time. Method (iii)
requires only the ability to read out a memory at controlled
times. Since a memory is not a mechanical syste~, it has
no inertia and can be read out in a discontinuous manner if
required. Accordingly, method (iii) is the preferred
method for line scan synchronisation in the present invention.
The frame scanner of Fig. 1 does not offer the same
options as does the line scanner due to the difficulties
of implementation. The alternative methods corresponding
to those described for -the line scanner are as ~Eollows:
(i) If the video signal is produced at a cons-tant
rate then the frame scan drive may be controlled to give the
required pointing direction. In this case the frame scanner
will be a position servomechanism driven by a sawtooth
waveform in which -the star-ting point o:E the ramp may vary in
a controlled manner and the slope of the ramp may vary in
a controlled manner in order to give a constant angular
sweep in free space when the projector mount is being
subjected to angular shifts.
(ii) The use of a supplementary mirror is impractical
in the frame scanner of Fi~. 1.
(iii) If -the frame scanner is driven with a sawtooth of
constant period, start poin-t and slope, then the read out
times from the frame store 20' may be adjusted to produce
the video signal when the scanner is at the required
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orientation in free space.
Of these three methods, method (i) requires adjustments
to the period and rate of a mechanical system which, due to
its construction, has a very low inertia. Hence, the
settling time following such a disturbance may be acceptable.
It can preserve -the instantaneous field of view constan-t
through the throughput delay period. Method (ii) is
impractical due -to -the physical constraints of the projection
lens and frame scanner assembly of Fig. 1. Method (iii)
involves adjustment to a system without inertia or the
requirements of continuity. However method (iii) reduces
the vir-tual field of view during the throughput delay period.
Continuing with the description of the apparatus of
Fig. 1, a synchronising pulse generator 106 supplies pulses
on line 108 to the throughput delay error control unit 100.
Line scan control signals are supplied -to the line
scanners of unit 42 from unit 92 by way of line 94. Frame
scan control signals are supplied to the frame scan motors
74, 76 from unit 96 by way of a flexible line 98. ~ideo
synchronisation timing pulses are fed to the frame buffer 20'
of the C.G.I. image genera-tor 20, from the unit 100 on
line 110. Control of the relative timings between the line
scan control 92, the frame scan control 96 and the C.G.I.
image generator frame buffer 20' is effected by the throughput
delay error compensation circuit 100 by way of lines 102,
104 and t10, respectively.
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It will be noted that the projection middle lines 66
a.nd 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 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 ma-terial, to result in a viewed
scene of much reduced brilliance. It is preferred therefore
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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The various units of -the apparatus, shown in the
block schernatic part of Fig. 1, will now be considered in
further detail in the following order:
C.G.I. Image Generator.
Laser Source.
Laser Beam Modulator.
Line Scanner.
Fibre Optic Light Guide Ribbon.
Frame Scanner.
Retro-reflective Screen.
Helmet-Head Orientation Sensor.
Throughput Delay Error Compensation Unit.
Line Scan Control.
Frame Scan Con-trol.
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C.G.I. Ima~e Generator
The displayed scene corresponds -to a real world scene
as it would be visible from the simulated aircraft during
fligh-t. In earlier visual display apparatus for ground-
based simulators, the visual image was generated using a
scale model and a closed-circuit television camera. `The
camera lens, comprising an optical probe, was moved over
the model correspondingly to the aircraft simulated position,
altitude, heading, pitch and roll. The generated image
was varied according to all these factors.
According to a more recent technique, now well
established, the same form of image is computer-generated.
The technique is explained in text books such as, for
example, "Principles of Interactive Computer Graphics",
by William M. Newman and Robert F. Sproull, published in 1973
by McGraw-Hill Book Con~pany, New York and elsewhereO
The signals available to the image generator computer
from the hos-t flight computer of the simulator are:
aircraft position, X.Y., alti-tude, heading3 pitch and roll.
C.G.I. image generators are known which generate the direct
ahead view from the aircraft according to the input data,
including solid~looking features with surface detail,
concealing hidden edge-lines and surfaces as the aircraft
flies around such objects and clipping and windowing the
display according -to the simulated field of view.
The image generator 20 of Fig. 1 is of this general
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type. Aircraft position and attitude data are supplied
from the host ~light computer on line 80. Aircra~t
heading, pi-tch and roll da-ta are supplied on line 81.
However, the image generated in the apparatus of
Fig. 1 is in the actual instantaneous line of view o~ the
pilot. This view is determined also by the pilot's line
of view heading and pitch and head roll relatively to
the aircraft axes. Head azimuth, head pitch and head
roll are determined by the head orientation sensor 22 and
these data are supplied on line 84 to the summing unit 82,
which adds these values -to the aircraft heading~ pitch
and roll values respectively. The output information
defining the pilot's line of view relatively to the
simulated terrain over~lown is supplied to the image
generator 20 on line 86.
The point midway between the pilot's eyes is a constant
position offset above and to the left of the aircraft
longitudinal axis. This offset requires only constant
values to be added to aircraft altitude and position
respectively throughout an entire exercise.
For the generation of separate zone images two similar
type image generators are included in the image generator 20.
The same data are continuously inpu-tted to both image
generators but each image genera-tor provides for an area of
image corresponding to the respective zone. The smaller
area zone provides for correspondingly greater detail within
the same video signal bandwidth.
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Laser Source Laser Beam ModulatorL Line ScannerL Fibre Optic
Li~ht Guide Ribbon and Frame Scanner
The basic components of a laser source, laser beam
modulator, line scanner, fibre op-tic light guide ribbon and
frame scanner combination, for the apparatus of Fig. 1, will
firs-t be described with reference to the simplified diagram
of Fig. 2 and Fig. 3.
Fig. 2 shows the laser beam source 30 which provides
-the output laser beam 31 directed through the full colour
modula-tor 38. Both the laser beam source 30 and the
modulator 38 are of known form. The full-colour modulated
beam output is shown at 31' in this figure, in which
intermediate beam-splitters are not shown. The line scanner
is shown generally at 42.
The line scanner comprises a synchronously-driven
polygonal section mirror drum 144 which rotates continuously
in the direction shown by the arrow 145 to sweep the beam 31'
over the scan path 44. One pass occurs for the movement of
each mirror facet of the mirror drum 144 past the beam 31'.
A fibre optic light guide, formed into a fla-t ribbon 52
over mos-t of its length, has individual groups of fibres
formed into an arc at -the input end 48 of the light guide.
The width of the line scan 44 may exactly cover the arc at 48.
The modulated beam 31' is then scanned along the entire arc
at 48 for each line of the image.
At the output end 56 of the fibre optic ligh-t guide 52,
the individual groups of fibres are similarly formed into
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an arc the fibre groups occurring in the same sequence at
the two ends 4~ and 56, so that the scanned image line at
the input end 48 is exactly reproduced at the output end 56.
The emergent rays from the output end 56 of the
light guide 52 are focussed by the spherical lens 62 onto
the face of the frame scanning mirror 60. As shown in
Fig~ 1, the mirror 60 is mounted on the pilot's helMet 12
in bearings provided by reciprocating motors 74 and 76.
With the mirror 60 stationary, the emergent rays
are reflected from the mirror 60, as shown ins-tantaneously
at 66, to form a single line of the image. As the mirror 60
is moved, successive lines of the image are projected to form
the entire scanned image corresponding to one zone of the
display.
Fig. 3 shows, in side view, the output end 56 of the
ligh-t guide 52, the spherical lens 62, the mirror 60 and
the reflected beam 66 as described above with reference -to
Fig. 2.
A second line scanner, comprising a second mirror drum,
produces a second line scan over the input end 50 of the
second fibre optic light guide 54, as is shown in Fig. 1
and in Fig. 5.
Referring now to Fig. 4, there is shown at 14 a part
of the screen 14 of Fig. 1 and the poin-t 17 at which the
pilot's line of view 67, Fig. 1, intersects the screen 14
is shown as the centre point of a small circle on the screen
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14 of Fig. 4.
A first zone 201 e~tends over an area represented by
the line 211 and corresponds to the pilot's field of view.
A smaller zone 202 extends over an area represented by the
broken line 212 and corresponds to a much smaller area within
which greater detail can be appreciated visually by a viewer.
The high-definition zone may be inset in the low-
definition zone in a number of alternative ways. Thus, the
low-definition zone 201 may be blanked out to leave a blank
~one 202 and the high-definition zone 202 may be optically
inset in the blank area so provided. Alternatively, the low-
defini-tion zone 201 may include -the high-definition zone 202
and the increased definition be provided by a high-definition
image electronically inset within the lower defini-tion image.
In either case, -the high-def`inition zone 202 is always
of smaller area than the low-definition zone 201. It may
either be central within the low-definition zone 201, if head
coupling only is used to position the image, or it may be
offset from centre, if head coupling is used to position the
boundary 211 of the low-definition image and eye coupling is
used to position the boundary 212 of the high-definition image.
The apparatus of Fig. 5 uses the same spherical lens 62
and frame scanner mirror 60, for both zones. A separate line
scanner and fibre optic guide for each zone.
Fig. 5 shows the laser source 30, beam-splitter and
reflector elemen-ts 32, 33 to provide two beams of equal
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intensity, as in Fig. 1. Laser beam 34 passes through the
modulator 38 and laser beam 36 passes through the
modulator 40, providing modulated output beams 34' and 36',
respectively, which are directed to the double line scanners
generally shown at 42 and 43, respectively.
In Fig. 5, the modulator 38 receives the high-definition
video input and the modulator 40 receives the low-definition
input. The fibre optic light guide 52 is composed of fine
fibres and transmits the high-definition image projected as
area 202, Fig. 4. The fibre optic light guide 53 is composed
of coarse fibres and transmi-ts the low-definition image
projec-ted as area 201, Fig. 4.
In the preferred system, the low-definition fibres are
each four times the diameter of -the high-definition fibres.
The line scanning polygons 144 and 144a, respectively, are
driven on a common shaft, not shown, and the high-definition
area scanner 144 has four times as many facets as the low-
defini-tion scanner 144a.
The high-definition line scan is thus one quarter the
length of the low-definition line scan. The high-definition
fibre guide 52 is capable of -transmitting a line scan
covering the whole field of view. Only one quarter is used
at any time and the portion selected determines where the
high-definition insert 202 is positioned across -the total
field of view 201, ~ig. 4. This selection is effected by a
mirror 402 mounted on a pivo-t 404 with rotational position 403
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determined by the viewer's eye movements relative to his
head, Since a common frame scanner 60 is used, this results
in a selected column of the screen being scanned by both a
high resolution spot and a low resolution spot. Video
signals may be routed to either modulator in order to display
the appropriate resolution image, with an appropriate delay
to compensate for the angular separation between the two
light guides at the projection lens end.
A complete column of high resolution image may be
projected if desired or the interval over which a high
resolution image is proJected may be determined by monitoring
vertical eye movement rela-tive to the head.
The system shown diagramma-tically in Fig. 5 is duplicated
to provide an image for each of the pilot's two eyes, and may
use the same video signal to produce a pseudo-collimated
image or correc-tly computed video to produce a s-tereo pair
of images. The apparatus of Fig. 7 uses two line scanners
for each eye but uses the same low-definition and high-
definition video information to provide a pseudo-collimated
image.
An alternative form of the apparatus uses only a single
high-definition fibre optic guide and line scanner. In this
case, the image is computed with two different resolutions and
stored in a buffer store as shown diagrammatically in Fig. 6.
The resolution of the display is equal to the resolution
of a high resolution computed picture element 406. A low
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resolution picture element 405 is 16 times the size of a
high resolution picture element. The image is computed as
a low-definition scene or as a high-definition scene and
stored in the appropriate portion of the bit map.
~; To read the memory, the address of each successive high
resolution element is computed in -the desired sequence and
the video corresponding to that address is used to modulate
the laser beam. When reading the low resolution portion of the
image store, the two least significant bits of the address
of both the line and picture element along that line are
ignored. Thus the video level will only change a-t the
boundaries of the coarse picture elements 405. When the high
resolution portion of the memory is accessed, all the
address bits are used, so allowing the video signal to change
at the fine picture element 406 bo~mdaries. The position of
-the boundary 212 between the two areas is determined during
picture composition and can either be fixed relatively to
the total field of view boundary or controlled by the measured
or predicted eye angular position with respect to head
orientation.
Referring to Fig. 7, which illustrates the form of the
invention which provides the pilo-t with a pseudo-collimated
display with an inset high-definition zone, it will be noted
that the apparatus is generally similar to that of Fig. 1,
except that two projectors are used, one above each eye of
the pilot. Thus, two projectors use respectively lenses 62
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and 64 -to projec~t the line image of a pair of fibre optic
light guide output ends onto the screen 14 by way of the
common frame scanning mirror 60.
The respective pairs of light guide ends 56, 57 and 58, 59
are relatively disposed as shown in Fig. 5.
The respective ends 56, 57 and 58, 59 -termina-te the
light guide pairs 52, 53 and 54, 55. The input ends 48, 50
and 49, 51 are scanned respectively by the high-definition
zone line scanner 42 and by the low-definition zone line
scanner 43.
The line scanner 42 scans -the modula~ted laser beam 34'
over the two light guide input ends 48 and 50. The line
scanner 43 scans the modulated laser beam 35' over the two
light guide input ends 49 and 51.
The respective high-defini-ti.on zone and low-defini-tion
zone laser beam ~odulators 38 and 40 bo-th receive their video
modulation signals from the store 20' of -the C.G.I. image
generator 20, under control of the pulses supplied by the
throughput delay error control unit 100 on line 110.
Fig. 8 shows line scanning apparatus alternative -to
that of Fig. 2 and Fig. 5 and includes a supplementary
mirror 202. The mirror 2C2 is pivotable on an axis 203 which
is parallel to the spin axis 204 of the polygon mirror
line scanner 144.
To effect image derotation for head movement in the
direction of line scan by the method (ii) described earlier,
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the mirror 202 is rota-tionally positioned about its axis 203
by a motor 205 in a controlled manner so tha-t the swep-t
arc 44 is positioned at -the required part of the arc 48 at
the input end of the fibre optic ligh-t guide 52. The
motor 205 is controlled from the throughpu-t delay error
control unit 100 by a signal on line 102.
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Mcdified Retro-Reflective Screen
_ _ . .. .
Retro-reflective projection screen material such as tha-t sold under
the name SCOTCHIITE (Registered Trade Mark) has a reflection characteristic such
that light incident upon the screen is reflected back along the line of inci-
dence. m at is to say, reflected light is brightest on the line of incidence,
falling in intensity rapidly as the eye is displaced from the line of inciden oe
in any direction. With one retro-reflective ma~erial, observed brightness falls
to one-half intensity at an angle of 0.8 displa oe ment from the line of inci-
dence. Stated in other w~rds, the area of half-brightness is the base area of a
cone which has its axis on the line of inciden oe 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 scamler 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. m us, with an unmcdified retro-reflective
screen, the projected image would be seen at half-brightness by the pilot.
In the app æ atus of the invention, it is preferred to modify the
reflection characteristic of the screen in order to increase the brightness of
the projected image on the pilot's line of view, while decreasing brightness
elsew~ere. This ~adification is effected by placing a diffraction grating in
front of the screen surface.
- 25 -
~J~
~7~173
- 26 -
Head/Helmet Orientation Sensor
Mechanical linkages have been proposed to sense the
orien-tation of a pilo~t's helmet relatively to an aircraf-t
cockpit. However, mechanical arrangements o~ any sort are
undesirable in the environment of an aircraft simulator
cockpit.
It is preferred to e~fect helmet orientation sensing
by non-contac-t means. Any suitable know,n head/helmet
orientation sensor may be used in apparatus of the present
invention to provide electrical signals defining instant-
aneous helmet orientation. One such sensor is that
described by R.G. S-toutmeyer and others in U.S. patent
No. ~,917,412, entitled "Advanced Helmet Tracker Using
Lateral Pho-tode-tection and Light-Emit-ting Diodes", Such
apparatus is further described by Edgar B. Lewis in U.S.
patent No. 4,028,725, entitled "High-Resolution Vision
System".
P. 20q9
~7~73
- 27 -
Throu~h~ut Delay Error Compensa-tion Unit, Line Scan Control
and Frame Scan Control
. ~
As has been explained earlier in ~the description, the
C.G.I. image generator 20 takes an appreciable time to
compute a new view for display when the pilot's line of
view is changed. The delay is of the order of 100 m secs.
However, when any viewer changes his line of view, by
extensive head movement, there is a delay before the
viewer appreciates the new view before him. This delay also
is of the same order of time as the image generator delay.
In a simplified form of the apparatus according to the
invention means are provided merely to ensure that the
old display is not projected in -t:he new line of view of
the changed head position.
In this simplified form of the apparatus, a large change
of head orien-tation signal on line 119is effective to blank
out the projected view for a period of some 100 m secs. until
the new view has been computed.
The apparatus of Fig. 1 provides means for the
derotation of the projected image upon rotation of the
pilot's head. Derotation is considered to be of especial
importance when head movement is such that the new field of
view is not separate from the old field of view but is
within it or overlaps it.
The displayed view is some 100 in azimuth and some
ln elevation, with respect -to -the pilot's line of view.
P.2099
~4~73
- 28 -
Although a viewer's field of view may exceed these angles,
the marginal areas are low-interest and the central area
of prime-interest may be a cone of perhaps only 5 about the
line of vision. It is therefore readily possible for the
pilot to change his line of view so as to move this area
of central interest within the initial displayed view area.
In the apparatus of Fig. 1, line scan is in a direction
across the screen 14 and frame scan is orthogonal thereto.
The head orientation sensor 22 provides signals resolved into
head azimuth movement and head pitch movement.
The synchronising pulse generator 106 provides a
line synchr~nising and frame synchronising pulse output of
equally spaced apart pulses. Upon change of head azimuth,
the output signal on line 119 causes the throughput delay
error control unit 100 to provide a relative change of phase
of the line synchronising pulses supplied by control unit 92
to the line scanner 42, and the video synchronising pulses
supplied on line 110 by the throughput delay error control
unit 100 to the frame buffer store 20', so controlling read
out from the store 20' in the sense to displace the displayed
image equally and oppositely to every change of head azimuth.
Similarly, the output signal on line 119 causes control
unit 100 together with frame scan control unit 96 to provide
a relative change of phase of the frame synchronisi~g pulses
supplied by control unit 96 to the frame scanning motors 74
and 76.
P.2099
7~73
- 29 -
Thereby, upon head rotation in azimuth or pitch or
both, -the displayed view is displaced oppositely. The
derotation is maintained for a period of some 100 m secs.,
until the new view is computed. The original relative
timing of the synchronising pulses is then restored, so that
-the new view is displayed in the direction of the new line
of view.
P.2099
