Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Field of the Invention
-
The present invention relates to a three-dimen-
sional display system, and in particular, to a display
system in which two-dimensional images are projected by
means of a light beam on an oscillating screen to generate
a three-dimensional display.
Backqround of the Invention
Three-dimensional displays will find a wide
range of applications in fields such as computer-assisted
tomography, air-traffic control, CAD/CAM, entertainment,
and the like. Many commercial and industrial processes
could benefit from a three-dimensional disp]ay of spatial
information.
Many techniques have been developed for pro-
ducing three-dimensional displays. For example, U.S.
Patent No. 4,472,737 (September 18, 1984, Iwasaki)
describes a stereographic tomogram observing apparatus
which comprises a projecting type CRT for projecting
light beams at varying intensities corresponding to
tomographic picture data, and a plurality of normally
transparent display screens. A switching circuit or
rotating means is provided to select one of the plural-
ity of screens on which a corresponding tomogram is
projected. U.S. Patent No. 3,970,361 (July 20, 1976,
DiMatteo et al) uses a series of transparent elements
which have varying optical thlckness. When they are
moved rapidly in front of a two-dimensional display,
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three-dimensional images can be seen. Another system
is described in U.S. Patent No. 4,607,255 (August 19,
1986, Fuchs et al). It employs a variable-focus mirror
whose curvature is adjusted synchronously with a display
on a CRT so that three-dimensional virtual images are
produced.
All of these techniques suffer from numerous
difficulties. The varifocal mirror, for example, exper-
iences mechanical fatigue and it is also very difficult
to precisely control the curvature, which is essential
for an accurate three-dimensional display. The plurality
of switchable screens of ~.S. Patent No. 4,472,737 are
complicated and expensive to manufacture and the rotatable
elements described by Di~atteo and Iwasaki limit the number
of discrete two-dimensional images employable, so that
the resulting three-dimensional displays are not very
finely defined in depth.
A serious difficulty common to the first and
third of these examples is that the information to be
displayed is accessed and displayed serially, point by
point, as the electron beam traverses the face of the
CRT. A great increase in the number of points that can
be displayed within the resolving time of the eye can
be achieved through the use of a display device which
permits the access and/or presentation of the data to
be performed at least partially in parallel mode, for
example, loading an entire line of an image into the
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display device at one time, and displaying an entire
two-dimensional image at one time.
The three-dimensional display system of the
present invention is technically straightforward and is
thus simple to manufacture, uses parallel data access,
and produces sharp images with many points and good
depth resolution.
Objects of the Invention
It is an object of the present invention to
provide a three-dimensional display system which employs
a two-dimensional magneto-optic display element.
It is another object of the present invention
to provide a three-dimensional display system which
includes a laser beam source in addition to the magneto-
optic display elements to produce bright, good resolutionthree-dimensional images.
It is still a further object of the present
invention to provide a three-dimensional display system
which includes an oscillating screen.
Summary of the Invention
Briefly stated, the three-dimensional display
apparatus according to the present invention comprises
a source of light producing a parallel beam of visible
light and modulation means for switching on/off the
parallel beam of light. Magneto-optic display means has
a two-dimensional array of pixels, each of which switches
on/off light transmitting therethrough and projecting
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means project the beam of light which has been trans-mitted
through the magneto-optic display means. The display
apparatus also includes an oscillating screen oscillating
along the optical path of the projected beam of light
and control means for controlling in synchronism the light
modulator means, the magneto-optic display means and the
oscillating screen.
Brief Description of the Drawings
For a more complete understanding of the present
invention and for further objects and advantages thereof,
references may now be made to the following description
taken in conjunction with the accompanying drawings in
which;
Figure 1 is a schematic illustration of the
present invention according to one embodiment; and
Figure 2 is a pulse train diagram showing the
image set-up and projection times.
Detailed Description of the Preferred Embodiments
Referring now to the drawings, Figure 1 is a
schematic illustration of a three-dimensional display
system according to one embodiment of the present inven-
tion. A laser light source 1 emits a parallel beam of
linearly polarized visible light toward a beam modulator
3 which blanks the beam rapidly and repetitiously to
produce light beam pulses. The light beam pulses are
expanded across the cross-section of the beam by a beam
expander 5 to correspond to the size of the two-dimen- --
sional magneto-optic chip module 7. The chip module 7
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-- 5
consists of a matrix of pixels in an (x,y~ array, each
of which pixels is accessible in parallel with all the
other pixels of the same line (x value), through x- and
y-leads, to control the linear polarization of the light
transmitted therethrough at a very fast switching rate.
Thus the chip module 7 produces two-dimensional time-
varying images with the help of a polarization analyser
9. Projection optics 11 project the two-dimensional
images to a synchronously oscillating screen 13 to pro-
duce a three-dimensional display. A computer 15 controls
generation of two-dimensional images in the magneto-optic
chip module. Synchronization among the beam modulator,
the chip module and the oscillating screen is achieved
by using electrical pulses generated by the moving screen
apparatus in synchronism with the movement of the screen.
These pulses are transmitted to the computer for timing
of the chip module, and to the modulator control unit
for timing the light pulses.
According to this embodiment, the beam source
is a low-cost He-Ne laser (about 35 mW) whose beam is
modulated by the electro-optic beam modulator at a rate
of 1440 pps. The magneto-optic chip module is, for
example, a product called "Sight-Mod"'~ (manufactured by
Semetex Corp., California, U.S.A.) and consists of a 48x48
pixel array. The screen oscillates at a frequency of
about 30 Hz to refresh each point about 30 times per
second, thus producing a display of 48x48x48 (110, 592)
points in a 15x15x15 cm volume. Because the screen
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moves continuously, the image produced by each pixel is
"smeared" over a distance corresponding to the distance
traveled by the screen during the light pulse, in the
direction of the screen motion. At the chosen pulse
length and display grid size, this amounts to 0.3 cm,
less than the distance separating adjacent image planes
in the display. If the beam power were increased, the
smearing could be reduced by shortening the light pulse
proportionately. This might necessitate using a more
powerful laser (e.g. gold vapour or double YAG laser),
or other mor~ intense light source, for example, a flash
lamp or high-intensity projector lamp.
In the magneto-optic chip module 7, a magnetic
field is applied to each of the pixels by means of cur-
rent pulses through small loops of conductor connectedto its x- and y-leads (nucleation), followed by a write
pulse in an external field coil (activation). This
changes the angle by which the pixel receiving such cur-
rent pulses will rotate the linear polarization of light
passing through it. The polarization analyzer following
the chip module in the optical path is adjusted so that
it transmit the light coming through those pixels having
a selected polarization state. The time required to acti-
vate all the pixels is a few hundred nanoseconds when
they are accessed a line at a time in parallel. This
is followed by a 10 microsecond write pulse to establish
the full optical activity of the pixels (activation).
This write pulse is applied to the external coil coplanar
with the chip module, affecting all the pixels simultane-
ously. All the pixels must be erased using a much longer
reverse current pulse in the same coil before generating
the next imaqe.
Figure 2 is a pulse train diagram which illus-
trates the various functions of the present invention
with respect to time. As shown therein, the image takes
about 268 microseconds to erase, set-up and write, through
a 200 microsecond erase pulse 21, a 58 microsecond image
set-up interval 23, and a 10 microsecond write pulse 25.
The laser beam pulse 27 is limited to 125 microseconds,
to limit blurring of the display. Thus the total time
required for image set-up and projection is 393 micro-
seconds. For display of 48x48x48 (110,592) points, 1440
(48x30) images must be projected in a second on the screen
which is oscillating at 30 Hz. Therefore 694 microseconds
separate each image. This exceeds the total time required
for image set-up and projection by a substantial margin
(301 microseconds).
The oscillating screen has an instantaneous
velocity given by:
x' = A~cos~t
where A is the oscillation amplitude and ~ is the angular
frequency and is equal to 2~f, where f is the frequency
in hertz. Thus,
x' = A~ = 2~Af
max
at the center of the screen oscillation range. In the
present embodiment, A = 12.7 cm, and f = 30 H2 so that
xmax = 2394 cm/sec. Thus, for a light pulse of 125
microseconds, the screen travels less than 0.30 cm, which
is less than the distance separating adjacent images
(0.32 cm).
In the 48x48 pixel chip interface provided
by the manufacturer, the pixel array is electronically
divided into 288 "addresses", each of which corresponds
to a vertical line of 8 pixels, representing one byte
of data in the computer memory. Eight data lines are
provided to set the optical state of each of the 8 pixels
in a byte. A low voltage (logical 0) level applied to
one of the data lines initiates a change in the optical
activity of (nucleates) the corresponding pixel of the
addressed byte. To build up a picture, the 288 addresses
are accessed sequentially and for each address, the data
lines corresponding to all the "opaque" pixels are pulsed
to activate them. Then a write pulse is sent to the
field coil to activate the pixels that have been activated.
Following activation, a laser pulse is passed through
the chip and focussed on the moving display screen.
The projection optics must have enough depth
of focus of the projected image to accommodate the 15 cm
depth of the display volume. The screen can be illumi-
nated by a nearly parallel beam to obviate this problem.
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The electronics for transferring the images
from the computer memory to the chip module is a modified
version of the SMD-48i research/demonstration box mar-
keted by the manufacturer. This box includes all the
components required to set up images on the chip module
in a cyclical fashion, under the control of any IBM -
compatible personal computer. Modifications are made
to increase the number of bits that can be set in each
line to the maximum value of 48, by the addition of
supplementary address, data, control and drive components.
According to another embodiment of the inven-
tion, the point density is increased by a factor of about
7 over that of the 48x48x48 display while remaining within
the heat dissipation range o~ the chip module (the cri-
terion which limits the number of images per second and
the total power of the projection beam), by using another
type of Sight-Mod'~ chip having 128x128 pixels, and pro-
jecting 48 images in the display volume as discussed
above.
Another embodiment would employ a multiplicity
of the 48x48 pixel chips to increase the point density
by, for example, a factor of 4 in a 96x96x48 point dis-
play.
Still another embodiment would employ an oscil-
lating screen enlarged to 30x30 cm or larger, to produce
a display volume of 30x30x30 cm or larger.
