Real image projection system

An image projection system uses a projection screen formed over at least a portion of a spherical surface, with either a retroreflective or a directional-specific reflecting material, along with a beamsplitter and at least one lens-projector combination for each real image input to the screen. More than one different input image may be simultaneously used, either with each of the different images provided to a different area of the screen for providing juxtaposed real images to a single observer, or with each of the different real images being provided to a different observer. A non-spherical screen may be used if a retroreflective screen material is used in a system for providing simultaneous real images to spatially-separated plural observers.

The present invention relates to image projection systems and, more 
particularly, to a novel video system for providing to at least one viewer 
a real image upon a projection screen. 
BACKGROUND OF THE INVENTION 
There are now several arts in which a large screen display of information 
to at least one viewer is desirable; even more desirable is the ability to 
simultaneously display multiple sets of different information upon a 
single viewing screen, either with all sets being viewable by a single 
individual or with each set being viewable by a different individual. In 
uses such as flight and vehicle combat simulators, a distortionless 
display, whether for one or several viewers, is desired so as to provide 
the realism necessary for best learning results. If several persons are 
simultaneously involved, the individuals are usually relative close 
together, in seating having fixed locations to the vehicle windows through 
which the visual information will be seen. It is then desirable to provide 
a display system in which each different viewer can observe a singular 
image on a projection screen surface where multiple images are being 
projected simultaneously; this system will present appropriate imagery to 
a specific observation volume without visually interfering with other 
imagery specifically intended to be observed from another viewing volume. 
This allows, for example, a pilot and copilot seated in a flight simulator 
to simultaneously observe geometrically corrected images on a common 
projection screen without parallax errors inherently introduced by 
pilot/copilot offset distances. System cost and complexity can often be 
minimized by allowing multiple image-for-single observer use or single 
image-for-each of multiple observers use to occur on a common projection 
screen. 
BRIEF SUMMARY OF THE INVENTION 
In accordance with the invention, an image projection system uses a 
projection screen formed over at least a portion of a spherical surface, 
with either a retroreflective or a directional-specific reflecting 
material, along with a beamsplitter and at least one lens-projector 
combination for each real image input to the screen. A plurality of input 
images may be used, either with each of plural different images provided 
to a different area of the screen for providing juxtaposed real images to 
a single observer, or with each of the plural different real images being 
provided to a different observer. A non-spherical screen may be used if a 
retroreflective screen material is used in a system for providing 
simultaneous real images to spatially-separated plural observers. 
In presently preferred embodiments, the reflector material has a non-zero 
optical gain only over a small viewing cone centered about the reflected 
ray from the screen surface. The screen is formed of retro-reflective 
material, to alleviate stringent smoothness requirements, and each 
projector is aligned with a viewing-aperture-setting lens and a 
beamsplitter, with the viewing position being established at a location 
along a line substantially complementary to the source projection line 
through the associated beamsplitter. 
The benefits and objects of our invention will now become apparent to those 
skilled in this art, upon reading the following detailed description of 
the present preferred embodiments of the invention, when considered in 
conjunction with the associated drawing FIGURES.

DETAILED DESCRIPTION OF THE INVENTION 
Referring initially to FIG. 1, a projection system 10 includes a relatively 
large screen 11 formed upon at least a portion of the interior surface of 
a substantially spherical cavity. An observer 12 is placed near to, 
although not necessarily at, the spherical surface center-of-curvature 
point 11c. A projection means 14, such as a video display monitor and the 
like, is located beyond an associated lens means 16, with respect to a 
beamsplitter means 18. A point, although not necessarily the mid-point, of 
the beamsplitter 18 is located substantially on the optical axis 11a 
between the observer O and the screen. The center of lens 16 is positioned 
along the reflected optical axis portion 11b, responsive to the 
beamsplitting action of means 18, and at a distance therefrom so that the 
image of the lens exit pupil is superimposed along the screen optical axis 
11a at the observer O position, by action of beamsplitter 18. The lens 
exit pupil diameter P thus provides a real image over a viewing volume 
determined by a diameter V. 
Referring to FIG. 1a, the screen 11 may be formed of any material having a 
high screen surface gain, given by curve 11g, over a range of screen cone, 
or bend, angles up to and including the angle .alpha. at which the screen 
gain falls to an average value G.sub.av, as at points 11e and 11f. Thus, 
the effectiveness of the redirection of energy is a function of the 
projection screen surface characteristics. Under ideal conditions, a 
screen surface material would be chosen such that all of the reflected 
light energy is directed within a selected beamwidth determined by the 
required viewing volume or cone angle .alpha.. The distinguishing 
characteristic of the screen material must be that it reflects and 
diffuses incident light energy in a directionally controlled manner 
resembling the gain function 11g, so as to allow projected imagery to be 
viewed only within a specifically designed viewing volume. If the screen 
interior surface 11s is formed of a material similar to the Protolite 
material described by Mihalakis et al. in U.S. Pat. No. 4,241,980, or the 
like, a spherical screen 11 must be used; if the screen surface 11s is 
formed of a retro-reflective material (such as might comprise microscopic 
glass spheres embedded in a reflective substrate, such as material 1761 
available from Minnesota Mining & Manufacturing (3M) and the like) then 
both spherical and non-spherical surface 11s shapes may be used. As an 
example, a high-gain screen surface can be provided with an average gain 
G.sub.av of 50 over bend angles up to .alpha.=7.1. Thus, the average bend 
angle is about 3.55, and a 6" aperture lens 16 can be imaged to an 
observer eyepoint 11c about 24" away from the screen surface 11s. 
In this situation, with the lens and eyepoint both located at the screen 
center-of-curvature, the optical magnification is about 2.07. The 
brightness B of the center of the image, at the eyepoint, can be found 
from 
EQU B=(S*T.sub.1 *BS.sub.r *BS.sub.t *G.sub.av)/(2*F#*(M+1)).sup.2 
where S is the source brightness (say, 55 ft-Lamberts from a 7.5" diagonal 
CRT), T.sub.1 is the lens transmission ratio (say, 0.88 for the 6" 
diameter lens with an F# of 1.3, and therefore a focal length of 7.8"), 
BS.sub.r is the beamsplitter reflection coefficient (say, 0.50), BS.sub.t, 
is the beamsplitter transmission coefficient (say, 0.45), and M is the 
magnification. For the above situation, with G.sub.av =50 and M=2.07, the 
eyepoint brightness is 8.55 ft-Lamberts. 
If the viewing area V is defined by the volume edge where the brightness 
falls to 50% of the maximum brightness (at the view center), then this 
volume will also be equal to the volume where the average gain, returned 
to the viewer, falls to 50% of the viewer-returned center gain value. For 
the above illustrational system, the viewing volume radius can be found to 
be 4.5" in the vertical direction, so that V=9.0". Various ratios of 
horizontal and vertical dimensions of the viewing volume can be obtained 
by varying the high-gain screen surface characteristics. For example, if 
the above screen has a gain which falls substantially to zero at bend 
angles of 17.7, then the perceived display field-of-view would be about 
22.5.times.30 with the 3:4 aspect ratio of the 7.5" diagonal CRT monitor 
as the image source. 
Referring now to FIG. 2, in a somewhat similar system 20 each of N multiple 
real images from different ones of a like number N of separate sources 
14i, where 1.ltoreq.i.ltoreq.N, can be juxtaposed together on a common 
screen 11' for viewing by the observer 12' situated near screen center of 
curvature point 11c'. Thus, a first source 14-1 operates with a first lens 
means 16-1 and a first portion 18a of a common beamsplitter means 18 to 
project a first real image towards the observer O'; a second source 14-2 
operates with a second lens means 16-2 and a second portion 18b of the 
common beamsplitter means 18 to project a second real image towards the 
observer O'. With proper placement of the beamsplitter portions 18a/18b, 
the reflected axes 11b'-1/11b'-2 pass through the lens 16-1/16-2 from the 
sources 14-1/14-2 and the reflected real images will be juxtaposed 
adjacent to one another when viewed by observer O' near COC point 11c'. 
It will now be apparent to those skilled in the art that this form of 
projection display system not only provides magnification of the image 
source picture size, with adequate brightness and viewing volume, but also 
allows a reduction in display envelope dimensions in the viewing 
direction, while providing a distortionless display due to the optical 
superposition of the lens pupil and viewer eyepoint. The juxtaposition of 
multiple real images, without gaps between adjacent images, is another 
benefit, although multiple source subsystems (source, lens and 
beamsplitter portions) are required. It will be understood that portions 
18a and 18b can comprise a single beamsplitter, of appropriate shape and 
area, or can be separate beamsplitter elements. 
Referring now to FIG. 3, in a system 30 a plurality M of different 
observers (say, an i-th observer O.sub.i and a j-th observer O.sub.j, 
where 1.ltoreq.i.ltoreq.j.ltoreq.M) can each be provided with at least one 
individually viewable real display image, which is viewable only within a 
certain associated viewing volume (volume V.sub.i for observer O.sub.i or 
volume V.sub.j for observer O.sub.j) and is therefore not viewable by any 
of the other spatially-separated viewer, if a suitable minimum viewer 
spacing is maintained. The screen 32 can have a viewing surface 32a of any 
shape, if a retro-reflective material is used. Each source means 33 (e.g. 
first source means 33i or second source means 33j) uses its own display 
means 34 and beamsplitter means 36, with its lens means 38 interposed 
therebetween (e.g. first source 33i has a first display means 34i, first 
lens means 38i and first beamsplitter 36i, while second source 33j has a 
second display means 34j, second lens means 38j and second beamsplitter 
36j). The lens pupil P.sub.i /P.sub.j size determines the viewing volume 
size V.sub.i /V.sub.j. The application of this method can be duplicated as 
many times as desired within the same spherical projection screen, 
provided spatial limitations of the projectors and observers are accounted 
for. Limitations of this projection approach are geometrically defined by 
the required offset distances between observers, and the corresponding 
desired screen bandwidth. Once an instantaneous observation point O.sub.k 
of any one k-th observer overlaps into the zone of reflectance of another 
observer, the two images will begin to interfere with each other. By using 
several optical projection means 34 and accompanying beamsplitters 36, 
each with its own projection lens means 38, with the same retro-reflective 
surface, each image will only be returned back towards its origination 
source. This allows for each subsystem 33 to project and receive its own 
imagery regardless of what is being concurrently projected onto the same 
area of the retro-reflective screen. 
Unfortunately, the helpful characteristic of retro-reflective screen 
material that returns light in the direction from which it came, also 
serves as a limitation because of the narrow viewing volume V in which it 
can be observed. To overcome this beamwidth limitation of the 
retro-reflective screen, the projection lens 38 can be designed with a 
large pupil P and placed appropriately in the optical path to enlarge the 
viewing volume as a function of something other than the beamwidth of the 
screen. This allows the advantages of the retro-reflective screen material 
to be obtained while simultaneously overcoming its limitations. 
For the most effective approach, each projector subsystem 33i must be such 
that all imagery emanating from that i-th projector be from a single exit 
pupil Pi in the optical source. It will be understood that if a 
multiple-pupil source were to be used, such as a CRT-type projector with 3 
exit pupils, the beamwidth characteristics of the screen material must be 
widened appropriately to account for the extreme exit pupil positions. A 
single-gun light-valve projector is a good example of a full color 
projector source with a single exit pupil in the projection optics ideally 
suited for this application. In theory, any projection display system 
which abides by the geometry and screen characteristics prescribed herein 
and consists of a single exit pupil projection subsystem as the lone 
source of imagery for each of the plurality of observers, is usable in 
this invention. 
If a spherical projection screen structure is used, then the screen must be 
as close to spherical in shape as possible given practical manufacturing 
limitations. Any deviation resulting in a screen tangent slope error must 
be accounted for in the determination of screen material beamwidth. In a 
theoretical case where the spherical projection screen is of perfect shape 
and the observation volume is reduced to a point, the screen material 
bandwidth can be reduced so that the reflective characteristics is like 
that of a mirror. The screen material must be reflective and diffuse to 
the degree necessary as determined by the system geometry and required 
viewing volume. Several materials have been previously developed to 
directionally control incident light reflectance. These range from 
specialized paints to minutely embedded concave/convex shapes in a highly 
reflective material. For optimum performance and maximizing usable viewing 
areas, the beamwidth of the screen material must be determined by the sum 
of the required viewing volume in addition to the slope error of the 
projection screen itself. The screen material best suited for this 
application is one which has a screen gain exhibiting a relatively flat 
peak and a decisive cutoff, typically referred to as a "top hat" function. 
If a projection screen material with direction reflectance properties is 
used, then simultaneous viewing of different information by different 
observers of a single screen must utilize a specific system geometry: a 
spherical-shaped projection screen is used with single-exit-pupil 
projection means as an imagery source for each observer. The beamsplitter 
may be eliminated, if observer and projector can be placed at conjugate 
points relative to the spherical projection screen center of curvature, so 
that the chief ray emitted from the projector is redirected back towards 
only the associated viewer, and the other viewers receive little or no 
light from the non-associated projectors. 
While several presently preferred embodiments of our novel multiple 
observer/common projection screen system have been described in detail 
herein, many variations and modifications will now occur to those skilled 
in the art. It is our intent, therefore, to be limited only by the scope 
of the appending claims, and not by way of the details and 
instrumentalities described with respect to these embodiments.