Real time autostereoscopic displays using holographic diffusers

Methods and apparatus for providing real time autostereoscopic displays using holographic diffusers are disclosed. In accordance with the method, a diffuse holographic optical element is fabricated whereby an image projected onto the diffused holographic optical element will be viewable only over a limited horizontal extent of less than the separation of a typical viewer's eyes. Then the holographic optical element is used as a projection screen for a plurality of projectors spatially positioned with respect to each other so that the horizontal position within which the image of each projector is viewable is substantially contiguous with the horizontal position of viewability of the respective adjacent projector whereby the autostereoscopic effect is achieved. Various embodiments, including the transmission mode and reflection mode holographic optical elements, are disclosed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the field of three-dimensional displays. 
1. Prior Art 
Traditional refractive optics has produced two screen-like mechanisms for 
three-dimensional viewing without headgear. The first screen mechanism is 
the Fresnel screen. This technique utilizes twin images projected onto a 
Fresnel lens, as if it were a projection screen. In a typical situation, 
the view is as far in front of the lens as the projectors are behind it. 
If the projectors are spaced the same as the viewer's eyes, then the 
viewer can be positioned so that each eye receives the appropriate image, 
and stereo vision results. Unfortunately, this process is limited by size 
of the system's exit pupil, and only one viewer is possible, since his 
head will fill the exit pupil. Also, to avoid loss of viewing, the 
viewer's movement must be restricted. 
The second screen mechanism uses lenticular plates in the form of sheets 
consisting of hundreds or thousands of narrow cylinder lenses arrayed in 
parallel, and produces some important improvements. In essence, the 
Fresnel screen of the first system is replaced by a sandwich of diffusing 
medium between two lenticular plates. The lenticules face outward and run 
vertically. Lenticules on the two plates are carefully aligned to be 
parallel to and in line with each other. Again, the twin images are 
projected onto the screen. In this case however, each lenticule on the 
first sheet images the strip of image falling on it onto the diffusing 
material. The lenticules of the second sheet then allow viewing of each 
total image only from particular angles. Because of the diffusion material 
and arrangement of lenticules, the vertical axis of the exit pupil is 
extended. Also, as lenticules in the second sheet image strips not only in 
line, but perhaps one or two strips adjacent, multiple exit pupils are 
created, allowing multiple viewers. An additional advantage of the 
lenticular screen is that a multiplicity of images projected onto the 
screen will be presented to the viewer as a continuous, autostereoscopic 
field. This permits the viewer to move within the field and to perceive 
motion parallax. A disadvantage to this technique, however, is the 
difficulty of manufacturing large lenticular sheets and performing the 
careful alignment needed to assemble the screen. 
The present invention preserves the advantages of a continuous 
autostereoscopic field for motion parallax and multiple viewer 
capabilities, but does so in a much more easily and inexpensively 
manufactured system. 
BRIEF SUMMARY OF THE INVENTION 
Methods and apparatus for providing real time autostereoscopic displays 
using holographic diffusers are disclosed. In accordance with the method, 
a diffuse holographic optical element is fabricated whereby an image 
projected onto the diffused holographic optical element will be viewable 
only over a limited horizontal extent of less than the separation of a 
typical viewer's eyes. Then the holographic optical element is used as a 
projection screen for a plurality of projectors spatially positioned with 
respect to each other so that the horizontal position within which the 
image of each projector is viewable is substantially contiguous with the 
horizontal position of viewability of the respective adjacent projector 
whereby the autostereoscopic effect is achieved. Various embodiments, 
including transmission mode and reflection mode holographic optical 
elements, are disclosed.

DETAILED DESCRIPTION OF THE INVENTION 
First referring to FIG. 1, a schematic diagram of an autostereoscopic 
display in accordance with the present invention may be seen. As shown, a 
linear array of projectors 20(a) through 20(n) superimpose individual 
images onto a diffuse holographic optical element 22. The particulars of 
each image, especially the viewpoint perspective, is determined by the 
angular position of the zone associated with its projector, the projectors 
in the preferred embodiment using monochromatic light sources. Because of 
the characteristics of the holographic optical element 22, each image is 
only visible from viewpoints in the associated viewing zone. By way of 
example, the image projected onto the back of the holographic optical 
element 22, a transmission holographic optical element, is viewable only 
in viewing zone 24(a), though within that zone one may focus on any part 
of that image, depending upon where on the holographic optical element 22 
one is looking. In general, the viewing zones, such as zone 24(a) will 
have a substantial vertical extent so that the exact elevation of the 
viewer's eyes will not be critical. The horizontal extent however, will 
normally be some fraction of the separation of a typical viewer's eyes so 
that only one eye may view the image viewable through viewing zone 24(a) 
at any one time. 
The image of camera 20(b) is also projected onto the holographic optical 
element 22 to overlay the image of camera 20(a). Because of the different 
angle from which the image is projected to the holographic optical element 
22, the viewing zone 24(b) for the image of projector 20(b) will be 
horizontally displaced from the viewing zone 24(a) of the image of 
projector 20(a). Preferably the various viewing zones are contiguous so 
that each eye of the viewer may always see one, but only one, image for 
any one position of the eye. Thus when a person is positioned such that 
each eye is in a separate viewing zone, as will normally be the case, 
parallax is seen and a single three dimensional image is perceived. 
Horizontal motion parallax is of course also achieved within the range of 
the totality of the viewing zones, with multiple viewers being limited 
only by the actual size of the combined viewing zones. 
In the lower limit, the number of projectors may be two, with the width of 
the viewing zones 24(a) and 24(b) each being on the order of 0.5 feet. So 
long as the viewer positions himself so that the junction between the two 
viewing zones 24(a) and 24(b) was located between the eyes of the viewer, 
a stereoscopic display would be achieved, and of course if movie 
projectors were used, a real time autostereoscopic display would be 
achieved. Ideally however, a relatively large number of relatively narrow 
viewing zones and thus a relatively large number of projectors should be 
used so that the difference between the images of adjacent projectors and 
thus adjacent viewing zones are slight, and no step change is perceived as 
the viewer moves his head in a horizontal direction within the combined 
viewing zone. Also, to the extent that viewing zones are relatively 
narrow, and thus adjacent images are quite similar, the viewing zones may 
in fact overlap, though in the region of overlap, the intensity of the 
perceived image may be greater as two overlayed images will actually be 
viewed in the overlapping region. 
For a stereoscopic display as illustrated in FIG. 3, a total viewing zone 
on the order of 12 to 18 inches wide might be desired. If each individual 
viewing zone were on the order of one inch wide, the number of projectors 
required would be in the range of 12 to 18. Assuming of course that movie 
projector-type projectors were used, such a large number of synchronized 
projectors would be relatively expensive. Obviously, however, other types 
of projectors or projection means which are less expensive and/or have 
other advantages may also be used. By way of example, rather than to use 
independent projector and reels of film which are synchronized to each 
other, a larger film strip capable of containing the images for all 
projectors could be used, which would enable the use of a single pair of 
reels, film advance system, etc., and which would automatically 
synchronize all images. Such a film strip in such a system might run in 
the approximate direction of the linear projector array, which of course 
is normally horizontal, though be sufficiently inclined thereto so that 
the images on the film strip, while appearing one after another for the n 
projectors, would be angled across the film strip so that successive 
images in the projector could be effectively nested, thereby advancing the 
film strip to the next images by only advancing the film strip some small 
fraction of the length of n successive individual images. Obviously such 
an arrangement, as noted before, would require only a single film handling 
and advancing system, and would provide automatic synchronization of the n 
images. While n sets of lenses would be required, and perhaps multiple 
light sources would be required for sufficient light intensity, these 
components would be of relatively low cost. 
Obviously other types of imaging systems may also be used. By way of 
example, to the extent that light from a display such as a cathode ray 
tube display can be made adequately monochromatic through the use of a 
proper display, filtering, etc., the linear array of "projectors" might 
actually be different horizontal strips of a high resolution computer 
controlled graphics display, imaged in an overlapping manner onto the 
holographic optical element 22. Such an arrangement would provide computer 
generated and controlled stereographic displays, allowing the viewer to 
interact with the stereographic display to rotate the object, alter the 
object form, etc. Similarly, a television display might be used, or a 
liquid crystal display might be used as a light mask to form the desired 
image strips. If the diffuse holographic element was made with the ground 
glass object tilted at the achromatic angle, the resulting hologram could 
be illuminated with broadband (white) light. The real image of the 
diffuser will be dispersed (chromatically smeared) along the achromatic 
angle. If the ground glass object is longer than the calculated 
dispersion, then there will be an area equal to the difference between the 
object and the dispersion where all colors of the spectrum overlap. This 
would serve to create a holographic diffuser that could be used in a full 
color display. 
The achromatic angle is found by using the diffraction focusing equations 
to determine the focus points for various colors, then drawing a line 
through the points to find the angle of dispersion. 
The creation of the diffuse holographic optical element is closely related 
to its operation. Referring to FIG. 2, one possible method of creating the 
diffuse holographic optical element may be seen. This figure shows the 
setup which could be used to expose holographic plate 26 to form the 
holographic optical element 22 of FIG. 1. Relating the orientation of the 
holographic plate 26 to the holographic optical element 22 of FIG. 1, FIG. 
2 would represent a top view of the setup. As shown in this figure, a 
monochromatic light source, generally a laser 28, directs a beam to a beam 
splitter 30 which allows part of the beam to pass therethrough to a 
spatial filter 32 and part of the beam to be reflected to reflectors 34 
and 36 to a second spatial filter 38. The spatial filter 32 spreads the 
beam to illuminate a diffuse plate, namely a ground glass plate 40 which 
in effect illuminates the holographic plate 26 with diffuse the 
monochromatic light from the ground glass plate 40. At the same time, 
spatial filter 38 spreads the beam reflected by the beam splitter 30 and 
mirrors 34 and 36, with the spread beam being reflected by a concave 
mirror 42 toward a focal point 44 behind the holographic plate 26. Thus 
the holographic plate 26 is simultaneously illuminated by an object beam 
providing a diffuse light pattern through ground glass plate 40, and a 
reference beam having a focal point at point 44. After exposure of the 
holographic plate 26, the plate of course is developed and may be used or 
reproduced for use as the holographic optical element 22 of FIG. 1. 
Relating the setup of FIG. 2 for exposure of the holographic plate 26 with 
the display system of FIG. 1, some of the geometric parameters may be 
readily identified. If the light source for exposing holographic plate 26 
is the same wave length as the source for the projector, the focal point 
44 of the reference beam is the virtual focal point for the projector 
associated with the primary or central viewing zone. The optical axis of 
this beam is the same as the optical axis of the primary projector with 
respect to the position of the holographic plate. The size (length and 
width) of the area of the ground glass plate 40 which is illuminated by 
the object beam determines the size of the individual viewing zone of each 
projector. Similarly, the distance of the ground glass plate 40 from the 
holographic plate 26 will be equal to the distance of the viewing zone 
from the diffuse holographic optical element in FIG. 1. Also, while the 
ground glass plate has been used in the exemplary setup herein described, 
other transmissive or reflective diffuse elements may be used such as, by 
way of example, an opaque matt finished material which of course would be 
lighted from the front rather than from the back as shown in FIG. 2. 
While the system illustrated with respect to FIGS. 1 through 3 utilizes a 
transmission mode diffuse holographic optical element, a reflection mode 
system may also be used. By way of example, a schematic illustration of 
such a system is presented in FIG. 4. As before of course the viewing zone 
50 of the display is in front of the diffuse holographic optical element 
52, though the projected images are projected to the front thereof rather 
than from the rear. For convenience, one or more mirrors 54 may be 
provided so that the projector array 56 may be positioned as desired out 
of the way of the viewer in any convenient location.