Abstract:
A projection apparatus comprising a three-dimensional screen, a convex mirror, two or more projectors, and an assemblage of flat mirrored surfaces that share at least one common edge or apex that faces the convex mirror. The flat mirrored surfaces fold the images from separate projectors into a common image that has single optical axis aligned with principal axis of the convex mirror so that the combined image is reflected from the convex mirror onto the three dimensional screen.

Description:
CROSS-REFERENCED TO RELATED APPLICATIONS 
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   FEDERALLY SPONSORED RESEARCH 
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   SEQUENCE LISTING OR PROGRAM 
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   BACKGROUND OF THE INVENTION 
   Projecting images onto spherical and other three-dimensional projection screens presents several issues that bear on the quality of the images. The most significant problem faced by early versions of spherical screens was adequate coverage of the screen surface with an image. For example, systems that used multiple projectors mounted inside a spherical rear projection screen such as the one described in U.S. Pat. No. 3,586,432 provided very limited coverage of the screen and made it difficult to access the projectors for maintenance. Systems using external projectors, such as the kinds described in U.S. Pat. No. 4,427,274 and U.S. Pat. No. 4,859,053, provided better access to the projectors but were either limited to producing an image on a hemispherical screen or required projectors to be located at opposite ends of a sphere. These limitations have been substantially overcome by systems in which the projectors project the image onto one or two convex mirrors within the sphere from which the image is reflected onto the surface of the screen. This approach is well described in U.S. Pat. No. 6,409,351. 
   Two significant challenges associated with three-dimensional projection systems are the brightness and resolution of the image formed of the screens. Image brightness is largely a function of the light output of the projector and the surface area of the screen, although other variables such as the characteristics of the screen can be important. Light output from a projector is usually specified in terms of lumens for digital projectors and in terms of lamp wattage for film projectors. Another aspect of brightness is the flatness of the illumination field (i.e., the reduction of brightness from the center of the image to the edges). Although an ideal projector would produce images that do not suffer from falloff of brightness at the edges, all projectors experience this problem to varying degrees. 
   Resolution is a dominant factor in determining the sharpness and clarity of the image on the screen. For film projection systems, resolution has traditionally been specified as the resolving power expressed as line pairs per millimeter (lp/mm). The resolving power of digital projection systems is usually expressed in terms of the number of pixels used to form the image, typically provided in two numbers, such as 800×600, where the first number refers to the number of pixels from across the width of the display area, and the second number refers to the number of pixels across the height of the display area. Both the brightness and perceived resolution of the image will decrease as the image size increases, and at some point the brightness or the resolution become unacceptable to viewers as the size of the screen increases. 
   One practical consideration for three-dimensional projection display systems is the cost of the projectors needed to produce images with adequate levels of brightness and resolution. The vast majority of projectors are manufactured in a few standard formats and are known generally as commodity projectors. Although it is possible to manufacture projectors with a custom format, such projectors are prohibitively expensive for most applications. In addition, the cost of commodity projectors increase markedly as the brightness and resolution capabilities are increased. For example, an LCD projector with a 3000 lumen output and native resolution of 1024×768 pixels will typically cost about three times as much as an LCD with a 1500 lumen output and native resolution of 800×600 pixels. 
   Another practical consideration is the aspect ratios of commodity projectors. The most common aspect ratio for digital projectors is the 4:3 ratio used in standard-format computer monitors, televisions, and digital audio-visual presentation systems. However, there is a strong trend towards wide-format projection systems such as the high-definition television (HDTV) aspect of 16:9. This presents a problem for three-dimensional projection systems project images onto screens with spherical, hemispherical, and like configurations, because such systems require the projection of a circular image onto a mirror and thus can only use the height of the display area. For example, a projection system using a projector with a resolution 1920×1080 pixels will only be able to project a circular image of 1080 pixels diameter, thereby wasting much of the display potential of the projector. In the context of flat screen systems, issues concerning brightness, resolution, and aspect ratio are often addressed by concurrently using multiple projectors. For example, it is possible to use two projectors together to superimpose identical images onto a common screen to increase the brightness of an image. Similarly, multiple projectors can be used to tile images onto a screen. In both cases, the images from the physically-separated projectors converge at the screen and the problem of adequately registering the images from the physically-separated projectors is addressed by means such as keystone correction (trapezoidal distortion) or lens shifting (physically moving a lens off the central optical axis). However, these approaches to using multiple projectors do not work well with three-dimensional projection systems featuring convex mirrors in the optical path because the images do not converge until they reach the convex mirror surface. Unless the images are combined prior to reaching a convex mirror, they will suffer from image distortion and registration problems that cannot be corrected with keystone correction or other means of compensating for distortion. 
   There is a need for a three-dimensional projection system that overcomes the problems described above, which the present invention addresses as discussed hereafter. It is an object of the present invention to provide an effective and economical means of combining images from multiple projectors in the context of three-dimensional projection systems. 
   SUMMARY OF THE INVENTION 
   The present invention is a projection system comprising a three-dimensional screen, a convex mirror, two or more projectors, and an assemblage of flat mirrored surfaces that share at least one common edge that faces the convex mirror. The general principle underlying the system is that the flat mirrored surfaces fold the images from separate projectors into a tiled image that has single optical axis aligned with principal axis of the convex mirror so that the combined image is reflected from the convex mirror onto the three dimensional screen. For example, an embodiment consisting of two projectors might have the projectors opposingly situated so that they face 90 degrees away from the principal axis of the convex mirror, with each projector transmitting an image to a flat mirrored surface set at 45 degrees to the projector. Because, the flat mirrored surfaces share a common edge on the side nearest to the convex mirror, they fold the images from each projector 90 degrees and combines them into a tiled image that is directed to the convex mirror along its principal axis. The single image is thereafter reflected by the convex mirror onto the screen in the form of a virtual image. 
   A critical requirement for two-projector systems is that the flat mirrored surfaces share a common edge with a point located on the principal axis of the convex mirror. An practical embodiment of such an assemblage is a triangular prism made of glass in which the two leg faces are aluminized and the hypotenuse base is left uncoated. For systems that use three or more projectors, the flat mirrored surfaces need to share a common point forming an apex. For example, a system encompassing four projectors could utilize a quadrilateral-pyramid form in which the four triangular faces are aluminized to reflect light. The number of mirrored surfaces on the reflecting assemblage will correspond to the number of projectors. For example, a two-projector system will utilize a mirror assemblage with two mirrored surfaces that face the convex mirror. 
   A significant advantage of the present invention is that it enables two or more projectors to be combined to project a tiled image on a three-dimensional screen at greater resolution and brightness than can be achieved with a single commodity projector. One of the significant limitations of commodity projectors is that they are not specifically designed for tiled displays and therefor it is impossible to align multiple projectors so that the images match perfectly at the edges. Another significant limitation is that their images suffer from falloff of brightness at the edges. Left unaddressed, these limitations necessitate that a tiled image will either have a gap between the images or a bright seam where they overlap. In the case of digital projectors, a practical solution is to overlap the images by a sufficient amount and use software to modify the pixels in the overlap region so as to reduce the brightness to make the seam blend with the adjacent portions of the image. For example, if two projectors with at resolution of 1920×1080 pixels are combined using the means of the invention, a circular image with a diameter of 1920 pixels can be projected onto the convex mirror with an overlap of the two images of 120 pixels. Because this width constitutes a useful interval over which to smoothly blend the seam into the adjacent portions of the images, the present invention allows for a very efficient use of the projector potential. Further refinements to the projected images can be made by using software to modify the video input so as to correct for geometric distortions and uneven brightness. 

   
     BRIEF DESCRIPTIONS OF THE DRAWINGS 
       FIG. 1  is a sectional side view illustrating an embodiment of the system employing two projectors. 
       FIG. 2  is a sectional side view illustrating an alternative embodiment of the system employing two projectors. 
       FIG. 3  is a top view illustrating an arrangement of four projectors directing images into a pyramidal-shaped assemblage of reflecting surfaces. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a system in accordance with the invention employing a first projector  1  and a second projector  11  for producing an image on an outer surface  22  of a three-dimensional screen  20  in the form of a hollow sphere substantially enclosing a spherical space. The basic arrangement of the preferred embodiment is as follows. The screen  20  is secured to a base  24 . A convex mirror  26  is suspended on a rod  28  extending downward from a mounting plate  30  attached to the screen  20 . The first projector  1  and the second projector  11  are horizontally disposed within the base  24  with their lenses  3  and  13  opposingly facing a mirror assemblage  9  in the shape of a triangular prism and which comprises a first reflective surface  5  and a second reflective surface  15  which share a common apex edge  8 . The apex edge  8  of mirror assemblage  9  is centered on the principal axis of convex mirror  26  and situated at 90-degree angles to the optical axes of lens  3  of projector  1  and lens  13  of projector  11 . The first projector  1  projects an off-axis image  7  horizontally onto reflective surface  5  which reflects the image upward through an aperture  19  in screen  20  and onto the lower surface of convex mirror  26 . Similarly, the second projector  11  projects an off-axis image  17  horizontally onto reflective surface  15  which reflects the image upward through an aperture  19  in screen  20  and onto the lower surface of convex mirror  26 . Because reflective surface  5  and reflective surface  15  share a common apex edge, image  7  and image  17  are reflected upward and combined into a single image  32  having a common optical axis centered on apex edge  8  and in coincident alignment with the principal axis of convex mirror  26 . The convex mirror  26  reflects the projected image  32 , dispersing it over most of the inner surface  34  of screen  20 . Because screen  20  is translucent, the image appears over most of the outer surface  22  of the screen except at the regions along the axis of projection obscured by the convex mirror  26  and the base  24 . 
   Projector  1  and projector  11  may be any kind of projector for producing still or moving images but in the preferred embodiment of the invention illustrated in  FIG. 1 , they are digital projectors. When desired, the images produced by projector  1  and projector  11  may be appropriately corrected by using means such as software and off-axis projection to compensate for any distortion arising out of the geometry of the screen  20 , convex mirror  26 , or the mirror assemblage  9 , so that the image is substantially free of distortions when projected on screen  20 . 
   The mirror assemblage  9  is preferably constructed by aluminizing the faces of a triangular prism made of glass. In the preferred embodiment, the apex angle of the mirror assemblage is 90 degrees and the optical axes of lenses  3  and  13  are aligned horizontally such that their optical axes strike the reflective surfaces  5  and  15  at an incident angle of 45 degrees. However, other means of construction and other combinations of apex angles and orientations of the projectors may be used to achieve a substantially equivalent effect. For example, a mirror assemblage in which the angle of the apex  8  is 120 degrees can be used in conjunction with projectors in which the optical axis is aligned 30 degrees from the horizontal and strikes a reflective surface at an incident angle of 60 degrees. Similarly, it is possible to construct a mirror assemblage in which the apex angle is adjustable and thus accommodating selectable variations in the orientation of the projectors to the mirror assemblage. 
     FIG. 2  shows and alternative embodiment in which projector  1  and projector  2  are vertically disposed within base  24 . The assembly of the screen  20 , convex mirror  26 , and base  24  are substantially the same as shown in  FIG. 1 . However, the lens  3  of the first projector  1  faces a first flat mirror  2  and the lens  13  of the second projector faces a second flat mirror  12 . The first projector  1  projects an off-axis image  7  vertically onto the first flat mirror  2  which directs the image horizontally onto reflective surface  5  which in turn reflects the image upward through an aperture  19  in screen  20  and onto the lower surface of convex mirror  26 . Similarly, the second projector  11  projects an off-axis image  17  vertically onto the second flat mirror  12  which directs the image horizontally onto reflective surface  15  which in turn reflects the image upward through an aperture  19  in screen  20  and onto the lower surface of convex mirror  26 . In this illustrative embodiment, reflective surface  5  and reflective surface  15  are incorporated into a mirror assemblage  9  in the shape of a triangular prism and share a common apex edge  8  as described above with respect to the preferred embodiment. The apex edge  8  of mirror assemblage  9  is centered on the principal axis of convex mirror  26  and thus the images  7  and  17  are combined into a single image  32  having a common optical axis centered on apex edge  8  and in coincident alignment with the principal axis of convex mirror  26 . The convex mirror  26  thus reflects the projected image  32  and disperses it over most of the inner surface  34  of screen  20 . 
   The invention is not limited to the use of two projectors.  FIG. 3  shows in top view an example of how the projectors could be arranged in an embodiment in which four projectors are used in a system. The mirror assemblage  50  comprises pyramidal shape with four triangular reflective faces  51 ,  52 ,  53 , and  54  which converge to an apex  55 . Projector  41  is oriented to project an image onto reflective face  51  and is oppositely situated to projector  45  which is oriented to project an image onto reflective face  53 . Projector  43  is situated at right angles to projector  41  and projector  45  and is oriented to project an image onto reflective face  52 . Projector  47  is oppositely situated to projector  43  and is oriented to project an image onto reflective face  55 . When the four images are reflected from the mirror assemblage  50 , they combine to form a common image with its optical axis centered over the apex  55  of the mirror assemblage. By centering the apex  55  beneath the principal axis of convex mirror such as shown in  FIGS. 1 and 2 , the combined image can be dispersed over a three-dimensional screen as described in those figures. 
   There are various other possible arrangements of elements that one skilled in the art could substitute and obtain similar results as achieved herein. Such arrangements include the addition of reflective surfaces as well as alternate configurations of the optical elements described above. Likewise, the system can be embodied to produce images viewable from the outside of the space enclosed by the screen (i.e., rear-screen projection) or to produce images viewable from within the space enclosed by the screen (i.e. front-screen projection). All these possibilities are within the scope and spirit of this disclosure.