Television – Panoramic
Reexamination Certificate
2000-01-04
2003-08-26
Philippe, Gims S. (Department: 2713)
Television
Panoramic
Reexamination Certificate
active
06611282
ABSTRACT:
1. FIELD OF THE INVENTION
This invention relates to panoramic image sensing of a super wide-angle field of view, and in particular, it relates to such image sensing using a two-mirror subsystem that is substantially self-corrected. The term “panoramic” means a 360° field of view in a horizontal plane while the term “super wide-angle” means a 120° or larger field of view in a vertical plane. Preferably, the field of view in a vertical plane is greater than about 1800. More preferably, it is greater than about 200°. And, for the desired apparatus, most preferably, it is greater than about 260°.
2. BACKGROUND OF THE INVENTION
A perspective imaging system collects rays of light from the scene that pass through a single point of reference and projects them onto a sensing element such as film or a charge coupled device (CCD). The single point of reference in the perspective imaging system is known as the viewpoint of the system. Yamazawa et al., “Omnidirectional Imaging with Hyperboloidal Projection”, IEEE International Conference on Robotics and Automation, 1993, by Nalwa, “A True Omnidirectional Viewer”, ATT Bell Laboratories Technical Memorandum, BL0115500-960115-01, January 1996 and by Nayar, “Omnidirectional Video Camera”, DARPA Image Understanding Workshop, May 1997, all incorporated herein by reference, describe the need for a single viewpoint. We have determined that the nature of light propagation through the imaging system and the shape of imaging sensor may introduce geometric transformations in the image projected onto the sensing element. In a large number of applications including surveillance, remote sensing, navigation, model acquisition, virtual reality, computer vision and robotics, it is desirable that these geometric transformations be corrected for the purposes of viewing and analysis. The lack of a single viewpoint introduces aberrations in pupils which manifests itself as uncorrectable geometric transformations (distortions).
A classification of imaging systems based on their field of view is:
1. Traditional imaging systems that image a narrow field of view, usually an octant of the sphere of view (up to 90 degrees).
2. Panoramic imaging systems that image a panorama of the scene. The field of view can be looked upon as a sphere truncated by two parallel planes giving a 360 degree field of view in the horizontal and a limited field of view in the vertical.
3. Omnidirectional imaging systems that image substantially spherical or substantially hemispherical fields of view.
A classification of the same based on their optical components is:
1. Dioptric systems that use only refractive elements;
2. Catoptric systems that use only reflective elements; and
3. Catadioptric systems that use a combination of reflective and refractive elements.
Perhaps the simplest system that provides perspective projection is a pinhole camera. Traditionally, lenses have been used in place of a pinhole simply because of their superior light gathering ability. But a lens, however wide angle it may be, is limited to a hemispherical field of view while still maintaining a single viewpoint, although it is physically challenging to design such wide-angle lenses.
Lenses that deviate from maintaining a single viewpoint have been described by E. H. Hall et al., “Omnidirectional Viewing using a Fish Eye Lens”, SPIE Vol. 728 Optics, Illumination and Image Sensing for Machine Vision, 1986, pp. 250, incorporated herein by reference. Such lenses have been known to achieve larger than hemispherical fields of view, up to 280 degrees in the vertical plane. However, these so-called fish eye lenses are significantly larger and more complex than conventional lenses, and suffer from severe geometric distortions and loss of resolution in the image over the field of view. Moreover, the lack of a single reference point for the rays of light imaged by the lens disqualifies their usage in a large number of applications, described above. U.S. Pat. No. 5,185,667 to Zimmerman and U.S. Pat. No. 5,359,363 to Kuban are descriptions of additional uses of fish eye lenses, all incorporated herein by reference. Thus, of the known dioptric systems, those that seek to preserve a single viewpoint are limited to a narrow field of view.
Using only reflective elements, catoptric imaging systems are the closest to ideal imaging systems. The lack of refractive elements removes the possibility of chromatic aberrations allowing these systems to operate under a wide range of illumination wavelengths. But the greatest advantage of catoptric systems is that the reflective elements can be matched to correct for almost all aberrations that plague imaging systems, including field curvature and spherical aberration in pupils. A disadvantage of such systems is their light gathering ability which requires them to operate with lower F-numbers.
Catadioptric systems have been known to span the entire gamut in terms of field of view, from omnidirectional to panoramic to traditional narrow fields of view. The simplest wide-angle catadioptric system comprises two optical components: a curved non-planar primary reflector placed in front of a objective lens. The complete class of mirror lens combinations that capture wide-angle views while maintaining a single viewpoint has been described by Nayar et al., “Catadioptric Image Formation”, IEEE International Conference on Computer Vision, January 1998, incorporated herein by reference. Further, for a mirror to have a single viewpoint it is necessary that the mirror be a surface of revolution of a two dimensional curve. Daniel Drucker et al., “A Natural Classification of Curves and Surfaces With Reflection Properties”, Mathematics Magazine, vol, 69, no. 4, pp. 249-256, 1996, incorporated herein by reference, have shown that the only two dimensional curves with focal properties are conic sections. Hence, the only mirrors that maintain a single viewpoint are conic sections of revolution. Further, those that can be realized in practice are paraboloids, hyperboloids and ellipsoids. It is worthwhile mentioning here that while the sphere is an intuitive solution it is impractical because the focus is at the center of the sphere, and so is the cone for the reason that the focus is at the apex of the cone.
A catadioptric omnidirectional imaging system has been described in U.S. Pat. No. 5,760,826 to Shree Nayar, incorporated herein by reference. The system uses a convex paraboloidal mirror, telecentric relay objective lens and a standard camera lens which projects an annular image of a substantially hemispherical scene captured from a single viewpoint onto a planar sensing device such as a CCD. A disadvantage of the system is that the use of an aspheric surface results in residual field curvature. This prevents its usage with a low F-number compact system.
A more complex catadioptric panoramic imaging system is one that uses two reflecting surfaces in conjunction with a relay objective lens. In such a system the primary reflector collects scene intensity information which is then reflected off a secondary reflector into the relay objective lens.
For the entire system to have a single viewpoint, while the primary mirror must have a single viewpoint (which is the viewpoint of the overall system) it is not necessary for the secondary mirror to have a single viewpoint. The tools for developing such systems of mirrors that have an overall single viewpoint have been described by Conbleet, “Microwave and Optical Ray Geometry”, Published by John Wiley and Sons, 1984, incorporated herein by reference. It can be shown that a variety of mirror pairs, some with exotic shapes, can be used to construct catadioptric imaging systems of interest. However we have determined that mirrors with complex shapes produce confounding optical aberrations. Moreover, even seemingly simple surfaces such as quadrics (surfaces of revolution of planar 2nd order algebraic curves) can produce complex optical aberrations. In our investigation we have found that the only quadrics that can form optically acceptable two mirror systems are conic s
Korein James
Nayar Shree K.
Peri Venkata N.
Trubko Sergey
Bateman Andrew J.
Katten Muchin Zavis & Rosenman
Philippe Gims S.
Remote Reality
Villacorta Gilberto M.
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