Omnidirectional visual sensor having a plurality of mirrors with surfaces of revolution

An omnidirectional visual sensor which can observe a 360-degree panoramic field area, obtain a sharp image and correctly transform a central projection with respect to the image and can be minimized as a whole includes a convex mirror with a surface of revolution having a focal point, a plurality of mirrors with surfaces of revolutions having at least one focal point, a photoreceiving lens system receiving light reflected by the convex mirror with the surface of revolution and the plurality of mirrors with surfaces of revolutions and an image acquisition surface receiving the light received in the photoreceiving lens system and converting the same to an electric signal, and the convex mirror with the surface of revolution and the plurality of mirrors with surfaces of revolutions are so arranged that the focal point of a first mirror included in the convex mirror with the surface of revolution and the plurality of mirrors with surfaces of revolutions aligns with the focal point of a second mirror, included in the convex mirror with the surface of revolution and the plurality of mirrors with surfaces of revolutions, further reflecting light reflected by the first mirror.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a visual sensor for observing all 
directions, and more particularly, it relates to an omnidirectional visual 
sensor utilizing a mirror with a surface of revolution. 
2. Description of the Background Art 
Conventionally known omnidirectional visual sensor utilizing a mirror with 
a surface of revolution include a visual sensor utilizing a conical 
mirror, a spherical mirror, a hyperboloidal mirror or a paraboloidal 
mirror. Referring to FIG. 1, an omnidirectional visual sensor employing a 
hyperboloidal mirror includes a vertically placed hyperboloidal mirror 
facing downward and a camera vertically placed under the mirror facing 
upward. The hyperboloidal mirror (hyperboloid of two sheets) has a 
geometric property of having two focal points, and a certain geometrical 
relation holds between the two focal points and the hyperboloid thereof. 
When aligning the lens center of the camera with the first focal position 
of the hyperboloid, therefore, it is possible to form an optical system 
such that extension of light from an arbitrary point P in the environment 
necessarily passes through the second focal point B of the mirror 
regardless of the position thereof as shown in FIG. 1, i.e., a perspective 
transformation coordinate system. In other words, when setting the 
principal point of the photoreceiving lens system on the second focal 
point B of the hyperboloidal mirror, an image acquired in the 
omnidirectional visual sensor employing the hyperboloidal mirror can be 
readily transformed into a coordinate system having the first focal point 
C of the mirror as a temporary lens center. An omnidirectional visual 
sensor utilizing a conical or spherical mirror cannot perform such 
transformation. 
Details related to a conventional omnidirectional visual sensor employing a 
hyperboloidal mirror are described in Yamazawa et al., "Omnidirectional 
Imaging with Hyperboloidal Projection", Proceedings of the 1993 IEEE/RSJ 
International Conference on Intelligent Robots and Systems, Yokohama, 
Japan, Jul. 26-30, 1993, and the description thereof is incorporated 
herein by reference. 
An omnidirectional visual sensor utilizing a paraboloidal mirror exhibits a 
property similar to that of the hyperboloidal mirror under a certain 
condition. When a photoreceiving system allowing a parallel projection can 
be prepared for a convex paraboloidal mirror, the viewpoint is located on 
a single point similarly to the hyperboloid for enabling expression in a 
spherical coordinate system having the origin on this point. In the 
conventional study, however, a correct perspective transformation 
coordinate system cannot be implemented due to employment of a 
photoreceiving system such as an optical system having a long focal length 
or a telecentric lens approximately allowing a parallel projection. 
Details related to a conventional omnidirectional visual sensor utilizing a 
paraboloidal mirror are described in U.S. Pat. No. 5,760,826, and the 
description thereof is incorporated herein by reference. 
On the other hand, every mirror with a surface of revolution generally has 
a problem of blurring due to influence by astigmatism or the like, thus 
requiring close attention in design of the optical system. In general, it 
is difficult to reduce the influence by the astigmatism with a single 
mirror or lens, and a plurality of lenses must be combined with each 
other. In general, there has been proposed an optical system like a 
reflecting telescope utilizing two mirrors having optimum curvatures 
employing a method of acquiring a virtual image formed by a secondary 
mirror with a general telecamera through a primary mirror in the form of a 
two-dimensional torus in order to minimize the influence by the 
astigmatism. 
The details thereof are described in Takeya et al., "Reflecting Wide-Angle 
Optical System", Proceedings of Annual Conference sponsored by the Society 
of Instrument and Control Engineers (SICE), Tokyo, Japan, Jul. 26-28, 
1994, and the description thereof is incorporated herein by reference. 
However, the aforementioned omnidirectional visual sensor employing a 
hyperboloidal mirror has a problem of blurring due to influence by 
astigmatism or the like. Therefore, it is difficult to implement a small 
visual sensor which can transform a central projection while obtaining a 
sharp image by merely adjusting the shape of the hyperboloidal mirror and 
the optical system of the telecamera forming the photoreceiving system. 
Further, it is hard to make the omnidirectional visual sensor utilizing a 
paraboloidal mirror smaller similarly to the omnidirectional visual sensor 
employing a hyperboloidal mirror, and it cannot correctly transform a 
central projection due to an approximate parallel projection by an optical 
system having a long focal length or a telecentric lens. 
In addition, the aforementioned reflection wide-angle optical system cannot 
transform a central projection dissimilarly to the omnidirectional visual 
sensor employing a hyperboloidal mirror, although it can optimize the 
imaging system. 
SUMMARY OF THE INVENTION 
The present invention has been proposed in order to solve the 
aforementioned problems, and an object thereof is to provide an 
omnidirectional visual sensor which can simultaneously implement a correct 
central projection, optimization of an imaging system and miniaturization 
of the sensor itself. 
An omnidirectional visual sensor according to an aspect of the present 
invention includes a convex mirror with a surface of revolution having a 
focal point, a plurality of mirrors with surfaces of revolutions having at 
least one focal point, a photoreceiving lens system receiving light 
reflected by the convex mirror with the surface of revolution and the 
plurality of mirrors with surfaces of revolutions and an image acquisition 
surface receiving the light received by the photoreceiving lens system and 
converting the same to an electric signal, and the convex mirror with the 
surface of revolution and the plurality of mirrors with surfaces of 
revolutions are so arranged that the focal point of a first mirror 
included in the convex mirror with the surface of revolution and the 
plurality of mirrors with surfaces of revolutions aligns with the focal 
point of a second mirror, included in the convex mirror with the surface 
of revolution and the plurality of mirrors with surfaces of revolutions, 
further reflecting light reflected by the first mirror. 
A visual sensor capable of transforming a central projection and obtaining 
a sharp image can be implemented in a small size due to the aforementioned 
arrangement of the mirrors. 
An omnidirectional visual sensor according to another aspect of the present 
invention includes a convex mirror with a surface of revolution having a 
focal point, a plurality of mirrors with surfaces of revolutions having at 
least one focal point, a photoreceiving lens system receiving light 
reflected by the convex mirror with the surface of revolution and the 
plurality of mirrors with surfaces of revolutions and an image acquisition 
surface receiving the light received by the photoreceiving lens system and 
converting the same to an electric signal, and the convex mirror with the 
surface of revolution and the plurality of mirrors with surfaces of 
revolutions are so arranged that a first mirror having only one focal 
point included in the convex mirror with the surface of revolution and the 
plurality of mirrors with surfaces of revolutions has the focal point on 
the side opposite to the reflecting surface of the first mirror, a second 
mirror, included in the convex mirror with the surface of revolution and 
the plurality of mirrors with surfaces of revolutions, having only one 
focal point and further reflecting light reflected by the first mirror has 
the focal point on the side opposite to the reflecting surface of the 
first mirror, and the rotation axis of the first mirror aligns with that 
of the second mirror. 
The omnidirectional visual sensor can be minimized due to the 
aforementioned structure, and can correctly transform a central projection 
while the conventional omnidirectional visual sensor utilizing a 
paraboloidal mirror transforms an approximate central projection. 
The foregoing and other objects, features, aspects and advantages of the 
present invention will become more apparent from the following detailed 
description of the present invention when taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 2, an omnidirectional visual sensor according to an 
embodiment of the present invention includes a primary mirror 1 formed by 
a hyperboloidal convex mirror, a secondary mirror 2 formed by a 
hyperboloidal convex mirror, an image acquisition optical system formed by 
a photoreceiving lens system 3 and an image acquisition surface 4 and a 
transparent tube 5. 
Referring to FIG. 2, the image acquisition optical system formed by the 
photoreceiving lens system 3 and the image acquisition surface 4 such as a 
general telecamera acquires a virtual image formed by the secondary mirror 
2 of a hyperboloidal convex mirror through the primary mirror 1 formed by 
a hyperboloidal convex mirror. The axes of the primary mirror 1 and the 
secondary mirror 2 are arranged on the same axial line to align a focal 
point A of the primary mirror 1 with that of the secondary mirror 2 while 
setting the primary point B of the photoreceiving lens system 3 forming 
the image acquisition optical system on another focal point of the 
secondary mirror 2. This arrangement enables perspective coordinate 
transformation centering on another focal point C of the primary mirror 1. 
The transparent tube 5 such as a cylindrical glass tube or a semispherical 
acrylic tube couples the primary mirror 1 and the secondary mirror 2 with 
each other. 
The aforementioned omnidirectional visual sensor adjusts the curvatures of 
the primary mirror 1 and the secondary mirror 2, thereby performing 
adjustment for imaging on the image acquisition surface 4. Thus, the 
omnidirectional visual sensor can transform a central projection, and is 
easy to minimize. 
In the omnidirectional visual sensor according to the embodiment shown in 
FIG. 2, the secondary mirror 2 may have a spherical or elliptic shape. 
While the omnidirectional visual sensor shown in FIG. 2 employs 
hyperboloidal convex mirrors, an omnidirectional visual sensor according 
to another embodiment shown in FIG. 3 employs paraboloidal mirrors. An 
image acquisition optical system formed by a photoreceiving lens system 3 
and an image acquisition surface 4 such as a general telecamera acquires a 
virtual image formed by a secondary mirror 2 of a parabolic concave mirror 
through a primary mirror 1 formed by a parabolic convex mirror. The 
principal point B of the lens system of the camera is set on the focal 
position of the secondary mirror 2, so that the secondary mirror 2 can 
image light incident in parallel. Light reflected by the primary mirror 1 
enters the secondary mirror 2 in parallel with the axis of the mirror. 
Thus, the omnidirectional visual sensor enables perspective coordinate 
transformation centering on the focal point C of the primary mirror 1. 
The aforementioned omnidirectional visual sensor adjusts the curvatures of 
the primary mirror 1 and the secondary mirror 2, thereby performing 
adjustment for imaging on the image acquisition surface 4. Thus, the 
omnidirectional visual sensor can correctly transform a central projection 
and is easy to minimize. 
While each of the omnidirectional visual sensors according to the 
aforementioned embodiments utilizes two mirrors, i.e., the primary mirror 
1 and the secondary mirror 2, an omnidirectional visual sensor according 
to still another embodiment shown in FIG. 4 utilizes three mirrors (a 
primary mirror 1 and two secondary mirrors 2). When the focal points can 
be coupled between the continuous mirrors, perspective coordinate 
transformation centering on the focal point C of the primary mirror 1 is 
finally enabled. Also when employing four or more mirrors, perspective 
coordinate transformation centering on the focal point of the primary 
mirror is finally enabled. 
In an omnidirectional visual sensor according to a further embodiment shown 
in FIG. 5, a plane mirror 6 is arranged between a secondary mirror 2 and 
an image acquisition optical lens system to bend an optical path. The 
plane mirror 6 may be arranged on any position, to finally enable 
perspective coordinate transformation centering on the focal point C of a 
primary mirror 1. 
FIG. 6 shows an omnidirectional visual sensor according to a further 
embodiment of the present invention. Referring to FIG. 6, a secondary 
mirror 2 is formed by an off-axial paraboloidal mirror. Also when 
employing an off-axial reflecting mirror for the secondary mirror 2, 
perspective coordinate transformation centering on the focal point C of a 
primary mirror 1 is finally enabled similarly to the aforementioned 
embodiments. 
The omnidirectional visual sensor according to the present invention having 
the aforementioned structure attains the following effects: 
An input image can be finally transformed into a coordinate system 
centering on the focal point of the primary mirror. 
Further, the omnidirectional visual sensor can be minimized while 
maintaining an imaging system. 
Although the present invention has been described and illustrated in 
detail, it is clearly understood that the same is by way of illustration 
and example only and is not to be taken by way of limitation, the spirit 
and scope of the present invention being limited only by the terms of the 
appended claims.