Abstract:
An imaging system includes an imaging body having an optical system and an imaging element, a power supplier configured to supply power to the imaging element, and a housing configured to hold the imaging body and the power supplier, wherein the optical system includes at least one optical element projecting from the housing, and a distance AP between a gravity center A of a portion including the optical system and a gravity center P of the entire imaging system and a distance BP between a gravity center B of the power supplier and the gravity center P of the entire imaging system satisfy the following condition. 
     
       
      
       AP&gt;BP

Description:
PRIORITY CLAIM 
       [0001]    The present application is based on and claims priority from Japanese Patent Application No. 2012-060242, filed on Mar. 16, 2012, and Japanese Patent Application No. 2012-277671, filed on Dec. 20, 2012, the disclosures of which are hereby incorporated by reference in their entirety. 
       BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an imaging system in which a lens surface projects from a housing. 
         [0004]    2. Description of the Related Art 
         [0005]    An imaging system using a plurality of wide-angle lenses such as a fisheye lens or a super-wide-angle lens is known as an imaging system which images all directions at one time In such an imaging system, an image from each lens is projected on the same or corresponding sensor, and the projected images are combined by an image process, so as to produce an omnidirectional image. 
         [0006]    When an imaging system is created with a small number of optical components, an angle of view assigned to each lens tends to be increased. For example, when photographing an omnidirectional image by using two fisheye lenses, each of the fisheye lenses requires a 180° or more angle of view. 
         [0007]    A wide-angle lens, however, tends to have a small curvature radius on the incident side, and project from a housing. In an imaging system in which a lens surface projects from a housing, a lens is easily damaged when dropping the imaging system. 
         [0008]    A technique described in Patent Document 1(JP S62-191838A), for example, is known as a technique which protects a lens from being damaged. Patent Document 1 discloses a camera with a lens cover in which a push button for opening and closing a lens cover is provided in a side face of a lens barrel cover on a grip side. Such a technique described in Patent Document 1 requires the push button for opening and closing a lens cover, resulting in an increase in costs. 
         [0009]    In the above-described imaging systems, in particular, an imaging system having a linear housing, an optical system, shutter button, and power supplier are often linearly arranged. A photographer holds such an imaging system between the gravity center of the imaging system and the position of the shutter button. When the arrangement of an optical system and a power supplier, which account for a substantial fraction of the weight of the imaging system, is inappropriate, camera shake easily occurs in the case of pushing the shutter button. It therefore becomes difficult for a photographer to stably perform photographing. 
       SUMMARY 
       [0010]    The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an imaging system in which a lens surface projects from a housing, and a balance of a center of a gravity is improved. 
         [0011]    Another object of the present invention is to provide an imaging system in which a lens surface projects from a housing, and a possibility of lens surface damage in the case of dropping the imaging system is preferably decreased without adding a new component. 
         [0012]    In order to achieve the above objects, one embodiment of the present invention provides an imaging system including an imaging body having an optical system and an imaging element, a power supplier configured to supply power to the imaging element, and a housing configured to hold the imaging body and the power supplier, wherein the optical system includes at least one optical element projecting from the housing, and a distance AP between a gravity center A of a portion including the optical system and a gravity center P of the entire imaging system and a distance BP between a gravity center B of the power supplier and the gravity center P of the entire imaging system satisfy the following condition. 
         [0000]    
       
      
       AP&gt;BP  
      
     
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The accompanying drawings are included to provide further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the specification, serve to explain the principle of the invention. 
           [0014]      FIG. 1  is an overall view illustrating an omnidirectional imaging system according to an embodiment of the present invention. 
           [0015]      FIG. 2  is a detailed view illustrating two imaging optical systems in an imaging body of the omnidirectional imaging system according to the embodiment of the present invention. 
           [0016]      FIG. 3  is a view describing a sag amount in a first lens LA 1 , LA 2 . 
           [0017]      FIG. 4  is an overall view illustrating an omnidirectional imaging system according to another embodiment of the present invention. 
           [0018]      FIG. 5  is an overall view illustrating an omnidirectional imaging system according to another embodiment of the present invention. 
           [0019]      FIG. 6  is view illustrating six planes of an omnidirectional imaging system according to another embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    Hereinafter, embodiments of the present invention will be described. However, the present invention is not limited to the following embodiments. In the following embodiments, an omnidirectional imaging system  10  including an imaging body having two fisheye lenses in an optical system and a battery as a power supplier is described as one example of an imaging system. 
         [0021]      FIG. 1  is an overall view illustrating the omnidirectional imaging system  10  according to the embodiment of the present invention. The omnidirectional imaging system  10  illustrated in  FIG. 1  includes an imaging body  12 , battery  14 , controller boards  16 A,  16 B, and housing  18  which holds these components  12 ,  14 ,  16 A,  16 B. In the embodiment illustrated in  FIG. 1 , the imaging body  12  includes two image-forming optical systems  20 A,  20 B and two imaging elements  24 A,  24 B. An imaging optical system is made of the combination of one image-forming optical system  20  and one imaging element  24 . 
         [0022]    Each of the image-forming optical systems  20 A,  20 B illustrated in  FIG. 1  is constituted as a fisheye lens of seven elements in six groups. The fisheye lens constituted by the image-forming optical system  20  includes an angle of view larger than 180° (=360°/n; n=2) in the embodiment illustrated in  FIG. 1 . It is preferable for the fisheye lens to include a 185° or more angle of view, and it is more preferable for the fisheye lens to include a 190° or more angle of view. With such an angle of view, images are synthesized by an image process based on an overlapped area. 
         [0023]      FIG. 2  is a view illustrating the detailed configuration of the two image-forming optical systems  20 A,  20 B in the imaging body  12  illustrated in  FIG. 1 . The image-forming optical systems  20 A,  20 B are cemented with the respective prisms as an axis as illustrated in  FIG. 1 . However, in  FIG. 2 , the two image-forming optical systems  20 A,  20 B are separated for the sake of simplicity. As illustrated in  FIG. 2 , the first image-forming optical system  20 A includes a front group having lenses LA 1 -LA 3 , a right angle prism PA as a reflection member, and a back group having lenses LA 4 -LA 7 . An aperture stop SA is disposed on the object side of the fourth lens LA 4 . In the first image-forming optical system  20 A, a filter F and an aperture stop SA are disposed on the image side of the seventh lens LA 7 . 
         [0024]    The image-forming optical system  20 B includes a front group having lenses LB 1 -LB 3 , a right angle prism PB, and a back group having lenses LB 4 -LB 7 . An aperture stop SB is disposed on the object side of the fourth lens LB 4 . A filter F and an aperture stop SB are disposed on the image side of the seventh lens LB 7 . 
         [0025]    In a specific embodiment, the lenses LA 1 -LA 3  of the front group of the first image-forming optical system  20 A are, in order form the object side, a negative meniscus lens (LA 1 ) made of an optical glass material, a negative lens (LA 2 ) made of a plastic resin material, and a negative meniscus lens (LA 3 ) made of an optical glass material. The lenses LA 4 -LA 7  of the back group are, in order from the object side, a biconvex lens (LA 4 ) made of an optical glass material, a cemented lens of a biconcave lens (LA 6 ) and a biconvex lens (LA 5 ) made of an optical glass material, and a biconvex lens (LA 7 ) made of a plastic resin material. 
         [0026]    In a specific embodiment, the lenses LB 1 -LB 3  of the front group of the second image-forming optical system  20 B are, in order from the object side, a negative meniscus lens (LB 1 ) made of an optical glass material, a negative lens (LB 2 ) made of a plastic resin material, and a negative meniscus lens (LB 3 ) made of an optical glass material. The lenses LB 4 -LB 7  of the back group are, in order from an object side, a biconvex lens (LB 4 ) made of an optical glass material, a cemented lens of a biconcave lens (LB 6 ) and a biconvex lens (LB 5 ) made of an optical glass material, and a biconvex lens (LB 7 ) made of a plastic resin material. 
         [0027]    In the first and second image-forming optical systems  20 A,  20 B, the negative lenses LA 2 , LB 2  in the front groups, which are made of a plastic resin material, and the biconvex lenses LA 7 , LB 7  in the back groups, which are made of a plastic resin material, have an aspheric surface on both surfaces. Each of the lenses made of an optical glass material is a spherical lens. 
         [0028]    It is preferable for each of the right angle prisms PA, PB disposed between the front group and the back group to be formed by a material having a refractive index of d-line (λ=587.6 nm) larger than 1.8. The right angle prism PA, PB internally reflects the light from the front group toward the back group. The optical path of the imaging light beams therefore passes through the right angle prism PA, PB in each of the image-forming optical systems  20 A,  20 B. By constituting the right angle prism with a material having a high refractive index, the optical path length in the right angle prism PA, PB is increased, and the optical path length between the front group and the back group in the front group, the right angle prism and the back group can be increased to be larger than the mechanical length. Thus, the fisheye lens can be downsized. 
         [0029]    By disposing the right angle prisms PA, PB near the aperture stops SA, SB, a right angle prism having a small outer diameter can be used, and the distance between the fisheye lenses can be reduced. Moreover, by adopting the arrangement of the right angle prisms PA, PB as illustrated in  FIG. 2 , the parallax of the two optical systems can be reduced. Furthermore, by disposing the two image-forming optical systems to face each other as illustrated in  FIGS. 1 ,  2 , the imaging system can be further downsized, and a non-imaging space can be reduced. 
         [0030]    The optical elements (lens, prism, filter and aperture stop) of each of the two image-forming optical systems  20 A,  20 B are held by the lens barrel  26  relative to each of the imaging elements  24 A,  24 B such that the optical axes of the optical elements are located orthogonal to the central portion of the light-receiving area of the corresponding imaging element  24 , and the light-receiving area becomes the imaging face of the corresponding fisheye lens. Namely, each of the image-forming optical systems  20  is positioned such that an image of an imaging target is imaged in the light-receiving area of the corresponding imaging element  24 . 
         [0031]    Each of the imaging elements  24  is a two-dimensional imaging element in which a light-receiving area forms an area, and converts the light collected by the corresponding image-forming optical system  20  into the image signals. Each of the imaging elements  24 A,  24 B includes a configuration in which tiny right-receiving areas are two-dimensionally arranged on the light-receiving surface. The information, which is photoelectrically converted in each of the tiny light-receiving areas, constitutes each pixel. 
         [0032]    In the embodiment illustrated in  FIG. 1 , the image-forming optical systems  20 A,  20 B have the same specification, and are combined to be opposite to each other such that the optical axes are aligned. The omnidirectional imaging system  10  is configured to image omnidirectional image information by combining the two image-forming optical systems  20 A,  20 B and the two imaging elements  24 A,  24 B. By adopting the configuration illustrated in  FIG. 1 , an object above the housing  18  can be photographed. 
         [0033]    The image obtained by the first image-forming optical system  20 A is imaged on the light-receiving area of the two-dimensional imaging element  24 A. The image obtained by the second image-forming optical system  20 B is also imaged on the light-receiving area of the two-dimensional imaging element  24 B. The imaging elements  24 A,  24 B convert the received light distribution into the image signals to be input to the controller boards  16 A,  16 B. 
         [0034]    A not-shown image processor and output unit are provided on the controller boards  16 A,  16 B. The image signals output from the imaging elements  24 A,  24 B are input to the image processor on the controller board  16 . The image processor synthesizes the image signals input from the imaging elements  24 A,  24 B into one image to obtain an image of solid angle of 4π radian (hereinafter referred to as an omnidirectional image), and outputs the image to the output unit. In the embodiment illustrated in  FIG. 1 , an omnidirectional image is formed, but a so-called panoramic image in which 360° only in a horizontal plane is photographed can be formed. 
         [0035]    As described above, since the fisheye lens includes a 180° or more angle of view, the overlapped image portion is used as a reference for combining the images as standard data showing the same image when forming an omnidirectional image by synthesizing image signals output from the imaging elements  24 A,  24 B. The output unit is, for example, a display device, printer, or external memory such as an SD card or compact flash (registered trademark), and outputs the synthesized omnidirectional image. 
         [0036]    The battery  14  is a power supplier which supplies power to a chip or a component on the controller boards  16 A,  16 B and the imaging elements  24 A,  24 B. The battery  14  is a primary battery such as an alkaline manganese primary battery or oxyride primary battery, or a secondary battery such as a lithium ion secondary battery, lithium ion polymer secondary battery or nickel hydride secondary battery. 
         [0037]    The omnidirectional imaging system  10  illustrated in  FIG. 1  includes a bar shape having one end provided with the image-forming optical system. The housing  18  includes a main body holding a module including the controller boards  16 A,  16 B and the battery  14 , and a lens holder holding the imaging body  12  and provided with an opening from which the first lenses LA 1 , LB 1  are exposed. The housing  18  includes flat housing faces  18 A,  18 B of the main body. 
         [0038]    In the image-forming optical systems  20 A,  20 B illustrated in  FIG. 1 , the first lenses LA 1 , LB 1  located on the most object side project from the housing faces  18 A,  18 B in the main body of the housing  18 . In a specific embodiment, the first lenses LA 1 , LB 1  are exposed outside the housing  18 . 
         [0039]    When the first lenses LA 1 , LB 1  are made of an optical glass material, the lens surfaces may get cracked under a dropping condition from a height of about 1.5 m in a dropping test of the imaging optical system  20 A. When the first lenses LA 1 , LB 1  are made of a plastic resin material, the lens surfaces may be scratched under a dropping condition similar to the above. Namely, when a photographer drops the omnidirectional imaging system  10  by accident, the first lens may be damaged. When the first lenses LA 1 , LB 1  are damaged, an image cannot be appropriately formed on the light-receiving surface of the imaging element  24 ; thus, it becomes difficult to obtain a preferable image. 
         [0040]    The above-described imaging body  12  and the battery  14  are main members which account for a substantial fraction of the weight of the omnidirectional imaging system  10  illustrated in  FIG. 10 . For this reason, the omnidirectional imaging system  10  according to the present embodiment includes the following features regarding the arrangement of the imaging body  12  and the battery  14  as the main members, which account for a substantial fraction of the weight of the omnidirectional imaging system  10 , based on the moment of the entire omnidirectional imaging system  10 . 
         [0041]    In the omnidirectional imaging system  10 , a distance AP between a gravity center A of the imaging body  12  and a gravity center P of the entire omnidirectional imaging system  10  and a distance BP between a gravity center B of the battery  14  and the gravity center P of the entire omnidirectional imaging system  10  satisfy the following condition 1. 
         [0000]        AP&gt;BP   (Condition 1)
 
         [0042]    By satisfying the above condition 1, the gravity center P of the entire omnidirectional imaging system  10  is biased on the battery  14  side. With this configuration, when the omnidirectional imaging system  10  is dropped from a hand, for example, a possibility that the omnidirectional imaging system  10  drops from the side of the imaging body  12  having the projected optical elements can be decreased. 
         [0043]    In a preferred embodiment, a shutter button can be disposed in a position S between the gravity center A of the imaging body  12  and the gravity center P of the entire omnidirectional imaging system  10 . The shutter button is an input unit for starting imaging, which is pushed by a photographer for inputting an instruction to start imaging. It is preferable for the imaging body  12 , shutter button and battery  14  to be arranged on the same straight line x in order of the gravity center A of the imaging body  12 , the position S of the shutter button and the gravity center P of the entire omidirecional imaging system  10 . The shutter button is arranged on the front face of the housing  18 . 
         [0044]    The arrangement of the shutter button is not limited to the arrangement illustrated in  FIG. 1 .  FIG. 5  is an overall view illustrating an omnidirectional imaging system  10  according to another embodiment. In the omnidirectional imaging system  10  illustrated in  FIG. 5 , the shutter button  22  is located on the left side of the straight line x, namely, below the left side of the image-forming optical system  20 . Similar to the arrangement in  FIG. 1 , the gravity center A of the imaging body  12 , the position S of the shutter button and the gravity center P of the omnidirectional imaging system  10  are arranged in order of the gravity center A, the position S and the gravity center P.  FIG. 6  is a view illustrating six planes of the omnidirectional imaging system  10  according to another embodiment. 
         [0045]    In order to stably hold the omnidirectional imaging system  10  it is preferable for a photographer to hold the omnidirectional imaging system  10  near the center N of the shape the omnidirectional imaging system  10 , namely, between the position S and the position P. A photographer pushes the shutter button disposed on the imaging body  12  side of the gravity center P with the above-described held condition. In this case, by adopting the arrangement in which the gravity center P is biased on the battery  14  side away from the imaging body  12 , camera shake which causes a deterioration in an image quality is controlled because the moment on the battery  14  side is larger even if the shutter button is pushed. A photographer therefore stably photographs an image by using the omnidirectional imaging system  10 . 
         [0046]    In addition, the three dimensional gravity center of each of the members  12 ,  14  can be specified by measuring the gravity center in the two-dimensional direction of each of the members multiple times by using a load cell (mass-measuring instrument). In the embodiment, the gravity center A is the gravity center of the entire imaging body  12  including the two image-forming optical systems  20 A,  20 B, lens barrel  26  and imaging elements  24 A,  24 B. However, in another embodiment, the gravity center of the portion including the two image-forming optical systems  20 A,  20 B and the lens barrel  26  without including the imaging elements  24 A,  24 B can be the gravity center A. In addition, the gravity center B is the gravity center of the battery  14  without including a cable which connects the battery  14  to the imaging element  24  in this embodiment. 
         [0047]    In the omnidirectional imaging system  10 , it is preferable for a weight m of the imaging body  12  and a weight M of the battery  14  to satisfy the following condition 2. 
         [0000]        m&lt;M   (Condition 2)
 
         [0048]    In the omnidirectional imaging system  10 , it is preferable for the weight m of the imaging body  12 , the weight M of the battery  14 , a distance AN between the gravity center A of the imaging body  12  and the center N of the shape of the entire omnidirectional imaging system  10 , and a distance BN between the gravity center B of the battery  14  and the center N to satisfy the following condition 3. 
         [0000]        m×AN&lt;M×BN   (Condition 3)
 
         [0049]    By satisfying the above conditions 2, 3, the gravity center P of the entire omnidirectional imaging system  10  is biased on the battery  14  side. With this configuration, when the omnidirectional imaging system  10  drops from a hand, for example, a possibility that the imaging system  10  is dropped from the side of the imaging body  12  having the projected optical elements can be decreased. 
         [0050]    The above described arrangement is especially effective for an imaging system in which the first lenses LA 1 , LB 1  project from the housing faces  18 A,  18 B, and also especially effective for an imaging system in which the sag amount of the first lenses LA 1 , LB 1  becomes 3 mm or more. This is because cracking becomes remarkable in a lens made of an optical glass material and scratching becomes remarkable in a lens made of a plastic resin material in the dropping test from 1.5 m when the sag amount of the first lenses LA  1 , LB 1  becomes 3 mm or more. In addition, the sag amount shows a sag amount in an effective diameter, and does not include a sag amount of a non-effective diameter. 
         [0051]    The above sag amount h is defined as illustrated in  FIG. 3 . It is preferable for a curvature radius r of the convex lens of the first lens LA 1 , LB 1  and an effective diameter (diameter) R of the first lens LA 1 , LB 1  to satisfy the following condition 4 by normalizing with the curvature radius r. 
         [0000]      1−1 cos {sin −1 ( R/ 2 r )}≧0.17  (Condition 4)
 
         [0052]    For example, when the curvature radius r of the convex lens of the first lens LA 1 , LB 1  is 18 mm and the effective diameter R of the first lens LA 1 , LB 1  is 20 mm, the sag amount h of the first lens becomes about 3.03 mm, and the value (h/r) in which the sag amount h is normalized by the curvature radius r becomes about 0.17. The above condition 4 therefore is satisfied. 
         [0053]    Moreover, when the curvature radius r of the convex lens of the first lens LA 1 , LB 1  is 17 mm and the effective diameter R of the first lens LA 1 , LB 1  is 20 mm, the sag amount h of the first lens becomes about 3.25 mm, and the value (h/r) in which the sag amount h is normalized by the curvature radius r becomes about 0.19. The above condition 4 therefore is satisfied. 
         [0054]    Furthermore, when the curvature radius r of the convex lens of the first lens LA 1 , LB 1  is 10 mm and the effective diameter R of the first lens LA 1 , LB 1  is 20 mm, the sag amount h of the first lens becomes about 10.00 mm, and the value (h/r) in which the sag amount h is normalized by the curvature radius r becomes about 1. The above condition 4 therefore is satisfied. In addition, the upper limit of the normalized value (h/r) is 1. 
         [0055]    Hereinafter, a material for achieving the arrangement of the imaging body  12  and the battery  14  satisfying the above conditions 1-3 will be described. 
         [0056]    In order to reduce the weight in of the imaging body  12  to be lower than the weight M of the battery  14 , namely, to satisfy the above condition 3, a material having a small specific gravity is adopted for the lens for use in the image-forming optical system  20  of the imaging body  12 . As described above, the image-forming optical system  20  includes seven lenses in six groups according to a specific embodiment. In such a configuration, the second lens LA 2 , LB 2  from the object side and the seventh lens LA 7 , LB 7  from the object side are made of a plastic resin material. In another embodiment, the plastic resin material is not limited to the second lens LA 2 , LB 2  and the seventh lens LA 7 , LB 7 , and all or a part of the lenses LA 1 -LA 7 , LB 1 -LB 7  can be made of a plastic resin material. 
         [0057]    It is preferable for the material of the lens to use a plastic resin material having a specific gravity of 2.5 g/cm 3  or below (unit will be hereinafter omitted). Such a plastic resin material includes cycloolefin resin (specific gravity 1.1), episulfide series resin (specific gravity 1.46), thiourethane series resin (specific gravity 1.35), (polyester) methacrylate (specific gravity 1.37), polycarbonate (specific gravity 1.20), (urethane) methacrylate (specific gravity 1.17), (epoxy)methacrylate (specific gravity 1.19), diallyl carbonate (specific gravity 1.23), diallyl phthalate series resin (specific gravity 1.27), urethane series resin (specific gravity 1.1), polymethylmethacrylate (specific gravity 1.18) and allyl diglycol carbonate (specific gravity 1.32). It is more preferable for the material of the lens to use a plastic resin material having a specific gravity of 1.1 or more and less than 1.25 such that the specific gravity is decreased twice or more the specific gravity (2.5) of glass. 
         [0058]    In order to satisfy the above condition 3, it is also preferable for the material of the lens barrel  26  holding a lens to use a material having a small specific gravity. It is preferable for the material of the lens barrel to use a plastic resin material having a specific gravity smaller than 2.7 g/cm 3 . A complex material of resin such as polycarbonate resin (PC), polyphenylene sulfide resin (PPS), acrylonitrile butadiene styrene resin (ABS), polybutylene terephthalate (PBT), polyethylene terephthalate resin (PET), polystyrene resin (PS), polyphenyleneether resin (PPE), and polyamide resin (PA), and filler such as glass fiber, carbon fiber, and carbon fiber, for example pitch series or PAN (polyacrylonitrile) series can be used as the plastic resin material of the lens barrel. 
         [0059]    It is more preferable for the plastic resin material of the lens barrel to use a plastic resin material having a specific gravity of 1.3 or more and less than 1.35 which is decreased twice or more the specific gravity (2.7) of aluminum. A polycarbonate material with glass can be used as the material for forming the lens barrel. 
         [0060]    As illustrated in  FIG. 1 , the omnidirectional imaging system  10  includes an impact absorber  30 . The impact absorber is provided near the battery  14  in the exterior of the housing  18 . A low-modulus rubber material such as low resilient urethane rubber or an impact absorbing gel molded material can be used for the impact absorber. 
         [0061]    With the above-described arrangement, when the omnidirectional imaging system  10  is dropped from a hand, the imaging system  10  tends to drop from the battery  14  side. The housing  18  and the module can be protected in such dropping by the impact absorber  30 . 
         [0062]    In the above embodiment, the omnidirectional imaging system which can photograph all directions by using the two imaging optical systems is described. However, the embodiment is not limited to the combination of the two imaging optical systems, and it can be applied to a monocular bar type camera. In the above description, the fisheye lens in which the distortion is not corrected is described as one example, but the omnidirectional imaging system can be constituted by using a super-wide-angle lens in which the distortion is corrected. 
         [0063]    Moreover, the above embodiment can be applied to an imaging system which can photograph all directions by using n-imaging optical systems where n is a natural number larger than 2. For example, an imaging system can be constituted by radially disposing three wide-angle lenses (image-forming optical system) having an angle of view larger than 360°/3=120° in the same plane, and combining the lenses with imaging elements, respectively. An image to be obtained with this system is not an omnidirectional image, but such a system can image a horizontal panoramic image of 360°, and is preferable for a car-mounted camera or security camera. The image can be a still image or moving image. 
         [0064]    In the above embodiment, the omnidirectional imaging system having a linear shape is described. However, the above-described arrangement can be applied to an omnidirectional imaging system having another shape.  FIG. 4  is a view illustrating an entire omnidirectional imaging system according to another embodiment. In addition, since an omnidirectional imaging system  50  according to the embodiment illustrated in  FIG. 4  includes a configuration similar to that of the omnidirectional imaging system  10  of the embodiment illustrated in  FIG. 1 , differences between the embodiment illustrated in  FIG. 1  and the embodiment illustrated in  FIG. 4  will be mainly described in the following description. 
         [0065]    The omnidirectional imaging system  50  according to another embodiment illustrated in  FIG. 4  includes an imaging body  52 , battery  54 , not-shown controller board, and housing  58  which holds these components. In the embodiment illustrated in  FIG. 4 , the imaging body  52  includes two image-forming optical systems as a fisheye lens having seven lenses in six groups and two imaging elements similar to the embodiment illustrated in  FIG. 1 . The image-forming optical system illustrated in  FIG. 4  includes a configuration similar to the embodiment illustrated in  FIGS. 1 ,  2 , but does not have a right angle prism between the front group and the back group, and the image-forming optical systems are combined to be opposite to each other with their optical axes aligned. 
         [0066]    The omnidirectional imaging system  50  illustrated in  FIG. 4  includes a spherical shape provided with an imaging optical system on both sides. The housing  58  includes a main body which holds the imaging body  52 , controller board and battery  54 . An opening from which the first lenses LA 1 , LB 2  are exposed is provided in the main body of the housing  58 . In the image-forming optical system illustrated in  FIG. 4 , the first lenses LA 1 , LB 2  located on the most object side project from the surface of the housing  58  and are exposed outside the housing  58 . 
         [0067]    Similar to the omnidirectional imaging system  10  illustrated in  FIG. 4 , the imaging body  52  and the battery  54  become the main members which account for a substantial fraction of the weight of the omnidirectional imaging system  50  illustrated in  FIG. 4 . In this case, in the omnidirectional imaging system  50  according to the embodiment illustrated in  FIG. 4 , the arrangement of the imaging body  52  and the battery  54  of the main members which account for a substantial fraction of the weight of the omnidirectional imaging system  50  has the following feature. 
         [0068]    In the omnidirectional imaging system  50  according to another embodiment illustrated in  FIG. 4 , a distance AP between a gravity center A of the imaging body  52  and a gravity center P of the entire omnidirectional imaging system  50  and a distance BP between a gravity center B of the battery  54  and the gravity center P of the entire omnidirectional imaging system  50  satisfy the above condition 1. 
         [0069]    By satisfying the above condition 1, the gravity center P of the entire spherical omnidirectional imaging system  50  illustrated in  FIG. 4  is biased on the battery  54  side as the omnidirectional imaging system  10  illustrated in  FIG. 1 . Moreover, by providing an impact absorber  70  near the battery  54  in the exterior of the housing  58 , the housing  58  can be preferably protected in dropping. 
         [0070]    Since a condition which defines another arrangement is similar to that illustrated in  FIG. 1 , the detailed description thereof will be omitted. 
         [0071]    As described above, according to the embodiments of the present invention, an imaging system in which the lens surface projects from the housing, and the balance of the gravity center is improved can be provided. In addition, in the imaging system, a possibility of lens surface damage when dropping the imaging system can be preferably decreased without adding a new member. 
         [0072]    Hereinafter, the imaging system according to the embodiments of the present invention will be described in details by using the following Embodiments. However, the present invention is not limited to the following Embodiments. 
       Embodiment 1 
       [0073]    The omnidirectional imaging system  10  having a linear shape illustrated in  FIG. 1  was obtained. Each of the image-forming optical systems  20 A,  20 B had seven lenses in six groups. In this configuration, the second lens LA 2 , LB 2  from the object side and the seventh lens LA 7 , LB 7  from the object side were a plastic lens. The plastic lens was formed by using a plastic resin material (specific weight 1.1) of E48R of Zeonex (registered trademark). The lens barrel  26  was formed by using a polycarbonate (PC+GF) material with glass having a specific weight of 1.3. 
         [0074]    The curvature radius r and the effective diameter R of the first lens LA 1 , LB 1  were 17 mm and 20 mm, respectively, and the sag amount h of the first lens LA 1 , LB I was about 3.25 mm. The projection amount of the first lens LA 1 , LB 1  from the housing surface  18 A,  18 A was about 5 mm. The above condition 4 therefore was satisfied. 
         [0075]    Measuring the gravity center A of the imaging body  12  in which the lenses LA 1 - 7 , LB 1 - 7 , right angle prisms PA, PB, lens barrel  26  and imaging elements  24 A,  24 B were combined, the gravity center P of the entire imaging system  10  and the gravity center B of the battery  14 , the distance AP was 38 mm, and the distance BP was 26 mm. The weight m of the imaging body  12  was 17 g and the weight M of the battery  14  was 25 g. Moreover, measuring the distance AN between the gravity center A of the imaging body  12  and the center N of the shape of the omnidirectional imaging system  10  and the distance BN between the gravity center B of the battery  14  and the center N of the form of the omnidirectional imaging system  10 , the distance AN was 35 mm and the distance BN was 29 mm. The above conditions 1-3 therefore were satisfied. 
         [0076]    The shutter button was provided in the position S in which the distance PS becomes 10 mm. Even when the shutter button in the position S was pushed by the pushing force of 10 g, the photographing was stably performed. 
       Embodiment 2 
       [0077]    The omnidirectional imaging system  50  having a spherical shape illustrated in  FIG. 4  was obtained. Each of the image-forming optical systems had seven lenses in six groups. In this configuration, the second lens LA 2 , LB 2  from the object side and the seventh lens LA 7 , LB 7  from the object side were a plastic lens of E48R of Zeonex (registered trademark). The lens barrel  26  was formed by using a polycarbonate (PC+GF) material with glass. 
         [0078]    The curvature radius r and the effective curvature radius R of the first lens LA 1 , LB 1  were 17 mm and 20 mm, respectively, and the sag amount h of the first lens LA 1 , LB 1  was about 3.5 mm. The above condition 4 therefore was satisfied. 
         [0079]    Measuring the gravity center A of the imaging body  52  in which the lenses LA 1 - 7 , LB 1 - 7 , right-angle prisms PA, PB, lens barrel and imaging elements were combined, the gravity center P of the entire imaging system  50  and the gravity center B of the battery  54 , the distance AP was 15 mm, and the distance BP was 10 mm. The weight m of the imaging body  52  was 17 g and the weight M of the battery  54  was 25 g. The distance AN was 0 mm and the distance BN was 25 mm. The above conditions 1-3 therefore were satisfied. 
       Embodiment 3 
       [0080]    The omnidirectional imaging system  10  having a linear shape illustrated in  FIGS. 5 ,  6  was obtained by using the lens barrel  26  and the image-forming optical systems  20 A,  20 B similar to Embodiment 1. The curvature radius r and the effective diameter R of the first lens LA 1 , LB 1  were 17 mm and 20 mm, respectively, and the sag amount h of the first lens LA 1 , LB 1  was about 3.25 mm. The projection amount of the first lens LA 1 , LB 1  from the housing surface  18 A,  18 A was about 5 mm. The above condition 4 therefore was satisfied. 
         [0081]    Measuring the gravity center A of the imaging body  12  in which the lenses LA 1 - 7 , LB 1 - 7 , right angle prisms PA, PB, lens barrel  26  and imaging elements  24 A,  24 B were combined, the gravity center P of the entire imaging system  10  and the gravity center B of the battery  14 , the distance AP was 47 mm, and the distance BP was 32 mm. The weight m of the imaging body  12  was 17 g and the weight M of the battery  14  was 25 g. Moreover, measuring the distance AN between the gravity center A of the imaging body  12  and the center N of the shape of the omnidirectional imaging system  10  and the distance BN between the gravity center B of the battery  14  and the center N of the form of the omnidirectional imaging system  10 , the distance AN was 42 mm and the distance BN was 37 mm. The above conditions 1-3 therefore were satisfied. The shutter button  22  was provided in the position S of the housing surface on the lens forming side in which the distance PS becomes 7.5 mm. Even when the shutter button in the position S was pushed by the pushing force of 10 g, the photographing was stably performed. 
         [0082]    Although the embodiments of the present invention have been described above, the present invention is not limited thereto. It should be appreciated that variations may be made in the embodiments described by persons skilled in the art without departing from the scope of the present invention.