Abstract:
An optical system having a pair of ellipsoidal shaped reflective surfaces or mirrors which receive light from an object and provide wide field view imaging for the object. The optical system directs the light onto a flat detector to a record a wide field of view image of the object on the flat detector.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to optical systems for image viewing. More particularly, the present invention relates to an optical system which utilizes a pair of ellipsoidal reflective surfaces to provide for wide field of view imaging.  
           [0003]    2. Description of the Prior Art  
           [0004]    Currently, wide field of imaging optics are used for a multitude of purposes including photographic, remote sensing and space surveillance imaging. Generally, wide field of view imaging systems are refractive or catadioptric, and thus suffer from chromatic aberration and are generally to heavy for applications such as space surveillance.  
           [0005]    Reflective optics do not suffer the drawbacks of refractive optics. Wide field of view imaging systems, however, share a common problem which is field curvature. This prevents a user from having the image sensed by a flat detector such as focal plane array or photographic film. In addition existing reflective wide field of view imaging systems are complex in nature requiring multiple mirrors to implement the system.  
           [0006]    An example of a prior art wide field of view optical system is disclosed in U.S. Pat. No. 4,566,763 for a “Panoramic Imaging Block For Three-Dimensional Space”, which issued Jan. 26, 1986. Disclosed in U.S. Pat. No. 4,566,763 is a wide field of view optical system which is suitable for pictorial recording and displaying based on a flat cylindrical perspective and reflective and refractive surfaces.  
           [0007]    Another example of a prior art wide field of view optical system is disclosed in U.S. Pat. No. 4,037,943 for a “Reflection Type Image Forming Optical System Having A Large Angle Of View”. Disclosed in U.S. Pat. No. 4,037,943 is a reflective type image forming optical system having a large angle of view which includes a convex mirror for reflecting incident light and a concave mirror. The convex mirror is disposed with its reflecting surface facing the reflective surface of the convex mirror. The concave mirror is arranged so that the distance between the center of curvature of the convex mirror and the center of curvature of the concave mirror is greater than half the radius of curvature of the concave mirror. There is also a stop disposed between the reflecting surfaces of the convex and concave mirrors.  
           [0008]    While the foregoing prior art wide field of view optical systems are fairly effective, there are still drawbacks with these systems including chromatic aberration, weight problems and complexity in design.  
           [0009]    Accordingly, there is a need for a relatively simple in design optical system which uses less than three mirrors to provide a high resolution wide field of view. In addition, the optical system should provide for a means whereby an image sensed by a flat detector such as focal plane array or photographic film is recorded by the flat detector.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention overcomes some of the disadvantages of the prior art including those mentioned above in that it comprises a relatively simple optical system which-includes a pair of mirror for receiving light from an object and directing the light onto a flat detector to record a wide field of view image of the object on the flat detector. The flat detector may be, for example, a photographic film.  
           [0011]    The optical system of the present invention is a two reflective surface optical system for imaging a wide field of view scene and recording the scene on a flat detector. The optical system utilizes a pair of ellipsoidal reflective surfaces or mirrors with different eccentricities such that the ellipsoidal reflective surfaces have shared foci or foci in close proximity to one another. Light from a wide field of view scene is reflected from the reflective surface of the first ellipsoid forming a virtual image near one of the shared foci. The reflected light is then directed toward the reflective surface of the second ellipsoid and is reflected from this surface forming a real image near the other shared foci. The elliptical shape of the second reflective surface directs light from the virtual image near one of the shared foci directing the light near the other shared foci.  
           [0012]    In the optical system of the present invention, the first reflective surface is convex and the second reflective surface is concave. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIGS. 1, 2 and  3  are schematic diagrams of a preferred embodiment of the present invention wherein a pair of elliptical shaped reflective surface for wide field of view imaging; 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]    Referring to FIG. 1, there is shown an optical system, designated generally by the reference numeral  10 , which is an all reflective, two mirror design capable of imaging a field of view 90° in azimuth and 25° to 75° in elevation. The optical system  10  includes a first elliptical shaped reflective surface  12  with surface  12  being convex and a second elliptical shaped reflective surface  14  with surface  14  being concave. Surfaces  12  and  14  form oblate ellipsoids  13  and  15 , respectively. Oblate ellipsoids  13  and  15  are ellipses rotated about a minor axis  16 . The ellipsoids  13  and  15  share a common minor axis  16  which is the optical axis for the optical system  10 .  
         [0015]    The reflective surfaces  12  and  14  may be, for example, mirrors which reflect incoming light from a wide field of view scene or other object. Aluminum may be used for the mirror surface of each mirror and may be shaped with a diamond turning machine.  
         [0016]    When there is a rotation about minor axis  16 , the foci of the ellipsoids  13  and  15  each trace out a circle or ring which is centered about minor axis  16  and lies in the major axis plane of the ellipsoids. The major axis for ellipsoids  13  and  15  is identified by the reference numeral  18 . The real image  22  is recorded on a flat detector  32  which may photographic film or a focal plane array.  
         [0017]    The eccentricities of the two ellipsoids  12  and  14  are chosen such that their ring foci are overlapping or nearly overlapping. When light (represented by parallel light rays  24 ) from an object reflects from the reflective surface  12  of ellipsoid  13  a virtual image  20  is created in proximity to one of the shared ring foci as shown in FIG. 1. The reflected light from surface  12  (represented by light rays  26 ) then reflect from surface  14  (represented by light rays  28 ) forming a real image  22  in proximity to the other shared foci.  
         [0018]    The elliptical shape of reflective surface  14  directs light from the virtual image  20  near one of the foci imaging it to form the real image near the other foci. The light rays of the object first diverge from reflective surface  12  and then converge from reflective surface  14  in the manner illustrated in FIG. 1.  
         [0019]    Mirror surface  12  may be defined by the following sag equation:  
               z   1     =       1       K   1     +   1            [       r   1     -         r   1   2     -       (       K   1     +   1     )          S   2             ]               (   1   )                               
 
         [0020]    where  
         [0021]    K 1  is the conic constant for ellipsoid  13   
         [0022]    r 1  is the radius of curvature for ellipsoid  13   
         [0023]    S 2 =x 2 +y 2  (x and y are transverse coordinates)  
         [0024]    In a like manner, mirror surface  14  may be defined by the same equation as follows:  
               z   2     =       1       K   2     +   1            [       r   2     -         r   2   2     -       (       K   2     +   1     )          S   2             ]               (   2   )                               
 
         [0025]    where  
         [0026]    K 2  is the conic constant for ellipsoid  15   
         [0027]    r 2  is the radius of curvature for ellipsoid  15   
         [0028]    S 2 =x 2 +y 2  (x and y are transverse coordinates)  
         [0029]    In the above equation x and y are lateral dimensions for a coordinate system where the z dimension for the coordinate system corresponds to the optical axis and the axis of revolution for mirrors  13  and  15 .  
         [0030]    The distance d between ellipsoids  13  and  15  is set forth by the following equation:  
             d   ≅             r   2   2          (       K   1     +   1     )       2     -         r   1   2          (       K   2     +   1     )       2                 r   2          (       K   1     +   1     )       2            (       K   2     +   1     )       3   /   2         +           r   1          (       K   2     +   1     )       2            (       K   1     +   1     )       3   /   2                     (   3   )                               
 
         [0031]    It should be noted that the distance d provided by equation 3 is an approximation.  
         [0032]    The optical system  10  comprising the present invention may be used for a wide range of imaging applications. Optical system  10  may be manufactured as a single piece injection molded plastic device with its mirror surfaces coated with a reflective material. The optical system  10  would therefore be lightweight and inexpensive to fabricate. Since it is an all reflective optical system it also has the capability of operating over a portion of the light spectrum from ultraviolet light to infrared light.  
         [0033]    In a preferred embodiment optical system  10  has a field of view of 90° in azimuth and 25° to 75° in elevation. Optical System  10  has an F# of approximately 1.7 with an effective focal length of 10.2 mm and an entrance pupil diameter of 6 mm. The total length of the imaging module (from the reflective surface  14  of mirror  15  to the image plane  22  is 25 mm.  
         [0034]    The diameter of the convex mirror  13  is 18.27 mm and the diameter of the concave mirror  15  is approximately 18 mm. The distance d between mirrors  13  and  15  is 16.03 mm. The value of K for the convex mirror  13  is 6.375 and the value of K for the concave mirror  15  is 2.33. The value of r for the convex mirror  13  is 28.616 mm and the value of r for the concave mirror  15  is −56.316 mm.  
         [0035]    Referring to FIG. 2, there is shown light rays from multiple objects which may be located in one scene being processed by optical system  10 . Light represented by parallel light rays  40  from a first object in a scene first reflects from the reflective surface  12  of mirror  13  as shown in FIG. 1. The reflected light from surface  12  (represented by light rays  42 ) then reflects from surface  14  of mirror  15  (represented by light rays  44 ) forming a real image  46  of the first object which is recorded on the flat detector  32  which may be photographic film or a focal plane array.  
         [0036]    In a like manner, light represented by parallel light rays  48  from a second object in a scene first reflects from the reflective surface  12  of mirror  13  as shown in FIG. 1. The reflected light from surface  12  (represented by light rays  50 ) then reflects from surface  14  of mirror  15  (represented by light rays  52 ) forming a real image  54  of the second object which is recorded on the flat detector  32 . Thus, it can be seen that multiple objects from a wide field of view scene can be recorded on photographic film or the like using the optical system  10  illustrated in FIGS. 1 and 2.  
         [0037]    Referring to FIG. 3, there is a wide field of view scene  60  which generates parallel light  62 ,  64  and  66  from multiple objects within scene  60 . The parallel light rays  62 ,  64  and  66  are first directed to reflected surface  12  and reflected from surface  12  to reflective surface  14 . Reflective surface  14  then directs the light rays  62 ,  64  and  66  to flat detector  32  where multiple real images  70 ,  72  and  74 . The multiple objects from the wide field of view scene  60  are then recorded on the flat detector  32  which again may be photographic film or a focal plane array.  
         [0038]    From the foregoing, it may readily be seen that the present invention comprises a new, unique and exceedingly useful confocal ellipsoidal mirror system for wide field of view imaging which constitutes a considerable improvement over the known prior art. Many modifications and variations of the present invention are possible in light of the above teachings. It is to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.