Abstract:
An optical system including an array of micro-optic afocal optical subsystems capable of receiving electromagnetic radiation from a source and of transmitting at least a portion of the electromagnetic radiation received from the source, and an imaging subsystem capable of receiving electromagnetic radiation from the an array of micro-optic afocal optical subsystems and imaging at least a portion of the received electromagnetic radiation onto an image plane.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims priority of U.S. Provisional Application 60/491,182 filed on Jul. 29, 2003, which is incorporated by reference herein. 

   BACKGROUND OF THE INVENTION 
   This invention relates to compact imaging systems, and, more particularly, to imaging systems having small telephoto ratios. 
   An imaging lens (or lens system, also referred to as an imager) is commonly used to observe objects or scenes, whereby light emitted or reflected by a given object or scene is imaged by some means onto a detecting element, where it can readily be observed or recorded. 
   Because the field of view of an imaging lens is related to its focal length, narrow field of view imagers tend to have longer focal lengths. It is therefore desirable in many cases to constrain the overall length of the optical system while maintaining a long focal length. The telephoto ratio of an imaging system is given as the ratio of the overall length, measured from the foremost optical element to the image plane (or detecting element), to its effective focal length. This is achieved in a number of ways including the classical solution comprised of positive and negative powered element groupings as well as folded mirror or catadioptric systems. While these approaches can provide some shortening of long focal length imaging systems, they have practical limitations. 
   An afocal lens system is a system that images an object, located a large distance away from the system, to an image plane, also located a large distance away from the system, with some angular magnification. Afocal attachments have conventionally been used to change the effective focal length of an optical imaging system by providing angular magnification at the pupil. 
   In many designs incorporating optical systems, there is a need for the overall system to be compact. Such needs stem from weight and space constraints in the application in which the system is used. Conventional afocal attachments have been used to change the focal length of an optical imaging system, but are limited in their ability to provide small telephoto ratios, since they contribute substantial size and mass to the imaging system in order to achieve the desired angular magnification and field of view. 
   There is therefore a need for an imaging system that is more compact in physical size than current imaging systems. 
   Furthermore, there is also a need for an imaging system that has a smaller telephoto ratio than current imaging systems. 
   Still further, there is a need for an imaging system that provides a combination of the characteristics described above with superior trade-offs than have been previously attainable. 
   BRIEF SUMMARY OF THE INVENTION 
   The needs for the invention set forth above as well as further and other needs and advantages of the present invention are achieved by the embodiments of the invention described hereinbelow. 
   An optical system of this invention includes an array of micro-optic afocal optical subsystems capable of receiving electromagnetic radiation from a source and transmitting at least a portion of the electromagnetic radiation received from the source, and an imaging subsystem capable of receiving electromagnetic radiation from the array of micro-optic afocal optical subsystems and imaging at least a portion of the received electromagnetic radiation onto an image plane. 
   In the present invention the afocal attachments are miniaturized and tiled into an array, which is placed in front of an imaging lens subsystem. These arrays can comprise, but are not limited to, micro-optic elements that are arranged in proximity to one another. The angular magnification of the afocal lens systems (which includes the array) can be used to convert a narrow field of view into a wider angular field for a shorter focal length imaging lens system than without the array. In this manner, the effective focal length of the imaging lens system can be greatly increased with little increase in overall length, thereby substantially reducing the telephoto ratio of the imaging lens systems. 
   For a better understanding of the present invention, together with other and further needs thereof, reference is made to the accompanying drawings and detailed description and its scope will be pointed out in the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic sectional view of a conventional imaging system, taken along the optical axis; 
       FIG. 2  is a schematic sectional view of a compact telephoto imaging system in accordance with an embodiment of the present invention, taken along the optical axis; 
       FIG. 3  is an expanded schematic sectional view of a section of the afocal imaging lens array component of the embodiment of the present invention illustrated in  FIG. 2  taken along the optical axis; 
       FIG. 4  is a schematic sectional view of a compact telephoto imaging system in accordance with a further embodiment of the present invention, taken along the optical axis; 
       FIG. 5  is a schematic sectional view of a compact telephoto imaging system in accordance with a still further embodiment of the present invention, taken along the optical axis; 
       FIG. 6  is an expanded schematic sectional view of a section of the afocal imaging GRIN rod lens array component of the embodiment of the present invention illustrated in  FIG. 5  taken along the optical axis; and 
       FIGS. 7A and 7B  are schematic representations of embodiment of a planar array of miniature (micro-optic) elements arranged in a periodic and non-periodic structure respectively. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Compact optical systems, having a small telephoto ratio, are disclosed hereinbelow. 
   The terms “micro-optics” and “micro-optical component” as used herein, refer to optical components having apertures substantially smaller than the entrance pupil of the conventional optical imaging subsystems discussed herein. The micro-optical components can be refractive, diffractive or reflective or any combination thereof. Exemplary micro-optical components include, but are not limited to, micro-lenses, GRIN rod lenses, micro-mirrors and micro-prisms. 
   The term “gradient index rod lens” as used herein, refers to radial gradient index optical components. 
   Reference is made to  FIG. 1 , which is a schematic sectional view of a conventional imaging system  10 , taken along the optical axis  20 . Electromagnetic radiation, typically in, but not restricted to, the ultraviolet, visible and/or infrared bands, hereinafter referred to generally as light, emitted or reflected by a given object, either real or virtual, hereinafter also referred to as a source, is imaged through an optical system  10  comprised of a combination of optical elements, in this embodiment consisting of refractive elements  30 ,  40 ,  50 , and  60 , onto an image plane  70 . In some applications, although not a limitation of this invention, a CCD array, phosphorescent screen, photographic film, microbolometer array, or other means of detecting light energy, hereinafter referred to generally as a detecting element, is substantially located at the image plane  70 . 
   Reference is made to  FIG. 2 , which is a schematic sectional view of an embodiment of this invention  100 , taken along the optical axis  20 . In the operation of this embodiment, light is imaged through an array of miniaturized (micro-optic) afocal optical systems  110 , in this embodiment consisting of two refractive planar lens array elements  120  and  130 , onto an imaging system, in this embodiment consisting of the conventional imaging system  10  illustrated in  FIG. 1 . The angular magnification provided by the afocal system of lens array elements  110  maps a modified field of view into the normal field of view of the conventional imaging lens system  10 . In this manner the effective focal length of the combined imaging system can be magnified with little increase to the overall length of the system and thereby provides a substantially small telephoto ratio. 
   Reference is made to  FIG. 3 , which is a schematic sectional view of a section of the array of miniaturized (micro-optic) afocal lens systems  110  in the embodiment of  FIG. 2 , taken along the optical axis. At each location in the array, there is a miniaturized afocal lens system, in this embodiment consisting of two refractive microlens elements  122  and  132 , which provides angular magnification to the system. It should be noted that, although only five miniaturized (micro-optic) afocal lens systems are shown, this is not a limitation of this invention and any pre-determined number of miniaturized (micro-optic) afocal lens systems can be utilized in any one-dimensional or two-dimensional pattern. 
   Reference is made to  FIG. 4 , which is a schematic sectional view of a further embodiment of this invention  200 , taken along the optical axis  20 . In the operation of this embodiment, light is imaged through an array of miniaturized afocal lens systems  210 , in this embodiment consisting of four refractive planar lens array elements  220 ,  230 ,  240 , and  250 , onto an imaging system, in this embodiment consisting of the conventional imaging system  10  illustrated in  FIG. 1 . Similar to  FIG. 3 , at each location in the array there is a miniaturized afocal lens system that provides angular magnification to the system. The angular magnification provided by the afocal system of planar lens array elements  210  maps a modified field of view into the normal field of view of the conventional imaging lens system  10 . In this manner the effective focal length of the combined imaging system can be magnified with little increase to the overall length of the system and thereby provides a substantially small telephoto ratio. 
   Reference is made to  FIG. 5 , which is a schematic sectional view of a still further embodiment  300  of this invention, taken along the optical axis  20 . In the operation of this embodiment, light is imaged through an array of miniaturized afocal lens systems  310 , in this embodiment consisting of two refractive planar gradient index lens array elements  320  and  330 , onto an imaging system, in this embodiment consisting of the conventional imaging system  10  illustrated in  FIG. 1 . The angular magnification provided by the afocal system of planar gradient index (GRIN) rod lens array elements maps a modified field of view into the normal field of view of the conventional imaging lens system  10 . In this manner the effective focal length of the combined imaging system can be magnified with little increase to the overall length of the system and thereby providing a substantially small telephoto ratio. 
   Reference is made to  FIG. 6 , which is a schematic sectional view of a section of the array of miniaturized (micro-optic) afocal lens systems  310  in the embodiment of  FIG. 5 , taken along the optical axis. At each location in the array, there is a miniaturized afocal lens system, in this embodiment consisting of two refractive gradient index (GRIN) rod lens elements  322  and  332 , which provides angular magnification to the system. It should be noted that, although only five miniaturized (micro-optic) a focal lens systems are shown in  FIG. 6 , this is not a limitation of this invention and any pre-determined number of miniaturized (micro-optic) afocal lens systems can be utilized in any one-dimensional or two-dimensional pattern. 
   In some embodiments, the angular magnification of the array of afocal lens systems can result in the entrance pupil of the imaging lens system being under-filled. Reference is made to  FIG. 7A , which represents a planar array of miniature (micro-optic) elements  400 , where the optical elements  410  are arrayed in a periodic structure  420 . If a periodic structure such as, but not limited to, the periodic array  400  is used in the array of afocal lens systems in these embodiments, a modulation of the system modulation transfer function (MTF) can result. Reference is made to  FIG. 7B , which represents a planar array of miniature (micro-optic) elements  500 , where the optical elements  510  are arrayed in a non-periodic structure  520  such that the autocorrelation of the array structure has a substantially uniform distribution. Embodiments utilizing a non-periodic array structure such as, but not limited to, the non-periodic array  500  could substantially reduce or avoid the modulation of the system modulation transfer function (MTF). 
   The form of the miniaturized afocal lens systems that make up the array can be any combination of refractive, diffractive, gradient index, or other optical element known in the art. These components need only be miniaturized and placed into arrays to form the miniaturized (micro-optic) afocal lens arrays described above. 
   It should be noted that, although the arrays of miniaturized (micro-optical) afocal optical systems disclosed above comprise two or four planar array elements, the number of planar array elements is not a limitation of this invention. 
   Although the invention has been described with respect to various embodiments, it should be realized that this invention is also capable of a wide variety of further and other embodiments within the spirit and scope of the appended claims.