Patent Publication Number: US-7724412-B2

Title: Variable aperture and actuator assemblies for an imaging system

Description:
FIELD OF THE INVENTION 
     The present invention relates to imaging systems, more particularly, to variable aperture assemblies and corresponding actuator assemblies for use in an imaging system having a radiation detector housing, such as a cooled infrared imaging system. 
     BACKGROUND OF THE INVENTION 
     Thermal infrared radiation (IR) is emitted from all objects as a function of their temperature. IR imaging systems are able to detect thermal signatures and identify target objects by analyzing the heat and profile emitted. The mid-wave and long-wave IR detectors used in IR imaging systems are typically housed in vacuum enclosures, commonly referred to as dewars, and cooled to cryogenic temperatures to improve target detectability and lower signal to noise ratios. 
     A conventional IR imaging system  100  employing a typical dewar system  102  to cryogenically cool an infrared detector  104  is depicted in  FIG. 1 . The infrared detector  104  is mounted on a substrate  106  attached to a cold stem  108  of the dewar system  102 . The cold stem  108  houses the refrigeration portion of a stirling cycle refrigerator which cools and maintains the detector  104  at cryogenic operating temperatures. The detector  104  is typically mounted within a coldshield  110  that is housed within a vacuum enclosure  112  of the dewar system  102 . The vacuum enclosure  112  includes a window  114  attached to the top  116  of the vacuum enclosure  112  that allows the detector  104  to receive radiation signals external to the vacuum enclosure  112 . The optics system of the IR imaging system  100  may be incorporated in the window  114  to the vacuum enclosure  112  or be positioned relative to the window  114  in the external housing (not shown in  FIG. 1 ) of the IR imaging system  100 . 
     The coldshield  110  typically includes a fixed aperture  116  that essentially forms the f-stop for the optics system of the IR while also serving as a radiation shield for the detector. Some conventional IR imaging systems are capable of switching between narrow and wide field of view window or optics system (e.g., window  114  or the optics system disposed in the external housing of the IR imaging system  100 ) to view various target scenes, which requires two different coldshield aperture sizes to effectively match the optics system. A large family of dewars with coldshields of different aperture sizes is currently needed to accommodate the broad range of IR camera system designs. Hence, the need arises for a dewar to have a single coldshield with a variable aperture assembly having two or more apertures that may be switched on command to accommodate the various optical systems that may be employed in an IR imaging system. Moreover, there is a need for an aperture actuator or control means that does not generate a significant amount of heat within the vacuum enclosure when powered on to drive the variable aperture assembly. 
     U.S. Pat. No. 7,157,706 to Gat et al. discloses variable aperture assemblies (each generally referenced as  122  in  FIG. 1 ) for use in an IR camera having a dewar system  200  with a detector  104  mounted in a coldshield  110  that is enclosed in a vacuum chamber  112  as shown in  FIG. 1 . However, each non-magnetic driven variable aperture assembly  122  disclosed by Gat requires modifying the vacuum chamber  112  wall to either (1) add an external aperture control means  120 , such as a worm gear system to drive a worm gear attached to the variable aperture assembly  122 , or (2) to accommodate a piezoelectric motor aperture control means that directly contacts a friction surface of an outer drive ring of the variable aperture assembly  122 . These conventional variable aperture assemblies and corresponding aperture control means are known to be extremely large in size (requiring significant space within the vacuum chamber or within the external housing of the IR imaging system) and require significant force and travel to control the size of the variable or swappable aperture to be used. 
     Accordingly, there is a need for an improved variable aperture assembly and aperture actuator assembly that overcomes the problems noted above and others previously experienced for implementing a variable aperture and actuator within a dewar system of a cooled IR imaging system or camera. 
     SUMMARY OF THE INVENTION 
     In accordance with systems consistent with the present invention, an imaging system is provided. The imaging system comprises a housing for a radiation detector (such as a cold shield housing), a variable aperture assembly, and an actuator assembly for the variable aperture. The radiation detector housing has a window disposed above and in axial alignment with the radiation detector. The variable aperture assembly includes a base ring having a first opening and mounted on the radiation detector housing such that the first opening is in axial alignment with the window of the radiation detector housing. The variable aperture assembly also includes a plate and at least one aperture blade (e.g., a single blade, two blades, or four blades) having a first aperture and adapted to engage the base ring such that the first aperture is disposed over the window. Each aperture blade is operatively coupled to the base ring so that the respective aperture blade is adapted to move laterally relative to the first aperture. The variable aperture further includes an aperture drive mechanism having a body and an actuator coupling member extending at an angle from the body. The body is operatively coupled to the base ring and to each aperture blade such that the aperture drive mechanism drives each aperture blade laterally away from the first aperture in response to the actuator coupling member being moved in a first lateral direction, and laterally over the first aperture to define a second aperture disposed over the window in response to the actuator coupling member being moved in a second lateral direction. The actuator assembly is disposed adjacent to the radiation detector housing in proximity to the actuator coupling member. The actuator assembly has an actuator and an actuator arm. The actuator arm has a first end operatively coupled to the actuator and a second end adapted to engage the actuator coupling member of the aperture drive mechanism so that the actuator controls the lateral movement of the actuator coupling member. 
     In accordance with articles of manufacture consistent with the present invention, a variable aperture assembly for use in an imaging system having a housing for a radiation detector. The housing has a window disposed above and in axial alignment with the radiation detector. The variable aperture assembly comprises a base ring, a plate disposed over the base ring, at least one aperture blade, and an aperture drive mechanism. The base ring has a first opening and is adapted to be mounted on the radiation detector housing such that the first opening is in axial alignment with the window. The plate has a first aperture and is adapted to engage the base ring such that the first aperture is disposed over the window. Each aperture blade is operatively coupled to the base ring so that each aperture blade is adapted to move laterally relative to the first aperture. The aperture drive mechanism has a body and an actuator coupling member extending at an angle from the body. The body is operatively coupled to the base ring and to each aperture blade such that the aperture drive mechanism drives each aperture blade laterally away from the first aperture in response to the actuator coupling member being moved in a first lateral direction, and laterally over the first aperture to define a second aperture disposed over the window in response to the actuator coupling member being moved in a second lateral direction. 
     In one implementation of the variable aperture assembly, the at least one aperture blade includes a first blade having a first end rotatably coupled at a pivot point to either the base ring or the plate, a second end adapted to be pivoted relative to the first end, and an inner portion disposed between the first and second ends. The inner portion defines the second aperture. The base ring or the plate to which the first blade is rotatably coupled has an upper surface, a first stop pin disposed on the upper surface away from the pivot point, and a second stop pin disposed on the upper surface across the first aperture from the first stop pin and substantially away from the pivot point. The first stop pin is adapted to engage the second end of the first blade to stop the lateral movement thereof when the first blade is moved laterally away from the first aperture so that the first aperture is exposed. The second stop pin is adapted to engage the second end of the first blade to stop the lateral movement thereof when the first blade is moved laterally over the first aperture so that the second aperture is disposed over the window. 
     In another implementation of the variable aperture assembly, the base ring has an outer diameter defining an outer surface and a flange extending from the outer surface. In this implementation, the body of the aperture drive mechanism corresponds to a drive ring adapted to rotate about the base ring in sliding contact with the flange of the base ring. In addition, the base ring may have a plurality of pivot pins circumferentially spaced on the base ring. The drive ring may have a plurality of drive pins circumferentially spaced on the drive ring relative to the pivot pins. In this implementation, the at least one aperture blade corresponds to two or more aperture blades each having a first end and a second end. The first end of each blade has a pivot opening adapted to receive a respective one of the pivot pins and a drive opening adapted to receive a respective one of the drive pins such that the second end of the respective blade is adapted to pivot relative to the first end when the drive ring is rotated about the base ring. 
     In another implementation of the variable aperture assembly, the drive ring has a plurality of stop pins circumferentially spaced on the drive ring such that each drive pin is disposed between a respective two of the stop pins. Each aperture blade has a top portion and a lower portion that collectively form a substantially L-shape having an external corner. The lower portion includes the first end and has an outer edge. The top portion includes the second end and has an external edge. The pivot opening and the drive opening of each aperture blade are disposed near the external corner. Each stop pin is adapted to engage the external edge of the top portion of a respective one of the aperture blades to stop the lateral movement thereof when the aperture blade is moved laterally away from the first aperture so that the first aperture is exposed. Each stop pin may also be adapted to engage the outer edge of the lower portion of a respective second of the aperture blades to stop the lateral movement thereof when the aperture blade is moved laterally over the first aperture so that the second aperture is disposed over the window. 
     In another implementation of the variable aperture assembly, the plate has a circular outer edge that defines a rim along an outer perimeter of the base ring and the body of the aperture drive mechanism corresponds to a drive ring adapted to rotate about the outer edge of the plate in sliding contact with the rim of the base ring. The plate has a first plurality of guide pins circumferentially spaced on the plate. The drive ring has a plurality of drive pins circumferentially spaced on the drive ring relative to the guide pins. In this implementation, the at least one aperture blade corresponds to two or more aperture blades. Each aperture blade has a first guide pin track running in a direction substantially parallel to a corresponding radial axis of the window, and a drive pin track running in a direction substantially diagonal to the first guide pin track of the aperture blade. Each of the plurality of drive pins is operatively coupled to the drive pin track of a corresponding one of the aperture blades such that each drive pin travels along the drive pin track of the corresponding aperture blade in response to the drive ring being rotated about the outer edge of the plate. Each of the first plurality of guide pins is operatively coupled to the first guide pin track of a corresponding one of the aperture blades such that each first guide pin travels along the first guide pin track of the corresponding aperture blade in response to the drive pin traveling along the drive pin track of the corresponding aperture blade. 
     In accordance with articles of manufacture consistent with the present invention, an aperture actuator assembly for use in actuating a variable aperture assembly disposed over a window of radiation detector housing in an imaging device is provided. The variable aperture assembly includes an aperture drive mechanism having a body and an actuator coupling member extending down at an angle from the body. The actuator coupling member is adapted to be moved in a first lateral direction so that the variable aperture assembly defines a first aperture over the window and in a second lateral direction so that the variable aperture assembly defines a second aperture over the window. The aperture actuator assembly comprises an actuator adapted to be disposed adjacent to the radiation detector housing below the actuator coupling member, and an actuator arm disposed between the actuator and the actuator coupling member. The actuator arm has a first end operatively coupled to the actuator and a second end adapted to engage the actuator coupling member of the aperture drive mechanism so that the actuator controls the lateral movement of the actuator coupling member. 
     In one implementation of the aperture actuator assembly, the actuator is a piezoelectric motor having an actuator rod operatively coupled to the first end of the actuator arm and adapted to be selectively moved between a first position to cause the actuator arm to move in the first lateral direction and a second position to enable the actuator arm to move in the second lateral direction. 
     In another implementation of the aperture actuator assembly, the aperture actuator assembly also includes a mounting bracket extending vertically relative to the radiation detector housing. The actuator arm is pivotally coupled to the mounting bracket such that the second end of the actuator arm is adapted to rotate in the first lateral direction and the second lateral direction. The actuator comprises a magnet and a voice coil motor having a wire coil operatively configured to receive a drive current. The wire coil is incorporated in the first end of the actuator arm and the magnet is disposed relative to the wire coil so that the magnet drives the first end of the actuator arm away from the magnet in a predetermined direction in response to the drive current flowing through the wire coil. The predetermined direction corresponds to one of the first lateral direction and the second lateral direction based on a direction of flow of the drive current through the wire coil. 
     In another implementation of the aperture actuator assembly, the actuator assembly further comprises a mounting bracket extending vertically relative to the radiation detector housing. The actuator arm is pivotally coupled to the mounting bracket such that the second end of the actuator arm is adapted to rotate in the first lateral direction and the second lateral direction. The actuator comprises an electromagnetic solenoid having a drive input and a piston adapted to move along a longitudinal axis of the solenoid between an extended position and a contracted position based on the drive input. The piston has an end operatively coupled to the first end of the actuator arm so that the piston drives the second end of the actuator arm in the first lateral direction when moving towards the extended position and in the second lateral direction when moving towards the contracted position. 
     Other systems, methods, features, and advantages of the present invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an implementation of the present invention and, together with the description, serve to explain the advantages and principles of the invention. In the drawings: 
         FIG. 1  is a schematic cross-sectional view of a conventional cooled conventional infrared imaging system having a radiation detector mounted within a radiation shield enclosed within a vacuum chamber, where the radiation shield has a variable aperture and a control means for the variable aperture is mounted external to the vacuum chamber; 
         FIG. 2  is a schematic cross-sectional view of an exemplary infrared imaging system employing a variable aperture assembly and an aperture actuator assembly consistent with the present invention; 
         FIG. 3A  is a perspective view of one embodiment of a variable aperture assembly and one embodiment of a corresponding aperture actuator assembly suitable for implementing the present invention in the infrared imaging system in  FIG. 2 , where the variable aperture assembly includes a plate having a first aperture disposed over a window of an exemplary radiation detector housing within the infrared imaging system and a single blade having a second aperture shown in a first position laterally away from the first aperture; 
         FIG. 3B  is another perspective view of the variable aperture assembly and the aperture actuator assembly in  FIG. 3A  (with a portion of the aperture actuator assembly in partial cut away view), where the single blade has been moved, via the aperture actuator assembly, in accordance with the present invention so that the second aperture is in a second position over the first aperture and the window of the radiation detector housing; 
         FIG. 3C  is an exploded view of the variable aperture assembly depicted in  FIGS. 3A and 3B ; 
         FIG. 3D  is a top view of the variable aperture assembly in which the second aperture of the single blade is shown in the first position as depicted in  FIG. 3A ; 
         FIG. 3E  is another top view of the variable aperture assembly in which the second aperture of the single blade is shown in the second position as depicted in  FIG. 3B ; 
         FIG. 3F  is a perspective view of an alternative aperture drive mechanism for the variable aperture assembly and a corresponding alternative actuator arm for the aperture actuator assembly in  FIGS. 3A and 3B ; 
         FIGS. 4A and 4B  are perspective views of a second embodiment of a variable aperture assembly and a second embodiment of a corresponding aperture actuator assembly suitable for implementing the present invention in the infrared imaging system in  FIG. 2 , where the variable aperture assembly includes a fixed plate having a first aperture as shown in  FIG. 4A  and two aperture blades operatively configured to define a second aperture over the first aperture when moved via the aperture actuator assembly as shown in  FIG. 4B ; 
         FIG. 4C  is an exploded view of the variable aperture assembly depicted in  FIGS. 4A and 4B ; 
         FIG. 4D  is a perspective view of the variable aperture assembly as depicted in  FIG. 4A , where a cover ring of the variable aperture assembly has been removed to provide a more complete view of the arrangement of the two blades when moved via the aperture actuator assembly to expose the first aperture of the fixed plate; 
         FIG. 4E  is a top view of the variable aperture assembly as depicted in  FIG. 4D ; 
         FIG. 4F  is a perspective view of the variable aperture assembly as depicted in  FIG. 4B , where the cover ring has been removed to provide a more complete view of the arrangement of the two blades when moved via the aperture actuator assembly to define the second aperture over the first aperture; 
         FIG. 4G  is a top view of the variable aperture assembly as depicted in  FIG. 4F ; 
         FIG. 4H  is an exploded view of the aperture actuator assembly depicted in  FIGS. 4A  and B; 
         FIG. 4I  is a perspective view of another implementation (or third embodiment) of a aperture actuator assembly suitable for implementing the present invention in the infrared imaging system in  FIG. 2 ; 
         FIG. 4J  is an exploded view of the aperture actuator assembly depicted in  FIG. 4I ; 
         FIGS. 5A and 5B  are perspective views of a third embodiment of a variable aperture assembly and a fourth embodiment of a corresponding aperture actuator assembly suitable for implementing the present invention in the infrared imaging system in  FIG. 2 , where the variable aperture assembly includes a fixed plate having a first aperture as shown in  FIG. 5A  and four aperture blades operatively configured to define a second aperture over the first aperture when moved via the aperture actuator assembly as shown in  FIG. 5B ; 
         FIG. 5C  is an exploded view of the variable aperture assembly depicted in  FIGS. 5A and 5B ; 
         FIG. 5D  is a perspective view of the variable aperture assembly as depicted in  FIG. 5A , where a cover ring of the variable aperture assembly has been removed to provide a more complete view of the arrangement of the four aperture blades when moved via the aperture actuator assembly to expose the first aperture of the fixed plate; 
         FIG. 5E  is a top view of the variable aperture assembly as depicted in  FIG. 5D ; 
         FIG. 5F  is a perspective view of the variable aperture assembly as depicted in  FIG. 5B , where the cover ring has been removed to provide a more complete view of the arrangement of the four aperture blades when moved via the aperture actuator assembly to define the second aperture over the first aperture; 
         FIG. 5G  is a top view of the variable aperture assembly as depicted in  FIG. 5F ; 
         FIG. 5H  is an enlarged view of one of the aperture blades of the variable aperture assembly depicted in  FIGS. 5A and 5B ; 
         FIG. 5I  is an exploded view of the actuator assembly depicted in  FIGS. 5A and 5B ; 
         FIG. 6  is an exploded view of another implementation (or fourth embodiment) of the variable aperture assembly shown in  FIGS. 5A-5G , where the fixed plate has been incorporated into a base ring of the variable aperture assembly and the base ring has been incorporated into a window of a radiation detector housing for the infrared imaging system; 
         FIGS. 7A and 7B  are perspective views of a fifth embodiment of a variable aperture assembly suitable for implementing the present invention, where the variable aperture assembly has four aperture blades operatively configured to be selectively adjusted via an aperture actuator assembly consistent with the present invention so as to vary an aperture defined by the aperture blades between a first size shown in  FIG. 7A  and a second size shown in  FIG. 7B ; 
         FIG. 7C  is an exploded view of the variable aperture assembly depicted in  FIGS. 7A and 7B ; 
         FIG. 7D  is a top view of the variable aperture assembly as depicted in  FIG. 7A ; 
         FIG. 7E  is another top view of the variable aperture assembly depicted in  FIGS. 7A &amp; 7B  in which the aperture blades have been adjusted via the aperture actuator assembly so that the aperture defined by the aperture blades has a corresponding size that is smaller than the first size shown in  FIG. 7A  and larger than the second size shown in  FIG. 7B ; 
         FIG. 7F  is a top view of the variable aperture assembly as depicted in  FIG. 7B ; 
         FIG. 8A  is a top view of another implementation (or sixth embodiment) of the variable aperture assembly shown in  FIGS. 7A-7G , where each aperture blade of the variable aperture assembly has an S-track or non-linear track for controlling the movement of the respective aperture blade and the size of the aperture collectively defined by each of the blades; 
         FIG. 8B  is a top view of one of the aperture blades of the variable aperture assembly shown in  FIG. 8A ; 
         FIGS. 9A and 9B  are perspective views of a seventh embodiment of a variable aperture assembly and a fifth embodiment of a corresponding aperture actuator assembly suitable for implementing the present invention in the infrared imaging system in  FIG. 2 , where the variable aperture assembly includes a fixed plate having a first aperture as shown in  FIG. 9A  and two aperture blades operatively configured to define a second aperture over the first aperture when moved via the aperture actuator assembly as shown in  FIG. 9B ; and 
         FIG. 9C  is an exploded view of the variable aperture assembly depicted in  FIGS. 9A and 9B . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to an implementation in accordance with methods, systems, and products consistent with the present invention as illustrated in the accompanying drawings. 
       FIG. 2  depicts a schematic cross-sectional view of an exemplary infrared imaging system  200  employing a variable aperture assembly  202  and an aperture actuator assembly  204  consistent with the present invention. The variable aperture assembly  202 , which has a low profile and low mass, is operatively configured to selectively switch between two or more apertures of different size so that the selected aperture is accurately positioned relative to a radiation detector  206  housed within the infrared imaging system  200 . The aperture actuator assembly  204  also has low mass, a compact profile, and a low power actuator that has limited thermal radiation output suitable for operation in a vacuum environment. 
     As shown in  FIG. 2 , the infrared imaging system  200  includes a housing  208  for the radiation detector  206 , which may be one or more pixels in a focal plane array mounted on a substrate  210 . The housing  208  has an aperture or window  212  having a fixed size. The window  212  may be an opening in the housing  208  with or without a transparent material adapted to permit radiation outside the housing  208  within the field of view of the window  212  to enter the housing  208 . The window  212  is disposed above and in axial alignment with the radiation detector  206 . In the implementation shown in  FIG. 2 , the housing  208  is a cold shield or radiation shield that is (along with the substrate  210 ) attached to a cold stem  214  of a dewar system  216 . In this implementation, the infrared imaging system  200  also includes a vacuum chamber  218  that encloses the radiation detector housing  208 . The vacuum chamber  218  has a sealed window  220  over which a variable or fixed lens  222  may be mounted. The cold stem  214  effectively cools and maintains the temperature of the housing  208  and the radiation detector  206 . However, the variable aperture assembly  202  and the aperture actuator assembly  204  as described herein may be employed in an uncooled imaging system without departing from the present invention. 
     In the implementation shown in  FIG. 2 , the radiation detector  206  and the housing&#39;s window  212  are centrally aligned with each other along a longitudinal or vertical axis  224  of the housing. The variable aperture assembly  202  is mounted over and axially aligned with the housing&#39;s window  212 . The aperture actuator assembly  204  is disposed adjacent to and extending vertically along a side  226  of the radiation detector housing  208  and below the variable aperture assembly  202 . Although the aperture actuator assembly  204  may employ different actuators as described herein, the aperture actuator assembly  204  has a compact profile, with a volume (e.g., width, height, and depth shown as w, h and d in  FIGS. 3A-B , w′, h′ and d′ in  FIGS. 4A-4B , and w″, h″, and d″ in  FIGS. 5A-5B ) within a range from 13 cubic mm to 3750 cubic mm based on a width (i.e., w, w′ or w″) ranging from 1.5 mm to 25 mm, a height (i.e., h, h′ or h″) ranging from 1.5 mm to 25 mm, and a depth of approximately 6 mm. The compact profile of the aperture actuator assembly  204  enables the aperture actuator assembly  204  to engage and actuate the variable aperture assembly  202  while disposed and operating in the vacuum chamber  218  of the imaging system  200 . 
     Various embodiments of the variable aperture assembly  202  consistent with the present invention are described herein. Each variable aperture assembly  202  ( 300  in  FIGS. 3A-3F ,  400  in  FIGS. 4A-4G ,  500  in  FIGS. 5A-5H ,  600  in  FIG. 6 ,  700  in  FIGS. 7A-7F ,  800  in  FIGS. 8A-8F , and  900  in  FIGS. 9A-9C ) may include a base ring  228  ( 306  in  FIG. 3C ,  404  in  FIG. 4C ,  504  in  FIG. 5C ,  604  in  FIG. 6 ,  704  in  FIG. 7C , or  904  in  FIG. 9C ) having a first opening  230  ( 308  in  FIG. 3C ,  406  in  FIG. 4C ,  506  in  FIG. 5C ,  706  in  FIG. 7C  or  906  in  FIG. 9C ) and mounted on the radiation detector housing  208  such that the first opening  230  is in axial alignment with the window  212  of the housing  208 . Each variable aperture assembly may include a plate  232  ( 310  in  FIG. 3C ,  426  in  FIG. 4C ,  526  in  FIG. 5C ,  726  in  FIG. 7C  or  926  in  FIG. 9C ) having a first aperture  234  ( 312  in  FIG. 3C ,  428  in  FIG. 4C ,  528  in  FIG. 5C ,  728  in  FIG. 7C  or  928  in  FIG. 9C ) of a fixed size, where the plate  232  is mounted to and/or adapted to engage the base ring  228  such that the first aperture  234  is disposed over and in axial alignment with the housing&#39;s window  212 . As discussed in reference to  FIG. 6 , the plate  232  may be incorporated into the base ring  228  and the base ring  228  affixed or incorporated into the top  236  of the radiation detector housing  208  so that the housing window  212  corresponds to the first aperture  234 . 
     Each variable aperture assembly also has one or more aperture blades  238  ( 332  in  FIG. 3C ,  438  and  440  in  FIG. 4C ,  538   a - 538   b  in  FIG. 5C ,  738   a - 738   b  in  FIG. 7C ,  838   a - 838   b  in  FIG. 8C  or  938   a - 938   b  in  FIG. 9C ) and an aperture drive mechanism  240  ( 334  in  FIG. 3C ,  410  in  FIG. 4C ,  510  in  FIG. 5C ,  710  in  FIG. 7C  or  910  in  FIG. 9C ). As explained in further detail below, each aperture blade  238  is operatively coupled to the base ring  228  or plate  232  so that the respective aperture blade  238  is adapted to move laterally relative to the first aperture  234 . The aperture drive mechanism  240  has a body  242  and an actuator coupling member  244  extending at an angle from the body  242 , allowing the actuator assembly  204  to be disposed next to (or abut) the radiation detector housing  208  and within the vacuum chamber  218 . 
     As described in further detail below, the body  242  of the aperture drive mechanism  240  is operatively coupled to the base ring  228  (or the plate  232  having the first aperture  234 ) and to each aperture blade  238  such that the aperture drive mechanism  240  drives each aperture blade  238  laterally away from the first aperture  234  in response to the actuator coupling member  244  being moved in a first lateral direction (e.g., direction reflected by arrow  370   a  in  FIGS. 3A ,  4 A and  5 A, arrow  757   a  in  FIG. 7A , or arrow  957   a  in  FIG. 9A ), and laterally over the first aperture  234  to define a second aperture  246  disposed over the radiation detector housing window  212  in response to the actuator coupling member  244  being moved in a second lateral direction (e.g., direction reflected by arrow  370   b  in  FIGS. 3B ,  4 B and  5 B, arrow  757   b  in  FIG. 7B , or arrow  957   b  in  FIG. 9B ). 
     Various embodiments of the aperture actuator assembly  204  consistent with the present invention are also described herein. Each actuator assembly  204  ( 302  in  FIGS. 3A-3B ,  402  in  FIGS. 4A-4B  and  4 H,  402   a  in  FIGS. 4I-4J ,  502  in  FIGS. 5A-5B  and  5 I, and  902  in  FIGS. 9A-9B ) may be employed in accordance with the present invention to drive any one of the variable aperture assemblies  202 ,  300 ,  400 ,  500 ,  600 ,  700 ,  800 , and  900 . As discussed in further detail below, each actuator assembly has an actuator  248  ( 380  in  FIGS. 3A-3B ,  3 F and  7 A- 7 B,  402  in  FIGS. 4A-4B  and  4 H,  402   a  in  FIGS. 4I-4J ,  502  in  FIGS. 5A-5B  and  5 I, and  902  in  FIGS. 9A-9B ) and an actuator arm  250  ( 354  in  FIGS. 3A-3B  and  7 A- 7 B,  364  in  FIG. 3F ,  416  in  FIGS. 4A-4B  and  4 H- 4 J,  516  in  FIGS. 5A-5B  and  5 I, and  966  in  FIGS. 9A-9B ). The actuator arm  250  has a first end  252  operatively coupled to the actuator  248  and a second end  254  adapted to engage the actuator coupling member  244  of the aperture drive mechanism  240  so that the actuator  248  controls the lateral movement of the actuator coupling member  244 . The actuator  248  of each actuator assembly  204  includes one or more interconnects  254  to route power inputs signals (e.g., a drive input signal or current and a return output signal or current for the actuator  248 ) and other signals between the respective actuator  248  and a drive motor or a backend processor (not shown in figures) which may be disposed external to the vacuum chamber  218  of the imaging system  200 . 
       FIGS. 3A-3E  depict one embodiment of a variable aperture assembly  300  and  FIGS. 3A-3B  depict one embodiment of a corresponding aperture actuator assembly  302  suitable for implementing the present invention in the infrared imaging system  200 . Components of the imaging system  200 , such as the vacuum chamber  218 , are not shown in  FIGS. 3A-3E  to avoid obscuring inventive aspects of the variable aperture assembly  300  and the aperture actuator assembly  302 . 
     The radiation detector housing  304  shown in  FIGS. 3A-3C  has a hour glass shape but is otherwise consistent with the housing  208  shown in  FIG. 2 . The radiation detector housing  304  has an aperture or window  206  disposed above and in axial alignment with the radiation detector  206  (not shown in  FIGS. 3A-3C ). The variable aperture assembly  300  includes a base ring  306  having a first opening  308  and mounted on the radiation detector housing  304  such that the first opening  308  is in axial alignment with the window  206 . The base ring  306  may be mounted to the radiation detector housing  304  via epoxy, soldering, or other fastening technique. A plate  310  having a first aperture  312  is engaged or attached to the base ring  306  such that the first aperture  312  is disposed over the window  206 . The base ring  306  and the plate  310  may each be comprised of a conductive metal or alloy that allows heat produced by the variable aperture assembly or received from radiation incident on the variable aperture assembly to be dissipated via the housing  304 , which may function as a cold shield as previously discussed. In the implementation shown in  FIGS. 3A-3E , the base ring  306  has an upper surface  314  and a plurality of pins  316 ,  318 ,  320  and  322  extending from the upper surface  314 . The plate  310  also has an upper surface  323  defining a number of openings  324 ,  326 ,  328  and  330  each operatively configured to receive and retain a corresponding one of the pins  316 ,  318 ,  320  and  322  so that the first aperture  312  is disposed over and accurately aligned with the window  206 . Alternatively, the base ring  306  may be formed to incorporate the plate so that the base ring  306  has the first aperture  312 . 
     The variable aperture assembly  300  also includes a single or first aperture blade  332  that is operatively coupled to the base ring  306  or the plate  310  so that the aperture blade  332  is adapted to move laterally relative to the first aperture  312 . In the implementation shown in  FIGS. 3A-3E , the aperture blade  332  has a first end  334 , a second end  336 , and an inner portion  338  disposed between the first and second ends where the inner portion  338  defines the second aperture  340 . The aperture blade  332  is rotatably coupled at a pivot point (e.g., pin  320 , which functions as a pivot pin) to either the base ring  306 , the plate  310 , or both. The second end  336  of the aperture blade  332  is adapted to be pivoted relative to the first end  334 . In the implementation shown in  FIGS. 3A-3E , the aperture blade  332  defines a pivot opening  341  that is adapted to receive and retain the pivot pin  320 . In an alternate implementation, the pivot opening  341  may be defined by the base ring  306  or the plate  310  having the first aperture  312  while the pivot pin  320  is on a lower surface of the aperture blade  332 . In either implementation, the pivot opening  341  and the pivot pin  320  engaged in the pivot opening  341  collectively define the pivot point for the aperture blade  332  relative to the first aperture  312 . 
     As shown in  FIGS. 3A-3E , the first end  334  and the second end  336  of the aperture blade  332  may be extensions or projections of the inner portion  338 . Alternatively, the first end  334  and the second end  336  may be opposing external sections of the inner portion  338 , which may have a substantially circular, elliptical, square or other shape. The inner portion  338 , however, has a size that is greater than the size of the first aperture  312  to enable the inner portion  338  to partially overlap the plate  310  when the aperture blade  332  is moved laterally over the first aperture  312  such that the second aperture  340  is axially aligned with the first aperture  312  and the underlying window  206  of the radiation detector housing  304  as shown in  FIGS. 3B and 3E . 
     In one implementation, a first stop pin (e.g., pin  318 ) is disposed on the upper surface  314  or  323  of either the base ring  306  or the plate  310  away from the pivot point (e.g., pin  320 ). A second stop pin (e.g., pin  316 ) is disposed on the upper surface  314  or  323  across the first aperture  312  from the first stop pin  318  and substantially away from the pivot point or pin  320 . As shown in  FIGS. 3A-3E , the pivot pin  320 , the first stop pin  318  and the second stop pin  316  are arranged in a triangular pattern about the first aperture  312 . The first stop pin  318  is positioned relative to the pivot pin  320  so that the first stop pin  318  is adapted to engage the second end  336  of the aperture blade  332  to stop the lateral movement thereof when the aperture blade  332  is moved laterally away from the first aperture  312  and the first aperture  312  is exposed. The second stop pin  316  is positioned relative to the pivot pin  320  so that the second stop pin  316  is adapted to engage the second end  336  of the aperture blade  332  to stop the lateral movement thereof when the aperture blade  332  is moved laterally over the first aperture  312  so that the second aperture  340  is disposed over and in axial alignment with the window  206  of the radiation detector housing  304 . 
     In one implementation, the plate  310  includes a lobed section  342  disposed relative to the first stop pin  318  and the pivot pin  320  so that the inner portion  338  of the aperture blade  332  rests on the lobed section  342  when the second end  336  of the aperture blade  332  engages the first stop pin  318  and the first aperture  312  is exposed. To further decrease the weight and mass of the variable aperture assembly  300 , a central portion  344  of the lobed section  342  may be removed. In addition, the lobed section  342  allows sufficient thermal conductivity with the plate  312 , base ring  306 , and the radiation detector housing  304  so as to maintain the aperture blade  332  at a uniform operational temperature that otherwise might be elevated due to the incident thermal radiation thereon when the inner portion  338  of the blade  332  is or is not resting on the lobed section  342 . 
     As shown in  FIGS. 3A-3E , the variable aperture assembly  300  also includes an aperture drive mechanism  344  having a body  346  and an actuator coupling member  348  extending at an angle  350  from the body  346  such that the actuator assembly  302  (or actuator assembly  402 ,  402   a ,  502  or  902  described herein) may engage the actuator coupling member  348  and drive the aperture drive mechanism  344  while being disposed next to the radiation detector housing  304  and within the vacuum chamber  218  of the imaging system  200 . In the implementation shown in  FIGS. 3A and 3B , the actuator coupling member  348  includes a rod  352  extending down from and perpendicular to the body  346  of the aperture drive mechanism  344 . In this implementation, the actuator assembly  302  includes an actuator arm  354  that has a recess  356  (e.g., defined by pins  358   a  and  358   b ) on one end  360  of the actuator arm  354 . The recess  356  is adapted to engage and laterally retain the rod  352  so that the actuator arm  354  controls the lateral movement of the aperture drive mechanism  344 . The actuator arm  354  may be comprised of titanium, plastic, or other poor thermal conducting material that is also low in friction to improve reliability. 
       FIG. 3F  is a perspective view of an alternative aperture drive mechanism  362  for the variable aperture assembly  300  and a corresponding alternative actuator arm  364  for the aperture actuator assembly  302 . The actuator arm  364  is consistent with the actuator arm  354  except that the actuator arm  364  includes a rod  365  instead of a recess  356 . The rod  365  extends from the end  366  of the actuator arm  364  toward the aperture drive mechanism  362 . The aperture drive mechanism  362  is consistent with the aperture drive mechanism  344  except that the actuator coupling member  367  of the aperture drive mechanism  362  has a flange  368  instead of a rod  352  extending down from and perpendicular to the body  346  of the aperture drive mechanism  362 . The flange  368  has an opening  369  adapted to receive and laterally retain the rod  365  on the actuator arm  364  of the actuator assembly  302  shown in  FIG. 3F  so that the actuator arm  364  controls the lateral movement of the aperture drive mechanism  344 . 
     As shown in  FIGS. 3A-3F , the body  346  of the aperture drive mechanism  344  and  362  is operatively coupled to the base ring  306  and to each aperture blade  332  such that the aperture drive mechanism  344  or  362  drives each aperture blade  332  laterally away from the first aperture  312  in response to the actuator coupling member  348  or  367  being moved in a first lateral direction (e.g., direction reflected by arrow  370   a  in  FIG. 3A ), and laterally over the first aperture  312  to define or move the second aperture  340  over the window  206  in response to the actuator coupling member  348  or  367  being moved in a second lateral direction (e.g., direction reflected by arrow  370   b  in  FIG. 3B ). 
     In the implementation shown in  FIGS. 3A-3F , the body  346  of the aperture drive mechanism  344  and  362  corresponds to a drive arm that includes a first end  372   a  and a second end  372   b . The first end  372   a  is pivotally coupled near the pivot point (e.g., pin  320  and pivot opening  341 ) to the aperture blade  332 . The first end  372   a  of the body  346  or drive arm has a drive pin  373  that is received and retained by a drive opening  374  (shown best in  FIG. 3C ) defined near the pivot opening  341  by the aperture blade  332 . In an alternate implementation, the first end  372   a  of the body  346  or drive arm may define the drive opening  374  and the drive pin  373  may be disposed on the aperture blade  332  near the pivot point corresponding to the pivot opening  341 . 
     The second end  372   b  of the body  346  or drive arm has a track  376  adapted to receive and control the lateral movement of a third stop pin (e.g., pin  322 ) disposed on the upper surface  316  or  323  of the base ring  306  or the plate  310  when engaged to the base ring  306 . The track  376  of the drive arm defines a lateral travel range for the aperture drive mechanism  344 . The track  376  has a first terminal  377   a  adapted to engage the third stop pin  322  when the actuator coupling member  348  is moved in the first lateral direction  370   a  so that the aperture blade  332  is rotated away from the first aperture  312  to a first position where the second end  336  of the aperture blade  332  is engaged by the first stop pin  318  as shown in  FIGS. 3A and 3D . The track  376  also has a second terminal  377   b  adapted to engage the third stop pin  322  when the actuator coupling member  348  is moved in the second lateral direction  370   b  so that the aperture blade  332  is rotated over the first aperture  312  to a second position where the second end  336  of the aperture blade  332  is engaged by the second stop pin  316  as shown in  FIGS. 3B ,  3 E and  3 F. 
     As shown in  FIGS. 3A ,  3 B and  3 F, the actuator arm  354  and  364  each have a first end  378  operatively coupled to an actuator  380  of the actuator assembly  302  and a second end  360 , which (as previously discussed) is adapted to engage the actuator coupling member  348  or  367  of the aperture drive mechanism  346  or  362  so that the actuator  380  controls the lateral movement of the actuator coupling member  348  or  367 . In the implementation shown in  FIGS. 3A and 3B , the actuator  380  includes a piezoelectric motor  381  having an actuator rod  382 . The actuator rod  382  is operatively coupled to the first end  378  of the actuator arm  354  or  364  and is adapted to be selectively moved between a first position (such as shown in  FIG. 3A ) to cause the actuator arm  354  or  364  to move in the first lateral direction  370   a  and a second position (such as shown in  FIG. 3B ) to enable the actuator arm  354  or  364  to move in the second lateral direction  370   b.    
     The actuator assembly  302  may also include a mounting bracket  383  extending vertically relative to the radiation detector housing  304 . The actuator arm  354  or  364  may be operatively coupled to the mounting bracket  383  such that the second end  360  of the actuator arm  354  or  364  is adapted to move in the first lateral direction  370   a  and the second lateral direction  370   b . In one implementation, the actuator assembly  302  includes an L-shaped linkage member  384  operatively coupled between the first end  378  of the actuator arm  354  or  364  and the actuator rod  382  of the piezoelectric motor  381 . The linkage member  384  has a first end  385 , a second end  386 , and a corner  387  that is pivotally attached to the mounting bracket  383  via a pin  398 , a torsion spring or other pivoting means. The first end  385  is pivotally coupled (e.g., via a pin  388 ) to the first end  378  of the actuator arm  354  or  364 . The linkage member  384  also has a flange  389  disposed at or near the second end  386  of the linkage member  384 . The actuator rod  382  of the piezoelectric motor  381  is disposed relative to the flange  389  so that the actuator rod  382  is adapted to engage the flange  389  when moving from the first position to the second position such that the first end  385  of the linkage member  384  pivots about the corner  387  and drives the second end  360  or  366  of the actuator arm  354  or  364  in a corresponding one of the first lateral direction  370   a  or the second lateral direction  370   b . For example, in the implementation shown in  FIG. 3B , the actuator rod  382  of the piezoelectric motor  381  is adapted to engage and drive the flange  389  downward when moving from the first position to the second position causing the first end  385  of the linkage member  384  to pivot about the corner  387  and drive the second end  360  or  366  of the actuator arm  354  or  364  in the second lateral direction  370   b.    
     As shown in  FIGS. 3A and 3B , the actuator assembly  302  may also include a bias member  390  (such as a spring, torsion bar, elastic band or other bias member) operatively coupled between the vertical mounting bracket  383  and a point (e.g., a pin  391  affixed to the linkage member  384 ) near the second end  386  of the linkage member  384  to bias the flange  389  vertically when the actuator rod  382  of the piezoelectric motor  381  is moved towards the first position. Accordingly, when the actuator rod  382  of the piezoelectric motor  381  is moved towards the first position and away from the flange  389 , the bias member  390  biases the first end  385  of the linkage member  384  to pivot about the corner  387  of the linkage member  384  so that the second end  360  of the actuator arm is driven in another of the first lateral direction  370   a  or the second lateral direction  370   b . For example, in the implementation shown in  FIG. 3A , when moving from the second position to the first position, the actuator rod  382  of the piezoelectric motor  381  moves away from the flange  389  and the bias member  390  biases the first end  385  of the linkage member so that the first end  385  of the linkage member  384  pivots about the corner  387  and drives the second end  360  of the actuator arm  354  or  364  in the first lateral direction  370   a.    
       FIGS. 4A-4G  depict a second embodiment of a variable aperture assembly  400  and  FIGS. 4A ,  4 B and  4 H depict a second embodiment of a corresponding aperture actuator assembly  402  suitable for implementing the present invention in the infrared imaging system  200  having a radiation detector housing  304 . Again, components of the imaging system  200 , such as the vacuum chamber  218 , are not shown in  FIGS. 4A-4H  to avoid obscuring inventive aspects of the variable aperture assembly  400  and the aperture actuator assembly  402 . 
     The variable aperture assembly  400  includes a base ring  404 , a plate  426  having a fixed aperture  428 , two aperture blades  438  and  440 , and an aperture drive mechanism  410 . The base ring  404  has a first opening  406  and is mounted on the radiation detector housing  304  such that the first opening  406  is in axial alignment with the window  206  of the housing  304 . The base ring  404  may be mounted to the radiation detector housing  304  via epoxy, soldering, or other fastening technique. The base ring  404  and the plate  426  may each be comprised of a conductive metal or alloy that allows heat produced by the variable aperture assembly or received from radiation incident on the variable aperture assembly to be dissipated via the housing  304 . The base ring  404  has an outer diameter  407  that defines an outer surface  408 . The base ring  404  also has a flange  409  extending from the outer surface  408 . In one implementation, the flange  409  is one of a plurality of flanges  409  circumferentially spaced about the base ring  404 . 
     The aperture drive mechanism  410  has a body  411  and an actuator coupling member  412  extending at an angle  413  from the body  411  such that the actuator assembly  402  (or actuator assembly  302 ,  402   a , or  502  described herein) may engage the actuator coupling member  412  and drive the aperture drive mechanism  410  while being disposed next to the radiation detector housing  304  and within the vacuum chamber  218  of the imaging system  200 . In the implementation shown in  FIGS. 4A-4G , the actuator coupling member  412  includes a rod  414  extending down from and perpendicular to the body  411  of the aperture drive mechanism  410 . In this implementation, the actuator assembly  402  includes an actuator arm  416  having a recess  418  (e.g., defined by pins  419  and  420  in  FIG. 4H ) on one end  422  (i.e., the second end) of the actuator arm  416 . The recess  418  is adapted to engage and laterally retain the rod  414  of the actuator coupling member  412  so that the actuator arm  416  controls the lateral movement of the aperture drive mechanism  410 . As further discussed below, the actuator assembly  402  includes a rotary actuator  423  operatively coupled to another end  424  (i.e., the first end) of the actuator arm  416  and adapted to control the rotational movement of the actuator arm  416 . The actuator arm  416  may be comprised of titanium, plastic, or other poor thermal conducting material that is also low in friction to improve reliability. 
     In an alternative implementation, instead of a recess  418 , the actuator arm  416  may have a rod  365  on the one end  422  (consistent with the rod  365  on the actuator arm  354  in  FIG. 3F ), where the rod  365  extends toward the aperture drive mechanism  410 . In this implementation, the actuator coupling member  412  of the aperture drive mechanism  410  may have a flange  368  having an opening  369  instead of a rod  414  consistent with the aperture drive mechanism  362  in  FIG. 3F . As previously discussed, the flange opening  369  is adapted to receive and laterally retain the rod  365  on the actuator arm of the actuator assembly so that the actuator arm controls the lateral movement of the aperture drive mechanism  410 . 
     In the implementation of the variable aperture assembly  400  shown in  FIGS. 4A-4G , the body  411  of the aperture drive mechanism  410  corresponds to a drive ring adapted to rotate about the base ring  404  in sliding contact with the one or more flanges  409  extending from the outer surface  408  of the base ring  404 . In this implementation, the drive ring  411  has an inner diameter  425  that is equal to or larger than the outer diameter  407  of the base ring  404 . 
     The plate  426  having the first aperture  428  is engaged or attached to the base ring  404  such that the first aperture  428  is disposed over the window  206  of the housing  304 . In the implementation shown in  FIGS. 4A-4G , the base ring  404  has an upper surface  430  and a plurality of pins  432  and  433  extending from the upper surface  430 . The plate  426  also has an upper surface  434  defining a number of openings  436  and  437  each operatively configured to receive and retain a corresponding one of the pins  432  and  433  so that the first aperture  428  is disposed over and accurately aligned with the window  206  of the housing  304 . Alternatively, the base ring  404  may be formed to incorporate the plate  426  so that the base ring  404  has the first aperture  428 . 
     Each of the two aperture blades  438  and  440  are operatively coupled to the base ring  404  or the plate  426  so that each aperture blade  438  and  440  is adapted to move laterally relative to the first aperture  428 . In the implementation shown in  FIGS. 4A-4G , each aperture blade  438  and  440  has a first end  442  or  443 , a second end  444  or  445 , and a front edge  446  and  447 . The first end  442  or  443  of each blade  438  and  440  has a pivot opening  450  adapted to receive a respective one of a plurality of pivot pins (e.g., pins  432  and  433 ) circumferentially spaced on the upper surface  430  of the base ring  404 . The first end  442  or  443  of each blade  438  and  440  also has a drive opening  452  adapted to receive a respective one of a plurality of drive pins  454  and  456  circumferentially spaced on the drive ring  411  relative to the pivot pins  432  and  433  on the base ring  404  such that the second end  444  or  445  of the respective blade  438  or  440  is adapted to pivot relative to the first end  442  or  443  when the drive ring  411  is rotated about the base ring  404 . As further described herein, the front edge  446  or  447  of each aperture blade  438  and  440  collectively define a second aperture  448  that is disposed over the radiation detector housing&#39;s window  206  in response to the actuator coupling member  412  (or rod  414 ) being moved in a the second lateral direction  370   b  so that the drive ring  411  is rotated in the same direction about the base ring  404  as shown in  FIGS. 4B ,  4 F and  4 G. 
     In an alternative implementation, each pivot opening  450  may be defined by the base ring  404  or the plate  426  having the first aperture  428  while a respective pivot pin  432  or  433  is disposed on a lower surface of the first end  442  or  444  of a respective aperture blade  438  or  440 . In either implementation, each pivot opening  450  and the pivot pin  432  or  433  engaged in the pivot opening  450  collectively define the pivot point for the respective aperture blade  438  or  440 . 
     The drive ring  411  may also have a plurality of stop pins  458  and  459  circumferentially spaced on the drive ring  411  such that each drive pin  454  and  456  is disposed between a respective two of the stop pins  458  and  459 . Each stop pin  458  and  459  is adapted to engage the second end  444  or  445  of a respective one of the aperture blades  438  or  440  to stop the lateral movement thereof when the aperture blade  438  or  440  is moved laterally away from the first aperture  428  so that the first aperture  428  is exposed as shown in  FIGS. 4A ,  4 D and  4 E. 
     Each pivot pin  432  and  433  (when received in the pivot opening  450  of the respective aperture blade  438  or  440 ) may also be adapted to engage the second end  444  or  445  of a respective second or adjacent one of the aperture blades  440  or  438  to stop the lateral movement thereof when the aperture blade  440  or  438  is moved laterally over the first aperture  428  so that the second aperture  448  is collectively defined by the front edges  446  and  447  of the blades and disposed over the window  206  of the radiation detector housing  304  as shown in  FIGS. 4B ,  4 F and  4 G. 
     As described herein, the drive ring  411  (i.e., the body of the aperture drive mechanism  410 ) is operatively coupled to the base ring  404  via sliding engagement with the flange  409  extending from the outer surface  408  of the base ring  404 . The drive ring  411  is also operatively coupled to each aperture blade  438  and  440  via a respective drive pin  454  or  456 . The aperture drive mechanism  410  is adapted to drive each aperture blade  438  and  440  about a respective pivot pin  432  and  433  so that each aperture blade  438  and  440  moves laterally away from the first aperture  428  in response to the actuator coupling member  412  (which extends from the drive ring  411 ) being moved in the first lateral direction (e.g., direction reflected by arrow  370   a  in  FIG. 4A ). In addition, the drive mechanism  410  is adapted to drive each aperture blade  438  and  440  in a reverse direction about a respective pivot pin  432  and  433  so that each aperture blade  438  and  440  moves laterally over the first aperture  428  to define the second aperture  448  over the window  206  in response to the actuator coupling member  412  being moved in the second lateral direction (e.g., direction reflected by arrow  370   b  in  FIG. 4B ). 
     As shown in  FIGS. 4A ,  4 B and  4 C, the variable aperture assembly  400  may also include a cover ring  460  having an inner aperture  462  that has a size equal to or greater than the size of the first aperture  428 . The cover ring  460  is disposed over and vertically retains or captivates the aperture blades  438  and  440  to the base ring  404  and/or the drive ring  410 . In one implementation, the cover ring  460  is attached to the pivot pins  432  and  433  so that the cover ring  460  is suspended above the aperture blades  438  and  440  so that each aperture blade  438  and  440  is adapted to freely rotate about a respective pivot pin  432  or  433  between a respective stop pin  458  or  459  and a respective drive pin  454  and  456 . 
     Turning again to the actuator assembly  402  shown in  FIGS. 4A ,  4 B and  4 H, the actuator assembly  402  includes a mounting bracket  464  extending vertically relative to the radiation detector housing  304 . The actuator arm  416  is pivotally coupled to the mounting bracket (e.g., via a pin  465 ) such that the second end  422  of the actuator arm is adapted to rotate in the first lateral direction  370   a  and the second lateral direction  370   b.    
     In the implementation shown in  FIGS. 4A ,  4 B and  4 H, the rotary actuator  423  is a voice coil actuator that includes a voice coil motor  466 . The voice coil motor  466  comprises a wire coil  467  and one or more magnets  468   a  and  468   b , each of which may be a permanent or non-permanent magnet. The wire coil  467  is incorporated in the actuator arm  416  near the first end  424 . The wire coil  467  is operatively configured to receive a drive current via a drive current motor (not shown in the figures). Each magnet  468   a - 468   b  is disposed relative to the wire coil  467  so that the respective magnet  468   a - 468   b  drives the first end  424  of the actuator arm  416  away from the magnet  468   a  or  468   b  in a predetermined direction in response to the drive current flowing through the wire coil  467 . The predetermined direction of the first end  424  of the actuator arm corresponds to either the first lateral direction  370   a  or the second lateral direction  370   b  (opposite to the lateral direction of movement of the second end  422  as shown in the figures) based on the direction of flow of the drive current through the wire coil  467 . 
     In the implementation shown in  FIGS. 4A ,  4 B, and  4 H, the wire coil  467  is disposed between a pair of magnets  468   a  and  468   b  so that, when a drive current is flowing through the wire coil  467 , the magnetic field produced by each of the magnets  468   a  and  468   b  drives or forces the first end  424  of the actuator arm  416  (which has the wire coil  467 ) about the pivot pin  465  in the predetermined direction corresponding to the direction of flow of the drive current. The second end  422  of the actuator arm  216  then drives the actuator coupling member  412  of the aperture drive mechanism  410  in a corresponding lateral direction (e.g., opposite to the lateral movement of the first end  424 ). The amount of lateral travel of each end  422  and  424  of the actuator arm  416  is controlled by the amplitude of the drive current received by and flowing through the wire coil  467 . 
     A first of the pair of magnets  468   a  is affixed to the vertical mounting plate  464 . A second of the pair of magnets  468   b  may be affixed to a cross bracket  470  that is attached to either or both sides  472   a  and  472   b  of the mounting bracket  464  such that the first end  424  of the actuator arm  416  may rotate freely about the pivot pin  465  and between the pair of magnets  468   a  and  468   b  without contacting or engaging the cross bracket  470 . Each magnet  468   a  and  468   b  may have an arcuate shape and a respective length that enables the wire coil  467  to remain at least partially between the pair of magnets  468   a  and  468   b  as the first end  424  of the actuator arm  416  is rotated about the pivot pin  465 . 
     In one implementation, the first end  424  of the actuator arm  416  has an arcuate outer surface  474  having a plurality of detents  475   a - 475   e  corresponding to a plurality of predetermined positions for the actuator arm  416 . The actuator assembly  402  further comprises a spring pin  476  having a first end  477  attached to one of the brackets  464  or  470 , or to one of the magnets  468   a  or  468   b . The spring pin  476  has a second end  478  disposed relative to the first end  424  of the actuator arm  416  such that the spring pin  476  is adapted to removably engage one (e.g.,  475   b ) of the plurality of detents  475   a - 475   e  to retain the actuator arm  416  in a corresponding one of the predetermined positions when the drive current is not flowing through the wire coil  467 . When the drive current is flowing through the wire coil  467 , the magnetic field produced by each magnet  468   a  and  468   b  (collectively or alone) may be sufficient to cause the actuator arm  416  to move and disengage the spring pin  476  from the one detent (e.g.,  475   b ). 
       FIGS. 4I and 4J  depict another implementation of an aperture actuator assembly  402   a  suitable for implementing the present invention in the infrared imaging system  200 . The aperture actuator assembly  402   a  has components consistent with the actuator assembly  402  except that, in lieu of a spring pin  476  and detents  475   a - 475   e  on the second end  424  of the actuator arm  416 , one of the actuator arm  416  and the mounting bracket  464  has a metal portion  482 , and the other of the actuator arm  416  and the mounting bracket  464  has a restraining magnet  483  disposed relative to and having an attraction for the metal portion  482 . In this implementation, the restraining magnet  483  has sufficient attraction to the metal portion  482  so that the restraining magnet  483  is adapted to retain the actuator arm  416  in its current rotated position when the drive current is not flowing through the wire coil. When the drive current is flowing through the wire coil  467 , the magnetic field produced by each magnet  468   a  and  468   b  (collectively or alone) may be sufficient to cause the actuator arm  416  to move and disengage the attraction of the restraining magnet  464  to the metal portion  483 . 
     Alternatively, as shown in  FIGS. 4I and 4J , in lieu of or in addition to the detents  475   a - 475   e , the actuator arm  416  may have a plurality of detent magnets  484   a - 484   e  corresponding to a plurality of predetermined positions for the actuator arm  416 . Each detent magnet  484   a - 484   e  has a first polarity. The restraining magnet  483  has a second polarity. The restraining magnet  483  is disposed on the mounting bracket  464  relative to the plurality of detent magnets  484   a - 484   e  such that the restraining magnet  483  is attracted to a closest one of the detent magnets  484   a - 484   e  to retain the actuator arm  416  in a corresponding one of the predetermined positions when the drive current is not flowing through the wire coil  467 . When the drive current is flowing through the wire coil  467 , the magnetic field produced by each magnet  468   a  and  468   b  (collectively or alone) may be sufficient to cause the actuator arm  416  to move and disengage the attraction of the restraining magnet  464  to the detent magnets  484   a - 484   e.    
     It is contemplated that the restraining magnet  483  may be disposed on the actuator arm  416  and the detent magnets  484   a - 484   e  may be disposed on the mounting bracket  464  relative to the restraining magnet  483  such that the restraining magnet  483  is attracted to a closest one of the detent magnets  484   a - 484   e  to retain the actuator arm  416  in a corresponding one of the predetermined positions when the drive current is not flowing through the wire coil. 
       FIGS. 5A-5H  depict a third embodiment of a variable aperture assembly  500  and  FIGS. 5A ,  5 B and  5 I depict a fourth embodiment of a corresponding aperture actuator assembly  502  suitable for implementing the present invention in the infrared imaging system  200  having a radiation detector housing  304 . The variable aperture assembly  500  includes a base ring  504 , a plate  526  having a fixed aperture  528 , four aperture blades  538   a - 538   d  and an aperture drive mechanism  510 . The base ring  504  has a first opening  506  and mounted on the radiation detector housing  304  such that the first opening  506  is in axial alignment with the window  206  of the housing  304 . The base ring  504  may be mounted to the radiation detector housing  304  via epoxy, soldering, or other fastening technique. As previously discussed, the base ring  504  and the plate  526  may each be comprised of a conductive metal or alloy that allows heat produced by the variable aperture assembly  500  or received from radiation incident on the variable aperture assembly  500  to be dissipated via the housing  304 . The base ring  504  has an outer diameter  507  that defines an outer surface  508  and a flange  509  extending from the outer surface  508 . The flange  509  may be one of a plurality of flanges  509  circumferentially spaced about the base ring  504 . 
     The aperture drive mechanism  510  has a body  511  and an actuator coupling member  512  extending at an angle  513  from the body  511  such that the actuator assembly  502  (or actuator assembly  302 ,  402 , or  402   a  described herein) may couple to the actuator coupling member  512  and drive the aperture drive mechanism  510  while being disposed next to the radiation detector housing  304  and within the vacuum chamber  218  of the imaging system  200 . In the implementation shown in  FIGS. 5A-5H , the actuator coupling member  512  includes a rod  514  extending down from and perpendicular to the body  511  of the aperture drive mechanism  510 . In this implementation, the actuator assembly  502  includes an actuator arm  516  having a recess  518  (e.g., defined by pins  519  and  520  as best seen in  FIG. 5J ) on one end  522  (i.e., the second end) of the actuator arm  516 . The recess  518  is adapted to engage and laterally retain the rod  514  of the actuator coupling member  512  so that the actuator arm  516  controls the lateral movement of the aperture drive mechanism  510 . As further discussed below, the actuator assembly  502  includes an actuator  523  operatively coupled to another end  524  (i.e., the first end) of the actuator arm  516  and adapted to control the rotational movement of the actuator arm  516 . 
     In an alternative implementation, instead of a recess  518 , the actuator arm  516  may have a rod  365  on the one end  522  (consistent with the rod  365  on the actuator arm  916  in  FIGS. 9A and 9B  discussed herein), where the rod  365  extends toward the aperture drive mechanism  510 . In this implementation, the actuator coupling member  512  of the aperture drive mechanism  510  may have a flange  368  having an opening  369  instead of a rod  514  consistent with the aperture drive mechanism  362  in  FIG. 3F  or the aperture drive mechanism  910  in  FIGS. 9A and 9B . As previously discussed, the flange opening  369  is adapted to receive and laterally retain the rod  365  on the actuator arm of the actuator assembly so that the actuator arm controls the lateral movement of the aperture drive mechanism  510 . 
     In the implementation of the variable aperture assembly  500  shown in  FIGS. 5A-5G , the body  511  of the aperture drive mechanism  510  corresponds to a drive ring adapted to rotate about the base ring  504  in sliding contact with the one or more flanges  509  extending from the outer surface  508  of the base ring  504 . To facilitate a sliding contact coupling between the drive ring  511  and the base ring  504 , the drive ring  511  may have an inner diameter  525  that is equal to or larger than the outer diameter  507  of the base ring  504 . 
     The plate  526  having the first aperture  528  is engaged or attached to the base ring  504  such that the first aperture  528  is disposed over the window  206  of the housing  304 . In the implementation shown in  FIGS. 5A-5G , the base ring  504  has an upper surface  530  and a plurality of pivot pins  532   a - 532   d  extending from the upper surface  530 . The plate  526  also has an upper surface  534  defining a number of openings  536   a - 536   d  each operatively configured to receive and retain a corresponding one of the pivot pins  532   a - 532   d  so that the first aperture  528  is disposed over and accurately aligned with the window  206  of the housing  304 . Alternatively, the base ring  504  may be formed to incorporate the plate  526  so that the base ring  504  has the first aperture  528 . 
     Each of the aperture blades  538   a - 538   d  is operatively coupled to the base ring  504  or the plate  526  so that each aperture blade  538   a - 538   d  is adapted to move laterally relative to the first aperture  528 . In the implementation shown in  FIGS. 5A-5H , each aperture blade  538   a - 538   d  has a first end  542   a - 542   d , a second end  544   a - 544   d , and a front edge  546   a - 546   d . The first end  542   a - 542   d  of each blade  538   a - 538   d  has a pivot opening  550  adapted to receive a respective one the pivot pins  532   a - 532   d  circumferentially spaced on the upper surface  530  of the base ring  504 . 
     The drive ring  511  has a plurality of drive pins  554   a - 554   d  circumferentially spaced on the drive ring  511  relative to the pivot pins  532   a - 532   d  on the base ring  504 . The first end  542   a - 542   d  of each blade  538   a - 538   d  has a drive opening  552  adapted to receive a respective one of the drive pins  554   a - 554   d  such that the second end  546   a - 546   d  of the respective blade  538   a - 538   d  is adapted to pivot relative to the respective first end  542   a - 542   d  when the drive ring  511  is rotated about the base ring  504 . 
     As further described herein, the front edge  546   a - 546   d  of each aperture blade  538   a - 538   d  collectively define a second aperture  548  disposed over the radiation detector housing&#39;s window  206  in response to the actuator coupling member  512  (or rod  514 ) being moved in a the second lateral direction  370   b  so that the drive ring  511  is rotated in the same direction about the base ring  504  as shown in  FIGS. 5B ,  5 F and  5 G. 
     In an alternate implementation, each pivot opening  550  may be defined by the base ring  504  or the plate  526  having the first aperture  528  while a respective pivot pin  532   a - 532   d  is disposed on a lower surface of the first end  542   a - 542   d  of a respective aperture blade  538   a - 538   d . In either implementation, each pivot opening  550  and the pivot pin  532   a - 532   d  engaged in the pivot opening  550  collectively define the pivot point for the respective aperture blade  538   a - 538   d.    
     In the implementation shown in  FIGS. 5A-5H , each aperture blade  538   a - 538   d  has a respective top portion  559   a - 559   d  and a respective lower portion  560   a - 560   d  that collectively form a substantially L-shape having an external corner  561   a - 561   d .  FIG. 5H  depicts an enlarged view of one of the aperture blades (e.g.,  538   d ) of the variable aperture assembly  500  to illustrate the top portion (e.g.,  559   d ) and the lower portion (e.g.,  560   d ) that define or form the L-shape and corresponding external corner (e.g.,  561   d ) of the respective aperture blade (e.g.,  538   d ). The lower portion  560   a - 560   d  of each aperture blade  538   a - 538   d  includes the first end  542   a - 542   d  and has an outer edge  562   a - 562   d . The top portion  559   a - 559   d  of each aperture blade  538   a - 538   d  includes the second end  544   a - 544   d  and has an external edge  563   a - 563   d . The pivot opening  550  and the drive opening  552  of each aperture blade  538   a - 538   d  is disposed near the respective external corner  561   a - 561   d.    
     The drive ring  511  may also have a plurality of stop pins  558   a - 558   d  circumferentially spaced on the drive ring  511  such that each drive pin  554   a - 554   d  is disposed between a respective two of the stop pins  558   a - 558   d . Each stop pin  558   a - 558   d  is adapted to engage the external edge  563   a - 563   d  of the top portion  561   a - 561   d  of a respective one of the aperture blades  538   a - 538   d  to stop the lateral movement thereof when the aperture blade  538   a - 538   d  is moved laterally away from the first aperture  528  so that the first aperture  528  is exposed. Each stop pin  558   a - 558   d  is adapted to engage the outer edge  562   a - 562   d  of the lower portion  560   a - 560   d  of a respective second of the aperture blades  538   a - 538   d  to stop the lateral movement thereof when the aperture blade  538   a - 538   d  is moved laterally over the first aperture  528  so that the second aperture  548  (as defined by the front edge  546   a - 546   d  of each aperture blade  538   a - 538   d ) is disposed over the window  206 . For example, in the implementation shown in  FIGS. 5A-5H , the stop pin  558   d  is disposed on the drive ring  511  between the drive pin  554   d  for one aperture blade  538   d  and the drive pin  554   c  for a second aperture blade  538   c . The stop pin  558   d  is adapted to engage the external edge  563   d  of the top portion  559   d  of the one aperture blade  538   d  to stop the lateral movement thereof when the aperture blade  538   d  is moved laterally away from the first aperture  528  to expose the first aperture  528  as shown in  FIGS. 5D and 5E . The same stop pin  558   d  is adapted to engage the outer edge  562   c  of the lower portion  560   c  of the second aperture blade  538   c  to stop the lateral movement thereof when the aperture blade  538   c  is moved laterally over the first aperture  528  so that the second aperture  548  is disposed over the window  206  as shown in  FIGS. 5F and 5G . 
     As described herein, the drive ring  511  (i.e., the body of the aperture drive mechanism  510 ) is operatively coupled to the base ring  504  via sliding engagement with the flange  509  extending from the outer surface  508  of the base ring  504 . The drive ring  511  is also operatively coupled to each aperture blade  538   a - 538   d  via a respective drive pin  554   a - 554   d . The aperture drive mechanism  510  is adapted to drive each aperture blade  538   a - 538   d  about a respective pivot pin  532   a - 532   d  so that each aperture blade  538   a - 538   d  moves laterally away from the first aperture  528  in response to the actuator coupling member  512  (which extends from the drive ring  511 ) being moved in the first lateral direction (e.g., direction reflected by arrow  370   a  in  FIG. 5A ). In addition, the drive mechanism  510  is adapted to drive each aperture blade  538   a - 538   d  in a reverse direction about a respective pivot pin  532   a - 532   d  so that each aperture blade  538   a - 538   d  moves laterally over the first aperture  528  to define the second aperture  548  over the window  206  in response to the actuator coupling member  512  being moved in the second lateral direction (e.g., direction reflected by arrow  370   b  in  FIG. 5B ). 
     As shown in  FIGS. 5A ,  5 B and  5 C, the variable aperture assembly  500  may also include a cover ring  570  having an inner aperture  572  that has a size that is equal to or greater than the size of the first aperture  528 . The cover ring  570  is disposed over and vertically retains or captivates the aperture blades  538   a - 538   d  to the base ring  504  and/or the drive ring  510 . In one implementation, the cover ring  570  is attached to the pivot pins  532   a - 532   d  so that the cover ring  570  is suspended above the aperture blades  538   a - 538   d  so that each aperture blade  538   a - 538   d  is adapted to freely rotate about a respective pivot pin  532   a - 532   d  between a respective pair of adjacent stop pins (e.g., pairs  558   a  &amp;  558   b ;  558   b  &amp;  558   c ,  558   c  &amp;  558   d , and  558   d  &amp;  558   a ). 
     Turning again to the actuator assembly  502  shown in  FIGS. 5A ,  5 B and  5 H, the actuator assembly  502  includes a mounting bracket  564  extending vertically relative to the radiation detector housing  304 . The actuator arm  516  is pivotally coupled to the mounting bracket (e.g., via a pin  565 ) such that the second end  522  of the actuator arm  516  is adapted to rotate in the first lateral direction  370   a  and the second lateral direction  370   b . The actuator arm  516  may be comprised of titanium, plastic, or other poor thermal conducting material that is also low in friction to improve reliability. 
     In the implementation shown in  FIGS. 5A ,  5 B and  5 I, the actuator  523  comprises one or more electromagnetic solenoids  523   a  and  523   b . Each solenoid  523   a  and  523   b  has a drive input  574   a , a return output  574   b , and a piston  576 . The drive input  574   a  and return output  574   b  of each solenoid  523   a  and  523   b  may be operatively coupled via the interconnect  254  to an external drive motor (not shown in figures) controlled by a backend processor (not shown in figures) both of which may be disposed external to the vacuum chamber  218  of the imaging system  200 . The piston  576  of each solenoid  523   a  and  523   b  is adapted to move along a longitudinal axis  577   a  or  577   b  of the respective solenoid  523   a  or  523   b  between an extended position (as shown for solenoid  523   a  in  FIG. 5A  and for solenoid  523   b  in  FIG. 5B ) and a contracted position (as shown for solenoid  523   b  in  FIG. 5A  and for solenoid  523   a  in  FIG. 5B ) based on the respective drive input  574   a . The drive input  574   a  of the first solenoid  523   a  is opposite in polarity to the drive input  574   a  of the second solenoid  523   b  (e.g., based on a respective drive signal that may be present on each drive input  574   a  as provided by the external drive motor) such that the piston  576  of the first solenoid  523   a  moves in opposition to the piston  576  of the second solenoid  523   b . For example, the piston  576  of the first solenoid  523   a  has an end  578  operatively coupled to the first end  524  of the actuator arm  516  so that the first solenoid&#39;s piston  576  (based on the polarity of its drive input  574   a ) is adapted to drive the second end  522  of the actuator arm  516  in the first lateral direction  370   a  when moving towards the extended position as shown in  FIG. 5A  and in the second lateral direction  370   b  when moving towards the contracted position as shown in  FIG. 5B . The piston  576  of the second solenoid  523   b  has an end  578  operatively coupled to the first end  524  of the actuator arm  516  so that the second solenoid&#39;s piston  576  (based on the polarity of its drive input  574   a ) is adapted to drive the second end  522  of the actuator arm  516  in the first lateral direction  370   a  when moving towards the contracted position as shown in  FIG. 5A  and in the second lateral direction  370   b  when moving towards the extended position as shown in  FIG. 5B . 
     In one implementation, in which the piston  576  of each solenoid  523   a  and  523   b  includes a metal or metal alloy having a magnetic attraction, the actuator  523  may also include one or more latching magnets  579   a  and  579   b  disposed relative to each solenoid  523   b . In this implementation, each latching magnet  579   a  and  579   b  is operatively configured to hold the piston  576  of a respective one or each solenoid  523   a  and  523   b  in the piston&#39;s current position (i.e., a first position for the first solenoid  523   a  and a second position for the second solenoid  523   b ) when the electrical bias present on the drive input  574   a  or  574   b  of the respective solenoid  523   a  or  523   b  is removed. The current position for the respective piston  576  of each solenoid  523   a  is between the extended position and the contracted position of the respective piston  576 . In an alternative implementation, the current position may be one of the contracted position or the extended position of the respective piston  576 . Thus, the aperture  528  or  548  may be maintained as each piston  576  is held in its current position without having an electrical bias or power applied to each solenoid  523   a  and  523   b , inhibiting potential thermal radiation from being generated by the actuator  523  during an image capture interval of the imaging system  200  (e.g., the interval when radiation is being collected by the radiation detector  206  for image processing). 
     In the implementation shown in  FIGS. 5A ,  5 B, and  5 I, the actuator arm  516  has a transverse member  580  disposed near the first end  524  of the actuator arm  516  and defining (relative to the actuator arm  516 ) a first moment arm  581   a  having a first distal end  582   a  and a second moment arm  581   b  having a second distal end  582   b . The end  578  of each piston  576  is operatively coupled to or near the distal end  582   a  or  582   b  of either the first moment arm  581   a  or the second moment arm  581   b . In one implementation, the distal end  582   a  and  582   b  of each moment arm  581   a  and  581   b  has a recess  584   a  or  584   b  as shown in  FIG. 5J  adapted to engage and retain the end  578  of a respective piston  578 . In this implementation, the actuator arm  516  having the transverse member  580  functions as a rocker arm actuated by the respective pistons  576  of each solenoid  523   a  and  523   b  to move the end  522  of the rocker arm  516  in the first lateral direction  370   a  or the second lateral direction  370   b , causing the actuator coupling member  512  (or  348  in  FIG. 3A ,  367  in  FIG. 3F ,  412  in  FIG. 4A ,  712  in  FIG. 7A  or  962  in  FIG. 9A ) to move in the same direction to laterally drive the aperture drive mechanism  510  (or  344 ,  410 ,  710  or  910 ) of the respective variable aperture assembly  500  (or  300 ,  400 ,  600 ,  700 ,  800  or  900 ). 
       FIG. 6  depicts another implementation of a variable aperture assembly  600  suitable for implementing the present invention in the infrared imaging system  200  having a radiation detector housing  602 . The variable aperture assembly  600  has components (including a base ring  604 , a drive ring  510 , aperture blades  538   a - 538   d , and cover plate  570 ) that are consistent with and function the same as the same components in the variable aperture assembly  500  except the plate  526  of the variable aperture assembly  500  is incorporated into the base ring  604  of the variable aperture assembly  600  so that the base ring  604  has the first aperture  528 . The base ring  604  is affixed to or incorporated into the radiation detector housing  602  such that the housing window (e.g., window  206  in  FIG. 5C ) corresponds to the first aperture  528 . In one implementation, the base ring  604  is formed into a cover of the housing  602 , via etching, a molding process, or other technique. In this implementation, the one or more flanges  509  extending from the outer surface  508  of the base ring  504  may be replaced by a lip  606  formed about the circumference of the base ring  604 . 
       FIGS. 7A-7F  depict a fifth embodiment of a variable aperture assembly  700  suitable for implementing the present invention. The variable aperture assembly  700  has a plurality of aperture blades operatively configured to be selectively adjusted via an aperture actuator assembly consistent with the present invention so as to vary an aperture defined by the aperture blades between a first size shown in  FIG. 7A  and a second size shown in  FIG. 7B . The variable aperture assembly  700  is depicted as operatively coupled to and actuated by the aperture actuator assembly  302  as illustrated in  FIGS. 7A and 7B . However, the aperture actuator assembly  302  may alternatively be operatively coupled to and actuated by the aperture actuator assembly  402 ,  402   a ,  502  or another aperture actuator assembly consistent with the present invention. 
     The variable aperture assembly  700  includes a base ring  704 , a plate  726  having a fixed aperture  728 , four aperture blades  738   a - 738   d  and an aperture drive mechanism  710 . Although the structure and operation of the variable aperture assembly  700  is described as having four aperture blades  738   a - 738   d , the variable aperture assembly  700  may be implemented using one blade (e.g.,  738   a ) or two or more aperture blades (e.g.,  738   a  and  738   c ) without departing from the present invention. 
     As shown in  FIGS. 7A-7F , the plate  726  is engaged or attached to the base ring  704  such that the first aperture  728  is disposed over the inner or first opening  706  of the base ring  704  and over the window  206  of the housing  304 . The first opening  706  of the base ring  704  is obscured from view in  FIG. 7C  by the plate  726  but may be defined to correspond to or have a larger size than the first aperture  728  of the plate  726 . The base ring  704  is mounted on the radiation detector housing  304  such that the first opening  706  of the base ring  704  and the first aperture  728  of the plate  726  are each disposed over and in axial alignment with the window  206  of the housing  304 . As previously discussed, the base ring  704  and the plate  726  may each be comprised of a conductive metal or alloy that allows heat produced by the variable aperture assembly  700  or received from radiation incident on the variable aperture assembly  700  to be dissipated via the housing  304 . The plate  726  has a circular outer edge  729  that defines a rim  707  along an outer perimeter  709  of the base ring  704 . 
     The aperture drive mechanism  710  has a body  711  and an actuator coupling member  712  extending at an angle  713  from the body  711  such that the actuator assembly  302 ,  402 ,  402   a , or  502  may couple to the actuator coupling member  712  and drive the aperture drive mechanism  710  while being disposed next to the radiation detector housing  304  and within the vacuum chamber  218  of the imaging system  200 . In the implementation shown in  FIGS. 7A-7F , the actuator coupling member  712  includes a rod  714  extending down from and perpendicular to the body  711  of the aperture drive mechanism  710 . In an alternative implementation, the actuator coupling member  712  of the aperture drive mechanism  710  may have a flange  368  having an opening  369  instead of a rod  714  consistent with the aperture drive mechanism  362  in  FIG. 3F  and the aperture drive mechanism  910  in  FIGS. 9A-9C . In either implementation, the second end  360 ,  422 , or  522  of the actuator arm  354 ,  416 , or  516  may be adapted to engage and laterally retain the rod  714  or the flange  368  of the actuator coupling member  712  so that the actuator arm  354 ,  416 , or  516  controls the lateral movement of the aperture drive mechanism  710  as described herein. 
     In the implementation of the variable aperture assembly  700  shown in  FIGS. 7A-7F , the body  711  of the aperture drive mechanism  710  corresponds to a drive ring adapted to rotate about the outer edge  729  of the plate  726  in sliding contact with the rim  707  of the base ring  704 . To facilitate a sliding contact coupling between the drive ring  711  and the base ring  704 , the drive ring  711  may have an inner diameter  725  that is equal to or larger than the outer diameter  713  of the plate  704 . 
     The plate  726  has an upper surface  730  and a first plurality of guide pins  732   a - 732   d  circumferentially spaced on the upper surface  730  of the plate  726 . The drive ring  711  has a plurality of drive pins  754   a - 754   d  circumferentially spaced on the drive ring relative to the guide pins  732   a - 732   d  on the plate  726 . In the implementation depicted in  FIGS. 7A-7F , the drive ring  711  has circumferentially spaced lobes  755   a - 755   d  that extend from an outer edge  756  of the drive ring  711 . In this implementation, each drive pin  754   a - 754   d  is disposed on a respective lobe  755   a - 755   d  of the drive ring  711 . 
     Each of the aperture blades  738   a - 738   d  is operatively coupled to the base ring  704  or the plate  726  so that each aperture blade  738   a - 738   d  is adapted to move laterally relative to the first aperture  728 . In the implementation shown in  FIGS. 7A-7F , each aperture blade  738   a - 738   d  has a first end  742   a - 742   d , a second end  744   a - 744   d , and a front edge  746   a - 746   d . Each aperture blade  738   a - 738   d  also has a first guide pin track  750   a - 750   d  running in a direction substantially parallel to a corresponding radial axis  260   a - 260   d  of the window  206  of the radiation detector housing  208  or  304  as shown, for example, in  FIG. 7C . In addition, each aperture blade  738   a - 738   d  has a drive pin track  752   a - 752   d  running in a direction substantially diagonal to the first guide pin track  750   a - 750   d  of the respective aperture blade  738   a - 738   d . Each of the drive pins  754   a - 754   d  is operatively coupled to the drive pin track  752   a - 752   d  of a corresponding one of the aperture blades  738   a - 738   d  such that each drive pin  754   a - 754   d  travels along the drive pin track  752   a - 752   d  of the corresponding aperture blade  738   a - 738   d  in response to the drive ring  711  being rotated about the outer edge  729  of the plate  726 . 
     Each of the first guide pins  732   a - 732   d  is operatively coupled to the first guide pin track  750   a - 750   d  of a corresponding one of the aperture blades  738   a - 738   d  such that each first guide pin  732   a - 732   d  travels along the first guide pin track  750   a - 750   d  of the corresponding aperture blade  738   a - 738   d  in response to the drive pin  754   a - 754   d  traveling along the drive pin track  752   a - 752   d  of the corresponding aperture blade  738   a - 738   d . Each aperture blade  738   a - 738   d  is adapted to move laterally away from the first aperture  728  along the radial axis  260   a ,  260   b ,  260   c , or  260   d  of the window  206  corresponding to the first guide pin track  750   a ,  750   b ,  750   c , or  750   d  of the aperture blade  738   a - 738   d  in response to the drive ring  711  being rotated about the plate  726  and the base ring  704  in the first lateral direction  757   a  as shown in  FIG. 7A . In addition, each aperture blade  738   a - 738   d  is adapted to move laterally over the first aperture  728  along the radial axis  260   a ,  260   b ,  260   c , or  260   d  of the window  206  corresponding to the first guide pin track  750   a ,  750   b ,  750   c , or  750   d  of the aperture blade  738   a - 738   d  in response to the drive ring  711  being rotated about the plate  726  and the base ring  704  in the second lateral direction  757   b.    
     In one implementation, to ensure each aperture blade  738   a - 738   d  does not rotate substantially when traveling along the radial axis  260   a - 260   d  of the window  206  corresponding to the first guide pin track  750   a ,- 750   d  of the respective aperture blade  738   a - 738   d , each aperture blade  738   a - 738   d  may include a second guide pin track  758   a - 758   d  running in a direction substantially parallel to the first guide pin track  750   a - 750   d  of the respective aperture blade  738   a - 738   d . In this implementation, the plate  726  has a second plurality of guide pins  760   a - 760   d  circumferentially spaced on the plate  726 . Each of the second guide pins  760   a - 760   d  is operatively coupled to the second guide pin track  758   a - 758   d  of a corresponding one of the aperture blades  738   a - 738   d  such that each second guide pin  760   a - 760   d  travels along the second guide pin track  738   a - 738   d  of the corresponding aperture blade  738   a - 738   d  in response to the drive pin  754   a - 754   d  traveling along the drive pin track  752   a - 752   d  of the corresponding aperture blade  738   a - 738   d.    
     The front edge  746   a - 746   d  of each aperture blade  738   a - 738   d  collectively define a second aperture  748  disposed over the radiation detector housing&#39;s window  206  in response to the actuator coupling member  712  being moved in a the second lateral direction  757   b  so that the drive ring  711  is rotated in the same direction about the plate  726  and base ring  704  as shown in  FIG. 7B . In one implementation, the front edge (e.g.,  746   a ) of each aperture blade (e.g.,  738   a ) overlays and aligns with the front edge (e.g.,  746   b  and/or  746   d ) of an adjacent aperture blade (e.g.,  738   b  and/or  738   d ) such that a portion of the front edge  746   a - 746   d  of each aperture blade  738   a - 738   d  defines the second aperture  748 . As illustrated in  FIGS. 7D-7F , the portion of the front edge  746   a - 746   d  of each aperture blade  738   a - 738   d  that defines the second aperture  748  decreases as each aperture blade  738   a - 738   d  is moved over the first aperture  728  along the radial axis  260   a ,  260   b ,  260   c , or  260   d  of the window  206  corresponding to the first guide pin track  750   a ,  750   b ,  750   c , or  750   d  of the respective aperture blade  738   a - 738   d.    
     In one implementation of the variable aperture assembly  700  as illustrated in  FIGS. 7A-7F , the drive pin track  752   a - 752   d  of each aperture blade  738   a - 738   d  defines a lateral travel range for the respective aperture blade  738   a - 738   d . In this implementation, each drive pin track  752   a - 752   d  has a first terminal  780   a  adapted to engage and stop the travel of a respective drive pin  754   a - 754   d  when the actuator coupling member  714  is moved in the first lateral direction  757   a  so that the respective aperture blade  752   a - 752   d  is driven laterally away from the first aperture  728  to a first position as shown in  FIGS. 7A and 7D . Each drive pin track  752   a - 752   d  also has a second terminal  780   b  adapted to engage and stop the travel of the respective drive pin  754   a - 754   d  when the actuator coupling member  714  is moved in the second lateral direction  757   b  so that the respective aperture blade  752   a - 752   d  is driven laterally towards and over the first aperture to a second position as shown in  FIGS. 7B and 7F . In an alternative implementation, instead of the drive pin track, the first guide pin track  750   a - 750   d  of each aperture blade  738   a - 738   d  may have respective terminal ends that define the lateral travel range for the respective aperture blade  738   a - 738   d.    
     In the implementation depicted in  FIGS. 7A-7F , the drive pin track  752   a - 752   d  of each aperture blade  738   a - 738   d  has a linear shape so that the respective blade  738   a - 738   d  may be incrementally driven in a linear manner by a corresponding drive pin  754   a - 754   d , enabling the second aperture  748  to be incrementally varied in size in accordance with the travel of each drive pin  754   a - 754   d  between the first and second terminals  780   a  and  780   b  of the respective drive pin track  752   a - 752   d . For example,  FIG. 7E  depicts the variable aperture assembly  700  in a state where the aperture blades  738   a - 738   d  have been adjusted via each drive pin  754   a - 754   d  traveling in the drive pin track  752 - 752  of a respective blade  738   a - 738   b  (where each drive pin  754   a - 754   d  may be actuated by the aperture actuator assembly  302 ,  402 ,  402   a , or  502  as described herein) between the first and second terminals  780   a  and  780   b  of the respective drive pin track  752   a - 752   d  so that the aperture  748  defined by the aperture blades  738   a - 738   b  has a corresponding size that is smaller than the first size (e.g., the size of the first aperture) shown in  FIG. 7A  and larger than the second size shown in  FIG. 7B . 
     In an alternative implementation, the drive pin track  752   a - 752   d  of each aperture blade  738   a - 738   d  may have a non-linear shape. For example, in the implementation depicted in  FIGS. 8A and 8B , an variable aperture assembly  800  is shown that has aperture blades  838   a - 838   b  consistent with the aperture blades  738   a - 738   d  of the variable aperture assembly  700 , except each aperture blade  838   a - 838   b  of the variable aperture assembly  800  has an “S” shaped track  852   a - 852   d  that is a non-linear track for controlling the movement of the respective aperture blade  838   a - 838   b  and the size of the aperture  748  collectively defined by each of the front edges  746   a - 746   d  of the aperture blades  838   a - 838   b  as discussed herein. In this implementation, the S-shape of each track  852   a - 852   d  enables greater sensitivity in varying between larger sizes of the aperture  748  as each drive pin  754   a - 754   d  travels near the first terminal  780   a  of the respective S-shaped drive track  852   a - 852   d  and between smaller sizes of the aperture  748  as each drive pin  754   a - 754   d  travels near the second terminal  780   b  of the respective S-shaped drive track  852   a - 852   d . This implementation may be desirable for a radiation detector housing  304  that may be employed in a family of imaging systems  200  in which each system requires switching between a narrow and a wide field of view optical system having range stops between, for example f/6 and f/3. 
     As shown in  FIGS. 7A ,  7 B and  7 C, the variable aperture assembly  700  may also include a cover ring  770  having an inner aperture  772  that has a size that is equal to or greater than the size of the first aperture  728  of the plate  726 . The cover ring  770  is disposed over and vertically retains or captivates the aperture blades  738   a - 738   d  to the base ring  704  and/or the drive ring  711 . In one implementation, the cover ring  770  is attached to one or more of the guide pins  732   a - 732   d  so that the cover ring  770  is suspended above the aperture blades  738   a - 38   d  so that each aperture blade  738   a - 738   d  is adapted to move relative to the guide pin tracks  750   a - 750   d  and  758   a - 758   d  of the respective aperture blade  738   a - 738   d.    
       FIGS. 9A-9C  depict a seventh embodiment of a variable aperture assembly  900  and a fifth embodiment of a corresponding aperture actuator assembly  902  suitable for implementing the present invention in the infrared imaging system  200  having a radiation detector housing  304 . The components of the aperture actuator assembly  902  correspond to and function the same as the aperture actuator assembly  502  as described herein, except the actuator assembly  902  employs an actuator arm  966  having a rod  365  instead of an actuator arm  516  having a recess  518  as further discussed below. 
     The variable aperture assembly  900  includes a base ring  904 , a plate  926  having a fixed aperture  928 , a plurality of aperture blades  938   a - 938   b , and an aperture drive mechanism  910 . The base ring  904  has a first opening  906  and mounted on the radiation detector housing  304  such that the first opening  906  is in axial alignment with the window  206  of the housing  304 . The base ring  904  may be mounted to the radiation detector housing  304  via epoxy, soldering, or other fastening technique. As previously discussed, the base ring  904  and the plate  926  may each be comprised of a conductive metal or alloy that allows heat produced by the variable aperture assembly or received from radiation incident on the variable aperture assembly to be dissipated via the housing  304 . 
     In the implementation shown in  FIGS. 9A-9C , the base ring  904  has an upper surface  907  defining a plurality of stop pin openings  908   a  and  908   b  each operatively configured to receive and retain a corresponding stop pin  909   a  and  909   b . Each stop pin opening  908   a - 908   b  and corresponding stop pin  909   a - 909   b  (when received in the respective stop pin opening) are disposed on the upper surface  907  of the base ring  904  so that each stop pin  909   a - 909   b  is adapted to engage and stop the lateral movement (relative to the first aperture  928 ) of one of the aperture blades  938   a - 938   b  when the blade  938   a - 938   b  is moved away from the first aperture  928  as shown in  FIG. 9A  and is adapted to engage and stop the lateral movement of another of the aperture blades  938   a - 938   b  when the blade  938   a - 938   b  is moved towards or over the first aperture  928  as shown in  FIG. 9B . 
     The upper surface  907  of the base ring  904  also defines a plurality of pivot pin openings  912   a ,  912   b  and  914  each operatively configured to receive and retain a corresponding pivot pin  916   a ,  916   b  or  918 . As further described below, each pivot pin  916   a ,  916   b , and  918  is operatively configured to engage either one of the aperture blades  938   a - 938   b  or the aperture drive mechanism  910  to allow the respective component to rotate relative to the base ring  904 . 
     The plate  926  also has an upper surface  930  defining a number of openings  932   a - 932   b  and  934  each operatively configured to receive and retain a corresponding one of the pivot pins  916   a ,  916   b  and  918  so that the first aperture  928  is disposed over and aligned with the base ring  904  and the window  206 . The plate  926  may be interchanged with other plates  926  that have a different sized aperture  928  to enable the fixed aperture  928  to be varied depending on the optics system (not shown in the figures) and the corresponding field of view of the imaging system  200 . Alternatively, the base ring  904  may be formed to incorporate the plate  926  so that the base ring  904  has the first, fixed aperture  928 . 
     Each of the aperture blades  938   a - 938   b  are operatively coupled to the base ring  904  or the plate  926  at a pivot point so that each aperture blade  938   a - 938   b  is adapted to move laterally relative to the first aperture  928  and the vertical axis  224  of the radiation detector housing  304 . In the implementation shown in  FIGS. 9A-9C , each aperture blade  938   a - 938   b  has a first end  942   a  or  942   b , a second end  944   a  or  944   b , and a front edge  946   a  or  946   b . The first end  942   a - 942   b  of each blade  938   a - 938   b  has a pivot opening  950  (e.g., the pivot point) adapted to receive a corresponding one of the pivot pins  916   a  and  916   b  to operatively couple the respective blade  938   a - 938   b  to the base ring  904  and/or the plate  926  so that the second end  944   a  or  944   b  of the aperture blade  938   a  or  938   b  is adapted to be pivoted relative to the first end  942   a  or  942   b  enabling the aperture blade  938   a - 938   b  to move laterally relative to the first aperture  928 . 
     The first end  942   a - 942   b  of each of the aperture blades  938   a - 938   b  is also operatively coupled to a respective one of two ends  956   a  and  956   b  of the aperture drive mechanism  910  so that the aperture drive mechanism  910  controls the lateral movement of each blade  938   a - 938   b  relative to the first aperture  928 . In the implementation shown in  FIGS. 9A-9C , the first end  942   a - 942   b  of each blade  938   a - 938   b  also has a drive opening  952  and each end  956   a - 956   b  of the aperture drive mechanism  910  has a corresponding drive opening  953 . In this implementation, the first end  942   a  or  942   b  of each of the aperture blades  938   a  and  938   b  is operatively coupled to a respective one of two ends  956   a  or  956   b  of the aperture drive mechanism  910  via a respective drive pin  954   a  or  954   b . Each drive pin  954   a  and  954   b  is inserted into and retained by the drive opening  952  of the respective aperture blade  938   a  and the corresponding drive opening  953  at either end  956   a  or  956   b  of the aperture drive mechanism  910 . 
     As further described herein, the front edge  946   a  or  946   b  of each aperture blade  938   a - 938   b  collectively define a second aperture  948  disposed over the radiation detector housing&#39;s window  206  in response to the aperture drive mechanism  910  (and the drive pins  954   a  and  954   b ) being moved in a second lateral direction  957   b  so that each aperture blade  938   a - 938   b  is moved towards or over the first aperture  928  as shown in  FIG. 9B . 
     The aperture drive mechanism  910  has a body  960  and an actuator coupling member  962  extending at an angle  963  from the body  960  such that the actuator assembly  902  (or actuator assembly  302 ,  402 ,  402   a , or  502  described herein) may couple to the actuator coupling member  962  and drive the aperture drive mechanism  910  while being disposed next to the radiation detector housing  304  and within the vacuum chamber  218  of the imaging system  200 . In the implementation shown in  FIGS. 9A-9C , the actuator coupling member  962  includes a flange  368  having an opening  369  extending down from and perpendicular to the body  960  of the aperture drive mechanism  910 . In this implementation, the actuator assembly  902  includes an actuator arm  966  having a rod  365  on one end  970  (i.e., the second end) of the actuator arm  966 . The flange opening  369  is adapted to engage and laterally retain the rod  365  of the actuator arm  966  so that the actuator arm  966  controls the lateral movement of the aperture drive mechanism  910 . 
     In an alternative implementation, instead of the actuator assembly  902 , the actuator assembly  502  may be employed to actuate the variable aperture assembly  900 . In this implementation, the actuator coupling member  962  of the aperture drive mechanism  960  may have a rod  514  (instead of a flange  368 ) consistent with the aperture drive mechanism  510  in  FIGS. 5A and 5B . As previously discussed, the recess  518  of the actuator arm  516  is adapted to receive and laterally retain the rod  514  on the aperture drive mechanism so that the actuator arm controls the lateral movement of the aperture drive mechanism  910 . 
     In one implementation, the body  960  of the aperture drive mechanism  910  defines an inner opening  972  that is larger in size than the first aperture  928  of the plate  926  and the second aperture  948  defined by the aperture blades  938   a - 938   b . The aperture drive mechanism  910  is disposed over the aperture blades so that the inner opening  972  encompasses the first aperture  928 . The pivot pin  918  pivotally couples the body  960  of the aperture drive mechanism  910  to the base ring  904  and/or the plate  926  at a pivot point (e.g., at a pivot opening  974  on the body  960 ) between the ends  956   a  and  956   b  of the aperture drive mechanism so that the actuator coupling member  962  is adapted to be moved in the first lateral direction  957   a  as shown in  FIG. 9A  or the second lateral direction as shown in  FIG. 9B . In this implementation, the ends  956   a  and  956   b  are arms extending laterally from the body  960  and the actuator coupling member  962  is disposed between the ends  956   a  and  956   b  across the inner opening  972  from the pivot point  974  of the aperture drive mechanism  910 . 
     When attached to the base ring  904  and/or the plate  926 , the aperture drive mechanism  910  may vertically retain or captivate the aperture blades  938   a - 938   b  to the base ring  904  and/or the plate  926  so that each aperture blade  938   a - 938   b  is adapted to laterally rotate about a respective pivot pin  916   a  or  916   b  between stop pins  909   a  and  909   b . As described herein, the aperture drive mechanism  910  is adapted to drive each aperture blade  938   a - 938   b  about a respective pivot pin  916   a  or  916   b  so that each aperture blade  938   a - 938   b  moves laterally away from the first aperture  928  in response to the actuator coupling member  962  (which extends from the aperture drive mechanism body  960 ) being moved in the first lateral direction  957   a  shown in  FIG. 9A . In addition, the drive mechanism  910  is adapted to drive each aperture blade  938   a - 938   b  in a reverse direction about a respective pivot pin  916   a  or  916   b  so that each aperture blade  938   a - 938   b  moves laterally over the first aperture  928  to define the second aperture  948  over the window  206  in response to the actuator coupling member  962  being moved in the second lateral direction  957   b  as shown in  FIG. 9B . 
     In the implementation shown in  FIGS. 9A-9C , each stop pin  909   a  and  909   b  (e.g., when received in a respective stop pin opening  908   a  or  908   b ) is disposed on the upper surface  907  of the base ring  904  between the pivot pins  916   a  and  916   b  coupling adjacent aperture blades  938   a - 938   b  to the base ring  904  and/or the plate  926 . Each stop pin  909   a  and  909   b  is positioned relative to a respective pivot pin  916   a  or  916   b  so that each stop pin  909   a  and  909   b  is adapted to engage an external edge  976   a  or  976   b  of one of the aperture blades  938   a  or  938   b  to stop the lateral movement thereof when the aperture blade  938   a  is moved laterally away from the first aperture  928  and the first aperture  928  is exposed as illustrated in  FIG. 9A . Each stop pin  909   a  and  909   b  is also adapted to engage the second end  944   a  or  944   b  of another of the aperture blades  938   b  or  938   a  to stop the lateral movement thereof when the other aperture blade  938   b  or  938   a  is moved laterally over the first aperture  928  so that the second aperture  948  is disposed over and in axial alignment with the window  206  of the radiation detector housing  304  as illustrated in  FIG. 9B . 
     While various embodiments of the present invention have been described, it will be apparent to those of skill in the art that many more embodiments and implementations are possible that are within the scope of this invention. Accordingly, the present invention is not to be restricted except in light of the attached claims and their equivalents.