Abstract:
A chopper for an imaging system includes a frame and a sheet that have different coefficients of thermal expansion. The frame has a space within it, and has at least one opening that allows radiation to pass through the space. The sheet has structure which influences radiation passing through the sheet, and the sheet is movably disposed within the space in the frame, in a manner so that the sheet can expand and contract relative to the frame in a direction approximately parallel to the sheet. The frame prevents any significant movement of the sheet in a direction transverse to the sheet.

Description:
This application is a continuation in part of U.S. Ser. No. 10/242,842 filed Sep. 12, 2002 now abandoned. 

   TECHNICAL FIELD OF THE INVENTION 
   This invention relates in general to imaging systems and, more particularly, to a chopper for use in an infrared imaging system. 
   BACKGROUND OF THE INVENTION 
   There are infrared imaging systems which have a lens assembly that images incoming infrared radiation onto an infrared detector. As the radiation travels from the lens assembly to the detector, it passes through a rotating chopper, and the chopper modulates the radiation by influencing it in a manner which varies as a result of the rotation of the chopper. For example, the chopper may alternately cause the radiation passing through it to be diffused to effect blurring, and permit the radiation to pass through it without any significant change. 
   One existing type of chopper has a frame that supports a polymer film. The polymer film has different portions which respectively include and are free of diffusion structure. Rotation of the chopper causes these portions to influence radiation passing through the chopper in an alternating manner. The film is a polymer material which is processed so that the polymer chains have a generally random orientation. The random orientation of the polymer chains has the effect of softening the material of the film to a point where, in order to keep the film flat, the film must be constrained to the frame at its perimeter, and then stretched taut within the frame. While existing choppers of this type have been generally adequate for their intended purposes, they have not been satisfactory in all respects. 
   For example, systems using this type of chopper are often expected to operate in a satisfactory and reliable manner across a relatively wide operational temperature range. However, the frame and the film have different coefficients of thermal expansion. As a result, temperature variations tend to induce tensile stresses within the film, due in part to the fact that the film is constrained at its perimeter. As a result, when the film has been subjected to several cycles of temperature variation, the tensile stresses often cause the film to tear, which in turn leads to degradation in the chopper&#39;s performance. 
   SUMMARY OF THE INVENTION 
   From the foregoing, it may be appreciated that a need has arisen for a method and apparatus for modulating radiation, in which a chopper can undergo many variations in temperature without experiencing physical damage and/or degraded performance. One form of the invention involves: providing a frame which has a first coefficient of thermal expansion, which has a space therein, and which has openings that allow radiation to pass through the space in a first direction; providing a sheet which has a second coefficient of thermal expansion different from the first coefficient of thermal expansion, and which includes structure that influences radiation passing through the sheet in the first direction; and responding to temperature changes by causing the sheet to undergo at least one of expansion and contraction relative to the frame within the space in a second direction approximately parallel to the sheet and approximately perpendicular to the first direction, the frame preventing significant movement of the sheet parallel to the first direction with respect to the frame. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the present invention will be realized from the detailed description which follows, taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a diagrammatic perspective view of an apparatus which is a chopper that embodies aspects of the present invention; 
       FIG. 2  is a diagrammatic perspective view of a lower frame part, which is a component of the chopper of  FIG. 1 ; 
       FIG. 3  is a diagrammatic perspective view of an upper frame part which is a component of the chopper of  FIG. 1 , but which is shown in  FIG. 3  in an inverted orientation; 
       FIG. 4  is a diagrammatic top view of a polymer film, which is a component of the chopper of  FIG. 1 ; and 
       FIG. 5  is a diagrammatic fragmentary sectional view taken along the section line  5 — 5  in  FIG. 1 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a diagrammatic perspective view of an apparatus which is a chopper  10 . The chopper  10  embodies aspects of the present invention, and is configured for use in a not-illustrated infrared imaging system of a known type. In the infrared imaging system, the chopper  10  is rotated about an axis of rotation  11  by a not-illustrated motor. The chopper  10  influences infrared radiation traveling from a lens assembly of the system to an infrared detector of the system, as discussed later. References in the following discussion to specific directions, such as up and down, relate to the orientation of the chopper  10  as it is shown in the accompanying drawings. It will be recognized that, when used in an imaging system, the chopper  10  could have any convenient orientation. 
   The chopper  10  includes a lower frame part  12 , an upper frame part  14 , and an approximately circular sheet of polymer film  16  disposed between the frame parts  12  and  14 . The frame parts  12  and  14  together define a frame. The frame parts  12  and  14  and the film  16  are each discussed separately below in more detail. 
   More specifically,  FIG. 2  is a diagrammatic perspective view of the lower frame part  12 . The lower frame part  12  has a circular main wall  31  which is perpendicular to the axis of rotation  11 , and which has a circular perimeter that is concentric with the axis of rotation  11 . The lower frame part  12  has a cylindrical side wall  32 , which is fixedly secured to and projects upwardly from the peripheral edge of the main wall  31 . The frame part  12  has twelve outer spacing protrusions  34 , which are equally angularly spaced. Each protrusion  34  is semi-cylindrical, has an axis which extends parallel to the axis  11 , has a lower end disposed at the main wall  31 , and has a flat side disposed at the main wall  32 . Each protrusion  34  projects upwardly from the main wall  31  and inwardly from the side wall  32 , and has a vertical height which is less than the vertical height of the side wall  32 . 
   The frame part  12  has, a short radial distance inwardly from the protrusions  34 , four openings  36 - 39  which are offset about the axis  11  by equal angular intervals, and which each have an approximately arcuate shape. The openings  36 – 39  are separated by respective thin radial spokes, one of which is identified by reference numeral  41 . The frame part  12  also has, a short radial distance inwardly from the openings  36 – 39 , eight inner spacing protrusions  43  which project upwardly from the main wall  31 , and which are offset about the axis  11  by equal angular intervals. Each protrusion  43  has an approximately cylindrical shape, extends approximately parallel to the axis  11 , and has one end disposed at the main wall  31 . Each protrusion  43  has a height which is less than the height of the side wall  32 . 
   The frame part  12  has, at the center of the main wall  31 , a cylindrical hub  51  which projects upwardly from the top surface of the main wall  31 . The hub  51  is concentric to the axis  11 , and has extending through it a central cylindrical opening  52 , which is also concentric to the axis  11 . The opening  52  can receive the shaft of the not-illustrated motor that can effect rotation of the chopper  10 . 
   Two approximately rectangular openings  56  and  57  are provided through the main wall  31  on diagrammatically opposite sides of the hub  51 . Two hook elements  61  and  62  are fixedly supported on diametrically opposite sides of the hub  51 , and each project outwardly over a respective opening  56  or  57 . Each hook element has on the bottom side thereof an approximately horizontal retaining surface, one of which is designated by reference numeral  67 , and also has an inclined side surface, one of which is designated by reference numeral  66 . 
   The frame part  12  also has, a short radial distance inwardly from the protrusions  43 , three cylindrical alignment posts  46 – 48 , which each project upwardly from the main wall  31 , in a direction parallel to the axis  11 . The posts  46 – 48  are offset about the axis  11  by equal angular intervals. 
   In the disclosed embodiment, the lower frame part  12  is a single integral part which is fabricated in its entirety from a 40% glass-filled polyphenylene material using injection molding techniques of a known type. However, the frame part  12  could alternatively be made in any other suitable manner, and could be made of any other suitable material or materials. 
     FIG. 3  is a diagrammatic perspective view of the upper frame part  14 , in an orientation which is inverted from the orientation shown in  FIG. 1 . In other words, the upper frame part  14  is upside down in  FIG. 3 , so that the bottom side of the frame part  14  faces upwardly. The upper frame part  14  includes a circular main wall  101 , which is perpendicular to the axis of rotation  11 , which has a circular perimeter that is concentric to the axis of rotation  11 , and which has an outside diameter that is slightly less than the inside diameter of the cylindrical side wall  32  on the lower frame part  12 . 
   The frame part  14  has twelve recesses or indentations  102  provided in the underside of the main wall  101 , at equally angularly spaced intervals along the periphery thereof. Each of the recesses  102  has an approximately semi-cylindrical shape, with an axis extending parallel to the axis  11 . Each of the recesses  102  has a depth which is approximately one-half the thickness of the main wall  101 . The frame part  14  also has, a short radial distance inwardly from the recesses  102 , four openings  106 – 109  which are offset by equal angular intervals about the axis  11 , and which each have an approximately arcuate shape. Between each adjacent pair of the openings  106 – 109  is a thin radial spoke, one of which is designated by reference numeral  111 . 
   The frame part  14  has, a short radial distance inwardly from the openings  106 – 109 , eight circular recesses or indentations  113 , which each have an axis extending parallel to the axis  11 , and which are offset about the axis  11  by equal angular intervals. The recesses  113  each have a depth which is approximately one-half the thickness of the main wall  101 . 
   The central portion of the main wall  101  has four parallel slots  121 – 124 , which each extend completely through the main wall  101 . The slots  121  and  122  are adjacent and disposed on one side of the axis  11 , and the slots  123  and  124  are adjacent and disposed on the other side of the axis  11 . The main wall  101  has a portion  127  which is disposed between and separates the slots  121  and  122 , and which serves as a beam that has a degree of resilient flexibility. In a similar manner, a beam  128  is provided between the slots  123  and  124 , and has a degree of resilient flexibility. 
   At the center of each of the beams  127  and  128  is a respective hook element  131  or  132 , which projects downwardly from the associated beam (upwardly in  FIG. 3 , since the frame part  14  is shown upside down). Each of the hook elements  131  and  132  has a portion which projects radially inwardly beyond the edge of the adjacent slot  122  or  123 , which has an upwardly facing retaining surface thereon (one of which is indicated at  137 ), and which has an inclined side surface thereon (one of which is indicated at  136 ). 
   The main wall  101  has in the center thereof an opening  141  which is approximately cylindrical, except that it communicates on diametrically opposite sides with center portions of each of the two slots  122  and  123 . The opening  141  is concentric to the axis  11 , and has a diameter which is slightly greater than the diameter of the hub  51  on the frame part  12  ( FIG. 2 ). The main wall  101  has two cylindrical alignment openings  146  and  147  extending completely therethrough. The alignment openings  146  and  147  are disposed on diametrically opposite sides of the axis  11 , and are each disposed slightly radially outwardly from a respective one of the slots  121  and  124 . 
   In the disclosed embodiment, the upper frame part  14  is a single integral part which is fabricated in its entirety from a 40% glass-filled polyphenylene material using injection molding techniques of a known type. However, the upper frame part  14  could alternatively be made in some other suitable manner, and could alternatively be made from some other suitable material or materials. 
     FIG. 4  is a diagrammatic top view of the film  16 . The film  16  is an approximately circular piece of sheet-like material, with an outside diameter which is somewhat less than the inside diameter of the cylindrical side wall  32  on the lower frame part  12 . In the disclosed embodiment, the film  16  is made from a known polymer material which is a high-density polyethylene. Applicants used a high-density polyethylene material obtained commercially under the tradename MONAX, type HD-A, from Tredegar Film Products of Richmond, Va. However, the film  16  could alternatively be made from any other suitable material, such as a material having the same infrared transmittance and mechanical properties. In the disclosed embodiment, the film has a thickness of approximately 0.002 inch, and the material of the film  16  has a high degree of orientation of its polymer chains, which in turn causes the film  16  to be some what stiffer than would be the case if the polymer chains had a random orientation. 
   The film  16  has twelve approximately semi-circular recesses  201  provided along the periphery thereof, at equally angularly spaced locations. Each of the recesses  201  has a diameter which is somewhat greater than the diameter of the protrusions  34  ( FIG. 2 ) on the lower frame part  12 . The radially outer half of the film  16  has an embossed portion  206 , which is bounded on its opposite sides by respective curves  207  and  208  that are each an Archimedes spiral, where the Archimedes spirals are 180° out of phase with each other. The embossed portion  206  has a pre-defined pattern, which includes a plurality of small defractive optical elements (DOEs). Infrared radiation which passes through the embossed portion  206  is diffused and thus blurred, in a known manner. In contrast, infrared radiation which passes through other portions of the film  16  is not subjected to diffusion or blurring. For clarity, the embossed portion  206  of the film  16  is not specifically depicted in  FIG. 1 . 
   The film  16  has at the center thereof an approximately circular opening  216  which is concentric to the axis  11 . The opening  216  has a diameter which is somewhat greater than the diameter of the hub  51  on the lower frame part  12  ( FIG. 2 ). The opening  216  has two extensions  217  and  218  which extend radially outwardly from diametrically opposite sides thereof, and which each have an approximately rectangular shape. The film  16  has three slots  221 – 223  which are located a short radial distance outwardly from the opening  216  and the extensions  217 – 218 . The slots  221 – 223  each extend completely through the film  16 , each extend approximately radially, and are offset from each other about the axis  11  by equal angular intervals. The slot  221  is angularly aligned with the extension  217 . The slots  221 – 223  each have a width which is slightly larger than the diameter of the posts  46 – 48  ( FIG. 2 ) on the lower frame part  12 . 
   The film  16  also has, a short radial distance outwardly from the slots  221 – 223 , eight cylindrical openings  231 , which each extend completely through the film  16 , and which are offset about the axis  11  by equal angular intervals. Each of the openings  231  has a diameter which is somewhat greater than the diameter of the inner spacing protrusions  43 . 
     FIG. 5  is a diagrammatic fragmentary sectional view taken along the section line  5 — 5  in  FIG. 1 . It will be noted from  FIG. 5  that an annular recess  251  is provided in the underside of the hub  51 , concentric to the axis  11 . The recess  251  is provided to reduce the amount of plastic material present in the hub  51 , and thus the cost of the frame part  12 , and is also provided to ensure that the plastic material of the hub  51  cures rapidly and uniformly while it is in the injection mold. 
   With reference to  FIGS. 1 and 5 , the chopper  10  is assembled by placing the film  16  into the lower frame part  12  above the main wall  31  thereof, in a manner so that the hub  51  projects upwardly through the opening  216 , the extensions  217  and  218  are respectfully disposed above the openings  56  and  57 , the posts  46 – 48  each extend through a respective slot  221 – 223 , the protrusions  43  each extend through a respective opening  231 , and the recesses  201  each receive a respective protrusion  34 . 
   The recesses  201  and openings  231  are oversized with respect to the protrusions  34  and  43 , so that the protrusions  34  and  43  do not engage the film  16  in any way that would serve to limit movement of the film  16  with respect to the lower frame part  12 . Similarly, the outer diameter of the film  16  and the diameter of its central opening  216  are selected so that the film  16  does not engage either the side wall  32  or the hub  51  of the frame part  12  in a manner that would limit relative movement of the film  16  and the frame part  12 . As mentioned above, the radial slots  221 – 223  are each only slightly wider than the corresponding post  46 – 48 . As a result, cooperation of the posts  46 – 48  with the slots  221 – 223  serves to maintain the film  16  in proper angular and radial alignment with respect to the lower frame part  12 . 
   After the film  16  has been properly positioned in this manner within the lower frame part  12 , the upper frame part  14  is moved vertically downwardly into engagement with the lower frame part  12 . As this occurs, the upper frame part  14  is oriented so that the openings  36 – 39  are respectively aligned with the openings  106 – 109 , the post  46  is aligned with and moves into the opening  146 , the posts  47  and  48  are aligned with and move into opposite ends of the slot  123 , hook elements  61  and  62  are aligned with and respectively move into slots  122  and  123 , and hook elements  131 – 132  are aligned with and respectively move into the openings  56  and  57 . 
   As the upper frame part  14  moves toward the lower frame part  12 , the inclined surface  66  on the hook element  62  engages the inclined surface  136  on the hook element  132 , causing the beam  128  to flex outwardly a small amount as the hook elements  62  and  132  move past each other, until the resilience of the beam  128  can return the beam  128  to its original position, with the hook elements  66  and  132  engaged in the manner shown in  FIG. 5 . In this position, the horizontal surfaces  67  and  137  on these hook elements face and engage each other, in order to prevent relative axial movement that would separate the frame parts  12  and  14 . At the same time that the hook elements  66  and  132  are moving into cooperating engagement in this manner, the other two hook elements  61  and  131  are moving into cooperating engagement in a similar manner. After this engagement is completed, the hook elements  61 – 62  and  131 – 132  effectively define a form snap-fit coupling that reliably couples the frame parts  12  and  14  to each other, without any need for separate fasteners that would add to the effective cost of the chopper. 
   As the upper frame  14  is moved downwardly toward the lower frame part  12  in the manner explained above, the protrusions  34  on the frame part  12  each move into a respective one of the recesses  102  in the frame part  14 , and the protrusions  43  on the frame part  12  each move into a respective recess  113  in the frame part  14 . When the hook parts  61 – 62  and  131 – 132  have become fully engaged in the manner discussed above, the upper end of each protrusion  34  will be urged against the inner end of the associated recess  102 , and the upper end of each protrusion  43  will be urged against the inner end of the associated recess  113 . This engagement between the protrusions  34  and  43  and the recesses  102  and  113  ensures that the facing surfaces of the main walls  31  and  101  of the frame parts  12  and  14  will be maintained in a position where there is a predetermined axial space therebetween (see  FIG. 5 ). In the disclosed embodiment, this axial space between the frame parts  12  and  14  is about 0.005 inch. As discussed above, the film  16  has a smaller thickness, which is about 0.002 inch. Consequently, the film  16  is not pinched between the facing surfaces of the frame parts  12  and  14  in a manner which would limit or restrict movement of the film  16  relative to the frame parts. Therefore, in directions parallel to the plane of the film  16 , the film  16  can effectively float with respect to the frame parts  12  and  14 , except to the extent that relative movement is limited by cooperation of the posts  46 – 48  with the slots  221 – 223 . 
   When the chopper  10  is installed in an infrared imaging system, a not-illustrated motor of the system effects rotation of the chopper  10  about the axis of rotation  11 . Infrared radiation travels along a path shown diagrammatically at  291  in  FIG. 1 , as it propagates from a not-illustrated lens assembly to a not-illustrated infrared detector. The path  291  extends approximately parallel to the axis  11 , and remains stationary in relation to the rotating chopper  10 . As the chopper  10  rotates, the pairs of aligned openings  36 – 39  and  106 – 109  will successively become aligned with the path  291 , so that radiation traveling along the path  291  will successively pass through each pair of aligned openings, and also the portion of the film  16  aligned with each pair of these openings. As this occurs, the embossed portion  206  of the film  16  will alternately move into and out of alignment with the path  291 . Consequently, radiation traveling along the path  291  will be alternately subjected to diffusion which blurs it, and freedom from such diffusion and blurring. 
   The film  16  has a coefficient of thermal expansion (CTE) which is greater that the CTE of the plastic material used for the frame parts  12  and  14 . During normal operational use, the chopper  10  may be subjected to a wide range of temperature variations. As discussed above, the film  16  is basically free to move or float within the annular space defined between the frame parts  12  and  14 , subject only to cooperation between the posts  46 – 48  and the slots  221 – 223 . Therefore, as the chopper  10  experiences temperature variations, the film  16  can expand and contract radially with respect to the frame parts  12  and  14 , because the radial orientation of the slots  221 – 223  facilitates this relative movement. The cooperating posts  46 – 48  and slots  221 – 223  maintain the film  16  in the proper angular position with respect to the frame parts  12  and  14 , and also maintain the film  16  in a proper position of concentricity with respect to the axis of rotation  11 . As a result, the differing CTEs of the film  16  and the frame parts  12  and  14  do not cause the film  16  to be subjected to any thermally-induced stresses. Accordingly, the film  16  will have a relatively long operational lifetime under normal operational conditions, despite any temperature fluctuations that may occur. 
   The present invention provides a number of technical advantages. One such advantage results from the provision of a film within a frame in a manner so that the film is free to expand and contract with respect to the frame in response to temperatures variations, without generation of stresses within the film. A further advantage is realized where the material of the film is a polymer material having polymer chains with a high degree of orientation, so to impart a greater degree of stiffness to the film than would be the case if the polymer chains had a generally random orientation. 
   Although one selected embodiment has been illustrated and described in detail, it will be understood that substitutions and alterations are possible without departing from the spirit and scope of the present invention, as defined by the following claims.