Abstract:
An image intensifier tube includes a microchannel plate (MCP) having conductive input and output surfaces disposed in a housing. A conductive lower support is in electrical contact with the output surface of the MCP, and a conductive upper support is disposed above the input surface of the MCP. A shape memory alloy (SMA) lockdown is disposed between the input surface of the MCP and the upper support. The SMA lockdown is configured to provide a lockdown for the MCP in the housing. An SMA upper surface is configured to provide an axial force against the upper support, and an SMA lower surface is in contact with the input surface of the MCP.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates, in general, to image intensifier tubes used in night vision goggle (NVG) devices. More specifically, the present invention relates to a shape memory alloy (SMA) for retaining a microchannel plate (MCP) at a precise location within the housing of the image intensifier tube. 
       BACKGROUND OF THE INVENTION 
       [0002]    Image intensifier devices multiply the amount of incident light they receive and provide an increase in light output, which can be supplied either to a camera or directly to the eyes of a viewer. Image intensifiers are constructed for a variety of applications and hence vary in both shape and size. These devices are particularly useful for providing images from dark regions and have both industrial and military applications. For example, image intensifiers are used in night vision goggles for enhancing the night vision of aviators and other military personnel performing covert operations. They are employed in security cameras and in medical instruments to help alleviate conditions such as retinitis pigmentosis (night blindness). 
         [0003]    As known, three major components of image intensifier tubes are the photocathode, phosphor screen (anode), and the MCP disposed between the photocathode and anode. These three components are positioned within an evacuated housing or vacuum envelope, thereby permitting electrons to flow from the photocathode through the MCP and the anode. In order for the image intensifier tube to operate, the photocathode and anode are normally coupled to an electric source, whereby the anode is maintained at a higher positive potential than the photocathode. Similarly, the MCP is biased and operates to increase the density of the electron emission set forth by the photocathode. Furthermore, since the photocathode, MCP and anode are all held at different electrical potentials, all three components are electrically isolated from one another, when retained within the vacuum housing. 
         [0004]    Referring to  FIG. 1 , there is shown a cross-sectional view of a conventional Gen III image intensifier tube  10  of the type manufactured by ITT Night Vision of Roanoke, Va. The prior art Gen III image intensifier tube  10  includes an evacuated housing  12  made from the assemblage of several separate components. Within housing  12 , positioned are photocathode  14 , microchannel plate (MCP)  16 , and an inverting fiber optic element  18 , the latter supporting phosphor screen  20 . The construction for vacuum housing  12  forms an air tight envelope between photocathode  14  and fiber optic element  18 . 
         [0005]    The photocathode  14  rests upon a conductive support ring  22  at one end of vacuum housing  12 . The abutment of photocathode  14  against support ring  22  creates an air tight seal, thereby closing one end of vacuum housing  12 . 
         [0006]    The lower end of vacuum housing  12  is sealed by the presence of an output screen flange  72 . The output screen flange  72  is joined to fiber optic element  18  and forms an air tight envelope, thereby closing the other end of vacuum housing  12 . 
         [0007]    Between support ring  22 , and screen flange  72  are additional elements providing annular spacers and electrical terminals for MCP  16  and fiber optic element  18 . These elements are described in detail in U.S. Pat. No. 5,994,824, which is incorporated herein by reference in its entirety. 
         [0008]    Completing the description of  FIG. 1 , an upper MCP terminal  32  extends into vacuum housing  12 , where it conductively engages a metal hold down ring  36  and a contact ring  38 . The contact ring  38  engages the conductive upper surface  42  of MCP  16 , while the hold down ring retains the MCP within the housing. Consequently, an electrical bias may be applied to upper surface  42  of MCP  16  by applying the electrical bias to upper MCP terminal  32  on the exterior of vacuum housing  12 . Similarly, a lower MCP terminal  48  extends into vacuum housing  12  and engages the lower conductive surface  44  of MCP  16 . As such, the lower conductive surface  44  of MCP  16  may be coupled to ground by connecting the lower MCP terminal  48  to a ground potential external of vacuum housing  12 . 
         [0009]    Referring next to  FIGS. 2 and 3 , two examples are shown of how MCP  16  is fixed into position and sandwiched between an upper electrical terminal contacting upper MCP surface  42  and a lower electrical terminal contacting lower MCP surface  44 . In  FIG. 2 , a ceramic ring  78  is positioned below support ring  22  ( FIG. 1 ), and joined to the support ring during a brazing operation. The brazing operation creates an impervious seal between support ring  22  and ceramic ring  78 . The ceramic ring  78  is part of ring assembly  86  for retaining the MCP. 
         [0010]    The ring assembly  86  includes ceramic ring  78 , conductive snap ring  77 , MCP ceramic ring  46 , and MCP lower support terminal  48 . The ceramic ring  78  includes a first metalized surface  88  in electrical contact with conductive snap ring  77 , and a second metalized surface  89  for providing electrical contact external to the housing and permit an electric source to be applied. The conductive snap ring  77  is formed of a metal or a metallic alloy. Snap ring  77  has a surface  77 B conductively engaging the upper surface  42  of the MCP, and another surface  77 A bonded to surface  88  of ceramic ring  78 . 
         [0011]    As illustrated, the conducting snap ring  77  is positioned between ceramic ring  78  and the MCP upper surface  42 . The MCP rests against and is retained by snap ring  77  and ceramic ring  78 . The MCP insulator ceramic ring  46  is positioned below and coupled to metalized surface  89  by a brazing ring (not shown) interposed between the two elements. The MCP insulator ceramic ring  46  is brazed to both metalized surface  89  and MCP lower support  48 . 
         [0012]    Thus, snap ring assembly  86  retains the MCP by using metalized ceramic  78  in combination with metalized snap ring  77  to provide both the lockdown and electrical contact. This feature eliminates the need for complex metal parts including mechanical rings and tabs used in other image intensifiers to hold the MCP in a fixed position. 
         [0013]    In another example, as shown in  FIG. 3 , the MCP lower support  48  is employed to both laterally center and axially support MCP  16 . The lower support structure provides a tab portion  48 A, which is disposed laterally to surface  16 C of the MCP, to prevent lateral dislocation of the MCP, and at the same time to maintain sufficient distance from the snap ring conductive surface in order to prevent short circuiting the device. The upper support structure  32 , on the other hand, is curved downwardly toward snap ring  77  at its end portion  32 A. The spring force of snap ring  77  is effective in forming a normal force against end portion  32 A, so that the snap ring becomes wedged between the MCP and the upper support structure  32 . In this manner, the upper surface  42  of the MCP is provided with an electrical potential by way of both the snap ring and the upper support structure. In addition, the MCP is fixed and locked down into position within the image intensifier tube. 
         [0014]    A top view of snap ring  77  is shown in  FIG. 4 . In the example shown, the outer diameter  77 G of the snap ring is 1.3 inches, whereas the ceramic ring  78  ( FIG. 2 ) has an aperture with an inner diameter of 1.24 inches (for example). As a result, the snap ring includes dual cavities  77 F, which are carved out of the ring&#39;s surface, for compressing the ring with the help of pliers. After compressing its outer diameter, snap ring  77  fits into the recess formed by the chamfer of metalized surface  88 . In this manner, snap ring  77 , at surface  77 A, conductively engages metalized surface  88 . In addition, at surface  77 B, the snap ring conductively engages the upper surface  42  of MCP  16 . The snap ring also secures and locks down the MCP into a fixed position within the housing. 
         [0015]    The above described method of securing the MCP with the snap ring results in some drawbacks. One drawback is the extra effort required to compress the snap ring with pliers, and properly release the compression after the snap ring is placed on top of the MCP. Another drawback is the possibility of cracking the MCP, when the compression of the snap ring is unevenly released. The present invention, as will be explained, provides a solution to these drawbacks. 
       SUMMARY OF THE INVENTION 
       [0016]    To meet this and other needs, and in view of its purposes, the present invention provides an image intensifier tube including a multichannel plate (MCP) having conductive input and output surfaces disposed in a housing. A conductive lower support is in electrical contact with the output surface of the MCP, and a conductive upper support is disposed above the input surface of the MCP. A shape memory alloy (SMA) lockdown is disposed between the input surface of the MCP and the upper support. The SMA lockdown is configured to provide a lockdown for the MCP in the housing. 
         [0017]    The SMA lockdown includes an SMA upper surface and an SMA lower surface. The SMA upper surface is configured to provide an axial force against the upper support, and the SMA lower surface is in contact with the input surface of the MCP. 
         [0018]    The SMA upper surface may include a chamfer at a peripheral portion of the SMA lockdown, so that the chamfer provides the axial force against the upper support. 
         [0019]    The SMA lockdown includes a memorized state and a deformed state, where the SMA lockdown has a larger diameter in the memorized state than in the deformed state. The SMA is configured to provide the axial force against the upper support in the memorized state. On the other hand, the SMA is configured free-of the axial force against the upper support in the deformed state. 
         [0020]    The SMA upper surface, the upper support and the input surface of the MCP are in electrical contact with each other. 
         [0021]    The SMA upper surface may be circular including a diameter of D 1 , and the SMA lower surface may be circular including a larger diameter of D 2 . The upper support may be circular including an inner aperture. The diameter D 2  is then smaller than the inner aperture for inserting the SMA lockdown through the inner aperture and disposing the SMA lower surface on top of the MCP. 
         [0022]    In another embodiment, the present invention includes an image intensifier tube having a lower support and an upper support in a housing. A multichannel plate (MCP) is disposed on top of the lower support; a shape memory alloy (SMA) element is disposed on top of the MCP; and the upper support is disposed above the SMA element. The SMA element is configured for locking down the MCP between the lower support and the upper support. 
         [0023]    The SMA element includes a deformed state and a memorized state. The SMA element is configured for locking down the MCP in the memorized state, and configured for insertion between the MCP and the upper support in the deformed state. 
         [0024]    The SMA element has a larger diameter in the memorized state than it does in the deformed state. The SMA element, the MCP and the upper support are in electrical contact with each other in the memorized state. The SMA element includes a peripheral portion facing another portion of the upper support, and the peripheral portion of the SMA element is configured to provide an axial force against the other portion of the upper support in the memorized state. 
         [0025]    The SMA element includes an SMA upper surface and an SMA lower surface. The SMA upper surface is circular including a diameter of D 1 ; the SMA lower surface is circular including a larger diameter of D 2 ; and the upper support is circular including an inner aperture. The diameter D 2  is smaller than the inner aperture for inserting the SMA element through the inner aperture and disposing the SMA lower surface on top of the MCP. The diameters D 1  and D 2  are of a first relative size in the deformed state for disposing the SMA element on top of the MCP. Furthermore, the diameters D 1  and D 2  are of a second relative size in the memorized state for locking down the MCP. The first relative size is smaller than the second relative size. 
         [0026]    In still another embodiment of the present invention an image intensifier tube includes a lower support and an upper support in a housing, and a shape memory alloy (SMA) element disposed on top of the lower support. A multichannel plate (MCP) is disposed on top of the SMA element, and the upper support is disposed above the MCP. The SMA element is configured for locking down the MCP between the lower support and the upper support. 
         [0027]    The SMA element includes a deformed state and a memorized state. The SMA element is configured for locking down the MCP in the memorized state, and is configured for insertion on top of the lower support in the deformed state. The SMA element includes a larger diameter in the memorized state than in the deformed state. The SMA element, the MCP and the lower support are in electrical contact with each other in the memorized state. The SMA element includes a peripheral portion facing another portion of the lower support, and the peripheral portion of the SMA element is configured to provide an axial force against the other portion of the lower support in the memorized state. 
         [0028]    The SMA element includes an SMA upper surface and an SMA lower surface. The SMA upper surface is circular including a diameter of D 1 ; the SMA lower surface is circular including a larger diameter of D 2 ; and the upper support is circular including an inner aperture. The diameter D 2  is smaller than the inner aperture for inserting the SMA element through the inner aperture and disposing the SMA lower surface on top of the lower support. 
         [0029]    It is understood that the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0030]    The invention may be understood from the following detailed description when read in connection with the accompanying figures: 
           [0031]      FIG. 1  is a cross-sectional view of a conventional image intensifier tube. 
           [0032]      FIG. 2  is a cross-sectional view of a conventional snap ring assembly inserted in the image intensifier tube of  FIG. 1 . 
           [0033]      FIG. 3  is cross-sectional view of another conventional snap ring assembly inserted in the image intensifier tube of  FIG. 1 . 
           [0034]      FIG. 4  is a top view of a conventional snap ring used in the image intensifier tube of  FIG. 1 . 
           [0035]      FIG. 5  is an example of a shape memory alloy (SMA) lockdown disposed between an upper support and a lower support of an image intensifier tube, in accordance with an embodiment of the present invention. 
           [0036]      FIG. 6  are top and front views of the exemplary SMA lockdown shown in  FIG. 5 , in accordance with an embodiment of the present invention. 
           [0037]      FIG. 7  is another embodiment of an SMA lockdown for insertion into the housing of an image intensifier tube, in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0038]    The Gen  2  and  3  image intensifier (I 2 ) tubes contain microchannel plates (MCPs) for electron amplification. The MCP is a thin glass disk, and must be held in place in the I 2  tube securely and in precise location. Typically, the MCP is placed on an annular ledge called the lower support, and is fixed in place with a locking element. Current MCP fixing methods use a tab lockdown, or a snap ring lockdown. The snap ring was described earlier by reference to  FIGS. 1-4 . 
         [0039]    The tab lockdown will now be briefly described. The tab lockdown includes a wavy washer, or equivalent, which is placed over the rim of the MCP. Next, a notched annular plate is placed over the wavy washer. The notches of the annular plate pass over tabs in the MCP upper support, and press against the wavy washer to preload it. With the plate in position, the upper support tabs are bent over, thereby, securing both the annular plate and wavy washer against the MCP. 
         [0040]    The snap ring lockdown, as previously described, includes a specially made snap ring ( FIG. 4 ). After MCP insertion into the housing, the specially made snap ring with a beveled edge is compressed with pliers and inserted through the upper support and released. As the pliers are released, the snap ring expands, and the beveled edge of the snap ring and the upper support engage each other. The beveled edge translates the radial expansion force of the snap ring into an axial thrust force against the MCP. 
         [0041]    The inventors discovered that both, the tab lockdown and the snap ring lockdown, have notable drawbacks. During operation, an image intensifier is subject to considerable shock and vibration. In addition, the MCP must be held very rigidly to prevent deformation. Modern gated power supplies also create high frequency voltage pulses that may cause the MCP to flex due to coulombic attraction. If the MCP is not secured all around its circumference, an audible tone may be produced. The MCP also has an extremely thin ion barrier film on its exposed face that may be damaged by the slightest touch. 
         [0042]    The tabbed lockdown requires two elements to be placed over the MCP, each with a chance of contacting the ion barrier film over the active area of the MCP. This contact may cause film damage that cannot be detected until the tube is sealed and operating. 
         [0043]    Furthermore, the tab bending operation is an additional opportunity for MCP damage, if the tool slips. Furthermore, the wavy washer may not adequately spread the lockdown force around the MCP. The unsupported areas may be free to flex, creating audibility and shock problems. The need to compress the wavy washer by hand, while bending the tabs, also limits the amount of axial thrust that the tab lockdown method may produce, resulting in reduced MCP rigidity. 
         [0044]    The inventors also discovered that while the snap ring lockdown may overcome some of the problems of the tab lockdown, the snap ring lockdown has produced a few new problems. The snap ring provides an even force distribution, but occasionally binds, resulting in a portion of the MCP being left unsupported. The snap ring must be compressed with pliers during insertion, but insufficient dexterity or visibility results in hitting the MCP active area. The MCP may also crack under the lugs of the snap ring. In addition, the snap ring has a complicated shape, and is difficult and expensive to manufacture. 
         [0045]    The present invention provides a new locking element, or pressure element for MCP lockdown in the image intensifier housing. As will now be explained, the new locking element eliminates problems associated with the tab lockdown element and the snap ring lockdown element. 
         [0046]    Referring now to  FIGS. 5 and 6 , there is shown an example of an element for MCP lockdown fabricated from shape memory alloys (SMAs). In the example shown in  FIG. 6 , SMA lockdown  93  includes a lower surface  93 C and an opposing upper surface  93 A. The upper surface has a circular shape of diameter D 1 ; and the lower surface has a circular shape with a larger diameter D 2 . An end portion of lower surface  93 C extends upwardly a first distance (not labeled) and then bends inwardly to meet the end portion of upper surface  93 A. In this manner, chamfer  93 B is formed. 
         [0047]    The exemplary SMA lockdown  93  may be inserted in an image intensifier (I 2 ) housing, designated as  90 , and placed on top of MCP  94 , as shown in  FIG. 5 . This may be accomplished without any force. During assembly, MCP  94  may be placed on top of lower support  92  (shown as an example). Next, SMA lockdown  93  may be placed on top of MCP  94 . Since the SMA lockdown, in its deformed state (as explained below), has a reduced diameter (D 1  and D 2 ), as compared to snap ring  77  ( FIGS. 3 and 4 ), the SMA lockdown may easily be placed on top of MCP  94  after being inserted through the aperture of upper support  91 . 
         [0048]    In the example shown in  FIG. 5 , in its deformed state (or pre-use state), the SMA lockdown  93  has a smaller diameter (shown in cross-section as top surface  93 A terminating in chamfer  93 B) than a diameter of the same SMA lockdown  93  in its memorized state (or normal-use state) (shown in cross-section as top surface  93 A terminating in chamfer  93 B′). In its memorized state, after being heated, SMA lockdown  93  expands to provide a clamping force against the MCP and an axial force against upper support  91 . In this manner, after transitioning into its memorized state, the SMA lockdown  93  provides a lock down mechanism for the MCP and fixes the MCP in place. 
         [0049]    As described above, the shaped memory element requires no force to install. The SMA lockdown element is shaped so that it is easily emplaced, and readily adaptable for automatic insertion. Only after insertion, and only after being heated, does the SMA lockdown element change shape to provide a clamping force to the MCP. 
         [0050]    Unlike the snap ring lockdown which has gaps in its structure, the shaped memory ring may be continuous without any gap that may lead to cesium migration problems. The reduced diameter needed to clear the upper support may be produced by deformation of the SMA lockdown, which may then be recovered to its larger memorized state after being heated. 
         [0051]    Because the SMA lockdown element is continuous and radially symmetric, it may be fabricated at low cost by stamping, or lathe turning without secondary machining. 
         [0052]    The SMA lockdown  93  is fabricated from shape memory alloys. These alloys may include nickel-titanium-based alloys, which attempt to return to a preset shape when heated. If the shape recovery is hindered, considerable stress is developed within the alloy. 
         [0053]    Shape memory alloys have a heat-treated set memory shape. After heat treatment, the element is generally soft and easily deformed, and stays in a deformed shape, or deformed state. If the deformed element is then heated above a recovery temperature, the SMA element changes crystal structure, and attempts to return to the original shape set during heat treatment, or its memorized state. If the recovery is constrained, the SMA element exerts a force on the constraint that is dependent on the geometry of the element, the temperature and the amount of deformation that the element has endured. 
         [0054]    In general, a shape memory alloy (SMA) is an alloy that “remembers” its original, cold, forged shape, and which returns to that shape after being deformed by applying heat. In addition to nickel-titanium (Ni—Ti) alloys, SMAs may include Ag—Cd alloys, Cu—Al—Ni alloys, Cu—Sn alloys, Cu—Zn alloys, Cu—Zn—Si alloys, Cu—Zn—Al alloys, In—Ti alloys, Ni—Al alloys, Fe—Pt alloys, Mn—Cu alloys, Fe—Mn—Si alloys, and the like. Currently, Ni—Ti alloys (also known as Nitinol) are considered a good SMA element. Generally, these SMA elements may be plastically deformed at a predefined temperature, and upon exposure to thermal manipulation, return to a preset memorized state. Some SMA alloys are considered to be one-way shaped memory alloys, and other SMA alloys are considered to be two-way shaped memory alloys. 
         [0055]    In a one-way shaped memory alloy, when in its cold state, the alloy can be bent or stretched and will hold those shapes until heated above the transition temperature. Upon heating, the shape changes to its original memorized shape. When the alloy cools again, it remains in the memorized shape, until deliberately deformed again. In a two-way shaped memory alloy, the alloy remembers two different shapes: one at low temperature and another at high temperature. 
         [0056]    In the present invention, a one-way shaped memory alloy is likely preferable over the two-way shaped memory alloy. The alloy may thus be fabricated into a memorized state (such as the larger diameter of SMA lockdown  93  shown in  FIG. 5 ). After heat treatment, the SMA lockdown is generally soft and may be easily deformed, and will stay in its deformed state (such as the smaller diameter of SMA lockdown  93  also shown in  FIG. 5 ). 
         [0057]    If the deformed SMA lockdown  93  is heated above the recovery temperature, the SMA lockdown changes crystal structure and returns to its original, memorized shape. The larger diameter of the SMA lockdown then stays in its memorized state and effectively provides a lock down for the MCP in its I 2  housing. 
         [0058]      FIG. 6  showed one example of an SMA lockdown.  FIG. 7  shows another example. As shown, SMA lockdown  96  includes upper surface  96 A and lower surface  96 C, which are joined by a tapered surface  96 B. The upper surface has an inner diameter D 3  and the bottom surface an outer diameter D 4 . As long as the deformed state shown in  FIG. 7  has an outer diameter D 4  which is smaller than the inner diameter (not labeled) of the aperture of upper support  91  ( FIG. 5 ), SMA lockdown  96  may be easily inserted into housing  90 , passed through the aperture of the upper support, and then placed on top of MCP  94 . This may be accomplished without any specialized tools, and without any force. The tapered surface  96 B may be formed so that it is parallel to the inner end portion of upper support  91 . 
         [0059]    After heat treatment, above its recovery temperature, SMA lockdown  96  changes crystal structure and returns to its memorized state, which may be similar to the deformed state shown in  FIG. 7 , except that SMA lockdown  96  is now expanded and includes larger diameters D 3  and D 4 , as compared to the diameters of its deformed state. The SMA lockdown stays in its memorized state and effectively provides a lock down for the MCP in the I 2  housing. 
         [0060]    Many other configurations and shapes of an SMA lockdown, of course, are possible and are contemplated within the scope of the present invention. The only limitations for the SMA lockdown are that the SMA lockdown have a deformed state that allows easy insertion into the I 2  housing and easy placement on top of the MCP. In addition, the SMA lockdown requires a surface that provides axial pressure against the upper support, when the SMA lockdown is in its larger memorized state. 
         [0061]    One possible configuration for an SMA lockdown includes a replacement element for the present snap ring. Such an SMA lockdown includes a shape which fits within the same volume of the present snap ring in the housing; no alteration is required to other I 2  tube components in the housing. Such a ring may include a circular cross section with one beveled corner. When deformed into a smaller diameter (either concentrically, or by 3 or more “puckers”), the ring fits easily between the MCP and the upper support. After being heated, the ring expands to fill the space between the MCP and the upper support. The final shape may be round, with a diameter large enough to interact with the upper support and produce an axial thrust force against the upper support. 
         [0062]    In another embodiment, the SMA lockdown may include a folded sheet metal which unfolds to contact the upper support. Friction or galling between the SMA lockdown and the upper support may cause the thrust force distribution to be less uniform, but is still likely to be better than the present snap ring. 
         [0063]    In yet another embodiment, the SMA lockdown may be a separate pressure plate consisting of a convoluted washer placed above or below the MCP. The washer would be pressed flat prior to insertion, and would recover to the convoluted shape which would apply force to the MCP. This embodiment may require a separate part to transmit the thrust force to the body, but this embodiment would have an advantage of distributing pressure more uniformly. 
         [0064]    Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.