Abstract:
An image intensifier such as a night vision goggle includes a tube module and a power supply module. The image quality may be characterized by a figure known as the “Figure of Merit” (FOM), which is an arithmetic product of screen resolution and signal-to-noise ratio (SNR). Both resolution and SNR are affected by the voltage supplied by the power supply module. By providing a power supply module with an adjustable voltage, the FOM may be varied. The adjustment mechanism may then be rendered tamper resistant, thereby enabling the FOM to be permanently reduced for devices intended for export.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provisional application 60/967,475, filed Sep. 5, 2007 and entitled “Image Intensifier with Adjustable Signal-to-Noise Ratio and Imaging Resolution and Method for its Operation and Production,” which is hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    This specification relates to the field of image intensifiers and more particularly to an image intensifier assembly with an adjustable figure of merit. 
         [0003]    Image intensifiers for use in night vision systems commonly use a measurement called Figure of Merit (FOM) for image quality. FOM is the arithmetic product of the resolution, measured in line pairs per millimeter (lp/mm) and signal-to-noise ratio (SNR), which is unitless. Resolution typically varies in the range of 50 to 72 lp/mm. SNR typically varies in the range of 20 to 25. So FOM typically varies in the range of 1,000 to 1,800, with a higher FOM generally representing a superior overall image quality. 
         [0004]    FOM may be important in some contexts because the United States government regulates the export of night vision systems by requiring that exported items have a FOM below a specified threshold. A common method of varying FOM is to provide a power supply that supplies a lower photocathode voltage, thus degrading both resolution and SNR. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a perspective view of a night vision system, which may employ an image intensifier constructed according to the present specification; 
           [0006]      FIG. 1A  is a perspective view of an image intensifier assembly for use with a night vision system; 
           [0007]      FIG. 1B  is a front view of a power supply for use with an image intensifier assembly; 
           [0008]      FIG. 1C  is a side view of an adjustment port that has been back filled to prevent tampering; 
           [0009]      FIG. 2  is an electrical schematic of an exemplary prior art power supply; 
           [0010]      FIG. 3  is an electrical schematic of an exemplary embodiment of an adjustable power supply in accordance with the present specification; and 
           [0011]      FIG. 4  is an electrical schematic of an exemplary opto-coupler in accordance with the present specification. 
       
    
    
     SUMMARY OF THE INVENTION 
       [0012]    In one aspect, an image intensifier for a night vision system may include a tube module and a power supply module. The image quality may be characterized by a figure known as the “Figure of Merit” (FOM), which is an arithmetic product of center limiting resolution and signal-to-noise ratio (SNR). Both resolution and SNR are affected by the voltage supplied by the power supply module. By providing a power supply module with an adjustable photocathode voltage, the FOM may be varied. The adjustment mechanism may then be rendered tamper resistant, thereby enabling the FOM to be permanently reduced for devices intended for export. 
       DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0013]    An adjustable FOM is useful for creating image intensifiers that are suitable for export outside of the United States. An image intensifier according to the disclosure in this specification may include a mechanically-variable adjustment mechanism such as an adjustment screw, typically as part of an adjustment potentiometer. 
         [0014]    Generally, a tube and a power supply may be encapsulated in a single package as an image intensifier for use with a night vision system. Because of variances inherent in the production process, it is difficult to predict in advance what FOM will be achieved by joining a particular tube to a particular power supply. But a mechanically-variable power supply can be joined with any tube and the FOM can then be adjusted accordingly. If a device is intended for export, the entire image intensifier assembly can be encapsulated in epoxy to ensure that the power supply cannot be separated from the tube without damage. After the final FOM adjustment, the adjustment screw or other mechanically-variable adjustment mechanism can be disabled. For example, an adjustment screw may be set with epoxy to ensure that it is securely fastened in position. The screw head can then be ground out to prevent any further manipulation, and the adjustment port can finally be back-filled with epoxy to prevent tampering. 
         [0015]    An image intensifier with adjustable figure of merit will now be described with more particular reference to the attached drawings. Hereafter, details are set forth by way of example to facilitate discussion of the disclosed subject matter. It should be apparent to a person of ordinary skill in the field, however, that the disclosed embodiments are exemplary and not exhaustive of all possible embodiments. Throughout this disclosure, a hyphenated form of a reference numeral refers to a specific instance or example of an element and the un-hyphenated form of the reference numeral refers to the element generically or collectively. Thus, for example,  102 - 1  may refer to a “pen,” which may be an instance or example of the class of “writing implements.” Writing implements may be referred to collectively as “writing implements  102 ” and any one may be referred to generically as a “writing implement  102 .” 
         [0016]      FIG. 1  discloses a perspective view of an night vision system  100 , which may be include an image intensifier assembly built according to the present specification. Night vision system  100  includes a lens  110  for permitting light to enter and an eyepiece  120  for viewing by a user. A focus knob  104  allows adjustment of the sharpness of the image, but does not permit adjustment of the FOM. Contained within image intensifier  100  is an image intensifier assembly  130  ( FIG. 1A ). Image intensifier assembly  130  ( FIG. 1A ) receives light through lens  110 . Its output is visible to a user through eyepiece  120 . As is commonly known, night vision system  100  may also include batteries, assorted controls, and objective and eyepiece optics for interface with the user. 
         [0017]      FIG. 1A  is a perspective view of an image intensifier assembly  130  for use with a night vision system  100 , and more particularly discloses the construction thereof. Image intensifier assembly  130  may include a tube  132  and a power supply  134 . In some embodiments, tube  132  and power supply  134  may be permanently joined as a single unit. For example, tube  132  and power supply  134  may be bonded together with epoxy or other strong material that will prevent disassembly without damaging the components. 
         [0018]      FIG. 1B  is a front view of power supply  134 , disclosing that power supply  134  may include a number of adjustment ports  140 . Adjustment ports may enable adjustment of certain parameters after manufacture, including FOM. For example, adjustment ports  140 - 1  and  140 - 2  may be used for other purposes, while adjustment port  140 - 3  may allow for adjustment of FOM. 
         [0019]      FIG. 1C  is a side view of adjustment port  140 - 3 . Adjustment screw  142  may be used to adjust FOM after manufacture of image intensifier assembly  130  ( FIG. 1A ). After final FOM adjustment has been completed, a small amount of epoxy may be used to permanently set adjustment screw  142 . Once the epoxy has set, a portion of it may be ground out, and then the head of adjustment screw  142  may also be ground out. This prevents end users from tampering with the adjusted FOM. Finally, the cavity may be enclosed in backfill material  144 . In some embodiments, hardened epoxy resin may be used. Other materials may also be used, for example, room-temperature vulcanizing (RTV) silicone rubber has been used in some applications. But softer materials such as RTV silicone rubber may be less effective in denying access to adjustment port  140 - 3 . 
         [0020]      FIG. 2  is an electrical schematic of an exemplary prior art power supply with a non-adjustable photocathode voltage. Voltage supplies V 1   210 , V BSP    240 , V 2   220  and V 3   230  provide the basic biasing potentials needed for an image intensifier. V 2   220  is an adjustable high voltage source used to bias the MCP  222 , which can be adjusted via an external means to allow factory or user adjustability, as well as by internal Automatic Brightness Control (ABC) circuitry  232 . ABC circuitry  232  senses the image intensifier&#39;s screen current  236 , which is proportional to the screen&#39;s  236  brightness, and adjusts V 2   220  to maintain a constant level of screen current  236  to prevent the output brightness from becoming too bright as a result of increasing input illumination. Typical adjustment range for V 2   220  is −750 VDC to −1250 VDC for the external adjustment and 0 to −1250 VDC for the internal adjustment via the ABC circuit  232 . V 3   230  is a fixed high voltage source used to bias the image intensifier&#39;s screen  234 . Typical V 3   230  operating voltage is between +4,000 VDC and +6,000 VDC. V 1   210  is a fixed high voltage source used to bias the image intensifier&#39;s photocathode  242 . V 1   210  is a floating potential referenced to V 2   220 , which is necessary to maintain the correct electrostatic field between the photocathode  242  and MCP  222  input independent of the operating level of V 2   220 , which is adjustable as described above. V BSP    240 , R BSP    212 , and D BSP    214  work in conjunction to provide bright source protection (BSP) for the image intensifier  100 . They do this by reducing the electrostatic potential between to photocathode  242  and MCP  222  to a predetermined level low enough to cause the vast majority of photoelectrons to be absorbed by the MCP&#39;s  222  ion barrier film, thus preventing an excessive flow of contaminating ions back toward the phtotocathode  242  from the MCP  222 . This operation is accomplished by the combination of the clamping voltage V BSP    240  (typically −30 to −60 VDC, with respect to V 2   220 ) connected to the output side of R BSP    212  (typically 5-18 GΩ) through a high voltage clamping diode D BSP    214 . During excessive input illumination, photocurrent flow through R BSP    212  causes the D BSP    214  to become forward biased, thereby conducting and effectively clamping the output voltage of R BSP    212  to a fixed level. 
         [0021]      FIG. 3  is an electrical schematic of an exemplary embodiment of an adjustable power supply in accordance with the present specification. In this case, an additional network is added, which allows the user to make adjustments of the floating V1 bias potential using a ground-referenced, user-adjustable interface. The high voltage isolation element is a dual-detector opto-coupler  320 , which consists of an LED emitter  324  monitored by two photodiodes PD FB    326  and PD SHUNT    322 . PD FB    326  provides a feedback current signal, which is sensed by a feedback resistor R FB    312  for voltage input to the V1 regulator op-amp  318 . It will be well within the ability of a person having ordinary skill in the art to select an appropriate resistor value for R FB    326 . PD SHUNT    322  is a high voltage photodiode that produces a shunting current across R SHUNT    330  located at the output of high voltage source V 1   210  in response to light emanating from the LED  324 . Photodiodes PD FB    326  and PD SHUNT    322  are of the same construction and are positioned so that the light emanating from the LED projects onto both. Accordingly, current through PD FB    326  is proportional to the current through PD SHUNT    322 , and thus an indication of the voltage present at the circuit node between R SHUNT    330  and R BSP    212 . By sensing and regulating the voltage developed across R FB    312  produced by the current through PD FB    326 , the V1 regulator op-amp  318  regulates the voltage at the circuit node between R SHUNT    330  and R BSP    212 , which is the voltage that is presented to the photocathode under low photocurrent (low light) conditions. Temperature drift effects of the LED and photodiodes, as well as aging effects on the current transfer ratios of the LED/photodiode combinations are effectively compensated by the fact that the photodiodes are of the same construction. Because the regulation is performed at ground level, rather than at a floating ground reference, user adjustment via variable resistor or other non-isolated means is possible. 
         [0022]      FIG. 4  is an electrical schematic of an exemplary opto-coupler  320  for use with an image intensifier as presently disclosed. In some embodiments, the opto-coupler  320  may be a single integrated circuit, such as Electronic Devices, Inc. part number ED2927. The following details are drawn to an exemplary opto-coupler only, and in no way are intended to limit the opto-coupler to the embodiment disclosed. 
         [0023]    In this embodiment, the opt-coupler is capable of enduring any or all of the following specified parameter ratings for unlimited periods of time without any permanent degradation resulting in failure to meet the required specification: 
         [0000]    
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Minimum Ratings 
               
             
          
           
               
                   
                 Paramter 
                 Symbol 
                 Requirement 
               
               
                   
                   
               
               
                   
                 Emitter 
                   
                   
               
               
                   
                 Reverse Voltage 
                 VR 
                 5 V 
               
               
                   
                 Forward Current 
                 IF 
                 50 mA 
               
               
                   
                 Power Dissipation 
                 P 1   
                 100 mW 
               
               
                   
                 Detector (each) 
               
               
                   
                 Peak Reverse Voltage 
                 VPR 
                 1600 Vdc 
               
               
                   
                 Forward Current, 
                 IF 
                 10 mA 
               
               
                   
                 Average 
               
               
                   
                 Forward Current, 
                 IS 
                 1 A (one cycle, 8.3 msec) 
               
               
                   
                 Surge 
               
               
                   
                 Coupler 
               
               
                   
                 Isolation Test Voltage 
                   
                 3000 Vdc 
               
               
                   
                 (in insulating 
               
               
                   
                 atmosphere) 
               
               
                   
                   
               
             
          
         
       
     
         [0024]    In this embodiment, the electrical characteristics are as follows (all at an operating temperature of 23° C. unless otherwise noted): 
         [0000]    
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Electrical Characteristics 
               
             
          
           
               
                   
                 Parameter 
                 Symbol 
                 Requirement 
               
               
                   
                   
               
               
                   
                 Operating Temperature 
                   
                 −51° C. to +52° C. 
               
               
                   
                 Storage Temperature 
                   
                 −51° C. to +85° C. 
               
               
                   
                 LED Emitter 
               
               
                   
                 Forward Voltage 
                 VFD 
                 4 V max @ 1 mA 
               
               
                   
                 Reverse Current 
                 IR 
                 10 nA max. @ 1200 Vdc 
               
               
                   
                 Detector (Each) 
               
               
                   
                 Forward Voltage 
                 VFD 
                 4 V max. @ 1 mA 
               
               
                   
                 Drop 
               
               
                   
                 Reverse Current 
                 IR 
                 10 nA max. @ 1200 Vdc 
               
               
                   
                 Coupler @ 3000 Vdc 
               
               
                   
                 Max. allowed leakage 
                 IL 
                 3 nA 
               
               
                   
                 @ 3 kV dc (emitter 
               
               
                   
                 LED to either 
               
               
                   
                 detector) 
               
               
                   
                 Current Transfer 
                 CTR 
                 0.02% min. @ IF = 1 mA 
               
               
                   
                 Ratio 
               
               
                   
                   
               
             
          
         
       
     
         [0025]    Opto-coupler  320  is capable of meeting all requirements after being subjected to 12 temperature cycles from +85° C. to −55° C. with two hour soaks at each temperature. The transition rate from −55° C. to +85° C. and from +85° C. to −55° C. is 3° C. per minute minimum. This requirement may be satisfied through sampling a single month&#39;s production per ANSI/ANSQCZI1.4-1993 Inspection Level S-1, 1.0 AQL. 
         [0026]    As disclosed in this specification, a single batch of image intensifiers, built from a batch of tube modules and power supply modules, may be produced for both domestic use and export. After production is complete, some of the image intensifiers will be selected for export, and the FOM will be adjusted and fixed as described herein. A FOM may also be selected for image intensifiers identified for domestic use by providing the maximum possible photocathode current. To make the image intensifiers resistant to tampering, for example by an enemy wanting to increase the photocathode voltage to increase FOM, by disabling the adjustment mechanism. In one embodiment, the method of disabling includes backfilling the adjustment port with epoxy or other rigid material that will fix the adjustment mechanism in place. This renders the FOM fixed for that image intensifier. And because the entire image intensifier is encapsulated in epoxy, a new power supply cannot be substituted to increase the FOM. In a non-adjustable power supply, there are sometimes two holes in this epoxy that provide access to two adjustment screw-heads that are used to adjust two operating parameters (not FOM) of the night vision tube. After adjustment, these two cavities may be backfilled with soft RTV so that these adjustment screw heads are not accessible. But in some embodiments, epoxy or other more rigid material may be preferred for backfilling the adjustment mechanism for the photocathode voltage. For further security, after setting the adjustment mechanism in epoxy, part of the epoxy may be drilled out to provide access to the adjustment interface (such as a screw head on a potentiometer), which may also be disabled, for example by drilling out the screw head. The drilled-out adjustment port may then again be backfilled with epoxy. This provides redundant security for the adjustment mechanism. 
         [0027]    Persons having skill in the art will recognize that there are numerous other methods for providing an adjustable voltage that can be disabled. For example, the mechanical adjustment mechanism, such as a screw, may receive a light coat of slow-acting adhesive (such as epoxy) before insertion, so that its position will be fixed after a time. In other embodiments, inserts may be used to physically block access to the port. In yet other embodiments, an electronic adjustment may be provided, and lead wires may be clipped or otherwise disabled before final encapsulation in epoxy, making the final product tamper resistant. 
         [0028]    While the subject of this specification has been described in connection with one or more exemplary embodiments, it is not intended to limit the claims to the particular forms set forth. On the contrary, the appended claims are intended to cover such alternatives, modifications and equivalents as may be included within their spirit and scope.