Flux monitor high light intensity cut-off circit for night vision devices
FIELD OF THE INVENTION 
     The present invention relates generally to night vision goggles, and more particularly, to a battery control circuit which monitors the flux of incoming light to remove battery power in the event the goggle is exposed to excessive flux. 
     BACKGROUND OF THE INVENTION 
     Night vision goggles enable a person to see objects in the darkness, which objects could not otherwise been seen by the naked eye. The principle by which night vision goggles operate is well known. Generally, some source of external light, such as the stars or moon, is capable of illuminating objects with very low light emitting therefrom. While the naked eye may not be able to detect such illuminations reflected from an object, the reflections can be electrically amplified by the night vision goggles so as to be visible to the observer&#39;s eye. 
     Therefore, the principle by which night vision goggles can make objects viewable in darkness is the electrical amplification of reflected light. An image of the object is displayed on a phosphorescent screen within the goggle. The battery powered photomultiplier or image intensifier tube is conventionally used to electrically amplify the light signals for presentation on the green phosphorous coated screen. The image is monochromatic, with the intensity of the phosphor representative of the amount of light reflected from the object. The phosphor coated screen is very sensitive and subject to accelerated degradation when exposed to high intensity light. Excessively lighted objects may thus overload the image tube and wash out the display on the phosphorous screen. Therefore, a need has arisen for a mechanism which automatically removes the battery power from the night vision goggle when subjected to periods of excessive light or to excessively lit objects. 
     In the prior art, this need has been addressed by circuits which remove battery voltage based solely on the intensity of the incident light. For example, in U.S. Pat. No. 4,755,725, there is disclosed one type of prior art light intensity monitor which senses the brightness of light to which the image intensifier tube is subjected. 
     As disclosed in the &#39;725 patent, when the light intensity incident on the image intensifier tube reaches a preset threshold value, such as when the goggles are turned on in a lighted room, a timer is activated. If the light intensity exceeds the threshold value for more than one minute, for example, the timer is operative to open a switch to remove automatically power from the image intensifier tube, as well as from other circuitry. 
     The switch is driven by control logic circuits in response to the light intensity monitor. The switch includes a field effect transistor (&#34;FET&#34;) placed in series with the battery and the image intensifier tube. To provide an extremely low series resistance, the switch is driven by a voltage which is larger than the battery voltage. The voltage multiplier is employed to boost the battery voltage to drive the FET switch. 
     An inherent disadvantage and limitation of prior art timer activated light intensity monitors, such as disclosed in the &#39;725 patent, is that there is still a high potential of damage to the image intensifier tube during the timer countdown phase when the timing circuit is measuring the time during which the high light intensity condition exists. The high flux conditions which can be damaging to the image intensifier tube during the period in which the timer is counting down will not switch off the image intensifier tube in the prior art device. Furthermore, low flux conditions, although over the threshold intensity, may cause the image intensifier tube to be shut down after the time period has elapsed even though the total flux has not caused any damage. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a flux monitor circuit which measures the flux of light incident on the image intensifier tube, rather than the intensity of light for a pre-set time, addresses and overcomes the limitations and disadvantages of the above discussed prior art. A threshold light intensity level is established so that when the illumination on the image intensifier tube exceeds this threshold level, a flux monitor circuit which measures the amount of luminous flux is activated. This measurement is accomplished by integrating light intensity over time, such as by charging a capacitor, or by other suitable devices. When a measured amount of luminous flux has been reached, the image intensifier tube is shut down. 
     In accordance with the concepts of the present invention, the image intensifier tube is shut down quicker in light intensity conditions which far exceed the threshold intensity level, while less damaging light intensity levels will allow the image intensifier tube to operate longer before shutdown. Some typical light intensity levels and times are exemplary shown as follows: 
     
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ILLUMINATION AT    TIME TO SHUT-                                          
PHOTOCATHODE (FT.C.)                                                      
                   DOWN (SE.CS)                                           
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0.0004             ∞                                                
0.004              ∞                                                
0.04               ∞                                                
0.4                60                                                     
4.0                6                                                      
40.0               0.6                                                    
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     More particularly, the flux monitor high light cut-off circuit of the present invention includes a photo detector and an integrator. Incident light striking the photo detector causes it resistance to change, resulting in a voltage change at the photo detector. As long as the photo detector voltage is below a first threshold voltage level, the integrator is inoperative. When the voltage at the photo detector exceeds the threshold voltage, this voltage is then applied to the input of the integrator. The integrating capacitor may now be charged resulting in the integration of the input current developed by the photo detector voltage to the integrator, providing an output integrator voltage proportional to the integral of the input current. 
     Thus, the higher the incoming light level, the larger the input voltage to the integrator is, thereby resulting in the output voltage of the integrator reaching a predetermined second threshold voltage quicker. Once the output of the integrator reaches a this second threshold voltage, it would cause, via an additional circuit, power to the image intensifier assembly to be interrupted. The power will remain interrupted until such time as the manual or automatic function is performed. 
     These and other objects, advantages and features of the present invention will become readily apparent to those skilled from a following description of the Exemplary Preferred Embodiment when read in conjunction with the attached Drawing and appended Claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a flux monitor high light cut-off circuit constructed according to the principles of the present invention showing an exemplary intended use thereof; 
     FIG. 2 is a circuit diagram of the signal conditioner and integrator of FIG. 1; 
     FIG. 3 is a block diagram, similar to FIG. 1, showing an alternative embodiment of the present invention; 
     FIG. 4 is a block diagram showing the flux monitor cut-off circuit of FIG. 1 incorporated into the electrical chassis of a night vision goggle; and 
     FIG. 5 is a circuit diagram of the flux monitor cutoff circuit of FIG. 4. 
    
    
     DESCRIPTION OF AN EXEMPLARY PREFERRED EMBODIMENT 
     Referring now to FIG. 1, there is shown a flux monitor high light cut-off circuit 10. The cut-off circuit 10 includes a photo detector 12, a signal conditioner 14, an integrator 16 and a voltage comparator 18. The cut-off circuit 10 is particularly useful in an environment to disable a light measuring device when the intensity of the input light could cause failure of the light measuring device. For example, the light measuring device may be an image intensifier tube 20, such as is used with night vision devices. The image intensifier tube 20 has a bias voltage applied thereto by a battery 22 and a supply switch 24. The supply switch 24, as will be described in greater detail hereinbelow, is controlled by the voltage comparator 18 to remove battery power from the image intensifier tube 20 when flux on the tube 20 becomes excessive and likely to cause damage. 
     The photo detector 12 and signal conditioner 14 in combination develop an input voltage to the integrator which is proportional to the intensity of the detected light. The integrator 16 will integrate this input voltage when the input voltage exceeds the predetermined first threshold voltage. The voltage comparator 18 in response to the integrated voltage disables the light measuring device when the integrated voltage exceeds a predetermined second threshold voltage. More particularly, the voltage comparator 18 opens the supply switch 24 thereby disconnecting battery voltage from the image intensifier 20. 
     The photo detector 12 has a resistance which changes in proportion to the light intensity incident thereon. The photo detector is biased such that a change in resistance causes the corresponding change in the voltage across the photo detector 12. 
     With particular reference to FIG. 2, the photo detector 12 is coupled in series with a bias resistor 26. The photo detector 12 is biased at a first voltage. The bias resistor 26 is biased at a second voltage. The input voltage is developed at a node 28 between the photo detector 12 and the bias resistor 26. Signal conditioner 14 compares the input voltage developed at the node 28 to the predetermined first threshold voltage. The signal conditioner 14 then develops an internal bias voltage when the input voltage exceeds this first threshold voltage. The signal conditioner 14 further will couple the input voltage at the node 28 to the integrator 16. 
     More particularly, signal conditioner 14 includes an amplifier 30 and a reverse biased Zener diode 32. The amplifier has an inverting input, an non-inverting input and an output. The input voltage from the node 28 is applied to the non-inverting input. The internal bias voltage is developed at the output of the amplifier 30. The Zener diode 32 is coupled to the inverting input of the amplifier 30, with the polarity as shown in FIG. 2. The Zener diode when reversed bias applies the first threshold voltage to this inverting input. 
     To couple the input voltage to the integrator 16, the signal conditioner 14 includes a normally on transistor switch 34. The transistor switch 34 when on shunts the input voltage to ground potential. The transistor switch 34 is turned off when the bias voltage developed within the signal conditioner 14 is applied thereto. 
     More particularly, the switch includes a PNP transistor 34 having a base to which the bias voltage is applied, a collector coupled to the integrator 16 and also to which the input voltage is applied and an emitter. A bias resistor 36 is coupled between the emitter of the PNP transistor 34 and ground potential. 
     The integrator 16 includes an amplifier 38 and integrating capacitor 40 and a bias resistor 42. The amplifier 38 has a non-inverting input to which the input voltage is applied, an inverting input and an output. The integrated voltage is developed at the output of the amplifier 38. The integrating capacitor 40 is coupled between the non-inverting input and the output of the amplifier 38. The bias resistor 42 is coupled between the inverting input of the amplifier 38 and ground potential. 
     With reference to FIG. 3, there is shown an alternative embodiment cut-off circuit 10&#39; which eliminates the use of the photo detector 12 to develop the input voltage for the signal conditioner 14 and hence the input current for the integrator 16. The image intensifier tube 20, as is well known in the art, converts incident light energy thereon to an electrical current. This current (or a current developed in response to sensing the current within the image intensifier tube 20) may be applied directly to the signal conditioner 14 as seen in FIG. 3. Similarly, when this current exceeds a predetermined threshold, this current may then be applied to the integrator 16 similarly as hereinabove described. The supply voltage for photo detector 12, signal conditioner 14, integrator 16 and voltage comparator 18 may also be provided by a conventional voltage doubler 46 which doubles the voltage of the battery 22. The voltage doubler 46 is used with either the first embodiment cut-off circuit 10 or the alternative embodiment cut-off circuit 10&#39;. 
     With reference now to FIG. 4, there is shown a block diagram useful to describe an exemplary manner in which how the cut-off circuit 10 described hereinabove may be integrated as an add on externally connected device to the electrical chassis of a commercially available night vision goggle, such as the AN/PVS-7A goggle commercially available from Litton Systems, Inc. 
     FIG. 5 is a detailed circuit diagram of the flux monitor cut-off circuit 10 of FIG. 4. The circuit of FIG. 5 also contains a full description of the elements shown generally in FIGS. 1 and 3 with additional features. 
     During power-up, the battery 22 has its anode connected to terminal 5 through an on-off switch 48 and its cathode connected to terminal 3. Capacitor C5 provides base drive current to transistor Q2, turning transistor Q2 on. Transistor Q2 turns on, in turn, series transistor Q3. When transistor Q3 turns on, it turns on voltage doubler circuit U2. The voltage doubler circuit U2 provides supply voltage (five volts) to the rest of the signal conditioner 14, integrator 16 and voltage comparator 18. The integrator U1,A with voltage comparator U1,B provides base drive to transistor Q2 via resistor R11 after cut-off circuit 10 is powered-up. When the battery 22 is disconnected, the capacitor C5 discharges through a resistor R14. Also during the power-up process, base drive is provided to transistor Q1 through capacitor C1 to reset the integrator 16. 
     The photo detector, shown as photo resistor R3, is parallel with resistor R4 and in series with resistors R1 and R2 and a voltage divider circuit. The voltage at point A (node 28) is a function of the photo resistor resistance and ranges between 2.5 volts and 1.2 volts. The circuit is balanced at a preset light level (2.5-3.0 foot candles) by adjusting resistor R1 to result in zero volts between A and B. 
     The integrator timing function is controlled by the following elements, series resistor R6, timing capacitor C2, divided voltage at point A controlled by the photo resistor and the threshold voltage a U1, B pin 5 which is the non-inverting input. 
     After the integrator 16 is reset, the output at U1,A terminal 1 is high and goes negative changing the integration cycle. At about 0.5 volts, the voltage comparator U1,B switches from low to high, cutting off the base drive to transistor Q2 thereby switching the battery switch transistor Q3 off. The photo tube switch transistor Q5 is only controlled by the doubler supply voltage and cuts off when transistor Q3 opens. 
     To enable one skilled in the art to construct the circuit of FIG. 5, the following table sets forth each element and its value or commercially available part number. 
     
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BM - FLUX CUT-OFF SWITCH                                                  
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Q1    TRANS           2N2222A                                             
Q2    TRANS           2N2907A                                             
Q3    TRANS           2N2222A                                             
Q4    TRANS           2N2222A                                             
Q5    TRANS Fet       MPF930                                              
Q6    TRANS           2N2222A                                             
Q7    TRANS           2N2222A                                             
CR1   DIODE Shotky    IN5817                                              
CR2   DIODE Shotky    IN5817                                              
CR3   DIODE Ref.      LM285 Z-12                                          
CR4   DIODE Sig.      IN 4148                                             
C1    CAP CER         .1    μf 50 V 10% (X7R)                          
C2    CAP TANT        2.2   μF 25 V 5%                                 
C3    CAP TANT        2.2   μF 10 V 20%                                
C4    CAP TANT        10    μF 6 V 20%                                 
C5    CAP TANT        .1    μF 57 V 17% (X7R)                          
C6    CAP CER         .1    μF 52 V 10% (X7R)                          
RI    RESISTOR POT    4.7K    20%    Cermet                               
R2    RESISTOR        2.21K   1%                                          
R3    PHOTO RESISTOR          CL905L CLAIREX                              
R4    RESISTOR        2.74K   1%                                          
R5    RESISTOR        220K    5%                                          
R6    RESISTOR        301K    1%                                          
R7    RESISTOR        47.5K   1%                                          
R8    RESISTOR        47.5K   1%                                          
R9    RESISTOR        47K     5%                                          
R10   RESISTOR        22.1K   1%                                          
R11   RESISTOR        10K     5%                                          
R12   RESISTOR        3.9K    5%                                          
R13   RESISTOR        47K     5%                                          
R14   RESISTOR        1M      5%                                          
R15   RESISTOR        100K    5%                                          
R16   RESISTOR        47K     5%                                          
R17   RESISTOR        4.7K    5%                                          
R18   RESISTOR        22.1K   1%                                          
R19   RESISTOR        15K     1%                                          
R20   RESISTOR        1M      5%                                          
R21   RESISTOR        47K     5%                                          
R22   RESISTOR        47K     5%                                          
R23   RESISTOR        100K    5%                                          
R24   RESISTOR        1M      5%                                          
R25   RESISTOR        18K     5%                                          
R26   RESISTOR        820.sup.                                            
                              10%                                         
R27   RESISTOR        100K    5%                                          
R28   RESISTOR        121K    1%                                          
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