JP2001319604A - Circuit for protecting photoelectric cathode of image intensifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for protecting intensifier that has a low power consumption and an appropriate cost by protecting the intensifier from overdriving and directly measuring a low current in the photoelectric cathode. SOLUTION: The protecting circuit that protects the photoelectric cathode of an image intensifier from being overdriven during operation has a high voltage supply that supplies a photoelectric cathode potential to the photoelectric cathode, a measuring circuit that measures current supplied to the photoelectric cathode, and a logic circuit that interrupts the photoelectric cathode potential fed to the photoelectric cathode when the measured current shows the photoelectric cathode is overdriven. In more specified manner, the measuring circuit contains a resistor connected between the high voltage supply and photoelectric cathode of the image intensifier, and a sensing circuit to sense a voltage on the resistor. As another property, the measuring circuit and the logic circuit are constituted so as to float with respect to the circuit ground, g.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the field of image intensification, and more particularly to a protection circuit for protecting an intensifier from illumination levels that can damage the tube or significantly shorten the life of the tube.

[0002]

BACKGROUND OF THE INVENTION Image intensifiers are used in a number of applications, including commercial, research, and military applications. One example is a night light application where an intensifier is used to amplify low levels of light to be viewed by an observer, and a good example of this application is night light binoculars. Another application is very fast, stop action,
Involving high speed image capture where shuttering is required, fast gating of intensifiers provides this functionality. However, with fast gating,
The exposure time is reduced. This is compensated for by the intensifier's ability to provide very high detected current amplification in the range of 50,000: 1 (illumination power amplification is on the order of 100). Another application uses the modulation of an intensifier microchannel plate to provide a heterodyne (mixing) effect with an incoming modulated optical signal. This is Sa
ndia National Laboratories (U.S. Pat.
5,616) used in LADAR (Laser Detection and Radar).

In operation, it is desirable to actively protect the image intensifier from being overdriven. Overdriving the intensifier is defined herein as excessive photocathode current generated by high input illumination that provides a standard voltage potential difference between the photocathode and the microchannel plate (MCP) input. Overdriving the intensifier can cause an end of life to be reached, requiring costly replacement. Various methods of actively protecting the intensifier already exist based on monitoring the current to the monitor's screen during operation, and the intensifier turns off the "gate" when the selected screen current level is exceeded. Is done. Monitoring the screen current is a possible protection method because the screen current is indirectly related to the photocathode current due to the MCP gain. This may represent an abnormal situation where there is low screen current but excessive photocathode current. Low screen current is obtained as a result of low MCP gain or incorrect screen voltage. Instantaneous conditions, such as turning on the light, can degrade the intensifier and significantly reduce tube life.

[0004] Another passive approach exists where the photocathode voltage is clamped to a reference voltage if the photocathode current exceeds a certain level. This generates a reference voltage and requires an additional high voltage supply current with a non-isolated circuit structure. This method uses M M that interferes with the photocathode current measurement.
Inaccuracy due to CP strip current. Further, it hinders the gating operation of the photocathode. An example of such a technique is disclosed in U.S. Pat. No. 5,146,077, which discloses that in response to a current drawn by a photocathode above a predetermined value indicating a clamp level on the tube,
A "bright source protection circuit" for the image intensifier modulates the voltage supplied to the photocathode of the tube. The problem addressed by this circuit is the loss of resolution at high light levels, with the goal of obtaining a constant brightness rather than protecting the tube. "Protecting the bright source" in the present context involves improving the resolution under the conditions of the bright source rather than protecting the tube itself. Indeed, the disclosed circuit allows the supply current to be delivered to the photocathode under adverse light conditions that may damage the tube.

The present invention is a technique for actively protecting an image intensifier from being overdriven during operation, especially under adverse light conditions that may damage the tube or shorten its life. Will be described. Thereby, the technique described in the present invention circumvents the constraints described above.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to preserve the life of a tube and to protect the intensifier from being overdriven.

It is a further object of the present invention to directly measure the low current flowing through the intensifier photocathode instead of using a presumed method of measuring the intensifier screen current. .

It is an object of the present invention to provide an intensifier protection method for low power consumption and reasonable cost.

[0009]

SUMMARY OF THE INVENTION The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, in accordance with one aspect of the present invention, a protection circuit that protects a photocathode of an image intensifier from being overdriven during operation includes a high voltage power supply that supplies a photocathode potential difference to the photocathode, A measurement circuit for measuring the current supplied to the cathode and a logic circuit for interrupting a photocathode potential difference supplied to the photocathode when the measured current indicates that the photocathode is overdriven. More specifically, the measurement circuit includes a resistor connected between the high voltage power supply and the photocathode of the intensifier, and a sensing circuit for sensing the voltage on the resistor. In another important aspect, the measurement and logic circuits are configured to float with respect to ground.

The present invention further provides a battery, a switching regulator connected to the battery to generate a circuit supply voltage that is floated with respect to circuit ground, and a power supply isolated from ground for turning on / off the power supply. And a power supply on / off portion connected to the switching regulator. This power supply is particularly adapted to supply the aforementioned cathode protection circuit with very low battery power.

The method and apparatus according to the present application provide a means for directly measuring the photocathode current regardless of the MCP gain or screen current. This advantage is a challenge when measuring photocathode currents: high voltage on the order of 800 volts, low current on the order of picoamps, fast enough to protect the unit, low cost, low power, Isolation to prevent leakage current and, finally, overcoming excessive component voltages.

[0012]

The above and other aspects, objects, and features of the present invention
The features and advantages will be more clearly understood and appreciated from a reading of the following detailed description of a preferred embodiment with reference to the accompanying drawings and the appended claims.

[0013] The description of the invention is directed in particular to elements which form part of or cooperate with the device according to the invention, since image intensifiers using protection circuits are known. Elements not specifically shown or described herein may be selected by one of ordinary skill in the art.

FIG. 1 shows a component 10 for protecting a photocathode in a circuit arrangement including a gating high voltage power supply 12 and an image intensifier 14. As in known intensifier applications, the gating high voltage power supply 12 generates the high voltage required to operate the internal elements of the intensifier 14. As shown in FIG. 3, the internal elements of the intensifier 14 include a photosensitive photocathode 16, a microchannel plate (MCP) 18 having a number of microchannels, and a phosphorescent screen 20. The gating high voltage power supply 12
A photocathode potential difference is applied to the photocathode 16 on the line 21a,
The potential difference MCP_in, line 21
Provide the potential difference MCP_out to the output surface of the MCP 18 above and the screen potential difference to the screen 20 on line 21d. The photocathode 16 has a function of emitting electrons generated by input illuminance. The emitted electrons are accelerated by applying a photocathode potential difference to the photocathode 16,
A potential difference is created for the input potential difference MCP_IN applied to the input surface on the microchannel plate 18, which gate-on potential difference is typically on the order of -800 volts. Applying a positive photocathode potential difference to the photocathode 16 with respect to the input potential difference MCP_in prevents the acceleration of electrons to the input surface 22, thereby causing the intensifier 14 to "gate" off and typically The typical gate-off potential difference is +40 dc. In normal operation,
The gating of the intensifier 14 is used to control the on / off state of the intensifier. Gating is known to be an effective means of brightness control by varying the on-time duty cycle during a given period (ie, pulse width modulation).

The electrons that enter the microchannels in MCP 18 are the channel design and the high voltage potential difference on MCP 18,
That is, it is amplified using secondary electron emission caused by a potential difference between MCP_in and MCP_out. This potential difference between input surface 22 and output surface 24 provides a means for gain and brightness control. The resulting electron flow (current) is further accelerated in the direction of the fluorescent coated screen 20 using a high potential difference applied to the screen (typically +4000+ to 6000 volts relative to the MCP output surface). . Impact of the screen 20 on the phosphor causes photoemission including a spectral distribution determined by the type of phosphor. The resulting output luminance from screen 20 represents an amplified image of the received input image.

The screen current is on the order of 100 nanometers to 200 nanometers when the MCP 18 is operated in its linear region. Considering the gain provided by MCP 18, the current from photocathode 16 is on the order of picoamps. Because of such low photocathode currents, higher output screen currents are typically used as a means to indicate an overdrive condition.
However, as a result, it is possible that high illumination can cause excessive photocathode current while having a low MCP voltage and thus a low screen current. The end of the life of the intensifier can be advanced, and the indications are low power intensity, high intensity spots and / or non-uniformity in the output image.

The present invention describes a technique for protecting the intensifier from being overdriven with the intention of conserving tube life. This approach involves a direct measurement of the low current flowing into the photocathode 16, instead of using a putative method of measuring the screen current. This embodiment also describes an approach aimed at low power consumption and reasonable cost.

The components 12 of the gating high voltage power supply, shown fully in FIG. 1 and partially in FIG. 2, are the circuit components that currently exist in known intensifier circuits. As a result, the gating high voltage power supply 12 includes a standard high voltage supply generating component 26, a standard screen current sensing component 28, and a standard gating component 30. The screen current sensing component 28 provides a monitoring signal to the high voltage supply generation component 26. If the screen current exceeds a predetermined value, the MCP voltage is reduced (however, this is not related to the present invention). It does not protect the photocathode in the manner described, and therefore does not protect the intensifier from overdriving the intensifier). As further shown in FIG. 1, the present invention provides a gating component 30 within the image intensifier 14 and the gating high voltage power supply 12.
And a photocathode protection component 10 arranged in series between FIG. 2 shows the photocathode protection component 10 and its interconnection with the gating component 30 in the high voltage power supply 12 in more detail. In the known gating component 30, the switch 32 switches the resistor 34 (with resistance R) during the initial turn-on time of the intensifier 14.
Is momentarily closed to bypass the second switch 3
6 applies a positive potential to the intensifier 14
Closed to turn off. This is a conventional gating process.

Following the initial activation of intensifier 14, switch 32 is opened and all current to photocathode 16 is switched from a negative high voltage source (eg, -800 volts) to a very high value (eg, -800 volts). , 2G-ohm). The rules for current in this description relate to the flow of electrons. The photocathode current flowing through resistor 34 generates a potential difference having the polarity shown on resistor R, which is sensed by photocathode protection component 10. Assuming an MCP gain of 10000: 1 and a limited channel current of 500 nanoamps, the input photocathode current will be about 50 picoamps. The potential difference developed on the resistor R used for the limited value will be 100 millivolts. The resistor 34 has two high voltage leads 5
Connected to the input of the photocathode component 10 using 0 and 52, these leads are specifically added to the high voltage power supply 12 for the purposes of the present invention.

The current measuring portion of the photocathode protection component 10 includes a scaler 42 connected on a differential amplifier 44 that produces an output that is evaluated by a comparator 46. The scaler 42 provides a high impedance means for adjusting the resistance R externally, allowing the input voltage level to be reduced by simple voltage division if reduction is required. Note that scaler 42 is in parallel with resistor R and does not create a load with respect to ground. A high resistance value for resistor T, typically on the order of 1 G-ohm to 5 G-ohm, is selected for scaler 42. Resistors having such resistance values are available on the free market from manufacturers / suppliers such as Vishay-Dale.

Photocathode protection component 10 is floated with respect to ground. This means that the connecting leads 50 and 52 are at a high potential (on the order of 800 volts dc).
, Allows the measurement of small potential differences generated on the resistor 34. The differential amplifier 44 is a very high impedance amplifier that allows measurement of the voltage drop across the resistor R without causing significant errors due to current loading or leakage due to the high input potential relative to ground gnd. Differential amplifier 44 is shown floating relative to circuit ground gnd (shown as an inverted triangle containing the letter F to indicate float).
The gain G of the differential amplifier 44 is sufficient to boost the input level and is responsible for providing additional sensitivity to lower level current sensing.

Two adjustment and filtering stages 54 and 5
6 is provided on the input side of the comparator 46. First stage 54
Provides adjustment and filtering of the amplitude of the signal from the differential amplifier 44. The conditioned signal is connected to the input of a comparator 46, which is also shown floating relative to circuit ground. The comparator 46 compares the adjusted signal with a threshold voltage _Vth . Second stage 56
_{Derives a} threshold voltage _Vth by scaling and filtering the adjusted reference voltage _Vref provided by comparator 46. When the threshold voltage is exceeded, the comparator output draws current through LED 57 in optocoupler 58, which is floating with respect to circuit ground gnd. The output of optocoupler 58 includes a transistor 59 that feeds an external optocoupler amplifier and logic stage 60 to generate a gate on / off on line 60a. When an overdrive condition is considered to exist, this gate on / off signal can be used in three ways, described below.

First, feeding the gate on / off signal back to the gate control input on the gating high voltage power supply 12 may simply turn off the photocathode voltage. Given a predetermined time delay, the intensifier 14 may be reactivated. This minimizes damage to the intensifier due to excessive photocathode current, which is likely to cause image anomalies. Second of gate on / off signal
Is used to "gate" the intensifier off for the remainder of a given period of the known clock frequency, ie, pulse width modulation. Following the end of each period, the tubes are reactivated. There may also be forced operation using a gate logic control signal applied to the gate control logic input 64 of the optocoupler amplifier and logic component 60. The duty cycle of the gate control signal can be varied according to the desired average level of output brightness. This is also a pulse width modulation method enabled by the embodiment. A third use of the gate on / off signal applies to frame capture systems. The intensifier may be "gated" off until the next frame is captured.

An integral part of the present invention is the power supply. A battery 70 is used to provide isolation and thus avoid very high potential differences on circuit components to ground. Only a single cell battery is needed. Another use for batteries is the use of acoustic transformers that can provide low power AC signals from which high isolation and DC power can be obtained. The battery design is such that long battery life can be achieved using low current / low operating voltage devices as currently available. Further, the supply can be powered down when not in use. Battery 70 is of the silver oxide type to provide a consistent nominal voltage of 1.55 volts. The end of battery life is 0.238 when operated for 24 hours per day
Defined as the time when the battery voltage reaches 1.3 volts (cut-off voltage) under a milliamp load, the estimated lifetime under these conditions is 734 hours.
The expected battery life when using the preferred embodiment for 5 hours per day is about 6 hours to 1 year.

The switching boost regulator 72 has a circuit supply voltage + V of 3.3 Vdc with respect to circuit ground (indicated by the inverted triangle containing the letter F to indicate float).
is used to generate _c, the supply voltage V _c is supplied to the amplifier 44 and the comparator 46. The circuit supply can be turned off using a power on / off command, which is connected to the optocoupler 7
4 to separate from ground. Receive amplifier 76
The output of is a stop signal used to turn off the switching regulator 72 and thus the circuit power. Note that optocoupler 74 and receive amplifier 76 are designed to use battery supply + _Vb directly for power to generate commands to the switching supply.

Battery-low component 78 monitors the battery level when the circuit is operating. In the event of a low level, indicated by Vmin, current is propagated to the LEDs in octocoupler 80. The LEDs are shown floating relative to circuit ground gnd (indicated by an inverted triangle containing the letter F to indicate floating above ground). The output is coupled to an optocoupler amplifier 82 from which a control signal is generated to signal a low battery alarm. The alarm indicator may simply be an onboard low current LED.

The invention has been described with reference to the preferred embodiment. However, it is understood that changes and modifications can be made by those skilled in the art without departing from the scope of the present invention.

[Brief description of the drawings]

FIG. 1 is a diagram showing a power supply for an image intensifier, generally showing a photocathode protection circuit connected to a power supply according to the present invention.

FIG. 2 is a detailed diagram of the photocathode protection circuit shown in FIG.

FIG. 3 is a diagram showing internal elements of an image intensifier.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Photocathode protection component 12 Gating high voltage power supply 14 Image intensifier 16 Photocathode 18 Microchannel plate (MCP) 20 Screen 21a-d line 22 Input surface 24 Output surface 26 High voltage supply generation component 28 Screen current sensing Component 30 Gating Component 32 Switch 34 Resistor 36 Second Switch 42 Scaler 44 Differential Amplifier 46 Comparator 50 First High Voltage Lead 52 Second High Voltage Lead 54 First Adjustment and Filtering Stage 56 Second Conditioning and Filtering Stage 57 LED 58, 74, 80 Optocoupler 59 Transistor 60 Optocoupler Amplifier and Logic Stage 62 Gate Control Input 64 Gate Logic Input 70 Battery 72 Switching Boost Regulator 76 Receive Amplifier 78 Low Battery Configuration Element 82 Optocoupler Amplifier

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Dennis J. Whipple USA New York 14615 Rochester Grenthorn Road 101 (72) Inventor Lawrence A. Ray United States of America New York 14610 Rochester Elmwood Avenue 3861 (72) Inventor Kenneth Jay Repichi United States New York 14450 Fairport Country Downs Circle 106

Claims (4)

[Claims] 1. A protection circuit for protecting a photocathode of an image intensifier from being overdriven during operation, comprising: a high-voltage power supply for supplying a photocathode potential difference to the photocathode; And a logic circuit for interrupting the photocathode potential difference supplied to the photocathode when the measured current indicates that the photocathode is overdriven. . 2. The protection circuit according to claim 1, wherein the measurement circuit and the logic circuit are configured to float with respect to a circuit ground g. 3. The measurement circuit includes a resistor connected between the high voltage power supply and the photocathode of the intensifier, and a sensing circuit for sensing a voltage on the resistor. 1
Protection circuit as described. 4. The protection circuit of claim 3, wherein said logic circuit interrupts said photocathode potential difference when said sensed voltage exceeds a threshold.