EP1139382A2

EP1139382A2 - Image intensifier photocathode protection circuit

Info

Publication number: EP1139382A2
Application number: EP01201005A
Authority: EP
Inventors: Louis R. c/o Eastman Kodak Company Gabello; Dennis J. c/o Eastman Kodak Company Whipple; Lawrence A. C/O Eastman Kodak Company Ray; Kenneth J. c/o Eastman Kodak Company Repich
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 2000-03-30
Filing date: 2001-03-19
Publication date: 2001-10-04
Also published as: JP2001319604A; EP1139382A3

Abstract

A protection circuit for protecting a photocathode of an image intensifier tube from being overdriven during operation includes a high voltage power supply for supplying the photocathode with a photocathode potential, a measurement circuit for measuring the current supplied to the photocathode, and a logic circuit for interrupting the photocathode potential supplied to the photocathode when the measured current indicates that the photocathode is being overdriven. More specifically, the measurement circuit includes a resistance connected between the high voltage power supply and the photocathode of the intensifier tube; and a sensing circuit for sensing a voltage across the resistance. In another feature, the measurement circuit and the logic circuit are configured to float with respect to ground.

Description

The invention relates generally to the field of image intensification, and in particular to protection circuits for protecting intensifiers from levels of illumination which could damage or significantly shorten tube life.
Image intensifier tubes are used in many ways involving commercial, research and military applications. One example involves a night vision application, where an intensifier is used to amplify low levels of light for viewing by an observer; night vision binoculars are a good example of this application. Another application involves high speed image acquisition where very fast, stop-action, shuttering is required; the high speed gating of the intensifier provides this function. Due to the high speed gating, however, there is reduced exposure time. This is compensated by the capability of the intensifier to provide very high detected current amplification, ranging in the neighborhood of 50,000:1 (amplification of illumination power ranges on the order of hundreds). Still another application uses modulation of the intensifier's microchannel plate to provide a heterodyning (mixing) effect with incoming modulated light signals. This has been used in LADAR (Laser Detection and Radar) as developed by Sandia National Laboratories (see U.S. Patent No. 4,935,616).
It is desirable to actively protect an image intensifier tube from being over-driven during operation. Over-driving the intensifier tube is defined herein as excessive photocathode current caused by a high input illumination given normal voltage potential between its photocathode and microchannel plate (MCP) input. Over-driving the intensifier tube can cause premature end-of-life which necessitates costly replacement. Various methods of actively protecting the intensifier tube already exist based upon monitoring current flow to its screen during operation; the intensifier is "gated" off beyond a selected screen current level. Monitoring the screen current is an inferred method of protection since the screen current is indirectly related to the photocathode current by the gain of the MCP. This can present an anomalous condition where there is low screen current, yet excessive photocathode current. Low screen current can be a result of low MCP gain or incorrect screen voltage. A momentary condition, such as turning a light on, could cause a degradation of the intensifier and significantly shorten the tube life.
Other passive approaches exist where the photocathode voltage is clamped to a reference voltage if the photocathode current exceeds a certain level. This requires additional high voltage supply current in order to develop the reference voltage and is of a non-isolated circuit configuration. The method contains inaccuracies due to MCP strip current flow, which interferes with the measurement of photocathode current. It also hinders the gating operation of the photocathode. An example of such a technique is described in U.S. Patent No. 5,146,077, in which a "bright source protection circuit" for an image intensifier tube modulates the voltage supplied to the tube's photocathode in response to current drawn by the photocathode that exceeds a predetermined value indicative of a clamp level for the tube. The problem addressed by this circuit is the reduction of resolution at high light levels; the idea here is to obtain constant brightness rather than to protect the tube. "Bright source protection" in this context refers to improving the resolution under bright source conditions, rather than protection of the tube itself. Indeed, the disclosed circuit enables supply current to feed the photocathode under adverse lighting conditions that might damage the tube.
The present invention describes a technique for actively protecting an image intensifier tube from being over-driven during operation, especially under adverse lighting conditions that might damage the tube or shorten its life. In doing so, the technique described in this invention circumvents the limitations described above.
It is an object of this invention to protect an intensifier tube from being over-driven, with the intent of preserving tube life.
It is a further object of the invention to directly measure the low currents flowing into the photocathode of the intensifier tube, in lieu of using the inferred method of measuring the screen current of the intensifier tube.
It is a further object of this invention to provide an intensifier protection approach targeted for low power consumption and moderate cost.
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, a protection circuit for protecting a photocathode of an image intensifier tube from being overdriven during operation includes a high voltage power supply for supplying the photocathode with a photocathode potential, a measurement circuit for measuring the current supplied to the photocathode, and a logic circuit for interrupting the photocathode potential supplied to the photocathode when the measured current indicates that the photocathode is being overdriven. More specifically, the measurement circuit includes a resistance connected between the high voltage power supply and the photocathode of the intensifier tube; and a sensing circuit for sensing a voltage across the resistance. In another important feature, the measurement circuit and the logic circuit are configured to float with respect to ground.
The invention further includes a power supply having a battery, a switching regulator connected to the battery for developing a circuit supply voltage floating with respect to circuit ground, and a power on/off section isolated from ground and connected to the switching regulator for turning the power supply on and off. This power supply is particularly adapted for supplying the aforementioned cathode protection circuit using very low battery power.
The method and apparatus presented herein provides a means of directly measuring the photocathode current, independently of the MCP gain or the screen current. The advantage is that it overcomes the challenges presented in measuring the photocathode current, namely, high voltage on the order of 800 volts, low currents on the order of picoamperes, sufficient speed to protect the unit, low cost, low power, isolation to prevent leakage currents, and lastly, excessive component voltages.
These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.
FIG. 1 is a diagram of a power supply for an image intensifier generally showing a photocathode protection circuit connected into the power supply according to the invention
FIG. 2 is a more detailed diagram of the photocathode protection circuit shown in Figure 1.
FIG. 3 is a diagram of the internal elements of an image intensifier.
Because image intensifier devices employing protection circuits are well known, the present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the present invention. Elements not specifically shown or described herein may be selected from those known in the art.
Figure 1 shows a photocathode protection component 10 in a circuit arrangement with a gating high voltage power supply 12 and an image intensifier 14. As in known intensifier applications, the gating high voltage power supply 12 develops the high voltages necessary to operate the internal elements of the intensifier 14. As shown in Figure 3, the internal elements of the intensifier 14 include a light-sensitive photocathode 16, a microchannel plate (MCP) 18 having a great multitude of microchannels and a phosphorescent screen 20. The gating high voltage power supply 12 provides a photocathode potential to the photocathode 16 on a line 21a, a potential MCP_in to an input face 22 of the MCP 18 on a line 21b, a potential MCP_out to an output surface 24 of the MCP 18 on a line 21c and a screen potential to the screen 20 on a line 21d. The photocathode 16 serves to emit electrons induced by input illumination. Freed electrons are accelerated by applying the photocathode potential to the photocathode 16, which generates a potential difference with respect to the input potential MCP_in applied to the input face 22 on the microchannel plate 18; this gate-on potential difference is typically on the order of -800 volts. Applying a positive photocathode potential to the photocathode 16 with respect to the input potential MCP_in prevents electron acceleration towards the input face 22 and thus "gates" the intensifier 14 off; a typical gate-off potential is +40Vdc. In normal operation, gating of the intensifier 14 is used to control the on/off state of the intensifier. Gating is also known to be an effective means of brightness control by varying the duty cycle of on-time during a given period (i.e. pulse width modulation).
Electrons entering micro-channels within the MCP 18 are amplified by means of secondary emission induced by the channel design and a high voltage potential across the MCP 18, i.e., the potential difference between MCP_in and MCP_out. This potential difference between the input surface 22 and the output surface 24 provides a means of gain control and brightness control. The resulting flow of electrons (current) is accelerated further towards the phosphor coated screen 20 by means of a high potential applied to the screen (typically +4000 to 6000 volts dc with respect to the MCP output surface). The bombardment onto the phosphors of the screen 20 causes photoemission with a spectral distribution as determined by the phosphor type. The resulting output luminance from the screen 20 presents an amplified image of the received input image.
Screen currents are on the order of 100 nanoamperes to 200 nanoamperes when the MCP 18 is operated in its linear region. Given the gains afforded by the MCP 18, the current from the photocathode 16 is on the order of picoamperes. Because of such low photocathode currents, the higher output screen current is typically used as a means to indicate an overdrive condition. As a result however, it is possible to have a low MCP voltage and thus low screen current while high illumination causes excessive photocathode current. Premature end-of-life of the intensifier tube can occur; exhibited symptoms are low output intensities, high intensity spots and/or non-uniformities in the output image.
This invention describes a technique of protecting the intensifier tube from being over-driven with the intent of preserving tube life. The approach involves direct measurement of the low currents flowing into the photocathode 16; this is in lieu of using the inferred method of measuring the screen current. The present embodiment also describes an approach targeted for low power consumption and moderate cost.
The gating high voltage power supply component 12, which is shown in full in Figure 1 and in part in Figure 2, is a circuit component that presently exists in known intensifier circuits. As a result thereof, the gating high voltage power supply 12 includes a standard high voltage supply generation 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 back to the high voltage supply generation component 26. If the screen current exceeds a predetermined value, the MCP voltage is reduced (this, however, does not protect the photocathode in the manner described in connection with the invention, and therefore does not protect the intensifier tube from being over-driven). As further shown in Figure 1, the invention includes the photocathode protection component 10, which is placed in series between the image intensifier 14 and the gating component 30 within the gating high voltage power supply 12. Figure 2 shows more detail of the photocathode protection component 10 and its interconnection with the gating component 30 in the high voltage supply 12. In the known gating component 30, a switch 32 is momentarily closed to bypass a resistor 34 (having a resistance R) during the initial turn-on time of the intensifier 14; a second switch 36 is closed to turn off the intensifier 14 by applying a positive potential. This is the conventional gating process.
Following the initial activation of the intensifier 14, the switch 32 is opened and all current to the photocathode 16 flows from the negative high voltage source (e.g. -800 volts) through the resistance R, which is a very high value (e.g., 2 G-ohms). The convention for current flow in this discussion refers to electron flow. The photocathode current flowing through the resistor 34 develops a potential across resistance R with the polarity as shown; it is this potential which is sensed by the photocathode protection component 10. Given an MCP gain of 10000:1 and a limiting channel current of 500 nanoamperes, the input photocathode current would be approximately 50 picoamperes. The potential developed across resistance R used for the limited value would then be 100 millivolts. The resistor 34 is connected to the input of the photocathode protection component 10 using two high voltage leads 50 and 52, which are specifically added to the high voltage power supply 12 for purposes of this invention.
The current measurement portion of the photocathode protection component 10 includes a scaler 42 connected across a differential amplifier 44 for producing an output that is evaluated by a comparator 46. The scaler 42 provides a high impedance means of adjusting the resistance R externally and also enables reducing the input voltage level, if such reduction is needed, by simple voltage division. Note that the scaler 42 is in parallel with resistance R and does not create loading with respect to ground. High resistance values for the scaler 42 relative to the resistance R, typically on the order of 1 G-ohms to 5 G-ohms, are chosen. Resistors with such resistance values are available in the open market, e.g., from a manufacturer/supplier such as Vishay-Dale.
The photocathode protection component 10 is floating with respect to ground. This enables measurement of the small potential developed across the resistor 34 even through the connecting leads 50 and 52 are at high potential (on the order of 800 volts dc). The differential amplifier 44 is a very high impedance amplifier that enables measurement of the voltage drop across the resistance R without significant error due to current loading or leakages due to high input potential with respect to ground, gnd. It is shown as floating relative to circuit ground, gnd (depicted by an inverted triangle with the letter, F, for float). The gain G of the differential amplifier 44 is sufficient to boost the input level; it also serves to provide further sensitivity to smaller levels of current detection.
Two adjustment and filtering stages 54 and 56 are provided on the input side of the comparator 46. The first stage 54 provides amplitude adjustment and filtering of the signal from the differential amplifier 44. The conditioned signal is connected to the input of comparator 46, which is also shown as floating relative to circuit ground. The comparator 46 compares the conditioned signal to a threshold voltage V_th. The second stage 56 derives the threshold voltage V_th by scaling and filtering a regulated reference voltage V_ref provided by the comparator 46. When the threshold voltage is exceeded, the comparator output drives current through an LED 57 in an optocoupler 58; the LED is floating relative to circuit ground, gnd. The output of the optocoupler 58 includes a transistor 59 that feeds into an external optocoupler amplifier and logic stage 60 to generate a gate on/off signal on a line 60a. When an over-drive condition has been deemed to exist, this gate on/off signal can be used in three fashions as described below.
First, feeding the gate on/off signal back into a gate control input 62 on the gating high voltage power supply 12 can simply turn off the photocathode voltage. Given a prescribed time delay, the intensifier 14 can then be reactivated. This will likely induce image anomalies but will minimize damage to the intensifier tube due to excessive photocathode current. The second use of the gate on/off signal is to "gate" the intensifier off for the remainder of a given period of a known clock frequency, i.e. pulse width modulation. Following the end of each period, the tube is reactivated. There can also be a forced operation using a gate logic control signal applied to a gate control logic input 64 of the optocoupler amplifier and logic component 60. The duty cycle of the gate logic control signal can be varied according to the average level of output luminance desired. Again, this is a pulse width modulation approach enabled by the embodiment. The third use of the gate on/off signal applies to a frame capture system. The intensifier can be "gated" off until the next frame is to be captured.
An integral part of the invention is the power supply. To provide isolation and to thus avoid very high potential across circuit components to ground, a battery 70 is used. Only a single cell battery is needed. An alternative to a battery is the use of an acoustic transformer, which can provide high isolation and a low power ac signal from which dc power can be derived. The battery design is such that using low current/low operating voltage devices as are currently available, long battery life can be achieved. In addition, the supply can be powered down when not in use. The battery 70 is a silver oxide type, in order to provide a consistent nominal voltage of 1.55 volts. End of life on the battery is defined as the time when the battery voltage reaches 1.3 volts (cutoff voltage) under a load of 0.238 milliamperes when operating 24 hours per day; the estimated life is 734 hours under these conditions. Expected battery life, using the preferred embodiment for 5 hours per day is approximately 6 months to 1 year.
A switching boost regulator 72 is used to develop a circuit supply voltage, +V_c, of 3.3Vdc relative to circuit ground (depicted by an inverted triangle with the letter, F, for float), which is supplied to the amplifier 44 and the comparator 46. The circuit supply can be turned off using a power on/off command; the command is isolated from ground by means of an optocoupler 74 connected to a receiver amplifier 76. The output of the receiver amplifier 76 is the shutdown signal used to turn off the switching regulator 72 and thus turn off the circuit power. Note that the optocoupler 74 and receiver amplifier 76 are designed to use the battery supply +V_b directly for power in order to generate the command to the switching supply.
A battery-low component 78 monitors the battery level when the circuitry is in operation. In the event of a low level as determined by Vmin, current is delivered to the LED in an optocoupler 80. It is also shown as floating relative to circuit ground, gnd (depicted by an inverted triangle with the letter, F, for floating ground). The output is coupled into an optocoupler amplifier 82 from which a control signal is generated to signal a low battery alarm. The alarm indicator could also simply be an on-board, low current LED.

Claims

A protection circuit for protecting a photocathode of an image intensifier tube from being overdriven during operation, comprising:

a high voltage power supply for supplying the photocathode with a photocathode potential;

a measurement circuit for measuring the current supplied to the photocathode; and

a logic circuit for interrupting the photocathode potential supplied to the photocathode when the measured current indicates that the photocathode is being overdriven.
The protection circuit as claimed in claim 1 wherein the measurement circuit and the logic circuit are configured to float with respect to circuit ground, g.
The protection circuit as claimed in claim 1 wherein the measurement circuit comprises:

a resistance connected between the high voltage power supply and the photocathode of the intensifier tube; and

a sensing circuit for sensing a voltage across the resistance.
The protection circuit as claimed in claim 3 wherein the logic circuit interrupts the photocathode potential when the sensed voltage exceeds a threshold.
A power supply for a control circuit, comprising:

a battery;

a switching regulator connected to the battery for developing a circuit supply voltage floating with respect to circuit ground;

a power on/off section isolated from ground and connected to the switching regulator for turning the power supply on and off.
The power supply as claimed in claim 13 further comprising:

a battery low detector that monitors the level of the battery and produces a low battery signal when the level is below a threshold;

an alarm generator for generating an alarm signal when the battery is low; and

an optocoupler floating with respect to circuit ground for coupling the low battery signal to the alarm generator.