US3848123A

US3848123A - Automatic brightness control for image intensifier tube

Info

Publication number: US3848123A
Application number: US00418920A
Authority: US
Inventors: W Parker; R Kryder
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1973-03-30
Filing date: 1973-11-26
Publication date: 1974-11-12
Anticipated expiration: 1991-11-12

Abstract

The brightness of an output display screen of an image intensifier tube is automatically controlled by a currentlimiting means connected between a direct current source and an oscillator. The oscillator supplies an alternating voltage to a voltage multiplier. The voltage multiplier provides accelerating voltages to the individual stages of the image intensifier assembly. The current-limiting means responds to a nonlinear, current sensing element to provide a substantially constant current source at input light intensity levels below a predetermined level, and to provide a substantially unlimited current source when the input light intensity exceeds the predetermined level.

Description

United States Patent 1 Parker et al.

[451 Nov. 12, 1974 AUTOMATIC BRIGHTNESS CONTROL FOR IMAGE INTENSIFIER TUBE [73] Assignee: RCA Corporation, New York, NY.

[22] Filed: Nov. 26, 1973 [21] Appl. No.: 418,920

Related U.S. Application Data [63] Continuation of Ser. No. 346,673, March 30, 1973,

abandoned.

[52] U.S. Cl. 250/213 VT, 313/96 [51] Int. Cl ..H01j31/50, l-lOlj 39/12 [58] Field of Search 250/213 VT; 313/108 R,

[56] References Cited UNITED STATES PATENTS 3,345,534 10/1967 Charles 250/213 VT CURRENT- REGULATOR 3,711,720 l/l973 Kryder 250/213 VT Primary Examiner.lames W. Lawrence Assistant ExaminerT. N. Grigsby Attorney, Agent, or Firm-George .l. Seligsohn; Edward J. Norton [57] ABSTRACT The brightness of an output display screen of an image intensifier tube is automatically controlled by a current-limiting means connected between a direct current source and an oscillator. The oscillator supplies an alternating voltage to a voltage multiplier. The voltage multiplier provides accelerating voltages to the individual stages of the image intensifier assembly. The current-limiting means responds to a nonlinear, current sensing element to provide a substantially constant current source at input light intensity levels below a predetermined level, and to provide a substantially unlimited current source when the input light intensity exceeds the predetermined level.

14 Claims, 2 Drawing Figures PATENTU, rmv 1 2 I974 CURRENT R OSCILLATOR O LTAG E M L H TIPLIER lO-O AUTOMATIC BRIGHTNESS CONTROL FOR IMAGE INTENSIFIER TUBE The invention herein described was made in the course of or under a contract or subcontract thereunder with the Department of the Army.

This is a continuation of application Ser. No. 346,673 filed Mar. 30, 1973.

BACKGROUND OF THE INVENTION This invention relates to image intensifier tubes and more particularly to an improved system for controlling the brightness at the output display screen.

An image intensifier tube is a device in which a visible image is produced in response to a radiant energy, such as ultra-violet light, visible light or infrared rays. In an image intensifier, the light reflected from a scene is imaged onto the photocathode of the image intensifier assembly. The photocathode converts the light image into an electron image and the electron image is focused and accelerated by a voltage applied to an associated anode of the intensifier assembly. The electron image ultimately impinges on a phosphor screen, on which the intensified visible image of the original scene is displayed. The resulting degree of intensification is dependent on the voltage difference between the anode and photocathode at each stage of the intensifier assembly.

Presently known image intensifier systems utilize an image intensifier tube, a voltage multiplier for applying a direct voltage between an input photocathode and an output electrode or screen with voltage connections to the additional intensifier stages therebetween. An alternating voltage is supplied to the voltage multiplier by an oscillator which is in turn coupled to a direct current source. In order to control the output display brightness over an extended range of input light intensities, various current-limiting means have been inserted between the direct current source and the oscillator to limit the direct current applied to the oscillator from the current source. As the light level and current demand in these prior art systems increases, a greater portion of the direct current voltage is dropped across the current-limiting means and a smaller voltage therefore appears at the oscillator input terminal. An example of one prior art system in U.S. Pat. No. 3,71 1,720 which issued to R. A. Kryder and which is assigned to the same assignee as the present invention. Another example of a current-limited prior art system is U.S. Pat. No. 3,581,098.

In the operation of these prior art systems, at higher input light levels, the output screen illuminance tends to logarithmically decrease as the input illumination logarithmically increases over the intended operating range. This undesired effect results from the currentlimiting action wherein the available current from the direct current source is insufficient to supply the stages of the image intensifier tube as the current demand increases.

SUMMARY OF THE INVENTION In an image brightness control system of the type comprising: an image intensifier tube, a voltage multiplier applying a direct voltage between the input and output electrodes of the tube, an oscillator circuit for applying an alternating voltage to the voltage multiplier, a direct current source, and current-limiting means connected between the direct current source and the oscillator; the improvement therewith, comprising: a nonlinear current sensing means coupled between the direct current source and an input electrode of the image intensifier tube for controlling the currentlimiting means to provide a substantially constant current source at input light intensity levels below a predetermined level, and to provide a substantially unlimited current source when the input light intensity exceeds the predetermined level.

BRIEF DESCRIPTION OF THE DRAWING:

The advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description when taken in conjunction with the accompanying drawing wherein:

FIG. 1 is a schematic representation of an image intensifier tube system incorporating the present invention; and

FIG. 2 is a graphical representation showing the improved response characteristics of an image intensifier tube system in accordance with the present invention.

DETAILED DESCRIPTION As shown in FIG. 1, an image intensifier system 10 includes: a three stage image intensifier tube 12, a voltage multiplier 14, an oscillator 16, a current regulator 18 and a direct current source or battery 20. A photocathode 12a of image intensifier tube 12 is coupled to a point of reference potential, schematically represented as ground, by way of serially coupled diodes 22a through 22d. Photocathode 12a is also coupled to ground by way of capacitor 24. The electrical junction point between capacitor 24 and diode 22a is coupled to an input of current regulator 18 by way of lead D. An anode 12a of the first image intensifier stage is coupled to a photocathode 12b of the second image intensifier stage. An anode 12b of the second stage is coupled to the photocathode of the third image intensifier stage. Similarly, an anode 12c of the third stage is coupled to a phosphor-coated anode 12d which serves as an output display screen.

Stepped accelerating and focusing voltages are supplied to the respective anode and photocathode electrodes between each stage by voltage multiplier 14. Voltage multiplier 14 comprises a plurality of capacitors 26 and diodes 28 arranged in a well-known voltage-doubler configuration. A first step connection V of voltage multiplier 14 supplies the accelerating voltage for the first image intensifier stage. Similarly, stepped voltage connections V and V provide accelerating potentials to the second and third image intensifier stages respectively. The current path between voltage multiplier 14 and the first image intensifier stage includes a resistor 30.

The oscillator 16 provides an alternating voltage to the voltage multiplier 14 by way of lead 32. Oscillator 16 may be any one of a number of known oscillators which convert a direct current input into an alternating voltage output. Voltage multiplier 14 in turn converts and multiplies this alternating voltage up to the direct voltages which are applied to image intensifier tube 12. By way of example, voltages V V and V may be 15Kv., 30Kv., and 45Kv. respectively.

The direct current provided by battery 20 is coupled to oscillator 16 by way of current regulator 18. Current from the positive terminal of battery 20 flows through the load provided by oscillator 16 and back to the negative terminal of battery 20 through the collectoremitter path of an NPN transistor 34. Forward bias for transistor 34 is provided by the biasing network comprising thermistor element 36a, sensistor element 36b and variable resistor 38. This biasing network provides a fixed or constant current drive to transistor 34 Which in turn acts to limit the current supplied to oscillator 16 to a fixed or constant value. Thermistor element 36a exhibits a negative temperature coefficient whereas sensistor element 36b exhibits a positive temperature coefficient. Accordingly, the values of temperature compensation elements 36a and 36b can be selected so as to track the temperature versus gain characteristics of transistor 34 in order to maintain a constant current output over a predetermined range of temperature variations. The input D of current regulator 18 is coupled to a second NPN transistor 40 by way of a variable resistor 42. It can be seen that the collector-emitter path of transistor 40 is serially coupled with the path of transistor 34. Accordingly, when a forward bias is applied to the base-emitter path of transistor 40, a low impedance path is provided between the positive terminal of battery 20 and the base-emitter path of transistor 34. Under these conditions, transistor 34 goes into a fully conductive or saturated condition. However, when the bias at the base input of transistor 40 is insufficient to cause conduction of transistor 40, the conduction of transistor 34 is controlled solely by the biasing network comprising elements 36a and 36b, and variable resistor The operation of image intensifier system 10 of FIG. 1 will now be described in conjunction with the improved response characteristics graphically represented by FIG. 2. At low light input levels, for example, less than 10 foot candles, only negligible current flows in the current path of the first image intensifier stage. This path includes serially coupled diodes 22a through 22d, resistor 30, the portion of voltage multiplier 14 which provides accelerating voltage V and the photo-current path between 12a and photocathode 12a of the first image intensifier stage. Thus, only a negligible voltage drop is developed between point D and ground. With only a negligible signal voltage being developed at point D, transistor 40 in current regulator 18 is therefore cut-off. Accordingly, the constant current being supplied by regulator 18 is controlled solely by the biasing network comprising elements 36a and 36b, and resistor 38.

Thus, at input light levels below 10 foot candles, the direct current being supplied to oscillator 16 is a substantially constant current. Resistor 38 is adjusted to provide a maximum desired brightness in the light input range of 10 foot candles to 10" foot candles. In currently preferred practice, resistor 38 is typically adjusted for a light output not to exceed 50 foot lamberts over the above-described light input range (i.e. lto foot candles).

At light input levels exceeding l0- foot candles, the first stage photocurrent is of a sufficie'nt level to cause a significant signal voltage to be developed across diodes 22a through 22d. As the light input increases from 10 foot candles, the signal voltage developed across the serially coupled diodes increases as a log function of the increase in input light levels or the corresponding photo-current. Stated differently, the photo-current is directly proportional to the light input. However, the light input, over the desired range of operation, increases several orders of magnitude. Thus, the current passing through diodes 22a through 22d also increases several orders of magnitude. Hence, advantage is taken of the nonlinear or logarithmic characteristics of the diodes wherein the voltage drop across the diodes is proportional to logarithmic changes in current passing therethrough rather than directly proportional to the current changes. It should be appreciated that a linear sensing device such as a resistor would develop a voltage thereacross which would increase linearly over the range of input light levels. Accordingly, under these conditions, the dynamic range of the developed voltage would be excessive, with reference to presently known image intensifier tubes, which therefore complicates and is not readily amenable to presently known techniques for controlling the associated current-limiting means.

When the signal voltage developed across diodes 22a through 22d is sufficient to forward bias the baseemitter junction of transistor 40, transistor 40 begins to conduct and continues to conduct in an increasing manner as the magnitude of the signal voltage increases. When the signal voltage is sufficient to fully forward bias the base-emitter junction of transistor 40, transistor 40 conducts heavily thereby causing transistor 34 to become fully conductive or saturated. Once transistor 34 saturates in this manner, the currentlimiting action of current regulator 18 is defeated, and battery 20 supplies the necessary operating current through oscillator 16 to maintain a relatively constant output display screen luminance. That is, since the photo-current passing through the individual stages of image intensifier tube 12 increases in proportion to the light input levels, the current demand of the image intensifier tube becomes correspondingly great at the higher light input levels. Hence, in accordance with the present invention, the substantially constant current source provided by current regulator 18 becomes a substantially unlimited current sourcethat is, unlimited relative to the current limited modeand, therefore the current source provides a direct current which varies in accordance with the current demand of the image intensifier tube when the input light levels exceed a predetermined level. In this manner, the current demand of the intensifier tube, at the higher input light levels, is satisfied.

Variable resistor 42 is used as a trimming device to compensate for various first stage image intensifier photocathode sensitivities-which may vary with different tubes; and to insure that the selected maximum light output, for example, 50 foot-lamberts, is not exceeded in the light input range from it) foot candles to 10.0 foot candles. The maximum input light level range is largely determined by the size (diameter) of the input photocathode electrode and by other physical considerations such as whether an aperture reducing device is inserted between the input light source and the input photocathode.

The function of resistor 30 in the first image intensifier stage is explained in detail in US. Pat. No.

3,711,720. Briefly, resistor 30 controls the voltage applied to the first stage in response to the intensity of the input light which thereby prevents a cut-off condition from occurring when the image intensifier tube tends to draw excessive current; resistor 30 also acts to imdeveloped across the sensing diodes(s) (s) must be sufficient to overcome the additive base-emitter voltage drops (V,,,,) of the transistors in the current-limiting means. Further, since the voltage developed across the sensing diode(s) is proportional to a logarithmic function of the forward diode current, the current level characteristics of the particular image intensifier will also dictate the number of sensing diodes required to provide the desired signal voltage.

While the present invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that other embodiments can be realized. For example, the nonlinear current sensing function can be provided by devices other than the specific type shown in the drawing. In this regard diodes 22a-22d can be replaced, for example, by a resistor having a Zener diode poled thereacross. Accordingly, a fixed Zener voltage would be developed at point D when the photo-current exceeded a predetermined level. However, this alternative embodiment would be less desirable as the attendant transistion from the current-limited mode to the unlimited mode would be significantly more abrupt than the preferred operation depicted in FIG. 2.

Further, the function provided by transistor 34 can be provided by a resistive impedance means other than the specific current-limiting semiconductor device means shown in the drawing. In this regard, a resistor having a fixed given value can be substituted for the resistive impedance provided by the collector-emitter path of transistor 34 in FIG. 1. Thus, the collectoremitter path of transistor 40 could be coupled across the resistor in this exemplary alternative embodiment. Accordingly, the effective impedance of the resistor would decrease as the intensity of the input light exceeded a predetermined level. However, the preferred resistive impedance means is a semiconductor device arranged to provide current-limiting. That is, as the current increases, the voltage drop across a resistor increases; whereas, the effective resistive impedance of a semiconductor current-limiter increases as the current demand increases in response to an effective decrease of the load resistance. The result is that a resistive impedance specifically comprising a semiconductor current-limiter provides a greater gain or light output level at low input light levels.

What has been taught then is an improved automatic brightness control for an image intensifier tube system which maintains a more constant light output level regardless of the light input level. In currently preferred practice, the brightness of the image displayed on the output screen is controlled well within a range of output light levels between and 50 foot lamberts. It should be appreciated that the output light level variations graphically represented by FIG. 2 are plotted with 6 reference to a linear ordinate. It will be appreciated by those skilled in the art, however, that prior art image intensifier tube systems exhibit a substantially greater output light level variation which is normally plotted with reference to a logarithmic ordinate.

What is claimed is:

1. In an image brightness control system for an image intensifier tube of the type comprising: an image intensifier tube having a photocathode input electrode emitting electrons in response to the intensity of light applied thereto and a phosphor display screen output electrode providing an image display in accordance with said electrons impinging thereon; a voltage multiplier applying a direct voltage between said input and output electrodes for accelerating said electrons and directing said accelerated electrons to said display screen for increasing the brightness of said image; an oscillator circuit applying an alternating voltage to said voltage multiplier, a direct current source; and currentlimiting means coupled between said direct current source and said oscillator to limit the direct current supplied to said oscillator from said direct current source to a predetermined current level for limiting the brightness of said image displayed on said screen; the improvement therewith comprising:

a nonlinear current sensing means coupled between said photocathode input electrode and said direct current source for developing a signal voltage which is substantially proportional to logarithmic changes of photo-current in said image intensifier tube; and

means coupling said current-limiting means to said current sensing means and responsive to said signal voltage for controlling said current-limiting means to provide a substantially constant current source to said oscillator at input light intensity levels below a predetermined level, and to provide current varying in accordance with the current demand of said image intensifier tube at input light intensity levels exceeding said predetermined level.

2. The image intensifier system according to claim 1, wherein said nonlinear current sensing means comprises at least one diode poled between said photocathode input electrode and said voltage multiplier.

3. The image intensifier system according to claim 2, further including a capacitor coupled across said diode.

4. The image intensifier system according to claim 3, wherein said current-limiting means includes temperature compensation means to substantially maintain said predetermined current level over a predetermined range of temperature variation.

5. The image intensifier system according to claim 1, including a serially coupled resistive impedance coupled between said photocathode input electrode and said voltage multiplier.

6. The image intensifier system according to claim 1 wherein:

said tube includes three cascaded image intensifier stages and stepped connections between said voltage multiplier and said stages, each stage including a photocathode input electrode and an output electrode; and

wherein the photo-current path of the first image intensifier stage includes said current sensing means, a resistive impedance, and the portion of said voltage multiplier which applies a direct voltage between said input and output electrodes of said first stage.

7. The image intensifier system according to claim 6 wherein said predetermined input light level is substantially equal to 10 foot candles.

8. The image intensifier system according to claim 7, wherein the brightness of said image displayed on said screen is controlled within a range of output light levels between 20 and 50 foot lamberts.

9. In an automatic image brightness control system for a light imaging device of the type comprising an image intensifier tube, a power supply circuit and means for coupling said power supply to said tube, said tube being adapted for operation within the range of input light illumination levels from l 'to foot candles;

wherein said tube includes a plurality of cascaded image intensifier stages each stage including a photocathode electrode and an anode electrode wherein each anode electrode is coupled to the succeeding photocathode electrode, the photocathode electrode of the first of said stages being a photocathode input electrode for emitting electrons in response to the intensity of light applied thereto, and the anode electrode of the last of said stages including a phosphor display screen output electrode providing an image display in accordance with said electrode impinging thereon;

wherein said power supply includes a direct current source, an oscillator circuit having an input and an output, a first resistive impedance serially coupling said direct current source to said input of said oscillator circuit wherein the alternating voltage at said output of said oscillator circuit decreases in proportion to the direct current being supplied to said oscillator circuit, and a diode-capacitor voltage multiplier having an input terminal and a plurality of discrete DC. voltage output terminals, said input terminal of said voltage multiplier being cou pled to said outputof said oscillator circuit; and wherein said means for coupling said power supply to said tube includes stepped connections respectively coupling output terminals of said voltage multiplier to said stages of said tube; the improvement therein comprising: nonlinear current sensing means coupled between said photocathode input electrode and said direct current source for developing a control signal wherein said signal is a substantially nonlinear function of the photo-current in the first of said stages; and means coupling said first resistive impedance to said current sensing means and responsive to said control signal for controlling the value of said first impedance, wherein the effective impedance of said first resistive impedance decreases as the intensity 5 of said input light exceeds a predetermined level,

thereby to control the output brightness at said screen within a predetermined range of output light levels when said input light exceeds said predetermined level.

10. The system according to claim 9, wherein said first resistive impedance is a resistor having a given fixed value and wherein said means for controlling the value of said first impedance comprises a semiconductor device having first and second main electrodes and a control electrode, said main electrodes being coupled across said resistor and said control electrode being coupled to said current sensing means.

11. The system according to claim 9, wherein said current sensing means comprises at least one diode.

12. The system according to claim 9, wherein said first resistive impedance comprises a semiconductor device having first and second main electrodes and a control electrode, said main electrodes coupling said direct current source to said input of said oscillator, said control electrode being coupled to said means for controlling the value of said first impedance, said semiconductor device providing a relatively constant current to said oscillator until said input light exceeds said predetermined level whereupon the effective resistive impedance of said semiconductor device decreases as the intensity of said input light exceeds said predetermined level.

13. The system according to claim 9, wherein the one of said connections coupling a given one of said output terminals of said voltage multiplier to said photocathode input electrode of said tube includes a serially connected second resistive impedance to individually control the voltage applied to the first of said stages in response to the intensity of the light being applied to said photocathode input electrode, said second resistive impedance having a given fixed value which at input light illumination levels substantially below l0 foot candles results in only a negligible voltage drop existing across said second resistive impedance and at input light illumination levels substantially above l0 foot candles results in a voltage drop across said second resistive impedance sufiicient to substantially reduce the voltage applied to the first of said stages.

14. The system according to claim 9, wherein said predetermined range of output light levels is to 50 foot lamberts.

Claims

1. In an image brightness control system for an image intensifier tube of the type comprising: an image intensifier tube having a photocathode input electrode emitting electrons in response to the intensity of light applied thereto and a phosphor display screen output electrode providing an image display in accordance with said electrons impinging thereon; a voltage multiplier applying a direct voltage between said input and output electrodes for accelerating said electrons and directing said accelerated electrons to said display screen for increasing the brightness of said image; an oscillator circuit applying an alternating voltage to said voltage multiplier, a direct current source; and current-limiting means coupled between said direct current source and said oscillator to limit the direct current supplied to said oscillator from said direct current source to a predetermined current level for limiting the brightness of said image displayed on said screeN; the improvement therewith comprising: a nonlinear current sensing means coupled between said photocathode input electrode and said direct current source for developing a signal voltage which is substantially proportional to logarithmic changes of photo-current in said image intensifier tube; and means coupling said current-limiting means to said current sensing means and responsive to said signal voltage for controlling said current-limiting means to provide a substantially constant current source to said oscillator at input light intensity levels below a predetermined level, and to provide current varying in accordance with the current demand of said image intensifier tube at input light intensity levels exceeding said predetermined level.

6. The image intensifier system according to claim 1 wherein: said tube includes three cascaded image intensifier stages and stepped connections between said voltage multiplier and said stages, each stage including a photocathode input electrode and an output electrode; and wherein the photo-current path of the first image intensifier stage includes said current sensing means, a resistive impedance, and the portion of said voltage multiplier which applies a direct voltage between said input and output electrodes of said first stage.

7. The image intensifier system according to claim 6 wherein said predetermined input light level is substantially equal to 10 1 foot candles.

9. In an automatic image brightness control system for a light imaging device of the type comprising an image intensifier tube, a power supply circuit and means for coupling said power supply to said tube, said tube being adapted for operation within the range of input light illumination levels from 10 5 to 102 foot candles; wherein said tube includes a plurality of cascaded image intensifier stages each stage including a photocathode electrode and an anode electrode wherein each anode electrode is coupled to the succeeding photocathode electrode, the photocathode electrode of the first of said stages being a photocathode input electrode for emitting electrons in response to the intensity of light applied thereto, and the anode electrode of the last of said stages including a phosphor display screen output electrode providing an image display in accordance with said electrode impinging thereon; wherein said power supply includes a direct current source, an oscillator circuit having an input and an output, a first resistive impedance serially coupling said direct current source to said input of said oscillator circuit wherein the alternating voltage at said output of said oscillator circuit decreases in proportion to the direct current being supplied to said oscillator circuit, and a diode-capacitor voltage multiplier having an input terminal and a plurality of discrete D.C. voltage output terminals, said input terminal of said voltage multiplier being coupled to said output of said oscillator circuit; and wherein said means for Coupling said power supply to said tube includes stepped connections respectively coupling output terminals of said voltage multiplier to said stages of said tube; the improvement therein comprising: a nonlinear current sensing means coupled between said photocathode input electrode and said direct current source for developing a control signal wherein said signal is a substantially nonlinear function of the photo-current in the first of said stages; and means coupling said first resistive impedance to said current sensing means and responsive to said control signal for controlling the value of said first impedance, wherein the effective impedance of said first resistive impedance decreases as the intensity of said input light exceeds a predetermined level, thereby to control the output brightness at said screen within a predetermined range of output light levels when said input light exceeds said predetermined level.

13. The system according to claim 9, wherein the one of said connections coupling a given one of said output terminals of said voltage multiplier to said photocathode input electrode of said tube includes a serially connected second resistive impedance to individually control the voltage applied to the first of said stages in response to the intensity of the light being applied to said photocathode input electrode, said second resistive impedance having a given fixed value which at input light illumination levels substantially below 10 2 foot candles results in only a negligible voltage drop existing across said second resistive impedance and at input light illumination levels substantially above 10 2 foot candles results in a voltage drop across said second resistive impedance sufficient to substantially reduce the voltage applied to the first of said stages.

14. The system according to claim 9, wherein said predetermined range of output light levels is 20 to 50 foot lamberts.