US3617801A

US3617801A - Intensity control and signal enhancement correlation tube

Info

Publication number: US3617801A
Application number: US731597A
Authority: US
Inventors: James J Hogan
Original assignee: Goodyear Aerospace Corp
Current assignee: Lockheed Martin Tactical Systems Inc
Priority date: 1968-05-23
Filing date: 1968-05-23
Publication date: 1971-11-02
Anticipated expiration: 1988-11-02

Abstract

The invention consists of a variable transmission field mesh which has the unique capability of isolating the uniform, accelerating electrostatic field in the photocathode region from the focus field in the drift tube while both regions are effectively isolated from the control signal. This isolation controls the electronic image intensity after the image has been generated by the photocathode, but before the image enters appreciably in the drift-tube region, without seriously degrading characteristics of the image tube such as sensitivity, resolution, and image distortion. The invention also contemplates intensity control of the output signal from the tube by actually sensing the intensity of the electron image as well as the transmisivity of the storage grid and feeding the resulting signal back to the variable transmission field mesh. A preferable construction is with a double-decked sandwich field mesh consisting of three conducting, aligned meshes separated by two insulating or vacuum layers. The middle mesh is the control mesh while the one facing the photocathode controls the field at the photocathode and the mesh facing the drift-tube maintains the drift-tube electrostatic field at essentially zero. The potential on the control mesh may vary from a value considerably below the potentials of the other two meshes for low transmission to considerably above for high transmission. Essentially, the invention relates to an electronic iris for controlling intensity of the signal in an electronic correlation tube through signal enhancement or intensity control utilizing a control mesh in association with the storage grid. Essentially, the invention relates to an electronic iris for controlling intensity of the signal in an electronic correlation tube through signal enhancement or intensity control utilizing a control mesh in association with the storage grid. Heretofore, it has been well known to utilize electronic image storage tubes for the purpose of correlation of similar optical images, such as shown in prior art U.S. Pat. Nos. 3,290,546 and 3,239,766. However, in these prior art systems there is no control for the particular intensity or light conditions of the optical input images. Therefore, for example, when the optical input image is an actual present image, the light intensity can vary considerably depending upon the atmospheric conditions and the actual location of the detector. Some type of iris control or shutter control is necessary, be it mechanical or electrical, to appropriately control the light intensity so that an image of constant average intensity is achieved. The general object of the invention is to meet the needs of the art by providing an electronic iris and intensity enhancement or reduction control in a correlation tube by controlling the signal in proportion to the intensity of the optical input signal, either before storage on the storage mesh, or after the storage mesh has performed its storage function, so as to provide an image of average intensity in both the write and read modes of operation. The aforesaid object of the invention and other objects which will become apparent as the description proceeds are achieved by providing in a correlation tube the combination of a closed tube drawn to a vacuum, a photocathode at one end of the tube, and an electrical anode at the other end thereof, means to direct and focus electrons emitted from the cathOde down the tube towards the anode, a storage mesh mounted in the tube between the cathode and anode adapted to collect electrons and represent an optical image depicted by the electrons as an electrical image, and means to control the storage capability of the storage mesh in proportion to the intensity of the light impinging on the photocathode.

Description

United States atem [72] Inventor James J. Hogan Akron, Ohio [21] Appl. No. 731,597 [22] Filed May 23, 1968 [45] Patented Nov. 2, 1971 [73] Assignee Goodyear Aerospace Corporation Akron, Ohio 541 INTENSITY CONTROL AND sicriAL ENHANCEMENT CORRELATION TUBE Primary Examiner-Rodney D. Bennett, Jr. Assistant E xaminerJoseph G. Baxter A tz0rneyJ. G. Pere ABSTRACT: The invention consis t s of a variable transmission field mesh which has the unique capability of isolating the uniform, accelerating electrostatic field in the photocathode region from the focus field in the drift tube while both regions are effectively isolated from the control signal. This isolation controls the electronic image intensity after the image has been generated by the photocathode, but before the image enters appreciably in the drift-tube region, without seriously degrading characteristics of the image tube such as sensitivity, resolution, and image distortion. The invention also contemplates intensity control of the output signal from the tube by actually sensing the intensity of the electron image as well as the transmisivity of the storage grid and feeding the resulting signal back to the variable transmission field mesh. A preferable construction is with a double-decked sandwich field mesh consisting of three conducting, aligned meshes separated by two insulating or vacuum layers. The middle mesh is the control mesh while the one facing the photocathode controls the field at the photocathode and the mesh facing the drift-tube maintains the drift-tube electrostatic field at essentially zero. The potential on the control mesh may vary from a value considerably below the potentials of the other two meshes for low transmission to considerably above for high transmission.

Essentially, the invention relates to an electronic iris for :controlling intensity of the signal in an electronic correlation tube through signal enhancement or intensity control utilizing a control mesh in association with the storage grid.

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AMPLIFIER CORRELATION x SIGNAL 60 AMPLIFIER INVENTOR JAMES J. HOGAN FIG.

ATTORNEYS INTENSITY CONTROL AND SIGNAL ENHANCEMENT CORRELATION TUBE Essentially, the invention relates to an electronic iris for controlling intensity of the signal in an electronic correlation tube through signal enhancement or intensity control utilizing a control mesh in association with the storage grid.

Heretofore, it has been well known to utilize electronic image storage tubes for the purpose of correlation of similar optical images, such as shown in prior art U.S. Pat. Nos. 3,290,546 and 3,239,766. However, inthese prior art systems there is no control for the particular intensity or light conditions of the optical input images. Therefore, for example, when the optical input image is an actual present image, the

light intensity can vary considerably depending upon the atmospheric conditions and the actual location of the detector.

Some type of iris control or shutter control is necessary, be it mechanical or electrical, to appropriately control the light intensity so that an image of constant average intensity is achieved.

The general object of the invention is to meet the needs of the art by providing an electronic iris and intensity enhancement or reductioncontrol in a correlation tube by controlling the signal in proportion to the intensity of the optical input signal, either before storage on the storage mesh, or after the storage mesh has performed its storage function, so as to provide an image of average intensity in both the write and read modes of operation.

The aforesaid object of the invention and other objects which will become apparent as the description proceeds are achieved by providing in a correlation tube the combination of a closed tube drawn to a vacuum, a photocathode at one end of the tube, and an electrical anode at the other end thereof,

means to direct and focus electrons emitted from the cathode down the tube towards the anode, a storage mesh mounted in the tube between the cathode and anode adapted to collect electrons and represent an optical image depicted by the electrons as an electrical image, and means to control the. storage capability of the storage mesh in proportion to the intensity of the light impinging on the photocathode.

For a better understanding of the invention reference should be had to the accompanying drawings wherein:

FIG. 1 is a schematic block diagram illustration of a correlation tube incorporating the features of the invention;

FIG. 2 is an enlarged, cross-sectional, schematic illustration of the particular control.grid construction utilized in the embodiment in FIG. 1;

FIG. 3 is another cross-sectional, enlarged schematic illustration of the control grid in another mode of operation; and

FIG. 4 is a broken-away, enlarged schematic block diagram showing a modified control operation to achieve the objects of the invention.

The art of image correlation generally involves providing a. reference image of a certain object, for example, a particular section of the earths terrain. Then a subsequent image of the same area of the terrain, normally called the present image, is matched or correlated with the reference image. Utilizing this technique, for example, an unmanned aircraft can fly a predetermined flight path governed by previously prepared reference image information on film, for example, where image correlation is used to determine error signals to correct the flight path of the aircraft. The instant invention deals with a single electronic tube to perform correlation functions, and particularly to an electronic system to control the light intensity of the present image in the tube.

With reference to the form of the invention illustrated in FIG. 1 of the drawings, there is shown an image matching system indicated generally by numeral 10. The basic element in this system is an electronic image correlation tube, indicated generally by numeral 12. The correlation tube 12 comprises a housing 12a transparent on an end 12b with a photocathode 14 positioned on the inside of end 1212. In the usual manner the cathode I4 is adapted to convert light energy impinging thereon into an electronic image to be accelerated for storage on a storage grid 18 or other appropriate chargable photoconductive surfaceby a voltage impressed on ah accelerating electrode 16 extending between the photocathode 14 and the storage grid 18. A permanent or electromagnetic coil or solenoid 20 is used to achieve a focus of the image onto the storage grid 18. This coil 20 may consist of a number of adjacent, individual elements to allow the currents causing the magnetic fields of each to be individually adjusted. The grid 18 is positioned substantially parallel to the photocathode 14, but spaced therefrom. A secondary emission or collecting screen 22 is provided in close-spaced adjacent relationship to the storage grid 18 to collect the electrons that are emitted from the grid 18 as a result of secondary emission during the writing of a charge pattern for storage on the grid 18.

The general use of a tube as shown in FIG. 1 is better understood with reference to U.S. Pat. No. 3,290,546.

In correlation use of the invention as shown, a reference image stored on film would be stored on grid 18, and compared to a present image nutated with respect thereto by a nutation yoke 24 so that a correlation signal as amplified through an'electron multiplier 30 is detected at an anode 26. on an electrical pickup 28, and further amplified through anamplifier 32 to provide a correlation signal 34. This technique is well explained in the above-identified patent.

The novelty in the instant invention resides in combining two

more grids

40 and 42 or other appropriate chargeable photoconductive surface in front of the accelerating electrode 16. A feedback from the collector mesh 22 to the center or control mesh 42 is provided with

appropriate amplifiers

44, 46 and 48, respectively, connected in a feedback line 50. Appropriate power sources 52' and 54 are incorporated into the feedback line 50 for amplification, together with

suitable resistors

52a and 54a, connected respectively to ground.

The average correlating current of signal 34 varies with the average light level corresponding to the live scene or present image areas which are projected via the photocathode 14 electronically to the storage mesh 18. The instant invention proposes that the charge on the collector mesh 22 may be used to control the correlated signal gain such that the gain is lowered when the collector current rises (coincident with the rise in the average light level of the corresponding portion of the image) and rises when the collector current decreases. If the same effective scene diameter is present at the collector mesh as at the storage mesh 18 (a condition which is generally true) the collector current is proportional to the average incident correlating current. Further, some fraction of electrons turned back from the storage mesh 18 are collected by the collector mesh 22. This fraction, by the basic process of correlation, is highest when two unlike scenes are correlated and lowest when two like scenes are correlated. Thus, the structure of the invention will enhance the correlation signal by always providing properly exposed signals for correlation.

Intensity feedback and signal enhancement are achieved by controlling the number of electrons passing down the tube to the storage mesh 18, and this is achieved by making mesh 42 a control mesh interposed between meshes l6 and 40.

The combined meshes l6, 4 0 and 42 comprise a variabletransmission field mesh in which the transmission is a function of the relative potential on the meshes, and yet which transmission does not change the electrostatic fields in the regions of the photocathode 14 or the drift-tube 15.

The assembly of

meshes

16, 40, and 42 shown in FIG. I is shown in greatly enlarged cross section in FIG. 2. Basically, it consists of three conducting meshes separated by two thin insulating layers such as vacuum so that the mesh grids and insulators are aligned to allow an optical transmission approximately equal to the optical transmission of any single mesh. The variable-transmission field mesh is proposed to replace the present single field mesh in the above-identified patent. The mesh total will require three potentials, V V,, and V which correspond to the grid 40 facing the photocathode, the middle or control grid 42, and the grid 16 facing the drift-tube, respectively. The potential V may be set equal or approximately equal to the drift-tube potential to control image rotation. This leaves V as the control voltage to vary the overall transmission of the field mesh. Two typical operating conditions are sketched in FIGS. 2 and 3, namely, a typical hightransmission and a typical low-transmission operation. Electrons will tend to follow the electric-flux lines indicated by dotted lines 56 in a direction opposite to the arrows.

The highest transmission may be considerably higher than the geometric transmission, and the lowest considerably lower even when the drift-tube electrostatic field is held to zero. This compares to a transmission of less than the geometric transmission for the same condition as in the above-identified patent.

The electrons collected by the collector 22 are proportional to the total number of incident electrons from the photocathode 14 and to the electrons reflected from the storage grid 18. Assuming all reflected electrons from the storage grid 18 are collected by the collector 22, the total collector current can be readily found to be PM um) where c collector current I, incident electron current (incident on collector 22 from photocathode 14) T collector transmission T,,.,, storage mesh transmission The incident current I, will typically vary with magnification change or as the projected photocathode area changes. The storage grid transmission can also be expected to change and must be maximum when the stored and live scenes are in match. Accordingly, for a constant incident electron current (incident on collector mesh from photocathode) and collector transmission, the collector current will be a minimum for the match condition.

Amplification of the collector current by chopper-stabilization or temperature compensation techniques provides the signal which may be used for feedback control of the live image or forward gain control of the correlation signal. The correlation signal is given by V,,,,=GI, T T where G gain of the correlation signal due to the electron multiplier and output amplifiers.

Forward control may be achieved by controlling the gain G with an amplified signal developed from the collector current as picked off on feedback line 56 such that G=G /(l+aI,,) where a is a constant. Thus, it is seen that cnr II c am[ l( v am)] which for sufficiently high values of 1, reduces to This equation shows that correlation is independent of I, and enhances the signal for maximum T by'the factor l/(l T,. T which is similar to the feedback equation.

FlG. 4 illustrates a feed forward control through an amplifier 60 to provide a control signal on the'gain-controlled amplitier 36a. Again an electron multiplier 39b can be utilized within the-tube.

The collector-current signal may be fed back to the variable-transmission mesh 42 to in effect create an electronic iris to achieve virtually the same result as a forward control. However, the feedback method has the advantage of eliminating the automatic gain control amplifier and maintaining a regulated correlation current.

FIG. 2 illustrates quite clearly that when V,,,,, is less than V, and V, is lessthan F the flux lines will tend to be represented by the solid arrows between the respective aligned portions of the meshes allowing more flux lines to pass between such alignment, which in effect results in a wide open or maximum electron transmission configuration. On the other hand, in FIG. 3, V is greater than V,,', with V, being less than V,,,,,. In this configuration, the flux lines all tend to flow into the mesh 42, as illustrated, and effectively clamp ofi or at least partially block off the transvisivity of the flux lines creating in effect a restriction to the electron intensity passing therebetween. The electrons not passing through are collected predominantly by mesh 40, although

meshes

42 and 16 may also collect some electrons. Of course, this type of restriction .becomes necessary when the intensity of light increases on the photocathode 14, as is well understood in the photography or camera art.

Thus, it should be understood that the invention resides in the variable-transmission field mesh with utilization of the collector current. The collector signal fed back to the field mesh provides an electronic iris. The following operational improvements with this electronic iris are possible:

1. Correlation signal will be independent of live-scene intensity variations.

2. The maximum transmission of the field mesh will be increased over that presently available.

3. Control of the live-scene intensity'with an electronic iris rather than a mechanical one, thus reducing package size, giving greater reliability, and providing much faster response to I a collector or field mesh) between them, aligning all three with the aid of a microscope, and suitably sealing the circumferences together. It should also be understood that the purposes of the invention, while preferably comprising three separate meshes in the control mesh combination, could be accomplished by a single mesh having variable voltage control dependent from the current of the collector mesh, or two meshes, again operating in the same manner.

Thus, it should be seen that the objects of the invention have been achieved by incorporating additional meshes in combination with the conventional field mesh in an electronic correlating tube, and controlling the voltage on the field mesh in accordance with the electric current characteristics on the collector mesh in a feedback technique, or utilizing the current on the collector mesh as an indication of intensity to in effect control the operation of the electronic multiplier which amplifies the correlation signal.

While in accordance with the patent statutes only one best known embodiment of the invention has been illustrated and described in detail, it is to be particularly understood that the invention is not limited thereto or thereby, but that the inventive scope is defined in the appended claims.

What is claimed is:

l. A correlation tube having intensity control which comprises an enclosed housing drawn to a vacuum,

a cathode at one end of the housing to convert light images into electronic images by the emission of electrons therefrom down the tube,

an electron chargeable surface positioned in spaced-parallel relation to the cathode inside the housing adapted when under no charge to receive the impingement of electrons to thereby effect an electrical charge thereon to represent the light images electronically,

an anode at the other end of the tube to receive any electrons passing through the electron chargeable surface and represent the number thereof as a variable voltage potential,

mesh means positioned between the cathode and the storage grid to control the number of electrons passing to the chargeable surface in proportion to the number of secondary emission electrons from the electron chargeable surface, and

a collector mesh position ed closely adjacent the chargeable surface towards the cathode, the potential on the field mesh varying in direct proportion to the potential on the collector mesh. 4

2. A correlation tube having intensity control which comprises an enclosed housing drawn to a vacuum,

a cathode at one end of the housing to convert light images into electronic images by emission of electrons therefrom down the tube,

a transmission field mesh positioned in spaced-parallel relation between the cathode and the electron chargeable surface to control the number of electrons passing through the chargeable surface,

means to provide the field mesh with variable current control, and

a collector mesh positioned closely adjacent the electronchargeable surface and toward the cathode, the current on the collector mesh controlling the means to provide variable current control to the field mesh.

3. A tube' according to claim 2 where the field mesh comprises three separate meshes in closely spaced, aligned insulated relationship with the potential on each outside mesh remaining constant to maintain electronic transmission from the photocathode to the chargeable surface uniform, and with the current from the collector mesh fed back to control the means to provide variable current control to the field mesh.

4: A correlation tube having intensity control which comprises an enclosed housing drawn to a vacuum,

an electron chargeable surface positioned in spaced-parallel relation to the cathode inside the housing adapted when under no charge to receive the impingement of electronsmeans to pick the current off the collector mesh and feed forward to the anode to control the amplification of the voltage potential from the anode.

5. A correlation tube comprising a closed tube drawn to a vacuum,

a photocathode at one end of the tube, and an electrical anode at the other end thereof,

means to direct and focus electrons emitted from the cathode down the tube toward the anode,

a storage mesh mounted in the tube between the cathode and anode adapted to collect electrons and represent an optical image deposited by the electrons as an electrical image,

a collector mesh positioned closely adjacent the storage mesh to detect secondary emission electrons, and I means to control the number of electrons impinging on the storage mesh in proportion to the intensity of the light impinging on the photocathode, the means comprising a transmission mesh which includes three separate meshes in closely spaced, aligned and insulated relationship and means to maintain an essentially constant voltage potential on the two outer meshes with the central mesh voltage proportional to the collector mesh potential.

Claims

1. A correlation tube having intensity control which comprises an enclosed housing drawn to a vacuum, a cathode at one end of the housing to convert light images into electronic images by the emission of electrons therefrom down the tube, an electron chargeable surface positioned in spaced-parallel relation to the cathode inside the housing adapted when under no charge to receive the impingement of electrons to thereby effect an electrical charge thereon to represent the light images electronically, an anode at the other end of the tube to receive any electrons passing through the electron chargeable surface and represent the number thereof as a variable voltage potential, mesh means positioned between the cathode and the storage grid to control the number of electrons passing to the chargeable surface in proportion to the number of secondary emission electrons from the electron chargeable surface, and a collector mesh positioned closely adjacent the chargeable surface towards the cathode, the potential on the field mesh varying in direct proportion to the potential on the collector mesh.

2. A correlation tube having intensity control which comprises an enclosed housing drawn to a vacuum, a cathode at one end of the housing to convert light images into electronic images by emission of electrons therefrom down the tube, an electron chargeable surface positioned in spaced-parallel relation to the cathode inside the housing adapted when under no charge to receive the impingement of electrons to thereby effect an electrical charge thereon to represent the light images electronically, an anode at the other end of the tube to receive any electrons passing through the electron chargeable surface and represent the number thereof as a variable voltage potential, a transmission field mesh positioned in spaced-parallel relation between the cathode and the electrOn chargeable surface to control the number of electrons passing through the chargeable surface, means to provide the field mesh with variable current control, and a collector mesh positioned closely adjacent the electron chargeable surface and toward the cathode, the current on the collector mesh controlling the means to provide variable current control to the field mesh.

3. A tube according to claim 2 where the field mesh comprises three separate meshes in closely spaced, aligned insulated relationship with the potential on each outside mesh remaining constant to maintain electronic transmission from the photocathode to the chargeable surface uniform, and with the current from the collector mesh fed back to control the means to provide variable current control to the field mesh.

4. A correlation tube having intensity control which comprises an enclosed housing drawn to a vacuum, a cathode at one end of the housing to convert light images into electronic images by emission of electrons therefrom down the tube, an electron chargeable surface positioned in spaced-parallel relation to the cathode inside the housing adapted when under no charge to receive the impingement of electrons to thereby effect an electrical charge thereon to represent the light images electronically, an anode at the other end of the tube to receive any electrons passing through the electron chargeable surface and represent the number thereof as a variable voltage potential, a collector mesh in closely space, adjacent relation to the chargeable surface between the chargeable surface and the cathode, and means to pick the current off the collector mesh and feed forward to the anode to control the amplification of the voltage potential from the anode.

5. A correlation tube comprising a closed tube drawn to a vacuum, a photocathode at one end of the tube, and an electrical anode at the other end thereof, means to direct and focus electrons emitted from the cathode down the tube toward the anode, a storage mesh mounted in the tube between the cathode and anode adapted to collect electrons and represent an optical image deposited by the electrons as an electrical image, a collector mesh positioned closely adjacent the storage mesh to detect secondary emission electrons, and means to control the number of electrons impinging on the storage mesh in proportion to the intensity of the light impinging on the photocathode, the means comprising a transmission mesh which includes three separate meshes in closely spaced, aligned and insulated relationship and means to maintain an essentially constant voltage potential on the two outer meshes with the central mesh voltage proportional to the collector mesh potential.