US3769539A

US3769539A - Camera tube

Info

Publication number: US3769539A
Application number: US00130440A
Authority: US
Inventors: C Catchpole
Original assignee: Bendix Corp
Current assignee: Bendix Corp
Priority date: 1969-02-24
Filing date: 1971-04-01
Publication date: 1973-10-30
Anticipated expiration: 1990-10-30
Also published as: FR2033382A1; FR2033382B1; GB1283794A; NL7002072A; DE2008358A1

Abstract

An intensified television camera tube including a channel multiplier array incorporated adjacent the target electrode to multiply the electrons being emitted from the source, the channel multiplier array being utilized as a mechanical support for the target electrode and as the output screen or readout mesh. Also, a system for eliminating spurious signals from the output video signal which includes a capacitance network connected in circuit with the input and output sections of the multiplier array.

Description

United States Patent [1 1 Catchpole Oct. 30, 1973 CAMERA TUBE [75] Inventor: Clive E. Catehpole, Southfield,

Mich.

[731 Assignee: The Bendix Corporation, Southfield,

Mich.

[22] Filed: Apr. 1, 1971 [21] Appl. No.: 130,440

Related US. Application Data [62] Division of Ser. No. 801,627, Feb. 24, 1969,

abandoned,

[52] US. Cl. 315/12, 313/68 R [51] Int. Cl. H0lj 29/41 [58] Field of Search 315/10, 11, 12; 313/68 R [56] References Cited UNITED STATES PATENTS 3,350,591 10/1967 Van Asselt 313/65 T 7/1969 Shoulders 10/1970 Maeda 313/104 X Wm [ix/MIX 1 3,039,017 6/1962 Brown et a1. 313/68 X 3,202,853 8/1965 Weimer 313/65 3,440,470 4/1969 Decker 313/103 3,497,759 2/1970 Manley 313/105 X Primary Examiner-Leland A. Sebastian Assistant ExaminerJ. M. Potenza AttorneyPlante, Hartz, Smith & Thompson and William F. Thornton [57] ABSTRACT 19 Claims, 6 Drawing Figures fir A /4 CAMERA TUBE CROSS REFERENCE TO RELATED APPLICATIONS This is a divisional of application Ser. No. 801,627 filed Feb. 24, 1969, now abandoned.

BACKGROUND AND SUMMARY OF THE INVENTION This invention relates generally to an improved image intensifier tube and associated output circuit arid more particularly to an image tube incorporating an electron multiplier assembly for amplifying the number of electrons striking the target electrode and for being utilized as a readout mesh, and support member and, further, an output circuit modification for substantially reducing spurious noise signals from the readout video circuit.

The features of the present invention will be described as associated with an image orthicon type television camera tube, however, it is to be understood that these features may be utilized in other types of systems. In an image orthicon tube, an optical image is focused on a photocathode, the photocathode being positioned immediately inside the glass envelope of the tube. The image focused on the photocathode causes electrons to be emitted therefrom, the electrons traveling from the photocathode to a target electrode located parallel to the photocathode and spaced therefrom approximately 1 /2 inches. With the proper bias voltages on certain elements, the electron image formed on the photocathode is drawn to the target. The impinging electrons on the target electrode give rise to secondary emission of electrons, the secondary electrons being collected in the prior art tubes, by a fine mesh screen located adjacent the surface of the target and between the photocathode and target electrodes. The target thereby becomes charged with a distribution proportional to the brightness of the optical image, and the value of this charge is several times greater than the impinging electron image, by virtue of the electron multiplication which occurs in the act of secondary emission of electrons at the target surface.

The target is made of very thin glass or other suitable material having high resistivity. The lateral resistance of the target electrode is sufficient to preserve the charge configuration on the target for the duration of the frame interval. The image may be sensed on the tar get by two methods of image removal, one being referred to as the return beam type of readout and the other the direct beam readout. In the return beam readout mode of operation, a scanning beam generated by an electron gun is directed against the opposite side of the target from the mesh screen and the photocathode. The scanning beam creates a return beam directed back toward the electron gun and the amount of readout beam returning to the electron gun is measured by means of an electron multiplier assembly positioned adjacent the electron gun. The amount of beam necessary to reestablish the target potential is, of necessity, missing or absent from the beam reflected from the target and this absent portion provides the video output signal. This return beam operation is illustrated in the drawings.

The drawings further illustrated the direct beam readout operation wherein the target is scanned by an electron beam. During this scanning, the potential change of the target as it is returned to gun cathode potential by the readout beam is sensed. This sensing is done, in the prior art by utilizing the readout mesh positioned between the photocathode and the target electrodes as a signal plate. The potential changes on the target are capacitively coupled to the signal plate for detection by a suitable amplifier in the video output circuit.

In accordance with the features of the present invention, an electron multiplier device in the form of a channel multiplier has been provided in lieu of the fine mesh screen, the channel multiplier array being utilized as the signal plate in this latter mode of readout operation, and as an electron multiplying system. Thus, the electron image is greatly amplified over that of prior art tubes.

Further, in accordance with certain other features of the present inventions, it is contemplated that the channel multiplier array additionally be utilized as a mechanical support for the thin target film, thus enabling the tube to withstand greater mechanical and electrical shock. In using this feature, the target is placed directly on the output portion of the multiplier array, thus being supported thereby. Further, with the use of this feature, the effective capacitance of the storage target is increased, thereby enabling a higher charge to be accumulated on the target electrode to give a higher output signal-to-noise ratio capability. Also, secondary electrons produced at the target are physically constrained from spreading to cause redistribution effects known as black halo.

In accordance with certain other features of the present invention, the channel multiplier array is provided with a film on the output section of the array to provide an insulating layer between the multiplier array and the target film, it being contemplated that this layer would be made up of magnesium fluoride or other like materials. This layer has been found to be helpful in reducing possible spurious signals in utilizing the channel multiplier array as a support structure. These signals exhibit themselves because the positive voltage of the channel multiplier array relative to the electron gun cathode causes a leakage through the storage film to appear as a signal on the image readout. If the target film is perfectly uniform, the signal is uniform and may be subtracted from the picture information electronically. However, in practical applications, it is not to be expected that the target film will be perfectly uniform, thus producing nonuniformities in the form of picture blemishes and shading.

Further, in utilizing a channel multiplier array to intensify the image being transmitted to the target electrode, it has been found that additional spurious signals are generated in the output circuit due to the presence of the array when it is operating with high electron gain. Accordingly, a spurious signal elimination circuit has been devised for substantially reducing the noise signals generated by the channel multiplier array. The signal elimination circuit takes the form, in the preferred embodiment, of a capacitor network interconnected between the two faces of the channel multiplier array to provide a shunt path for the spurious signals, thus eliminating these signals from the output circuit.

Accordingly, it is one object of the present invention to provide an improved image intensified camera tube.

It is another object of the present invention to provide an improved camera tube having capability of am- 3 plifying the image while eliminating the necessity for a readout screen within the camera.

It is a further object of the present invention to provide an improved camera tube incorporating an image intensifying system which may also be utilized as a readout mesh.

It is a further object of the present invention to provide an improved camera tube having improved mechanical and electrical shock characteristics.

A further object of the present invention is to provide an improved camera tube having improved charge distribution characteristics on the target electrode.

It is still a further object of the present invention to provide an improved camera tube wherein the thin target film is mechanically supported on a heretofore unused support structure.

It is a further object of the present invention to increase the effective capacitance of the storage target electrode, thus enabling a higher charge to be accumulated on the target to give a higher output signal-tonoise ratio capabiity.

It is still a further object of the present invention to provide a camera tube having means for physically constraining the secondary electrons produced at the target electrode.

It is a further object of the present invention to provide an improved camera tube wherein picture blemishes and shading are eliminated in the case of a nonuniform target electrode.

It is still another object of the present invention to eliminate spurious signals from the output circuit of image camera tube.

And, it is a further object of the present invention to provide a camera tube having a channel multiplier array positioned adjacent the target electrode, the channel multiplier array being utilized as a mechanical support for the target film, an electron multiplying device and a readout screen mesh.

Further objects, features and advantages of this invention will become apparent from a consideration of the following description, the appended claims and the accompanying drawing in which:

FIG. 1 is a schematic diagram illustrating portions of a camera tube incorporating certain features of the present invention, the readout being in the return beam mode of operation;

FIG. 2 is a schematic diagram similar to FIG. 1, but illustrating the direct beam readout mode of operation;

FIG. 3 is a detail diagram illustrating the portion in circle A of FIG. 1;

FIG. 4 is a modification of the details of FIG. 3;

FIG. 5 is a further modification of the details of FIG. 3; and

FIG. 6 is a schematic diagram illustrating a preferred form of spurious signal eliminator circuitry incorporating certain features of the present invention.

Referring now to the drawing, in particular FIG. 1, there is illustrated a schematic diagram of certain portions of a camera tube 10 which includes a photocathode 12 on which an input optical image is focused, and a target electrode 14 on which the electron image is focused. The tube 10 further includes an intervening electron multiplier array 16 for increasing the number of electrons emitted from the photocathode prior to the impingement thereof on the target electrodes 14. The above noted elements of the camera tube are enclosed in an evacuated envelope, as is well known in the art.

The photocathode is adapted to emit electrons in response to the focusing of an image through the face of the camera tube and on the photocathode surface. The electrons emitted from the photocathode are focused, either magnetically, electrostatically, or by proximity, on to the microchannel plate multiplier. The multiplied electrons emitted from the microchannel plate are incident on the charge storage target. The target can be fabricated from a thin glass disc, typically 0.0001 inch thick. Other materials can also be used for a charge storage target. The principles of operation of the tube as described here with reference to a charge storage target made from a thin glass disc have been shown to hold for charge storage targets made from thin films of Aluminum Oxide and Magnesium Oxide, typically 500A thick. The principle has also been shown to be valid for charge storage layers made from insulating materials, such as Potassium Chloride or Magnesium Fluoride, evaporated in a gaseous atmosphere to form a porous spongy layer about 10 microns thick. These, and other, methods of charge storage target preparation are well known in the art. The surface of the target 14 is, in certain embodiments of the invention, supported with its surface approximately 0.002 inches away from the output of the channel multiplier array 16. Thus, the electrons being emitted from the photocathode pass into the channel array 16, are amplified by the secondary emission within the channel multiplier array, and the output electrons strike the target electrode 14.

The output electrode of the array is normally held at a positive bias potential; the value of this potential depends upon the target material used, but a potential of the order of 10 volts is usually satisfactory. The electrons emerging from the microchannel array have the property that they have sufficient energy to pass the potential barrier between the array and the target and still have sufficient energy to liberate on the average of more than one secondary electron from the target. The secondary electrons are collected by the multiplier array output electrode, to be discussed more in detail in conjunction with the descriptions in FIGS. 3 to 5, thus leaving the target with a positively charged image. This positive charge is capacitively coupled to the opposite side of the target and hence varies the electron scanning beam conventionally utilized to scan the target 14.

The scanning beam is generated from an electron emitting gun 20 which supplies electrons towards the target 14 in a direct beam. For purposes of discussion, the return beam operation mode of FIG. 1 will be described first and the direct beam mode of operation will be described in conjunction with the description of FIG. 2. The electron scanning stream is directed through a multiplier array 22, in the form of a dynode structure, and thence impinges on the target electrode 14. If the target electrode requires electrodes to return it to its normal potential, due to the charge capacitance coupled from the charged surface on the opposite side of the target electrode, a certain portion of the direct beam will be utilized in furnishing these electrons. The remaining portion will return to the electron gun and will be intercepted by the dynode structure 22. The dynode 22 includes a plurality of plates 24 which are utilized to multiply the electrons being fed therethrough, and the output from the dynode structure is derived from a video output conductor 26.

A similar operation exists in the system of FIG. 2, which also includes a photocathode 12, a channel multiplier array 16, and a target electrode 14. Further, a stream of scanning electrons are supplied by an electron gun 20. However, the charge on target electrode 14 is sensed by capacitive coupling between the target electrode 14 and a conductive output face of the channel multiplier array 16, as will be more fully explained in connection with the explanation of FIGS. 3 to 5. The output from the conductive surface of the channel multiplier array 16 is fed to a video output circuit, including a coupling capacitor 28 and a resistor 30, the latter of which is connected to a positive voltage biasing source. This operation is conventionally known as the direct beam readout operation.

In this direct beam readout mode of operation, the potential change of the target, as it is returned to gun cathode potential by the read-out scanning beam, is detected by a signal plate. In the case of the system of the present invention, this signal plate takes the form of a conductive surface on the output side of the channel output multiplier array 16. In the case of prior art tubes, the "signal plate takes the form of a readout mesh screen. The potential changes on the target are capacitively coupled to the signal plate, the output electrode being connected with a suitable amplifier to amplify the video signal.

Referring now to FIGS. 3, 4, and 5, there are illustrated three variations of the channel multiplier arrays which may be utilized in conjunction with the systems of FIGS. 1 and 2. Specifically, the target electrode 14 is mounted spaced, in the first embodiment, from the channel multiplier array. The array takes the form of a body member 34 fabricated of glass or other suitable material, the output end of which is rendered conductive by means of a conductive coating 36 formed thereon. It is to be understood that the conductive coating 36 is coextensive with the output end of the channel multiplier array 16. In the case of FIG. 3, the target electrode 14 must be suitably mechanically and electrically mounted on some structure to withstand shocks of a preselected degree.

Referring to FIG. 4, there is illustrated the channel multiplier array 16 of FIG. 3 with the target electrode 14 supported directly on the conductive portions of the output end of the channel multiplier array 16. In this way, the target electrode 14 is rigidly supported by the array to permit it to withstand greater mechanical and electrical shock than prior camera tubes were capable of withstanding. Further, the effective capacitance of the storage target is greatly increased, thereby enabling a higher charge to be accumulated on the target, with the result of an improved signal-to-noise ratio capability. Further, secondary electrons produced at the target are physically constrained from spreading, thus reducing redistribution effects.

FIG. 5 illustrates a channel multiplier array 16 which incorporates the desirable features of both the assemblies of FIGS. 3 and 4. In FIG. 5, the body member 34 is provided with a conductive coating 36 at the output end thereof, as was the case with FIGS. 3 and 4. However, the array 16 is also provided with an insulating coating 40, the insulating coating 40 providing a spacing between the conductive coating 36 and the target electrode 14. With this arrangement, it has been found that the desirable features of FIG. 3, also incorporate the desirable features of FIG. 4 including confining the secondary electron emission, higher capacitance capabilities and greater mechanical and electrical shock withstanding characteristics. Additionally, the arrangement shown in FIG. 5 is advantageous when a target is used which relies for continuous operation on the conductivity between its faces, as is the case when a target made from a thin glass film is used. The advantage is that the positive potential necessary on the microchannel plate output electrode 36 for proper collection of the secondary electrons from the target is not able to leak through to the target thence to the scanned surface of the target, to produce a spurious positive potential.

In utilizing a channel multiplier array of the type described above, it has been discovered that spurious signals are generated at the output terminal due to the electron travel down the channel, which causes displacement current to be intercepted by a channel multiplier plate output electrode. This output signal appears as a negative polarity signal. Thus appearing as random white spots on the television monitor screen. The number of these spots is proportional to the input signal level to the photocathode. The spurious signal producing these white spots is only evident when the microchannel plate multiplier is operating with high electron gain. The magnitude of the spurious signal is proportional to the electron gain of the microchannel plate. When the microchannel plate is operating with low gain, the spurious signals are sufficiently small that they are masked by noise sources in the system. In previous systems, the equal and opposite positive signal generated along the channel walls was not coupled to the video output terminal due to the distributed capacitance and resistance along the channel. In order to substantially eliminate these spurious signals, the circuit of FIG. 6 was devised to preclude any changes in charge distribution along the channels from being coupled into the video output circuit.

In the improved video output circuit, the output terminal 48 is coupled to the array through a coupling capacitor 50 similar to that described in conjunction with FIG. 2, the other terminal of the capacitor 50 being connected to the output side of the channel multiplier array. Bias voltage for the system is provided through a first resistor 52 and a second resistor 54 which are connected to a bias conductor 56, the bias conductor 56 being connected to a source of positive DC potential. The distributed capacitance and resistance generated signal is shunted by means of a capacitor 60 and resistor 62 combination which are added to the prior circuit to couple the equal and opposite positive signal generated along the channel walls to the video output.

The channel multiplier plate electrodes are connected together in an alternating current sense by the capacitor 60 to form in effect a Faraday cage, and the input electrode is decoupled by the resistor 62. In practice, the capacitor 60 may be the capacitor plates formed by the two electrodes of the channel multiplier array, i.e., the input and output electrodes. One combination of capacitor 60 and resistor 62 which have been found to provide satisfactory results are where the input and output electrodes of the channel multiplier array, i.e., the input and output electrodes provide a ten picofarad capacitance and a resistor was provided in a range of approximately 470 Kilohms. However, it is to be understood that both resistor 62 and the capacitor 60 may have a wide range of values as long as the frequency l/CR is much lower than the high frequency cutoff of the video amplifier circuit. Actual choice of circuit parameters will be determined by consideration of space, convenience and loading of other circuit parameters.

While it will be apparent that the embodiments of the invention herein disclosed are well calculated to fulfill the objects of the invention, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope or fair meaning of the subjoined claims.

I claim:

1. In a camera tube system for deriving an output signal in response to an electron image of the type having an electron emissive member and a target member separated therefrom, and an electron gun for scanning said target member, the improvement comprising means for amplifying the electron emission and deriving an output signal in response to the scanning of said target comprising:

an electron multiplier disposed between said emissive member and said target member, said electron multiplier having an input face and an output face, an output conductive surface formed on said output face and an input conductive surface on said input face,

said target member being a charge storage layer positioned in the proximity of said output conductive surface so that said target member and said conductive surface are capacitively coupled;

output terminal means connected to said conductive surface for receiving voltages created on said conductive surface by said capacitive coupling;

and detection circuit means responsive to said voltages and yielding said output signal, said detection circuit means including means providing capacitive coupling between said input conductive surface and said output conductive surface.

2. Th improvement of claim 1 wherein said target member is spaced from said conductive surface.

3. The improvement of claim 1 wherein said conductive surface physically supports said target member.

4. The improvement of claim 3 further including an insulating layer between said conductive surface and said target member, said insulating layer being contiguous with both said conductive surface and said target member.

5. The improvement of claim 1 wherein said coupling is an alternating current coupling.

6. The improvement of claim 1 wherein said electron multiplier is a channel multiplier array having a body member, a plurality of channels formed in said body member and extending between said input surface and said output surface, and the interior surface of each of said channels being covered with electron emissive material.

7. The improvement of claim 6 wherein said target member is spaced from said conductive surface.

8. The improvement of claim 6 wherein said conductive surface physically supports said target member.

9. The improvement of claim 8 further including an insulating layer between said conductive surface and said target member, said insulating layer being contiguous with both said conductive surface and said target member.

10. The improvement of claim 1 wherein said detection circuit further includes impedance means electrically coupled to said input and output conductive surfaces for shunting spurious signals produced at said output surface by a high gain operation of said electron multiplier.

11. The improvement of claim 10 wherein said impedance means is connected to said input conductive surface for electrically isolating said input surface from ground so that the potential of said input surface can vary in response to shunted spurious signals and thereby eliminate said spurious signals.

12. The improvement of claim 11 wherein said impedance provides said capacitive coupling between said input conductivesurface and said output conductive surface, and said impedance means includes resistive means connected to said input conductive surface for isolating said spurious signals from said detection circuit means.

13. The improvement of claim 12 wherein said capacitive coupling is provided by the capacitance inherently formed by said input and output conductive surfaces.

14. In a camera tube system for deriving an output signal in response to an electron image of the type having an electron emissive member a target member separated therefrom, and an electron gun for scanning the target member, the improvement comprising means for amplifying the electron emission and deriving an output signal in response to the scanning of the target comprising:

an electron multiplier array disposed between said emissive member and said target member, said electron multiplier having an input face and an output face, an output conductive surface formed on said output face;

output terminal means coupled to said conductive surface for receiving image signals created on said conductive surface;

and output detection circuit means responsive to said image signals and yielding said output signal.

15. The camera tube of claim 14 wherein said elec tron multiplier further includes an input conductive surface on said input face, and said detection circuit means includes means providing capacitive coupling between said input conductive surface and said output conductive surface.

16. The camera tube of claim 15 wherein said detection circuit further includes impedance means electrically coupled to said input and output conductive surfaces for shunting spurious signals produced at said output surface by a high gain operation of said electron multiplier.

17. The camera tube of claim 16 wherein said impedance means is connected to said input conductive surface for electrically isolating said input surface from ground so that the potential of said input surface can vary in response to shunted spurious signals and thereby elimimate said spurious signals.

18. The camera tube of claim 17 wherein said impedance provides said capacitive coupling between said input conductive surface and said output conductive surface, and said impedance means includes resistive means connected to said input conductive surface for isolating said spurious signals from said detection circuit means.

19. The camera tube of claim 18 wherein said capacitive coupling is provided by the capacitance inherently formed by said input and output conductive surfaces. =l

Claims

1. In a camera tube system for deriving an output signal in response to an electron image of the type having an electron emissive member and a target member separated therefrom, and an electron gun for scanning said target member, the improvement comprising means for amplifying the electron emission and deriving an output signal in response to the scanning of said target comprising: an electron multiplier disposed between said emissive member and said target member, said electron multiplier having an input face and an output face, an output conductive surface formed on said output face and an input conductive surface on said input face, said target member being a charge storage layer positioned in the proximity of said output conductive surface so that said target member and said conductive surface are capacitively coupled; output terminal means connected to said conductive surface for receiving voltages created on said conductive surface by said capacitive coupling; and detection circuit means responsive to said voltages and yielding said output signal, said detection circuit means including means providing capacitive coupling between said input conductive surface and said output conductive surface.

12. The improvement of claim 11 wherein said impedance provides said capacitive coupling between said input conductive surface and said output conductive surface, and said impedance means includes resistive means connected to said input conductive surface for isolating said spurious signals from said detection circuit means.

14. In a camera tube system for deriving an output signal in response to an electron image of the type having an electron emissive member a target member separated therefrom, and an electron gun for scanning the target member, the improvement comprising means for amplifying the electron emission and deriving an output signal in response to the scanning of the target comprising: an electron multiplier array disposed between said emissive member and said target member, said electron multiplier having an input face and an output face, an output conductive surface formed on said output face; output terminal means coupled to said conductive surface for receiving image signals created on said conductive surface; and output detection circuit means responsive to said image signals and yielding said output signal.

15. The camera tube of claim 14 wherein said electron multiplier further includes an input conductive surface on said input face, and said detection circuit means includes means providing capacitive coupling between said input conductive surface and said output conductive surface.

19. The camera tube of claim 18 wherein said capacitive coupling is provided by the capacitance inherently formed by said input and output conductive surfaces.