US2517807A - Neutralization of deflection field between photocathode and mosaic of pickup tube - Google Patents

Description

Aug. 8, 1950 G c szlKLAl 2,517,807

NEUTRALIZATION F EFLECTION FIELD BETWEEN PHoTocATl-IODE AND MOSAIC 0E PICKUP TUBES Filed May 30. 1945 difieren/v elf/v 6s @ANN/N6 i 74 INVENTOR.

'fa/eos .Sz/KM/ Patented Aug. 8, 1950 NEUTRALIZATION '0F DEFLECTION FIELD BETWEEN PHOTOCALTHODE AND MOSAIC F PICKUP TUBE George C. Sziklai, Princeton, N. J., assigner to Radio 'Corporation of America, a corporation of Delaware Application May 30, 1945, Serial No. 596,686

y iClaims. l.

The present invention relates to image-multiplier camera tube apparatus of the type used in television transmitting systems, and more particularly to a method and means for improving the resolution of such tubes and hence the quality of the reproduced image.

Television pickup or camera tubes which operate upon the image-multiplication principle generally include a photocathode electrode upon which an optical image to be transmitted is focused by means of a lens system. Light, falling upon the photocathode, causes electrons to be released from the inner surface thereof in the form of an electron image. The density of the electrons at any point in this electron image corresponds to the brilliance of the homologously located point on the optical image.

While an electron image is generally produced in image-multiplier television camera tubes in the above manner, it is converted into video or image signal outputs in several ways depending upon the species of camera tube employed. In that type of camera tubeknown as the image Iconoscopef for example, the electrons released from the photocathode are focused electrostatioally or electro-magnetically by means of an electron lens upon a mosaic surface remotely spaced therefrom but usually located in a plane parallel to the photocathode. When the electron image is caused to impinge upon the mosaic as a relatively high velocity electron stream, it releases secondary electrons and so produces on the mosaic electrostatic charges which are stored and which are of magnitudes substantially proportional to the light intensity of the initial image at an homologously related point.

The mosaic is then scanned by a high velocity cathode ray beam developed within the camera tube. The scanning operation brings about the release of additional secondary electrons from the mosaic, with the number of additional 'seca ondary electrons released being dependent upon the magnitude of the electrostatic charge possessed by the particular elemental area of the mosaic being instantaneously scanned by the cathode ray beam.

The mosaic electrode of the image Iconoscope is provided With a metallic signal coating or plate on the side thereof remote from that scanned by the cathode ray beam. The changes in capacity between the elemental areas of the mosaic and this coating or plate due to the scanning action above explained provide a changing voltage at the signal plate so that the signal output Qi @he camera tube is obtained by connecting a load circuit thereto.

In another type of television camera tube known as the image Orthicon, the electrons released from the photocathode surface are drawn longitudinally of the tube under the influence of an accelerating eld to impinge on one side of a mosaic electrode. The impacting electrons striking the mosaic at moderately high velocity cause secondary electrons to be released from the mosaic at the impacted areas, with the release of electrons being proportional to the number of impacting electrons. The released secondary electrons are collected upon a screen electrode positioned adjacent the mosaic surface. The mosaic electrode is of a somewhat different variety from that utilized in the previously described image Iconoscopa One preferred form of mosaic structure is described in the copending application of Albert Rose, Serial No. 407,132, led August 8, 1941, now U. S. Patent No. 2,403,239. As set forth in the Rose application. the mosaic is, broadly speaking, a glass sheet of minute thickness which receives on one side the impacting electrons released from the photocathode. The impacting electrons cause electrostatic charges representative of the optical image to be transmitted to be built up on the mosaic.

The charges are then released by the scanning action of a cathode ray scanning beam developed in the image Orthicon, which instead of scanning that side of the mosaic electrode impinged by the electrons emitted from the photocathode, as in the image Iconoscope above referred to, scans the reverse side of the mosaic. The scanning beam electrons, traveling at a relatively low velocity, impinge on this reverse side of the mosaic and restore each elemental mosaic area to an equilibrium state by neutralizing the negative charge deficiency existing thereon. The number of scanning beam electrons collected at any instant depends on the magnitude of the negative charge deficiency possessed by the particular surface area of the mosaic being instantaneously scanned by the cathode ray beam. Due to the differences in the amounts of positive charge acquired by the various elemental areas of the mosaic surface, the total number of electrons present in the scanning beam is not always required to neutralize a particular mosaic area. The excess beam electrons, or, in other words, those not collected by the mosaic, are returned toward that portion of the camera tube in which the scanning beam is developed, where they are collected. The currents which are caused to flow in a connected load circuit as a result of such collection of the electron stream constitute the signal output of the tube.

Each of the particular camera tube types above referred to accordingly is so designed that electrons emitted from a photocathode electrode in response to the illumination thereof by a light image are caused to impinge on a mosaic electrode which is spaced apart from the photocathode. In passing between the photocathode and mosaic, the electrons are in the form of an electron image representative of the optical image. This electron image must be preserved substantially without change in order to avoid distortion in the optical image reproduced by the television receiver.

The scanning cathode ray beam, which is developed in each of the particular types of camera tube above referred to, is periodically deflected horizontally at a relatively high rateV in order to scan the mosaic electrode. Each scanning movement of the cathode rav beam occurs (for a 525 line image raster repeated 30 times per second) within a time interval of l/isflo of a second, and part of this time interval (usually less than is used to return the scanning beam to the starting point of the next scanning trace. While the line deflection (usually horizontal) of the cathode ray beam may be produced by electrostatic means, it is also frequently accomplished electro-magnetically, in which case a split horizontal deflection coil is placed across the neck of that part of the camera tube in which the cathode ray scanning beam is developed. A sawtooth wave of current, iiowing through this deflection coil, `produces an electromagnetic iield which, by varying linearly with time, eiiects the desired linear line deiiection traces of the cathode ray beam relative to the impacted target.

Both of the camera tube types discussed above are customarily proportioned so that the line or horizontal deflection coil is located in close proximity to the image section of the camera tube, or, in other Words, to that section of the camera tube containing the photocathode and mosaic electrodes. This design of the camera tube is made necessary in part by limitations on the physical lengths of the paths traversed by the electron image and the cathode ray scanning beam, as the difficulty of controlling an electron path usually increases with an increase in the length of the path.

Due to the high rate at which the current in the line or horizontal deflection coil changes in value, and due also to the proximity of this deflection coil to the image section of the camera tube, the magnetic eld surrounding the line or horizontal deection coil has a detrimental eiect on the accuracy with which the stored electrostatic charges on the mosaic are-caused to represent an electron image corresponding point-forpoint to the optical image focused on the photocathode. This results from the fact that the varying magnetic eld produced by the deiiection coils gives rise to a corresponding varying lateral displacement of the electron image emitted from the photocathode prior to the impingement of this electron image on the mosaic. This high. rate of lateral oscillation, or jiggling, of the electron image during a frame sequence causes the reproduced optical image viewed at the receiver to be blurred, since the electrons in the electron image representing a single elemental light area of the optical image not onlyv impinge on that one particular elemental mosaic surface area corresponding to the single light area, but also on the elemental mosaic surface areas contiguous to that one particular elemental mosaic area. This brings about a condition in which each elemental light area of the optical image is, in effect, enlarged on the mosaic so that adjacent elemental areas overlap one another. Consequently, the resolution of the camera tube is adversely ailected, and the quality of the reproduced optical image at the receiver correspondingly lowered.

Attempts have been made to overcome the eiect of the scanning eld on the image section of an image-multiplier television camera tube by shielding the deflection coil. While this expedient is satisfactory under certain circumstances, it has been found that in many instances, especially in the case of small image Orthicon tubes, no practical shielding will provide adequate isolation of the electron image from the scanning field.

According to a feature of the present invention, means are provided for overcoming the effects on the image section o an image-multiplier television camera tube of the magnetic iield produced by the line or horizontal deiection coil. Furthermore, the means of the present invention eliminates the necessity for employing any shielding between these parts of the camera tube assembly.

One object of the present invention, therefore, is to provide a method and means for improving the resolution of a television camera tube of the image-multiplier type.

Another object of the present invention is to improve the accuracy with which the electrostatic charges on the mosaic electrode of an imagemultiplier television camera tube are caused to represent an electron image corresponding pointfor-point to the optical image focused on the photocathode of the camera tube.

A still further object of the present invention is to provide a method and means for overcoming the eiects on the image section of an imagemultiplier television camera tube of the electromagnetic field produced by the line or horizontal deflection coil associated therewith.

An additional object of the invention is to eliminate the necessity for shielding the line or horizontal deflection coil associated with an imagemultiplier television camera tube from the image section of the tube.

Other objects and advantages will be apparent from the following description of preferred forms of the invention and from the drawings,in which:

Fig.1 illustrates schematically one form of the present invention as applied to an image-multiplier television camera tube of the type wherein a high Velocity scanning beam is utilized, such, for instance, as in the well known Iconoscope; and,

Fig. 2 illustrates schematically one form of the present invention as applied to an imagemultiplier television camera tube of the type wherein a low velocity scanning beam is utilized, such, for instance, as in the well known Orthicon.

Referring rst to Fig. 1, there is shown an image-multiplier television camera tube l0 of the high velocity scanning beam type, such as the Iconoscope Many of the component parts of camera tube l0 for developing a video signal output are known in the art, and hence will not herein be described in detail. However, tube I0 will be understood to include a photocathode electrode I2 on which an optical image of an Object i4 iS focused by means such as a lens I6. `Illumination falling on photocathode I2 causes anemission of electrons from the inner surface thereof, such emission, as is well known in the art, being in the form of an electron image each point of which substantially corresponds in density to the intensity of the illumination on the corresponding point of photocathode I2.

Electrons thus emitted from the inner surface of photocathode I 2 are focused by means such asan electron lens I8 on one surface of a mosiac electrode 20, as is known in the art. When the electrons` released from photocathode I2 impinge on mosiac 20, they release secondary electrons from the mosiac, and so form a charge deflciency on `the mosaic. The secondary electrons Ware collected'by a collector anode (not shown) which may be a metallic coating on a portion of the inner wall of tube I0.

In another section of camera tube I 0, and spaced apart from the photocathode I2 and lens I8, is an electron gun 22 w-hich may be of any suitable type. Gun 22 develops a cathode ray scanning beam indicated in the drawing by the reference character 24, The cathode ray beam 24 scans the mosaic electrode 20, releasing from the latter additional secondary electrons. The variations in electrostatic charge on the mosaic 20 as a result of the scanning operation above set forth are capacitively transferred to a signal plate or metallic coating 26 on the side of mosaic 2D remote from that scanned by the cathode ray beam 24, and separated therefrom by a suitable dielectric layer. The effects of the potential changes on the signal plate 26 are then transferred over an output conductor 28 to an appropriate load circuit (not shown) to constitute the video signal output of the camera tube l0.

The cathode ray scanning beam 2'4 is deflected at a relatively high rate in order to effect a lineby-line scanning of the mosaic 20. A similar deflection is brought about in a mutually perpendicular direction to produce the field deflection so that the complete image raster is traced. This is not shown for reasons of simplicity and because it is well understood in the art.

The means for producing this line or horizontal deflection includes a deflection coil 30 connected to a line or horizontal deflection generator 32. Deflection coil 30 is generally of the split or two-section type, and is usually disposed relative to the neck of that portion of the camera tube 50 containing the electron gun 22 so that the longitudinal axis of the split coil is substantially perpendicular to the normal :centered or undeflected position of the cathode ray beam 24.

The current output of generator 32 has a waveform of substantially sawtoo-th configuration, as indicated by reference character 34. This current, flowing through the deflection coil 30, produces an electro-magnetic field, which, by varying substantially linearly with time, effects the desired linear deflection of the cathode ray beam 24.

The cyclically varying magnetic field associated with the deflection coil 30, however, causes the electron image emitted from the photocathode I2 to deviate from its normal path as explained above. This condition is overcome in the present invention through the use of a neutralizing coil 36 connected to the deflection generator 32 in series with the cathode ray beam deflection coil 3|). The neutralizing coil 36 preferably has rela- 4 tively few turns compared to the deflection coil 30, and is arranged to produce an electro-magnetic field, which, in the vicinity of the image section of cameratube I 0, is in opposition to the magnetic field created by the line or horizontal cathode ray beam deflection coil 30. By proper design and positioning of the neutralizing coil 36 relative to the image section of camera tube I0, the auxiliary electro-magnetic field produced by the coil 36 may be made of equal strength and in opposition to the line or horizontal deflection eld. Accordingly, the electro-magnetic elds of the two coils 30 and 36 will effectively cancel one another in that section of camera tube I0 traversed by the electron image emitted from photocathode I2. The strength of the electro-magnetic field produced by the neutralizing coil 36, however, is insufficient to have'any appreciable effecten 4the deflec-YW tion of the cathode ray beam 24 due to the action of the line or horizontal deflection coil 30.

Fig. 2 shows one form of the present invention as applied to an image-multiplier television camera tube 5D of the low velocity scanning beam type, such as that known in the art as the Orthiconf This type of tube, like the high beam velocity species referred to above, includes a photocathode electrode 52 on which an optical image of an object 54 is focused by means of a lens or lens system 56. Illumination falling on photocathode 52 causes an emission of electrons from the inner surface thereof in the form of an electron image similarly to the production of the electron image by the photocathode I2 of the camera tube IIJ illustrated in Fig. 1.

The velocity of the electrons thus emitted from the surface of photocathode 52 is increased by an accelerating electrode 58 toward a mosaic electrode 6U. The various electrodes of tubes I0 and 50 in Figs. 1 and 2 respectively are maintained at their proper operating potentials by any suitable t-ype of power supply, which has been omitted from the drawing for the sake of clarity of illustration. A television camera tube of the low velocity type which is similar in many respects to the camera tube 50 of Fig. 2, and including means for maintaining the proper operating potentials of the tube electrodes, is described in a copending application of Robert R. Thalner, Serial No. 593,153, filed May 11, 1945, now U. S. Patent No. 2,451,640, and the structure of the mosaic electrode may be as disclosed in the abovementioned Rose application.

The electron image impacting mosaic 6U causes secondary electrons to be released therefrom. These secondary electrons are collected by a screen E2. The release of secondary electrons by a particular elemental area of mosaic 5U leaves such area with a positive charge, the value of which is dependent upon the density of the electron image at that particular point.

The side of the positively charged mosaic 60 opposite to that impacted by the electrons released from photocathode 52 is then scanned by means of a low velocity cathode ray scanning beam 64 produced by an electron gun B at the opposite end of tube 50 from the photocathode 52. The structure of the electron gun B6 may be of any suitable type known in the art.

As the cathode ray beam 64 scans the side of mosaic 60 opposite to that impacted by the electrons released from photocathode 52, electrons from the scanning beam 64 neutralize the positively charged mosaic elements, the scanning beam supplying sufcient electrons to make up the negative charge deficiency existing on each elemental mosaic area. If a particular mosaic element is not positively charged, or, if such positive charge is small enough so that all lof the electrons available in the scanning beam during the instant of passage are not required to make up the negative charge deciency on that mosaic element, then the remaining electrons in the scanning beam, or, in other words, those not employed to neutralize the electrostatic charge representing each image point or element, are caused to return along a path substantially parallel to the scanning beam Sli toward the end of camera tube 50 from which they were emitted. Upon arriving at the end of tube 50 containing the electron gun 66, the returned electrons are collected by a signal plate 68 forming a part of the tube output circuit 10. v

The cathode ray scanning beam 64 of camera tube 50 is deiiected to scan mosaic $0 line-byline and also more slowly in a mutually perpendicular path in substantially the same manner as is the scanning beam 2l! of camera tube IE! in Fig. 1. The line or horizontal deecting means in Fig. 2 includes a line or horizontal deflection coil 12 mounted externally of tube 53 and located between mosaic Sil and the electron gun 65. The line or horizontal deflection coil 12 may be split into two sections as illustrated, and is usually so positioned that the longitudinal axis of the coil is substantially perpendicular to the normal centered or undeiiected position of the cathode ray scanning beam 54, in a manner analogous to the positioning of the coil in Fig. 1. A focusing coil (not shown) may be employed, if desired, to create an axial electro-magnetic eld, so that the amplitude of deflection of the cathode ray beam 64 is proportional to the magnitude and axial length of the electro-magnetic iield of the deflection coil 12, and inversely proportional to the magnitude of the axial electro-magnetic field produced by the focusing coil.

Cyclically varying current for the line or horizontal deflection coil 12, having a waveform of substantially sawtooth configuration such as indicated by the reference character 1t, is supplied by a line or horizontal deflection generator 16. A neutralizing coil 18 is also connected to the line or horizontal deflection generator 16 in series with the line or horizontal deection coil 12. The neutralizing coil 18 is located exterior of the camera tube 5U, and adjacent to the image section thereof. The position of neutralizing coil 18 is so arranged relative to the envelope of camera tube 5l] that the electro-magnetic eld produced by the coil 18 is substantially in opposition to the electro-magnetic field produced by the cathode ray beam deflection coil 12 in the vicinity of the image section of the tube 5U. The electro-magnetic iield of the neutralizing coil 18, however, does not have any appreciable effect on the deflection of the cathode ray beam 64 due to the action of the line or horizontal deflection coil 12.

In order to completely neutralize or cancel the oscillatory or jiggling effect of the electromagnetic iield of the deflection coil 12 on the electron image emitted from photocathode 52, the electro-magnetic field of the neutralizing coil 18 in the vicinity of the image section of camera tube 58 must not only be in opposition to the electro-magnetic iield of the scanning coil 12, but it must be equal thereto in strength. In order to premit a variation in the strength of the electro-magnetic eld produced by the neutralizing coil 18, so that the eld may be made equal to the electro-magnetic iield established by deilection coil 12 in the vicinity of the image section of tube 50, an adjustable resistor is provided in parallel with the neutralizing coil 18.` By varying resistor 80, the strength of the counter-deecting electro-magnetic eld produced by the neutralizing coil '18 may be made equal to the strength of the electro-magnetic field produced by the deectio-n coil 12 insofar as its elect on the electron image emitted from photocathode 52 is concerned. Complete lateral stability of the electron image is thus brought about, and the resolution of camera tube 50 is materially improved as well as the quality of the optical image reproduced at the television receiver.

It will be clear that, if desired, an adjustable resistor element, such as indicated by the reference character 80 in Fig. 2, may be employed in parallel with the neutralizing coil 36 in Fig. 1. This permits adjustment of the strength of the electro-magnetic eld produced by the neutralizing coil 36 in the same manner that the resistor 8U regulates the strength of the electro-magnetic iield of the neutralizing coil 18 in the circuit of Fig. 2.

Having thus described my invention, I claim:

l. In a television system including a camera tube of the type having a photocathode adapted to release electrons in response to the reception thereby of light from an optical image, said electrons, in the form of an electron ima/ge corresponding point-for-point in density to said-optical image, being then caused to impinge upon a mosaic electrode within said camera tube, said system also including means for developing and electro-magnetically deecting a cathode ray beam Within said tube to scan said mosaic electrode to thereby produce an output signal from said tube representative of said optical image, the combination of neutralizing means adapted to produce an electro-magnetic field, means for energizing said neutralizing means synchronously with said cathode ray beam deflecting means and means for positioning said neutralizing means relative to said camera tube so that the electromagnetic field produced by said neutralizing means will tend to oppose the electro-magnetic field of said cathode ray beam deflecting means substantially only in that portion of said camera tube between and including said photocathode and said mosaic electrode.

2. A television system in accordance with claim 1, in which said cathode ray beam deiiecting means includes at least one coil positioned adjacent said camera tube and means for causing cyclically varying current to ow through said coil to thereby produce a periodically varying electro-magnetic eld surrounding said coil. which acts to deiiect said cathode ray beam, in which said neutralizing means also includes at least one coil, and in which said positioning means includes means for positioning ythe coil of said neutralizing means adjacent said camera tube and so arranged with respect to the coil of said cathode ray beam deecting means that the varying electro-magnetic eld produced by the coil of said neutralizing means in response to the synchronous energization of both said coils will tend to oppose the varying electro-magnetic iield produced by the coil of said cathode ray beam deflecting means substantially only in that portion of said camera tube between and including said photocathode and said mosaic electrode.

3. In a television system, a camera tube of the type in which electrons are released from a photocathode element to impinge on a mosaic electrode which is then scanned by an electron beam developed Within said tube, said electron beam being deflected to scan said mosaic electrode by the action of an electro-magnetic field produced as a result of cyclically varying currentv flowing through a iirst coil positioned exterior of and adjacent to that portion'of said camera tube in which said electron beam is developed, a second coil positioned exterior of and adjacent to that portion of said camera tube traversed by the electrons released from said photocathode element in passing to said mosaic electrode. means for causing at least a portion of the cyclically varying current flowing through said iirst coil to also flow through said second coil. and means for so positioning the said coils thatthe electro-magnetic elds of the coils in that portion of said camera tube traversed by the electrons released from said photocathode are substantially in opposition.

4. In television apparatus, the method which comprises producing an electronic current image representative of a subject, focusing the produced electronic current image upon a target area there to develop a charge image under the control of the electronic current image, developing an electron beam and directing it toward the charge image on the said target area, developing a iirst electromagnetic leld to deect the electron beam according to a selected s-cansion pattern to scan the charge image and thereby to release signals representative thereof to an output circuit, and developing a second electro-magnetic eld concurrently with the rst electro-magnetic field and of equal strength and opposite phase thereto in the region wherein the electronic current image ows so that the eifect of the said iirst electro-magnetic field upon the electronic current image is neutralized and nullified.

5. In television apparatus, the combination of means for producing an electronic current 'image representative of a subject, means for focussing the produced electronic current image upon a target area there to develop a charge image under the control of the electronic current image, means for developing an electron beam and for directing it toward the charge image on the said target area, means for developing a rst electro-magnetic field to deflect the electron beam according to a selected scansion pattern to scan the charge image and thereby to release signals representative thereof to an output circuit, and means for developing a second electro-magnetic leld concurrently with the first electro-magnetic eld and in the region wherein the electronic current image flows so that the effect of the said first electromagnetic field upon the electronic current image in that region is neutralized and nullied.

GEORGE C. SZIKLAI.

REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS Number Name Date 2,085,742 Farnsworth July 6, 1937 2,164,906 Deserno et al. July 4, 1939 2,220,303 Tingley Nov. 5, 1940 2,355,110 Rosenthal Aug. 8, 1944 2,387,608 Paumier Oct. 23, 1945 FOREIGN PATENTS Number Country Date 481,944 Great Britain Mar. 18, 1938 526,954 Great Britain Sept. 30, 1940 806,582 France Dec. 19, 1936

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