US3221209A - Electron-image tube apparatus with improved acuteness of the image corners - Google Patents

Description

Nov. 30, 1965 H. GREIN 3,221,209

-IMAGE TUBE ELECT AR US WITH IMPROVED UTENESS T IMAGE CORNERS File ay 13, 1965 a a a 4 5 Fig.1 1

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Jnvenfol Attorney United States Patent 3,221,209 ELECTRON-IMAGE TUBE APPARATUS WITH IMPROVED ACUTENESS OF THE IMAGE CORNERS Hermann Greiner, Darmstadt, Germany, assignor to Fernseh G.m.b.l-l., Darmstadt, Germany Filed May 13, 1963, Ser. No. 279,879 Claims priority, application Germany, May 12, 1962, F 36,788 9 Claims. (Cl. 315-10) This invention relates to electron-image tube apparatus in which an electron image is transferred between plane electrodes. The invention has particular, though not exclusive application in connection with television pickup tubes, in relation to which it will be chiefly described.

In modern television cameras employing 3-inch imageorthicon pickup tubes the resolution has already approximately reached the theoretical value for 5 mc./ s. In the central zone of the picture field the resolution has in general a value of more than 80% of the black-white interval at 500 kc./s. A certain loss of resolution arises, however, at the edges of the picture, and in particular at the corners Where it may amount to only some 50% of the maximum black/white interval. It is therefore desirable as far as possible to equalize the resolution in the corners to that at the centre-of the picture.

By careful adjustment of the potential applied to the photocathode of an image-orthicon pickup tube it may be determined that the maximum resolution for the image section of the tube occurs at different settings for the centre of the picture and for the margins and corners, where the photocathode requires a different potential applied to it. If, for example, the photocathode voltage for optimum resolution at the centre of the picture is -460 v., the voltage for optimum resolution in the corners of the picture of the photocathode potential may be found to be some -485 v.; in other words, a certain amount of field curvature arises in the image transfer process. In practical operation precedence is always given to the centre of the picture, so that the effect of the field curvature is at its maximum.

The origin of the field curvature mentioned may be understood from the following discussion. Because of the electrode arrangement employed in the image-converter section of an image-orthicon pickup tube the equipotential lines in the image section do not lie in planar surfaces. In front of the photocathode they are often curved to somewhat of a bell form. As the electrons are withdrawn from the photocathode, owing to their very low initial velocity they at first describe a path running exactly perpendicular to the equipotential surfaces. Since the ratio of the electric field strength to the magnetic field strength at the edges of the photocathode is very high in comparison with that at the centre, an electron from the centre of the photocathode will follow a rectilinear path to the target, while an electron emitted from an edge of the photocathode will follow an outwardly curved path. As a first result of this the paths of marginal electrons are therefore somewhat lengthened. More important, however, these electrons now cross the lines of force of the magnetic field at an angle which is not negligible, so that they now follow spirally curved paths resulting in a substantial elongation of the path length. Marginally-emitted photoelectrons therefore form nodes before reaching the target. This effect can apparently only be abolished if it is possible to match the paths of the electrons and the lines of force of the magnetic field to one another, so that the electrons move as far as possible along the magnetic lines of force and the marginal electrons are prevented from following 3,221,209 Patented Nov. 30, 1965 spiral paths. The resultant shortening of the paths of the marginal electrons ensures that nodes are formed in the plane of the target by marginal electron bundles as well as by axial bundles.

Since the equipotential surfaces of the electric field are determined by the arrangement of the electrodes within the image-orthicon tube and, in view of the complexity of the whole system which could only be altered only at great expense, the only alternative is to modify the paths of the magnetic lines of force in such a manner that the field curvature is equalized.

Focusing systems for the image converters of television camera tubes have already been proposed in which an additional magnetic field is produced in the neighbourhood of the photocathode in order to improve the image forming conditions. In a supericonoscope, for example, in which a magnetic field concentrated in the neighbourhood of the photocathode is employed, a ferromagnetic ring has been arranged within the magnetic focusing coil so that its inner margin extended to the neighbourhood of the picture field. The purpose of this arrangement was to suppress the image aberrations resulting from the S-formed distortion due to the use of a short magnetic focusing coil. It has also been attempted to improve the image quality by the inclusion of a winding in the interior of the magnetic core. In image-orthicon pickup tubes it has also been attempted to increase the magnetic field strength in the plane of the photocathode as com pared with that in the plane of the target electrode by arranging on the end surface of the focus coil surrounding the image-orthicon tube a winding of disc form which is traversed by an adjustable current. This arrangement was employed especially with 4 /z-inch image-orthicon tubes in order to produce a divergent magnetic field through which the image magnification between photocathode and storage target was increased. Increase of the resolution in the corners of the picture was not the purpose of this system and according to present experience such an effect was not in fact produced. Finally, such an additional winding placed on the end face of the main focusing coil was used in order to make the magnetic field in the image-converter section of the pickup tube as homogeneous as possible.

The underlying purpose of the invention, of improving the resolution in the corners of the picture, is also made difficult because, owing to the already specified optical conditions in the television camera and the fixed size of the scanning pattern on the target electrode, the conditions under which the image is transferred from the photocathode to the target electrode can be altered only very little. The use of an iron pole-piece, possible with an additional Winding embedded in it, or an-auxiliary coil applied to the end face of the deflection coils, does not therefore lead to the required result, for these measures introduce an undesirable modification of the image forming conditions and also yield only an inadequate reduction of the loss of resolution in the corners of the picture.

To solve the problem underlying the invention, of improving the marginal resolution of the electron image projected on to the storage target in an image-orthicon pickup tube, in accordance with the present invention there is placed directly upon the faceplate of the pickup tube which carries the photocathode an auxiliary coil of small radial height which is traversed by current in the same direction as that in the main focusing coil and is surrounded by this main coil. Preferably there exists between the outer margin of the auxiliary coil and the inner surface of the main focusing coil a distance which is at least as great as the radial height of the auxiliary coil winding. This arrangement ensures that the magnetic field is modified chiefly in the rear neighbourhood of the auxiliary coil and a marked restriction of the magnetic field occurs in the neighbourhood of the corners of the picture field on the photocathode, while the field strength at the centre of the photocathode is relatively little changed.

This arrangement may be still further improved by adding yet another auxiliary coil in the interspace between the first-mentioned auxiliary coil and the main focusing coil, this further auxiliary coil being traversed by a current in the direction opposite to that in the firstmentioned auxiliary coil. The result of this arrangement is that a radial dipole field is produced, which allows a high field-strength to be generated in the immediate neighbourhood of the edges of the picture field on the photocathode, this field decaying rapidly towards the centre of the photocathode. This simple arrangement allows a parabolic field distribution to be superimposed upon the substantially constant field strength of the main focusing coil. It has been found that in the case of the 3-inch irnage-orthicon an increase of the field strength at the corners of the picture of some 15% as compared with that at the centre of the picture is necessary in order to eliminate the field curvature. With a voltage on the photocathode suitable for obtaining maxim-um resolution at the centre of the picture, the central resolution produced is then more than 80%, as before, while the marginal resolution is at least 65%. Variation of the voltage applied to the photocathode shows that the field curvature is made practically to vanish by the use of the present invention, since it i-s not possible to find a photocathode voltage at which the corner resolution can be further increased.

The invention will now be more particularly described with reference to the accompanying drawings, comprising FIGURES 1 and 2, of which:

FIGURE 1 is a schematic representation of a partial longitudinal section through an image-orthicon pickup tube and its associated focusing coil, such as has been employed hitherto, showing the equipotential surfaces of the electrostatic field and the lines of force of the magnetic focusing field, together with the resultant electron paths, and

FIGURE 2 is a schematic representation of a partial longitudinal section through a 3-inch image-orthicon pickup tube provided, in accordance with the invention, with an auxiliary focusing coil of the dimensions found by experiment to be the most advantageous.

In FIGURE 1, 1 is the envelope of an irnage-orthicon pickup tube which carries on its faceplate a photocathode 2. In addition the tube contains a storage target 3 and an accelerating electrode 4, which serves to produce a suction field for electrons emitted from the photocathode. Other electrodes contained in the tube, which serve to produce a scanning beam and for the collection of the return beam forming the picture signal are not shown, since the construction of an image-orthicon tube is familiar to the expert and since the present invention is not concerned with these portions of the tube.

The tube is surrounded by a long focusing coil 5, the field strength of which is so designed that both the electrons in the image-converter section formed by the electrodes 2, 3 and 4 and also the electrons in the scanning beam are focused upon the storage target. The magnetic lines of force of the focusing coil 5 are indicated by broken lines a, while the equipotential surfaces of the electrostatic field in front of the photocathode are indicated by the broken lines b. These equipotential surfaces in front of the photocathode will be seen to be somewhat of bell shape. As mentioned in the introduction, an electron leaving the photocathode at a point near its edge will follow an outwardly curved path as indicated by dotted line A, while an electron emitted from the centre of the photocathode will travel rectilinearly to the target as shown by do ted line B. The curvature of A the path of the marginal electron is exaggerated in the drawing for the sake of clarity in illustration. The result of these field conditions in the known arrangement is that a loss of resolution occurs at the edges of the picture.

FIGURE 2 shows an arrangement according to the present invention. The figure shows to an enlarged scale the image converter section of an image-orthicon pickup tube, the electrodes within the tube being omitted for the sake of simplicity in illustration. In this embodiment of the invention there is placed on the faceplate of the envelope 1 which carries the photocathode a member 6 in the form of a frame, which is provided with an auxiliary coil 7. The inner edge of auxiliary coil 7 lies closely adjacent to the area of the photocathode occupied by the optical image which in operation is projected thereupon, and its outer edge has a substantial separation from the inner edge of the main focusing coil 5. The frame member 6 is provided at its periphery with several projections which engage in corresponding grooves in the main focusing coil 5, so that the frame member 6 is firmly anchored to the coil 5. The magnetic field produced by the coil 7 has the same main direction as that of the coil 5, and the coil 7 may be so designed that it is traversed by the same focusing current.

The advantage yielded by the invention is obtained because the image forming field of the photocathode interacts with that of the coil to produce a magnetic field with a radial component which gives rise to a restriction of the main magnetic field. Owing to this restriction the magnetic lines of force become curved towards the margins in a manner similar to the paths of the electrons shown by dotted line A in FIGURE 1. In this manner it is ensured that, with the magnetic field strength correctly adjusted, the electrons are imaged in the plane of the storage target both at the edges and in the centre of the target electrode 3, so that the field curvature is substantially abolished. In certain types of tube, in which it is desirable to vary as little as possible the image magnification between photocathode and storage target, a further improvement may be obtained if the focusing field produced by the auxiliary coil 7 is compensated, as least as regards electrons emitted at the centre of the photocathode, by a further auxiliary coil 8 fitted in the space between the auxiliary coil 7 and the margin of the frame member 6.

For the former 3-inch image-orthicon cameras there is specified for a distance of mm. between photocathode and storage target, a picture diagonal 10 of some 45 mm. and a raster height 11 on the storage target of some 35 mm. These conditions are necessary in view of the fact that even within the image space the magnetic field of the main focusing coil diverges somewhat. Since, however, owing to unavoidable winding tolerances, the lines of force of the coil 5 are often found to be much too divergent, a certain reduction of the divergence is desirable. This is likewise produced by the field of the auxiliary coil if the dimensions given below are adopted: Outer diameter of coil 7 =distance between photocathode and target Ratio of radial height to width of winding section=5:2 Radial height of winding=4 to 5 mm.

The measures described above for improving the corner resolution may naturally also be employed with advantage in image converter tubes, in which there likewise exists the problem of transferring an electron image between plane surfaces, of which in this case one is a photocathode and the other is a fluorescent screen lying at a lu'gh potential or a permeable electron-multiplying membrane.

What is claimed as new and desired to be secured by Letters Patent is:

1. Electron image tube apparatus, comprising an electron image tube, said electron image tube includmg an envelope having a planar member, a first plane electrode positioned on said planar member in said envelope, a second plane electrode in said envelope, means for producting electrostatic and electromagnetic fields in said envelope, and means for transferring an electron image having a central area and peripheral corner areas from said first plane a first auxiliary coil positioned adjacent the first plane electrode of said electron image tube, said first auxiliary coil being of substantially annular configuration and having an inner periphery, an outer periphelectrode to said second plane electrode under the 5 ery and a radial width between said inner and outer combined effect of said electrostatic and electromagperipheries, the inner periphery of said first auxiliary netic fields; coil lying adjacent the corners of the area of said an elongated main magnetic focusing coil surrounding first plane electrode occupied by said image, the outer said electron image tube; and periphery of said auxiliary coil being spaced from an auxiliary coil positioned immediately adjacent the 10 said main focusing coil by a distance at least as great first plane electrode of said electron image tube, said as the radial width of said first auxiliary coil, said auxiliary coil being of substantially annular confirst auxiliary coil being adapted to be traversed by figuration and having an inner periphery, an outer a current flowing in the same direction as a current periphery and a radial width between said inner and flowing in said main focusing coil; and outer peripheries, the inner periphery of said auxsecond auxiliary coil positioned coaxially with and iliary coil lying adjacent the corners of the area of around the outer periphery of said first auxiliary coil said first plane electrode occupied by said image, the within said main focusing coil and adapted to be outer periphery of said auxiliary coil being spaced traversed by a current flowing in a direction opposite from said main focusing coil by a distance at least. that a Current flowing n said ma n focusing coilas great as the radial width of said auxiliary coil, Electron image tube apparatus, comprising said auxiliary coil being adapted to be traversed by a n electron image tube, said tr n image tube incurrent flowing in the same direction as a current cluding an envelope having a planar member, a first flowing in said main focusing coil to provide an plane electrode positioned on said planar member in auxiliary electromagnetic field interacting with the said envelop a second Plalle tr d in said nelectromagnetic field produced in said electron image v l p ns f r producing electrostatic and electrotube to reshape the electromagnetic field produced magnetic fields in said envelope, and means for transin said electron image tube in a determined manner ferring an electron image having a central area and in the corner areas of said image to substantially peripheral corner areas from said first plane electrode eliminate field curvature and to substantially avoid to said second plane electrode under th bin d variation of the electromagnetic field produced in effect of said electrostatic and electromagnetic fields; said electron image tube in the central area of said an elongated main magnetic using l surr unding image. said electron image tube; 2. Electron image tube apparatus, comprising a first auxiliary coil positioned adjacent the first plane an electron image tube, said electron image tube inelectrode of said electron image tube, said first auxcluding an envelope having a planar member, a first iliary coil being of substantially annular configuraplane electrode positioned on said planar member in tion and having an inner periphery, an outer said envelope, a second plane electrode in said en- P P Y and a radial Width hetWeen said invelope, means for producing electrostatic and electronet and outer peripheries, the inner periphery of magnetic fields in said envelope, and means for transsaid first ellXiiiary Coil lying adjacent the Corners ferring an electron image having a central area and of the area of said first plane electrode occupied by peripheral corner areas from said first plane electrode Said image, the Outer periphery of said first auxiliary to said second phase electrode under the combined Coil being spaced fmm said main focusing Coil y effect of said electrostatic and electromagnetic fields; a stan t l as as great as the radial Width of said an elongated main magnetic focusing coil surrounding first allXiiiafY Coil, said first allXilialY Coil being said electron image tube; adapted to be traversed by a current flowing in the a first auxiliary coil positioned adjacent the first plane same direction as a Current flowing in said main electrode of said electron image tube, said first auxfocusing coil to Provide an auxiliary electromagnetic iliary coil being of substantially annular configuration field interacting With the electromagnetic field P and having an inner periphery, an outer periphery duced in said electron image tube to reshape the and a radial width between said inner and outer 5O eieetromagnetic field Produced in said electron image peripheries, the inner periphery of said first auxiliary tube in a determined manner in the corner areas of coil lying adjacent the corners of the area of aid fir t said image to substantially eliminate field curvature plane electrode occupied by said image, the outer and to substantially avoid variation of the electroperiphery of said first auxiliary coil being spaced magnetic field produced in said electron image tube from said main focusing coil by a distance at least in the central area of said image; and as great a the radial width of aid fi t auxiliary second auxiliary coil positioned coaxially with and coil; and around the outer periphery of said first auxiliary coil second auxiliary coil positioned coaxially with and Within said main focusing Coil and adapted to be around the outer periphery of said first auxiliary traversed y a current flowing in a direction Opposite coil within said main focusing coil and adapted to be that of a current flowing in said main focusing coiltraversed by a current flowing in a direction opposite Electron image tube pp Comprising that of a current flowing in said main focusing coil. an electron image tube, said electron image tube 3. Electron image tube apparatus, comprising eluding an envelope having a planar member, a first an electron image tube, said electron image tube includ- Plane electrode Positioned on said Planar member ing an envelope having a planar member, a first in said envelope, a second plane electrode in said enplane electrode positioned on said planar member in VeiOPe, said first and second Plane electrodes being id envelope, at second plane l t d i id spaced a determined distance from each other, means velope, means for producing electrostatic and electrofor Producing eiectiostatic and electromagnetic fields magnetic fields in said envelope, and means for transin Said envelope, and means for transferring an elecferring an electron image having a central area and tion image having a Central area and Peripheral peripheral corner areas from said first plane electrode corner areas from Said first Plane electrode to said to id second plane electrode under the bi d second plane electrode under the combined effect of effect of said electrostatic and electromagnetic fields; said electrostatic and electromagnetic fields; an elongated main magnetic focusing coil surrounding an elongated main magnetic focusing coil surrounding said electron image tube; said electron image tube; and

an auxiliary coil positioned immediately adjacent the first plane electrode of said electron image tube, said auxiliary coil being of substantially annular configuration and having an inner periphery, an outer periphery, a radial width between said inner and outer peripheries and an outer diameter of said outer periphery substantially equal to the distance between said first and second plane electrodes, the inner periphery of said auxiliary coil lying adjacent the corners of the area of said first plane electrode ocner areas from said photocathode to said raster electrode under the combined effect of said electrostatic having an inner peripheral area with grooves formed therein;

a frame member positioned adjacent the planar member of the envelope of said image orthicon tube outside the envelope thereof and having an outer peripheral area with projections extending therefrom and adapted to cooperate with corresponding grooves in said main focusing coil; and

an auxiliary coil positioned in said frame immediately adjacent the photocathode of said image orthicon cupied by said image, the outer periphery of said 10 tube, said auxiliary coil being of substantially annuauxiliary coil being spaced from said main focusing lar configuration and having an inner periphery, an coil by a distance at least as great as the radial width outer periphery and a radial width between said inner of said auxiliary coil, said auxiliary coil being and outer peripheries, the inner periphery of said adapted to be traversed by a current flowing in the auxiliary coil lying adjacent the corners of the area same direction as a current flowing in said main of said photocathode occupied by said image, the focusing coil to provide an auxiliary electromagnetic outer periphery of said auxiliary coil being spaced field interacting with the electromagnetic field profrom said main focusing coil by a distance at least as duced in said electron image tube to reshape the great as the radial width of said auxiliary coil, said electromagnetic field produced in said electron image auxiliary coil being adapted to be traversed by a tube in a determined manner in the corner areas of current flowing in the same direction as a current said image to substantially eliminate field curvature flowing in said main focusing coil to provide an auxand to substantially avoid variation of the electroiliary electromagnetic field interacting with the elecmagnetic field produced in said electron image tube tromagnetic field produced in said image orthicon in the central area of said image. tube to reshape the electromagnetic field produced 6. Image orthicon tube apparatus, comprising in said image orthicon tube in a determined manner an image orthicon tube, said image orthicon tube inin the corner areas of said image to substantially cluding an envelope having a planar member, an eliminate field curvature and to substantially avoid illuminated planar photocathode in said envelope variation of the electromagnetic field produced in positioned on said planar member in said envelope, 3 said image orthicon tube in the central area of said a planar raster electrode in said envelope, means for image. producing electrostatic and electromagnetic fields in 8. Image orthicon tube apparatus, comprising said envelope, and means for transferring an electron an image orthicon tube, said image orthicon tube inimage having a central area and peripheral corner cluding an envelope having a planar member, an areas from said photocathode to said raster electrode illuminated planar photocathode in said envelope under the combined effect of said electrostatic and positioned on said planar member in said envelope, a electromagnetic fields; planar raster electrode in said envelope, means for an elongated main magnetic focusing coil surrounding producing electrostatic and electromagnetic fields in said image orthicon tube; and said envelope, and means for transferring an electron an auxiliary coil positioned immediately adjacent the image having a central area and peripheral corner photocathode of said image orthicon tube, said auxareas from said photocathode to said raster electrode iliary coil being of substantially annular configuraunder the combined effect of said electrostatic and tion and having an inner periphery, an outer peelectromagnetic fields; riphery and a radial width between said inner and an elongated main magnetic focusing coil surrounding outer peripheries, the inner periphery of said auxsaid image orthicon tube; and iliary coil lying adjacent the corners of the area of an auxiliary coil positioned immediately adjacent the said photocathode occupied by said image, the outer photocathode of said image orthicon tube, said auxperiphery of said auxiliary coil being spaced from iliary coil being of substantially annular configurasaid main focusing coil by a distance at least as great tion and having an inner periphery, an outer periphas the radial width of said auxiliary coil, said auxcry and a radial width between said inner and outer iliary coil being adapted to be traversed by a curperipheries, the inner periphery of said auxiliary coil rent flowing in the same direction as a current flowing lying adjacent the corners of the area of said photoin said main focusing coil to provide an auxiliary cathode occupied by said image, the outer periphery electromagnetic field interacting with the electromagof said auxiliary coil being spaced from said main netic field produced in said image orthicon tube to focusing coil by a distance at least as great as the reshape the electromagnetic field produced in said radial width of said auxiliary coil, the distance beimage orthicon tube in a determined manner in the tween the outer periphery of said auxiliary coil and corner areas of said image to substantially eliminate id main focusing coil being approximately three fiel c rv re and to n i lly avoid yar'latlon times the radial width of said auxiliary coil, said Of the eleclfomagnetlc field produced In 531d Image auxiliary coil being adapted to be traversed by a orthicon tube in Central area of Said g current flowing in the same direction as a current Imagfi orthicon Tube pp Comprising flowing in said main focusing coil to provide an an image orthicon tube, Said image orthicon tube auxiliary electromagnetic field interacting with the cluding an envelope having a Planat member, an electromagnetic field produced in said image orthiilluminated Planar photocathode in Said envelope con tube to reshape the electromagnetic field propositioned 011 said Planar member in Said fill/$10139, duced in said image orthicon tube in a determined a planar raster electrode in said envelope, means for manner i h Comer areas f i image to P 'P electrostatic and electromag fields in stantially eliminate field curvature and to substansaid envelope, and means for transferring an electially avoid variation of the electromagnetic field tron image having a central area and peripheral corproduced in said image orthicon tube in the central area of said image.

. 9. Image orthicon tube apparatus, comprising,

and electromagnetic fields; an image orthicon tube, said image orthicon tube inan elongated mam magnetic focusing coil surrounding cluding an envelope having a planar member an said image orthicon tube, said main focusing coil illuminated planar photocathode in said envelope positioned on said planar member in said envelope, a planar raster electrode in said envelope, means for producing electrostatic and electromagnetic fields in said envelope, and means for transferring an electron image having a central area and peripheral corner areas from said photocathode to said raster electrode under the combined effect of said electrostatic and electromagnetic fields;

an elongated main magnetic focusing coil surrounding said image orthicon tube; and

an auxiliary coil positioned immediately adjacent the photocathode of said image orthicon tube, said auxiliary coil being of substantially annular configuration and having an inner periphery, an outer periphery and a radial width between said inner and outer peripheries, the inner periphery of said auxiliary coil lying adjacent the corners of the area of said photocathode occupied by said image, the outer periphery of said auxiliary coil being spaced from said main focusing coil by a. distance at least as great as the radial width of said auxiliary coil, the distance between the outer periphery of said auxiliary coil and said main focusing coil being approximately three times the radial width of said auxiliary coil, said auxiliary coil having a cross-sectional area formed by said radial width and the thickness of said auxiliary coil, said radial width being approximately 2 times said thickness, said auxiliary coil being adapted to be traversed by a current flowing in the same direction as a current flowing in said main focusing coil to provide an auxiliary electromagnetic field interacting with the electromagnetic field produced in said image orthicon tube to reshape the electromagnetic field produced in said image orthicon tube in a determined manner in the corner areas of said image to substantially eliminate field curvature and to substantially avoid variation of the electromagnetic field produced in said image orthicon tube in the central area of said image.

References Cited by the Examiner UNITED STATES PATENTS 8/1962 Polinko 3l3252 X DAVID G. REDINBAUGH, Primary Examiner.

Claims (1)

1. ELECTRON IMAGE TUBE APPARTUS, COMPRISING AN ELECTRON IMAGE TUBE, SAID ELECTRON IMAGE TUBE INCLUDING AN ENVELOPE HAVING A PLANAR MEMBER, A FIRST PLANE ELECTRODE POSITIONED ON SAID PLANAR MEMBER IN SAIE ENVELOPE, A SECOND PLANE ELECTRODE IN SAID ENVELOPE, MEANS FOR PRODUCTING ELECTROSTATIC AND ELECTROMAGNETIC FIELDS IN SAID ENVELOPE, AND MEANS FOR TRANSFERRING ANE ELECTRON IMAGE HAVING A CENTRAL AREA AND PERIPHERAL CORNER AREAS FROM SAID FIRST PLANE ELECTRODE TO SAID SECOND PLANE ELECTRODE UNDER THE COMBINED EFFECT OF SAID ELECTROSTATIC AND ELECTROMAGNETIC FIELDS; AN ELONGATED MAIN MAGNETIC FOCUSING COIL SURROUNDING SAID ELECTRON IMAGE TUBE; AND AN AUXILIARY COIL POSITIONED IMMEDIATELY ADJACENT THE FIRST PLANE ELECTRODE OF SAID ELECTRON IMAGE TUBE, SAID AUXILIARY COIL BEING OF SUBSTANTIALLY ANNULAR CONFIGURATION AND HAVING AN INNER PERIPHERY, AN OUTER PERIPHERY AND A RADIAL WIDTH BETWEEN SAID INNER AND OUTER PERIPHERIES, THE INNER PERIPHERY OF SAID AUXILIARY COIL LYING ADJACENT THE CORNERS OF THE AREA OF SAID FIRST PLANE ELECTRODE OCCUPIED BY SAID IMAGE, THE OUTER PERIPHERY OF SAID AUXILIARY COIL BEING SPACED FROM SAID MAIN FOCUSING COIL BY A DISTANCE AT LEAST AS GREAT AS THE RADIAL WIDTH OF SAID AUXILIARY COIL, SAID AUXILIARY COIL BEING ADAPTED TO BE TRAVERSED BY A CURRENT FLOWING IN THE SAME DIRECTION AS A CURRENT FLOWING IN SAID MAIN FOCUSING COIL TO PROVIDE AN AUXILIARY ELECTROMAGNETIC FIELD INTERACTING WITH THE ELECTROMAGNETIC FIELD PRODUCED IN SAID ELECTRON IMAGE TUBE TO RESHAPE THE ELECTROMAGNETIC FIELD PRODUCED IN SAID ELECTRON IMAGE TUBE IN A DETERMINED MANNER IN THE CORNER AREAS OF SAID IMAGE TO SUBSTANTIALLY ELIMINATE FIELD CURVATURE AND TO SUBSTANTIALLY AVOID VARIATION OF THE ELECTROMAGNETIC FIELD PRODUCED IN SAID ELECTRON IMAGE TUBE IN THE CENTRAL AREA OF SAID IMAGE.

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