US2203734A - Electron lens - Google Patents

Description

H. G. LUBszYNsKI ELECTRON LENS Filed May l2 1938 June 1l, 1940.

|NVENTQR H. G. L z/sz YNSK/ B 5w ATTORNEY A' :UNITED STATES `Patented June 11,1940

PATENT OFFICE ELECTRON LENS Hans Gerhard Lubszynsk-i, Hillingdon, Middlesex, England, assigner to Electric & Musical Inl `dustries Limited, Hayes, Middlesex, England, a

company of Great Britain Application May 12, 193s, serial No. 207,470 w v 11n Great Britain May 20, 1937 l,

The present invention relates to electron discharge devices comprising electron optical systems.` o, t

",I'heinvention is; particularly but not exc1usively concerned with the position of. lenses for useyin electron dischargedevices in` which an optical ,image oianrobject is `projected on Ito a photo-sensitive cathode and `the photoelectron image emanating from the cathode being formed by a lens on to arionfphoto` sensitive mosaic elec-` trode or screen which is arranged to be scanned bye cathode ray beam to produce picture signals, for example, in a television transmission system. An electron discharge device of this lrind, termed an image transmitter," is disclosed inthe speci,n fication of British Patent No. 442,666.

There Iare'twomfactors whiohcontrol the choice of electron lenses used in the above mentioned devices, namely `the deiinition required in the electron `image and the distortion produced by the electron` lenses.` From theV point of view of i definition and from energy considerations, it is required that the area of the electron image and `mosaic electrode by an electromagnetic coil, the

distortion in the electron image increases with the size of the image to an extent depending on the ratio of the coil diameter to the diameter oi the electron image. The coil diameter is restricted due to the fact that with a large coil the magnetic field produced would ,spread too far along the i axis of the coil, and would tend to interfere with the scanning beam, and also with some constructions the length of the lens coil used and/or the cross-section ci the electron beam mayl be limited by the geometry of the envelope or tube enclosing the electrodes.` Also in the case of an electrostatic lens, if the crosssection of the elec,-

o tron beam is large in comparison with the separation of the plates by which the lens iielcl is produced, a serious amount of spherical aberration will appeal` in the image obtained.

The object of the present invention is to provide an improved electron optical system which may beused in electron discharge devices to focus a large electron image with small distortion or to produce an electron image in cases Where the `cross-section of the electron beam isrestricted in some parts of its path.

` According to the `present invention, an electron discharge device is provided comprising a source of electrons from which a `beam of electrons is projected along a path, a long frusto-conioal electro-,magnetic coil with its larger end directed towards said source arranged about said` path and adapted to be energized so `as toset up a magnetic lens field which causes the cross-section of the :electron beam` projected from said source to become `reduced and the beam ,to become `or tend to become focused, there beinga further electron lens `constituting device arranged `to be energized `to set up a further lens `field for `acting on said electron beam after it-has .been acted on by the rst lens field to produce an electron 15 image.

Themethod of carrying the invention intorlpractice will be readily understood from the following description in detail, reference being made to the single figure oi the drawing which represents diagrainmatically an arrangement according to the invention as applied by way of example to an image transmitter of the kind referred to above. In the arrangement shown in the drawing, P is a large photo-cathode on which the imageoi an object O is projectedby an optical system repre sented by lens L. This photo-cathode P is ar ranged on the end of a conical neck portion l of the enclosing envelope 2 of a scanning tube, the electron gun `of which is shown at G. The neck portion of the tube tapers away from the photo-cathode. In body portion 3 of the envelope is arranged a mosaic electrode M which is adapted to be scanned by a cathode ray, de veloped by the electron gun G. Surrounding the conical neck portion l and extending slightly beyond the photoathode P, as shown, is a. conica-1 electromagnetic coil 4 which, as described, for example, by Coeterier and Teves in Physica v vol. 3, 1936, pages 698-976, reduces the crosssection of the electron beam emitted from the 4o photo-cathode P as indicated by the dotted lines in the drawing. Surrounding the body portion 3 of the envelope is a further short electro-magnetic coil 5, by which the electron beam of reduced crosssection is focused `on the mosaic electrode M, the focused electron image being of such size as to occupy practically the whole of the available area on the mosaic electrode. With this arrangement the cross-section of the 50 electron beam after the beam has passed through the coil 4 will be small and there will only be a small amount of distortion present, and the image produced by the coll 4 will be found to be both large and free from serious distortion.` Moreover, the use of the conical coil 4 as described enables the cross-section of the neck I at its point of junction with the body portion 3 to be made relatively small. Y

While the invention described above is pri marily (intended for application to image transmitters, it Wil1 be appreciated that ,the invention may also be applied to any arrangement in'which an electron image is required to be produced on an extended surface, as for example, in an elec tron telescope or microscope where no scanning is involved. y

lIf desired, the magneticfcoil 5`might be replaced by a suitable arrangement adapted to be charged to different potentials to set upan electrostatic iield constituting an electrostatic electron lens rather than an electromagnetic lens as shown in the drawing, in which case also, the arrangement of the invention may be used to. lessen the distortion or aberration in the image produced and to overcome diiiiculties due to the construction i of the tube.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is: i

l. An electron discharge device including a photo-cathode electrode upon which an optical image may be projected and from which an electron current image may be derived, means for directing the photo-electrons constituting the current image along a predetermined path including a long `frustro-conical electromagnetic coil arranged about the path with its 'larger end directed toward said photo-cathode and adapted to be energized so as to set up a magnetic electron lens eld which causes the cross-section of the photo-electron beam emitted by said photocathode to become materially reduced, means including a further electron lens device adapted to be energized to set up a further magnetic electron lens field for acting on said photo-electron beam, after it has been acted on bytherst ,lens eld, to cause divergence of the beam? to jfproduce an electron image, and a mosaic electrode upon which the4 produced electron image .may be focused, whereby-because of the reduction in the cross-section of he beam under the action of the iield set up injthe vfrustro-conical coiL-fthedistortion arising is less than that which Would be produced by saidV `further lens device acting alone.

2. An electron discharge device including a photo-cathode electrode upon which an optical image may be projected and from which an electron current image may be derived, means for directing the photo-electrons constituting the current image along a predetermined path including a long frustro-conical electromagnetic coil arranged concentrically about the path with` its larger end directed toward said photo-cathode and adapted to be energized so as to set up a magnetic lens iield for causing the cross-sectional area of the photo-electron beam to becomematerially reduced, means including a second elcctromagnetic coil adapted to be energized so as to set up asecond magnetic lens eld for acting duced by reason of the convergence and divergy.

ence of the electron current image.

HANS GERHARD IlUBSZYNSKI.`

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