US2212208A

US2212208A - Electron lens

Info

Publication number: US2212208A
Application number: US221920A
Authority: US
Inventors: Klemperer Otto
Original assignee: EMI Ltd
Current assignee: EMI Ltd; Electrical and Musical Industries Ltd
Priority date: 1937-08-04
Filing date: 1938-07-29
Publication date: 1940-08-20
Anticipated expiration: 1957-08-20
Also published as: GB496603A

Description

Aug. 20, l940 o. KLEMPERER ELECTRON LENS Filed July 29, 1938 AAAIZAAA vvvvvvvv LHP-m- INVENTOR. 70 KLEMPERER ATTORNEY Patented Aug. 20, 1940 UNITED STATES PATENT OFFICE ELEc'rRoN LENS Britain Application July 29, 1938, Serial No. '221,920 In Great Britain August 4, 1937 'l Claims.

This invention relates to improvementsv in or modifications of the electron lenses described in the specification of United States application Serial No. 195,010, filed March 10, 1938.

In the specification of that application an electron lens is described comprising a pair of coaxial cylindrical electrodes the ratio of the effective diameters of which is between 1.5:1 and 2.511 and in operation the smaller diameter electrode l is arranged to contain the nearly parallel beam of electrons and a higher potential is applied to such electrode compared with the potential that is applied to the other electrode. A lens constructed and operating in this manner is found to provide a focussed beam which is substantially free from spherical aberration over about onefth of the diameter of the aperture of the larger diameter electrode. In the preferred form of the invention described in the specification referred 20 to the ratio of the effective diameters of the electrodes is 2:1. In one specific construction the electrodes comprise cylinders of substantially the same ldiameter and a diaphragm is provided at that end of one cylinder which is adjacent the 25 end of the other cylinder, the diameter of the diaphragm determining the effective diameter of the electrode with which it is associated. As stated above, lenses constructed in accordance with the (3o-pending application are found to 3()` be substantially free from spherical aberration up to about one-fifth of the diameter of the larger electrode.

It is the object of the present invention to pro- Vide an improved electron lens in which longi- 35 tudinal `spherical aberration is corrected to a greater extent than in the lenses according to the specification previously referred to.

According to one feature of the present invention an electron lens is provided comprising a pair of co-axial cylindrical electrodes the ratio of the effective diameters of which is between 1.5:1 and 2.5:1 and a source of potential maintaining a potential on one of said electrodes and a substantially different potential on the other of 45. said electrodes, the ratioI of the potentials determining the focussing ratio, the smaller electrode containing the nearly parallel beam of electrons and being maintained at the higher potentials, said smaller diameter electrode having a dia- 50 phragm which Adetermines its effective diameter,

the aperture in said diaphragm being displaced from the end of the electrode with which the diaphragm is associated a distance such that longitudinal spherical aberration is reduced.

55 According to another feature of the invention (Cl. Z50-162) an .electron discharge device is provided incorporating a source of electrons, a pair of co-axial cylindrical electrodes adapted to form an electron lens through which a beam of electrons from the source may pass, the ratio of the effective I5 diameters of said electrodes lying between 1.5:1 and 2.5:1, a screen and means whereby different potentials can be applied to said electrodes, the smaller diameter electrode having a diaphragm which determines its effective diameter, the aperture in said diaphragm being displaced from the end of the electrode with which the diaphragm is associated a distance such that spherical aberration is reduced, said screen being so positioned in relation to the electrodes "that when a more positive potential is applied to the electrode of smaller diameter the beam is focussed upon said screen when the electrode of smaller dia-meter is arranged to contain the nearly parallel beam of electrons.

The diaphragm may be a plane diaphragm, the whole diaphragm being displaced from the end of the smaller effective diameter electrode, or alternatively, the diaphragm may be of a substantially frusto-conical or equivalent form which projects into the interior ofthe smaller effective diameter electrode.

Preferably the actual diameters of the two cylindrical electrodes are the same, but this is not essential, since the diameter of the aperture in the diaphragm determines the effective diameter of the electrode with which `it is associated. The ratio of the effective diameters is preferably 2:1.

It is found that in lenses constructed in accordance with the invention spherical aberration can be reduced and that a lens can be constructed which is substantially corrected up to about onequarter of the diameter of the larger electrode.

In order that the said invention may be clearly1 understood and readily carried into effect, it will now be more fullydescribed with reference to the accompanying drawing in which:

Figure 1 illustrates diagrammatically a lens constructed according to one form of the invention,

Figure 2 illustrates diagrammatically a lens constructed in accordance with another embodiment ofthe invention, and

Figure 3 illustrates the application of the invention to a cathode ray television transmitting tube.

As shown in Figure l, the lens comprises a pair of

cylindrical electrodes

8 and 9 of substantially the same diameter, the electrode 8 having a plane diaphragm l0, the aperture in the diaphragm determining the effective diameter of the electrode 8, the effective diameter of said electrode being half the eifective diameter of the electrode 9. The diaphragm I 0, as shown, is displaced from the end of the electiode 8 adjacent the end of the electrode 9 so that its aperture is disposed at a distance from the end of the electrode 8 corresponding approximately to the radius of the larger diameter electrode. In operation the electrode 8 is maintained at a higher potential than the electrode 9 and is arranged to contain the nearly parallel beam of electrons as described in the specification of United States Patent application Serial No. 195,010. The ratio of the potentials applied to the two electrodes determines the focussing ratio. With a lens constructed as described in Figure 1 optimum results have been produced, that is to say, the lens is substantially corrected for longitudinal spherical aberration up to about one-quarter of the diameter of the larger electrode 9. It is found that lenses in which the aperture in the diaphragm is displaced to an extent about one-half of the radius of the larger electrode 9 or twice the radius of that electrode, longitudinal spherical aberration is reduced cornpared with lenses described in the specication referred to, but for optimum results the aperture in the-diaphragm I0 should be displaced a distance corresponding approximately to the radius of the electrode 9, as statedabove.

Figure 2 of the drawing illustrates a modified form of the invention in which the diaphragm I0, instead of being of the form shown in Figure 1, is of a frusto-conical shape, the frusto-conical portion projecting into the interior of the electrode 8. The diaphragm is attached to the end of the electrode 8, the frusto-oonical portion commencing at a short distance from the periphery of the electrode 8, the semi-angle of the frustoconical portion being about 'Ihe extent to which the aperture in the frusto-conical portion projects into the interior of the electrode 8, i. e.,

the distance indicated by the arrow Illa in Figure 2, is approximately equal to one-half of the radius of the larger electrode 9. With such a construction the results obtained are substantially the same as those obtained with the arrangement shown in Figure 1. Instead of providing the frusto-conical portion, equivalent configurations may be used, as for example, the inwardly projecting portion of the diaphragm may be a portion of the sphere.

The focussing ratio which determines the focal strength of the lens in practical cases will be determined by the geometry of the electron discharge device to which the lens is applied and hence the potentials applied to the electrodes will be governed by this factor. In practical cases the focussing ratio will probably vary vbetween 2:1 and 8:1.

The electrons may be incident on the high potential side of thev lens so as to be decelerated by, and brought to a focus at the low potential side, or may be incident on the low potential side, accelerated by the higher potential electrode to a nearly parallel but converging beam.

The electron lens'in accordance with the invention may be applied toany type of cathode ray tube or electron discharge device where "found desirable and Figure 3 of the accompanying drawing illustrates a lens in accordance with the invention applied by way of example to a cathode ray picture scanner'for use in television transmitting systems. Thepicture scanner comprises an evacuated envelope I I of bulbous form having a tubular neck I2 in which the lens is arranged. The lens shown is of the kind illustrated in Figure 1 and comprises two equal diameter

cylindrical electrodes

8 and 9, the electrode 8 having a diaphragm I0. In the bulbous portion of the tube II a photo-sensitive mosaic screen II is arranged associated with a conductive signal plate I 4. An optical image of a subject for transmission is projected onto the mosaic screen I3 through an optical system indicated generally at I5, projection of the optical image onto the mosaic screen causing the elements to acquire electro-static charges according to the intensity of the picture from point to point. On scanning the mosaic screen vby a cathode ray beam the elements of the mosaic screen are restored to a datum potential, restoration of the elements to the datum potential generating currents in the signal plate I4, which are applied to a suitable amplifier from across a resistance I6 connected to the signal plate I4. rI'he electron lens, in accordance with the invention, is employed for the purpose of focussing an electron beam emanating from a cathode I'I, such electron beam being employed for scanning the screen `I3 and being deflected in known manner by the provision of electrostatic scanning plates or by the provision of electromagnetic scanning coils, not shown. The required potentials for the

electrodes

8 and 9 may be derived from tapping points on a potentiometer I8 connected across the poles of a battery. The potentiometer I8 may supply potentials to other electrodes of the picture scanner.

I claim:

1. An electron gun comprising a cathode followed by in the order name-d, a control electrode in register with the cathode, a first annular cylindrical electrode of predetermined internal diameter in register with both the cathode and control electrode, a second annular cylindrical electrode in register with the first cylindrical electrode, the effective ratio of diameters of said two cylindrical electrodes lying between 1.5 to 1 and 2.5 to 1 an apertured diaphragm Within said second cylindrical electrode and positioned at a distance of the order of one half the internal diameter of the first cylindrical electrode from the end of the secondcylindrical electrode nearest the first cylindrical electrode, said aperture having a diameter of the order of one half the internal diameter of said first cylindrical electrode to reduce longitudinal spherical aberration.

2. An electron gun comprising a cathode followed by in the order named, a control electrode in register with the cathode, a irst annular cylindrical electrode of predetermined internal diameter in register with both the cathode and control electrode, a second annular cylindrical electrode in register with the rst cylindrical electrode, said'two cylindrical electrodes having substantially the same diameter, an apertured diaphragm within said second cylindrical electrode and positioned at a distance of the order of one half the internal diameter of the rst cylindrical electrod-e from the end of the second cylindrical electrode nearest the first cylindrical electrode, said aperture having a diameter of the order of one half the internal diameter of said first cylindrical electrode to reduce longitudinal spherical aberration.

3. An electron gun comprising a cathode followed byl in the order named, a control electrode in register with the cathode, a first annular cylindrical electrode of predetermined internal diameter in register with both the cathode and control electrode, a second annular cylindrical electrode in register with the first cylindrical electrode, the effective ratio of diameters of said two cylindrical electrodes lying between 1.5 to 1 and 2.5 to 1, and an apertured diaphragm within said second cylindrical electrode, said aperture lying in a plane located at a distance of the order of one-half the internal diameter of said first cylindrical electrode from the end of the second cylinrical electrode nearest the first cylindrical electrode, said aperture having a diameter of the `order of one-half .the internal diameter of the first cylindrical electrode to reduce longitudinal spherical aberration.

4. An electronv gun` comprising a cathode followed by in the order named, a control electrode in register with the cathode, a first annular cylindrical electrode of predetermined internal diameter in register with both the cathode and control electrode, a second annular cylindrical electrode in register with the rst cylindrical electrode, the effective ratio of diameters of said two cylindrical electrodes lying between 1.5 to 1 and 2.5 to 1, a frustro-conical apertured .diaphragm located at the end of the second cylindrical electrode nearest the first cylindrical electrode, the plane of the minimum diameter of the aperture lying at' a distance of the order of onehalf the internal diameter of the first cylindrical electrode from the end of the second cylindrical electrode to reduce longitudinal spherical aberration.

5. An electron gun comprising a cathode followed by in the order named, a control electrode in register with the cathode, a rst annular cylindrical electrode of predetermined internal diameter in register with both the cathode and control electrode, a second annular cylindrical electrode in register with the first cylindrical electrode, said two cylindrical electrodes having substantially the same diameter, and a frustroconical apertured diaphragm located at the end of the second cylindrical electrode nearest the first cylindrical electrode, the plane of the minimum diameter of the aperture lying at a distance of the order of one-half the internal diameter of the rst cylindrical electrode from the end of the second cylinrical electrode to reduce longitudinal spherical aberration.

6. An electron gun comprising a cathode followed by in the order named, a control electrode in register with the cathode, a rst annular cylindrical electrode of predetermined internal diameter in register with both the cathode and control electrode, a second annular cylindrical electrode in register with the first cylindrical electrode, the effective ratio of diameters of said two cylindrical electrodes lying between 1.5 to l and 2.5 to 1, and a planar apertured diaphragm within said second cylindrical electrode and positioned at a distance of the order of one-half the internal diameter ofthe first cylindrical electrode from the end of the second cylindrical electrode nearest the first cylindrical electrode,

^ said planar aperture having a diameter of the order of one-half the internal diameter of said rst cylindrical electrode to reduce longitudinal spherical aberration.

7. An electron gun comprising a cathode folowed by in the order named, a control electrode in register with the cathode, a rst annular cylindrical electrode of predetermined internal diameter in register with both the cathode and control electrode, a second annular cylindrical electrode in register with the first cylindrical electrode, said two cylindrical electrodes having substantially the same diameter, and a planar ap-ertured diaphragm within said second cylindrical electrode and positioned at a distance of the order of one-half the internal diameter of the first cylindrical electrode `fromy the end of the second cylindrical electrode nearest the rst cylindrical electrode, said planar aperture having a diameter of the order of one-half the internal diameter of said iirst cylindrical electrode to reduce longitudinal spherical aberration.

OTTO HEIVIPERER.