US2673305A

US2673305A - Image-reproducing device

Info

Publication number: US2673305A
Application number: US229013A
Authority: US
Inventors: Constantin S Szegho
Original assignee: Rauland Borg Corp
Current assignee: Rauland Borg Corp
Priority date: 1951-05-31
Filing date: 1951-05-31
Publication date: 1954-03-23
Anticipated expiration: 1971-03-23

Description

March 23,A 1954 c. s. szEGHo IMAGEREPRODUCING DEVICE 2 Sheets-Sheet 2 Filed May 51, 1951 m22; o. A.' mN-m ....On-w

300 GRID 2 VOLTAGE-+- 300 BEAM CURRENT b. b. E YQVPP. o o o o llllo m O O 0 O O O 5w Tm A A., A, ,m w 5 A um v X X v. ,Y u vw o EH V D@ 7. o O/ 4 N D A |l R 6. L G. A m H s L/ MW 7 8 O O $22: o. we s Allmm Foam Almm Sam Y E e OE G p :los iA m 4A U u o) U O Vm i V EN ./O D 8 I I Nil R k G A( kG G L A O llllBW .111.5 F l- F IN V EN TOR.

Patented Mar. 23, 1954 UNITED STATES ATENT OFFICE IMAGE-REPRODUCING DEVICE Constantin S. Szegho, Chicago, Ill., assigner to The Rauland Corporation, a corporation of Illinois 14 Claims.

This invention relates to image-reproducing devices and more particularly to electrostatically focused cathode-ray tubes foi` use as picturereproducing devices in television receivers and, the like.

The great majority of commercially produced picture tubes for television receivers have been of the magnetically focused type, although electrostatic focusing has been employed for special applications such as cathode-ray oscillographs, radar, electron microscopes and the like. Magnetic focusing has been preferred for television picture tubes, in spite of its inherently higher cost, primarily for its ready adjustability, comparative freedom from lens defects such as spher- 1 ical aberration, astigmatism and coma, and adaptability to mass production techniques. On the other hand, electrostatic focusing systems are attractive for their economy and simplicity, as

well as for the reduction in space and weight requirements and material conservation as compared with magnetic focusing arrangements.

One of the chief deterrente to the use of electrostatic focusing systems in television picture tubes has been the commercial adoption of iontrap arrangements for removing negative ions from the electron beam to prevent fatigue of the fluorescent screen at an excessively rapid rate. While several types of ion-trap arrangements have been devised and employed commercially, a

characteristic of each is that the mixed beam of electrons and ions originating at the cathode is directed along a path displaced at least in part from the tube axis, and the electrons alone are subsequently deflected to the axis by means of one or more external magnetic fields. In order to obtain quality of focusing with an electrostatic system comparable to that achieved by magnetic arrangements, it is necessary that the electron beam be centrally projected in an axial direction through the electrostatic focusing lens; otherwise, excessive astigmatism and/or coma are encountered. These defects are accentuated with an increase in the strength of the individual lens components of the focusing system employed.

While several distinct types of electrostatic -focusing systems are known, the only one which has been commercially adapted for use in modern television picture tubes is the so-called unipotential electrostatic lens system. Fundamentally, such a lens system comprises three electrodes, the outermost two of which are operated rat a common high positive potential, while the intermediate one is operated at a lower poten-- tial to provide the desired electrostatic eld distribution comprising two convergent lens components and an intermediate divergent lens component having a net convergent effect on the electron beam. In general, the potential of the intermediate electrode has been established at from 20% to 25% of the final anode voltage. In order to employ a picture tu'be comprising a focusing arrangement of this type in a television receiver, it is necessary to provide special circuit components in the receiver to develop the required focusing voltage. While it is generally recognize-d that an electrostatic focusing system capable of providing optimum focus at focusing voltages of 509 volts or less, corresponding to the ordinary direct-voltage supply or B-supply of the receiver, would permit the elimination of the additional focusing-voltage supply circuit, no commercially acceptable television picture tube having an ion-trap type of electron gun and provided with such a focusing arrangement has been heretofore proposed. Indeed, statements may be found in the literature to the effect that an ion-trap picture tube having a focusing system which meets these specifications is beyond the realm of commercial feasibility.

It is a primary object of the present invention to provide a new and improved image-reproducing device of the cathode-ray type, having an ion-trap type of electron gun, which employs an electrostatic focusing system requiring no operating potential between that of the receiver B- supply and the nal anode voltage and yet is capable of affording beam focusing comparable to that obtainable with the best magneti-c focusing arrangements.

`The present invention is largely predicated on the principle that undesirable lens distortion, characterized generically as multiplicity of focus and including specifically spherical aberration, astigmatism, and coma, is greatly aggravated as the strength of the individual lens components of the focusing system is increased and may be effectively minimized, even for exceedingly strong electrostatic lens components, by accurate centering of the electron beam. The problem is particularly aggravated in the case of television picture tubes employing ion traps, since the electron beam must be directed to the axis from a path displaced therefrom after separation of the ions. Consequently, the quality of focusing is largely dependent on the accuracy of alignment of the ion trap. Precise ion trap alignment by ordinary methods most widely employed in industry at the present time is extremely diflicult and time consuming, even with a highly skilled operator, and as a practical matter, is quite impossible of achievement on a large scale production basis; the best that may be achieved in commercial practice is an alignment within a range of operable conditions. Slight misalignment of the ion trap may be tolerated in a system employing a focusing voltage of 20% or more of the nal anode voltage, but results in prohibitively great lens distortion with a focusing system employing stronger lens components.

In accordance with the present invention, a new and improved electrostatically focused image-reproducing device of the cathode-ray type comprises an evacuated envelope having av uorescent-screen-bearing target portion and a neck portion which is at least partly transparent. An apertured diaphragm is positioned with its aperture symmetrical with respect to a reference axis extending longitudinally of the envelope. An electron gun is housed in the neck portion of the envelope and includes a cathode, a control electrode, and rst and second accelerating electrodes for projecting an electron beam toward the fluorescent-screen-bearing target portion of the envelope, these electrodes being adapted to cooperate with a predetermined magnetic eld to constitute an ion trap for intercepting negative ions originating at the cathode and for directing electrons through the diaphragm aperture along the longitudinal reference axis from a path displaced from that axis. An electrostatic focusing system comprises an additional electrode adapted to be operated at a high potential with respect to that of the cathode and a lens electrode adapted to be operated at r a potential less than 5% of the first-mentioned potential, the additional electrode and the lens electrode being disposed in mutually coaxial relation with respect to the longitudinal tube axis. A fluorescent coating is included within the neck portion of the envelope and is associated with the electron gun for accurately indicating the condition of establishment of the predetermined magnetic field required for accurate axial alignment of the electron beam in the diaphragm aperture to effect optimum focusing of the electron beam on the fluorescentscreen-bearing target portion without excessive multiplicity of focus.

As employed throughout the specification and the appended claims, the terminology a potential substantially equal to that of the cathode, as applied1 to the opera-ting potential for the focusing lens electrode, is to be construed as meaning an operating potential which differs from the cathode potential by no more than the ordinary B-supply voltage employed in the associated apparatus. Conventional television receivers utilize B-sunply voltages ranging from about 250 to about 500 volts and a nal anode voltage for the picture tube of from 8 to 14 kilovolts. Conseouently, it is apparent that a focusing voltage of 590 volts or less is generally less than 5% of the nal anode voltage and may properly be considered substantially equal to the cathode potential by comparison with the nal anode voltage. The term low potential is employed to describe a voltage not exceeding that of the receiver B-supply. A low-potential" electrostatic focusing system is one which requires. no operating voltage intermediate the B- supply voltage and the final anode voltage.

The features ofthe present invention which are believed to be novel are set forth with particularity in the appended` claims. The inveni tion, together with further objects and advantages thereof, may best be understood, however, by reference to the following description taken in connection with the accompanying drawings, in the several figures of which like reference numerals indicate like elements, and in which:

Figure 1 is a fragmentary side elevation, partly in cross-section and partly cut away, of an image-reproducing device constructed in accordance with the present invention;

Figure 2 is an exploded perspective view of one of the electrodes of the device of Figure 1;

Figure 3 is a schematic diagram of a television receiver embodying the image-reproducing device` ofV Figure 1; and

Figures 4-9 are graphical representations of operating characteristics of the device of Figure 1.`

The image-reproducing device of Figure 1 comprises a fluorescent screen Il] afxed to the glass target portion H of a; cathode-ray tube envelope which also comprises a glass neck portionv |72 enclosing an electron gun and an electrostatic focusing system. The electron gun comprises a cathode I3, a control electrode I4, and first and second accelerating electrodes I5 and I6 respectively. A diaphragm I7 having a central aperture I8 is disposed across the outlet end of second accelerating electrode I6, and aperture I8 is symmetrically centered with respect to the tube axis A-A perpendicular to the center of the uorescent screen ID. Second accelerating electrode I5 is laterally offset from first accelerating electrode I5 to provide a transverse electrcstatic deflection field in the region between these two electrodes, and the entire electron gun structure is tilted with respect to the. tube. axis A-A by an angle 0.

An electrostatic focusing system of the unipotential lens type is` disposed between the electron gun and the fluorescent screen. The focusing system comprises diaphragm Il, a lens electrode I9, and an additional electrode 2!) which are all coaxially mounted with respect to the tube axis A-A. Diaphragm I1 and additional electrode 2'0 are maintained at a common operat'ng potential by means of connecting strips 2|, while lens electrode I9 is provided with 'a separate lead 22 extending through the base 23 of the tube, Additional electrode 20 is further maintained at a common potential with a conduotive coating 24 of aquadag or the like on the inner wall of the tube envelope, by means of etal spacer springs 25. Conductive Acoating 24 extends toward the base only as far as electrode 2'1 to avoid undesirable spark discharge between that coating and lens electrode I9, and lead 22 may be provided with an insulating glass bead (not shown) to prevent spark discharge to electrode I6.

For convenience, electrodes I4, I5. I6, I9 and 28 may be termed grids and may be designated by number starting with control electrode I4 as the first grid and progressing in the'direction of beam travel to additional electrode 20 which is tle fth grid. All five grids are supported in predetermined mutually spaced relation by means of a pair of glass pillars 21B, of which only one is shown, in a manner which will be apparent to those skilled in the art. Separate leads for grids I, 2 and 4 extend through the base 23 of the tube, as do the supply leads for the cath- 0de I3 and its associated heater element (not shown). Operating voltage for the conductive coatingV 24',- and therefore for the third-and fifth grids, may be supplied by means of a conventional contact button if the envelope is of the all-glass type, or directly to the metal cone member if the tube is of the glass-metal variety.

An external permanent magnet 2l, supported in a spring clamp 28 which fits snugly around the neck of the tube and is movable both axially and rotationally, is provided to develop a magnetic eld within the tube to provide separation of the negative ions from the electron beam. Moreover, a fluorescent coating 29 on the outer surface of the second accelerating electrode I9 (grid 3) is provided for facilitating alignment of ion-trap beam-bender magnet 21.

The tube is evacuated, sealed and based in accordance with well-known procedures which require no further explanation, and suitable getters 30 are supported from grid 5 to absorb residual gases after evacuation.

In operation, a mixed. beam of electrons and negative ions originating at cathode I3 is projected through the aperture in first accelerating electrode I 5. When the mixed beam emerges from grid 2, it encounters an electrostatic eld having a transverse component due to the lateral offset of grid 3 with respect to grid 2. Consequently, electrons and ions are both deflected upwardly in the view of Figure 1. The magnetic field imposed by beam-bender magnet 2'! serves to deflect the electrons in a downward direction as viewed in Figure 1 without substantially affecting the path taken by the negative ions. rIlhus, when beam-bender magnet 2'! is accurately adjusted, the beam of electrons is projected centrally through aperture I8 of diaphragm l1 in a direction along the tube axis A--A, while the negative ions are intercepted by the metallic portions of grid 3 and diaphragm I1. The ion-trap mechanism is disclosed and claimed in the copending application of Willis E. Phillips et al., Serial No. 156,746, led April 19, 1950, now Patent No. 2,596,508, issued May 13, 1952, for "Electron Gun .for Cathode-Ray Tubes and assigned to the pres- The general construction and operation of lenses of this type are Well understood by those skilled in the art as indicated by an article entitled AMeasured properties of strong unipotential electron lenses by G. Liebmann, Proceedings of :the Physical Society, section B, volume 62, part 4, pages 213-228 (April 1, 1949) The required operating voltage for the lens .electrode (grid 4) is determined by the dimen :sions of and the spacing between the electrodes constituting the unipotential lens. `relationships are not necessarily linear, the re- Althougfh the quired focusing voltage varies directly with the length and inversely with the diameter of grid 4,

and inversely with the separation between grid 4 and

grids

3 and 5. Certain limitations on these .parameters are imposed by practical consideran tions; if the diameter of grid e' is made too small, excessive spherical aberration is encountered, and if the-separation between grids 3 and di is made too great, the deflecting influence exerted by the asymmetrical electrostatic field established between lead wire 22 and grid 3 becomes objectionable.

Unipotential electrostatic lens systems have lpreviouslybeen employed in cathode-ray tubes.

6 Such lens systems have been found quite satisfactory and readily adaptable to mass production techniques when the electron gun and the focusing system are coaxial and the path of the beam is restricted to the tube axis during its entire progress from the cathode through the focusing system. 'However, according to present commercial practice, nearly all television picture tubes are provided with an ion-trap mechanism of one sort or another for removing negative ions from the electron beam in order to avoid deterioration of the fluorescent screen. Such types of ion-trap arrangements as are commonly employed provide ion separation by subjecting the mixed beam to opposed electrostatic and magnetic elds; both electrons and ions are transversely deflected by the electrostatic field, while the electrons only are substantially deflected in the opposite direction by the impressed magnetic field. It is apparent that the practical requirement for ion trapping results in a displacement of the electron beam from the tube axis, and it is necessary that the beam be directed to the axis before it enters the focusing system. The accuracy with which this is accomplished is of importance when magnetic focusing is employed or when a unipotential electrostatic lens system is employed with a focusing voltage source which may be varied over a relatively wide range, but slight misalignment may be compensated by varying the focusing voltage. However, the ion trap alignment becomes much more critical when satisfactory operation of the electrostatic focusing system is required within a narrow focusing voltage range between cathode potential and the B-supply voltage of the associated apparatus, owing to the increased strength of the individual lens components constituting the unipotential focusing system.

In general, if the electron beam is inaccurately centered or approaches the focusing system in an angular manner, multiplicity of focus is encountered. The most troublesome form in which this manifests itself is that of astigmatism and/or corna. In order to obtain focusing comparable with that provided by magnetic focusing systems, while operating the lens electrode at or near cathode potential, it has been found essential that the electron beam be centrally directed along the axis in its passage through the focusing system. In other words, the ion trap must be precisely adjusted for satisfactory focusing with a low-potential electrostatic lens system. Such precise alignment of the ion trap is facili tated by providing a fluorescent coating 29 on the outer wall of second accelerating electrode I6. This fluorescent coating serves as an iontrap adjustment indicator; in practice, perma nent magnet 2'! is moved both axially and rota tionally until the glow from fluorescent coating 2d is reduced to a minimum, thus indicating precise ion trap alignment. The fluorescent coating must of course be so situated that it is excited into fluorescence whenever the electron beam is entirely or partially intercepted by diaphragm Il', and the fluorescent glow must be visible, either directly or by reflection, through the transparent neck portion i2 of the tube envelope. Other possible locations for fluorescent coating 29 include the inner wall of neck portion i2 near the space between

grids

2 and 3, the inner surface of diaphragm I1, and the surface of diaphragm 20 facing the cathode. The ion-trap indicator is described and claimed in .the copending applications of Constantinl S.

Szegho, Serial No. 134,725'led December 23, 1949, now U. S. Patent No. 2,564,737, issued August 2l, 1951, and of Constantin S. Szegho et al., Serial No. 162,906, led May 19, 1950, now U. S. Patent No. 2,565,533, issuedAugust 28, 1951, both entitled Cathode-Ray Tube, and both assigned to: the present assignee.

In order to obtain satisfactory focusing with the system shown in Figure 1, it is necessary to maintain rather stringent manufacturing tolerances with respect to the dimensions and spacings of the several electrodes constituting the focusing system. In addition, since grid 4 is to be operated at a potential substantially equal to that of the cathode, extremely high voltage gradients are produced between grid 4 and

grids

3 and 5. In order to avoid undesirable corona effects and field emission,

grids

3 and 5 are each provided with corona rings 3I and 32 in the form of rolled flanges of stainless steel or the like which are welded or otherwise secured to the respective electrodes, and grid 4 is constructed by rolling the two ends of a metal cylinder 33 over the edge of a large aperture in a metal disc 34. The particular construction of grid 4 is specifically claimed in the copending application of Robert W. Shawfrank, Serial No. 234,920, filed July 3, 1951, now U. S. Patent No. 2,627,049, issued January 27, 1953, for Cathode-Ray Tube Electrodes and assigned to the p-resent assignee. The construction of grid 5 is illustrated in the exploded perspective view of Figure 2, and the outlet end of grid 3 is similarly constructed. Grid 5 comprises a cylindrical portion 35, an outwardly extending flange portion 36 at one end of cylindrical portion 35, and a centrally apervtured diaphragm 3'I across the other end of cylindrical portion 35. Corona ring 32 is supported internally of cylindrical portion 35 and is provided with a rolled rim 33 overlapping and terminally abutting flange portion 35. Thus, the elect-rode comprises a cylindrical portion, an apertured diaphragm across one end of the cylindrical portion, and a substantially planar ange portion extending outwardly from the cylindrical portion in a plane including the other end of the cylindrical portion; the electrode further comprises a smoothly curved corona-'inhibiting portion between the cylindrical portion and the flange portion and tangential to a plane parallel 'to and spaced from the above-mentioned plane.

glass pillars 26, the gun assembly being properly oriented in the tube neck by means of other jigs in the usual manner. It has been found that these precautions suiiice to insure satisfactory operation of the completed structure, any small deviations in dimensions and spacings being readily compensated by adjustment of the ion- As compared with the somewhat similar constructions now employed in magnetically focused television picture tubes, the electron gun of the tube of Figure l has been modified in two further structural respects. In the rst place, the angle 0 by which the entire gun is tilted with respect to the tube axis A-A is reduced from about 6 de- 8; grees to about`4% degrees, and theY amount "of lateral offset of grid 3 with respect to' grid` 2 is reduced from 1.9 mm. to 1.6 mm. In the Vsecond place, the length of the tubular portion of grid 3 is reduced from about 38 millimeters to about 30 millimeters.

These two parameters influence the performance of the electrostatic focusing system in the following manner. In order to obtain satisfactory focusing with the reduced focusingvoltage range, it is desirable to make the electron beam more nearly parallel as it enters the focusing system. This may be accomplished by locating the unipotential lens system nearer to the cathode and thereby decreasing its focal length; for this reason grid 3 is shortened. However, it is not possible to shorten grid 3 indefinitely because a certain minimum length is required to insure ion trapping at normal operating voltages. To compensate for the impaired performance of the ion trap occasioned by shortening grid 3, the amount of lateral offset between

grids

2 and 3 and the angle 0 by which the entire electron gun is tilted with respect to the tube axis A-A may each be increased; however, increasing the angle of gun tilt results in an increase in the angle at which the beam enters the focusing system, resulting in greater astigmatism and/or coma, so that from this point of View the angle of gun tilt should be made as small as possible consistent with the requirement for ion-trapping with a single beam-bender magnet. The condition represented by the relationships set forth in the preceding paragraph has been found to provide good focusing while retaining ion trapping at reasonable operating voltages. In this connection, it is observed that the particular type of offset ion trap employed in the tube of Figure 1 has the important advantage over other types of ion-trap gun, such as that employing coaxial electrodes with a slanted aperture between

grids

2 and 3, that a greater angle of gun tilt may be employed for a given amount of lens distortion Ysince the cathode is closer to the tube axis.

Moreover, the offset type of ion trap is free of the characteristic elliptical distortion associated with the coaxial slanted-aperture type.

For best results, it has been found that the apertures in grids I, 2, 3 and 5 should be in marginally overlapping alignment in a direction parallel to the tube axis A-A. In other words, all of these apertures should intercept an imaginary straight line parallel to referenceaxis A-A, and the apertures in grids I and 2 should intercept that line asymmetrically. Fulfillment of this condition is dependent upon the angle 0 by which lthe entire electron gun is tilted with respect to the tube axis, and also upon the length of the electron gun from the cathode to aperture I8 in diaphragm II. If the angle 0 and/or the length of the gun is increased to such an extent that the apertures in grids I, 2, 3 and 5 are no longer in marginally overlapping alignment in a direc'- tion parallel to the tube axis, increased multiplicity of focus is encountered, and the performance of the focusing system is inferior. On the other hand, if the angle 0 is decreased so that the apertures are in complete coaxial alignment, ion trapping may no longer be conveniently accomplished with a single beam-bender magnet.

In a television receiver schematically illustrated in Figure 3, incoming composite television signals are received and separated into videosignal components and synchronizing-signal components by means of conventional, receiver ynarily between 250 and 450 volts. range, the spot size decreases with increasing circuits 4U which may include a radio-frequency amplifier, an oscillator-converter, an intermediate-frequency amplifier, a video detector, a video amplifier and a synchronizing-signal separator, as well as suitable circuits for reproducing the sound portion of the received signal. The detected composite video signal from receiver circuits Il@ is applied between the control electrode i4 and the grounded cathode i3 of an image-reproducing device 4l of the type shown in YFigure 1.

synchronizing-signal components of the detected composite video signal are employed to drive a synchronizing system t2 of convention-- al construction which supplies the line-frequency and field-frequency deflection coils 43 and 44 with suitable scanning currents to control the scansions of the cathode-ray beam of device A high-voltage power supply 45, which may also 4be of conventional construction, is employed to provide a suitable high operating voltage for the conductive coating 215 to which grids 3 and e are rinternally connected. Lens electrode i9 (grid 4) is connected to a variable tap 46 associated with a potentiometer resistor 4l connected between the receiver D. C. voltage supply source, conventionally designated B+, and ground. With the tube construction shown and described in connection with Figure l, optimum focus of the cathode-ray beam is achieved when the minimum glow of constant. The solid curve 5t illustrates the beam diameter versus beam current characteristic of the electrostatically focused tube shown in Figure 1, while the dotted curve 5l, plotted on the same axes for purpose of comparison, represents a similar characteristic for a conventional magnetically focused cathode-ray tube. It is apparent from a consideration of the two curves that the beam diameter is less dependent on beam current in the case of the electrostatically focused tube of the present invention (in other words, the beam is maintained more nearly parallel) than with the conventional magnetically focused tube; While the spot size at the fluorescent screen is somewhat larger than that obtainable by magnetic focusing, it is also more unij form.

Figure 5 is a curve of spot size at the fluorescent screen as a function of the voltage applied to the first accelerating electrode l5 (grid 2) the remaining operating conditions being maintained constant. Grid 2 is conventionally operated at the receiver B-supply voltage, which is ordivVithin this grid-2 voltage in a manner comparable with the 'performance of conventional magnetically focused tubes.

Figure 5 is a plot of the focusing voltage applied to the lens electrode I9 (grid 4) for optimum focusing conditions as a function of the operating voltage of grid-2. It is apparent from this characteristic that the B-supply voltage of the associated television receiver, within the `range ordinarily employed, has no effect on the 10 Voltage required at the lens electrode for minimum lens distortion.

Figure 7 is a graphical representation of the spot size at the fluorescent screen as a function of the voi-tage applied to

grids

3 and 5 and conductive coating i4, the beam current being maintained constant at 200 microarnperes and the focusing voltage applied to grid d being varied within the range of from 150 to 25() volts to maintain optimum focusing. Figure S is a curve i1- lustrating the voltage required for grid i to maintain optimum focusing, plotted as a function of final anode voltage, the beam current also being maintained constant at 200 microamperes. From a consideration of the curves of Figures 7 and 8, it is apparent that the tube of Figure l may be operated within wide range of final anode voltages while still retaining satisfactory focusing with a focusing voltage between the Al3- supply voltage and that of the picture tube cathode.

Figure 9 is a graphical representation of the spot size at the fluorescent screen as a function of focusing voltage and beam current, the nal anode voltage being maintained constant at 14 kilovolts and the grid-2 voltage at 300 volts. From this family of curves, it is apparent that optimum focusing is obtained for any beam current at a focusing voltage below 300 volts, and that the effect of beam current on spot size for any given focusing condition is no more pronounced than in the case of conventional magnetically focused tubes.

Merely by way of illustration and in no sense by way of limitation, it may be desirable to tabulate certain critical dimensional relationships in an operative embodiment of the invention constructed in the manner shown and described in connection with Figure 1. The operating characteristics of Figures 4-9 were obtained with a tube of the type shown in Figure 1 having the following dimensional relationships:

Angle 0 of gun tilt 445' Length of grid 3 mm 30 Lateral offset of grid 3 with respect to grid 2 inch .063 Longitudinal spacing between grid 2 and grid 3 inch .080 Diameter of aperture in grid l do. .040 Diameter of aperture in grid 2 do .075 Diameter of aperture i8 do .100 Diameter of aperture in grid 5 do .100 Inner diameter of grid 4 do .500

Inner diameter of corona rings 3i and 32 inch .470 Axial length of grid 4 do .125 Axial spacing between grid 3 and grid 4 inch .110 Axial spacing between grid 4 and grid 5 inch .110

'The dimensions of

grids

3, 4 and 5 and the spac- '.000 inch; while these tolerances are extremely stringent, and would be practically impossible to maintain with ordinary parts, the reinforcing veifect of the corona rings on

grids

3 and 5 permit ordinary manufacturing techniques to be em- 11 ployed while maintaining the specified close tolerances.

While the invention has been shown and described in connection with an embodimentJ which utilizes an ion-trap type electron gun comprising laterally offset first and second accelerating electrodes, and While the entire electron gun has been shown in a position tilted with respect to the longitudinal axisof the' tube, the invention is also applicable to cathode-ray tubes employing other types of ion-trap arrangements. For example, the ion trap may be of the type comprising a pair of adjacent electrodes having complementary slanted apertures, or of the type in which the entire electron gun is curved or bent, or of any other known construction. Moreover, the electron gun may be mounted either on or parallel to the longitudinal axis of the tube, in which case a pair of beam bender magnets may be required. In any tube employing an ion-trap type electron gun, the same problems are presented vvhen it is attempted to employ a lowpotential electrostatic focusing system; in order to obtain efcient ion-trap action, the electrons must be directed to the tube axis from a path displaced from that axis, and accurate centering of the beam in the electrostatic focusing system is essentialifundue multiplicity of focus is to be avoided; By employing the ion-trap indicator,

accurate axial alignment of the electron beam in f the focusing system is insured, and satisfactory focusing may be achieved event/ith an operating potential for the lensV electrode which is substantially equal to, that ofthe cathode.

While a particular embodiment of the present invention has been shown and described, it is apparent that various changes and modifications may be made, and it is therefore contemplated in the appended claims to cover all such changes and modifications. as fall within the true spirit and scope of the invention.V

I claim:

1. An electrostatically focused image-reproducing device of the cathode-ray type comprising: an evacuated envelopeenclosing a target portion and having a neck portion which is at least partially transparent; a diaphragm having an aperture symmetrical with respect to a reference axis extending longitudinally of said envelope; an electron gun housed in said neck portion of said envelope for projecting an electron beam toward said target portion of saidenvelope and including a cathode, a control electrode, and first and secondV accelerating electrodes adapted to cooperate with a predetermined magnetic field to constitute an ion trap for intercepting negative ions originating at said cathodeand for directing electrons through said aperture along said reference axis from a path displaced from said reference axis; an electrostatic focusing system including an additionalelectrode adapted to beoperated at a high potential with respect to that of said cathodeV anda lens electrode adapted to beoperated at a potential less than 5% of said first-mentioned potential, saidradditiona-l electrode and said lensv electrode being disposed in mutually coaxial relation with respect to Said reference axis; andra fluorescent coating within said neck portion of said envelope and associated with said electron gun for accurately indicating the condition of establishment of said predetermined magnetic eld to eiect optimum focusing of said beam on said'target portion without excessive multiplicity of focus.

2. An electrostatically focused image-reproducing device of thecathode-ray type comprising: an evacuated envelopeenclosing a target portion and having a neck portion which is at least partially transparent; a diaphragm having an aperture symmetrical with respect to a reference axis perpendicular tothe center of said target portion of said envelope; an electron gun housed in said neck portion of said envelope for projecting an electron beam toward said target portion of said envelope and including a cathode, a control electrode, and rst and second accelerating electrodes adapted to cooperate-With a predetermined magnetic field to constitute an ion trap for intercepting negative ions originatingl at said cathodeand for directing electrons through said aperture along said reference axis from a path displaced' from Isaid reference axis; an electrostatic focusing system including an additional electroden adapted to` be operated at a high potential With respect to that of said cathode and a lens electrode adapted to be operated at a potentialless than 5% of said firstmentioned potential, said additional electrode and said lens electrode being disposed in mutually coaxial relation With respect to said reference axis; and aA fluorescent coating within said neck portion of said envelope and associated with said electron gunfor accurately indicating the condition of establishment ofV said predetermined magnetic eld to-.effect optimum focusing of said beam on said target portion without cxcessive multiplicity of focus.

3. An electrostatically focused image-reproducing device of the cathode-ray type comprising: an evacuated envelope enclosing a target portion and havinga neck portion which is at least partially transparent; a diagram having an aperture symmetricalwith respect to a reference axis extending longitudinallyV of said envelope; an electron gun housed in said neck portion of said envelope for projecting an electron beam toward said target portion of said envelope and including a cathode, a control electrode and first and second accelerating electrodes adapted to cooperate vvithaV predetermined magnetic field to constitute an iontrapfor intercepting negative ions originating at said cathode and for directing electrons through said aperture along said reference. axis from a path displacedfrom said reference axis; a; unipotential electrostatic focusing system comprising said diaphragm, an additional electrode adaptedto be operated at a high potential with-respect to that of said cathode, and a lens electrode adapted to be operated atA a. potential less. than 5% of said first-mentioned potential; said diaphragm, said additional electrode, and saidlens electrode being disposed inmutually coaxial relation with respect to said axis;iandra fluorescentrcoating within said neck portion of said envelope and associated with said electron gunfor accuratelyv indicating the condition` of establishment ofv said predetermined magnetic field to effect; optimum focusing of said beam on said target portion without excessive multiplicity. of focus.

4. An electrostatically focused image-reproducing device of the cathode-ray type comprising: an evacuated envelopeenclosing a target portion and having aneck portion4 which is at least partially transparent; a diaphragm having an aperture symmetrical with respect to a reference axis extending longitudinally of said envelope; an electron gun housed in said neck portion of saidenvelope for projecting an electron beam towardsaid targetportion of said envelope 13 'and including a cathode, a control electrode, and vfirst and second accelerating electrodes individually supported with their axes inclined to said reference axis, said electrodes being adapted to cooperate with a predetermined magnetic field to constitute anion trap for intercepting negative ions originating at said cathode and for directing electrons through said aperture along said reference axis from a path displaced from said reference axis; an electrostatic focusing system including an additional electrode adapted to be operated at a high potential with respect to that of said cathode and a lens electrode adapted to be operated at a potential less than of said firstmentioned potential, said additional electrode and said lens electrode being disposed in mutually coaxial relation with respect to said reference axis; and a iiuorescent coating within said neck portion of said envelope and associated with said electron gun for accurately indicating the condition of establishment of said predetermined -magnetic field to effect optimum focusing of said 'beam on said target portion without excessive multiplicity of focus.

5. An electrostatically focused image-reproducing device of the cathode-ray type comprising: an evacuated envelope enclosing a target portion and having a neck portion which is at least partially transparent; a diaphragm having an aperture symmetrical with respect to a reference axis extending longitudinally of said envelope; an electron gun housed in said neck portion of said envelope for projecting an electron beam toward said target portion of said envelope and including a cathode, a control electrode and `a first accelerating electrode each supported in substantially coaxial alignment with said cathode, and a second accelerating electrode supported with its axis parallel to that of said iirst accelerating electrode but offset therefrom by a distance small in comparison with the diameters of said electrodes, said electrodes being adapted to cooperate with a predetermined magnetic held to constitute an ion trap for intercepting negative ions originating at said cathode and for directing electrons through said aperture along `said reference axis from a path displaced from .disposed in mutually coaxial relation with respect to said reference axis; and a fluorescent coating within said neck portion of said envelope and associated with said electron gun for accurately indicating the condition of establishment of said predetermined magnetic field to veffect optimum focusing of said beam on said target portion Without excessive multiplicity of focus.

6. An electrostatically focused image-reproducing device of the cathode-ray type comprising: an evacuated envelope enclosing a target portion and having a neck portion which is at jleast partially transparent; a diaphragm having an aperture symmetrical with respect to a reference axis extending longitudinally of said envelope; an electron gun housed in said neck portion of said envelope for projecting an electron beam toward said target portion of said v,envelope and including a cathode, a control electrode and a. first accelerating electrode supported in substantially coaxial alignment with said cathode and with their axes inclined to said reference axis, and a second accelerating electrode supported with its axis parallel to that of said first accelerating electrode but offset therefrom by a distance small in comparison vvithr the diameters of said electrodes, said electrodes being adapted to cooperate with a predetermined magnetic eld to constitute an ion trap for intercepting negative ions originating at said cathode and for directing electrons through said aperture along said reference axis from a path displaced from said reference axis; an electrostatic focusing system including an additional electrode adapted to be operated at a high potential with respect to that of said cathode and a lens electrode adapted to be operated at a potential less than 5% of said first-mentioned potential, said additional electrode and said lens electrode being disposed in mutually coaxial relation with respect to said reference axis; and a fluorescent coating Within said neck portion of said envelope and associated with said electron gun for accurately indicating the condition of establishment of said predetermined magnetic field to effect optimum focusing of said beam on said target portion Without excessive multiplicity of focus.

7. An electrostatically focused image-reproducing device of the cathode-ray type comprising: an evacuated envelope enclosing a target portion and having a neck portion which is at least partially transparent; a diaphragm having an aperture symmetrical with respect to a reference axis extending longitudinally of said envelope; an electron gun housed in said neck portion of said envelope for projecting an electron beam toward said target portion of said envelope and including a cathode, a control electrode, and first and second accelerating electrodes adapted to cooperate with a predetermined magnetic eld to constitute an ion trap for intercepting negative ions originating at said cathode and for directing electrons through said aperture along said reference axis from a path displaced from said reference axis; an electrostatic focusing system including a lens electrode and an additional electrode disposed in mutually coaxial relation with respect to said reference axis; means for operating said additional electrode at a high potential with respect to that of said cathode; means for operating said lens electrode at a potential less than 5% of said firstrnentioned potential; and a fluorescent coating Within said neck portion of said envelope and associated with said electron gun for accurately indicating the condition of establishment of said predetermined magnetic field to effect optimum focusing of said beam on said target portion Without excessive multiplicity of focus.

8. An electrostatically focused image-reproducing device of the cathode-ray type comprising: an evacuated envelope enclosing a target portion and having a neck portion which is at least partially transparent; a diaphragm having an aperture symmetrical with respect to a reference axis extending longitudinally of said envelope; an electron gun housed in said neck portion of said envelope for projecting an electron beam toward said target portion of said envelope and including a cathode, a control electrode, and first and second accelerating electrodes adapted to cooperate with a predetermined magnetic field to constitute an ion trap for intercepting negative ions originating at said cathode and for directing electrons through said aperture-.alongsaidi. reference axis from-.a` path displaced. from; said. reference axis; a unipotential. electrostatic. focusing system including, said diaphragm, a. lens electrode,nand an additional electrode disposed in. mutuallyY coaxial relation associated with said electron gun for accurately indicatingthe condition of establishment of said predetermined magnetic field. to effect optimum focusing of said beam on said target portion without excessive multiplicity of focus.-

9. An electrostatioallyI focused image-reproduoing device ofthe cathode-ray typecomprising: an evacuated. envelopeV enclosing a target portionand having a neck` portion which is at leastv partially transparent; a; diaphragm having an aperture symmetrical with respect to a reference axis extending longitudinally of said. envelope; an electronl gun housed in said neck portion .of said envelope for projecting aneectron beam toward said target portion of said envelope and including a cathode, a control electrode, and rstland second accelerating` electrodes; adjustable means for applying a magnetic field which cooperates with said'electrodes to'constitute an ion trap for intercepting negative ionsoriginating at said` cathode and. for directing electrons through said aperture-along said reference axis from a path displaced .from said reference axis; anY electrostatic focusing. system including an additional electrode adapted to be operated at a high potential with respect4 to that of said cathode and' a lens electrode adapted to be operated at a potential less than 5% of said iirstmentioned potential, said additional electrode and said lens electrode being disposed in mutually coaxial relation with respect to said reference axis; and-aiiuorescent coating within said neel; portion of said envelope and associated with said electron gun for accurately indicating the reference axis extending longitudinally of said envelope; an electron gun housed. in saidneck portion ofsaidenvelope forprojecting anelec.- tron beam toward said target portion of said envelope and including a cathode, a controlelectrode, a rst accelerating electrode, and a second accelerating electrode including said diaphragm, said electrodes being adapted to cooperate with a predetermined magnetic field to constitute an ion trap for intercepting negative ions originating at said cathode andl fordirecting electrons through said aperture along said reference axis from a path displaced from said reference axis; an electrostatic focusing system including an additional electrode adapted to be operated at a high potential withA respect to-that of. said cathode and a lens electrodev adapted to be operated at apotential less than 5% of said rstmentioned potential, said. additional.A electrode and saidlens electrode being disposed inL mutually coaxial relation with respect toA said-refer;- ence axis; and a fluorescent coating within'said neck portion of said envelope and associated-with said electron gunfor accurately indicatingthe condition ofestablishment of said ypredetermined magnetic eld to eect optimum focusing of said beam on said target portion without excessive multiplicity of focus 11. An electrodek for a cathode-ray tube comprising: a cylindrical portion; an apertured diaphragm across one end of said cylindrical portion; a substantially planar ange portion extending outwardly from said cylindrical portion in a plane including the other end ofl said cylindrical portion; and a smoothly curved corona-inhibiting portion between saidy otherend of said cylindrical portion and said flange portion and tangential to a plane parallel to and spaced from said Erst-mentioned plane.

12. An electrode for a cathode-ray tube comprising: a cylindrical portion; an outwardlyV extending flange portion at one end of said cylindrical portion; an apertured diaphragm across the other end of said cylindrical portion;A and a corona ring supported internally of saidcylindrical portion and having a rimoverlappingand terminally abutting said flange portion for inhibiting corona discharge between said electrode and an adjacent electrode of said tube and for mechanically reinforcing said electrode against warping and bending,

13. An electrode for a cathode-ray tube comprising: a cylindrical portion; a substantially planar outwardly extending ange portion atl one endof said cylindricalportion; an aperturedcdiaphragm across the other.. endiof. said.. cylindrical portion; and a corona'iring supported internally of said'cylindrical portionandihaving arolled rim overlapping and terminally. abuttingE said flange portion for inhibitingV corona discharge between said electrode and an adjacent electrode of said tube' and for mechanically-'reinforcing said electrode against warping and bending.

14. An electrostatically focused image-repro.. duoing device of the'cathode-ray tub'e type comprising: anevacuated envelope enclosing a target portion and having a neck portionwhich isat least partially transparent; a diaphragmohaving an aperture symmetrical with respect` to. a reference axis extending longitudinally of saidenve.- lope; an electron gunjhoused insaid neck portion of said envelope-for projecting an electron beam towardsaid target portion: of. said envelopefand including a4 cathode; and a. pair of electrodes adapted to cooperate :with Ya'predeterminedmagnetic eld'tdconstitute anion trap for intercepting negative ions originatingat said cathode and for directing electrons through lsaid `aperture along sa'idueferencel axis-from a path' displaced from said reference axis; an'electrostatic -focusing system including an additional electrode adapted'to be operatedfat a high potential with respectto that of saidcathode anda lens electrode. adapted. to f be .operated f at a. potential 'less than 5%, of said first-mentioned potential, said additional electrode and said .lens electrodebeing disposedinmutually coaxial relation Withrespect to saidreference axis ;..and.a uorescent. coating Within said neck, portionl of. said. envelope.. and

associated with said electron-gunfor. accurately indicating the condition, ofestablishment of f said predetermined magnetic heldv to .V effect` optimum 17 18 focusing of said beam on said target portion Number Name Date Without excessive multiplicity of focus. 2,484,721 Moss Oct. 11, 1949 CONSTANTIN S. SZEGHO. 2,555,850 Glyptis June 5, 1951 2,564,737 Szegho et al. Aug. 21, 1951 References Cited in the le of this patent 5 2,555,533 Szegho et a1 Aug. 23, 1951 UNITED STATES PATENTS 2,575,835 Pohle Nov. 20, 1951 Number Name Date 2,604,599 Breeden July 22, 1952 .2,058,194 Rudenberg Oct. 27, 1936 OTHER REFERENCES 2,070,319 Rudenbe'g Feb- 9: 1937 10 Article by Bowie, Proceedings of the Institute gom leb- 2g of Radio Engineers, vol. 36, No. 12, December 1948, i OgOWS ugpp. 1482-1486. f 2,363,359 Ramo NOV- 21 1944 Industrial Electronics and Control by R. G. 2,452,919 Gabor NOV- 2, 194'8 Kipefer, copyright 1949, published by John W.

vudbg ITTOV- 2 15 Wiley & C0., see in particular page 453, Fig. 2. oo r1 ge une