US3462638A

US3462638A - Electron beam correction apparatus for color picture tube

Info

Publication number: US3462638A
Application number: US743223A
Authority: US
Inventors: Tokita Tetsuo; Minoru Morio; Masakatsu Tobari
Original assignee: Sony Corp
Current assignee: Sony Corp
Priority date: 1967-07-10
Filing date: 1968-07-08
Publication date: 1969-08-19
Anticipated expiration: 1986-08-19
Also published as: NL162809B; NL6809771A; GB1226614A; FR1573580A; DE1762569B2; NL162809C; DE1762569A1

Description

Aug. 19, 1969 ToK TETSUQ ET AL 3,462,638

ELECTRON BEAM CORRECTION APPARATUS FOR COLOR PICTURE TUBE Filed July 8 1968 5 Sheets-Sheet 1 IN VENTORS rsrsuo 10mm MINORU MORIO MASAKATSU TOBARI skuvmi \SSYQ ga n \SQT Q AT TORNE Y g- 1969; TOKITA TETSUO ETAL 3,462,638

' ELECTRON BEAM CORRECTION APPARATUS FOR COLOR PICTURE TUBE 5 Sheets-Sheet 2 Filed July 8, 1968 1 /9 AAAA A/VAAA/I AAA FIG.'5A. L VVVVVL VVV V FIG. 5B.

@Q/ ggW INVENTORS TETSUO TOKITA MINORU MORIO B MASAKATSU TOBARI ATTORNEY Aug. 19,1959 ToKlTA suo ET AL 3,462,638

ELECTRON BEAM CORRECTION APPARATUS FOR COLOR PICTURE TUBE Filed July 8, 1968 5 Sheets-Sheet 5 Fl ll.

FIG. IO.

5 I R m m mm NmR o u V TMB mouA TMM ATTORNEY 15 Claims ABSTRACT OF THE DISCLOSURE In a color picture tube of the single-gun, plural-beam type in which the plural beams are passed through the optical center of an electron lens by which all of the beams are focussed on the color phosphor screen with at least two of the beams being angled relative to the optical axes of the lens so as to emerge therefrom along paths divergent to the axis, and convergence deflecting plates are arranged along the divergent paths and have a voltage difference applied thereto to deflect the respective beams and cause convergence of all beams at a common point on the beam selecting grid or mask; a magnetic flux is produced parallel to the optical axis at a location along the latter where beams are angled with respect to the axis so that such beams have imparted thereto a twisting displacement about the axis to compensate for misconvergence that may result from mechanical misalignment of the beam generating cathodes or from spherical aberration of the deflection yoke which causes the beams to scan the screen. The magnetic flux may be produced by a suitably adjusted DC. current flow in a suitably wound coil around the tube neck and/ or from a modulated current flow synchronized with the scanning of the screen by the beams. Further, magnetic fluxes in planes perpendicular to the optical axis may be provided to deflect all of the beams for ensuring proper passage between the convergence deflecting plates and/ or centering of the point of convergence of the beams with respect to the apertures of the grid or mask for obtaining maximum color purity.

This invention relates generally to color picture tubes of the single-gun, plural-beam type, and particularly to tubes of that type in which the plural beams are passed through the optical center of a common electron lens by which the beams are focussed on the color phosphor screen.

In single-gun, plural beam color picture tubes of the described type, for example, as specifically disclosed in the co-pending U.S. application Ser. No. 697,414, filed J an. 12, 1968 and having a common assignee herewith, three laterally spaced electron beams are emitted by a beam generating or cathode assembly and directed in a common substantially horizontal plane with the central beam coinciding with the optical axis of the single electron focussing lens and the two outer beams beingconverged to cross the central beam at the optical center of the lens and thus emerge from the latter along paths that are divergent from the optical axis. Arranged along such divergent paths are pairs of convergence deflecting plates nited States Patent having voltages applied thereacross to laterally deflect the divergent beams in a substantially horizontal plane for causing all beams to converge at a point on the apertured beam selecting grid or shadow mask associated with the color screen.

If in assembling a color picture tube of the above described single-gun, plural beam type, there is an angular deviation or twist of the plane in which the three beams are emitted relative to the plane in which the convergence deflecting plates deflect the divergent beams, then the three beams will be vertically spaced at the beam selecting grid or mask and produce defective color by impinging, at least to some extent, on color phosphors that do not correspond to the particular beams. A similar misconvergence of the beams will occur at certain positions in the masker, particularly when the beam is directed at corners of the screen during scanning thereof, by reason of spherical aberration of the deflection yoke by which the scanning is effected.

Further, in a color picture tube of the above described type, due to the earths magnetic field or mechanical error in the alignment of the beam generating assembly with respect to the convergence deflecting plates, the beams to be converged by the latter may impinge thereon rather than passing freely therebetween. It is also possible, within normal manufacturing tolerances, for the landing spots of the converged beams to be eccentrically located with respect to the apertures of the beam selecting grid or shadow mask with the color purify being adversely affected thereby.

Accordingly, it is generally an object of this invention to avoid the above mentioned problems encountered in color picture tubes of the described type.

More specifically, it is an object of this invention to correct or compensate for the misconvergence of the beams in a color picture tube of the described type that results from twist of the beam generating assembly relative to the assembly of convergence deflecting plates.

Another specific object is to dynamically compensate for the misconvergence of the beams that results, particularly at the corners of the screen, from spherical aberration of the deflection yoke for causing the beams to scan the screen.

According to an aspect of the invention, a single-gun, plural-beam color picture tube as described is provided with means to produce magnetic flux parallel to the optical axis of the focussing lens at a location where beams either converge toward or diverge from the optical center of the lens to impart a twisting displacement to such beams for correcting a twist of the beam generating assembly and/or the misconvergence that results from spherical aberration of the deflecting yoke. Such means to produce magnetic flux parallel to the optical axis may desirably include a suitably wound coil to receive a DC. current adjusted to correct for twist of the beam generating assembly and/or to receive a current varied in synchronism with scanning of the screen to correct for misconvergence due to spherical aberration of the deflection yoke.

Still another object is to ensure proper passage of the beams through the convergence deflecting plates, particularly by means producing magnetic flux in a plane perpendicular to the optical axis at a location between the beam generating assembly and the convergence defleeting plates.

A further specific object is to achieve accurate centering of the landing spots of the converged beams with respect to the apertures of the beam selecting grid or shadow mask, particularly by means producing magnetic flux in a plane perpendicular to the optical axis at a location between the convergence deflecting plates and such grid or mask.

The above, and other objects, features and advantages of the invention, will be apparent in the following detailed description of an illustrative embodiment which is to be read in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic sectional view in a horizontal plane passing through the axis of a single-gun, pluralbeam color picture tube according to this invention;

FIGS. 2A and 2B are diagrammatic transverse sectional views respectively illustrating an ideal arrangement of the beams as emitted from the beam generating assembly, and a twisted beam arrangement;

FIGS. 2A and 2B are diagrammatic views showing the pattern formed by the beams on the grid or shadow mask for the beam arrangements of FIGS. 2A and 2B, respectively;

FIG. 3 is a schematic view illustrating the effect of a magnetic flux produced according to this invention to compensate for the condition of FIGS. 2B and 2B;

FIGS. 4A and 4B are similar schematic views respectively showing misconvergence that results at the corners of the color screen due to spherical aberration of the deflecting yoke, and a condition of desired or ideal convergence of the beams at all positions of the screen;

FIGS. 5A, 5B and 5C are graphic representations of saw-tooth wave signals used to generate a magnetic flux compensating for the condition of FIG. 4A according to this invention;

FIG. 6 is a schematic view of a coil used to produce a flux for ensuring proper passage of the beams through the convergence deflecting plates;

FIGS. 7, 8 and 9 are respectively a side elevational -view, an end elevational view, and a sectional view taken on the line 9-9 on 'FIG. 8 of an assembly of two magnetic flux producing coils according to the invention; and

FIGS. 10, 11 and 12 are respectively an end elevational view, a side elevational view and the opposite end elevational view of an assembly with the deflection yoke of another coil for producing magnetic flux according to this invention.

Referring to the drawings in detail, and initially to FIG. 1 thereof, it will be seen that the invention is there shown applied to a single-gun, plural-beam color picture tube 10 comprising a glass envelope 11 (indicated in broken lines) having a neck 12 and cone 13 extending from the neck to a color screen S provided with the usual arrays of color phosphors S S and S and with an apertured beam selecting grid or shadow mask G Disposed within neck 12 is an electron gun A having cathodes K K and K each of which is constituted by a beam-generating source with the respective beamgenerating surfaces thereof disposed as shown in a plane which is substantially perpendicular to the axis of the electron gun A. In the embodiment shown, the beamgenerating surfaces are arranged in a straight line so that the respective beams B B and B emitted therefrom are directed in a substantially horizontal plane containing the axis of the gun, with the central beam B being coincident with such axis. A first grid G is spaced from the beam-generating surfaces of cathodes K K and K and has apertures g g and g formed therein alignment with the respective cathode beam-generating surfaces. A common grid G is spaced from the first grid G and has apertures g g and g formed therein in alignment with the respective apertures of the first grid G Successively arranged in the axial direction away from the common grid G are open-ended, tubular grids or electrodes G G and G respectively, with cathodes K K and K grids G and G and electrodes G G and G being maintained in the depicted, assembled positions thereof, by suitable, non-illustrated support means of an insulating material.

For operation of the electron gun A of FIG. 1, appropriate voltages are applied to the grids G and G and to the electrodes G G and G Thus, for example, a voltage of 0 to minute 400 v. is applied to the grid G a voltage of 0 to 500 v. is applied to the grid G a voltage of 13 to 20 kv. is applied to the electrodes G and G and a voltage of 0 to 400 v. is applied to the electrode G with all of these voltages being based upon the cathodes voltage as a reference. As a result, the voltage distributions between the respective electrodes and cathodes, and the respective lengths and diameters thereof, may be substantially identical with those of a unipotential-single beam type electron gun which is constituted by a single cathode and first and second, single-apertured grids.

With the applied voltage distribution as described hereinabove, an electron lens field will be established between grid G and the electrode G to form an auxiliary lens L as indicated in dashed lines, and an electron lens field will be established around the axis of electrode 6.; by the electrodes G G and G to form a main lens L, again as indicated in dashed lines. In a typical use of electron gun A, bias voltages of v., 0 v., 300 v., 20 kv., 200 v. and 20 v. may be applied respectively to the cathodes K K and K the first and second grids G and G and the electrodes G G and G Further included in the electron gun A of FIG. 1 are electron beam convergence deflecting means P which comprise shielding plates P and P disposed in the depicted spaced, relationship at opposite sides of the gun axis, and axially extending, deflector plates Q and Q which are disposed, as shown, in outwardly spaced, opposed relationship to shielding plates P and P, respectively. Although depicted as substantially straight, it is to be understood that the deflector plates Q and Q may, alternatively, be somewhat curved or outwardly bowed, as is well known in the art.

The shielding plates P and P are equally charged and disposed so that the central electron beam B will pass substantially undeflected between the shielding plates P and P, while the deflector plates Q and Q have negative charges with respect to the plates P and P so that respective electron beams B and B will be convergently deflected as shown by the respective passages thereof between the plates P and Q. More specifically, a voltage V which is equal to the voltage applied to the electrode G may be applied to both shielding plates P and P, and a voltage V which is some 200 to 300 v. lower than the voltage V is applied to the respective deflector plates Q and Q to result in the respective shielding plates P and P being at the same potential, and to result in the application of a deflecting voltage difference or convergence deflecting voltages V between the respective plates P and Q and P and Q and it is, of course, this convergence deflecting voltage V which will impart the requisite convergent deflection to the respective electron beams B and B In operation, the respective electron beams B B and B which emanate from the beam generating surfaces of the cathodes K K and K will pass through the respective grid apertures g g and g to be intensity modulated with what may be termed the red, green and blue intensity modulation signals applied between the said cathodes and the first frid G The respective electron beams will then pass through the common auxiliary lens L to cross each other at the center of the main lens L. Thereafter, the central electron beam B will pass substantially undeflected between shielding plates P and P since the latter are at the same potential. Passage of the electron beam B between the plates P and Q and of the electron beam B between the plates P and Q will, however, result in the convergent deflections thereof as a result of the convergnce deflecting voltage V applied therebetween, and the system of FIG. 1 is so arranged that the electron beams B B and B will desirably converge or cross each other at a common spot centered in an aperture between adjacent grid wires g of the beam selecting grid or mask G so as to diverge therefrom to strike the respective color phosphors of a corresponding array thereof on screen S. More specifically, it may be noted that the color phosphor screen S is composed of a large plurality of sets or arrays of vertically extending red, green and blue phosphor stripes or dots S S and S with each of the arrays or sets of color phosphors forming a color picture element as in a chromatron type color picture tube. Thus, it will be understood that the common spot of beam convergence corresponds to one of the thusly formed color picture elements.

The voltage V may also be applied to the lens electrodes G and G and to the screen S as an anode voltage in conventionalmanner through a non-illustrated graphite layer which is provided on the inner surface of cone 13 of the tube envelope. The grid wires of screen grid G may have a post-focussing voltage V ranging, for example, from 6 to 7 kv. applied thereto as indicated. Thus, to summarize the operation of the depicted color picture tube of FIG. 1, the respective electron beams B B and B will be converged at screen grid G and will diverge therefrom in such manner that electron beam B will strike the blue phosphor S electron beam B will strike the green phosphor S and electron beam B will strike the red phosphor S of the array or set corresponding to the grid aperture at which the beams converge. Electron beam scanning of the face of the color phosphor screen is effected in conventional manner, for example, by horizontal and vertical deflection yoke means indicated in broken lines at D and which receives horizontal and vertical sweep signals whereby a color picture will be provided on the color screen. Since, with this arrangement, the respective electron beams are each passed, for focussing, through the center of the main lens L of the electron gun A, the beam spots formed by impingement of the beams on the color phosphor screen S will be substantially free from the effects of coma and/ or astigmatism of the said main lens, whereby improved color picture resolution will be provided.

It will be noted that the plane H (FIG. 2A) in which beams B B and B are emitted is intended to coincide with the horizontal plane in which beams B and B are deflected laterally by convergence deflecting means F. If such intended relationship exists, the lateral deflection of beams B and B can in fact result in convergence of all three beams at a common spot on grid G as shown on FIG. 2A. However, if as shown on FIG. 2B, the plane H in which the beams are emitted is angularly deviated from the plane H in which beams B and B are deflected by means F, for example due to a manufacturing error, the lateral deflections of beams B and B by convergence deflecting means P will only bring beams B and B into vertical alignment with central beam B at the grid G but the beams will be vertically spaced, as shown on FIG. 213'. Such failure to attain accurate convergence in the vertical, as well as in the horizontal directions, will affect the quality and fidelity of the color picture that is obtained.

In accordance with the present invention, such vertical spacing of the beams at grid or mask G is compensated for and corrected by providing means 14 to produce lines of magnetic flux 15 parallel to the optical axis of focussing lens L (FIG. 3), and hence parallel to central beam B at a location along the optical axis between auxiliary lens L and convergence deflecting means F, that is, at a location where beam B and B are oppositely angled with respect to the optical axis so as to have components directed across the lines of flux. With reference to Flemings left-hand rule, it will be apparent that the oppositely angled electron beams B and B are oppositely twisted or displaced perpendicular to plane H in directions to restore the coincidence of the plane H to the plane H so as to eliminate the vertical deviations of the beam spots shown on FIG. 28'. Of course, the central beam B which extends parallel to the lines of magnetic flux will not be displaced or deflected thereby.

The means 14 for producing magnetic flux 15 parallel to the optical axis may be conveniently constituted by a coil 16 extending around neck 12 and being suitably Wound, for example, circumferentially around the optical axis, so that current flow through the coil 16 will result in the desired magnetic flux. The current flow through coil 16 may be derived from an adjustable D.C. source 17 of reversible polarity, whereby to permit variation of the flux density and change of direction of the flux parallel to the optical axis for effecting the necessary twist correction or compensation of the beams B and B It will be apparent that, when beams B B and B are deflected by yoke means D from a point of convergence at the center of screen S, as during scanning of the screen, the distances that such beams travel through the magnetic fields of yoke means D are relatively varied and spherical aberration results, that is, the beams undergo diflerent degrees of deflection resulting in misconvergence of the beams, particularly when the latter are directed at corner portions of the screen. Such misconvergence of the beams has a vertical component and a horizontal component by reason of the magnetic fields produced by yoke means D for deflecting the beams in vertical and horizontal directions. The horizontal component of the described rnisconvergence can be compensated for by means, forming no part of this invention, which apply across plates P and Q and plates P and Q, in addition to a static convergence deflection voltage, a dynamic convergence deflection voltage synchronized with the horizontal sweep signal so as to suitably vary the deflections of beams B and B caused by convergence deflection means F for compensating or eliminating the horizontal component of misconvergence. However, as shown on FIG. 4A, the uncorrected vertical component of misconvergence resulting from the described spherical aberration of deflecting yoke means D would still remain when the beams are directed at portions of the screen away from the center thereof and particularly at the corner portions of the screen.

In accordance with this invention, the vertical component of misconvergence of beams B B and B particularly at the corners of the screen, as shown on FIG. 4A, is also eliminated so as to obtain accurate convergence at all portions of the screen being scanned, as shown on FIG. 43, by providing a magnetic field having its lines of flux parallel to the optical axis at a location along the latter between auxiliary lens L and convergence deflecting means F, that is, where beams B and B are angled to the optical axis, and the intensity and direction of such field are varied in correspondence with the scanning of the screen S so that, at any instant, the twisting displacement of beams B and B that is thereby caused is just sufficient to return the beams to accurate convergence in the vertical direction.

As shown, the last described magnetic field having its lines of flux parallel to the optical axis is also preferably produced by the passage of current through coil 16, which current is a dynamic convergence correcting current varied in synchronism with the scanning of the screen. The dynamic convergence correcting current is provided by a source 18 in which there are generated a first saw-tooth wave signal 19 (FIG. 5A) synchronized with the horizontal sweep signal and having the same period T and a second saw-tooth wave signal 20 (FIG. 5B) synchronized with the vertical sweep signal and having the same period T with the signal 29 being used to modulate the signal 19 by means of a balanced modulator, whereby the output signal of source 18 fed to coil 16 has the waveform indicated at 21 on FIG. 5C. It will be apparent from FIG.

C that the magnetic field produced by signal 21 changes from maximum intensity in one direction to maximum intensity in the opposite direction during each horizontal sweep period, and that the magnitudes of the maximum intensities of the field are decreased and then increased during each vertical sweep period. Thus, the extent of the twisting displacement of beams B and B that results from signal 21 applied to coil 16 is maximum when the beams are directed at corners of screen S and decreases to zero at the center of the screen.

In order to ensure that the beams B and B will be similarly affected by the magnetic flux produced by coil 16, the latter is preferably positioned along the optical axis close to the plane of the optical center of focussing lens L so that the beams all pass through the center of magnetic fields produced by coil 16.

In a small size color picture tube of the type illustrated, the spacing between plates P and Q and between plates P and Q of convergence deflecting means F is so small that the eflect of the earths magnetism on the electron beams or a small mechanical error may cause one or the other of beams B and B to impinge against one of the respective convergence deflecting plates. In order to avoid such impingement, the tube according to this invention may be further provided with means.22, at a location between the beam generating cathodes K K and K and the convergence deflecting means F, to produce magnetic flux in a plane perpendicular to the optical axis with the lines of magnetic flux being directed substantially vertically, that is, perpendicular to the plane H in which the beams are emitted, whereby to deflect all of the beams laterally toward one side or the other to the extent necessary to cause free passage of beams B and B between plates P and Q and between plates P and Q. In the embodiment shown, the means 22 for producing the described magnetic flux may consist of a ring-shaped core 23 (FIG. 6) extending around neck 12 and having series-connected coils 24a and 24b wound, in opposite directions, on the portions of core 23 disposed at the opposite sides of a vertical plane extending through the optical axis. The series-connected coils 24a and 24b are further connected to an adjustable DC. current source 25 (FIG. 1) of reversible polarity so that the flow of DC current through such coils will produce a uniform magnetic flux, as indicated in broken lines at 26 on FIG. 6, with the direction of the lines of flux being determined by the polarity of source 25 and the intensity being determined by the magnitude of the current. The magnetic flux 26 in a plane perpendicular to the optical axis and in directions perpendicular to the plane in which beams B B and B are generated, causes simultaneous lateral displacement, in one direction or the other, of the electron beams, thereby to properly align the beams with the respective spaces between the convergence deflection plates P, P, Q and Q.

The magnetic flux 26 is also preferably produced at a location adjacent the plane of the optical center of main focussing lens L so that the several electron beams will pass through the center of the magnetic field. In fact, in a particular embodiment of the invention shown on FIGS. 7, 8 and 9, the coil 16 and the coils 24a and 241) are included in a coil assembly 27 having a supporting member 28 of insulating material, such as a polyester resin, formed with a sleeve 29 dimensioned to fit over neck 12 and provided with longitudinal slots 30 at one end portion which receives a clamp ring 31 for securing member 28 on neck 12. Sleeve 29 has a radial flange 32 at the end thereof remote from slots 30, and a radial flange 33 spaced axially from flange 32 and formed with an annular rim 34 directed axially away from flange 32. As shown, coil 16 is wound on sleeve 29 between flanges 32 and 33, and the core for coils 24a and 24b is formed in halves 23a and 23b (FIG. 8) having holes 35 in their ends to receive support pins 36 projecting axially from flange 33 in the direction away from flange 32. Thus,

coils 24a and 24b wound on core halves 23a and 23b, respectively, are accommodated in the annular space between sleeve 29 and rim 34 at the side of flange 33 away from flange 32, and are held in assembled position by adhesive, indicated in broken lines at 377 (FIGS. 8 and 9Q.

Terminals

38 and 39 extend from rim 34 and are connected to coil 16 and to coils 24a and 23b, respectively, for connection of the respective coils to sources 17 and 18 and to source 25.

In a color picture tube of the described character, there is further the possibility that, although the electron beams B B and B are properly converged at a point on the beam selecting grid or shadow mask G such point will not be horizontally centered with respect to the apertures of the grid or mask with resultant deterioration of the color purity. Such deviation of the landing points of the converged beams from the centers of the grid or mask apertures can result from manufacturing or assembling errors. Accordingly, a color picture tube embodying this invention preferably further has means 40 (FIG. 1) similar to the previously described means 22 to produce magnetic flux in a plane perpendicular to the optical axis at a location between convergence deflection means F and grid or mask G with the directions of the lines of flux thereof extending vertically, that is, perpendicular to the plane in which beams B and B are deflected for convergence, whereby the produced magnetic flux is eflective to laterally displace all of beams B B and B to the extent necessary for aligning the point at which the beams converge with the centers of the grid or mask apertures during scanning of the screen.

As shown on FIGS. 10, 11 and 12, the means 40 for ensuring color purity may include coils 41a and 41b which are oppositely wound on core halves 42a and 42b and connected in series with a suitable source (not shown) of an adjustable DC. current of reversible polarity. More particularly, coils 41a and 41!) on cores 42a and 42b are desirably mounted adjacent the horizontal and vertical deflection yoke D by which scanning of screen is effected. Such yoke D is shown to include a frusto-conical support sleeve 43 of insulating material adapted to fit over neck 12 and onto the adjacent section of cone 13, and which has

flanges

44 and 45 extending radially outward from its opposite ends. Toroidal windings 46a and 46b are provided on an annular magnetic yoke 47 which extends around sleeve 43 between

flanges

44 and 45 to constitute the usual vertical deflection coil (FIGS. 11 and 12),

saddle windings

48a and 48b (FIG. 10) are arranged against the outer face of flange 45 and extend axially therefrom into sleeve 43 to constitute the usual horizontal deflection coil. The core halves 42a and 42b, which together form a ring, are mounted, at their ends, on screws 49 extending through flange 45 so as to dispose the respective coils 41a and 41b radially outside the portions of

windings

48a and 48b which lie against flange 45. A terminal board 50 is mounted in back of flange 45, as by the screws 49, and carries the terminals 51 for connecting the coils 41a and 41b, the windings 46a and 46b, and the

windings

48a and 48b to the respective energizing circuits.

From the above description of an illustrative embodiment of the invention, it will be apparent that a color picture tube in accordance therewith is arranged to correct or compensate for vertical spacing of the beams at the beam selecting grid or mask arising by reason of manufacturing error and spherical aberration of the deflection yoke, for impingement of a beam or beams against the convergence deflection plates, and further for improper positioning of the beam convergence point with respect to the centers of the grid or mask apertures, whereby to obtain color pictures of enhanced clarity and color purity with a single-gun, plural-beam tube.

Although electro-magnetic devices have been disclosed to constitute the means 22 and 40 by which magnetic fluxes are produced in planes perpendicular to the optical axis of focussing lens L, it will be apparent that such means 22 and 40 may be each constituted by a pair of ring magnets (not shown) having diametrically opposed poles and being relatively rotatable about the optical axis for varying the intensity and direction of the resultant lines of magnetic flux.

Having described a particular embodiment of the invention with reference to the accompanying drawings, it will be understood that the invention is not limited to such precise embodiment, and that various changes and modifications, only some of which have been mentioned above, may be made therein without departing from the scope or spirit of the invention as defined in the appended claims.

What is claimed is:

1. In a single-gun, plural beam color picture tube which includes a color screen having arrays of color phosphors and beam selecting means provided with apertures corresponding ,to said arrays, beam generating means for directing a plurality of electron beams toward said color screen for impingement on respective phosphors of each array through the corresponding aperture, lens means for focusing said electron beams on said color screen and having an optical center through which said beams are all passed with at least two or said beams being angled with respect to the optical axis of said lens means to enter the latter along paths that are convergent to said optical axis and to emerge from said lens means along paths divergent to said axis, and electron beam convergence deflecting means interposed between said lens means and said beam selecting means and being operative to deflect said beams emerging along said divergent paths for convergence of said beams at an aperture of said beam selecting means; the improvement comprising means to produce magnetic flux parallel to said optical axis at a location along the latter where said at least two beams are angled with respect to said axis, whereby said magnetic flux eflects a twisting displacement of said at least two beams around said axis to insure accurate convergence of said beams at said aperture.

2. A color picture tube according to claim 1, in which said means to produce said magnetic flux includes coil means extending concentrically around said axis at said location and being wound to produce said flux parallel to the axis upon a current flow through said coil means, and means for supplying current to said coil means.

3. A color picture tube according to claim 2, in which said beam generating means directs said beams in a common plane containing said optical axis with one of said beams lying in said axis and said two beams being generated at opposite sides of said one beam, said beam deflecting means enables the 'undeflected passage therethrough of said one beam and deflects said two beams passing therethrough in a plane containing said optical axis, and said means for supplying current to said coil means includes a source of an adjustable D.C. static convergence correction current so that said DC. current may be varied tocause said twisting displacement to compensate for any angular deviation of said plane in which said beams are directed by said generating means from said plane in which said two beams are deflected.

4. A color picture tube according to claim 3, in which deflection yoke means are provided between said convergence deflecting means and said beam selecting means and receive horizontal and vertical sweep signals generating magnetic fields to cause said beams to scan'said color screen, said screen is of generally rectangular configuration and has said optical axis directed at the center thereof so that when said beams are directed toward corners of said screen, the varying distances that the beams travel through said magnetic fields of the yoke means produce misconvergence having a component in directions perpendicular to said plane in which said two beams are deflected, and said means for supplying current to said coil means further includes a source of a dynamic convergence correcting current varied in synchronism with the scanning of said screen by said beams for modulating said twisting displacement so as to ensure accurate con vergence of said beams at each of said apertures irrespective of the positions of the latter with respect to said center of the screen.

5. A color picture tube according to claim 4, further comprising means at a location between said beam generating means and said convergence deflecting means to produce a second magnetic flux in a plane perpendicular to said axis and in directions substantially perpendicular to said plane in which the beams are directed by said beam generating means whereby to laterally displace said beams in said plane in which the latter are directed for ensuring proper passage of said beams through said convergence deflecting means.

6. A color picture tube according to claim 5, in which said means to produce said second magnetic flux includes coil means wound to produce the respective flux upon a current flow therethrough and an adjustable D.C. source of said current flow for varying the lateral displacement of said beams with respect to said convergence deflecting means.

7. A color picture tube according to claim 6, in which said coil means to produce the first mentioned magnetic flux and said coil means to produce said second magnetic flux are mounted adjacent each other.

8. A color picture tube according to claim 7, in which said coil means to produce said first magnetic flux and said coil means to produce said second magnetic flux are located adjacent a plane which passes through said optical center of the lens means and is perpendicular to said axis.

9. A color picture tube according to claim 5, further comprising means at a location between said convergence deflecting means and said beam selecting means to produce a third magnetic flux in a plane perpendicular to said axis and in directions substantially perpendicular to said plane in which the beams are deflected by said convergence deflecting means whereby to laterally displace said beams for ensuring that the point of convergence thereof at said beam selecting means is centered in an aperture of the latter tor attaining maximum color purity.

10. A color picture tube according to claim 9, in which sa d means to produce said third magnetic flux includes C011 means wound to produce said third flux upon a current flow therethrough and an adjustable D.C. source of said current flow for varying the lateral displacement of said beams with respect to said apertures of the beam selecting means.

11. A color picture tube according to claim 9, in which said means to produce said third magnetic flux is mounted adjacent said deflection yoke means.

12. A color picture tube according to claim 4, in which said source of a dynamic convergence correcting current includes means generating a saw tooth'wave signal having a period corresponding to that of said horizontal sweep signal and modulating said saw tooth wave signal with a second saw tooth wave signal having a period corresponding to that of said vertical'sweep signal.

13. A color picture tube according to claim 1, further comprising means at a location between said beam generating means and said convergence deflecting means to produce a second magnetic flux in a plane perpendicular to said axis and in directions to displace said beams for ensuring proper passage of said beams through said convergence deflecting means.

14. A color picture tube according to claim 1, in which said location of the means to produce magnetic flux is adjacent said optical center of said lens means.

15. A color picture tube according to claim 1, further comprising means to effect raster scanning of said screen by said beams, and means at a location between said convergence deflecting means and said beam selecting means to produce magnetic flux in a plane perpendicular to said axis and in directions to cause lateral displacement of said 1 l 1 2 beams for ensuring centering of the point of convergence 2,737,608 3/1956 Sziklai 315-43 X of said beams with respect to the apertures of said beam 2,907,915 10/1959 Gleichauf 315-13 selecting means during said scanning, whereby to attain maximum color purity. RODNEY D. BENNETT, JR., Primary Examiner References Cited 5 M. F. HUBLER, Asslstant Examlner US. Cl. X.R.

UNITED STATES PATENTS 313 70 79 2,679,614 5/1954 Friend 31513