VIDEOCOLOR (RCA Technology) CORRECTING COLOR PURITY METHOD.

CORRECTING COLOR PURITY METHOD.


A cathode ray tube of a color television receiver includes a magnetic material located adjacent to a neck portion. A magnetizing apparatus is used for establishing the color purity of three in-line electron beams within the cathode ray tube. The magnetizing apparatus comprises at least two elongated conductor loops arranged for positioning about the neck in proximity to the magnetic material and capable of being energized by a magnetizing current for creating permanently magnetized regions within the material. The magnetized regions produce a color purity magnetic field within the cathode ray tube for establishing the color purity of the three in-line electron beams. An elongated portion of each of the conductor loops follows along a portion of the periphery of the neck.




What is claimed is:

1. A magnetizing apparatus for use in establishing the color purity of three in-line electron beams within a cathode ray tube of a color television receiver, said cathode ray tube including a magnetic material located adjacent to a neck portion, comprising:
at least two elongated conductor loops arranged for positioning about said neck portion in proximity to said magnetic material and capable of being energized by a magnetizing current for creating permanently magnetized regions within said magnetic material for producing a color purity magnetic field within said cathode ray tube for establishing the color purity of said three in-line electron beams, an elongated portion of each of said conductor loops following along a portion of the periphery of said neck portion.


2. A magnetizing apparatus according to claim 1, wherein said magnetizing apparatus creates magnetized regions near the in-line axis of said three in-line electron beams sufficiently extensive and of sufficient pole strength to produce a pincushion-shaped color purity magnetic field in a plane perpendicular to the central axis of said cathode ray tube adjacent to said magnetic material.

3. A magnetizing apparatus according to claim 2, wherein the ends of each of said conductor loops are located within approximately 5 degrees of said in-line axis.

4. A magnetizing apparatus for use in establishing the color purity of three in-line electron beams within a cathode ray tube of a color television receiver, said cathode ray tube including a magnetic material located adjacent to a neck portion, comprising:
at least two conductor loops arranged for positioning about said neck portion in proximity to said magnetic material and capable of being energized by a magnetizing current for creating permanently magnetized regions within said magnetic material for producing a color purity magnetic field within said cathode ray tube for establishing the color purity of said three in-line electron beams, said conductor loops shaped in a manner that will produce within said magnetic material upon energization by said magnetizing current a magnetic field that includes a substantial component that is tangential to a circumference of said neck portion.


5. A magnetizing apparatus according to claim 4, wherein said magnetizing apparatus creates magnetized regions angularly located near the in-line axis of said three in-line electron beams sufficiently extensive and of sufficient pole strength to produce a pincushion-shaped color purity magnetic field in a plane perpendicular to the central axis of said cathode ray tube adjacent said magnetic material.

6. A magnetizing apparatus according to claim 4, wherein each of said conductor loops is shaped to extend tangentially to a circumference of said neck portion.

7. A magnetizing apparatus according to claim 6, wherein the ends of said conductor loops is located within approximately 5 degrees of said in-line axis.

8. A cathode ray tube including a magnetic material located adjacent to a neck portion of said cathode ray tube with magnetized regions for establishing the color purity of three color electron beams within said cathode ray tube, said magnetic material including permanently magnetized regions fixedly located with polarities and pole strengths of the permanent regions selected to establish a color purity electron beam moving field, the permanent magnetic field within said magnetic material including a substantial component that is tangential to a circumference of said neck portion.

9. A cathode ray tube according to claim 8, wherein said magnetic field is of a pincushion shape in a plane perpendicular to the central axis of said cathode ray tube adjacent said magnetic material.

10. A cathode ray tube according to claim 9, wherein said magnetized regions extend adjacent to said in-line axis.

11. A cathode ray tube including a magnetic material located adjacent to a neck portion of said cathode ray tube with permanently magnetized regions fixedly located with polarities and pole strengths of the permanently magnetized regions establishing the color purity of three in-line electron beams within said cathode ray tube, said magnetic material including permanently magnetized regions that extend adjacent to the in-line axis of said electron beams.

12. A cathode ray tube including a magnetic material located adjacent a neck portion, said magnetic material including magnetized regions created by the magnetizing apparatus of claim 1.

13. A magnetizing apparatus for use in establishing the color purity of three in-line electron beams within a cathode ray tube of a color television receiver, said cathode ray tube including a magnetic material located adjacent to a neck portion, comprising:
an elongated conductor loop arrangement capable of being positioned about said neck portion in proximity to said magnetic material and capable of being energized by a magnetizing current for creating fixedly located permanently magnetized regions within said magnetic material for producing a color purity magnetic field within said cathode ray tube for establishing the color purity of said three in-line electron beams, said conductor loop arrangement shaped in a manner that will produce within said magnetic material upon energization by said magnetizing current a magnetic field that includes a substantial component that is tangential to a circumference of said neck portion.


14. A method of establishing the color purity of three in-line electron beams of a cathode ray tube comprising the steps of:
locating a magnetic material adjacent a neck portion of said cathode ray tube;
positioning about said neck portion at least two elongated conductor loops with an elongated portion of each of said conductor loops following along a portion of the periphery of said neck portion for creating permanently magnetized regions within said magnetic material upon the coupling of magnetizing current to said elongated conductor loops;
determining the amount of color purity correction required; and
coupling magnetizing current of predetermined magnitudes and directions to said elongated conductor loops for establishing the color purity of said three in-line electron beams.


15. A method according to claim 14, wherein said elongated conductor loops are positioned about said neck portion in a manner creating magnetized regions near the in-line axis of said three in-line electron beams sufficiently extensive and of sufficient pole strength to produce a pincushioned-shaped color purity magnetic field in a plane perpendicular to the central axis of said cathode ray tube adjacent to said magnetic material.



BACKGROUND OF THE INVENTION
This invention relates to color purity adjustment of cathode ray tubes for color television receivers.
Color display systems such as utilized in color television receivers include a cathode ray tube in which three electron beams are modulated by color-representative video signals. The beams impinge on respective color phosphor areas on the inside of the tube viewing screen through apertures in a shadow mask. To accurately reproduce a color screen, the three beams must be substantially converged at the screen at all points on the raster. The deflection center of each of the three beams must be correctly located in the yoke deflection plane to establish color purity. Incorrectly located deflection centers, due to such factors as incorrect placement of the deflection yoke, tolerances in the manufacture of the electron beam guns, and their assembly into the cathode ray tube neck, frequently result in color misregistration.
Many color purity devices include structure for producing adjustable magnetic fields. The devices are placed over the neck of the cathode ray tube, and the magnetic fields are appropriately adjusted to provide for color purity of the electron beams. Such adjustment is accomplished by moving magnetic field producing elements, by rotating magnetized rings about the cathode ray tube neck, or by rotating cylindrical magnets about an axis.
Other color purity devices, such as disclosed in German Provisional Pat. No. 2,611,633, filed Mar. 19, 1976, published Oct. 21, 1976, by Piet Gerard Joseph Barten et al., produce permanent nonadjustable magnetic fields. In a first step, an auxiliary device having eight coils circumferentially located is placed around the cathode ray tube neck. Appropriately valued DC currents flowing through the coils establish a magnetic field which provide for color purity of the electron beams. The values of the DC currents provide data to a magnetizing apparatus which in a second step magnetizes regions within a sheath or strip of magnetic material producing the aforementioned permanent nonadjustable magnetic fields. The magnetized strip, when placed over the neck of the cathode ray tube, establishes the color purity of the electron beams.
It is desirable, when using such a magnetic strip, to eliminate the step of utilizing an auxiliary device for determining the locations within the strip where magnetized regions are to be established. A magnetizing apparatus, not utilizing such an auxiliary device, should have magnetizing areas arranged to facilitate uncomplicated operation when directly performing color purity operations.
For an in-line cathode ray tube with three in-line electron beams and a slot shadow mask with vertical slot apertures, color purity correction requires only horizontal, like-direction motion of all three beams. The magnetic field produced by the permanently magnetized regions need only have a vertical component perpendicular to the in-line axis of the cathode ray tube to produce the horizontal motion.
As color purity correction may require large motions, the magnetic strip must be capable of producing a sufficiently strong color purity magnetic field. Furthermore, the correction introduced by the color purity magnetic field must not introduce any substantial misconvergence of the electron beams, that is, the motion of all three electron beams should be in substantially identical directions and of substantially identical magnitudes.


SUMMARY OF THE INVENTION
A magnetizing apparatus is used for establishing the color purity of three in-line electron beams within a cathode ray tube of a color television receiver, the cathode ray tube including a magnetic material located adjacent to a neck portion. The apparatus comprises at least two conductor loops arranged for positioning about the neck portion in proximity to the magnetic material. Each of the conductor loops is capable of being energized by a magnetizing current for creating permanently magnetized regions within the magnetic material to produce a color purity magnetic field within the cathode ray tube for establishing the color purity of the three in-line electron beams. An elongated portion of each of the conductor loops follows along a portion of the periphery of the neck portion.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a top elevation view of a cathode ray tube, magnetic material, and magnetizing apparatus according to the invention;
FIG. 2 is a magnified cross-sectional view of a portion of the cathode ray tube of FIG. 1 which illustrates the color purity of three in-line beams of the cathode ray tube;
FIG. 3 is a perspective view of the magnetizing appartus of FIG. 1 with a portion of the cathode ray tube and magnetic material removed;
FIGS. 4 and 5 illustrate magnetic field lines and forces produced by the magnetizing apparatus of FIG. 3; and
FIGS. 6-11 illustrate various magnetic field producing configurations.
DESCRIPTION OF THE INVENTION
In FIG. 1, a magnetic material comprising a magnetizable strip or sheath 20 is placed adjacent a neck portion 21 of cathode ray tube 22. Strip 20 is long enough to be wrapped around neck 21 providing only a small gap 23 at the top to avoid overlying of material. The composition of the magnetic material for strip 20 may be conventional barium ferrite mixed in a rubber or plastic binder material. Strip 20 may be held in a fixed relation to neck 21 by gluing or by wrapping around the strip a thin nonmagnetic tape.
Cathode ray tube 22 includes three in-line guns 24, 25 and 26 for producing blue, green and red electron beams, respectively. The green gun is illustratively along the central axis 53 of the tube. To obtain a raster, a deflection apparatus 27, which may comprise conventional horizontal and vertical windings, is placed around neck 21. Static or center convergence is achieved, as illustrated by the magnified cross-sectional view 99 of FIG. 2, when all three in-line beams intersect in the plane of a shadow mask 61 through an appropriate aperture 62 to impinge on a common phosphor trio of a phosphor screen 67 deposited on a faceplate 63 of cathode ray tube 22.
To obtain color purity, permanently magnetized regions of appropriate polarity and pole strength are created in magnetic strip 20. These regions produce an interior color purity magnetic field for moving the three in-line beams onto their respective color phosphor stripes 64-66, as illustrated in FIG. 2.
To create these regions, a magnetizing apparatus 28 is placed around magnetic strip 20. Magnetizing apparatus 28 comprises an annular housing 29 of nonmagnetic material within which inner surface are embedded four conductor wires 30-33 so shaped as to extend tangential to the circumference of neck 21, as illustrated in FIG. 3. Wires 30-33 may be either circular or square in cross-section. Spacers 97 & 98 separate wires 30 & 31 from wires 32 & 33. Connecting wires 34 and 35 couple together ends of wires 30 and 33 and 31 and 32, respectively. The other ends of wires 30 and 31 are coupled together by a connecting wire 36. The other ends of wires 32 and 33 are coupled to terminal wires 37 and 38, respectively. Terminal wires 37 and 38 may be coupled to a source of magnetizing current, not shown, of a selectable polarity, magnitude, and duration for creating appropriate permanently magnetized regions for establishing color purity.
With the wire coupling as described, the four wires form two elongated conductor current loops 39 and 40. Each of the conductor loops is therefore shaped to extend tangentially along the periphery of neck 21. If the conductor loops are energized by a peak magnetizing current I flowing in the direction of the arrows of FIG. 3, the current flows in each of the conductor loops in the direction indicated by the arrows in FIG. 4, the connecting and terminal wires 34-38 being functionally represented by end turns 41-44.
The magnetizing current creates magnetized regions in the material of the magnetic strip which, in turn, will produce the vertical field lines 45-47 intersecting the beams 24-26 along the in-line axis 51. The field lines produce horizontal forces and motions 48-50 for establishing the color purity of the three in-line beams. The color purity misregistration is observed on the screen of cathode ray tube 22. Current pulses of appropriate peak magnitude and direction are coupled to terminal wires 37 and 38 producing the desired beam motions. If any misregistration still exists, the above procedure is repeated until the desired degree of color purity is achieved. A method of coupling magnetizing current pulses to magnetizing apparatus 28 that will stabilize the magnetic material within strip 20 and prevent demagnetization of the magnetized mass with the magnetized regions is disclosed in co-pending U.S. patent application entitled, MAGNETIZING METHOD FOR USE WITH A CATHODE RAY TUBE, Ser. No. 819,095, filed concurrently herewith, by Joseph Leland Smith.
Barium ferrite used as the magnetic material for strip 20 has a relatively permeability near 1. Thus, as shown in FIG. 5, the magnetic field lines 52 pass through the material of strip 20 without substantial shaping or distortion. Field lines of sufficient intensity will impress a similar color purity permanent magnetic field into the material for establishing the color purity of the beams.
By shaping the elongated conductor loops 39 and 40 to extend lengthwise along a portion of the periphery of neck 21 so that the ends of the conductor loops are located adjacent the horizontal in-line axis 51, the interior magnetic field 52, in a plane perpendicular to the central axis, becomes a pincushion-shaped field, that is, a field that increases in intensity along the line of deflection of the central beam, as illustrated in FIG. 5. Such a field is desirable to offset the barrel shaped fields produced by magnetic strips 20 in planes perpendicular to central axis 53 but located at some distance from the strip. Such an arrangement provides for substantially identical magnitude motions of the three beams.
As shown in FIG. 6, magnetized regions, such as 54 and 55 which extend near the vertical center line 60, contribute to establishing a barrel field, while regions, such as 56-59 of FIG. 7 which extend closer to the in-line axis 51, contribute to establishing a pincushion field. The ends of elongated conductor loops 39 and 40 of FIG. 5 are located within approximately 5 degrees of the horizontal in-line axis 51 producing sufficiently extensive magnetized regions near the in-line axis for establishing the desired net pincushion shaped field. Alternatively or supplementarily, the pincushion shape of the field may be enhanced by diminishing or removing magnetized areas in strip 20 near the vertical center line 60, accomplished, for example, by decreasing the width of conductor loops 39-40 near the vertical center line.
Color purity correction for some cathode ray tubes may require up to ±5 mils of register correction as measured at the center of the screen in the horizontal direction. Magnetizing apparatus 28 must be capable of creating magnetized regions within strip 20 that are able to provide such magnitude motions. If a substantial component of the magnetic field within strip 20 is tangential to the periphery or circumference of neck 21, a sufficiently strong magnetic field can be created to provide these large magnitude beam motions.
Consider magnetic regions 61 and 62 within strip 20, which contain no tangential field lines but only radial lines 63, as illustrated in FIG. 8. Such field lines may be produced, for example, by placing solenoid coils near regions 61 and 62 with appropriate polarity currents flowing through the coils. For a strip thickness d and outer radius r of FIG. 8, an equivalent bar magnet configuration is shown in FIG. 9. The separation of the poles N-S of bar magnet 61a and S'-N' of bar magnet 62a is d. This separation is relatively small, typically 60 mils or less, when compared to the radius r, typically about 0.6 inch. Field line 63a, connecting the inner poles N & S', will only be slightly greater in relative magnitude than field line 63b, connecting the outer poles S & N'. The net field, represented by field line 63, will be quite small unless the pole strengths of bar magnets 61a and 62a are made relatively large.
Equivalently stated, the magnetizing current through the solenoid coils must be relatively great to produce a sufficient field intensity at the electron beam locations for providing any significant beam motion. It is even possible that a magnetizing apparatus using solenoidal coils may be incapable of producing the relatively intense fields required at the beam locations.
Furthermore, at certain locations along the central axis, field direction reversal may occur, resulting in beam motions opposite to the desired direction. Field direction reversal will occur if, at a given point on the central axis, the field lines connecting the S & N' poles of magnets 61a and 62a, respectively, are more intense than the field lines connecting the other poles of the magnets. An even stronger overall field will be needed to provide the required net motion.
Consider, however, a strip 20 with a magnetized region 64 having only tangential field lines 65, as illustrated in FIG. 10. The equivalent bar magnet configuration comprising a portion of a C-shaped magnet 64a is illustrated in FIG. 11. The poles of magnet 64a are separated by a relatively large distance subtending an angle θ. The net field 65a is sufficiently intense to provide the required beam motions.
A relatively uncomplicated method of obtaining a magnetic field within strip 20 having a substantial tangential component is to so shape conductor loops 39 and 40 as to extend tangentially to the periphery of neck 21. As illustrated in FIG. 5, the magnetic field within strip 20 has substantial tangential components, such as component 52a of field line 52. Substantial motions for beams 24-26 can be provided without requiring relatively large magnetizing currents flowing through conductor loops 39 and 40.
Should more intense magnetic fields be desired, but without increasing the magnetizing current amplitude, added conductor loops may be positioned tangential to the neck periphery. These added loops need not extend angularly as close to the in-line axis of the first loops do. The amount of added pincushion shaped field, however, will be correspondingly less.
Typical characteristics for a magnetic strip 20, cathode ray tube 22 and magnetizing apparatus 28 are as follows:
Magnetic Strip: length 3.8", width 0.675", thickness 0.060", gap width 0.100" maximum; material--barium ferrite mixed in a rubber binder with a B-H characteristic of 1.1×10 ⁶ gauss-oersteads minimum such as General Tire Compound 39900 from the General Tire & Rubber Company, Evansville, Indiana.
Cathode Ray Tube: 13 V in-line, 90° deflection, slot mask, 25 KV ultor, gun separation of 0.26", neck diameter 1.146".
Magnetizing Apparatus: four conductor loops, each loop of 0.040" square copper wire; width along central axis 225 mils, extension along the neck periphery 1.94" for an angular extension to within 5° of in-line axis; maximum beam motion required ±5 mils of register correction, peak magnetizing current needed for maximum register correction 2800 amps, magnetizing current pulse duration 15 μsec.
Static convergence correction may be performed by using conventional adjustable magnetic ring members, such as disclosed in U.S. Pat. No. 3,725,831 granted to R. L. Barbin. It may alternatively be performed by further creating appropriately magnetized regions in magnetic strip 20. A magnetizing unit capable of creating such regions is disclosed in copending U.S. Patent Application, entitled, MAGNETIZING APPARATUS & METHOD FOR PRODUCING A STATICALLY CONVERGED CATHODE RAY TUBE & PRODUCT THEREOF, Ser. No. 819,093, filed concurrently herewith, by Joseph Leland Smith. For certain cathode ray tubes, the magnetized regions for color purity correction should be those most forward of the electron guns thereby producing the least amount of beam defocussing.


In-line electron gun PRECISION IN LINE TECHNOLOGY p.i.l. :
The three co-planar beams of an in-line gun are converged near the screen of a cathode ray tube by means of two plate-like grids transverse to the beam paths and having corresponding apertures for the three beams. The three beam apertures of the first grid are aligned with the three beam paths. The two outer beam apertures of the second grid are offset outwardly relative to the beam paths to produce the desired convergence. The three sets of apertures also provide separate focusing fields for the three beams. The second plate-like grid is formed with a barrel shape, concave toward the first grid, to minimize elliptical distortion of beam spots on the screen due to crowding of the adjacent focusing fields. Each of the two outer beams is partially shielded from the magnetic flux of the deflecting yoke by means of a magnetic ring surrounding the beam path in the deflection zone, to equalize the size of the rasters scanned on the screen by the middle and outer beams. Other magnetic pieces are positioned on opposite sides of the path of the middle beam, to enhance one deflection field while reducing the transverse deflection field for that beam.

1. In a color picture tube including an evacuated envelope comprising a faceplate and a neck connected by a funnel, a mosaic color phosphor screen on the inner surface of said faceplate, a multiapertured color selection electrode spaced from said screen, an in-line electron gun mounted in said neck for generating and directing three electron beams along co-planar paths through said electrode to said screen, and a deflection zone, located in the vicinity of the junction between said neck and said funnel, wherein said beams are subjected to vertical and horizontal magnetic deflection fields during operation of said tube for scanning said beams horizontally and vertically over said screen; said electron gun comprising: 2. The structure of claim 1, wherein said electron gun further comprises a pair of magnetic elements positioned in said deflection zone on opposite sides of the middle beam path and in a plane transverse to the common plane of said paths for enhancing the magnetic deflection field in said middle beam path transverse to said common plane and for reducing the magnetic deflection field in said middle beam path along said common plane, thereby increasing the dimension of the raster scanned by the middle beam in said common plane while reducing the dimension of said raster in said transverse plane. 3. In a color picture tube including an evacuated envelope comprising a faceplate and a neck connected by a funnel, a mosaic color phosphor screen on the inner surface of said faceplate, a multi-apertured color selection electrode spaced from said screen, an in-line electron gun mounted in said neck for generating and directing three electron beams along co-planar paths through said electrode to said screen, and a deflection zone, located in the vicinity of the junction between said neck and said funnel, wherein said beams are subjected to vertical and horizontal magnetic deflection fields during operation of said tube for scanning said beams horizontally and vertically over said screen, and wherein the eccentrity of the outer ones of said beams in the deflection fields causes the sizes of the rasters scanned by the outer beams to tend to be larger than the size of the raster scanned by a middle beam, said electron gun comprising; 4. The tube as defined in claim 3, including two small discs of magnetic material located at the fringe of the deflection zone on opposite sides of the middle beam transverse to the plane of the three beams, whereby the magnetic flux on the middle beam transverse to the plane of the three beams is enhanced and the flux in the plane of the three beams is decreased thereby increasing the middle beam dimension in the plane of the three beams while reducing the middle beam dimension in the plane of the three beams.

Description:

BACKGROUND OF THE INVENTION

The present invention relates to an improved in-line electron gun for a cathode ray tube, particularly a shadow mask type color picture tube. The new gun is primarily intended for use in a color tube having a line type color phosphor screen, with or without light absorbing guard bands between the color phosphor lines, and a mask having elongated apertures or slits. However, the gun could be used in the well known dot-type color tube having a screen of substantially circular color phosphor dots and a mask with substantially circular apertures.

An in-line electron gun is one designed to generate or initiate at least two, and preferably three, electron beams in a common plane, for example, by at least two cathodes, and direct those beams along convergent paths in that plane to a point or small area of convergence near the tube screen. Various ways have been proposed for causing the beams to converge near the screen. For example, the gun may be designed to initially aim the beams, from the cathodes, towards convergence at the screen, as shown in FIG. 4 of Moodey U.S. Pat. No. 2,957,106, wherein the beam apertures in the gun electrodes are aligned along convergent paths.

In order to avoid wide spacings between the cathodes, which are undesirable in a small neck tube designed for high deflection angles, it is preferable to initiate the beams along substantially parallel (or even divergent) paths and provide some means, either internally or externally of the tube, for converging the beams near the screen. Magnet poles and/or electrostatic deflecting plates for converging in-line beams are disclosed in Francken U.S. Pat. No. 2,849,647, Gundert et al. U.S. Pat. No. 2,859,378 and Benway U.S. Pat No. 2,887,598.

The Moodey patent referred to above also includes an embodiment, shown in FIG. 2 and described in lines 4 to 23 of column 5, wherein an in-line gun for two co-planar beams comprises two spaced cathodes, a control grid plate and an accelerating grid plate each having two apertures aligned respectively with the two cathodes (as in FIG. 2) to initiate two parallel co-planar beam paths, and two spaced-apart beam focusing and accelerating electrodes of cylindrical form. The focusing electrode nearest to the first accelerating grid plate is described as having two beam apertures that are offset toward the axis of the gun from the corresponding apertures of the adjacent accelerating grid plate, to provide an asymmetric electrostatic field in the path of each beam for deflecting the beam from its initial path into a second beam path directed toward the tube axis.

Netherlands U.S. Pat. application No. 6902025, published Aug. 11, 1970 teaches that astigmatic aberration resulting in elliptical distortion of the focused screen spots of the two off-axis beams from an in-line gun, caused by the eccentricity of the in-line beams in a common focusing field between two hollow cylindrical focusing electrodes, can be partially corrected by forming the adjacent edges of the cylindrical electrodes with a sinusoidal contour including four sine waves. A similar problem is solved in a different manner in applicant's in-line gun.

Another problem that exists in a cathode ray tube having an in-line gun is a coma distortion wherein the sizes of the rasters scanned on the screen by a conventional external magnetic deflection yoke are different, because of the eccentricity of the two outer beams with respect to the center of the yoke. Messineo et al. U.S. Pat. No. 3,164,737 teaches that a similar coma distortion caused by using different beam velocities can be corrected by use of a magnetic shield around the path of one or more beams in a delta type gun. Barkow U.S. Pat. No. 3,196,305 teaches the use of magnetic enhancers adjacent to the path of one or more beams in a delta gun, for the same purpose. Krackhardt et al. U.S. Pat. No. 3,534,208 teaches the use of a magnetic shield around the middle one of three in-line beams for coma correction.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, at least two electron beams are generated along co-planar paths toward the screen of a cathode ray tube, e.g., a shadow mask type color picture tube, and the beams are converged near the screen by asymmetric electric fields established in the paths of two beams by two plate-like grids positioned between the beam generating means and the screen and having corresponding apertures suitably related to the beam paths. The apertures in the first grid (nearest the cathodes) are aligned with the beam paths. Two apertures in the second grid (nearest the screen) are offset outwardly with respect to the beam paths to produce the desired asymmetric fields. In the case of three in-line beams, the two outer apertures are offset, and the middle apertures of the two grids are aligned with each other. The pairs of corresponding apertures also provide separate focusing fields for the beams. In order to minimize elliptical distortion of one or more of the focused beam spots on the screen due to crowding of adjacent beam focusing fields, at least a portion of the second grid may be substantially cylindrically curved in a direction transverse to the common plane of the beams, and concave to the first grid. Each of the two outer beam paths of a three beam gun may be partially shielded from the magnetic flux of the deflection yoke by means of a magnetic ring surrounding each beam in the deflection zone of the tube, to minimize differences in the size of the rasters scanned on the screen by the middle and outer beams. Further correction for coma distortion may be made by positioning magnetic pieces on opposite sides of the middle beam path for enhancing one field and reducing the field transverse thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view, partly in axial section, of a shadow mask color picture tube in which the present invention is incorporated;

FIG. 2 is a front end view of the tube of FIG. 1 showing the rectangular shape;

FIG. 3 is an axial section view of the electron gun shown in dotted lines in FIG. 1, taken along the line 3--3 of that figure;

FIG. 4 is an axial section view of the electron gun taken along the line 4--4 of FIG. 3;

FIG. 5 is a rear end view of the electron gun of FIG. 4, taken in the direction of the arrows 5--5 thereof;

FIG. 6 is a transverse view, partly in section, taken along the line 6--6 of FIG. 4;

FIG. 7 is a front end view of the electron gun of FIGS. 1 and 4;

FIG. 8 is a similar end view with the final element (shield cup) removed; and

FIGS. 9 and 10 are schematic views showing the focusing and converging electric fields associated with two pairs of beam apertures in FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a plan view of a 17V-90° rectangular color picture tube, for example, having a glass envelope 1 made up of a rectangular (FIG. 2) faceplate panel or cap 3 and a tubular neck 5 connected by a rectangular funnel 7. The panel 3 comprises a viewing faceplate 9 and a peripheral flange or side wall 11 which is sealed to the funnel 7. A mosaic three-color phosphor screen 13 is carried by the inner surface of the faceplate 9. The screen is preferably a line screen with the phosphor lines extending substantially parallel to the minor axis Y-Y of the tube (normal to the plane of FIG. 1). A multi-apertured color selection electrode or shadow mask 15 is removably mounted, by conventional means, in predetermined spaced relation to the screen 13. An improved in-line electron gun 19, shown schematically by dotted lines in FIG. 1, is centrally mounted within the neck 5 to generate and direct three electron beams 20 along co-planar convergent paths through the mask 15 to the screen 13.

The tube of FIG. 1 is designed to be used with an external magnetic deflection yoke, such as the yoke 21 schematically shown, surrounding the neck 5 and funnel 7, in the neighborhood of their junction, for subjecting the three beams 20 to vertical and horizontal magnetic flux, to scan the beams horizontally and vertically in a rectangular raster over the screen 13. The initial plane of deflection (at zero deflection) is shown by the line P--P in FIG. 1 at about the middle of the yoke 21. Because of fringe fields, the zone of deflection of the tube extends axially, from the yoke 21, into the region of the gun 19. For simplicity, the actual curvature of the deflected beam paths 20 in the deflection zone is not shown in FIG. 1.

The in-line gun 19 of the present invention is designed to generate and direct three equally-spaced co-planar beams along initially-parallel paths to a convergence plane C--C, and then along convergent paths through the deflection plane to the screen 13. In order to use the tube with a line-focus yoke 21 specially designed to maintain the three in-line beams substantially converged at the screen without the application of the usual dynamic convergence forces, which causes degrouping misregister of the beam spots with the phosphor elements of the screen, the gun is preferably designed with samll spacings between the beam paths at the convergence plane C--C to produce a still smaller spacing, usually called the S value, between the outer beam paths and the central axis A--A of the tube, in the deflection plane P--P. The convergence angle of the outer beams with the central axis is arc tan e/c+d, where c is the axial distance between the convergence plane C--C and the deflection plane P--P, d is the distance between the deflection plane and the screen 13, and e is the spacing between the outer beam paths and the central axis A--A in the convergence plane C--C. The approximate dimensions in FIG. 1 are c = 2.7 inches, d = 9.8 inches, e = 0.200 inch (200 mils), and hence, the convergence angle is 55 minutes and s = 157 mils.

The details of the improved gun 19 are shown in FIGS. 3 through 8. The gun comprises two glass support rods 23 on which the various electrodes are mounted. These electrodes include three equally-spaced co-planar cathodes 25, one for each beam, a control grid electrode 27, a screen grid electrode 29, a first accelerating and focusing electrode 31, a second accelerating and focusing electrode 33, and a shield cup 35, spaced along the glass rods 23 in the order named.

Each cathode 25 comprises a cathode sleeve 37, closed at the forward end by a cap 39 having an end coating 41 of electron emissive material and a cathode support tube 43. The tubes 43 are supported on the rods 23 by four straps 45 and 47 (FIG. 6). Each cathode 25 is indirectly heated by a heater coil 49 positioned within the sleeve 37 and having legs 51 welded to heater straps 53 and 55 mounted by studs 57 on the rods 23 (FIG. 5). The control and screen grid electrodes 27 and 29 are two closely-spaced (about 9 mils) flat plates having three pairs of small (about 25 mils) aligned apertures 59 centered with the cathode coatings 41 to initiate three equally-spaced coplanar beam paths 20 extending toward the screen 13. Preferably, the initial paths 20a and 20b are substantially parallel and about 200 mils apart, with the middle path 20a coincident with the central axis A--A.

Electrode 31 comprises first and second cup-shaped members 61 and 63, respectively, joined together at their open ends. The first cup-shaped member 61 has three medium-sized (about 60 mils) apertures 75 close to grid electrode 29 and aligned respectively with the three beam paths 20, as shown in FIG. 4. The second cup-shaped member 63 has three large (about 160 mils) apertures 65 also aligned with the three beam paths. Electrode 33 is also cup-shaped and comprises a base plate portion 60 positioned close (about 60 mils) to electrode 31 and a side wall or flange 71 extending forward toward the tube screen. The base portion 69 is formed with three apertures 73, which are preferably slightly larger (about 172 mils) than the adjacent apertures 67 of electrode 31. The middle aperture 73a is aligned with the adjacent middle aperture 67a (and middle beam path 20a) to provide a substantially symmetrical beam focusing electric field between apertures 67a and 73a when electrodes 31 and 33 are energized at different voltages. The two outer apertures 73b are slightly offset outwardly with respect to the corresponding outer apertures 67b, to provide an asymmetrical electric field between each pair of outer apertures when electrodes 31 and 33 are energized, to individually focus each outer beam 20b near the screen, and also to deflect each beam, toward the middle beam, to a common point of convergence with the middle beam near the screen. In the example shown, the offset of each beam aperture 73b may be about 6 mils.

The approximate configuration of the electric fields associated with the middle and outer apertures are shown in FIGS. 9 and 10, respectively, which show the equipotential lines 74 rather than the lines of force. Assuming an accelerating field, as shown by the + signs, the left half 75 (on the left side of the central mid-plane) of each field is converging and the right half 77 is diverging. Since the electrons are being accelerated, they spend more time in the converging field than in the diverging field, and hence, the beam experiences a net converging or focusing force in each of FIGS. 9 and 10. Since the middle beam 20a passes centrally through a symmetrical field in FIG. 9, it continues in the same direction without deflection. In FIG. 10, the outer beam 20b traverses the left half 75 of the field centrally, but enters the right half 77 off-axis. Since this is the diverging part of the field, and the electrons are subjected to field forces perpendicular to the equipotential lines or surfaces 74, the beam 20b is deflected toward the central axis (downward in FIG. 10) as it traverses the right half 77, in addition to being focused. The angle of deflection, or convergence, of the beam 20b can be determined by the choice of the offset of the apertures 73 b and the voltages applied to the two electrodes 31 and 33. For the example given, with an offset of 6 mils, electrode 33 would be connected to the ultor or screen voltage, about 25 K.V., and electrode 31 would be operated at about 17 to 20 percent of the ultor voltage, adjusted for best focus. The object distance of each focus lens, that is, the distance between the first cross-over of the beams near the screen grid 29 and the lens, is about 0.500 inch; and the image distance from the lens to the screen is about 12.5. inches.

The above-described outward offset of the beam apertures to produce beam convergence is contrary to the teaching of FIG. 3 of the Moodey patent described above, and hence, is not suggested by the Moodey patent.

The focusing apertures 67 and 73 are made as large as possible, to minimize spherical aberration, and as close together as possible, to obtain a desirable small spacing between beam paths. As a result, the fringe portions of adjacent fields interact to produce some astigmatic distortion of the focusing fields, which produces some ellipticity of the normally-circular focused beam spots on the screen. In a three-beam in-line gun, this distortion is greater for the middle beam than for the two outer beams, because both sides of the middle beam field are affected. In order to compensate for this effect, and minimize the elliptical distortion of the beam spots, the wall 69, or at least the surface thereof facing the electrode 31, is curved substantially cylindrically, concave to electrode 31, in the direction normal or transverse to the plane of the three beams, as shown at 79 in FIG. 3. Preferably, this curvature is greater for the middle beam path than for the outer beam paths, hence, the wall 69 may be made barrel-shaped. In the example given, the barrel shape may have a stave radius of 8 inches (FIG. 4) and a hoop radius of 2.28 inches (FIG. 3), with the curvature 79 terminating at the outer edges of the outer apertures 73b.

The shield cup 35 comprises a base portion 81, attached to the open end of the flange 71 of electrode 33, and a tubular wall 83 surrounding the three beam paths 20. The base portion 81 is formed with a large middle beam aperture 85 (about 172 mils) and two smaller outer beam apertures 87 (about 100 mils) aligned, respectively, with the three initial beam paths 20a and 20b.

In order to compensate for the coma distortion wherein the sizes of the rasters scanned on the screen by the external magnetic deflection yoke are different for the middle and outer beams of the three-beam gun, due to the eccentricity of the outer beams in the yoke field, the electron gun is provided with two shield rings 89 of high magnetic permeability, e.g., an alloy of 52 percent nickel and 48 percent iron, known as 52 metal, are attached to the base 81, with each ring concentrically surrounding one of the outer apertures 87, as shown in FIGS. 4 and 7. These magnetic shields 89 by-pass a small portion of the fringe deflection fields in the path of the outer beams, thereby making a slight reduction in the rasters scanned by the outer beams on the screen. The shield rings 89 may have an outer diameter of 150 mils, an inner diameter of 100 mils, and a thickness of 10 mils.

A further correction for this coma distortion is made by mounting two small discs 91 of magnetic material, e.g., that referred to above, on each side of the middle beam path 20a. These discs 91 enhance the magnetic flux on the middle beam transverse to the plane of the three beams and decrease the flux in that plane, in the manner described in the Barkow patent referred to above. The discs 91 may be rings having an outer diameter of 80 mils, an inner diameter of 30 mils, and a thickness of 10 mils.

Each of the electrodes 27, 29, 31 and 33 are mounted on the two glass rods 23 by edge portions embedded in the glass. The two rods 23 extend forwardly beyond the mounting portion of electrode 33, as shown in FIG. 3. In order to shield the exposed ends 93 of the glass rods 23 from the electron beams, the shield cup 35 is formed with inwardly-extending recess portions 95 into which the rod ends 93 extend. The electron gun 19 is mounted in the neck 5 at one end by the leads (not shown) from the various electrodes to the stem terminals 97, and at the other end by conventional metal bulb spacers (not shown) which also connect the final electrode 33 to the usual conducting coating on the inner wall of the funnel 7.