Was formed from an Italian CRT Fabricant called ERGON which was sold to Thomson in 1971 and the technology further called PIL (Precision In Line) was produced by a collaboration with RCA. (ERGON S.P.A., ANAGNI, FROSINONE).
They have patented several technologyes like the LICHT-KOLLIMATOR
and various methods to improve the fabrication of shadowmasks in CRT
Tubes like the invention of a process of manufacturing a cathode-ray
tube (CRT)
having an anti-glare, anti-static, dark coating on an external surface
of a faceplate panel thereof, and more particularly, to the formulation
of such a coating. Further Inventions were related to inventions formulated for the control of electron beam for adjustment of, for example, static convergence and/or purity in a picture tube and others invention relates to a shadow mask or color selection electrode for a color television picture tube, as well as the support frame making it possible to stiffen or rigidify the mask.
Videocolor CRTs were widely used by many fabricants on European scale and even around the world.
Example of Videocolor CRTs were the P.I.L. (Precision In Line) the PIL S4 the PIL PLANAR the PIL MP the PIL FS10.......
In 2005 Videocolor was sold to Videocon An Indian monkeys dotheads conglomerate (WTF !) wich has converted it to Plasma Lcd (cheapshit Crap) manufacturing, resulting in a total FAIL !!
Now Videocolor has Stopped the production, it's gone (Forever-dead) !!
Here below the images of the rest of the completely abandoned Anagni VIDECOLOR Production Factory
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° rectangu
lar 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.
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.
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° rectangu
lar 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 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 o
f 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.
Color picture tube having an improved expanded focus lens type inline
electron gun -
VIDEOCOLOR (RCA) P.I.L. S4 CRT TUBE FAMILY TECHNOLOGYAn improved color picture tube has an inline electron gun for generating and directing three electron beams, a center beam and two side beams, along coplanar paths toward a screen of the tube. The gun includes a main focus lens for foc
using the electron beams. The main focus lens is formed by two spaced electrode members each having three separate inline apertures therein. Each electrode also includes a peripheral rim. The peripheral rims of the two electrodes face each other. The apertured portion of each electrode is within a recess set back from the rim. The width of the recess in at least one of the electrodes is wider at the side beam paths than at the outer beam path, measured perpendicular to the plane containing the electron beam paths.1. In a color picture tube having an inline electron gun for generating and directing three electron beams, a center beam and two side beams, along coplanar paths toward a screen of said tube, said gun including a main focus lens for focusing said electron beams, the main focus lens being formed by two spaced electrode members each having three separate inline apertures therein, each electrode also including a peripheral rim, the peripheral rims of the two electrodes facing each other, and the apertured portion of each electrode being within a recess set back from the rim, the improvement comprising
the width of the recess at least at the rim in at least one of the electrodes being wider at the side beam paths than at the center beam path, measured perpendicular to the plane containing the electron beam paths.
2. In a color picture tube having an inline electron gun for generating and directing three electron beams, a center beam and two side beams, along coplanar paths toward a screen of said tube, said gun including a main focus lens for focusing said electron beams, the main focus lens being formed by two spaced electrode members each having three separate inline apertures therein, each electrode also including a peripheral rim, the peripheral rims of the two electrodes facing each other, and the apertured portion of each electrode being within a recess set back from the rim, the improvement comprising
the width of the rim in at least one of the electrodes being narrower at the side beam paths than at the center beam path, the rim width being the rim thickness measured perpendicular to the plane containing the electron beam paths.
Description:
BACKGROUND OF THE INVENTION
The present invention relates to color picture tubes having improved inline electron guns, and particularly to such guns having an improved expanded focus lens for reduced spherical aberration.
An inline electron gun is one designed to generate or initiate preferably three electron beams in a common plane and direct those beams along convergent paths in that plane to a point or small area of convergence near the tube screen. In one type of inline electron gun shown in U.S. Pat. No. 3,873,879, issued to R. H. Hughes on Mar. 25, 1975, the main electrostatic focusing lenses for focusing the electron beams are formed between two electrodes referred to as the first and second accelerating and focusing electrodes. These electrodes include two cup-shaped members having bottoms facing each other. Three apertures are included in each cup bottom to permit passage of three electron beams and to form three separate main focus lenses, one for each electron beam. In a preferred embodiment, the overall diameter of the electron gun is such that the gun will fit into a 29 mm tube neck. Because of this size requirement, the three focusing lenses are very closely spaced from each other, thereby providing a severe limitation on focus lens design. It is known in the art that the larger the focus lens diameter, the less will be the spherical aberration which restricts the focus quality.
In addition to the focus lens diameter, the spacing between focus lens electrode surfaces is important, because greater spacing provides a more gentle voltage gradient in the lens which also reduces spherical aberration. Unfortunately, greater spacing between electrodes beyond a particular limit (typically 1.27 mm) generally is not permissible because of beam bending from electrostatic charges on the neck glass penetrating into the space between the electrodes, which causes electron beam misconvergence.
In copending U.S. patent application Ser. No. 201,692, filed Oct. 29, 1980 by R. H. Hughes and B. G. Marks, now Pat. No. 4,370,592, an electron gun is described wherein the main focus lens is formed by two spaced electrodes. Each electrode includes a plurality of apertures therein equal to the number of electron beams and also a peripheral rim, with the peripheral rims of the two electrodes facing each other. The apertured portion of each electrode is located within a recess set back from the rim. The effect of this main focus lens is to provide the gentle voltage gradient sought to reduce spherical aberration. Because of the asymmetrical shape of the peripheral rims of the two electrodes, described in patent application Ser. No. 201,692, horizontal and vertical focus voltage components for the inner and outer guns are not the same. In the vertical direction, the center electron beam sees more a slot, and experiences more focusing action, than the sides, where the focusing geometry is bounded in part by a circular arc. This is because the field penetrates the slot in the vertical direction more easily than an inscribed circular boundary. Likewise, the horizontal focusing component at the outer electron beams may be more active than at the center beam, because the field in the horizontal direction falls away more rapidly at the side ends of the peripheral rims than within the center of the recessed cavity. Therefore, there is a need to modify the peripheral rim geometry to unitize the focus voltages.
SUMMARY OF THE INVENTION
An improved color picture tube has an inline electron gun for generating and directing three electron beams, a center beam and two side beams, along coplanar paths toward a screen of the tube. The gun includes a main focus lens for focusing the electron beams. The main focus lens is formed by two spaced electrode members each having three separate inline apertures therein. Each electrode also includes a peripheral rim. The peripheral rims of the two electrodes face each other. The apertured portion of each electrode is within a recess set back from the rim. The width of the recess in at least one of the electrodes is wider at the side beam paths than at the center beam path, measured perpendicular to the plane containing the electron beam paths.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view partly in axial section, of a shadow mask color picture tube embodying the invention.
FIG. 2 is a partial axial section view of the electron gun shown in dashed lines in FIG. 1.
FIG. 3 is an axial sectional view of the G3 and G4 electrodes of the electron gun of FIG. 2.
FIG. 4 is a front view of the G4 electrode taken at line 4--4 of FIG. 3.
FIG. 5 is a plan view of the stigmators on the G4 electrode taken at line 5--5 of FIG. 2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 is a plan view of a rectangular color picture tube having a glass envelope 10 comprising a rectangular faceplate panel or cap 12 and a tubular neck 14 connected by a rectangular funnel 16. The panel comprises a viewing faceplate 18 and peripheral flange or sidewall 20 which is sealed to the funnel 16. A mosaic three-color phosphor screen 22 is carried by the inner surface of the faceplate 18. The screen is preferably a line screen with the phosphor lines extending substantially perpendicular to the high frequency raster line scan of the tube (normal to the plane of FIG. 1). A multiapertured color selection electrode or shadow mask 24 is removably mounted, by conventional means, in predetermined spaced relation to the screen 22. An improved inline electron gun 26, shown schematically by dotted lines in FIG. 1, is centrally mounted within the neck 14 to generate and direct three electron beams 28 along coplanar convergent paths through the mask 24 to the screen 22.
The tube of FIG. 1 is designed to be used with an external magnetic deflection yoke, such as the yoke 30 in the neighborhood of their junction. When activated, the yoke 30 subjects the three beams 28 to magnetic fields which cause the beams to scan horizontally and vertically in a rectangular raster over the screen 22. 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 30. Because of fringe fields, the zone of deflection of the tube extends axially, from the yoke 30 into the region of the gun 26. For simplicity, the actual curvature of the deflection beam paths in the deflection zone is not shown in FIG. 1.
The details of the gun 26 are shown in FIGS. 2 through 5. The gun comprises two glass support rods 32 on which the various electrodes are mounted. These electrodes include three equally spaced coplanar cathodes 34 (one for each beam), a control grid electrode 36 (G1), a screen grid electrode 38 (G2), a first accelerating and focusing electrode 40 (G3), and a second accelerating and focusing electrode 42 (G4), spaced along the glass rods 32 in the order named. Each of the G1 through G4 electrodes has three inline apertures therein to permit passage of three coplanar electron beams. The main electrostatic focusing lens in the gun 26 is formed between the G3 electrode 40 and the G4 electrode 42. The G3 electrode 40 is formed with four cup-shaped elements 44, 46, 48 and 50. The open ends of two of these elements, 44 and 46, are attached to each other, and the open ends of the other two elements, 48 and 50, are also attached to each other. The closed end of the third element 48 is attached to the closed end of the second element 46. Although the G3 electrode 40 is shown as a four-piece structure, it could be fabricated from any number of elements, including a single element of the same length. The G4 electrode 42 also is cup-shaped, but has its open end closed with an apertured plate 52.
The facing closed ends of the G3 electrode 40 and the G4 electrode 42 have large recesses 54 and 56, respectively, therein. The recesses 54 and 56 set back the portion of the closed end of the G3 electrode 40 that contains three apertures, 58, 60 and 62, from the portion of the closed end of the G4 electrode 42 that contains three apertures, 64, 66 and 68. The remaining portions of the closed ends of the G3 electrode 40 and the G4 electrode 42 form rims 70 and 72, respectively, that extend peripherally around the recesses 54 and 56. The rims 70 and 72 are the closest portions of the two electrodes 40 and 42. It has been found that the vertical focusing action on the center electron beam can be decreased by reducing the width of the rim 72 on the G4 electrode 42, the divergent side of the electrostatic lens formed in and between the recesses 54 and 56. As shown in FIG. 4, the recess 56 in the G4 electrode 42 is wider at the side beam path than at the center beam paths, the width being measured perpendicular to the plane containing the electron beam paths. It also has been found that the horizontal focusing action on the two outerbeams can be decreased by decreasing the length of the recess 56 in the G4 electrode.
The electron gun 26 of FIG. 2 provides a main focusing lens having substantially reduced spherical aberration compared to that of prior guns discussed above. The reduction in spherical aberration is caused by an increase in the size of the main focus lens. This increase in lens size results from recessing the electrode apertures. In most prior inline guns, the strongest equipotential lines of the electrostatic field are concentrated at each opposing pair of apertures. However, in the gun 26 of FIG. 2, the strongest equipotential lines extend continuously from between the rims 70 and 72, so that the predominant portion of the main focus lens appears to be a single large lens extending through the three electron beam paths. The remaining portion of the main focus lens is formed by weaker equipotential lines located at the apertures in the electrodes. The performance and advantages of an electron gun similar to the electron gun 26 are discussed in previously cited copending U.S. patent application Ser. No. 201,692.
There is a slot effect astigmatism formed by the main focusing lens as a result of penetration of the vertical focusing field through the open areas of the recesses. This effect is caused by the greater compression of vertical equipotential lines than of horizontal equipotential lines. The field penetration causes the focus lens to have greater vertical lens strength than horizontal lens strength. A correction is made for this astigmatism in the electron gun 26 of FIG. 2 by the inclusion of a horizontal slot opening at the exit of the G4 electrode 42. One particular embodiment has the slot width one-half the lens diameter and is spaced from the opposite surface of the G4 electrode at 86 percent of the lens diameter. This slot is formed by two strips 96 and 98, shown in FIGS. 2 and 5, welded to the apertured plate 52 of the G4 electrode 42 so as to extend across the three apertures therein in the plate 52.
To statically converge the two outer beams with the center beam, the length "E" of the recess 56 in the G4 electrode 42 is slightly greater than the length "F" of the recess 54 in the G3 electrode 40 (FIG. 3). The effect of the greater recess length in the G4 electrode 42 is the same as that discussed with respect to the offset apertures in U.S. Pat. No. 3,772,554, issued to Hughes on November 13, 1973.
Some typical dimensions for an electron gun such as the electron gun 26 of FIG. 2, but without the slot formed by strips 96 and 98, are presented in the following table.
In various other inline electron gun embodiments, the depth "G" of the recesses in the electrodes 40 and 42 may vary from 1.30 mm to 2.80 mm and the depth of the recesses in the two electrodes 40 and 42 may vary from each other.
The present invention relates to color picture tubes having improved inline electron guns, and particularly to such guns having an improved expanded focus lens for reduced spherical aberration.
An inline electron gun is one designed to generate or initiate preferably three electron beams in a common plane and direct those beams along convergent paths in that plane to a point or small area of convergence near the tube screen. In one type of inline electron gun shown in U.S. Pat. No. 3,873,879, issued to R. H. Hughes on Mar. 25, 1975, the main electrostatic focusing lenses for focusing the electron beams are formed between two electrodes referred to as the first and second accelerating and focusing electrodes. These electrodes include two cup-shaped members having bottoms facing each other. Three apertures are included in each cup bottom to permit passage of three electron beams and to form three separate main focus lenses, one for each electron beam. In a preferred embodiment, the overall diameter of the electron gun is such that the gun will fit into a 29 mm tube neck. Because of this size requirement, the three focusing lenses are very closely spaced from each other, thereby providing a severe limitation on focus lens design. It is known in the art that the larger the focus lens diameter, the less will be the spherical aberration which restricts the focus quality.
In addition to the focus lens diameter, the spacing between focus lens electrode surfaces is important, because greater spacing provides a more gentle voltage gradient in the lens which also reduces spherical aberration. Unfortunately, greater spacing between electrodes beyond a particular limit (typically 1.27 mm) generally is not permissible because of beam bending from electrostatic charges on the neck glass penetrating into the space between the electrodes, which causes electron beam misconvergence.
In copending U.S. patent application Ser. No. 201,692, filed Oct. 29, 1980 by R. H. Hughes and B. G. Marks, now Pat. No. 4,370,592, an electron gun is described wherein the main focus lens is formed by two spaced electrodes. Each electrode includes a plurality of apertures therein equal to the number of electron beams and also a peripheral rim, with the peripheral rims of the two electrodes facing each other. The apertured portion of each electrode is located within a recess set back from the rim. The effect of this main focus lens is to provide the gentle voltage gradient sought to reduce spherical aberration. Because of the asymmetrical shape of the peripheral rims of the two electrodes, described in patent application Ser. No. 201,692, horizontal and vertical focus voltage components for the inner and outer guns are not the same. In the vertical direction, the center electron beam sees more a slot, and experiences more focusing action, than the sides, where the focusing geometry is bounded in part by a circular arc. This is because the field penetrates the slot in the vertical direction more easily than an inscribed circular boundary. Likewise, the horizontal focusing component at the outer electron beams may be more active than at the center beam, because the field in the horizontal direction falls away more rapidly at the side ends of the peripheral rims than within the center of the recessed cavity. Therefore, there is a need to modify the peripheral rim geometry to unitize the focus voltages.
SUMMARY OF THE INVENTION
An improved color picture tube has an inline electron gun for generating and directing three electron beams, a center beam and two side beams, along coplanar paths toward a screen of the tube. The gun includes a main focus lens for focusing the electron beams. The main focus lens is formed by two spaced electrode members each having three separate inline apertures therein. Each electrode also includes a peripheral rim. The peripheral rims of the two electrodes face each other. The apertured portion of each electrode is within a recess set back from the rim. The width of the recess in at least one of the electrodes is wider at the side beam paths than at the center beam path, measured perpendicular to the plane containing the electron beam paths.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view partly in axial section, of a shadow mask color picture tube embodying the invention.
FIG. 2 is a partial axial section view of the electron gun shown in dashed lines in FIG. 1.
FIG. 3 is an axial sectional view of the G3 and G4 electrodes of the electron gun of FIG. 2.
FIG. 4 is a front view of the G4 electrode taken at line 4--4 of FIG. 3.
FIG. 5 is a plan view of the stigmators on the G4 electrode taken at line 5--5 of FIG. 2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 is a plan view of a rectangular color picture tube having a glass envelope 10 comprising a rectangular faceplate panel or cap 12 and a tubular neck 14 connected by a rectangular funnel 16. The panel comprises a viewing faceplate 18 and peripheral flange or sidewall 20 which is sealed to the funnel 16. A mosaic three-color phosphor screen 22 is carried by the inner surface of the faceplate 18. The screen is preferably a line screen with the phosphor lines extending substantially perpendicular to the high frequency raster line scan of the tube (normal to the plane of FIG. 1). A multiapertured color selection electrode or shadow mask 24 is removably mounted, by conventional means, in predetermined spaced relation to the screen 22. An improved inline electron gun 26, shown schematically by dotted lines in FIG. 1, is centrally mounted within the neck 14 to generate and direct three electron beams 28 along coplanar convergent paths through the mask 24 to the screen 22. The tube of FIG. 1 is designed to be used with an external magnetic deflection yoke, such as the yoke 30 in the neighborhood of their junction. When activated, the yoke 30 subjects the three beams 28 to magnetic fields which cause the beams to scan horizontally and vertically in a rectangular raster over the screen 22. 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 30. Because of fringe fields, the zone of deflection of the tube extends axially, from the yoke 30 into the region of the gun 26. For simplicity, the actual curvature of the deflection beam paths in the deflection zone is not shown in FIG. 1.
The details of the gun 26 are shown in FIGS. 2 through 5. The gun comprises two glass support rods 32 on which the various electrodes are mounted. These electrodes include three equally spaced coplanar cathodes 34 (one for each beam), a control grid electrode 36 (G1), a screen grid electrode 38 (G2), a first accelerating and focusing electrode 40 (G3), and a second accelerating and focusing electrode 42 (G4), spaced along the glass rods 32 in the order named. Each of the G1 through G4 electrodes has three inline apertures therein to permit passage of three coplanar electron beams. The main electrostatic focusing lens in the gun 26 is formed between the G3 electrode 40 and the G4 electrode 42. The G3 electrode 40 is formed with four cup-shaped elements 44, 46, 48 and 50. The open ends of two of these elements, 44 and 46, are attached to each other, and the open ends of the other two elements, 48 and 50, are also attached to each other. The closed end of the third element 48 is attached to the closed end of the second element 46. Although the G3 electrode 40 is shown as a four-piece structure, it could be fabricated from any number of elements, including a single element of the same length. The G4 electrode 42 also is cup-shaped, but has its open end closed with an apertured plate 52. The facing closed ends of the G3 electrode 40 and the G4 electrode 42 have large recesses 54 and 56, respectively, therein. The recesses 54 and 56 set back the portion of the closed end of the G3 electrode 40 that contains three apertures, 58, 60 and 62, from the portion of the closed end of the G4 electrode 42 that contains three apertures, 64, 66 and 68. The remaining portions of the closed ends of the G3 electrode 40 and the G4 electrode 42 form rims 70 and 72, respectively, that extend peripherally around the recesses 54 and 56. The rims 70 and 72 are the closest portions of the two electrodes 40 and 42. It has been found that the vertical focusing action on the center electron beam can be decreased by reducing the width of the rim 72 on the G4 electrode 42, the divergent side of the electrostatic lens formed in and between the recesses 54 and 56. As shown in FIG. 4, the recess 56 in the G4 electrode 42 is wider at the side beam path than at the center beam paths, the width being measured perpendicular to the plane containing the electron beam paths. It also has been found that the horizontal focusing action on the two outer
beams can be decreased by decreasing the length of the recess 56 in the G4 electrode. The electron gun 26 of FIG. 2 provides a main focusing lens having substantially reduced spherical aberration compared to that of prior guns discussed above. The reduction in spherical aberration is caused by an increase in the size of the main focus lens. This increase in lens size results from recessing the electrode apertures. In most prior inline guns, the strongest equipotential lines of the electrostatic field are concentrated at each opposing pair of apertures. However, in the gun 26 of FIG. 2, the strongest equipotential lines extend continuously from between the rims 70 and 72, so that the predominant portion of the main focus lens appears to be a single large lens extending through the three electron beam paths. The remaining portion of the main focus lens is formed by weaker equipotential lines located at the apertures in the electrodes. The performance and advantages of an electron gun similar to the electron gun 26 are discussed in previously cited copending U.S. patent application Ser. No. 201,692.
There is a slot effect astigmatism formed by the main focusing lens as a result of penetration of the vertical focusing field through the open areas of the recesses. This effect is caused by the greater compression of vertical equipotential lines than of horizontal equipotential lines. The field penetration causes the focus lens to have greater vertical lens strength than horizontal lens strength. A correction is made for this astigmatism in the electron gun 26 of FIG. 2 by the inclusion of a horizontal slot opening at the exit of the G4 electrode 42. One particular embodiment has the slot width one-half the lens diameter and is spaced from the opposite surface of the G4 electrode at 86 percent of the lens diameter. This slot is formed by two strips 96 and 98, shown in FIGS. 2 and 5, welded to the apertured plate 52 of the G4 electrode 42 so as to extend across the three apertures therein in the plate 52.
To statically converge the two outer beams with the center beam, the length "E" of the recess 56 in the G4 electrode 42 is slightly greater than the length "F" of the recess 54 in the G3 electrode 40 (FIG. 3). The effect of the greater recess length in the G4 electrode 42 is the same as that discussed with respect to the offset apertures in U.S. Pat. No. 3,772,554, issued to Hughes on November 13, 1973. Some typical dimensions for an electron gun such as the electron gun 26 of FIG. 2, but without the slot formed by strips 96 and 98, are presented in the following table.
| TABLE |
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| External diameter of tube neck 29.00 mm Internal diameter of tube neck 24.00 mm Spacing between G3 and G4 electrodes 40 and 42 1.27 mm Center-to-center spacing between adjacent apertures in G3 electrode 40 (A in FIG. 3) 6.6 mm Inner diameter of apertures 58, 60 and 62 in G3 electrode 40 (B in FIG. 3) 5.4 mm Width at center beam path of recess 56 in G4 electrode 42 (C in FIG. 4) 6.30 mm Width near outer beam paths of recess 56 in G4 electrode 42 (D in FIG. 4) 7.02 mm Length of recess 56 in G4 electrode 42 (E in FIG. 3) 20.7 mm Length of recess 54 in G3 electrode 40 (F in FIG. 3) 20.2 mm Depth of recess in the electrodes 40 and 42 (G in FIG. 3) 1.65 mm Width of G3 electrode 6.99 mm |
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