US3639795A

US3639795A - Color cathode-ray tube with plural cathodes and three common grids each having same plurality of beam-defining apertures

Info

Publication number: US3639795A
Application number: US66375A
Authority: US
Inventors: Piet Gerard Joseph Barten
Original assignee: US Philips Corp
Current assignee: US Philips Corp
Priority date: 1970-08-24
Filing date: 1970-08-24
Publication date: 1972-02-01
Anticipated expiration: 1989-02-01

Abstract

A single gun cathode-ray tube for the display of color television images including a separate cathode for each electron beam and common first, second, third and fourth grids, each having apertures for each beam. The distance between successive centers of the apertures in the third grid is less than 7 mms. and the dimension of the third grid at the apertures in the direction of the axis of the gun is not more than 4 mms.

Description

O United States Patent us] 3,639,795

Barten 1 Feb. 1, 1972 [54] COLOR CATHODE-RAY TUBE WITH [58] FleldolSe'areh ..3l3/69 C. 70C

PLURAL CATHODES AND THREE COMMON cums EACH HAVING SAME [561 PLURALITY OF BEAM-DEFINING UNITED STATES PATENTS APERTURES 2,957,l06 l0/l960 Moodey ..s|s/|s 72 Inventor; pm G 1 5 m Emmasingd, 3,254,25l 5/l966 Hughes ..3l3/70 Eindhoven, Netherlands Primary Examiner-Robert Segal [73] Assrgnee: U.S. Philips Corporation, New York, N.Y. Anomey prank T if i 22 F1 d: A 24 1970 N 571 7 ABSTRACT v l 1 PP A single gun cathode-ray tube for the display of color televi- Related U.S. Application D n sion images including a separate cathodefor each electron beam and common first, second, third and fourth grids, each [63] -re of 743,765, July having apertures for each beam. The distance between succes- 1963, commuatlon'm'pan of 5 July sive centers of the apertures in the third grid is less than 7 I968- mms. and the dimension of the third grid at the apertures in the direction of the axis of the gun is not more than 4 mms. [52] US. Cl. ..3l3/70 C, 3 l 3/82 R [5 1] Int. Cl. ..l 0lj 29/50, H0 1 j 29/00 6 Claims, 3 Drawing Figures PATENTED FEB H972 3.639.795

sum 1 BF 2 INVENTOR.

BY PIET G.J. BARTEN AGEN PATENTEUFEB 11972 3.639.795

SHEET 2 [IF 2 I I I I I fig.3

INVENTOR.

BY PIET G.J. BARTEN gM d L AGEN 3 COLOR CATHODE-RAY TUBE WITH PLURAL CATI-IODES AND THREE COMMON GRIDS EACH HAVING SAME PLURALITY F BEAM-DEFINING APERTURES ,This is a continuation-in-part of application Ser. No. 743,165 filed July I0, [968 and application Ser. No. 745,551 filed July 17, 1968.

This invention relates to a cathode-ray tube having an electron gun for producing a plurality of electron beams and a lu-. minescent screen on the gun side of which and at a short distance therefrom a color-selecting electrode is arranged, in the plane of which the various electron beams are converged. The electron gun comprises a separate cathode for each electron beam. Further, a common first grid is provided, with an aperture for each beam. A common second grid is also provided, which serves as an accelerating electrode and is provided with an aperture for each beam. in addition, a common third grid having an aperture for each beam, and a common fourth grid of circular cross section, are also provided.

Such a cathode-ray tube in which the various electron beams are produced by only one gun affords, in comparison with a cathode-ray tube in whicha separate gun is present of electron beam, the advantage of amore compact arrangement of the gun so that a smaller spacing between the beams is possible. In one embodiment the common third grid comprises a circular-cylindrical portion closed at each end by a plate having an aperture for each beam and which includes another apertured plate between the said plates, the common fourth grid also being circular-cylindrical. The portion of the envelope between the fourth grid and the screen has a conductive coating. If the voltage of the conductive coating is higher than that of the common fourth grid and'if the latter voltage is higher than that of the common third grid, a focusing lens action for each electron beam occurs between the third and fourth grids. The electron beams at the same time, however, undergo a diverging lens action with respect to one another, whereas the lens action betweenthe fourth grid and the conductive coating substantially converges the electron beams with respect to one another in the plane of the color-selecting electrode, for example, a shadow mask electrode, and also focuses each electron beam still further.

However, in this case, the spacing between the various electron beams at the area of the lens action between the fourth grid and the conductive coating is large in relation to the diameter of the fourth grid. It has been found that in this case great errors in the focusing of the beam occur owing to the spherical aberration of this lens, since'this spherical aberration is proportional to the square of the ratio between the distance of the beam from theaxis of the gun andthe internal diameter of the fourth grid. Another disadvantage is that, owing to the beams not being close enough to one another, errors in convergence arise on passing through the deflection coil, so that only common dynamic convergence is insut'ficient. Furthermore, problems are also involved with regard to the impact areas at the edge of the luminescent screen which are associated with one mask hole. In that case the centers of the beams form the comers of an equilateral triangle in the gun and the triangle formed by the centers of these impact areas deviates too much from an equilateral triangle.

However, the dynamic convergence required may in principle be dispensed with when using a specific kind of deflection coil, if the electron beams within the gun are substantially coplanar. If this were brought about while retaining the spacing between the beams, then thespacing of the electron beams which do not lie on the axis of the-gun is at thearea of the lens.

action between the fourth grid and the conductive coating which is large in relation to the diameter of the fourth grid. In the coplanar case large errors in the focusing of the beam would occur owing to the spherical aberration of this lens, since this spherical aberration is proportional to the square of the ratio between the distance from the beam to the axis of the.

gun and the internal diameter of the, fourth grid, Another dis passing through the deflection coil sothat a dynamic correction of convergence would still be necessary. Furthermore,

it is possible that this correction cannotbe effected in common. Also, the original geometry of the beams is disturbed owing to the aberrations of the beams on passing through the deflection coil, so that problems would arise with regard to the impact areas at the edge of the luminescent screen which are associated with one mask hole.

The. present invention is based on the discovery that, in order to avoid these disadvantages, thedistancebetween the centers of the beams and the axisof the gun' in the field of the converging lens must be as smaller. possible, but not smaller than predetermined. dimensions which depends upon the diameter of the beam locally occurring at the maximum current, since otherwise. complete color selection on the. luminescent screen athigh currents would not 'occur. In the case of a triangular gun, arrangement this distanc'e should be less than 7 mm. In the case, of coplanarbeams this implies that the spacing between the centers of the successive beams inthe field of theconverging lens must be as small as possible, preferably less than 2 mm. Because of the diverging lens action between the third and fourth grids, the distance between the centers of the beams which do not lie on'the axis of the gun and the axis of the gun in the field of the converging lens is greater than the distance between the axis of the relevant apertures in the third grid and the axis of the gun. To obtain a small spacing between the beams in the field of the converging lens, the relevant aperturesin the third grid must therefore be advantage would bethat, owing to the beams not being close.

enough to one another, errorsin convergence wouldarise on placed closer to the'axis of the gun. However, the apertures cannot approach the axisof'the gun without limit, since otherwise too few conductive parts of the grid would be present in the vicinity of the beams on the side of the axis of the gun. This is also true of the preceding grids, but-the difficultyarises especially for the third grid since the diameters of the beams are greater in situ and hence require larger apertures in the grid. To ensure that at the thirdgrid sufficiently conductive parts are still present on the side of the axis 'of the gun, the diameters of the beams on entering this grid must be as small as possible and must increase as little as possible on passing through it. The latter can be achieved by making the grid as shortaspossible. 1 a

According to the invention the distance between the centers of the apertures in the third grid (which centers li e' substantially in a plane passing through the axis of the gun and the axis of the gun is less than 4 mm. and thedimension of the. third grid at the. apertures in the direction of the axis of the gun is at most equal to this distance, and preferably less than. 1 mm. It is .noted that centers which lie substantially in a plane passing through the axis of the gun include both triangular and coplanar aperture arrangements. The errors which occur in the focusing of each beam owing to spherical aberration of the converging lens are in this case considerably smaller. Furthermore, fewer errors in convergence occur on passing through the deflection coil so that, if a suitable deflection system is used, only common dynamic convergence is sufiicient. The smaller aberrations of the beams on passing through the deflection coil also cause the original geometry of the beams being retained better so that the problems relating to the impact area on the luminescent screen become smaller and hence a larger angle of deflection, forexample in the tube is possible. In the case of acommon third grid which is short'in the axial direction, the focusing lens action for each electron beam takes place between the second and fourth grids. This focusing lens action must be such that an image of the crossover of each beam is produced substantially at the point of intersection of the axis of the beamupon entering the third grid. andthe axis of the gun. Only in this case can complete convergence of the beams in theplane-of the colorselecting electrode as well as complete focusing of each beam in this planebe obtained at a specific strength of the converginglens. In the particular case in which the axes of the beams upon entering the third grid extend in parallel with the axis of the gun, the said .pointof intersection. lies at infinity and an image of the crossover of each beam must be produced there. In the latter case the centers of the apertures in the first, second and third grids lie on lines parallel to the axis of the gun. This embodiment will generally be preferred for structural reasons. The axes of the cathodes need not then coincide with those of the apertures in the grids and the minimum distance permissible between the cathodes need not in themselves be a limiting factor in the choice of the distance between the apertures in the third grid.

The converging lens may be an accelerating lens comprising two electrodes, i.e., a common fourth grid and either a common fifth grid or a conductive coating or it may be a threeelectrode lens, for example, of the so-called unipotential type. An accelerating lens affords in itself the advantage that its spherical aberration is smaller than that of a three-electrode lens of the same diameter and strength. More particularly the electron gun includes a common, substantially circular-cylindrical fifth grid. This hasthe advantage that the fifth grid can be satisfactorily centered relative to the other components of the electron gun. If the accelerating lens comprises the common fourth grid and a conductive coating, difficulties may arise because the inner wall of the glass neck which carries the conductive coating, must not exhibit deviations from the cylindrical shape and the axis of the gun must coincide with the axis of the neck with high precision.

For the focusing lens action on each electron beam it is desirable that the focusing field for each electron beam should be as far as possible rotationally symmetrical about the axis of the electron beam. In the tube the focusing lens action takes place between the second and fourth grids. The axis of the common fourth grid lies eccentrically of the axes of the electron beams, resulting in principle in disturbance of the rotational symmetry about the axes of the beams. This disturbance may be inhibited by taking a special step for the third grid. Preferably the third grid is provided, on the side adjacent the fourth grid, with an annular elevation having a diameter which is greater than the enveloping area of the apertures and at most equal to that of the portion of the fourth grid which is adjacent the third grid.

The third grid, which is short in the axial direction, furthermore permits the beams at the first, second and third grids to be brought closer to each other than would be strictly necessary because of their distance from the axis of the gun in the field of the converging lens. While retaining the last-mentioned distance a stronger divergence of the beams is thus possible at the area of the lens action between the third and fourth grids, which affords certain advantages. This stronger divergence can in this case be obtained without making the fourth grid shorter. Shortening the fourth grid is in itself objectionable since as a first electrode of the converging lens it must always be longer than a predetermined length, in order to ensure that the focusing field occurring at the third grid is sufficiently separate from the converging field. The stronger diverging lens action between the third and fourth grids may be obtained by reducing the section of the fourth grid at an unchanged voltage on this grid. However, the section of the fourth grid at the converging lens should be maximum, in order that the spherical aberration is minimum. More particularly, the section of the fourth grid is therefore smaller on the side of the third grid than on the other side. The convergence of the beams diverging more strongly has naturally to be stronger. In the case of an accelerating lens this may be achieved with a lower voltage at the fourth grid, which in itself is advantageous, and in the case of a three-electrode lens by lengthening the central electrode. In either case the advantage arises that the dynamic convergence can be operated at a voltage of smaller amplitude. It has been found that a smaller section of the electron spot on the luminescent screen is also obtained in this manner. Although the lower voltage on the fourth grid, which is desirable for the stronger convergence in the case of an accelerating lens, in itself attenuates the diverging lens action between the third and fourth grids, this attenuation is considerably smaller than the increase in diverging lens action resulting from the reduced section. The specified shape of the fourth grid can be obtained in several other ways. The fourth grid may comprise at least two interconnected portions of substantially circular-cylindrical shape. In another form the fourth grid is conical at least in part.

If the fourth grid comprises at least two interconnected portions of substantially circular-cylindrical shape, very advantageous results have been obtained with a structure in which the length of the portion located on the side of the third grid is substantially equal to the inner radius of this portion.

The invention will now be described in detail with reference to the accompanying drawing, in which:

FIG. 1 is a sectional view of a cathode-ray tube.

FIG. 2 shows a cross-sectional view of an electron gun according to the invention.

FIG. 3 shows a cross-sectional view of another embodiment of the invention.

The cathode-ray tube 1 has a gun 2, shown diagrammatically which produces three electron beams with centers at an gles of 120 with the axis of the gun as shown in Figure 2, or with centers coplanar with the axis of the gun. The gun 2 converges the three electron beams on a shadow mask 3 to thereafter strike certain portions of a luminescent screen 4. The screen is scanned by a deflection device 5, shown diagrammatically. I

Figure 2 shows, in part section, the neck portion of the tube. One of the three cathodes, designated 6, of the gun can be seen, the two other cathodes being rotated through angles of 120 relating to the axis 7 of the gun. The gun comprises a common first grid 8 having an aperture 9 for the beam originating from the cathode 6, a common second grid 10 having an aperture 11 for the said beam, and a common third grid. 12 having an aperture 13 for the said beam. The centers of the apertures 9, l1 and 13 lie on a line 19 parallel to the axis of the gun, which in this case coincides with the axis of cathode 6. The gun also includes a common fourth grid 14 and a common circular-cylindrical fifth grid 15. The fourth grid comprises two

interconnected portions

16 and 17 of circular-cylindrical shape. The plate-shaped third grid 12 has an annular elevation 18 on the side adjacent the fourth grid.

In a specific case the distance between the cathode 6 and the first grid 8 is 0.09 mm., that between the first grid 8 and the second grid 10 is 1.0 mm., and that between the second grid 10 and the third grid 12 is 1.5 mm. The thickness of the first grid 8, of the second grid 10 and the third grid 12 are 0.20 mm., 0.50 mm. and 0.50 mm. respectively. The

circular apertures

9, 11 and 13 have diameters of 0.75 mm., 0.75 mm. and 2.0 mm., respectively. The distance of line 19 which passes through the centers of the

apertures

9, 11, and 13, from the axis 7 of the gun is 2.5 mm. The

portions

16 and 17 of the fourth grid 14 have internal diameters of 14 mm. and 22 mm., respectively. The diameter of the annular elevation 18 is 14 mm. and is thus equal to the internal diameter of the portion 16 of the fourth grid 14, which is adjacent the third grid 12. The internal diameter of the fifth grid 15 is 22 mm. The dimension of the elevation 18 in the direction of the axis of the gun is 1.5 mm., that of the portion 16 in this direction is 7 mm., that of the portion 17 in this direction is 22 mm., and that of the fifth grid 15 in this direction is 10 mm. The distance between the elevation 18 and the portion 16 is 2 mm. and between the portion 17 and the fifth grid 15 is also 2 mm. This gun can be operated at the following voltages:

cathode between 0 volt and volts first grid 0 volt second grid 750 volts third grid 350 volts between 3,400 volts and 4,200 volts fifth grid 25,000 volts The variable voltage on the cathode serves to control the beam and that on the fourth grid serves to converge the beams as a function of their impact areas on the screen.

Figure 3 shows, the neck portion of the tube in a section through the axis of the gun. The gun comprises three cathodes fourth grid 6', 7' and 8' in common first grid 9' having apertures of ll and 12 for the beams originating from the cathodes 6', 7' and 8', a common second grid 13' having apertures l4, l5 and I6, and a common third grid 17' having apertures l8, l9 and 20. The centers of the apertures ll, and 19' lie on the axis 21 of the gun whereas the centers of the apertures l0, l4 and 18' lie on a line 22 parallel to the axis 21 and the centers of the apertures 12', 16' and lie on a line 23 likewise parallel to the axis 21. In this case the line 22 coincides with the axis of cathode 6', the axis 21 coincides with the axis of cathode 7' and the line 23 coincides with the axis of cathode 8'. The gun also includes a common fourth grid 24 and a common circular-cylindrical fifth grid 25. The fourth grid .com-

prises two

interconnected portions

26 and 27 of circular-cylindrical shape. The plate-shaped third grid 17' has an annular elevation 28 on the side adjacent the fourth grid 24.

In a specific case the distance between the cathodes 6', 7' and 8 and the first grid 9' is 0.09 mm., between the first grid 9" and the second grid 13 is 1.0 mm. and between the second grid 13' and the'third grid 17' is 1.5 mm. The thicknesses of the first grid 9', the second grid .13 and the third grid 17' are 0.20 mm., 0.50 mm. and 0.50 mm. respectively. The circular apertures 10', 11 and 12' in the first grid, 14, 15 and 16 in the second grid, and 18, 19' and 20 in the third grid have diameters of 0.75 mm., 0.75 mm. and 2.0 mm., respectively. The distance of line 22 through the'centers of the apertures l2, l6 and 20 from the axis 21 of the gun is 3.5 mm. The

portions

26 and 27 of the fourth grid 24 have internal diameters of 14 mm. and 20 mm.,'respectively. The diameter of the annular elevation 28 is 14 mm. and is thus equal to the internal diameter of the portion 26 of the fourth grid 24 which is adjacent the third grid 17'. The internal diameter of the fifth grid 25 is 20 mm. The dimension of the elevation 28 in the direction of the axis 21 of the gun' is 1.5 mm., that of the portion 26 in this direction is 7 mm, that of the portion 27 in this direction is 18 mm. and that of the fifth grid 25 in this direction is 10 mm. The distance-between the elevation 28 and the portion 26 is 2 mm. and between the portion 27 and the fifth grid 25 is also 2 mm. This gun can be operated with the following voltages:

cathode between 0 volt and 140 volts first grid 0 volt second grid 1,700 volts third grid 350 volts fourth grid 4,300 volts fifth grid 25,000 volts The variable voltage on the cathode serves to control the beams.

lclaim:

l. A cathode-ray tube having an electron gun for producing a plurality of electron beams, a luminescent screen positioned to intercept said beams, a color-selecting electrode between the electron gun and screen and spaced from the luminescent screen in a plane in which the various electron beams are converged,-the electron gun comprising a separate cathode for each electron beam, a common first grid provided with an aperture for each beam, a common second grid which serves as an accelerating electrode and is provided with an aperture for each beam, a common third grid having an aperture for each beam and a dimension at the aperture in the direction of the axis of the gun which is less than 4mm. said third grid being provided on the side remote from the second grid with an annular elevation having a diameter which is greater than the enveloping area of the apertures and substantially equal to the diameter of a fourth grid positioned adjacent the third grid, said fourth grid having a circular cross section, the section of the fourth grid being smaller on the side of the third grid than on the other side, and a common fifth electrode of substantially circular-cylindrical shape, the apertures in the third grid lying substantially in a plane passing through the axis of the gun.

2. A cathode-ray tube as claimed in claim 1 wherein the apertures in the third grid are arranged to form a triangle in a plane at right angles to the axis of the gun 3. A cathode-ray tube as claimed in claim 1 wherein the apertures in the third grid are arranged coplanar with the axis of the gun.

4. A cathode-ray tube as claimed in claim 2 wherein the distance of the centers of the apertures from the axis of the gun is less than 4 mm.

5. A cathode-ray tube as claimed in claim 3 wherein the distance between the centers of the apertures is less than 7 6. A cathode-ray tube as claimed in claim 4 in which the distance of the centers of the apertures from the axis of the gun is less than I mm.

a: s a s 2 3 3 UNITED STATES PATENT OFFTCE CERTIFICATE @F CORRECTION Patent No. 3,639 795 I Dated Februaryl, 1972 Inventoflfi) PIET G. J. BARTEN It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Aft-or Section [21] insert the following [30] Foreign ApplicatiooPriority Data July 18, 1967 Netherlands.... ..6709978 June 29, 1968 Netherlands .6809258 Signed and sealed this ZLUzh day of October 1972.

(SEAL) Attest:

EDWARD M.FLETCHEIR ,JR. ROBERT GOTTSCHALK Attesting; Officer Cominissioner of Patents

Claims

1. A cathode-ray tube having an electron gun for producing a plurality of electron beams, a luminescent screen positioned to intercept said beams, a color-selecting electrode between the electron gun and screen and spaced from the luminescent screen in a plane in which the various electron beams are converged, the electron gun comprising a separate cathode for each electron beam, a common first grid provided with an aperture for each beam, a common second grid which serves as an accelerating electrode and is provided with an aperture for each beam, a common third grid having an aperture for each beam and a dimension at the aperture in the direction of the axis of the gun which is less than 4 mm. said third grid being provided on the side remote from the second grid with an annular elevation having a diameter which is greater than the enveloping area of the apertures and substantially equal to the diameter of a fourth grid positioned adjacent the third grid, said fourth grid having a circular cross section, the section of the fourth grid being smaller on the side of the third grid than on the other side, and a common fifth electrode of substantially circular-cylindrical shape, the apertures in the third grid lying substantially in a plane passing through the axis of the gun.

2. A cathode-ray tube as claimed in claim 1 wherein the apertures in the third grid are arranged to form a triangle in a plane at right angles to the axis of the gun.

3. A cathode-ray tube as claimed in claim 1 wherein the apertures in the third grid are arranged coplanar with the axis of the gun.

5. A cathode-ray tube as claimed in claim 3 wherein the distance between the centers of the apertures is less than 7 mm.

6. A cathode-ray tube as claimed in claim 4 in which the distance of the centers of the apertures from the axis of the gun is less than 1 mm.