US3610992A - Cathode-ray tube having end electrodes of three electrodes connected by helical coil coaxial with tube axis - Google Patents

Abstract

In a cathode-ray tube, for example, a color picture tube in which a plurality of electron beams are made to converge or cross each other substantially at the optical center of an electrostatic focusing lens by which the beams are focused on the electron-receiving screen of the tube; the focusing lens includes first and second axially spaced, annular electrodes extending around the tube axis and being at the same potential, a third annular electrode extending between the first and second electrodes and being at a different potential to establish the focusing electric field, and an auxiliary electrode disposed within the third electrode and connected with the first and second electrodes to modify the electric field so that its equivalent optical lens has relatively flatter surfaces for further reducing aberrations of the beam or beams focused thereby.

Description

United States Patent lnventor Senri Miyaoka Kanagawa-ken, Japan Appl. No. 846,533 Filed July 31, 1969 Patented Oct. 5, 1971 Assignee Sony Corporation Tokyo, Japan Priority Dec. 19, 1968 CATHODE-RAY TUBE HAVING END ELECTRODES OF THREE ELECTRODES CONNECTED BY HELICAL COIL COAXIAL WITH TUBE AXIS Primary Examiner-Robert Segal AttorneysAlbert C. Johnston, Robert E. Isner, Lewis H.

Eslinger and Alvin Sinderbrand ABSTRACT: In a cathode-ray tube, for example, a color picture tube in which a plurality of electron beams are made to converge or cross each other substantially at the optical center of an electrostatic focusing lens by which the beams are focused on the electron-receiving screen of the tube; the focusing lens includes first and second axially spaced, annular electrodes extending around the tube axis and being at the same potential, a third annular electrode extending between the first and second electrodes and being at a different potential to establish the focusing electric field, and an auxiliary electrode disposed within the third electrode and connected with the first and second electrodes to modify the electric field so that its equivalent optical lens has relatively flatter surfaces for further reducing aberrations of the beam or beams focused thereby.

Ill/Illlll/ll/lll/l/lI/Il PATENTEU um 5:971

SHEET 3 0r 3 INYEN'IOR.

SENRI MIYAOKA CATIIODE-RAY TUBE HAVING END ELECTRODES OF THREE ELECTRODES CONNECTED BY HELICAL COIL COAXIAL WITH TUBE AXIS This invention generally relates to cathode-ray tubesand is particularly directed to improvements in the electrostatic lens of such tubes by which the electron beam or beams are focused on the electron-receiving screen of the tube.

In cathode-ray tubes having an electron gun of the unipotential type, each electron beam is passed through the electric field of the electrostatic focusing lens so as to be focused thereby on the screen, and such lens usually comprises first and second axially spaced, annular'electrodes extending around the tube axis and being at the same potential, and a third annular electrode extending between the first and second electrodes and being at a different potential, for example, at a substantially lower potential, to establish the focusing electric field. In order to minimize spherical aberrations of the beam focused on the screen, it is desirable that the electrostatic focusing lens be equivalent to an optical lens of large diameter and, hence, having relatively flat surfaces, that is, surfaces with large radii of curvature.

In an electrostatic focusing lens, the surface curvatures of the equivalent optical lens are dependent upon the gradient of the potential along the optical axis between the first and second electrodes, and the gradient of the potential is, in turn, dependent upon the potential difference between the first and second electrodes and the third or intermediate electrode and also upon the axial distance between the first and second electrodes and the diameter of the third electrode. Since the electron gun is positioned within the neck of the cathode-ray tube envelope, it will be apparent that the diameter of the third or intermediate electrode of the electrostatic focusing lens is limited by the diameter of the neck. Thus, the diameter of the equivalent optical lens can be increased by increasing the diameter of the intermediate electrode only to a limited extent. If the axial distance between the first and second or end electrodes of the electrostatic focusing lens is reduced to decrease the potential gradient in the field along the optical axis, and hence to increase the radii of curvature of the surfaces of the equivalent optical lens, the focusing effect of the lens is decreased and, therefore, it is necessary to undesirably increase the distance from the focusing lens to the screen and also the overall length of the tube. If the potential difference between the intermediate electrode and the end electrodes is to be reduced, it becomes necessary to apply a relatively high voltage to the intermediate electrode, bearing in mind that the end electrodes are usually at the anode voltage which is usually in the range of from 13 to 20 kv. The application of a relatively high voltage, such as, a voltage of 4 to kv. or more, to the intermediate electrode is disadvantageous in that it requires additional circuitry for producing that high voltage, and further in that there is the possibility of discharges occurring between the closely spaced leads and pins that supply the voltages to the intermediate electrode and to the grids by which the beam is produced and modulated.

The need for an electrostatic focusing lens equivalent to an optical lens of large diameter and having surfaces of large radii of curvature is particularly acute in the case of single-gun, plural-beam cathode-ray tubes of the type disclosed in U.S. Pat. No. 3,448,316, issued June 3, 1969, and having a common assignee herewith.

In such cathode-ray tubes adapted for use as a color picture tube in a television receiver, a cathode structure emits electrons which are formed into a plurality of electron beams and such beams are made to converge or cross each other substantially at the optical center of an electrostatic focusing lens which is common to all the beams and focuses-the beams on the electron-receiving screen. The fact that all of the beams pass through the center of the focusing lens diminishes the aberrations introduced by the latter as compared with earlier proposed arrangements in which at least two of the beams pass through the focusing lens at substantial distances from the optical axis. However, optimum reduction of aberration again requires that the electrostatic focusing lens for focusing the beams which converge to cross each other at its optical center be equivalent to an optical lens of large diameter having surfaces of large radii of curvature.

Accordingly, it is an object of this invention to provide a unipotential, focusing type electron gun for a single-beam or plural-beam cathode-ray tube in which the electrostatic focusing lens is made equivalent to an optical lens of large diameter having surfaces of large radii of curvature, without unduly increasing the diameter of the tube neck or the length of the tube and further without unduly reducing the potential difference between the electrodes constituting the lens.

In a cathode-ray ray tube according to an aspect of this invention, for example, a color picture tube in which a plurality of electron beams are made to converge or cross each other substantially at the optical center of an electrostatic focusing lens by which the beams are all focused on the electronreceiving screen of the tube; the electrostatic focusing lens is constituted by first and second axially spaced, annular electrodes extending around the tube axis and being at the same potential, for example, approximately the anode voltage of the tube, a third annular electrode extending between the first and second electrodes and being at a different potential, for example, a voltage substantially lower than the anode voltage, to establish the focusing electric field, and an auxiliary electrode disposed within the third electrode and connected with the first and second electrodes so as to reduce the potential gradient in the focusing electric field and thereby make the electrostatic lens equivalent to a large diameter optical lens having surfaces with large radii of curvature.

In an electrostatic focusing lens for a cathode-ray tube, as aforesaid, the auxiliary electrode has open areas to permit the electric field to penetrate therethrough, and such open areas may be defined by forming the auxiliary electrode as a helical coil with spaced turns, as a cylindrical sleeve having apertures, or as a circular array of straight conductors spaced from each other and extending parallel with the tube axis.

The above, and further objects, features and advantages of the invention, will appear from the following detailed description of illustrative embodiments of the invention which is to be read in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagrammatic axial sectional view of a conventional single-beam, unipotential focusing electron gun;

FIG. 2 is a fragmentary axial sectional view of an electrostatic focusing lens in accordance with an embodiment of the present invention;

FIGS. 3a, 3b, and 3e are graphical illustrations of the potential distributions in electrostatic focusing lenses of the prior art and according to this invention.

FIGS. 4a, 4b, and 4c are diagrammatic perspective views of other embodiments of the present invention; and

FIG. 5 is an axial sectional view of a color picture tube employing an electrostatic focusing lens according to the present invention.

Referring to the drawings in detail, and initially to FIG. 1, thereof, it will be seen that a conventional single-beam unipotential electron gun 10 for a cathode-ray tube is there shown to include a cathode ll constituting an electron beam generating source, first and second control grids 12 and 13 having aligned apertures 14 and 15, respectively, and an electrostatic focusing lens 16. The lens 16 includes first and second end electrodes 17 and 18 which are an annular, axially spaced from each other and coaxial with the tube axis, and a relatively larger diameter third or intennediate annular electrode 19 which is also coaxial with the tube axis and extends between end electrodes 17 and 18 and axially overlaps the latter.

In operating the electron gun 10, appropriate voltage are applied to grids l2 and 13 and to electrodes l7, l8 and 19. For example, with the voltage of cathode 11 as a reference a voltage of 0 to 400 v. is applied to first grid 12 for modulating the beam, a voltage ofO to 500 v. is applied to grid 13 to cause the bundle of electrons of the single-beam to converge to a point source substantially within aperture 15, a voltage of 13 to 20 kv. is applied to electrodes 17 and 18 which may be connected to each other by a conductor 20 extending outside of intermediate conductor 19, and a relatively low voltage of to 600 v. is applied to the intermediate electrode 19.

The voltage applied to electrodes 17 and 18 may conveniently be the anode voltage applied to the conductive layer 24 at the inner surface of the tube which further has a phosphor screen 23 for receiving the electrons of the single beam 25. With the distribution of applied voltages, as given above, the bundle or rays of electrons of beam 25 which diverge from the tube axis x-x after passing through aperture are converged or focused at a point on screen 23 by passage through the electric field thus established within electrode 19 between electrodes 17 and 18, and which is equivalent to an optical lens represented in broken lines at L on FIG. 1. and centered between electrodes 17 and 18.

When the axial distance between electrodes 17 and 18 is sufficient to ensure that the overall length of the tube is not undesirably large and the diameter of electrode 19 is suitable to permit a reasonable diameter of the tube neck, the field of electrostatic focusing lens 16 has a steep potential gradient as indicated by the curve P on FIG. 3A which represents the potentials along the tube axis xx at various distances from the plane y--y passing through the optical center of lens 16 or its equivalent optical lens L. With such a steep potential gradient, the equivalent optical lens L is of limited diameter and has surfaces with relatively small radii of curvature. The small radii of curvature of the surfaces of equivalent optical lens L result from the presumed generation of such surfaces at right angles to the lines p of equal potential within the electric field of electrostatic focusing lens 16, which lines p as shown on FIG. 3B are at substantial angles with respect to axis x-x.

Thus, in focusing beam 25, the conventional electrostatic lens 16 may impart spherical aberrations to the beam with resultant poor resolution of the picture produced when the beam is made to scan screen 23, as by the usual deflection yoke (not shown).

In accordance with this invention, the above-mentioned spherical aberrations are substantially diminished by providing an auxiliary electrode within the field of electrostatic lens 16a to reduce the angles of the line p (FIG. 3C) of equal potential with'respect to the tube axis x-x and to decrease the potential gradient along such axis, as indicated at P on FIG. 3A, whereby to make the electrostatic lens equivalent to an optical lens L (FIG. 3A) of relatively large diameter and having surfaces with large radii of curvature.

As shown on FIG. 2, in which the general parts of an electrostatic focusing lens 160 are identified by the same reference numerals employed in connection with the above description of FIG. 1, but with the letter "a appended thereto, the auxiliary electrode 30 provided according to this invention is there shown to be in the form of a helical conductive coil extending around tube axis xx between end electrons 17a and 18a and having its ends mounted on the latter so as to be supported within intermediate electrode 19a. The turns 31 of coil 30 are spaced axially to define open areas 32 therebetween through which the electric field may penetrate. The effect of coil 30, which is at the same relatively low potential as electrodes 17a and 18a by reason of its connection to the latter, is to very substantially reduce the potential gradient along the tube axis between electrodes 17a and 18a and to decrease the angles with respect to the tube axis of the lines of equal potential within the field particularly adjacent the tube axis, with the result that the equivalent optical lens L' (FIG. 3A) is of large diameter and has surfaces of large radii of curvature, as is desired.

In a particular example of the embodiment of FIG. 2, the voltage applied to electrodes 17a 18a and to the auxiliary electrode 30 thcrebetween is kv., the voltage applied to intermediate electrode 19a is 0 v., the coil constituting electrode 30 and electrodes 17a 18a have a diameter of 8 mm., electrode 190 has a diameter of 18 mm., and coil 30 has a pitch of 1 mm. and is formed of a wire having a diameter of 0.2 mm. With the foregoing dimensions and voltages, the potential gradient adjacent the midplane y-y on FIG. 3A is found to be 1 l6 v./mm., whereas, with the same dimensions and voltages but in the absence of the auxiliary electrode 30, the potential gradient at such location is found to be in the range of from 1,000 v./mm. to 2,000 v./mm.

Further, it will be apparent that, in the conventional arrangement of FIG. 1, a substantial radial clearance must be maintained between the tube neck and electrode 19 to accommodate conductor 20 for, in the absence of such substantial clearance, a discharge can occur from the highly charged inner surface of the tube neck through conductor 20 to electrode 19, and such discharge can result in the application of the high anode voltage to transistor containing circuits with resultant destruction of the transistors. However, in the electrostatic focusing lens according to the invention, auxiliary electrode 30 electrically connects electrodes 17a and 18a to each other so that conductor 20 can be omitted to permit an increase in the diameter of electrode 19a without the mentioned discharge hazard. The diametric increase of electrode 19a further increases the diameter of the equivalent optical lens L without a corresponding increase in the tube neck diameter.

It should be noted that the auxiliary electrode provided according to this invention may take many forms other than the helical coil form of FIG. 2. Thus, for example, as shown on FIG. 4A, in which the intermediate electrode 19 is omitted for clarity of illustration of electrode 30!) according to this invention, such electrode 3011 may be constituted by a circular array of spaced conductors 31b arranged around axis x-x and extending parallel with the latter between electrodes 17b and 18b to which the ends of conductors 31b are suitably secured. The spaced conductors 31b define the necessary open areas 32b therebetween.

In another embodiment shown on FIG. 4B, the auxiliary electrode 30c is in the form of a cylindrical sleeve which may be integral with electrodes 17c and 18c, or otherwise joined thereto, and which has apertures 32c therein to define the necessary open areas. Similarly, as shown on FIG. 4C, the cylindrical sleeve constituting an auxiliary electrode 30d may be formed of a conductive mesh constituted by axially and circumferentially extending wires 31d to define the necessary open areas 32d in the auxiliary electrode 300! extending between and connecting electrodes 17d and 18d.

It will be apparent that, in all of the above-described embodiments of the invention, the degree or extent of penetration of the electric field through the auxiliary electrode can be varied, so as to vary the radii of curvature of the surfaces of the equivalent optical lens, by changing the proportion of the overall surface area of the auxiliary electrode constituted by its open areas, for example, by changing the pitch and wire diameter of the coil forming electrode 30 in FIG. 2. Of course, the surface radii of the equivalent optical lens can also be changed by changing the distance between electrodes 17a and 18a and the potential difference between electrodes 17a and 18a and electrode 190. Thus, the invention permits an electrostatic focusing lens to be obtained that is equivalent to an optical lens with precisely desired surface radii.

Although the invention has been described above with reference to its application to single-beam cathode-ray tubes, reference to FIG. 5 will show the application of the invention to a single-gun, plural-beam cathode-ray tube of the type disclosed in detail in U.S. Pat. No. 3,448,316.

In the cathode-ray tube of FIG. 5, three electrically separated cathodes K K and K have red," green and blue" video signals respectively supplied thereto. The three cathodes are arranged with their electron emitting surfaces in a straight line so as to be aligned with similarly arranged apertures in a first grid 6,. A second cup-shaped grid G, has an end plate disposed adjacent grid G and formed with apertures aligned with the apertures of first grid G,. Arranged in order G, are successive, open-ended tiibular grids or electrodes 117, l

119 and 118 constituting an electrostatic focusing lens 116. Electrode 117 includes a relatively small diameter end portion 117a, and is supported with each end portion extending into cup-shaped grid G and spaced radially from the sidewall of the latter.

When voltages similar to those indicated for the cathoderay tube of FIG. 1 are applied to grids G and G and electrodes 117, 118, and 119 beams B B and B emitted by cathodes K K and K,, are modulated with the three different video signals applied between grid G and the respective cathodes. Grid G and the end portion 117a of electrode 117 cooperate to provide an electric field defining an electrostatic beam converging lens illustrated in broken lines by its optical equivalent 1 and which is operative to converge beams B and B toward beam B so that the three beams cross each other substantially at the location of the optical center of the focusing lens 116.

In order to cause convergence of the beams B and B which emerge from electrode 118 along divergent paths, the electron gun of FIG. 5 further has deflecting means 33 that includes shielding plates 34 and 34' provided in spaced opposing relationship to each other and extending axially away from the free end of electrode 118. Deflecting means 33 further includes converging deflector plates 35 and 35, which may be flat, as shown, or outwardly convexly bent or curved, and which are mounted in spaced opposing relation to the outer surfaces of shielding plates 34 and 34, respectively. The plates 34 and 34' and the plates 35 and 35' are disposed so that the beams B B and B pass between the plates 34 and 35, between the plates 34 and 34 and between the plates 34' and 35', respectively. The outer plates 35 and 35 may be mounted by attachment to electrode 118, as shown, while plates 34 and 23' are supported from plates 35 and 35 and insulated therefrom, as by insulating supports 36.

A high anode voltage V,,, for example, of 13 to 20 kv., provided by a source 37 is applied by way of an anode button 38 to the usual conductive layer 39 lining the tube envelope, and a spring contact 40 extends from plate 34 into engagement with layer 39. The high voltage V, thus applied to plate 34 is transmitted to plate 34 by a conductor 41 therebetween. A voltage (V -V which is lower than the voltage V,, by 200 to 300 v., constituting a convergence voltage, is applied to outer plates 35 and 35'. The source of the convergence voltage V is indicated at 42 and may provide a static convergence voltage and also, if desired, a dynamic convergence voltage varied in accordance with the scanning action. As shown, the voltage (V,,Vc) may be applied by way of a button 43 in the tube neck 44 and a conductor 45 to electrode 117 of focusing lens 116. Further, as hereinafter described, electrode 118 is electrically connected with electrode 117 to receive the voltage (V -V) and, since outer plates 35 and 35 are mounted directly o'ri electrodeTI fplates 35 and 35' also receive the" voltage (V,,V Thus, convergence voltage differences V are applied between plates 34 and 35 and between plates 34 and 35, so that beams B and B are deflected thereby to converge with beam B which is undeflected by reason of plates 34 and 34 being at the same potential. The system is so arranged that beams B and B will cross each other and beam B at a common spot on an aperture grill or mesh 46 and diverge therefrom to strike respective color phosphors b, r and g arranged in suitable arrays to constitute the color screen 48 on the faceplate 49 of the tube. A deflection yoke 50 is also provided to cause beams B B and B to simultaneously scan screen 48 in the usual manner.

Since beams B B and B all pass substantially through the optical center of electrostatic focusing lens 116 so as to be focused thereby on screen 48, lens 116 imparts diminished aberration to the resulting beam spots on the screen as compared with earlier arrangements in which, for example, beams B and 8,, pass through the focusing lens at substantial distances from its optical axis. However, since beams B and B pass through lens 116 at substantial angles to the optical or tube axis, optimum reduction or avoidance of aberrations of the beam spots requires that lens 116 be equivalent to a large diameter optical lens having surfaces with large radii of curvature. Thus, in accordance with this invention, the optical lens L equivalent to electrostatic focusing lens 116 is made to have a large diameter and surfaces with large radii of curvatures by providing lens 116 with an auxiliary electrode 130. As shown, electrode is in the form of a helical coil with spaced turns and extends between electrodes 117 and 118 with electrode 119. The electrode 130 functions in the same way as has been described with respect to the similar electrode 30 of FIG. 2 and further serves to electrically connect electrode 118 with electrode 117 so that electrode 118 also receives the voltage (V,-V,.). Of course, the embodiment shown on FIGS. 4A, 4B and 4C may be substituted for the form of auxiliary electrode 130 shown on FIG. 5.

Although illustrative embodiments of electrostatic lenses according to this invention have been described in detail herein with reference to the accompanying drawings, it is the understood that the invention is not limited to those precise embodiments and that various changes and modifications may be made therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.

1. A cathode-ray tube having beam producing means generating a plurality of electron beams and a phosphor screen positioned to have the beams impinge thereon said beams being made to intersect each other at a location in the tube between said beam producing means and said screen; electron focusing lens means for focusing the beam on the screen having an optical center substantially at said location where the beams intersect each other and comprising:

first annular electrode means coaxial with the longitudinal axis of the tube and having one end facing the same way as the direction of movement of the electrons, second annular electrode means coaxial with said axis of the tube and having an end facing said one end of said first annular electrode means and axially spaced therefrom,

third annular electrode means coaxial with said axis of the tube and extending over the space between said ends of said first and second electrode means and partly over said first and second electrode means,

said third electrode means being at a potential different from the potential of said first and second electrode means to provide an electric focusing field therebetween, and auxiliary electrode means including a helical coil with spaced turns extending conductively between said first and second electrode means and coaxially with the longitudinal axis of the tube within said third electrode means and operative to reduce the potential gradient of said field.

Claims (1)

1. A cathode-ray tube having beam producing means generating a plurality of electron beams and a phosphor screen positioned to have the beams impinge thereon said beams being made to intersect each other at a location in the tube between said beam producing means and said screen; electron focusing lens means for focusing the beam on the screen having an optical center substantially at said location where the beams intersect each other and comprising: first annular electrode means coaxial with the longitudinal axis of the tube and having one end facing the same way as the direction of movement of the electrons, second annular electrode means coaxial with said axis of the tube and having an end facing said one end of said first annular electrode means and axially spaced therefrom, third annular electrode means coaxial with said axis of the tube and extending over the space between said ends of said first and second electrode means and partly over said first and second electrode means, said third electrode means being at a potential different from the potential of said first and second electrode means to provide an electric focusing field therebetween, and auxiliary electrode means including a helical coil with spaced turns extending conductively between said first and second electrode means and coaxially with the longitudinal axis of the tube within said third electrode means and operative to reduce the potential gradient of said field.

Images