This type the 45AX FST TUBE BY PHILIPS WAS WIDELY USED AROUND THE WORLD and fabricated form more than 22 YEARS.
Picture display system including a deflection unit with a double saddle coil system
PHILIPS 45AX SYSTEM
Self-convergent picture display system with a color display tube and an electromagnetic deflection unit including a field deflection coil and a line deflection coil which are both of the saddle type and are wound directly on a support. The deflection unit includes a pair of magnetically permeable portions which are arranged symmetrically with respect to the plane of symmetry of the field deflection coil on either side of the tube axis. The magnetically permeable portion draws magnetic flux from the end of the yoke ring in order to extend the vertical deflection field. A self-convergent system can be realized with different screen formats by choosing different lengths of the magnetically permeable portions.
What is claimed is:
1. A picture display system including a colour display tube having a neck accommodating an electron gun assembly for generating three electron beams, and an electromagnetic deflection unit surrounding the paths of the electron beams which have left the electron assembly, said deflection unit comprising
a field deflection coil of the saddle type having a front and a rear end for deflecting electron beams generated in the display tube in a vertical direction;
a line deflection coil of the saddle type likewise having a front and a rear end for deflecting electron beams generated in the display tube in a horizontal direction, and a yoke ring of ferromagnetic material surrounding the two deflection coils and having front and rear end faces extending transversely to the tube axis, the electron beam traversing the coils in the direction from the rear to the front ends when the deflection unit is arranged on a display tube, characterized in that the deflection unit also has first and second magnetically permeable portions arranged symmetrically with respect to the plane of symmetry of the field deflection coil on either side of the tube axis, each magnetically permeble portion having a first end located opposite the rear end face of the yoke ring and a second end located at the neck of the display tube in the proximity of the location where the electron beams leave the electron gun assembly, the length of the first and second magnetically permeable portions and their distance to the yoke ring being dimensioned for providing a self-convergent picture display system.
2. A picture display system as claimed in claim 1 characterized in that regions of the rear end of the yoke ring located on either side of the plane of symmetry of the line deflection coil are left free by the rear end of the field deflection coil and in that the first ends of the magnetically permeable portions are located opposite said regions.
3. A picture display system as claimed in claim 1 characterized in that the field deflection coil and the line deflection coil are directly wound on a support.
4. Apparatus for adapting a self-convergent deflection unit of the type mountable on the neck of a display tube and including a saddle type field deflection coil screen end and a gun end extending away from said tube in a plane disposed at an angle to a tube axis, and a yoke ring having a screen end and a gun end, for use with display tubes having different screen formats comprising:
format adjustment means disposed adjacent to the gun end of the yoke ring for coupling flux from the yoke ring to the neck of the tube to supplement the field produced by the vertical deflection coil to uniformly increase the vertical deflection field to produce a raster having a different format from the raster produced by said deflection unit alone.
5. The apparatus of claim 4 wherein said field deflection coil is arranged symmetrically about a plane of symmetry passing through said neck and said format adjustment means comprises first and second magnetically permeable members arranged symmetrically about said plane of symmetry, each of said magnetically permeable members having a first end disposed adjacent the gun end of the yoke ring and a second end disposed adjacent the neck of the display tube.
6. The apparatus of claim 5 wherein each of said first and second magnetically permeablel members comprises a first end located opposite a gun end face of the yoke ring, and a second end located at the neck of the display tube adjacent the location where the electron beams leave the electron gun assembly.
7. The apparatus of claim 6 wherein said first end comprises a portion of said permeable member disposed parallel to the neck of the displaya tube and said second end comprises a portion of said magnetically permeable member located perpepndicular to the neck of the display tube.
8. The apparatus of claim 7 wherein said second endsn of said magnetically permeable members have inwardly extending arms subending a first angle.
9. The appaaratus of claim 8 wherein said angle is large so that the supplemental field has a positive sixpole component.
10. The apparatus of claim 8 wherein said angle is very small, so that said supplemental field has a dipole component and a negative sixpole component.
11. Apparatus for adapting a self-convergent deflection unit of the type used on the neck of a display tube having an electron gun disposed in a neck of said tube, said deflection unit including a field deflection coil of the saddle type having a rear end portion disposed at an angle to the axis of said tube, comprising means disposed adjacent to said neck between said electron gun and said deflection unit, and coupled to said deflection unit for changing the distance between the line and field deflection points for causing said deflection unit to produce a different screen format.
CRT TUBE PHILIPS 45AX TECHNOLOGY Method of Production / manufacturing a color display CRT tube and color display tube manufactured according to said method.
A ring is provided to correct the convergence, color purity and frame errors of a color display tube which ring is magnetized as a multipole and which is secured in or around the tube neck and around the paths of the electron beams.
The magnetization of such a ring can best be carried out by energizing a magnetization unit with a combination of direct currents thereby generating a multipole magnetic field and then effecting the magnetization by generating a decaying alternating magnetic field which preferably varies its direction continuously.
1. A method of manufacturing a color display tube in which magnetic poles are provided in or around the neck of said tube and around the paths of the electron beams, which poles generate a permanent static multipole magnetic field for the correction of errors in convergence, color purity and frame of the display tube, which magnetic poles are formed by the magnetisation of a configuration of magnetisable material provided around the paths of the electron beams, the method comprising energizing a magnetisation device with a combination of direct currents with which a static multipole magnetic field is generated, and superimposing a decaying alternating magnetic field over said static multipole magnetic field which initially drives said magnetisable material into saturation on either side of the hysteresis curve thereof, said decaying alternating magnetic field being generated by a decaying alternating current. 2. The method as claimed in claim 1, 6 or 7, wherein the decaying alternating magnetic field is generated by means of a separate system of coils in the magnetisation device. 3. The method as claimed in claim 2, wherein the decaying alternating magnetic field varies its direction continuously. 4. The method as claimed in claim 3 wherein the frequency of the decaying alternating current is approximately the standard line frequency. 5. A colour display tube manufactured by means of the method as claimed in claim 4. 6. The method as claimed in claim 1 which further comprises erasing any residual magnetism in said configuration, prior to said magnetisation, with an alternating magnetic field. 7. The method as claimed in claim 6 which further comprises correcting the errors in convergence, color purity and frame of the display picture with a combination of direct currents applied to said magnetisation device and then reversing said direct currents while increasing the magnitudes thereof and applying these adjusted direct currents to said magnetisation device for the magnetisation of said configuration.
BACKGROUND OF THE INVENTION
The invention relates to a method of manufacturing a color display tube in which magnetic poles are provided in or around the neck of the envelope and around the paths of the electron beams, which poles generate a permanent multipole magnetic field for the correction of the occurring errors in convergence, color purity and frame of the color display tube, which magnetic poles are formed by the magnetisation of a configuration of magnetisable material provided around the paths of the electron beams, which configuration is magnetized by energising a magnetising device with a combination of currents with which a static multipole magnetic field is generated.
The invention also relates to a color display tube manufactured according to said method.
In a color display tube of the "delta" type, three electron guns are accommodated in the neck of the tube in a triangular arrangement. The points of intersection of the axes of the guns with a plane perpendicular to the tube axis constitute the corner points of an equilateral triangle.
In a color display tube of the "in-line" type three electron guns are arranged in the tube neck in such manner that the axes of the three guns are situated mainly in one plane while the axis of the central electron gun coincides substantially with the axis of the display tube. The two outermost electron guns are situated symmetrically with respect to the central gun. As long as the electron beams generated by the electron guns are not deflected, the three electron beams, both in tubes of the "delta" type and of the "in-line" type, must coincide in the center of the display screen (static convergence). Because, however, as a result of defects in the manufacture of the display tube, for example, the electron guns are not sealed quite symmetrically with respect to the tube axis, deviations of the frame shape, the color purity and the static convergence occur. It should be possible to correct said deviations.
Such a color display tube of the "in-line" type in which this correction is possible, is disclosed in Netherlands Pat. application No. 7,503,830 laid open to public inspection. Said application describes a color display tube in which the deviations are corrected by the magnetisation of a ring of magnetisable material, as a result of which a static magnetic multipole is formed around the paths of the electron beams. Said ring is provided in or around the tube neck. In the method described in said patent application, the color display tube is actuated after which data, regarding the value and the direction of the convergence errors of the electron guns, are established, with reference to which the polarity and strength of the magnetic multipole necessary to correct the frame, color purity and convergence errors are determined. The magnetisation of the configuration, which may consist of a ring, a ribbon or a number of rods or blocks grouped around the electron paths, may be carried out in a number of manners. It is possible, for example, first to magnetise the configuration to full saturation, after which demagnetisation to the desired value is carried out with an opposite field. A disadvantage of this method is that, with a combination of, for example, a 2, 4, and 6-pole field, the polarity and strength of the demagnetisation vary greatly and frequently, dependent on the place on the ring, and hence also the polarity and strength of the full magnetisation used in this method. Moreover it appears that the required demagnetising field has no linear relationship with the required correction field. Due to this non-linearity it is not possible to use a combined 2, 4 and 6-pole field for the demagnetisation. It is impossible to successively carry out the 2, 4 and 6-pole magnetisation since, for each magnetisation, the ring has to be magnetised fully, which results in the preceding magnetisation being erased again. The possibility of successively magnetising various places on the ring is very complicated and is not readily possible if the ring is situated in the tube neck since the stray field of the field necessary for the magnetisation again demagnetizes, at least partly, the already magnetised places.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a method with which a combined multipole can be obtained by one total magnetisation.
According to the invention, a method, of the kind described in the first paragraph with which this is possible, is characterized in that the magnetisation is effected by means of a decaying alternating magnetic field which initially drives the magnetisable material on either side of the hysteresis curve into saturation. After the decay of the alternating magnetic field, a hard magnetisation remains in the material of the configuration which neutralizes the externally applied magnetic field and is, hence, directed oppositely thereto. After switching off the externally applied magnetic field, a magnetic multipole field remains as a result of the configuration magnetized as a multipole. The desired magnetisation may be determined in a number of manners. By observing and/or measuring the deviations in the frame shape, color purity and convergence, the desired multipole can be determined experimentally and the correction may be carried out by magnetisation of the configuration. If small deviations are then still found, the method is repeated once or several times with corrected currents. In this manner, by repeating the method according to the invention, it is possible to produce a complete correction of the errors in frame, color purity and convergence. Preceding the magnetisation, residual magnetism, if any, in the configuration is preferably erased by means of a magnetic field.
The method is preferably carried out by determining the required correction field prior to the magnetisation and, after the erasing of the residual magnetism, by correcting the errors in the convergence, the color purity and the frame of the displayed picture by means of a combination of currents through the magnetising device, after which the magnetisation is produced by reversing the direction of the combination of currents, increasing the current strength and simultaneously producing the said decaying alternating magnetic field.
The correction field, obtained with the magnetizing device and measured along the axis of the electron beams, is generally longer than the multipole correction field generated by the configuration. So the correction of the deviations will have to be carried out over a shorter distance along the axis of the tube, which is possible only with a stronger field. During the magnetisation, a combination of currents, which in strength and direction is in the proportion of m:1 to the combination of currents which is necessary to generate a correction multipole field with the device, where m is, for example, -3, should flow through the magnetisation device. The value of m depends on the ratio between the length of the correction multipole field, generated by the magnetizing device, to the effective field length of the magnetized configuration. This depends upon a number of factors, for example, the diameter of the neck, the kind of material, the shape and the place of the configuration, etc., and can be established experimentally. If it proves, upon checking, that the corrections with the magnetized configuration are too large or too small, the magnetisation process can be repeated with varied magnetisation currents.
The decaying alternating magnetic field can be generated by superimposing a decaying alternating current on the combination of currents through the magnetisation device (for example, a device as disclosed in Netherlands Pat. application No. 7,503,830 laid open to public inspection). The decaying alternating magnetic field is preferably generated in the magnetisation device by means of a separate system of coils. In order to obtain a substantially equal influence of all parts of the configuration by the decaying alternating field, it is recommendable not only to cause the alternating field to decay but also to cause it to vary its direction continuously. The system of coils therefore consists preferably of at least two coils and the decaying alternating currents through the coils are shifted in phase with respect to each other. Standard line frequency (50 or 60 Hz) has proven to give good results. The phase shift, when using coils or coil pairs, the axes of which enclose angles of 120° with each other, can simply be obtained from a three-phase line.
DESCRIPTION OF THE DRAWINGS
The invention will now be described in greater detail with reference to a drawing, in which
FIG. 1 is a diagrammatic sectional view of a known color display tube of the "in-line" type having an external static convergence unit,
FIG. 2 shows the pinion transmission used therein,
FIGS. 3 and 4 are two diagrammatic perpendicular cross-sectional views of the color display tube with a ring, which has not yet been magnetized, and in which the outermost electron beams do not converge satisfactorily,
FIGS. 5 and 6 are two diagrammatic perpendicular sectional views of a color display tube in which convergence by means of the magnetisation device has been obtained,
FIGS. 7 and 8 show the magnetisation of a ring arranged in the system of electron guns,
FIGS. 9 and 10 show two diagrammatic perpendicular sectional views of a color display tube with a magnetized ring with which the convergence error, as shown in FIG. 4, is removed,
FIGS. 11 and 12 show two types of devices suitable for magnetisation according to the invention, and
FIGS. 13 to 18 show parts of another type of magnetisation unit.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a diagrammatic sectional view of a known color display tube of the "in-line" type. Three electron guns 5, 6 and 7, generating the electron beams 8, 9 and 10, respectively, are accommodated in the neck 4 of a glass envelope 1 which is composed of a display window 2, a funnel-shaped part 3 and a neck 4. The axes of the electron guns 5, 6 and 7 are situated in one plane, the plane of the drawing. The axis of the central electron gun 6 coincides substantially with the tube axis 11. The three electron guns are seated in a sleeve 16 which is situated coaxially in the neck 4. The display window 2 has on the inner surface thereof a large number of triplets of phosphor lines. Each triplet comprises a line of a phosphor luminescing green, a line of a phosphor luminescing blue, and a line of a phosphor luminescing red. All of the triplets together constitute a display screen 12. The phosphor lines are normal to the plane of the drawing. A shadow mask 12, in which a very large number of elongate apertures 14 are provided through which the electron beams 8, 9 and 10 pass, is arranged in front of the display screen 12. The electron beams 8, 9 and 10 are deflected in the horizontal direction (in the plane of the drawing) and in the vertical direction (at right angles thereto) by a system 15 of deflection coils. The three electron guns 5, 6 and 7 are assembled so that the axes thereof enclose a small angle with respect to each other. As a result of this, the generated electron beams 8, 9 and 10 pass through each of the apertures 14 at said angle, the so-called color selection angle, and each impinge only upon phosphor lines of one color.
A display tube has
a good static convergence if the three electron beams, when they are not being deflected, intersect each other substantially in the center of the display screen. It has been found, however, that the static convergence often is not good, no more than the frame shape and the color purity, which may be the result of an insufficiently accurate assembly of the guns, and/or sealing of the electron guns, in the tube neck. In order to produce the static convergence, so far, externally adjustable correction units have been added to the tube. They consist of a number of pairs of multipoles consisting of magnetic rings, for example four two-poles (two horizontal and two vertical), two four-poles and two six-poles. The rings of each pair are coupled together by means of a pinion transmission (see FIG. 2), with which the rings are rotatable with respect to each other to an equal extent. By rotating the rings with respect to each other and/or together, the strength and/or direction of the two-, four- or six-pole field is adjusted. It will be obvious that the control of a display tube with such a device is complicated and time-consuming. Moreover, such a correction unit is material-consuming since, for a combination of multipoles, at least eight rings are necessary which have to be provided around the neck so as to be rotatable with respect to each other.
In the Netherlands Pat. application No. 7,503,830, laid open to public inspection, the complicated correction unit has, therefore, been replaced by one or more magnetized rings, which rings are situated in or around the tube neck or in or around the electron guns.
However, it has proved difficult with the magnetising methods known so far to provide a combination of multipoles in the ring by magnetisation.
The method according to the invention provides a solution.
For clarity, identical components in the following figures will be referred to by the same reference numerals as in FIG. 1.
FIG. 3 is a diagrammatic sectional view of a display tube in which the electron beams do not converge in the horizontal direction. As is known, the outermost electron beams can be deflected more or less in the opposite direction by means of a four-pole, for example, towards the central beam or away therefrom. It is also possible to move the beams upwards and downwards. By means of a six-pole the beams can be deflected more or less in the same direction. For simplicity, the invention will be described with reference to a display tube which requires only a four-pole correction. The convergence errors in the horizontal direction of the electron beams 8 and 10 are in this case equally large but opposite.
FIG. 4 is a sectional view of FIG. 3. On the bottom of sleeve 16, a ring 18 is provided of an alloy of Fe, Co, V and Cr (known as Vicalloy) which can be readily magnetized. It will be obvious that the ring may alternatively be provided in other places around the guns or in or around the tube neck. Instead of a ring it is alternatively possible to use a ribbon or a configuration of rods or blocks of magnetisable material.
In FIG. 5 a device 19 for generating a controllable multipole magnetic field is provided around the neck 4 and the ring 18 according to the method of the invention. 2-, 4- or 6-poles and combinations thereof can be generated by means of the device 19. For the tube shown in FIG. 3, only a four-pole correction is necessary. The coils of the device 19, which device will be described in detail hereinafter, are in this case energized as four-poles until the point of intersection S of the three electron beams 8, 9 and 10, which in FIG. 3 was situated outside the tube 1, lies on the display screen 12. The current I through the coils of the device originates from a direct current source B which supplies a current -mI 1 (m being an experimentally determined constant >1) to the coils via a current divider and commutator A. The current can be adjusted per coil so as to generate the desired multipole. In this phase of the method, an alternating current source C does not yet supply current (i=0).
FIG. 6 is a perpendicular sectional view of FIG. 5. The current I 1 is a measure of the strength of the required correction field. The correction field of the multipole of the device 19 extends over a larger length of the electron paths than the magnetic field generated later by the magnetized ring. Therefore the field of the ring is to be m-times stronger.
FIG. 7 shows the step of the method in which the ring 18 is magnetized as a four-pole. As follows from the above, in this preferred embodiment of the method, the current through the coils of the device must be -mI 1 during the magnetisation, so must traverse in the reverse direction and be m-times as large as the current through the coils during the correction. Moreover, the alternating current source C supplies a decaying alternating current (i=i 1 >0) to the device 19, with which current the decaying alternating field is generated. When the alternating current is switched on, it must be so large that the ring 18 is fully magnetized on either side of the hysteresis curve. When the alternating field has decayed, the ring 18 is magnetized, in this case as a four-pole. It is, of course, alternatively possible to magnetise the ring 18 as a six-pole or as a two-pole or to provide combinations of said multipoles in the ring 18 and to correct therewith other convergence errors or color purity and frame errors. It is also possible to use said corrections in color display tubes of the "delta" type.
FIG. 9 shows the display tube 1 shown in FIG. 3, but in this case provided with a ring 18 magnetized according to the method of the invention as shown in FIGS. 5 and 7. The convergence correction takes place only by the magnetized ring 18 present in sleeve 16. The provision of the required multipole takes place at the display tube 1 factory and complicated adjustments and adjustable convergence units (FIG. 2) may be omitted.
FIG. 10 is a cross-sectional view perpendicular to FIG. 9. FIG. 11 shows a magnetisation device 19 comprising eight coils 20 with which the convergence (see FIG. 5) and the magnetisation (see FIG. 7) are carried out. For generating the decaying alternating magnetic field, two pairs of coils 21 and 22, extending in this case at right angles to each other, are incorporated in the device 19. The current i a through the pair of coils 21 is shifted in phase through 90° with respect to the current i b through the other pair of coils 22, so that the decaying alternating magnetic field changes its direction during the decay and is a field circulating through the ring 18. FIG. 12 shows a magnetisation device known from Netherlands Pat. application No. 7,503,830 laid open to public inspection. In this case, the decaying alternating current may be superimposed on the direct current through the coils 23 so that extra coils are not necessary in the device. The coils 23 are wound around a yoke 24.
The magnetisation device 19 may alternatively be composed of a combination of electrical conductors and coils, as is shown diagrammatically in FIGS. 13 to 18.
FIG. 13 is a sectional view of the neck 4 of a display tube 1 at the area of a ring 18 to be magnetised. A two-pole field for corrections in the horizontal direction is generated in this case by causing currents to flow through the conductors 25, 26, 27 and 28 in the direction as shown in the figure. Said conductors may be single wires or wire bundles forming part of one or more coils or turns, and extending parallel to the tube axis at the area of the ring 18.
FIG. 14 shows how, in an analogous manner, a four-pole field for corrections of the outermost beams 8 and 10 in the horizontal direction can be generated by electrical conductors 29, 30, 31 and 32. A four-pole field for corrections of the outermost beams 8 and 10 in the vertical direction is substantially the same. However, the system of conductors 29, 30, 31 and 32 is rotated through 45° with respect to the neck 4 and the axis of the tube 1.
FIG. 15 shows, in an analogous manner, a six-pole for corrections in the horizontal direction with conductors 33 to 38. By means of a combination of conductors (wires or wire bundles) with which 2-, 4- and 6-poles can be generated, all combinations of two-, four- and six-pole fields with the desired strength can be obtained by variations of the currents through said conductors 33 to 38.
The decaying alternating magnetic field in a magnetisation unit with conductors as shown in FIGS. 13, 14 and 15 can be obtained by means of coils positioned symmetrically around the neck 4 and the conductors as shown in FIGS. 16 and 17 or 18. By energizing the coils 39 and 40, shown in FIG. 16, with a decaying alternating current, a decaying alternating magnetic field is generated. A better influencing of the ring 18 by the decaying alternating field is obtained when a system of coils having coils 41 and 42 in FIG. 17 is provided which is rotated 90° with respect to the coils 39. In this case, 40 and the decaying alternating current through the coils 41 and 42 should then preferably be shifted 90° in phase with respect to the decaying alternating current through the coils 39 and 40.
It is alternatively possible to generate the decaying alternating magnetic field with one or more systems of coils as shown in FIG. 18. The coils 43, 44 and 45 are situated symmetrically around the tube axis and are energized with decaying alternating currents which are shifted 120° in phase with respect to each other (for example from a three-phase line).
CRT TUBE PHILIPS 45AX TECHNOLOGY Method of manufacturing a static convergence unit, and a color display tube comprising a convergence unit manufactured according to the method, PHILIPS 45AX INTERNAL STATIC CONVERGENCE SYSTEM Application technology:
IMACO RING (Integrated Magnetic Auto Converging )
The method according to the invention consists in the determination of data of the convergence errors of a color display tube, data being derived from the said determinations for determining the polarity and the intensity of magnetic poles of a structure. The structure thus obtained generates a static, permanent, multipole magnetic field adapted to the convergence errors occurring, so that the errors are connected.
The invention relates to a method of manufacturing a magnetic convergence device for the static convergence of electron beams which extend approximately in one plane in a neck of a colour display tube, and to a colour display tube provided with a permanent magnetic device for the static convergence of electron beams in the colour display tube. A known device, described in U.S. Pat. No. 3,725,831, consists of at least four permanent magnetic rings arranged in pairs which generate a magnetic field that can be adjusted as regards position and intensity. The adjustability is obtained by turning the two rings of a pair in the same direction with respect to the electron beams and by turning the one ring in the opposite direction with respct to the other ring. The adjustability necessitates that the rings be arranged on a support which is arranged about the neck of the colour display tube and which should include facilities such that the adjustability of each pair of rings, independent of the position of the other rings, is ensured. The invention has for its object to provide a method whereby a device for converging electron beams can be manufactured which need not be mechanically adjustable, so that it can have a very simple construction, and to provide a colour display tube including such a device.
To this end, the method according to the invention is characterized in that the colour display tube is activated, after which data concerning the extent and the direction of the convergence error of each electron beam are determined, on the basis of which is determined the polarity and intensity of magnetic poles of a structure for generating a permanent, multi-pole, static magnetic field for the correction of the convergence errors occurring in the colour display tube, about the neck of the colour display tube there being provided an auxiliary device for generating variable magnetic fields in the neck of the colour display tube, the auxiliary device being subsequently adjusted such that a magnetic field with converges the electron beams is produced, data being derived from the adjustment of the auxiliary device thus obtained, the said data being a measure for the convergence errors and being used for determining the structure generating the permanent static magnetic field.
Using the described method, a device can be manufactured which generates a magnetic field adapted to the colour display tube and which thus constitutes one unit as if it were with the colour display tube. If desired colour purity errors as well as convergence errors can be eliminated by this method. The convergence errors visible on the screen can be measured and expressed in milimeters of horizontal and vertical errors. The errors thus classified represent data whereby, using magnetic poles of an intensity to be derived from the errors, there can be determined a structure of a magnetic multi-pole which generates a permanent magnetic field adapted to the determined convergence errors.
As a result of the generation of a desired magnetic field by means of an auxiliary device and the derivation of data therefrom, it is possible to determine a device adapted to the relevant colour display tube. Simultaneously, it is ensured that the convergence of the electron beams can be effected.
A preferred version of the method according to the invention is characterized in that for the auxiliary device is used an electromagnetic convergence unit which comprises a number of coils wherethrough electrical currents are conducted in order to generate a magnetic field required for the convergence of the electron beams, the values of the electrical currents producing the data for determining an annular permanent magnetic structure. Because the electrical currents whereby the auxiliary device is actuated are characteristic of the magnetic field generated, the intensity and the position of the poles of the magnetic multi-poles to be used for the colour display tube are determined by the determination of the values of the electrical currents.
The data obtained from the auxiliary device can be used in various manners. The data from the auxiliary device can be stored in a memory, or the data from the auxiliary device can be used immediately for controlling a magnetizing unit which magnetizes an annular magnetizable structure. Alternatively it is possible to convert the data into a code; on the basis thereof an annular permanent magnetic structure having a desired magnetic field strength can be taken or composed from a set of already magnetized structural parts. Obviously, the latter two possibilities can be performed after the data have been stored in a memory.
A simplification of the method is achieved when the device is formed from a magnetizable mass which is provided in the form of a ring on at least one wall of the neck of the colour display tube. The device to be magnetized is thus arranged around the electron beams to be generated. Subsequently, a construction which comprises the auxiliary device and the magnetizing unit is arranged around the neck of the colour display tube. The auxiliary device is then adjusted, after which the construction can possibly be displaced, so that the magnetizing unit encloses the device. The magnetizing unit is actuated on the basis of the data received from the auxiliary device, and magnetizes the device.
In order to make the construction of a magnetizing unit as simple and as light as possible, it is advantageous to polarize material of the structure to be magnetized one area after the other by means of the magnetizing unit. A suitable alternative of the method for which use can be made of the described construction of the magnetizing unit is characterized in that the device consists of a non-magnetizable support and a number of permanent magnetic bipoles. It was found that any feasible magnetic field required for the static convergence of electron beams in a neck of a colour display tube can be comparatively simply generated using at least one eight-pole electromagnetic convergence unit. Similarly, any desired magnetic field can be generated using a twelve-pole electromagnetic convergence unit. It is to be noted that electromagnetic convergence units have already been proposed in U.S. Pat. No. 4,027,219.
The invention will be described in detail hereinafter with reference to a drawing.
FIG. 1 is a diagrammatic representation of a first version of the method according to the invention.
FIG. 2 is a diagrammatic representation of a second version of the method according to the invention.
FIG. 3 shows a preferred embodiment of an auxiliary device.
FIG. 4 is a side elevation of a first embodiment of a device manufactured using the method according to the invention.
FIG. 5 is a cross-sectional view of the embodiment shown in FIG. 4.
FIG. 6 is a side elevation of a further embodiment of a device manufactured using the method according to the invention.
FIG. 7 is a cross-sectional view of the device shown in FIG. 6.
FIG. 8 is a diagrammatic perspective view of a magnetizing device and a convergence unit arranged therein.
FIG. 9a is a cross-sectional view of a convergence unit manufactured using a method according to the invention.
FIG. 9b is a partial side elevation of part of a support of the convergence unit shown in FIG. 9a.
FIG. 9c shows a permanent magnetic structural part of the device shown in FIG. 9a.
The method according to the invention will be described with reference of FIG. 1. An electromagnetic auxiliary device 5 is arranged around the neck 3 of the colour display tube 1. The auxiliary device 5 will be described in detail with reference to FIG. 3. Electrical currents which generate a magnetic field are applied to the auxiliary device 5. When the electrical currents are adjusted to the correct value, a magnetic field adapted to the colour display tube 1 as regards position and intensity is generated. The electrical currents are measured by means of the measuring unit 9. The electrical currents represent data which completely describe the magnetic field generated by the auxiliary device 5. The data are stored in a memory 19 (for example, a ring core memory) in an adapted form (digitally). The data can be extracted from the memory 19 again for feeding a control unit 11. The control unit 11 actuates a magnetizing unit 13. A magnetic field is impressed on the device 15 arranged inside the magnetizing unit 13 (shown to be arranged outside this unit in FIG. 1), the said magnetic field equalling the magnetic field generated by the auxiliary device 5 at the area of the electron beams. The auxiliary device 5 is then removed from the neck 3 and replaced by the device 15.
The method is suitable for the application of an automatic process controller 17. The storage of the data in the memory 19, the retrieval thereof, the determination and the feeding of the data to the control unit 11 are operations which are very well suitable for execution by an automatic controller. Similarly, the process controller 17 can dispatch commands at the correct instants to mechanisms which inter alia arrange the auxiliary device 5 on the display tube 1, arrange the device 15 to be magnetized in the magnetizing unit 13, remove the auxiliary device 5 from the display tube 1, and arrange the device 15 on the neck 3 of the display tube 1. Besides these controlling functions, checking functions can also be performed by the process controller, such as the checking of:
the position of the display tube 1 with respect to the auxiliary device 5.
the determination of the number of data by the measuring unit 9.
the actuation of the magnetizing unit 13.
the position of the device 15 with respect to the display tube 1.
The method shown in FIG. 2 is an alternative to the method described with reference to FIG. 1. The auxiliary device 5 and the magnetizing unit 13 are accommodated together in one construction 6. Before the auxiliary device 5 and the magnetizing unit 13 are arranged around the neck 3 of the colour display tube 1, the as yet unmagnetized device 15 is arranged in a desired position. The auxiliary device 5 is activated and adjuste so that a magnetic field converging the electron beams is produced. Subsequently, the measuring unit 9 determines the necessary data whereby the control unit 11 is adjusted. The auxiliary device 5 may be shifted so that the magnetizing unit 13 encloses the device 15. After the current to the auxiliary device 5 has been interrupted, the magnetizng unit 13 is activated by the control unit 11. After magnetization of the device 15, the auxiliary device 5 and the magnetizing unit 13 are removed. A convergence unit which has been exactly adjusted as regards position and strength has then been arranged on the neck 3 of the tube 1.
FIG. 3 more or less diagrammatically shows an embodiment of an auxiliary device 5. The auxiliary device 5 comprises an annular ferromagnetic core 21 having formed thereon eight pole shoes a, b, c, d, e, f, g, and h which are situated in one plane and radially orientated. Each pole shoe has provided thereabout a winding wherethrough a direct current I to be adjusted is to be conducted.
In the space enclosed by the core 21 an eight-pole static magnetic field is generated whose polarity and intensity can be controlled. The value and the direction of the direct currents Ia, Ib, Ic, Id, Ie, If, Ig and Ih can be adjusted on the basis of the value and the direction of the deviations of the electron beams to be converged. The corrections required for achieving colour purity and convergence can be derived from the value and the direction of the direct currents Ia and Ih which form the data from which the necessary corrections are determined.
A similar embodiment can be used for the magnetizing unit, but because the electrical currents required for converging electron beams are smaller than the currents required for magnetizing the device, the conductors of the coils of the magnetizing unit must be constructed in a different manner which takes account the higher current intensities. If a similar embodiment of the auxiliary device has been made suitable for higher current intensities, it can also operate at lower current intensities. It follows that it is possible also to use the magnetizing unit as the auxiliary device, which is in one case connected to the measuring unit and in the other case to the control unit.
FIG. 4 shows a partly cut-away neck 3 having an envelope 31 of a colour display tube, the flared portion and the adjoining display screen not being shown. At the end of the neck 3 there are provided contact pins 33 to which cathodes and electrodes of the system of electron guns 35 are connected. The device 15 for the static convergence of the electron beams generated by the system of guns 35 consists of a support 15A of synthetic material and a ferrite ring 15B. On the jacket surface of the support 15A is provided a ridge 15c which extends in the longitudinal direction; the ferrite ring 15B is provided with a slot which co-operates therewith and which opens into the edge of the ring on only one side, so that the ring 15B can be secured to the carrier 15A in only one way. FIG. 5 is a cross-sectional view which clearly shows the ridge 15C and the slot of the device 15. The references used in FIG. 5 correspond to those used in FIG. 4.
FIG. 6 shows the same portions of the neck 3 of a colour display tube as FIG. 4. Instead of a support on which a ferrite ring is secured, the device consists only of a layer of ferrite 15 which is secured directly to the inner wall 37 of the neck 3 by means of a binding agent. This offers the advantage that a support which requires space and material can be dispensed with. FIG. 7 is a cross-sectional view and illustrates the simplicity of the device 15. The references used correspond to the references of FIG. 6. The device 15 can also be mounted (not shown in the Figure) on the rear of a deflection unit of the colour display tube. It is alternatively possible to arrange the device on grids or on the cathodes in the neck of the colour display tube.
FIG. 8 diagrammatically shows a magnetizing unit 13 whereby the device 15 arranged thereon is magnetically polarized one location after the other. The extent of the polarization is dependent of the value and direction of the used direct current Im and of the number of ampere-turns of the coil 41 arranged about the core of the magnetizing unit 13. The core consists of two portions 43 and 45 which form a substantially closed magnetic circuit. Between a concave pole shoe 47 and a convex pole shoe 49 of the core portions 43 and 45, respectively, there is a space wherein a portion of the device 15 to be magnetized is arranged. The concave and convex pole shoes 47 and 49 preferably are shaped to follow the curved faces 51 and 53 of the device substantially completely. In order to enable easy arrangement and displacement of the device between the pole shoes 47 and 49, the core portions 43 and 45 are provided with ground contact faces 55 and 57 which are perpendicular to each other. The pole shoes 47 and 49 can be moved away from and towards each other, the core portions 43 and 45 always returning to the same position relative to each other due to the faces 55 and 57 perpendicularly extending to each other. At the same time, the magnetic contact resistance at the faces 55 snd 57 is low and constant, so that the necessary unambiguous relationship between the current Im and the magnetic field generated in the core is ensured.
FIGS. 9a, b and c show a preferred embodiment and details of a static convergence device 15. The device 15 consists of a support 61 of synthetic material, for example, polycarbonate, wherein eight ferromagnetic discs (or "inserts") 63 are equidistantly arranged along the circumference. It will be obvious that this embodiment is particularly suitable for being actuated in a magnetizing unit as shown in FIG. 8. The holes 65 provided in the support 61 are slightly elliptical so as to lock the capsules 63 firmly in the holes 65. To this end, the width b is chosen to be slightly smaller than the height h which equals the diameter d of the round discs (or "inserts") 63. The narrow portions 67 of the support 61 with clamp the disc 63 in the hole 65 due to their elastic action. It is, of course, possible to magnetize the disc 63 before they are arranged in the support 61; the sequence in which the disc 63 are arranged in the support 61 should then be carefully checked.
Color television display tube with coma correction ELECTRON GUN STRUCTURE PHILIPS CRT TUBE 45AX
A color television display tube including an electron gun system (5) in an evacuated envelope for generating three electron beams whose axes are co-planar. The beams converge on a display screen (10) provided on a wall of the envelope and are deflected in the operative display tube across the display screen into two orthogonal directions. The electron gun system (5) has correction elements for causing the rasters scanned on the display screen by the electron beams to coincide as much as possible. The correction elements include annular elements (34) of a material having a high magnetic permeability which are positioned around the two outer beams. In
1. A color display tube comprising an envelope containing a display screen, and an electron gun system for producing a central electron beam and first and second outer electron beams having respective axes which lie in a single plane and converge toward a point on the screen, the electron gun system including an end from which the electron beams exit into a deflection field region of the envelope where a field deflection field effects deflection of the beams in a direction perpendicular to said plane and a line deflection field effects deflection of the beams in a direction parallel to said plane, said line deflection field producing a positive lens action;
characterized in that the electron gun system includes field coma-correcting means comprising:
(a) first and second deflection field shaping means of magnetically-permeable material arranged adjacent the respective outer electron beams, at the end of the electron gun system, for cooperating with the positive lens action of the line deflection field to anisotropically overcorrect the field coma error of said outer electron beams relative to that of the central electron beam; and
(b) a third deflection field shaping means of magnetically-permeable material arranged adjacent the central electron beam, at a position in the electron gun system further from the screen than the first and second field shaping means, for cooperating with the positive lens action of the line deflection field to reverse-anisotropically correct the field coma error of the central electron beam by an amount sufficient to compensate for the overcorrection by the first and second field shaping means, thereby effecting production of a central-electron-beam- produced raster which is substantially identical to the outer-electron-beam-produced rasters.
characterized in that the electron gun system includes field coma-correcting means comprising:
(a) first and second deflection field shaping means of magnetically-permeable material arranged adjacent the respective outer apertures in the first plate-shaped part for cooperating with the positive lens action of the line deflection field to anisotropically overcorrect the field coma error of said outer electron beams relative to that of the central electron beam; and
(b) a third deflection field shaping means of magnetically-permeable material arranged adjacent a central aperture in a second plate-shaped part of the electron gun for passing the central electron beam, at a position in the electron gun system further from the screen than the first plate-shaped part, for cooperating with the positive lens action of the line deflection field to reverse-anisotropically correct the field coma of the central electron beam by an amount sufficient to compensate for the overcorrection by the first and second field shaping means, thereby effecting production of a central-electron-beam-produced raster which is substantially identical to the outer-electron-beam-produced rasters.
BACKGROUND OF THE INVENTION
The invention relates to a colour television display tube comprising an electron gun system of the "in-line" type in an evacuated envelope for generating three electron beams. The beam axes are co-planar and converge on a display screen provided on a wall of the envelope while the beams are deflected across the display screen into two orthogonal directions by means of a first and a second deflection field. The electron gun system is provided with field shapers for causing the rasters scanned on the display screen by the electron beams to coincide as much as possible. The field shapers comprise elements of a magnetically permeable material positioned around the two outer beams and placed adjacent the end of the electron gun system closest to the screen.
A colour television display tube of this type is known from U.S. Pat. No. 4,196,370. A frequent problem in colour television display tubes incorporating an electron gun system of the "in-line" type is what is commonly referred to as the line and field coma error. This error becomes manifest in that the rasters scanned by the three electron beams on the display screen are spatially different. This is due to the eccentric location of the outer electron beams relative to the fields for horizontal and vertical deflection, respectively. The Patent cited above sums up a large number of patents giving partial solutions. These solutions consist of the use of field shapers. These are magnetic field conducting and/or protective rings and plates mounted on the extremity of the gun system which locally strengthen or weaken the deflection field or the deflection fields along part of the electron beam paths.
In colour television display tubes various types of deflection units may be used for the deflection of the electron beams. These deflection units may form self-convergent combinations with tubes having an "in-line" electron gun system. One of the frequently used deflection unit types is what is commonly referred to as the hybrid deflection unit. It comprises a saddle line deflection coil and a toroidal field deflection coil. Due to the winding technique used for manufacturing the field deflection coil it is not possible to make the coil completely self-convergent. Usually such a winding distribution is chosen that a certain convergence error remains, which is referr
ed to as field coma. This coma error becomes clearly noticeable in a larger raster (vertical) for the outer beams relative to the central beam. The vertical deflection of the central beam is smaller than that of the outer beams. As has been described, inter alia, in the U.S. Pat. No. 4,196,370 cited above, this may be corrected by providing elements of a material having a high magnetic permeability (for example, mu-metal) around the outer beams. The peripheral field is slightly shielded by these elements at the area of the outer electron beams so that these beams are slightly less deflected and the field coma error is reduced.
A problem which presents itself is that the correction of the field coma (Y-coma) is anisotropic. In other words, the correction in the corners is less than the correction at the end of the vertical axis. This is caused by the positive "lens" action of the line deflection coil (approximately, quadratic with the line deflection) for vertical beam displacements. (The field deflection coil has a corresponding lens action, but it does not contribute to the relevant anisotropic effect). The elimination of such an anisotropic Y-coma error by adapting the winding distribution of the coils is a cumbersome matter and often introduces an anisotropic X-coma.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a display tube in which it is possible to correct field coma errors on the vertical axis and in the corners to an equal extent without requiring notable adaptation of the winding distribution of the coils.
To this end a display tube of the type described in the opening paragraph is characterized in that the elements placed at the display screen end of the electron gun system are constructed to overcorrect field coma errors and that the field shapers comprise a further element positioned around the central electron beam at an area of the electron gun system further away from the display screen which operates oppositely to the elements at the end.
The invention is based on the recognition of the fact that the problem of the anisotropic Y-coma can be solved by suitably utilizing the Z-dependence of the anisotropic Y-coma.
This dependence implies that as the coma correction is effected at a larger distance (in the Z-direction) from the "lens" constituted by the line deflection coil the operation of said "lens" becomes more effective, so that the coma correction acquires a stronger anisotropic character. With the coma correction means placed around the outer beams at the gun extremity closest to the screen, the coma is the overcompensated to such a large extent that it is overcorrected even in the corners. The coma is then heavily overcorrected on the vertical axis. The correction is anisotropic. A stronger anisotropic anti-correction is brought about by performing an anti-coma correction at a still greater distance from the lens. By adding this stronger anisotropic anti-correction the coma on the vertical axis can be reduced to zero without the coma in the corners becoming anisotropic. The coma on the vertical axis and the corners is then corrected to an equal extent.
The further element may have the basic shape of a ring and may be mounted around the central aperture of an apertured electrode partition. However, restrictions then are imposed on the positioning of the further element. As will be further described hereinafter, there will be more freedom in the positioning of the further element when in accordance with a preferred embodiment of the invention the further element comprises two strips of a magnetically permeable material which extend parallel to and symmetrically relative to the plane through the electron beam axis around the axis of the central beam.
The effectiveness of these strips may be improved under circumstances when according to a further embodiment of the invention their extremities are provided with outwardly projecting lugs.
The strips may further be separate components or form one assembly with a magnetic material cup-shaped part of the electron gun system, which facilitates mounting.
An effective embodiment of the invention is characterized in that the further element is positioned in, or in front of, the area of the focusing gap of the electron gun. This may be realized in that the further element consists of a ring of magnetically permeable material which is mounted around the central aperture of an apertured partition in the focussing electrode.
The principle of the invention is realised in a given case in that the field shapers adjacent the display screen facing end of the electron gun system consist of two rings mounted on the apertured lid of a box-shaped centering bush, while the further element in that case may advantageously consist of a ring of magnetically permeable material which is mounted around the central aperture in the bottom of the centering bush.
The display tube according to the invention is very suitable for use in a combination with a deflection unit of the hybrid type, particularly when a combination is concerned which should be free from raster correction.
BRIEF DESCRIPTION OF THE DRAWING
The invention will now be further described by way of example, with reference to the accompanying drawing figures in which
FIG. 1 is a perspective broken-up elevational view of a display tube according to the invention;
FIG. 2 is a perspective elevational view of an electron gun system for a tube as shown in FIG. 1;
FIG. 3a is an elevational view of a vertical cross-section through part of FIG. 2 ; and
FIG. 3b is a cross-section analogous to FIG. 3a of a further embodiment according to the invention; and
FIG. 3c is a cross-section analogous to FIG. 3a of a further embodiment according to the invention;
FIGS. 4a, b, c and d show the field coma occurring in the different deflection units;
FIG. 4e illustrates the compensation of the field coma according to the invention;
FIG. 5a schematically shows the beam path on deflection in a conventional dislay tube, and
FIG. 5b schematically shows the beam path on deflection in a display tube according to the invention; and
FIGS. 6a, b, c and d are longitudinal sections of different embodiments of an electron gun system for a display tube according to the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a perspective elevational view of a display tube according to the invention. It is a colour television display tube of the "in-line" type. In a glass envelope 1, which is composed of a display window 2, a cone 3 and a neck 4, this neck accommodates an integrated electron gun system 5 generating three electron beams 6, 7 and 8 whose axes are co-planar prior to deflection. The axis of the central electron beam 7 coincides with the tube axis 9. The inside of the display window 2 is provided with a large number of triplets of phosphor elements. These elements may be dot shaped or line shaped. Each triplet comprises an element consisting of a blue-luminescing phosphor, an element consisting of a green-luminescing phosphor and an element consisting of a red-luminescing phosphor. All triplets combined constitute the display screen 10. Positioned in front of the display screen is a shadow mask 11 having a very large number of (elongated) apertures 12 which allow the electron beams 6, 7 and 8 to pass, each beam impinging only on respective phosphor elements of one colour. The three co-planar electron beams are deflected by a system of deflection coils not shown. The tube has a base 13 with connection pins 14.
FIG. 2 is a perspective elevational view of an embodiment of an electron gun system as used in the colour television display tube of FIG. 1. The electron gun system has a common cup-shaped electrode 20, in which three cathodes (not visible in the Figure) are secured, and a common plate-shaped apertured grid 21. The three electron beams whose axes are co-planar are focused with the aid of a focussing electrode 22 and an anode 23 which are common for the three electron beams. Focussing electrode 22
consists of three cup-shaped parts 24, 25 and 26. The open ends of parts 25 and 26 are connected together. Part 25 is coaxially positioned relative to part 24. Anode 24 has one cup-shaped part 27 whose bottom, likewise as the bottoms of the other cup-shaped parts, is apertured. Anode 23 also includes a centering bush 28 used for centering the electron gun system in the neck of the tube. This centering bush is provided for that purpose with centering springs not shown. The electrodes of the electron gun system are connected together in a conventional manner with the aid of brackets 29 and glass rods 30.
The bottom of the centeri
ng bush 28 has three apertures 31, 32 and 33. Substantially annular field shapers 34 are provided around the apertures 31 and 33 for the outer electron beams. The centering bush is for example 6.5 mm deep and has an external diameter of 22.1 mm and an internal diameter of 21.6 mm in a tube having a neck diameter of 29.1 mm. The distance between the centers of two adjacent apertures in the bottom is 6.5 mm. The annular elements 34 are punched from 0.40 mm thick mu-metal sheet material. (Conventional elements generally have a thickness of 0.25 mm).
FIG. 3a is an elevational view of a vertical cross-section through the cup-shaped part 25 of the electron gun system of FIG. 2 in which the plane through the beam axes is perpendicular to the plane of the drawing. Two (elongated) strips 35 of a magnetically permeable material such as mu-metal are provided symmetrically relative to the aperture 37 for the central electron beam.
FIG. 3b shows a cross-section analogous to the cross-section of FIG. 3a of a further embodiment of the strips 35. In this case each strip has projecting lugs 36.
The strips 35 which produce a coma correction in a direction opposite to the direction of the coma correction produced by the elements 34 are shown as separate components secured to the focussing electrode 22 (for example, by means of spotwelding). If the cup-shaped part 24 has a magnetic shielding function and is therefore manufactured of a magnetically permeable material, the strips 35 may be formed in an alternative manner as projections on the cup-shaped part 24.
FIG. 3c is an elevational view of a cross-section at a different area through the anode 22 in an alternative embodiment of the electron gun system of FIG. 2. In this alternative embodiment the strips 35 are absent. They have been replaced by an annular element 38 of a magnetically permeable material positioned around the center beam. The annular element 38 is provided on an additional apertured partition 39 accommodated between the cup-shaped parts 25 and 26.
In this embodiment there is a restriction that such an additional partition cannot be accommodated in any arbitrary position. The embodiments shown in FIGS. 3a and 3b do not have such a restriction. The strips 35 may be provided in any axial position of the component 22 dependent on the effect to be attained. A plurality of variants based on the embodiment shown in FIG. 3c is, however, possible. For this purpose reference is made to FIG. 6.
The effect of the invention is demonstrated with reference to FIG. 4. In FIG. 4a the rasters of the outer electron beams (red and blue) and the central beam (green) are shown by means of a solid and a broken line, respectively, in a display tube without field shapers and provided with a self-convergent deflection coil. The reference bc indicates the field coma.
Correction of the coma with the means hitherto known results in the situation shown in FIG. 4b. The field coma is zero at the ends of the Y-axis (the vertical axis or picture axis), but in the corners the field coma is still not zero.
Overcompensation of the field coma causes the situation shown in FIG. 4c. Overcompensation is realised, for example, by adapting the external diameter of the annular elements 34 shown in FIG. 2, or by placing them further to the front.
A coma correction in the opposite direction is realised with the aid of the elements 35 or the element 38 in a position located further to the rear in the electron gun system. The effect of this "anti"-coma correction by itself is shown in FIG. 4d.
The combined effect of the corrections as shown in FIGS. 4c and 4d is shown in FIG. 4e. The effect of the invention can clearly be seen; the field coma is corrected to an equal extent on the vertical axis and in the corners.
Elaboration of the step according to the invention on the beam path of the electron beams in a display tube is illustrated with reference to FIGS. 5a and b. FIG. 5a is a longitudinal section through a display tube 40 in which the outer electron beams R, B and the central electron beam G are deflected in a conventional manner. The reference L indicates the position where the "lensing action" of the deflection coils is thought to be concentrated. Upon generating a change in direction, a displacement (ΔY) of the outer beams relative to the central beam occurs in the "lens".
The step according to the invention ensures that there is no displacement in the lens of the outer beams relative to the central beam when generating a change in direction (FIG. 5b).
When using an annular element provided around the central aperture in an apertured partition, such as the element 38, for ensuring an anti-coma correction, there are different manners of positioning the element in a suitable place in addition to the manner of positioning previously described with reference to FIG. 3c. Some of these manners are shown with reference to FIGS. 6a, b, c and d showing longitudinal sections through different electron gun systems suitable for use in a display tube according to the invention. The plane through the axes of the electron beams is in the plane of the drawing.
FIG. 6a shows the same situation as FIG. 3c. An additional apertured partition 39 on which a ring 38 of a magnetically permeable material is mounted around the central aperture is provided between the parts 25 and 26 of the focussing electrode 22 (G3). If no additional partition 39 is to be accommodated, it is possible to provide an anti-coma correction ring 38' around the central aperture on the bottom 41 of the cup-shaped part 24. However, one should then content oneself with the effect that is produced by the ring positioned in this particular place.
As FIG. 6b shows, an alternative manner is to provide an additional partition 42 between the electrode parts 24 and 25 and mount a ring 38' of a magnetically permeable material on it. This is, however, only possible when the cup-shaped part 24 does not have a shielding function.
There is a greater variation in the positioning possibilities of the anti-coma correction element when the electron gun system is of the multistage type, as is shown in FIG. 6c. Broken lines show that one or more rings of a megnetically permeable material may be provided in different positions around the axis of the central beam.
The closer the correction elements 34 around the outer beams are placed towards the display screen, the better it is in most cases. To meet this purpose, an electron gun system having a special type of centering bush as shown in the electron gun system of FIG. 6d can be used. In that case the centering bush 28 is box-shaped and provided with an apertured end 46 on the side facing the display screen.
The apertured end 46 has three apertures 43, 44 and 45. Rings 34 of a magnetically permeable material are mounted on the outside of the end 46 at the aperture 43 and 45 for the outer beams. An optimum position, viewed in the longitudinal direction of the electron gun system, can then always be found for the ring 38 of a magnetically permeable material which is to be positioned around the central beam. This may be the position of ring 38 in FIG. 6d, but also a more advanced position indicated by the ring 38". Even a still more advanced position indicated by ring 38"' is possible. Generally, a position of the ring around the central beam in, or in front of the area of the focusing gap 47 of the electron gun, that is to say, in or in front of the area of the transition from part 26 to part 27 is very suitable. The rings around the outer beams should then be located further to the front, into the direction of the display screen.