MAGNETIC FOCUSING LENS FOR A CATHODE RAY TUBE

Abstract: A magnetic ray tube comprises in a glass envelope 1 which consists of a display window 2, a cone 3 and a neck 4, an electron gun 5 in said neck 4 for generating an electron beam 6. The electron beam 6 is focused by a magnetic focusing lens 12 consisting of two non-identical magnets 13 and 14 which are assembled at the neck 4. The magnetic field of said magnetic focusing lens is essentially asymmetrical with respect to the centre of the lens and the magnetic fields of the magnets attenuate each other more strongly in the space where the electron beam is deflected than on the other side of the focusing lens.

Description: The invention relates to a cathode ray tube comprising at least an electron gun for generating an electron beam, a system of deflection coils for the electron beam and a magnetic focusing lens for generating a magnetic field for focusing the electron beam and consisting of two permanent magnets which have at least substantially opposite magnetic orientations, surround the electron beam and generate magnetic fields, which magnetic fields are at least substantially symmetrical with respect to an axis at least substantially coaxial with the axis of the electron beam prior to deflection and attenuate each other beyond the magnetic focusing lens. Such cathode ray tubes may be used in black-and-white televisions, colour televisions and projection televisions, in data display apparatuses and in other apparatuses in which a cathode ray tube is used. A cathode ray tube of the construction described in the opening paragraph is disclosed in United States Patent Specification 3387158. In this construction the magnetic focusing lens which focuses the electron beam to form a spot on the display screen is composed of two coaxial permanent ring magnets the magnetic fields of which are at least substantially identical. The magnets are magnetized axially, that is to say with the direction of magnetization parallel to the axis. In this known construction the magnetic field of said lens present beyond the magnetic focusing lens, that is, the stray field of the lens, is reduced considerably with respect to a magnetic focusing lens consisting of one single magnet. The size of the spot on the display secreen of a cathode ray tube, and hence the definition, is also determined by the magnification. In order to make this as small as possible, the spot distance, the distance between lens and display screen, should be as small as possible. Since the distance between the system of deflection coils and the display screen is fixed, the distance between the system of deflection coils and the magnetic focusing lens should therefore be as small as possible. However, picture defects arise due to overlap of the magnetic field of the magnetic focusing lens present beyond the magnetic focusing lens, the stray field of the magnetic focusing lens, with the deflection field of the system of deflection coils in the space where the electron beam is deflected, the deflection space. Unless otherwise stated, we consider the magnetic focusing lens to be bounded by the front and rear faces of the front and rear magnets, respectively. The overlap of the magnetic field of the magnetic focusing lens with the deflection field of the system of deflection coils, and hence the picture defects caused thereby, increase as the distance between the magnetic focusing lens and the system of deflection coils decreases and as the stray field increases. The stray field of the magnetic focusing lens on the side of the deflection coil should therefore be reduced as much as possible so as to be able to reduce maximally the distance between the magnetic focusing lens and the system of deflection coils for given picture defects. The stray field of the magnetic focusing lens in the construction disclosed in US-A-3387158 can be reduced by reducing the distance between the two permanent magnets. However, this also results in a reduction of the strength of the lens. Since the strength of the lens required is determined by the distance between the electron gun and the lens and the distance between the lens and the display screen, (the object and spot distances), an increased strength of the lens produced by increasing the magnetization or the size of the magnets would have to be used to compensate for any such reduction of the strength of the lens. An additional disadvantage of reducing the distance between the magnets is that said magnets should be made of a magnetically harder material to prevent demagnetization of the magnets. It is an object of the invention to reduce considerably the stray field of the lens in the deflection space without reducing considerably the strength of the lens. The basic idea underlying the invention is that this object can be achieved by means of a magnetic focusing lens which is composed of two essentially non-identical magnetic fields which are generated by two different permanent magnets. A cathode ray tube of the type mentioned in the opening paragraph is characterized according to the invention in that the axial magnetic field of the focusing lens is essentially asymmetrical with respect to the centre of the lens and that the magnetic fields of the magnets attenuate each other more strongly in the space where the electron beam is deflected by the field of the system of deflection coils than in the space on the other side of the focusing lens. An important aspect of the invention resides in the fact that the stray field of the lens in the deflection space is considerably reduced by using a magnetic focusing lens described hereinbefore. Picture defects are considerably reduced without the strength of the lens being considerably reduced. An embodiment is characterized in that the absolute extremum of the axial magnetic field of the magnet present nearest to the deflection space coincides at least substantially with the extremum, of equal sign, of the axial magnetic field of the other magnet inside the magnetic focusing lens. The magnetic fields of the magnets neutralize each other within the magnetic lens as little as possible. Various picture defects are to be distinguished: A first picture defect which occurs is coma. This picture defect is caused by the electron beam being deflected away from the axis by the deflection field, while the electron beam is also focused by the stray field of the magnetic focusing lens. A second picture defect which occurs is picture rotation. This is produced by the stray field of the magnetic focusing lens causing the beam to rotate as a result of Lorentz forces on the moving electrons, while said beam is already being deflected. A further embodiment is characterized in that the magnets are adapted to substantially minimize coma. Again a further embodiment is characterized in that the magnets are adapted to substantially minimize picture rotation. A construction which substantially minimizes coma and hence substantially optimizes the picture definition is advantageous for high resolution data display. A construction which substantially minimizes picture rotation improves the picture display. Within the scope of the claims, the magnets may advantageously be magnetized after assembly on or in the neck. The advantage of in situ magnetization is that the properties of the magnetic focusing lens are not a priori restricted by the choice of the magnetizations of the magnets and as a result of this it is possible, after permanent assembly of the magnets, to magnetize the magnets in such a manner that a magnetic focusing lens is formed which is suitable for a cathode ray tube according to the invention. The at least substantially optimum magnetizations of the magnets can be determined empirically. A few embodiments of the invention will now be described in greater detail with reference to the drawings, in which Figure 1 is a perspective view, partly broken away, of a cathode ray tube according to the invention; Figure 2 is a sectional view of a magnetic focusing lens consisting of a permanent ring magnet magnetized in the axial direction; Figure 3 shows the axial magnetic field of the magnetic focusing lens shown in figure 2; Figure 4 is a sectional view of a magnetic focusing lens consisting of two identical permanent ring magnets, magnetized in the axial direction and positioned with opposite directions of magnetization; Figure 5 shows the axial magnetic field of the magnetic focusing lens shown in figure 4; Figure 6 is a sectional view of a magnetic focusing lens suitable for a cathode ray tube according to the invention; Figure 7 shows the axial magnetic field of the magnetic focusing lens shown in figure 6; Figures 8, 9, 10 and 11 a , 11 b , 11 c and 11 d are sectional views of further embodiments of magnetic focusing lenses suitable for a cathode ray tube according to the invention; Figure 11 e is a partial perspective view of a magnetic focusing lens suitable for a cathode ray tube according to the invention; Figure 12 a shows the total lens strength, the beam rotation, and the lens strength of the stray field for a magnetic focusing lens consisting of two identical permanent magnets as a function of the mutual distance; Figure 12 b is a sectional view of the magnetic lens corresponding to figure 12 a ; Figure 13 a shows the total lens strength, the beam rotation, and the lens strength of the stray field for a magnetic focusing lens consisting of two non-identical magnets, as a function of the inner radius of one of the magnets; Figure 13 b is a cross-sectional view of the magnetic lens corresponding to figure 13 a ; Figure 14 a shows the overall lens strength, the beam rotation, and the lens strength of the stray field for a magnetic focusing lens consisting of two non-identical magnets as a function of the thickness of one of the magnets; Figure 14 b is a sectional view of the magnetic lens corresponding to figure 14a; Figure 15 shows a magnetic focusing lens assembled on the neck of the cathode ray tube; and Figure 16 shows a magnetic focusing lens assembled in the neck of the cathode ray tube. Figure 1 is a perspective view, partly broken away, of a cathode ray tube according to the invention, in this case of a projection television tube. The invention may also be applied to camera, black-and-white and colour television, to data display apparatuses and to other apparatuses in which a cathode ray tube is used. This tube comprises in a glass envelope 1 which consists of a display window 2, a cone 3 and a neck 4, an electron gun 5 in said neck 4 for generating an electron beam 6. Said electron beam 6 is focused on a display screen 7 to form a spot 8. The display screen 7 is provided on the inside of the display window 2. The electron beam 6 is deflected over the display screen 7 in two mutually perpendicular directions x , y by means of a system of deflection coils 9. The tube comprises a base 10 having connections 11. The electron beam 6 is focused by a magnetic focusing lens 12 consisting of two non-identical magnets 13 and 14 assembled on the neck 4. Figure 2 is a sectional view of a magnetic focusing lens not according to the invention consisting of a permanent ring magnet 15, magnetized in the axial direction. The example shown is a ring magnet having a thickness D, an inner radius R₁ and an outer radius R₂. The axis of symmetry which in this example is the axis of rotational symmetry of the magnet 15 is situated on the z -axis. Figure 2 also shows a few of the magnetic field lines 16 of said magnet 15. In this figure and in the following figures the magnetic orientation in a magnet is indicated by arrows. The axial magnetic field H z (15) of said magnet 15 in relation to the axis z is shown in figure 3. Figure 4 shows a magnetic focusing lens 17 of the type as described in US-A-3387158 consisting of two identical permanent ring magnets 18 and 19 which in this example are identical to the permanent magnet 15 shown in figure 2. Said ring magnets 18, 19 are positioned coaxially with opposite directions of magnetization. The magnets 18, 19 may be positioned oppositely to each other both with their north poles (N) and with their south poles (Z). The axial magnetic field of said magnetic focusing lens 17, H z (17), is shown in figure 5 by the solid line. The axial magnetic fields of the magnet 18, H z (18), and of the magnet 19, H z (19), are indicated by broken lines in figure 5. Comparing the axial magnetic field with that of the magnetic focusing lens 15 consisting of a single ring magnet it is seen that the stray field on both sides of the magnetic focusing lens 17 is decreased. The stray field beyond the magnetic focusing lens 17 can further be decreased by reducing the distance between the magnets 18 and 19. However, this also results in the strength of the magnetic field within the lens 17, and hence the strength of the lens 17, decreasing. Figure 6 is a sectional view of a magnetic focusing lens 20 according to the invention. Said magnetic lens consists of two non-identical permanent magnets 21 and 22 positioned coaxially with opposite directions of magnetization. For this embodiment, and for each of the further embodiments, it holds that the magnets may be positioned oppositely to each other both with their north poles (N) and with their south poles (Z). In this example magnet 22 is identical to the permanent magnet 15 of figure 2. Magnet 22 has the same magnetization as magnet 21 and in this example also the same inner and outer radius. In this example the thicknesses of the magnets differ. The axial magnetic field of said magnetic lens 20, H z (20), is shown by the solid line in figure 7. The axial magnetic fields of the magnet 21, H z (21), and of magnet 22, H z (22), are indicated in figure 7 by the broken lines. In this example the absolute extremum A of the axial magnetic field H z (21) and the extremum B of equal sign situated inside the magnetic focusing lens of the axial magnetic field H z (22) coincide at lest substantially. In this manner the magnetic fields of the magnets extinguish each other between the magnets as little as possible. The last passage through zero of the axial magnetic field in this figure is indicated by Z₀. Comparing the axial magnetic field of the known magnetic focusing lens 17 in figure 5 which is composed of two identical magnets 18 and 19, it can be seen that the stray field is attenuated on one side and is intensified on the other side. The maximum axial magnetic field is substantially equal. In the invention the magnetic focusing lens 20 is positioned so that magnet 22 is farthest away from the deflection coil, so that the stray field in the deflection space is decreased. Figures 8, 9 and 10 show alternative embodiments of the magnetic focusing lens 20 in which the inner radius R₁, the outer radius R₂ and the magnetization of the magnets 21 and 22 are different. The difference in magnetization is expressed in figure 10 by different densities of magnetic field lines 16. Other possible embodiments, one of which is shown in figure 11 a , are formed by a combination of two or more of the differences between the magnets shown in figures 6, 8, 9 and 10, which means that both the inner radius and the outer radius, inner radius and thickness, outer radius and thickness, and thickness and magnetization, etc., may differ. Magnetic focusing lenses suitable for a cathode ray tube according to the invention are not restricted to magnetic focusing lens consisting of axially magnetized magnets or magnets having a rectangular cross-section. Magnets having a different cross-section or magnetization may also be suitable as a magnetic focusing lens as described hereinbefore. These magnetic focusing lenses are not restricted to rotationally symmetrical designs either. Figures 11 b , 11 c , 11 d and 11 e show a few possible further embodiments. Figure 11 b is a sectional view of a magnetic focusing lens 20 consisting of two non-identical toroidal magnets 21 and 22. Figure 11 c is a sectional view of a magnetic focusing lens 20 consisting of two partially axially and partially radially magnetized non-identical magnets 21 and 22. Figure 11 d is a sectional view of a magnetic focusing lens 20 consisting of two radially magnetized non-identical magnets 21 and 22. Figure 11 e is a partial perspective view of a magnetic focusing lens 20 consisting of two non-identical non-rotationally symmetrical magnets 21 and 22 having axes of symmetry z . The decrease of the stray field in the deflection space and the associated decrease of the picture defects, and more particularly coma and picture rotation, which can be realized by means of the invention, will now be described in greater detail with reference to a few examples. Two picture defects result from the overlap of the stray field of the magnetic focusing lens with the deflection field of the system of deflection coils. A first picture defect which occurs is coma. This defect is determined by the lens strength of the stray field in the deflection space. Said lens strength is to a first approximation proportional to the square of the axial magnetic field strength integrated over the z-axis in the deflection space: Said integral is hereinafter denoted L c . A second picture defect which occurs is picture rotation. This picture rotation is caused by beam rotation as a result of the stray field. Said beam rotation is proportional to the axial magnetic field strength integrated over the z-axis in the deflection space: This integral is hereinafter denoted B. The overall lens strength of the magnetic focusing lens is proportional to This integral is hereinafter denoted L. So these integrals determine the overall strength, the coma and the picture rotation of the lens. In the following examples in which the coma and picture rotation of a few lenses shown in figures 12 b , 13 b and 14 b are compared, it is assumed, unless otherwise stated, that the deflection space begins at point F indicated in figures 12 b , 13 b and 14 b , said point being identical for all lenses. This corresponds to a situation in which the lenses are placed at the same distance from the system of deflection coils and in such a manner that the deflection space begins approximately at point Z₀ indicated in figure 7. In figures 12 a , 13 a and 14 a , L, B/L and L c /L are shown for three different magnetic lenses 23, 24 and 25 as shown in figures 12 b , 13 b and 14 b , as a function of the distance Z between the two identical magnets 26 and 27 in figure 12 a , as a function of the thickness S of the magnet 28 in figure 13 a , and as a function of the inner radius R₁ of magnet 29 in figure 14 a , respectively. The broken line in these figures denotes a value of 2 for L, 0.1 for B/L and 0.01 for L c /L. The distance between the magnets 26 and 28 and between 26 and 29, respectively, has been chosen to be so that the absolute extremum of the axial magnetic field of magnets 28 and 29, respectively, coincides at least substantially with the extremum of equal sign of the axial magnetic field of magnet 26 within the magnetic lens. The distance between the magnets in these examples hence is equal to the distance between the maximum and the minimum of the axial magnetic field of the magnet 26. B/L and L c /L show minima as a function of S or R₁, which minima occur for approximately equal S or R₁. With equal L, said minima are many times smaller than the values for a magnetic focusing lens consisting of two identical magnets, as is obvious from a comparison of figures 12 a with figures 13 a and 14 a . By means of a construction according to the invention, coma and picture rotation can therefore be reduced by a considerable extent as compared with the known construction without considerably reducing the lens strength. This reduction of the picture defects can also be achieved by varying the outside radius or the magnetization of one of the magnets or choosing a combination of the differences between the two magnets. It is also possible to choose magnets having a non-rectangular cross-section. Decrease of the distance between the identical magnets of the known magnetic lens has the additional disadvantage that said magnets should be made of a magnetically harder material so as to prevent demagnetization of the magnets. A construction which substantially minimizes coma and hence optimizes the picture definition substantially is possible by means of a magnetic focusing lens according to the invention. In figures 13 a and 14 a this corresponds substantially to the minima in L c /L. Such a construction is advantageous for high resolution data display. A construction which minimizes the picture rotation substantially is also possible by means of a magnetic focusing lens according to the invention. Since picture rotation is undesired, this improves the picture display. In graphs 13 a and 14 a this corresponds substantially with the minima in B/L. A construction is also possible which minimizes a combination of said errors substantially, in order to obtain a substantially optimum display on the display screen. The substantially optimum choice for S or R₁ will then generally be in the region or immediately near the region indicated by the minima in L c /L and B/L. The lens strength is generally determined by the distances between the electron gun and the lens and between the lens and display screen. Those skilled in the art may choose a construction of a magnetic focusing lens according to the invention which for given lens strengths substantially optimizes the coma or the picture rotation or the total quantity of magnetic material or the minimum distance between the magnets or a combination of said properties. The possibility presented to those skilled in the art by the invention to design a magnetic focusing lens which satisfies said different criteria is an important advantage of the invention. Figure 15 is a sectional view of the neck 4 of the cathode ray tube on which a magnetic focusing lens 30 according to the invention and consisting of two non-identical magnets 31 and 32 is connected. Figure 16 is a sectional view of the neck 4 of a cathode ray tube in which a magnetic focusing lens 33 according to the invention and consisting of two non-identical magnets 34 and 35 is connected. The magnets may comprise means which enable the distance between magnets 31 and 32, or 34 and 35, to be varied and/or the distance between the lens and the system of deflection coils 36 to be varied, for example, to obtain an optimum display on the display screen which may possibly be established empirically. These means are shown diagrammatically in figures 15 and 16 by elements 37 and 38. It is possible, for example, if the shapes and the magnetization(s) of the magnets of the magnetic focusing lens are defined a priori, to set the mutual position of the magnets and/or their position with respect to the deflection coil in such a manner that the coma or the picture rotation or a combination of these properties is minimized. In order to simplify a coaxial arrangement of the magnets, both magnets may comprise means, shown diagrammatically in figures 15 and 16 by elements 39 and 40, which enable a substantially coaxial arrangement. An arrangement of the magnets which is not substantially coaxial may result in display defects. Both magnets may comprise, for example, substantially identical inner and/or outer radii, in which case the magnets can be arranged substantially coaxially on or in the neck of the envelope in a simple manner if the inside or outside diameter of the neck has been made accurately. In order to screen the stray field of the magnetic focusing lens in the radial direction, magnetic screening boxes 41 and 42, respectively, may be present. The magnets may be or become magnetized either prior to, during, or after their provision on or in the neck. The use of already premagnetized magnets is simpler than magnetization in situ of said magnets. In situ magnetization, however, has the advantage that the properties of the magnetic focusing lens are not restricted a priori by the choice of the magnetizations of the magnets. It is possible, for example, if one of the magnets of the magnetic focusing lens has a fixed magnetization, to magnetize the other magnet in situ in such a manner that coma or picture rotation or a combination of these properties is minimized.

Claims: 1. A cathode ray tube comprising at least an electron gun for generating an electron beam, a system of deflection coils for the electron beam, and a magnetic focusing lens for generating a magnetic field for focusing the electron beam and consisting of two permanent magnets which have at least substantially opposite magnetic orientations, surround the electron beam and generate magnetic fields, which fields are at least substantially symmetrical with respect to an axis at least substantially coaxial with the axis of the electron beam prior to deflection and attenuate each other beyond the magnetic focusing lens, characterized in that the axial magnetic field of the focusing lens is essentially asymmetrical with respect to the centre of the lens and that the magnetic fields of the magnets attenuate each other more strongly in the space where the electron beam is deflected by the field of the system of deflection coils than in the space on the other side of the focusing lens. 2. A cathode ray tube as claimed in Claim 1, characterized in that the absolute extremum of the axial magnetic field of the magnet which is nearest to the deflection space coincides at least substantially with the extremum, of equal sign, of the axial magnetic field of the other magnet inside the magnetic focusing lens. 3. A cathode ray tube as claimed in Claim 1 or 2, characterized in that the magnets are adapted to minimize coma substantially. 4. A cathode ray tube as claimed in Claim 1 or 2, characterized in that the magnets are adapted to minimize picture rotation substantially. 5. A device comprising a cathode ray tube as claimed in any one of the preceding Claims. 6. A method of manufacturing a cathode ray tube as claimed in any one of the preceding Claims, characterized in that the permanent magnets, after assembly on or in the neck of the cathode ray tube, are magnetized in situ.

Inventors: SPANJER, TJERK GERRIT
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1988
Main classification: H01J29
IPC codes: H01J29