DISPLAY TUBE

Abstract: Display tube such as a projection television dis­ play tube or a DGD tube (DGD = D ata G raphic D isplay), com­ prising an electron gun (5) in an evacuated envelope (1) for generating an electron beam (6) which is focussed to a spot (8) on a display screen (7) with the aid of an electro­ static focussing lens (27, 30) and which is deflected across said display screen (7) in two mutually perpendicular directions (x, y). If a correction structure (37) consist­ ing of magnetic half-hard material is provided in such a tube in the proximity of the focussing lens (27, 30) and coaxially of the gun axis (20) in which at least one mag­ netic 2N pole (N ≧ 2) is induced for correcting spot shape errors, a high spot quality can be realized with low-cost deep-drawn lens electrodes (27, 30).

Description: The invention relates to a display tube comprising an electron gun in an evacuated envelope for generating an electron beam which is focussed on a display screen with the aid of an electrostatic focussing lens and which is deflect­ ed across said display screen in two mutually perpendicular directions. A display tube of this type is used, for example, in a device for displaying symbols and/or characters gene­ rated by, for example, a computer. Such a tube is also re­ ferred to as a D.G.D. tube (DGD = Data Graphic Display). Such a display tube may, however, alternatively be a pro­ jection television display tube or another type of display tube in which only one electron beam is generated. A display tube of this type is known from "Philips Data Handbook", Electron tubes, part 8, July 1983, Monitor Tubes. An electron beam spot of very high quality is de­ sired on the display screens of both projection television display tubes and DGD tubes. This is a spot having very de­ termined, preferably small dimensions and without a halo surrounding the spot. For example, the spot must be circular. In the gun types known hitherto having focussing lens elec­ trodes deep-drawn from sheet material it has been difficult to realize the desired spot circularity. Further, an asymmetrical halo (haze) may occur around the core of the spot when the triode grids of the electron gun are not exactly in alignment. This asymmetry of the core halo results in an enlargement of the spot upon focussing to the starting point of the halo (the situation in which the halo has completely disappeared). It is therefore an object of the invention to provide a display tube in which it is possible to use low-­ cost electrodes deep-drawn from sheet material in which a high spot quality is realized. According to the invention a display tube of the type described in the opening paragraph is characterized in that a correction structure of magnetic half-hard material is provided in the proximity of the focussing lens and co­ axially of the gun axis in which at least one magnetic 2N pole is induced, with N 2. The invention is based on the recognition of the fact that, inter alia , the mechanical misalignment of the apertures in the focussing lens electrodes has four-pole and higher order pole effects on the electron beam. These effects cause the electron beam, and hence the spot on a display screen to be non-circular. These influences can be compensated by means of oppositely oriented magnetic 4-pole fields and magnetic multipole fields of an order higher than 4 (6-pole; 8-pole; 10-pole, etc.). These fields, which depend on the way in which the shape of the apertures in the focussing lens electrodes deviates from the circular shape, are generated, within the scope of the invention, by means of a correction structure in which at least one magnetic 2N pole (N ≧ 2) is induced. If this structure is present in the display tube, it may advantageously serve a second purpose. By inducing also a magnetic dipole therein, it is possible to correct centring errors. A centring error occurs when the non-­ deflected electron beam is not focusssed to a spot in the centre of the display screen. This is a result of misalign­ ment of the gun. Due to possible eccentricities and tilts of the gun electrodes located close to the cathode, an unwanted beam deflection may be effected so that the beam does not pass through the centre of the focussing lens. In that case the beam is askew and eccentric in the focussing lens. When the anode voltage and/or focussing voltage is varied, the spot on the display screen will thereby change its position (referred to as beam displacement). The fact that the elec­ tron beam does not pass centrally through the focussing lens results in a non-symmetrical halo round the spot. This type of error can be corrected by providing a bipolar field in a second structure of magnetizable material, in the triode part of the gun close to the cathode. A first preferred embodiment of a display tube according to the invention is characterized in that, viewed in the direction of propagation of the electron beam, an electron beam alignment structure of magnetic half-hard material in which a magnetic dipole is induced is provided coaxially round the gun axis just behind the cathode. By this structure the electron beam can be passed through the centre of the focussing lens and the core halo asymmetry can be corrected. A display tube according to the invention can be constructed in such manner that the focussing lens, viewed in the direction of propagation of the electron beam, con­ sists of a first and a second cylindrical focussing lens electrode, said first electrode extending coaxially into the second electrode, the said correction structure being secured to the edge of the second electrode remote from the first electrode. By securing the correction structure behind the lens gap of this accelerating focussing lens, the electron beam shape can be corrected in a very effective manner. In fact, the corrected beam does not sub­ sequently pass through an electron lens where it could be distorted again. In addition, it is easy in practice to se­ cure a correction structure in the form of a ring to the edge of a cylindrical electrode. The (annular) structure can alternatively be provided in the second focussing lens electrode near the edge of the first focussing lens elec­ trode. It is even possible to provide the (annular) structure just in front of the focussing lens, for example, at the cathode-facing extremity of the second focussing lens elec­ trode. The corrected beam then, however, still passes through the focussing lens. A display tube according to the invention may fur­ thermore be constructed in such manner that the electron gun comprises a cathode, a control electrode, an anode, a pre­ focussing electrode and a first focussing lens electrode and that the beam alignment structure is secured coaxially round the gun axis to the cathode-facing side of the first focussing lens electrode. The invention will now be described in greater de­ tail by way of example with reference to the drawing in which Fig. 1 is a perspective view, partly broken-away, of a display tube according to the invention and Fig. 2 is a longitudinal section through an elec­ tron gun for a display tube as shown in Fig. 1. The display tube shown in Fig. 1 comprises a glass envelope 1 consisting of a display window 2, a cone 3 and a neck 4 which accommodates an electron gun 5 for generating an electron gun 5 for generating an electron beam 6. This electron beam 6 is focussed to a spot 8 on a display screen 7. The display screen 7 is provided on the inside of the display window 2. The electron beam is deflected across the display screen 7 in two mutually perpendicular directions X, Y with the aid of the deflection coil system 9. The tube is provided with a base 11 having connection pins 12. Fig. 2 is a longitudinal section through an elec­ tron gun 5 as shown in Fig. 1. This electron gun comprises, centred along an axis 20, a cathode 21 having an emitting surface 22, a control electrode 24 provided with an aper­ ture 23, a first anode 25 provided with an aperture 255, a prefocussing electrode 26 provided with an aperture 266, a first cylindrical focussing lens electrode 27 having a bottom 28 with an aperture 29 and a second cylindrical focussing lens electrode 30. The electrodes 24, 25, 26, 27 and 30 are supported on glass rods 33 by means of brackets 31 and electrode pins 32 sealed therein. The entire electron gun assembly is secured by means of the mounting pins 34 in a glass bottom plate 35 provided with an exhaust tube 36 and connection pins 12. The connection wires between the various gun electrodes and the connection pins are omitted so as not to make the drawing unnecessarily complicated. A ring 37 of a magnetic half-hard material as described in German Patent Specification 2,612,607, details of which are included herein by reference, is provided on the edge of the second focussing electrode 30. This material consists of, for example, an alloy of Fe, Co, V and Cr, which alloy is known under the trade name Koërflex (a trade­ mark of the firm of Krupp). No welding operation may be per­ formed on this ring, because otherwise its magnetic proper­ ties change. Therefore the ring is secured by means of a number of clamps not shown in the drawing. The gun assembly shown in Fig. 2 is inserted into the neck 4 of the tube (See Fig. 1), positioned and subsequently sealed with glass plate 35. Subsequently at least one magnetic 2N pole (N ≧ 2) and a magnetic dipole are externally induced in the ring 37 after the tube is finished, dependent on the observed errors in the spot shape and location of the spot of the non-deflected electron beam. The ring 37 is magnetized, for example, in a manner and with the aid of a magnetizing device as described in the United States Patent Specificat­ ion 4,220,897 already referred to. As N is larger, the re­ quired strength of the 2N pole generally decreases, in other words, when a 4-pole is present, this pole has the greatest strength. When a ring 38 which is also magnetizable is pro­ vided close to the cathode 21 and with this ring appropriate­ ly magnetised, it is possible to pass the electron beam accurately through the centre of the focussing lens con­ stituted by the electrodes 27 and 30. This is effected by magnetizing the ring 38 as a dipole. The distance between the two magnetized rings is 63 mm in this case. Conventio­ nal voltages on the electrode are shown in the Figure. The operation of one and two magnetized rings will now be des­ cribed in detail with reference to the following Table: The first column states three values of electron beam currents I (in mA). The second column states (under A) the spot di­ mensions ds (mm) in the x and y directions and the associated potential at focussing electrode 27, referred to as the fo­ cussing voltage V foc (kV) for a gun in which the rings had not yet been magnetized. The third comumn states (under B) also the spot di­ mensions ds (mm) in the x and y directions and the asso­ ciated V foc . In this case ring 37 has been magnetized in such a manner that the beam and the spot were circular in the fo­ cussed state. In the case of larger beam currents the spot is also smaller in surface area than in the situation shown under A. The fourth column states (under C) the spot di­ mensions ds (mm) in the x and y directions and the asso­ ciated V foc . In this case ring 38 had also been magnetized optimally (as a dipole). Particularly in the case of larger beam currents the spot dimensions considerably decrease with respect to the situation shown in the second column (under A), The diameter of electrode 27 is 10 mm in its narrow­ est part and 16 mm in its wider part. The length of electrode 27 is 53.5 mm. The diameter of electrode 30 is 20 mm. The diameter of both aperture 255 and aperture 23 is 0.4 mm. The diameter of aperture 266 is 1.5 mm and that of aperture 29 is 2 mm. The distance between the cathode surface and electrode 24 is 0.065 mm. The distance between the electrodes 24 and 25 is 0.150 mm. The distance between the electrodes 25 and 26 is 0.65 mm and that between the electrodes 26 and 27 is 1.4 mm. The thickness of electrode 24 is 0.1 mm. The thickness of electrode 25 is 0.25 mm, of electrode 26 0.4 mm, of electrode 27 0.25 mm and of electrode 30 also 0.25 mm. The Figure is an approximately 1.5 to 2 times en­ larged illustration of the actual electron gun. It stands to reason that the magnetizable struct­ ure is not limited to a ring and may alternately have a different shape. It is, for example, possible to position a plurality of magnetizable elements in a ring consisting of non-magnetic material and subsequently mount this ring in the gun. It is also possible for the focussing lens to be a unipotential lens or a multi-stage lens.

Claims: 1. A display tube comprising an electron gun in an eva­ cuated envelope for generating an electron beam which is focussed on a display screen with the aid of an electrosta­ tic focussing lens and which is deflected across said display screen in two mutually perpendicular directions, character­ ized in that a correction structure of magnetic half-hard material is provided in the proximity of the focussing lens and coaxially of the gun axis in which at least one magnetic 2N pole is induced, with N ≧ 2. 2. A display tube as claimed in Claim 1, character­ ized in that a magnetic dipole is also induced in the cor­ rection structure. 3. A display tube as claimed in Claim 1 or 2, cha­ racterized in that, viewed in the direction of propagation of the electron beam, an electron beam alignment structure of magnetic half-hard material in which a magnetic dipole is induced is provided coaxially of the gun axis just be­ hind a cathode. 4. A display tube as claimed in Claim 1 or 2, cha­ racterized in that the focussing lens, viewed in the di­ rection of propagation of the electron beam, consists of a first and a second cylindrical focussing lens electrode, said first electrode extending coaxially into the second electrode, said correction structure being secured to the edge of the second electrode remote from the first electrode. 5. A display tube as claimed in Claim 3, character­ ized in that the gun comprises a cathode, a control electrode, an anode, a prefocussing electrode and a first focussing lens electrode and that the electron beam alignment struct­ ure is secured coaxially round the gun axis at the ca­ thode-facing side of the first focussing lens electrode. 6. A display tube as claimed in any one of the pre­ ceding Claims, characterized in that the structure or structures are annular.

Inventors: MENSIES, OTTO C/O INT. OCTROOIBUREAU B.V.
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1986
Main classification: H01J29
IPC codes: H01J29