Electron gun for colour cathode-ray tube

Abstract: An electron gun for a colour cathode-ray tube has a correction electrode to reduce astigmatism. This is achieved with a simple plate electrode 11 having at least one horizontally elongate electron beam aperture, the plate being fixed in a second accelerating/focusing electrode 5 at a location between an inner shield 10 fixed to the second accelerating/focusing electrode 5 and a shield cup 8. This corrects astigmatism without giving rise to assembly or alignment difficulties.

Description: The present invention relates to a correction electrode suitable, for instance, for use in association with a shield cup in an electron gun fixed to the neck of a funnel and for emitting electron beams. Such correction electrodes are aimed at preventing the electron beams from being distorted at the centre and periphery of the screen by reducing the astigmatism of the main lens. The basic components of electron guns of this type are shown in Fig. 1. Disposed in a horizontal line are an electron-beam-forming portion having a cathode 1 for emitting thermions according to red, green and blue electric input signals after being heated by a heater, a first grid electrode 2 installed on one side of the cathode for controlling the electron beams emitted from the cathode, and a second grid electrode 3 installed on one side of the first grid electrode for attracting and accelerating the thermions gathered around the cathode; and a first accelerating/focusing electrode 4 and a second accelerating/focusing electrode 5 constituting a main focusing lens for finely focusing the electron beams serially incident from the electron-beam-forming portion and thereby forming electron beam spots. Fig. 2 shows a refinement of the basic concept, namely the electron gun of the multilevel-focusing type. Here third and fourth grid electrodes 6 and 7 for front-stage focusing are added to form a front-stage focusing lens, between the electron-beam-forming portion and the electrodes of the main focusing lens. The electrodes each having three electron beam passing holes for passing the red, green and blue electron beams produced from the cathode 1 are integrally fixed by a pair of glass beads at a predetermined interval. In the conventional electron guns, as the cathode 1 is heated by the heater and thermions are emitted from it, electron beams are controlled by the first grid electrode 2 and simultaneously accelerated by the second grid electrode 3. They then pass through the main lens, i.e. the first accelerating/focusing electrode 4 and the second accelerating/focusing electrode 5. During their passage the electron beams are finely focused and accelerated as a result of the difference in the voltages applied to the first accelerating/focusing electrode 4 and the second accelerating/focusing electrode 5, thereby causing the phosphor coating on the inner surface of the screen to luminesce. This realises an image on the screen. In these conventional electron guns, the electron beam passing holes are perforated in almost a full circle from the first grid electrode 2 to the second accelerating/focusing electrode 5 so that the main focusing lens formed by the first and second accelerating/focusing electrodes 4, 5 becomes a rotationally symmetric lens. Thus, when voltages required in the operation of electron guns are applied, the electron beams passing the electron beam passing holes are converged rotationally symmetrically according to Lagrange's law so that the electron beams are circular when starting from the electron guns, and thinly focused in a circle when they reach the centre of the screen. At this stage, the electron beam forms a small circular spot. Images are realised as the electron beams emitted from the electron guns are projected at different points on the screen by the magnetic field of a deflection yoke. This deflection field has a certain stray component which can adversely influence the course of the electron beam, as described below. In the above operation, when the electron beams pass through the second accelerating/focusing electrode 5, if there is no correction electrode (not shown) for shielding and weakening the effect of the stray magnetic field of the deflection yoke on the electron beams, the convergence can still be properly adjusted by changing the shape and location of an inner shield fixed in the second accelerating/focusing electrode 5. However, the astigmatism cannot be properly adjusted in this manner and the diverging field is weakened in the diverging area of the second accelerating/focusing electrode 5, reducing the electron beams' vertical divergence. This creates a halo phenomenon at the centre and periphery of the screen. In order to overcome this problem there was proposed a technique in which a correction electrode is installed. Such an electrode arrangement appears in Figs. 3 and 4, the correction electrode 9 being shown between the second accelerating/focusing electrode 5 and a shield cup 8. The effect of the correction electrode is that the convergence is not affected but the astigmatism is varied optimally. This correction electrode has a divergence field which is strong in the divergence area of the second accelerating/focusing electrode 5, increasing the electron beams' vertical divergence. Therefore, it corrects the astigmatism without the convergence being affected, so that a good beam spot at the centre and periphery of the screen is obtained. In other words, the correction electrode diverges the electron beams vertically so as to elongate the electron beams vertically at the centre of screen but to obtain circular beam spots at its periphery. Here, the astigmatism represents the difference between the vertical and horizontal voltages of a spot beam formed on the screen. It implies that as the difference becomes greater, the astigmatism also becomes greater. The astigmatism is calculated from the difference between the vertical focus voltage and the horizontal focus voltage. If the vertical focus voltage is higher than the horizontal focus voltage, the astigmatism is negative, and vice versa. If the astigmatism falls within the range 100-300 (positive), the best electron beam spot can be obtained at the centre of screen as well as at the periphery. However, if the astigmatism is negative, the halo phenomenon is severe at the centre and periphery of the screen. Turning to Figs. 3 and 4 in more detail, Fig. 3 is a partially cut away perspective view of an electron gun in which the conventional electrode is fixed in the shield cup, and Fig. 4 is a vertical cross-sectional view of Fig. 3. In this drawing, the correction electrode 9 in the form of a horizontal barrier is welded at the upper and lower portions of electron beam apertures or passing holes 8a formed in the shield cup 8, and the shield cup 8 with the correction electrode 9 fixed is itself inserted and fixed to the second accelerating/focusing electrode 5. The second electrode 5, which is of generally oval cross-section, contains a shield electrode 10 with three apertures corresponding to the passing holes 8a but much larger. With this arrangement, when the electron beams emitted from the cathode pass through the second accelerating/focusing electrode 5, the magnetic field produced by the deflection yoke can be sufficiently shielded and the astigmatism corrected without the convergence being affected. In this structure, however, the processing of the shield cup 8 in order to fix the correction electrode 9 is not without problems, since it is hard to make an even connection surface for fixing the correction electrode and to align the electrode 9 with the electron beam passing holes. This puts the welding points of the correction electrode out of true so that the passage of the electron beam is altered and precise processing of the correction electrode is difficult. This diminishes the resolution. It is therefore an object of the present invention to provide an electron gun for a colour cathode-ray tube for reducing astigmatism and for which processing and assembly is easier. This can be done by varying the structure and installation position of the correction electrode. For instance, in EP-A-0104674 (N.V. Philips), there is provided an electron gun for a colour cathode-ray tube having, in line sequentially from the cathode to the screen, a grid electrode assembly, an accelerating/focusing electrode assembly and a shield cup, wherein a correction electrode having at least one electron beam aperture is fixed in the second accelerating/focusing electrode assembly for reducing astigmatism, the apertures in the correction electrode being in some embodiments elongate in the horizontal direction. The elongate shape of the electron beam aperture or apertures provides the necessary compensation for the astigmatising tendency of the stray field from the deflection yoke. In the invention as defined in claim 1 there is also present an inner shield fixed to the accelerating/focusing electrode assembly, for adjusting the convergence of the electron beams, the correction electrode being parallel to the inner shield and located between the inner shield and the shield cup closer to the inner shield than to the shield cup. The correction electrode can be made simply in the form of a plate; this means that it can be easily fixed and aligned within the accelerating/focusing electrode. A better understanding of the present invention will be obtained from the following description of a preferred embodiment with reference to the attached drawings, in which: Fig. 1 is a vertical cross-sectional view of an example of a type of electron gun; Fig. 2 is a vertical cross-sectional view of a more specific type of electron gun; Fig. 3 is a partially cut-away perspective view of a gun in which a conventional correction electrode is fixed to a shield cup; Fig. 4 is a vertical cross-section of Fig. 3; Fig. 5 is a perspective view of an electron gun in accordance with the present invention; Fig. 6 is a vertical cross-section of Fig. 5; and Figs. 7A - 7E are front views of a variety of correction electrodes applied within the scope of the present invention. Referring to Figs. 5, 6 and 7, like numerals are applied to like components as in the conventional configuration. A plate-like correction electrode 11 in which a single horizontally elongate electron beam passing hole 11a is formed is fixed in the second accelerating/focusing electrode 5, at an axial location between the inner shield 10 and the shield cup 8 fixed to the second accelerating/focusing electrode 5. This plate correction electrode 11 is, like the inner shield 10, fixed at its edge to the surrounding electrode 5 at a distance from the cup 8, so that it does not need to be fixed to the cup. This means that any irregularities in the surface of the cup do not affect the mounting of the correction electrode, and likewise the alignment of the electrode 11 is automatic by virtue of its edge location within the accelerating/focusing electrode 5. The electron beam aperture 11a formed on the correction electrode 11 is in Fig. 5 in the form of a single slot encompassing the three beams. However, a variety of forms is possible: the total aperture may consist of several individual holes of rectangular section as shown in Fig. 7A, of oval-rectangular form with both ends semicircular as shown in Fig. 7B, or elliptical as shown in Fig. 7C. Alternatively, the electron beam aperture can be formed as a horizontally elongated single hole with both ends semicircular or expanded as shown in Figs. 7D and E respectively. It would be possible in theory to subdivide the individual holes to have more than one hole per electron beam. In the correction electrode 11, since the height V os the electron beam passing holes lla is lower than their width, or than the equivalent width of the part of the single aperture per electron beam, the electron beam's vertical divergence is increased. The greater the thickness of the correction electrode, the more the divergence effect is increased. It is preferable that the thickness of the correction electrode fall within 0.5-1.0 mm. It is further necessary according to the present invention that the correction electrode 11 be closer to the inner shield 10, than to the shield cup 8. This is because the closer the correction electrode is to the inner shield, the more the electron beams are diverged vertically. As described above, in embodiments of the present invention, the correction electrode has one or more horizontally elongate electron beam passing holes, arranged in a horizontal line to correspond to the row of electron beams. This line need not in principle be gravitationally horizontal, though this will be the usual configuration, as dependent on the astigmatic effect of the stray field of the deflection yoke. The correction electrode is fixed around the inner shield, which itself is fixed to the second accelerating/focusing electrode so that the electron beams are diverged more vertically in order to correct the astigmatism. This realises a good-quality image.

Claims: An electron gun for a colour cathode-ray tube having, in line sequentially from the cathode to the screen, a grid electrode assembly, a first accelerating/focusing electrode assembly (4, 5) and a shield cup (8), wherein the accelerating/focusing electrode assembly includes first and second accelerating/focusing electrodes (4,5), a correction electrode (11) located in the second of these electrodes and having at least one electron beam aperture for reducing astigmatism, the aperture (11a) in the correction electrode being elongate in the horizontal direction, characterised in that an inner shield (10) is fixed to the accelerating/focusing electrode assembly, for adjusting the convergence of the electron beams, the correction electrode being parallel to the inner shield and located between the inner shield (10) and the shield cup (8), the correction electrode (11) being located closer to the inner shield (10) than to the shield cup (8). An electron gun as claimed in claim 1, in which the correction electrode (11) is in the form of a flat plate. An electron gun as claimed in claim 1 or 2 and having a plurality of independently formed horizontally elongate electron beam apertures in the correction electrode. An electron gun as claimed in claim 3, wherein each electron beam aperture is rectangular, or is oblong with both ends semicircular. An electron gun as claimed in claim 3, wherein each electron beam aperture is elliptical. An electron gun as claimed in claim 1 or 2 and having an electron beam aperture formed as a single horizontally elongate hole. An electron gun as claimed in claim 6, wherein the ends of the aperture are expanded. An electron gun as claimed in any preceding claim, in which the grid assembly comprises a first grid electrode (2), a second grid electrode (3), a third grid electrode (6), and a fourth grid electrode (7).

Inventors: CHOI JIN YEOL, CHOI, JIN YEOL
Applicants: LG ELECTRONICS INC, LG ELECTRONICS INC.
Application year: 1995
Main classification: H01J29
IPC codes: H01J29