GB2046988A - Colour television tube guns - Google Patents

Abstract

In a shadow-mask colour television tube having guns with a triode section 47, 49, 51 and a focussing section 53, 55, 57, 59, coma is reduced by including in each section at least one electrode having a non-circular aperture - e.g. elliptical - having a major and a minor axis, the two major axes being mutually orthogonal, and preferably with that of the triode section aperture being horizontal. <IMAGE>

Description

SPECIFICATION Television picture tube This invention relates to a picture tube and more particularly to a color picture tube with an electron gun having non-circular apertures.

In a shadow mask type color picture tube, it is possible to converge three electron beams by means of a non-uniform magnetic deflecting field without the use of a separate convergence coil. This deflecting field should consist of both a horizontal field having a pincushion-shaped configuration and a vertical field having a barrel-shaped configuration.

While the pincushion field causes the beams to converge in front of the phosphor screen (i.e., over convergence) for both the horizontal and vertical directions, the barrel shaped field causes a different convergence in the vertical and horizontal directions.

That is, the action of the horizontal components of the barrel-shape field, causes the three beams to converge behind the phosphor screen (i.e., under convergence) while the vertical components of the barrel-shaped field causes convergence in front of the phosphor screen on the side facing the electron gun (i.e., over convergence). This difference in convergence in both the horizontal and vertical directions affects the electron current distribution of the beam itself which causes coma aberration.

As illustrated in Figure 1, an electron beam 1 generated from an electron gun 2 has an electron current distribution comprising a high current density portion 3 at the centre and a low current density portion 4 around the high current density portion 3.

As the beam approaches the phosphor screen 5, is passes through the vertical deflecting field 6. The electrons positioned near the centre of the deflecting field 9 are subject to a stronger deflecting force that the electrons positioned away from the center field.

Therefore, the low current density portion 4 of the beam will be focused in front of the screen as shown by focal point 7 (i.e. over focused). Consequently, the beam spot on the screen at the periphery thereof comprises a high current density portion 3 and a tail shaped haze portion 8 around the high density portion 3. This taii portion is referred to as coma aberration.

Reducing the coma aberration can be obtained by using a large focusing lens or a complex focusing lens. Any reduction in aberration, however, produced by such structure will be restricted by the size of the electrodes that can be used with the narrow neck color picture tubes currently used.

This invention seeks to provide a picture tube with an electron gun structure for reducing coma aberration at the periphery or corners of the screen.

In accordance with the present invention there is provided an electron gun which comprises a triode section and a focusing section; each of these sections being provided with at least one electrode containing a non-circular aperture having major and minor axes of symmetry. Each major axis is preferably positioned substantially at right angles to at least one other major axis, and the major axis of at least one non-circular aperture within the focusing section is position substantially parallel to the vertical direction. While the use of non-circular apertures are known in the prior art, they have not been used in the way disclosed by the instant invention. For example, U.S. Patent No. 3,997,763 discloses the use of a single non-circular aperture in a picture tube to compensate for the astigmatic effect caused by a quadrupolar lens used for deflection amplification.

U.S. Patent No. 3,524,094 discloses a picture tube which generates a narrow elongated spot and the use of a single non-circular aperture to minimize the twist of the spot during deflection. British Patent No.

1,421,865 discloses the use of two non-circular apertures positioned in the triode of an electron gun to obtain a narrow elongated spot for an indexingtype tube.

This invention seeks to provide a picture tube with an electron gun structure for reducing coma aberration at the periphery or corners of the screen.

In a preferred form of this invention, the picture comprises an envelope with a panel containing a phosphor screen on its inner wall and an electron gun. The electron gun generates an electron beam which scans the phosphor screen and is deflected by a beam deflecting system in both horizontal and vertical directions along the phosphor screen. The electron gun comprises a cathode for emitting the electron beam, a triode section and a focusing section wherein each of these sections contain electrodes placed along the path of the electron beam. At least one of the electrodes within each section contains a non-circular aperture having a major axis and minor axis of symmetry, and each major axis being positioned substantially at a right angle with the major axis of one at least one other non-circular aperture electrode.Finally, the major axis of at least one non-circular aperture within the focusing section is positioned substantially parallel to the vertical direction.

One embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure lisa schematic illustration of an electron beam generated by a prior art electron gun; Figure2 is a perspective, partially in section, of a color picture tube embodying this invention; Figure 3 is a schematic illustration showing the relative position and orientation of the non-circular apertures of this invention; Figure 4 is another schematic illustration showing the elevational aspect of the non-circular apertures of this invention superimposed; and Figure 5 is an axial section view of an electron gun embodying this invention with the schematic association of the main lens and sub-lens formed by the focusing section.

Referring to the drawings, wherein like reference numerals designate identical corresponding parts of the embodiment, and more particularly to Figure 2 thereof. Figure 2 shows a shadow-maktype color picture tube of the invention comprising a glass envelope 11 which comprises a panel 13, a funnel portion 15 extending from panel 13, and a neck portion 17 extending from funnel portion 15.

On the inner wall of panel 13 there is a phosphor screen 19 containing a combination of phosphor stripes 21, 23 and 25 emitting red, green and blue light respectively. A color selecting electrode (i.e. a shadow-mask 27) is positioned close the phosphor screen 19 within the panel 13. The color selecting electrode 27 is provided with a plurality of slots 29 for selecting electron beams 31,33 and 35 generated from an electron gun 37 within neck portion 17. The electron beams will impinge the phosphor screen 19 after passing through a magnetic deflecting field and slots 29. The deflecting field is formed by deflecting coils 39 attached to the outside of funnel portion 15 and neck portion 17, for deflecting the beams in both a horizontal direction X and a vertical direction Y.

Electron gun 37 comprises three electron gun units 41,43 and 45 (see Figure 2) arranged in a line (i.e., the X direction). One of these units is shown in detail in Figure 5. Each electron gun unit comprises a cathode 47, a first electrode 49, a second electrode 51, a third electrode 53, a fourth electrode 55, a fifth electrode 57 and a sixth electrode 59. Cathode 47 and first and second electrode 49 and 51, respectively, constitute a triode section. The third through sixth electrode 53-59 constituting a focusing section. First and second electrode 49 and 51 have a plate-like shape and the third through sixth electrodes 53-59 have a cylindrical shape. The second and the fourth electrodes 51 and 55 are interconnected and the third and the fifth electrodes 53 and 57 are interconnected.Sixth electrode 59 is connected to a wall electrode (not shown) disposed on the inner wall of funnel portion 15. In operation, 25KV (i.e., anode voltage) is applied to the sixth electrode 59, 7KV (i.e.

focus voltage) is applied to the third and fifth electrodes 53 and 57, and 500V is applied to the second and the fourth electrodes 51 and 55. As illustrated in Figure 5, the focusing section forms both a mains electrostatic focusing lens 61 of a bi-potential type between electrodes 57 and 59, and a sub-electrostatic focusing lens 63 of a uni-potential type among electrodes 53-57 along path 65 of the electron beam.

In such a composite lens system, the electron beam 31 generated by cathode 47 is focused by sub-lens 63 before being focused by main lens 61.

Sub-lens 63 functions to reduce the spherical aberration of the electron beam.

As shown in Figures 3 and 4, electrode 49 within the triode section contains a non-circular aperture 67 (e.g., having an elliptical or rectangular shape) having a major axis 69 and minor axis 68. Electrode 55 within the focusing section contains a noncircular aperture 71 (e.g., an ellipical or rectangular shape) on the side facing fifth electrode 57; aperture 71 also contains a major axis 73 and minor axis 72.

Aperture 67 has dimensions of 0.65 to 0.7 mm (major diameter) and 0.6 mm (minor diameter), while aperture 71 has dimensions of 5.55 to 5.6 mm (major diameter) and 5.5 mm (minor diameter). All the other apertures of the electrodes are circular. As shown in Figure 3, major axis 69 of aperture 67 is positioned substantially at a right angle with respect to the major axis 73 of aperture 71. Furthermore, major axis 73 is positioned substantially parallel to vertical direction Ywhich causes sub-focus lens 63 to be a non-rotationally symmetrical lens. As a result, the amount of electron beam focusing in the vertical direction is smaller than the focusing in the horizontal direction; consequently, the beam will be underfocused in the vertical direction.

As previously mentioned, the instant invention is utilized with a vertical deflecting field having a barrel-shaped configuration. Due to such a field, the electron beam is subject to over-focusing in the vertical direction as it passes through the field, as shown by the position of focal point 7 in Figure las a result coma aberration of the low density portion is produced. The instant invention, however, reduces this coma aberration by verticaliy elongating the low current density portion 4 of the electron beam within the focusing section of the electron gun. Therefore, as the low current density portion 4 of the electron beam is vertically elongated, the focal point 7 of the beam approaches the screen 5. In otherwords, the instand invention makes it possible to control the position of the focal spot in order for it to be focused on the screen.As a result, coma aberration is reduced even at the corner or periphery of the screen.

This non-rotationally symmetric sub lens 63 produces an electron beam having a cross section which is longitudinally elongated; consequentiy the beam spot on the centre of the screen will be slightly non-circular. However, since the major axis 69 of non-circular aperture 67 electrode 49 is at a right angle with the major axis 73 of the other non-circular aperture 71, the electron beam is given a cross section which is horizontally elongated. Utilizing non-circular electrode 49 in combination with noncircular 71 tends to compensate for any noncircularity of the beam spot.

As mentioned above, non-circular apertures are utilized, one in the triode section (i.e. electrode 49) and one in the focusing section (i.e., electrode 55).

The use of non-circular apertures are also applicable to the other electrodes within the triode section and focusing section. For example, the aperture of the fifth electrode 57 within the focusing section (i.e., the aperture facing the sixth electrode 59) can also be made non-circular.

Alternatively, since there must be at least two circular apertures, the first, second and fourth electrodes 49, 51 and 55, respectively, can also contain non-circular apertures.

Although not necessary, for operation of the invention it is desirable that the particular noncircular shape and dimension of the aperture of the second electrode 51 be the same as or substantially similar to the non-circular aperture of the first electrode 49. It should be noted, that each major axis of the non-circular apertured electrode utilized should be positioned substantially at a right angle with the major axis of at least one other electrode.

It should be understood that according to the invention, an electron beam having a very narrow width can be obtained, and beam spots without coma aberration can be obtained at the corner of the screen.

It should also be understood that the invention can be used in many types of electron guns, i.e., unti-potential, bi-potential, tri-potential and periodic potential. In fact, any electron gun comprising a triode section and a focusing section can obtain the benefits of the invention by utilizing at least two non-circular apertures, at least one in each section, wherein each major axis is positioned substantially at a right angle with at least the major axis of one other non-circular electrode.

It should be further understood that the invention can be applied to an unitary type electron gun in lieu of a discrete type electron gun.

Claims (9)

1. A television picture tube including at least one electron gun comprising a triode section for generating a beam of electrons, and a focusing section including a plurality of electrodes arranged along the path of the beam for focusing the beam, each of said sections including at least one electrode having a non-circular aperture with major and minor axes of symmetry.

2. Atelevision picture tube as claimed in claim 1, which further comprises a beam deflecting system for deflecting the electron beam in horizontal and vertical directions, and in which the said non-circular aperture of the or each electrode of the focusing section is arranged wth its major axis vertical and the non-circular aperture of the or each electrode of the triode section is arranged with its major axis horizontal.

3. A television picture tube according to claim 2 in which the vertical beam-deflecting field has a barrel-shaped configuration.

4. Atelevision picture tube according to any preceding claim in which the said apertures are elliptical.

5. Atelevision picture tube according to any preceding claims, further comprising a phosphor screen on the inner suface of the front panel of the tube, the screen comprising a plurality of parallel phosphor stripes extending in a vertical direction.

6. Atelevision picture tube according to claim 5, in which the said stripes are arranged in groups of three, one phosphor of each group being a redemitting phosphor, one being a green-emitting phosphor, and one a blue-emitting phosphor.

7. Atelevision picture tube according to claim 6, having three electron guns arranged in a horizontal line, one for each of the said three colours.

8. A television picture tube according to claim 6 or claim 7 further comprising a colour-selecting electrode arranged close to the said screen.

9. A television picture tube substantially as herein described with reference to to the accompanying drawings.