In-line color picture tube

Details In-line color picture tube In-line color picture tube

1991-02-28

1992-09-15

O'Shea, Sandra L.

Electric lamp and discharge devices

Cathode ray tube

Plural beam generating or control

313447, H01J 2948

Patent

active

051480816

DESCRIPTION:

BRIEF SUMMARY
DESCRIPTION

The present invention relates to an in-line colour picture tube in accordance with the precharacterizing part of claim 1.
Colour picture tubes provided with a self-converging deflection system suffer from the drawback that the non-homogeneous multiple fields produced by the said system cause distortions of the electron spot produced on the screen, the distortions being such that a circular spot at the centre of the screen, following deflection to the lateral edge of the screen, will be underfocused in the horizontal direction, thereby giving rise to a supine (i.e. horizontal) ellipse with an aureole (or corona) at both the top and the bottom. Though the round shape of the spot remains relatively well preserved after vertical deflections, an aureole is produced as the upper or lower edge of the screen is approached; this aureole always faces inwards, i.e. towards the centre of the screen, and therefore corresponds to overfocusing.
Such spots are produced with electron-beam systems in which the beam apertures are consistently circular. For the reasons just discussed, however, such systems cannot be used.
With a view to avoiding the formation of an aureole, there are known electron-beam systems (cfr. the paper entitled "A High Performance Color CRT Gun With an Asymmetrical Beam Forming Region" by H. Y. Chen and R. H. Hughes, RCA publication ST-5105, presented at the IEEE Chicago Spring Conference on Consumer Electronics, 1980, Chicago, Ill.) in which the apertures of the second grid facing the first grid are rectangular and have a length-to-breadth ratio of about 3:1. Given such apertures in the second grid, one obtains a smaller focusing voltage in the vertical direction than in the horizontal direction and therefore a flatter beam pattern in the area of the upper and lower edge of the screen, this being particularly true as regards the edge rays of the electron beam. In this way one obtains a circular spot in the case of vertical deflection and a spot in the form of an ellipse lying on its side--though with a relatively large axial ratio--in the case of horizontal deflection. The form of the spot at the centre of the screen, however, will in this case be distorted into a standing ellipse with an axial ratio of, say, 1:1.4. Subject to accepting a worsening of the resolution at the centre of the screen, one can thus obtain an improvement in the edge areas.
An electron beam formation system is also known (EP-A 111 872) in which the apertures in the first grid are either of an elongated or a rectangular shape. In this known electron-beam formation system, however, the apertures in the second grid facing the first grid are either circular or elongated in shape, while on the side facing away from the first grid there is provided a rectangular aperture that is common to all three electron beams. This measure is intended to improve the resolution of a colour picture tube of the type described at the beginning hereof by modifying the form of the spot into a horizontal ellipse and preventing the formation of aureoles, above all in the edge areas of the screen. In the known solution this is obtained by means of two asymmetrical lens systems, arranged in sequence and consisting of the grids 1/2 and 2/3, and a symmetrical lens system consisting of the grids 3/4.
An in-line system intended to avoid the formation of vertical aureoles when the spot is deflected horizontally is known from FR-A- 2 437 062. To this end a plate is arranged on the side of the second grid that faces the first grid. This plate is provided with an elongated slit extending in the plane of the electron beam for each of the beams. The apertures in the second grid permitting the passage of the electron beams also terminate in these slits. The effect of these slits arranged in the plane of the beams and their shape is that, as compared with the focusing in the beam plane itself, the electron beams are underfocused in a plane at right angles thereto. This ensures that the spot will be deformed into a standing (vertical) ellipse at the scree

REFERENCES:
patent: 4251747 (1981-02-01), Burdick
patent: 4629933 (1986-12-01), Bijma et al.

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