CATHODE RAY TUBE INCLUDING MEANS FOR VERTICALLY EXTENDING THE SPOT

Abstract: Cathode ray tube, particularly a (blue luminescing) pro­ jection television display tube, comprising in an evacuated envelope (1) an electron gun (5) for generating an electron beam (6) which is focused to a spot (8) on a display screen (7) by means of an electrostatic focusing lens (27, 30) and which is deflected across this display screen (7) in two mu­ tually perpendicular directions (x, y). A structure (37) mag­ netized as a four-pole and consisting of a magnetic half-­ hard material is provided at the area of the focusing lens (27, 30) and coaxially around the gun axis (20), the axes of the four-pole being located centrally between the said de­ flection directions. The structure (37) extends the spot in the vertical direction so that phosphor saturation, particularly in blue projection tubes is reduced.

Description: The invention relates to a monochromatic cathode tube comprising an evacuated envelope containing a display screen and an electron gun for producing an electron beam for exciting a spot on the display screen, which electron gun comprises an electrostatic focusing lens, the electron beam being deflected during operation across said display screen in two mutally perpendicular directions to effect production on the screen of mutually-adjacent lines. A cathode ray tube of this type may be a projection television display tube or a different type of display tube in which only one electron beam is generated. Phosphor saturation is a problem in projection tubes, particularly for the zinc sulphide phosphor used for blue. It is therefore desirable to provide a cathode ray tube with means to reduce the phosphor saturation. This object is achieved by a monochromatic cathode ray tube according to claim 1. It is known that the human eye is less critical to the definition of the blue partial image than that for that of the green and red partial images because blue has a very low luminance contribution and because the eye has less resolving power for blue. The invention utilizes this fact by enlarging the spot in a (blue luminescing) projection tube, thus reducing the current density and hence the phosphor saturation. Particularly, the dimension of the (blue) spot in the vertical direction (the direction transverse to the line scan direction) is enlarged. The conventional blue phosphor has a very short persistence period (some tens of microseconds). A vertical spot enlargement then results in the phosphor being excited with a reduced current density for a number of times with intervals of 64 µs by overlapping of picture lines. As this interval is of the same order or is longer than the persistence period, less phosphor saturation will occur. On the other hand, a horizontal spot enlargement would only lead to a distribution of the current load over several hundred nanoseconds (much shorter than the persistence period) and thus would not contribute to a reduction of the saturation, but uselessly sacrifice definition. It is therefore favourable to give the (blue) spot a vertically extended shape, which can be achieved by causing an electric or magnetic quadripolar field to act on the beam. In the older application EP-A-197573 there is already proposed a monochromatic cathode ray tube according to the preamble of claim 1 provided with means to obtain an electron beam spot having very determined dimensions. Preferably, the spot on the display screen should be circular. From US-B-381074 an index tube is known in which the dimension of the spot in the direction of the phosphor strips exceeds the dimension at right angles to the phosphor strips in order to obtain a picture of adequate brightness and to avoid saturation of the phosphors by an excessive current density. The invention can be realised in various manners. A simple and elegant embodiment is possible if a permanent magnetic structure on the gun is used in a (blue luminescing) projection tube. In this structure (which preferably has the shape of a ring) a fixed four-pole component can be induced to achieve the above-described object. The advantage is that no extra components or deviating constructions are required and that otherwise the gun designs for red, green and blue luminescing tubes may be identical. A preferred embodiment of the cathode ray tube according to the invention is therefore characterized in that the means for extending the electron beam comprise a permanent magnetized structure of a magnetic half-hard material in which a four-pole is induced whose axes are located centrally between the line scan direction and the field scan direction. A further preferred embodiment of a display tube according to the invention is characterized in that the focusing lens, viewed in the direction of propagation of the electron beam, consists of a first and a second cylindrical focusing lens electrode, which first electrode extends coaxially in the second electrode and in that the permanent magnetized structure is secured to the edge of the second electrode remote from the first electrode. By securing the structure after the lens gap of this accelerating focusing lens, the desired distortion of the electron beam can be effected in a very effective manner. In fact, the distorted beam does not subsequently pass any longer through an electron lens where it could be distorted again. In addition, it is easy in practice to secure, for example, a ring to the edge of a cylindrical electrode. The structure or ring may of course also be provided in the second focusing lens electrode at the area of the edge of the first focusing lens electrode. It is also possible to provide the structure or ring just in front of the focusing lens, for example, at the end of the second focusing lens electrode facing the cathode. However, the distorted beam then still passes through the focusing lens. The invention will now be described in greater detail by way of example with reference to the drawing in which: Fig. 1 is a perspective elevational view, partly broken up, of a display tube according to the invention, and Fig. 2 is a longitudinal sectional view of an electron gun for a display tube according to Figure 1. Fig. 1 is a perspective elevational view, partly broken up, of a display tube according to the invention. This tube comprises a glass envelope 1 consisting of a display window 2, a cone 3 and a neck 4, which neck accommodates an electron gun 5 for generating an electron beam 6. This electron beam 6 is focused to a spot 8 on a display screen 7. The display screen 7 is provided on the inner side of the display window 2. The electron beam is deflected across the display screen 7 in two mutually perpendicular directions x, y by using the deflection coil system 9. The tube has a base 11 with connection pins 12. Fig. 2 is a longitudinal section of an electron gun 5 according to Figure 1. This electron gun comprises a cathode 21 centred along an axis 20, which cathode has an emitting surface 22, a control electrode 24 having an aperture 23, a first anode 25 having an aperture 255, a prefocusing electrode 26 having an aperture 266, a first cylindrical focusing lens electrode 27 having a bottom 28 with an aperture 29 and a second cylindrical focusing lens electrode 30. The electrodes 24, 25, 26, 27 and 30 are sealed in glass rods 33 by means of brackets 31 and pins 32. The entire electron gun assembly is secured in a glass bottom plate 35 by means of the pins 34, which plate is provided with an exhaust tube 36 and connection pins 12. The connection wires between the various gun electrodes and the connection pins have been omitted so as not to make the drawing needlessly complicated. A ring 37 of a magnetic half-hard material as described in German Patent 2,612,607 is provided at the edge of the second focusing electrode 30. This material consists of, for example, an alloy of Fe, Co, V and Cr which alloy is known under the tradename of Koërflex (a trademark of Messrs. Krupp). No welding operation may be performed on this ring, because otherwise the magnetic properties change. Therefore the ring is secured with a number of clamps not shown. The gun assembly shown in Fig. 2 is inserted into the neck 4 of the tube (see Fig. 1), positioned and then sealed to the glass plate 35. Subsequently a four-pole is externally induced in the ring 37 after the tube is finished, with the axes of the four-pole being located centrally between the line scan direction (x) and the field scan direction (y) (see Fig. 3). A four-pole is provided in the ring 37, for example, in a manner and by means of a magnetising device as described in U.S. Patent 4,220,897. It stands to reason that magnetisable structure is not limited to a ring and may alternatively have a different shape. Thus it is possible to place a number of magnetisable elements in a ring of a non-magnetic material and subsequently mount them to the gun. The focusing lens may alternatively be a unipotential lens or a "multistage" lens.

Claims: A monochromatic cathode ray tube comprising an evacuated envelope (1) containing a display screen (7) and an electron gun (5) for producing an electron beam (6) for exciting a spot (8) on the display screen (7), which electron gun (5) comprises an electrostatic focusing lens, the electron beam (6) being deflected during operation across said display screen (7) in two mutually perpendicular directions (x, y) to effect production on the screen (7) of mutually-adjacent lines, characterized in that the cathode ray tube comprises means for widening the spot (8), in a direction transverse to the lines, sufficiently to overlap at least an adjacent line. A cathode ray tube as claimed in Claim 1, characterized in that the means for widening the electron beam comprise a permanently magnetized structure (37) of a magnetic half-hard material for producing a four-pole field whose axes are located centrally between the two deflection directions. A cathode ray tube as claimed in Claim 2, characterized in that the focusing lens, viewed in the direction of propagation of the electron beam, consists of a first (27) and a second (30) cylindrical focusing lens electrode, which first electrode (27) extends coaxially in the second electrode (30), and in that the permanently magnetized structure (37) is secured to an end of the second electrode (30) which is remote from the first electrode (27). A cathode ray tube as claimed in Claim 2 or 3, characterized in that the permanently magnetized structure has an annular shape.

Inventors: GERRITSEN, JAN
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1987
Main classification: H01J29
IPC codes: H01J29