Colour display tube comprising an inline electron gun

Abstract: A colour display tube comprising an in-line electron gun which has a first and a second lens electrode which produce an electron-optical lens field for focusing the electron beams on a display screen of the cathode ray tube, one of said lens electrodes having a correction element (29) for correcting the astigmatism of the lens field, the correction element having three apertures (30,31,32), the outer apertures of which are formed such that, in operation, an electric field having a six-pole component is generated for the outer electron beams in the correction element. As a result thereof, the outer electron beams produce an improved image on the display screen. The six-pole fields generated in the correction element correct the negative effect of six-pole fields occurring in the lens field. In an embodiment, the outer apertures (30,32) are in the shape of a trapezium.

Description: The invention relates to a colour display tube according to the preamble of Claim 1, comprising an in-line electron gun generating three electron beams in an evacuated envelope, which electron gun comprises a first and a second lens electrode, each with three apertures for passing the electron beams, said apertures facing each other, which first and second lens electrodes, in operation, produce an electron-optical lens field between the first and second lens electrodes for focusing the electron beams on a display screen, one of the first and second lens electrodes having a correction element for correcting the astigmatism of the lens field, the correction element comprising three collinear apertures. Such a colour display tube is known from European Patent Specification EP-O 104 674 B1. The lens field which is formed by the lens electrodes and which, in turn, forms an electron optical lens may be astigmatic owing to manufacturing errors or other causes. As a result thereof, the electron beams are astigmatically focused on the display screen, i.e. when the electron beams are focused, for example, in the horizontal direction, they are vertically out of focus. Due to this, the spots of the electron beams on the display screen are not optimal. Thus, it is impossible to sharply focus the electron beams on the display screen. To correct this adverse effect, one of the lens electrodes comprises a correction element. Said correction element has a plate-shaped portion in which oval or rectangular apertures are formed for passing the electron beams. It has been found that for certain applications this correction is insufficient. In particular in the corners of the display screen, an improvement of the dimensions of the spots, i.e. a sharper focusing, is desired. It is an object of the invention to provide inter alia, a colour display tube of the type mentioned in the opening paragraph, in which tube the above problem is at least partly overcome. For this purpose, the colour display tube according to the invention is characterized in that the outer apertures of the correction element are formed such that in operation an electric field having a six-pole component is generated for the outer electron beams for compensating, at least partly, the disturbing effects, exerted on the outer electron beams, of six-pole components in the lens field produced by the first and second lens electrodes. The invention is based, inter alia, on the insight that six-pole fields having a disturbing effect on the outer electron beams are generated in the lens field of the electron-optical lens. Said six-pole fields negatively influence the focusing of the outer electron beams in directions between the vertical and the horizontal directions. The disturbing effect of the six-pole fields in the focusing lens can at least partly be compensated by generating compensating six-pole fields for the outer electron beams in the correction element. An embodiment of the colour display tube according to the invention is characterized in that the outer apertures of the correction element have a trapezoidal shape. By virtue of the trapezoidal shape of the outer apertures in the correction element, six-pole fields can be generated in the correction element in a simple manner, said six-pole fields at least partly compensating the disturbing effect of the six-pole components of the lens field. Within the framework of the invention, trapezoidal is to be understood to mean a shape which is mirror-symmetrical relative to the in-line plane, and a shape such that, when the trapezoidal aperture is divided in four imaginary parts which are bounded on the one hand by the in-line plane and on the other hand by a line extending transversely to the in-line plane through the point of the in-line plane which is equidistant from both intersections of the in-line plane with the boundary lines of the aperture, the centroids of the four parts thus formed of the aperture form a trapezium. The strength of the six-pole field is determined by the difference between the long side and the short side of the trapezium. The invention will be explained in greater detail by means of several exemplary embodiments and with reference to the accompanying drawing, in which Fig. 1 is a sectional view of a colour display tube according to the invention. Fig. 2 is a sectional view of an electron gun suitable for a colour display tube according to the invention. Fig. 3 is an elevational view of a correction element for electron gun shown in Fig. 2. Fig. 4a up to and including 4d show several embodiments of correction elements. Fig. 5 shows the beam diameter as a function of the intensity I of the beam. Fig. 6 shows the shape of the spots when they are focused so that they have the smallest possible diameter. The Figures are not drawn to scale, corresponding parts generally bearing the same reference numerals. Fig. 1 is a sectional view of a colour display tube according to the invention. Colour display tube 1 comprises an evacuated envelope 2 which consists of a display window 3, a conical portion 4 and a neck 5. In the neck 5 there is provided an electron gun 6 for generating three electron beams 7, 8 and 9 which are located in one plane, the in-line plane, in this case the plane of the drawing. A display screen 10 is provided on the inside of the display window. Said display screen 10 comprises a large number of phosphor elements luminescing in red, green and blue. Said phosphor elements may be in the form of, for example, dots or strips. On their way to the display screen 10, the electron beams 7, 8 and 9 are deflected across the display screen 10 by means of a deflection unit 11 and pass through a colour selection electrode 12 which is arranged in front of the display window 3 and which comprises a thin metal plate with apertures. The three electron beams 7, 8 and 9 pass through the apertures 13 at a small angle with each other and, consequently, impinge each on phosphor elements of only one colour. The colour selection electrode is suspended by means of suspension means 14. Fig. 2 is a sectional view of an electron gun which is suitable for a colour display tube according to the invention. The electron gun 21 comprises three cathodes 22, 23 and 24 for emitting three electron beams 7, 8 and 9. The electron gun further comprises a first, joint electrode 25 for the three electron beams, a second joint electrode 26, a third joint electrode 27 and a fourth joint electrode 28. In operation, the electrodes 27 and 28 form an electron-optical field. Said electron-optical field focuses the electron beams on the display screen of the colour display tube. Electrode 28 comprises a correction element 29 having apertures 30, 31 and 32. Fig. 3 is a top view of a correction element for an electron gun as shown in Fig. 2. The apertures 30 and 32 for the outer electron beams are formed such that, in operation, an electric field having a six-pole component is generated for the outer electron beams. In this example, the apertures have a trapezoidal shape. Figs. 4a up to and including 4d show further examples of correction elements for an electron gun which is suitable for a colour display tube according to the invention, the outer apertures of the correction element being trapezoidally shaped. Within the framework of the invention, trapezoidal is to be understood to mean a shape, as indicated in aperture 32 in electrode 29 of Fig. 4a, such that the centroids 44, 45, 46 and 47 of the parts I, II, III and IV form a trapezium. The parts I, II, III and IV are bounded by two imaginary planes, the in-line plane 42 and plane 43 which extends transversely to the in-line plane and passes through point A. Fig. 5 shows the beam diameter (vertical axis) as a function of the intensity I of the beam (horizontal axis). Line 51 represents the beam diameter for an electron gun having a correction element according to the state of the art. Line 52 represents the beam diameter when a correction element is used which can be suitable used in an electron gun for a colour display tube according to the invention. Fig. 6 shows the shape of the spots when they are focused to the smallest possible diameter. Line 61 represents the shape of the outer electron beams according to the state of the art. The maximum beam circumference is represented by the dotted line 62. By use of trapezoidal apertures in the apertures for the outer beams, the spots of the outer beams are reduced in this example to approximately circular spots as represented by line 63. The maximum beam diameter is reduced. It is noted that Fig. 6 illustrates the effect of trapezoidal apertures relative to a substantially circular spot. In fact, the shape of the spot varies somewhat across the screen, the shape of the apertures in the correction element being selected such that the best possible compromise is found for the size and the shape of the spots in the centre and in the corners. It will be obvious that within the scope of the invention as claimed many variations are possible to those skilled in the art.

Claims: A colour display tube (1) comprising an in-line electron gun (6) generating three electron beams (7, 8, 9) in an evacuated envelope (2), which electron gun comprises a first (27) and a second (28) lens electrode, each of said lens electrodes comprising a cup-shaped part the bottom of which has three apertures for passing the electron beams, said apertures facing each other, which first and second lens electrodes, in operation, produce an electron-optical lens field between the first and second lens electrodes for focusing the electron beams on a display screen (1), one of the first and second lens electrodes having a correction element (29) provided separate from said apertures in the respective cup-shaped part for correcting the astigmatism of the lens field, the correction element comprising three collinear apertures (30, 31, 32), characterized in that the outer apertures (30, 32) of the correction element (29) are formed such that in operation an electric field having a six-pole component is generated for the outer electron beams (7, 9) for compensating, at least partly, the disturbing effects, exerted on the outer electron beams, of six-pole components in the lens field produced by the first and second lens electrodes. A colour display tube as claimed in Claim 1, characterized in that the outer apertures of the above-mentioned apertures have a trapezoidal shape.

Inventors: HIMMELBAUER ERICH EDUARD, HIMMELBAUER, ERICH EDUARD
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1991
Main classification: H01J29
IPC codes: H01J29