Cathode-Ray Tube

The invention relates to a cathode-ray tube comprising, in an evacuated envelope, an electron gun for generating an electron beam directed onto a target, the electron gun being comprised successively of a cathode centred around an axis, a first grid and a second grid, the first grid forming with the second grid a non-rotationally symmetrical electron lens, mainly a quadrupole lens.

Such a cathode-ray tube may be used for displaying television pictures or in an oscilloscope. In that case the target is a display screen having a phosphor layer or a pattern of phosphor elements luminescing in different colours. Such a tube may also be used for recording pictures. In that case the target is a photoconductive layer. In all such applications it is desirable to have a spot of certain dimensions and without a surrounding haze.

Such a cathode-ray tube is disclosed in U.S. Pat. No. 3,497,673, Hasker. The first grid of the electron gun of the tube there described is provided with an elongated aperture which, in cooperation with the second grid, forms a non-rotationally symmetrical electron lens, which asymmetry is expressed mainly in a quadrupole lens action. After deflection of the electron beam, the spot obtained with such a gun, generally has, in the corners and at the edge of the target, a considerable haze partially or entirely surrounding the spot. The haze produces a region of low but disturbing light intensity near the spot.

It is therefore an object of the invention to provide a cathode-ray tube with considerably less haze around the spot than in the known cathode-ray tube.

According to the invention, the cathode-ray tube comprises, in an evacuated envelope an electron gun for generating an electron beam the electron gun includes, in succession, a cathode centered around an axis, a first and second grid each having an aperture therein, the first grid forming, in cooperation with the second grid, a non-rotationally symmetrical electron lens. The cross-section normal to the axis of the aperture opening in the first grid at the side facing the second grid is larger than and transversely elongated relative to the cross-section of the aperture opening at the side of the first grid facing the cathode.

In known cathode-ray tubes, the cross-sections of the first grid normal to the axis are substantially the same. This is because one continuous aperture has the same elongate shape on the sides of cathode and anode. As a result of this a non-symmetrical lens is also formed in cooperation with the cathode, which asymmetry is expressed in a quadrupole lens action. The quadrupole lens acts in the same direction as the previously mentioned quadrupole lens. In the cathode-ray tube according to the invention such quadrupole asymmetry is not or substantially not present on the cathode side. That is, the shape of the aperture opening on the side of the first grid adjacent the cathode is such that the electric field in the region between the first grid and the cathode is substantially axially symmetric. The cross-section of the aperture in the first grid on the cathode side may be circular. In order to influence the shape of the beam and hence of the target, however, the aperture may also be square or slightly oval. The surface of the aperture on the cathode side, however, is always much smaller than the surface of the aperture in a cross-section on the side of the second grid.

A non-symmetrical funnel-shaped aperture in the first grid may be constructed in a large number of ways. It is possible, for example, to provide it by etching, spark erosion and the like. However, it is alternatively possible for the first grid to be made from two plate-shaped parts which are secured together and extend normally to the axis. The plate-shaped part situated on the side of the second grid has an elongated aperture opening so that the increased aperture area and elongated shape is obtained in a simple manner.

The present invention will now be described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a sectional view of a cathode-ray tube made in accordance with the invention,

FIG. 2 is a perspective view of a three-fold electron gun for a cathode-ray tube in accordance with the invention,

FIG. 3 is a longitudinal sectional view of one of the three guns shown in FIG. 2,

FIGS. 4 and 5 are cross-sectional views on the lines IV--IV and V--V respectively of FIG. 3, FIGS. 6 to 8 are views of another preferred embodiment of the first grid,

FIG. 9 is a perspective view of a further embodiment of a first grid,

FIG. 10 is a perspective view of yet another embodiment of a first grid and

FIGS. 11 and 12 are cross-sectional views of deflected electron beams, FIG. 11 being produced in a prior art cathode-ray tube and FIG. 12 in a cathode-ray tube made in accordance with the present invention.

FIG. 1 is a diagrammatic sectional view of a cathode-ray tube in accordance with the invention, in this case a colour display tube of the in-line type. In a glass envelope 1 which is composed of a display window 2, a funnel-shaped part 3 and a neck 4, three electron guns 5, 6 and 7 are provided in the neck and generate the electron beams 8, 9 and 10, respectively. In a colour display tube of the in-line type the axes of the electron guns are located in one plane, in this case the plane of the drawing. The axis of the central electron gun 6 coincides substantially with the tube axis 11. The three electron guns debouch in a sleeve 16 which is positioned coaxially in the neck 4. The display window 2 has a large number of triplets of phosphor lines on its inside surface. Each triplet comprises a line consisting of a green-luminescing phosphor, a line consisting of a blue-luminescing phosphor and a line consisting of a red-luminescing phosphor. All triplets together constitute the display screen 12. The phosphor lines extend normally to the plane of the drawing. Provided in front of the display screen is a shadow mask 13 having a very large number of elongate apertures 14 which are parallel to the phosphor lines through which the electron beams 8, 9 and 10 pass. The electron beams are deflected in the horizontal direction (in the plane of the drawing) and in the vertical direction (normal thereto) by the deflection coil system 15. The three electron guns are assembled so that the axes thereof enclose a small angle with each other. As a result of this the generated electron beams pass through the apertures 14 at that angle, the so-called colour selection angle, and each impinges only on phosphor lines of one colour. The three electron guns 5, 6 and 7 may have one or more electrodes in common, as described, for example, in U.S. Pat. No. 3,772,554. It will be obvious that the invention may also be used in such a system of electron guns.

FIG. 2 is a perspective view of the three electron guns 5, 6 and 7. The electrodes of the electron gun system are positioned with respect to each other by means of metal strips 17 which are sealed in glass assembly rods 18. Each gun comprises a first grid 21 and three other grids 22, 23 and 24.

FIG. 3 is a longitudinal sectional view of one of the guns shown in FIG. 2. A rapidly heating cathode 19 is positioned in the first grid 21. A coiled heating wire 28 is disposed in a cathode shaft or cylinder 29 which has an emissive surface consisting of a barium-strontium oxide layer opposite an aperture 34 in the first grid 21. The cathode cylinder is secured to a supporting cylinder 33 by means of three thin metal strips 30. The supporting cylinder is mounted in the first grid 21 by means of glass 31 secured in a metal ring. The supporting rods 32 are also sealed in the glass 31 so as to secure the filament in the cathode. Aperture 34 is provided in the first grid by means of an etching process.

FIG. 4 is a sectional view along line IV--IV of FIG. 3 showing the surface 36 of the first grid 21. On this side of grid 21, the aperture opening 34 is of circular cross-section.

FIG. 5 is a view along line V--V of FIG. 3 showing the surface 35 of the first grid 21. On this side the aperture has an elongate shape in that an elongate pit 37 is etched in the first grid. By using such a first grid, a spot in the corners and at the edge of the display screen is obtained which is surrounded by a negligible haze.

FIG. 6 is a cross-sectional view of another embodiment of a first grid which can be obtained in a simple and cheap manner. In this case the first grid comprises a plate-shaped part 38 having a rectangular aperture 39, as shown in FIG. 7, which is a view on the line VII--VII of FIG. 6, and a plate-shaped part 40 placed against it and having therein a small square aperture 41, as shown in FIG. 8 which is a view on the line VIII--VIII of FIG. 6. By making the shape of aperture 41 square, the shape of the spot can be influenced. As a result of the elongate aperture 39 a quadrupole lens action on the side of the second grid is obtained so that the haze around the spot in the corners and at the edge of the display screen is considerably reduced.

FIG. 9 is a perspective view of another embodiment of a first grid. In a metal part 48 a V or U-shaped groove 49 and an aperture 50 are provided. The first grid is obtained by mounting the part 48 normal to the axis of the gun in such manner that the side 51 containing the groove 49 faces the second grid.

FIG. 10 is also a perspective view of a first grid. The grid comprises a plate 52 having a circular aperture 53. On its side facing the second grid, the plate comprises two parallel metal strips 54 and 55. As a result of this the aperture in the cross-section of the first grid on the side of the second grid becomes infinitely long.

FIG. 11 shows a spot 56 of a deflected electron beam at the edge of a display screen of a known cathode-ray tube. The spot is surrounded by a haze 57, a region having a low but disturbing light intensity.

FIG. 12 shows a spot 58 of a deflected electron beam, which electron beam is generated by means of an electron gun having a first grid of the type described with reference to FIGS. 3 to 10. As indicated haze 59 is negligible and is unlikely to cause annoyance.