Picture display tube

Abstract: Picture display device comprising a display tube with an electron gun having a tubular structure with an outer surface and an inner surface on which a helical resistance structure of a material having a high electrical resistance constituting a focusing lens is provided. The outer surface is provided with an electrode of readily electrically conducting material arranged opposite at least a part of the focusing lens for capacitively controlling the focusing lens by means of a dynamically variable voltage the amplitude of which is a funciion of the position of the electron beam spot on the display screen.

Description: The invention relates to a picture display device comprising a picture display tube having a display screen and an electron gun facing said screen and having a cathode centred along an electron-optical axis and a plurality of electrodes which jointly constitute a beam-shaping part for generating an electron beam directed at said screen, said gun further comprising a tubular structure of insulating material, having an outer surface and an inner surface on which inner surface a helical resistance structure of a material having a high electrical resistance constituting a focusing lens for focusing said beam is provided, said tubular structure having a coaxial input portion and a coaxial output portion. A focusing lens constituted by a helical high-ohmic resistance structure for use in display tubes in order to obtain a low spherical aberration is known from literature (cf. EP-A-0 275 611). However, problems occur if a dynamic focusing signal is to be applied to this type of focusing lens. Correction by means of applying a dynamic focusing voltage may be necessary, for example, in the case of large deflection angles so as to keep the electron beam in focus throughout the screen (a different focusing voltage is required in the corners than in the centre of the screen). The high resistance of the helical lens structure causes particular problems with regard to the dynamic focus if the frequency of the focusing signal exceeds 16 kHz. This is a result of the large intrinsic RC time of the resistance layer, even at those areas where the layer does not constitute a helical structure, but is homogeneous. It is a.o. an object of the invention to provide a picture display tube comprising an electron gun and a focusing lens of the type described above, which is suitable for using dynamic focus. A picture display device according to the invention is therefore characterized in that the outer surface is provided with an electrode of an electrically readily conducting material arranged opposite to at least a part of the focusing lens, said electrode, said part of the focusing lens and the insulating material which is disposed between said electrode and said part of the focusing lens forming a capacitor, for capacitively controlling the focusing lens through said capacitor by means of a signal applied to said electrode and varying in time, the intensity of said signal being a function of the position of the electron beam spot on the display screen, and in that the device comprises voltage supplying means for applying said signal. An essential aspect of the invention is that the dynamic correction signal is capacitively coupled in with a metal electrode on the outer side of the envelope in which the helical focusing lens structure is present. This electrode may comprise, for example a (preferably closed) coaxial metal cylinder in a sheet or foil shape or in the shape of a deposited layer. It appears that dynamic focusing signals up to frequencies in the MHz range can be used in this manner. Some embodiments of a picture display tube according to the invention will now be described in greater detail with reference to the accompanying drawings in which Fig. 1 is a diagrammatic cross-section of a picture display tube according to the invention; Fig. 2 is an elevational view of a longitudinal section of an electron gun suitable for use in the tube of Fig. 1; Fig. 3 shows a focusing field which can be generated by means of an electron gun of the type shown in Fig. 2; Fig. 4 shows an alternative embodiment of a picture display tube according to the invention in a cross-section; Fig. 5 shows an electrical analogon of the dynamic control of the electron gun of Fig. 2; Fig. 6 shows a part of a focusing lens structure suitable for a display tube according to the invention; and Fig. 7 shows an alternative for the structure of Fig. 6. The device shown in Fig. 1 comprises a cathode ray tube having, inter alia, a glass envelope 1 which is composed of a display window 2, a conical portion 3 and a neck 4. This neck accommodates a plurality of electrode structures 8, 9, which together with a cathode 7 constitute an electron gun. The electron-optical axis 6 of the electron gun is also the axis of the envelope. An electron beam 12 is successively formed and accelerated by the cathode 7 and the electrode structures 8, 9. The reference numeral 10 denotes a tubular structure whose inner side bears a helical structure of a material having a very high electrical resistance which constitutes a focusing lens 11 focusing the beam on a display screen 14 on the inner side of the display window 2. Conventionally applied voltages are, for example cathode 7 50 V electrode 8 0 V electrode 9 500 V entrance side of focusing lens 11 7 kV exit side of focusing lens 11 30 kV Generally, the potential at the exit side of the focusing lens is a factor of 2 to 10 higher than the potential at the entrance side. The electron beam 12 is deflected from the axis 6 across the display screen 14 by means of a system 5 of deflection coils. Display screen 14 comprises a phosphor layer which is coated with a thin aluminium film which is electrically connected to the end of electrode 10 via a conducting coating on the inner wall of the conical portion 3. Fig. 3 diagrammatically shows an example of a focusing lens field which can be generated by the focusing lens 11. The curved lines represent the lines of intersection of the equipotential planes which are produced by applying a voltage difference across the ends of the helical resistance track, in the plane of the drawing. Each equipotential plane represents a plane having an equal refractive index. The centre of the lens is the point A. This is the point at which the second derivative of the potential variation as a function of the location on the axis is zero. The focal lengths f₁ and f₂ are the distances between the focus F₁ and the first main surface H₁ and the distance between the focus F₂ and the second main surface H₂, respectively. The foci F₁ and F₂ are located at distances F′₁ and F′₂ respectively, from the centre A. The focusing lens has a diverging input portion and a converging output portion. The focusing lens constituted by the electrode structure 11 is partly located within the deflection coil system 5. Since the focusing lens is less far remote from the display screen than in tubes in which the focusing lens is located in front of the deflection coils, the angular aperture of the beam on the display screen is larger if the electron beam diameter in the focusing lens remains the same, thus with equal aberrations and a given cathode load, so that a smaller electron spot is realised on the display screen. This results in a better resolving power. In Fig. 1 the focusing lens 11 is partly located in the field of the deflection coils because this is favourable for the resolving power of the picture display tube 1. However, the invention is not limited to such a mutual positioning. The invention may be used to advantage in all picture display devices comprising cathode ray tubes with one electron beam and using magnetic deflection, such as monochrome television display tubes, certain colour television display tubes (chromatons and penetrons) but particularly in projection television display devices. Fig. 2 shows an electron gun of a type suitable for use in the display tube of Fig. 1. The type in question comprises a tubular (glass) envelope 15. A high-ohmic resistance layer 16 is provided on the inner side of the envelope 15, in which layer a helical structure is formed near one end, constituting a focusing lens field 17 when a suitable electric voltage is applied to the ends. The high-ohmic resistance layer 16 may be, for example glass enamel with a small amount (for example, several % by weight) of metal oxide (particularly ruthenium oxide) particles. The layer 16 may have a thickness of between 1 and 10 µm, for example 3 µm. The resistance per square of such a layer depends on the concentration of metal oxide and the firing treatment to which the layer is subjected. Resistance per square varying between 10⁴ and 10⁸ Q have been realised in practice. A desired resistance per square can be realised by adjusting the relevant parameters. A resistance per square of the order of 10⁶ Q is very suitable for the present application. The total resistance of the helical structure formed in the layer 16 (which structure may be a continuous helix or a plurality of separate helixes connected by segments without a helical structure - 5 in the embodiment of Fig. 2 -) may be of the order of 10 GQ, which means that a current of several micro-amperes will flow across the ends at a voltage difference of 30 kV. The electron gun of Fig. 2 comprises a beam-shaping part 18 in front of the focusing lens 17, which part generally comprises a cathode 19, a grid electrode 20 and an anode 21. The components of the beam-shaping part 18 may be mounted in the tubular envelope 15 of the focusing lens 17, as in the gun shown in Fig. 2. Alternatively, they may be mounted outside the tubular envelope of the focusing lens in the display tube, for example, by securing them to axial glass-ceramic mounting rods. The tubular envelope 15 may advantageously be constituted by the neck of the display tube. Such a display tube 22 is shown diagrammatically in Fig. 4. In this case a high-ohmic resistance layer with a helical structure 23 constituting the focusing lens is provided on a part of the inner side of the envelope 24 of the display tube 22. It may be necessary to correct occurring picture errors (particularly the curvature of field) by means of dynamic focusing. The power of the electron lens for focusing the electron beam is adjusted as a function of the deflection to which the electron beam is subjected at that moment. This makes it possible to have the then prevailing main surface of the image intersect the display screen at that area where the electron beam impinges upon the display screen. This correction method necessitates an extra circuit in the control device for generating the correct dynamic focusing voltages on the electrodes of the focusing lens. Since the material of the helical resistance track has such a high electrical resistance (for example, 10 GQ), the RC time is large (for example, 10 msec.). As a result, the effect of the dynamic focusing voltage hardly penetrate the helical resistance structure. The invention provides a solution in the form of a capacitive electrode 25 (Fig. 1); 26 (Fig. 2) and 21 (Fig. 4) which is separated by an isolator and surrounds the high-ohmic focusing lens structure. The principle of this solution will be elucidated with reference to Fig. 2. The high-ohmic resistance layer 16 on the inner surface of the tubular structure 15 has parts in which a helical pattern is provided and parts without such a pattern so that an optimally static focusing field, particularly with respect to minimum spherical aberration, is obtained when applying a voltage. The dynamic focusing signal is applied to a (tubular) electrode 26 of a satisfactory electrically conducting material. In the embodiment shown this electrode 26 is electrically connected at the point 28 to a supply lead 27 through which the static focusing signal is supplied. A DC voltage applied to point 28 acts on the focusing lens as a normal static focusing voltage. However, the focusing lens behaves in a different manner if the focusing voltage is modulated in time. The part of the inner wall of the tubular structure 15 facing the electrode 26 will tend to follow the potential changes of the electrode 26. The inner wall and the electrode 26 may be considered to be a capacitor, one terminal of which is connected to the focusing signal supply lead and the other terminal of which is connected to the exterior via the helical resistors (R₁ and R₂) at the ends of the tubular structure. Together with these resistors, the capacitor constitutes an RC network. Variations of the focusing voltages V dyn which are (much) faster than the corresponding RC time cannot be attenuated and will be coupled in via the capacitor. The electrical analogon is shown diagrammatically in Fig. 5. In this Figure the capacitor plate C coupled to the supply voltage V dyn represents the electrode 26 shown in Fig. 2, the resistors R₁ and R₂ represent the helical parts of the resistance layer at the side of the beam shaping part (17a) and at the side of the screen (17b). A practical example will be described with reference to Fig. 6. In a specific case a metal electrode 29 having a length of 45 mm and a diameter of 11 mm is present on the outer surface of a glass tube 30 having a wall thickness of 0.6 mm. A high-ohmic resistance layer 31 with a helical structure 32 constituting a pre-focusing lens and a helical structure 33 constituting a main focusing lens are provided on the inner surface. The capacitive electrode 29 bridges the space between the structures 32 and 33 and at least a part of the structure 33. The capacitance of the capacitor constituted by the inner wall and the electrode 29 is approximately 45 pF in this case and the total resistance of the helical structures 32, 33 is approximately 0.5 x 1010 Ohm, with a resultant RC time of approximately 240 msec. This means that the inner wall of the tube will follow all voltage variations of the electrode 29 whose characteristic time is shorter than 100 msec. Unlike Fig. 2, the static focusing voltage V stat and the dynamic focusing voltage V dyn are separately applied in the situation shown. The construction of Fig. 1 is realised by providing a metal (for example aluminium) foil between two coaxial tubes which constitute the tubular structure 15 after softening and drawing on a mandril. The electrical contact 28 can be established by pressing the contact strip (27) (Fig. 2) of the focusing electrode supply lead against the aluminium foil during the drawing process. Alternatives for the above-described use of an aluminium foil between two coaxial tubes are, for example, vapour deposition of a layer of satisfactory electrically conducting material on the outer surface of a tubular structure 30 as is shown in Fig. 6, or providing a metal cylinder around such a tubular structure 30. If in the latter case a magnetically conducting material (for example, nickel-iron) is used as a material for the cylinder, the cylinder may also serve as a magnetic shield. The invention is not limited to rotationally symmetrical dynamic focusing. Interesting possibilities are provided if the capacitive electrode is non-rotationally symmetrical because it has certain elements such as holes, (slanting) slits and the like. These elements may be used for generating dynamic multipole fields in the static focusing lens region. In this way, for example, dynamic dipoles (for beam displacement) and dynamic quadripoles (for correcting astigmatism) can be added. The internal high-ohmic resistance structure is preferably adapted to the correction elements in the external capacitive electrode by means of a meandering or strip-shaped pattern which must ensure that the conducting power of those areas in the resistance layer where the non- rotationally symmetrical corrections are performed is minimal in the rotationally symmetrical direction. Preferably these corrections will be performed in the non-helical part 17c of the focusing device, as is shown in Fig. 2. An embodiment is shown in Fig. 7. In this case the capacitive electrode 34 has a non-rotationally symmetrical recess 35. The part of the resistance layer 36 located under this recess is formed as a meandering pattern whose longitudinal direction is parallel to the tube axis 37 of the (glass) cylinder 38. The picture display tube according to the invention may be used advantageously as a projection TV tube, but in principle it may also be used in colour display tubes. It may alternatively be used for oscilloscope tubes in which the high-frequency deflection could take place, for example by means of a signal which is capacitively coupled in.

Claims: A picture display device comprising a picture display tube having a display screen and an electron gun facing said screen and having a cathode centred along an electron-optical axis and a plurality of electrodes which jointly constitute a beam-shaping part for generating an electron beam directed at said screen, said gun further comprising a tubular structure (10) of insulating material, having an outer surface and an inner surface on which inner surface a helical resistance structure of a material having a high electrical resistance constituting a focusing lens (11) for focusing said beam is provided, said tubular structure having a coaxial input portion and a coaxial output portion, characterized in that the outer surface is provided with an electrode of an electrically readily conducting material arranged opposite to at least a part of the focusing lens, said electrode, said part of the focusing lens and the insulating material which is disposed between said electrode and said part of the focusing lens forming a capacitor, for capacitively controlling the focusing lens through said capacitor by means of a signal applied to said electrode and varying in time, the intensity of said signal being a function of the position of the electron beam spot on the display screen, and in that the device comprises voltage supplying means for applying said signal.

Inventors: VAN ALPHEN WILLEM MEIJNDERT, VRIJSSEN GERARDUS ARNOLDUS HER, VAN ALPHEN, WILLEM MEIJNDERT, VRIJSSEN, GERARDUS ARNOLDUS HERMAN MARIA
Applicants: PHILIPS NV, N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1990
Main classification: H01J29
IPC codes: H01J29