Picture display device provided with an electron gun, and...

Electric lamp and discharge devices – Cathode ray tube – Plural beam generating or control

Reexamination Certificate

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Details

C313S412000

Reexamination Certificate

active

06271625

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a picture display device comprising an evacuated envelope, a first side of which is provided with an electroluminescent display screen, an opposite side is provided with an electron gun and in which deflection means are arranged between the electron gun and the display screen with which means, at least during operation, an electron beam generated by the electron gun can be deflected, the electron gun having a portion generating at least an electron beam and being provided with a main lens system having a first electrode, a final electrode and at least one intermediate electrode across which a main lens voltage is gradually applied step-wise during operation so as to form an electron-optical main focusing lens. The invention also relates to an electron gun for use in such a device.
Such a device is known from European Patent Specification 302 657. The electron gun described in this Specification comprises two intermediate electrodes between the first electrode and the final electrode of the main lens system and, in comparison with other, more conventional electron guns, it comprises a relatively large number of electrodes For this reason such a main lens is commonly referred to as DML (Distributed Main Lens), MSFL (Multi-Stage Focus Lens) or MEL (Multi-Element Lens). The separate electrodes of the main lens system in the known device are interconnected by means of a resistive voltage divider so that the main lens voltage is gradually distributed step-wise across the electrodes during operation in order to reduce the magnitude of potential jumps in the main lens system. This leads to considerably improved lens properties as compared with more conventional guns in which the main lens voltage is entirely applied across only two electrodes. Notably spherical aberrations can be adequately suppressed to relatively large electron beam currents without an increase of the lens diameter being required.
To enhance the lens action, the intermediate electrodes of the main lens system of the electron gun of the known device are made of a relatively thick sheet material. A thickness of minimally approximately 1-2 mm appears to be desirable in this respect. A drawback of the use of such a thick sheet material is, however, that (mechanical) steps of processing this material are impeded. For example, in such a thick sheet material it is difficult to punch, for example the apparatus for passing the electron beam(s) in the electrodes and consequently considerably longer and more costly techniques such as, for example laser cutting will have to be used. Moreover, the risk of deviations and errors in the apertures and the edges of the electrodes is larger as the sheet material is thicker. These drawbacks are all the more troublesome if the electron gun is to be mass-produced.
SUMMARY OF THE INVENTION
It is, inter alia, an object of the invention to provide a device of the type described in the opening paragraph in which one or more relatively thick intermediate electrodes are used which do not have, or hardly have the above-mentioned drawbacks.
According to the invention, a device of this type described in the opening paragraph is therefore characterized in that the at least one intermediate electrode of the main lens system comprises a plurality of separate plate-shaped parts whose main faces are positioned against each other and are interconnected. The separate plate-shaped parts of the intermediate electrode(s) of the main lens system may thus remain sufficiently thin so that there will be no problems in mass production. For example, the forces exerted, for example when punching the parts for providing, for example apertures for passing electron beams can be maintained sufficiently low so as to limit irregularities and deviations to a minimum. By composing a sufficient number of such parts in conformity with the invention to an intermediate electrode, the thickness of the electrode can be arbitrarily increased so as to obtain a satisfactory lens action.
In this respect parts having a thickness of between 0.3 and 1.5 mm are preferably used in accordance with a special embodiment of the device according to the invention. Such a thickness is sufficiently small to be used without any problems, even in mass production, and is sufficiently large to realise a sufficiently large total thickness of the relevant intermediate electrode, even when a relatively low number of parts is used.
As regards the specific thickness of the intermediate electrodes, a number of factors play a role. For example, the mutual distance between different electrodes is subjected to a given minimum so as to avoid electrostatic breakdown. On the other hand, a too large mutual distance increases the risk of electromagnetic disturbances of the electron beams which are less well shielded in this case. Moreover, an optimum lens filling, i.e. the quotient of the diameter of the beam and the diameter of the lens aperture and the electrode (hereinafter briefly referred to as lens diameter) appears to apply, which is approximately 70% when a colour gun is used. With a smaller lens filling, the definition of the ultimate spot of the electron beam(s) on the display screen deteriorates due to internal repellency of the electrons in the beam, whereas with larger values the beam will experience too much interaction with the metal of the electrodes.
It has surprisingly been found that for a device of the type described a given optimum number of intermediate electrodes can be found, below which number the lens quality clearly deteriorates and above which number there is no essential improvement of the lens quality, while the cost price of the gun increases due to the larger number of electrodes. In connection with the afore-mentioned peripheral conditions, this recognition has led to a special embodiment of the device according to the invention, in which the thickness of the intermediate electrodes is between 30% and 40% of the lens diameter and is more particularly approximately 35% of the lens diameter.
Although manufacturing tolerances in the intermediate electrode(s) of the main lens system in accordance with the invention are limited considerably, such tolerances will inevitably remain. To inhibit their detrimental consequences, a preferred embodiment of the device according to the invention is characterized in that the intermediate electrode comprises an odd number of plate-shaped parts, in which an even number of at least substantially uniform first parts is arranged in a mirrored relationship around a central, second part. In that case the electron-optical influence of possible manufacturing errors in a first part is at least partly compensated by an opposite influence of the mirror image thereof in the form of the uniform first part which is placed at the other side of the second part.
A special implementation of the preferred embodiment of the device according to the invention is characterized in that the central, second part is provided with securing means for securing said second part to an insulating support.
BRIEF DESCRIPTION OF THE INVENTION
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.


REFERENCES:
patent: 3295001 (1966-12-01), Burdick et al.
patent: 4833364 (1989-05-01), Izumida et al.
patent: 4935663 (1990-06-01), Shimoma et al.
patent: 0302657 (1994-03-01), None
patent: 61-8832A (1986-01-01), None

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