Electric lamp and discharge devices – Cathode ray tube – Plural beam generating or control
Reexamination Certificate
1995-08-22
2001-07-10
Day, Michael H. (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Plural beam generating or control
C313S456000
Reexamination Certificate
active
06259195
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a cathode ray tube comprising an electron gun having a beam-shaping portion having at least one electron source for generating an electron beam, a prefocusing system of electrodes across which a prefocusing voltage is supplied during operation so as to form an electron-optical prefocusing lens, the electron gun further comprising a main lens system of electrodes whose outer electrodes are provided with means for supplying, at least during operation, a main focusing voltage between said electrodes so as to form an electron-optical main focusing lens, said electrodes being provided with at least one aperture for passing the electron beam and with securing means with which the electrodes are secured to an insulating supporting body.
The invention also relates to an electrostatic lens system comprising a prefocusing system of electrodes across which a prefocusing voltage is supplied during operation so as to form an electron optical prefocusing lens, a main lens system of electrodes whose outer electrodes are provided with means for supplying, at least during operation, a main focusing lens, said electrodes being provided with at least one aperture for passing an electron beam and with securing means with which the electrodes are secured to an insulating supporting body.
A device of this type is known from U.S. Pat. No. 4,168,452. The electron gun used in this device is of the tripotential type, in which the outer electrodes of the main lens system are provided with means in the form of electric connections for supplying therebetween, at least during operation, a comparatively large main focusing voltage of the order of 15-20 kV. As in other types of guns, in which a comparatively large main focusing voltage is supplied between the outer electrodes of the main lens system, as in, for example most conventional bipotential guns, this generally has the consequence that during operation only a moderate potential is present on the first electrode of the main lens system, in contrast to unipotential guns in which the outer electrodes of the main lens system both convey the same high potential. Since the first electrode of the main lens system generally also constitutes the last electrode of the prefocusing system, this leads to a moderate prefocusing action of the gun and hence to a poor. beam definition in a device of the type described in the opening paragraph.
To avoid the latter phenomenon, an additional electrode is used in the prefocusing system in the electron gun of the known device, which electrode conveys a higher potential during operation than the first electrode of the main lens system. Thus, a higher field strength in the prefocusing system is realised so that a better prefocusing is obtained, which leads to a sharper electron beam.
A drawback of the known device is, however, that an extra electrode is required for the realised improvement of the beam definition, which does not only have a cost-increasing effect but also requires more space and, under circumstances, an extra electrical connection in the gun.
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 the prefocusing is improved without the addition of an extra electrode.
According to the invention, a device of the type described in the opening paragraph is therefore characterized in that the prefocusing system of electrodes comprises a pair of adjacent electrodes, at least one of which protrudes towards the other in such a way that the distance between the two electrodes at the location of the apertures therein is smaller than at the location of the respective securing means thereof, and in that the electrodes at the location of the apertures provided therein are spaced apart substantially at a distance which is limited by electrostatic breakdown. Within the scope of the invention, a distance which is still just justified with a view to electrostatic breakdown and within the prevailing positioning and other process tolerances is maintained between the two electrodes at the location of the apertures.
The invention is based on the recognition that the minimum mutual distance between adjacent electrodes is mainly determined by the mutual distance between the securing means in the insulating supporting body. Generally, the electrostatic breakdown voltage along the insulating material of the supporting body is considerably lower than the breakdown voltage in the prevailing vacuum. For securing means which are in alignment with the electrodes, the maximum field strength between the electrodes is therefore limited to said breakdown voltage along the insulating support. By placing the securing means of the relevant pair further apart in accordance with the invention, rather than in alignment with the electrodes, it is achieved that both electrodes can be placed closer together and that their minimum mutual distance at the location of the apertures therein is only limited by the electrostatic breakdown voltage in the prevailing vacuum. In practice this means that the distance between the two electrodes at the location of the aperture therein approaches the theoretical minimum value of the quotient of the prefocusing voltage and the maximum electric field strength in the vacuum prevailing in the envelope, while allowing for inevitable positioning tolerances of the electrodes and ambient fluctuations in the device. The field strength to which an electron beam between the two electrodes is subject and hence the prefocusing may consequently be increased without an extra electrode having an extra (high) voltage being required for this purpose. Thus, in practice a considerable improvement of the beam homogeneity and hence the spot quality can thus be achieved.
In most conventional cases, the maximum electric field strength along the supporting body is approximately a factor of three lower than that in the prevailing vacuum. A particular embodiment of the device according to the invention is therefore characterized in that the distance between the two electrodes at the location of the apertures therein is approximately a factor of the smaller than at the location of the respective securing means thereof.
Generally, such a vacuum prevails in the evacuated envelope of a device of the type described in the opening paragraph that a maximum field strength of between 7 and 12 kV/mm can be achieved, while the maximum field strength along the insulating supporting body is generally limited to approximately 3 kV/mm. A further embodiment of the device according to the invention is therefore characteried in that the distance between the two electrodes is between one-seventh and one-twelfth of the prefocusing voltage, while the distance between the securing means is larger than one-third of the prefocusing voltage, with said distances being expressed in millimeters and said prefocusing voltage being expressed in kilovolts. In that case the electron gun can be used without any problems, in substantially any conventional device of the type described in the opening paragraph.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
REFERENCES:
patent: 2907916 (1959-10-01), Beam et al.
patent: 2935636 (1960-05-01), Knechtli
patent: 2975315 (1961-03-01), Szegho
patent: 3772554 (1973-11-01), Hughes
patent: 4168452 (1979-09-01), Christensen et al.
patent: 4731563 (1988-03-01), Bloom et al.
patent: 5300855 (1994-04-01), Kweon
Solid Insulators in Vacuum: A Review by R. Hawley Vacuum, vol. 18 No. 7 Pergamon Press 1968, pp. 383-390 (no month).
Montie Edwin A.
Van Der Wilk Ronald
Van Engelshoven Jeroen
Day Michael H.
U.S. Philips Corporation
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