Electric lamp and discharge devices – Cathode ray tube – Plural beam generating or control
Reexamination Certificate
1999-12-21
2001-12-11
Day, Michael H. (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Plural beam generating or control
C313S414000
Reexamination Certificate
active
06329746
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a cathode ray tube (CRT), and particularly to a method of correcting deflection defocusing in a cathode ray tube which is capable of improving focus characteristics and thereby obtaining a sufficient resolution over the entire phosphor screen and over the entire electron beam current region; a cathode ray tube employing the method; and an image display system including the cathode ray tube.
A cathode ray tube such as a picture tube or a display tube includes at least an electron gun having a plurality of electrodes and a phosphor screen (screen having a phosphor film, which is also referred to as phosphor film or simply as “screen” hereinafter), and it also includes a deflection device for allowing an electron beam emitted from the electron gun to scan on the phosphor screen.
As for such a cathode ray tube, there have been known the following techniques for obtaining a desirable reproduced image on the entire phosphor screen from the center to the peripheral portion.
Japanese Patent Publication No. Hei 4-52586 discloses an electron gun emitting three in-line electron beams in which a pair of parallel flat electrodes are disposed on the bottom face of a shield cup in such a manner as to be positioned above and below paths of the three electron beams in parallel to the in-line direction and to extend toward a main lens.
U.S. Pat. No. 4,086,513 and its corresponding Japanese Patent Publication No. Sho 60-7345 disclose an electron gun emitting three in-line electron beams in which a pair of parallel flat electrodes are disposed above and below paths of the three electron beams in parallel to the in-line direction in such a manner as to extend from a facing end of one of a pair of main-lens-forming electrodes toward a phosphor screen, thereby shaping the electron beams before the electron beams enter a deflection magnetic field.
Japanese Patent Laid-open No. Sho 51-61766 discloses an electron gun in which an electrostatic quardrupole lens is formed between two electrodes and the strength of the electrostatic quardrupole lens is made to vary dynamically in synchronization with the deflection of an electron beam, thereby achieving uniformity of an image over the entire screen.
Japanese Patent Publication No. Sho 53-18866 discloses an electron gun in which an astigmatic lens is provided in a region between a second grid electrode and a third grid electrode forming a prefocus lens.
U.S. Pat. No. 3,952,224 and its corresponding Japanese Patent Laid-open No. Sho 51-64368 discloses an electron gun emitting three in-line electron beams in which an electron beam aperture of each of first and second grid electrodes is formed in an elliptic shape, and the degree of ellipticity of the aperture is made to differ for each beam path or the degree of ellipticity of the electron beam aperture of the center electron gun is made smaller than that of the side electron gun.
Japanese Patent Laid-open No. Sho 60-81736 discloses an electron gun emitting three in-line electron beams in which a slit recess provided in a third grid electrode on the cathode side forms a non-axially-symmetrical lens, and an electron beam is made to impinge on the phosphor screen through at least one non-axially-symmetrical lens in which the axial depth of the slit recess is larger for the center beam than for the side beam.
Japanese Patent Laid-open No. Sho 54-139372 discloses a color cathode ray tube having an electron gun emitting three in-line electron beams in which a soft magnetic material is disposed in fringe portions of the deflection magnetic field to form a pincushion-shaped magnetic field for deflecting the electron beams in the direction perpendicular to the in-line direction of each electron beam, thereby suppressing a halo caused by the deflection magnetic field in the direction perpendicular to the in-line direction.
The desirable focus characteristics of a cathode ray tube include a desirable resolution over the entire screen and over the entire electron beam current region; a characteristic without generation of moire in a small current region; and uniformity in resolution over the entire screen and over the entire electron beam current region. The design of an electron gun for simultaneously satisfying a plurality of these focus characteristics requires a high technique.
The studies by the present inventors showed that an electron gun having a combination of an astigmatic lens and a large-diameter main lens is essential to give the above focus characteristics to a cathode ray tube.
In the above-described related arts, however, a dynamic focus voltage has been required to be applied to a focus electrode of an electron gun for obtaining a desirable resolution over the entire screen using an electrodes forming astigmatic lens, that is, non-axially symmetrical lens in the electron gun.
FIG. 80
is a side view of the entire configuration of one example of an electron gun used for a cathode ray tube; and
FIG. 81
is a partial sectional view seen in the direction of an arrow of
FIG. 80
showing an essential portion of the electron gun.
The electron gun of this type has a plurality of electrodes including a cathode K, a first grid electrode (G1)
1
, a second grid electrode (G2)
2
, a third grid electrode (G3)
3
, a fourth grid electrode (G4)
4
, a fifth grid electrode (G5)
5
, a sixth grid electrode (G6)
6
, and a shield cup
100
integrally attached to the sixth grid electrode (G6)
6
. In addition, the fifth grid electrode (G5)
5
is composed of two electrodes
51
,
52
.
A focus voltage is applied between the third grid electrode
3
and the fifth electrode
5
, and an anode voltage is applied only to the sixth electrode
6
, so that an electron beam produced by a so-called triode portion composed of the cathode K, the first grid electrode
1
and the second grid electrode
2
is accelerated and focused by an electron lens formed by the third grid electrode
3
to the sixth grid electrode
6
, to project toward a phosphor screen.
Effects on an electron beam of electric fields determined by lengths of the electrodes, and diameters of electron beam apertures in the electrodes of this electron gun differ from electrode to electrode. For example, the shape of the electron beam aperture of the first grid electrode near the cathode K exerts an effect on the spot shape of an electron beam in a small-current region; however, the shape of the electron beam aperture of the second grid electrode exerts an effect on the spot shape of an electron beam in a wide current region from the small-current region to the large-current regions.
In the electron gun in which a main lens is formed between the fifth grid electrode
5
and the sixth grid electrode
6
by applying an anode voltage to the sixth grid electrode
6
, the shape of the electron beam aperture of each of the fifth grid electrode
5
and the sixth grid electrode
6
forming the main lens exerts a large effect on the shape of the electron beam in a large-current region but exerts a smaller effect on the shape of the electron beam in a small-current region than in the large-current region.
The axial length of the fourth grid electrode
4
of the electron gun exerts an effect on the magnitude of the optimum focus voltage and also exerts a large effect on a difference in the optimum focus voltage between a small-current region and a large-current region. The effect of the axial length of the fifth grid electrode
5
, however, is significantly smaller than that of the fourth grid electrode
4
.
Accordingly, it is required for optimizing the characteristics of each electron beam to optimize the structure of each electrode to be most effective to each characteristic of the electron beam.
In the case where a shadow mask pitch in the direction perpendicular to the electron beam scanning direction is made smaller or the density of electron beam scanning lines is increased for enhancing resolution in the direction perpendicular to the electron beam scanning direction of a cathode ray tube, an interference is
Antonelli Terry Stout & Kraus LLP
Day Michael H.
Hitachi , Ltd.
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