Electric lamp and discharge devices – Cathode ray tube – Beam deflecting means
Reexamination Certificate
2001-01-10
2002-10-22
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Beam deflecting means
C313S491000, C313S631000
Reexamination Certificate
active
06469432
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a cathode-ray tube which includes an electron gun comprising a cathode and a plurality of electrodes and, particularly, to a color cathode-ray tube which includes a a plurality of cathodes corresponding to colors.
2. Description of the Related Art
Heretofore, as an electron gun of a color cathode-ray tube, there has been mainly used an electron gun
50
of a multi-beam single-electron gun system in which a plurality of electron beams EB are intersected with each other within the electron gun and are emitted as shown in
FIG. 1A
, or an electron gun
60
, a so-called inline electron gun of a system in which a plurality of electron beams EB are arrayed substantially in parallel within the electron gun and the electron beams are not intersected with each other within the electron gun and are emitted as shown in FIG.
1
B.
In the electron guns
50
,
60
of any of the systems, three electron beams EB are generated from three cathodes KR, KG, KB corresponding to a display of the three colors red, green and blue. These electron beams pass through respective electrodes of the electron guns
50
,
60
and are introduced into the surface of a fluorescent screen of the cathode-ray tube. These electron beams are introduced into adjacent red, green and blue phosphors.
Specifically, as
FIG. 2
shows, an electron gun
73
(
50
,
60
) is disposed in a necked-down portion
72
c
of a cathode-ray tube assembly
72
formed of a glass, for example, of a cathode-ray tube
71
. Three electron beams EB are emitted from the electron gun
73
. The three electron beams EB travel through a color-selecting mechanism
75
, such as a so-called aperture grille, before being incident onto a fluorescent screen
76
formed on the inner surface of a panel portion
72
a
of the cathode-ray tube assembly
72
. These electron beams have to be incident on phosphors emitting red, green and blue light, which are adjacent to each other, although not shown.
It has been customary that the trajectories of the two side electron beams should be changed by suitable means, and the three electron beams go into convergence so that the side beams, i.e., two electron beams, e.g., electron beams usually corresponding to a red and blue display, may intersect with each other on the color-selecting mechanism
75
disposed ahead of the fluorescent screen
76
as-seen from the side of the electron gun
73
.
In the electron gun
50
of the system shown in
FIG. 1A
, a convergence plate
56
is disposed between a focus lens of the electron gun and the color-selecting mechanism so that a stationary electric field acts on the side beams to (change the trajectories of the side beams, and thereby the three electron beams establish a convergence.
In the electron gun
60
of the system shown in
FIG. 1B
, a convergence of a plurality of electron beams is established by various systems.
In general, there is used a system in which a so-called dog-bone-like, large, overlapping electrostatic lens electric field is formed in the X direction (horizontal direction) on a main lens formed between a third electrode
63
(G
3
) and a fourth electrode (G
4
). That is, its lens action is used to change the convergence of each of the electron beams EB and the trajectories of the side beams, thereby establishing the convergence.
As another example, there is known a method of changing the trajectories of the side beams by displacing the axes of the side beams between the opposing electrodes.
In any of the above methods, it has been customary that the convergence is established by the action of the electric field.
As a method of changing the amperage of each electron beam current in response to a video signal, there is generally used a so-called cathode-drive system for changing a cathode-drive voltage. Precisely, the amperage of an electron-beam current should be changed in order to change a liminance. The amperage of an electron-beam current can be changed by changing the cathode voltage. Specifically, when the luminance increases, the cathode potential should be decreased. When the luminance decreases, the cathode potential should be increased.
Incidentally, a constant high-voltage potential is applied to the fluorescent screen or electrodes of other electron guns regardless of the change of the cathode potential.
However, the energy of an electron beam passing through the main lens changes in response to the change of the cathode potential so that the velocity of an electron beam changes.
Specifically, when the luminance increases, the cathode potential decreases and the potential difference increases relatively so that the velocity of the electron beam increases. So, the trajectories of the side beams change.
FIGS. 3A and 3B
show the manner in which the trajectories of the side beams change as the velocity of the electron beam increases.
FIG. 3A
shows the case of the electron gun of the system shown in FIG.
1
A.
FIG. 3B
shows the case of the electron gun of the system shown in FIG.
1
B.
When the velocity of the electron beam increases as described above, in the convergence plate
56
shown in
FIG. 3A
, or in the overlapping electrostatic lens formed between the third electrode
63
(G
3
) and the fourth electrode
64
(G
4
) shown in
FIG. 3B
, the sensitivity with which the electron beam changes its trajectory as the electron beam is deflected is lowered.
As a consequence, the trajectory of the electron beam changes, as shown arrows in the figures, with the result that the side beams which are coincident with each other at the color-selecting mechanism
75
are displaced from each other.
When the amperage of the electron beams' currents of red, green and blue increase, a so-called repulsion action influences each electron beam more strongly.
A repulsion action which influences the electron beam
57
for diplaying blue emitted from the cathode KB of the electron gun
50
of the system shown in
FIG. 1A
will be described with reference to FIG.
4
.
The electron beam
57
is influenced by a repulsion action
59
acting on the electron beam from other two electron beams
58
while it travels toward the fluorescent screen
76
.
This repulsion action
59
is generated by an electric field acting on the electron beams. The repulsion action
59
becomes strong when the amperage of the respective electron beam's current increases in order to increase the luminance of each color.
While the electron beam
57
is influenced by a repulsion action
59
′ acting on the electron beams within the electron gun
50
, this repulsion action is generally weaker than the repulsion action
59
which influences the blue electron beam between the electron gun
50
and the color-selecting electrode
75
.
The above-described two actions, i.e., a decrease in the sensitivity when the trajectory of the electron beam is changed in response to the velocity and the repulsion action, cause misconvergences in the same direction, respectively.
This influence becomes remarkable for an image with a higher luminance, such as a teletext, compared with an ordinary broadcasting, for example. Specifically, a color shading occurs and characters are displayed doubly, which, therefore, causes picture quality to be deteriorated.
As a method of solving this problem, there have been proposed a method of adding an auxiliary electrode (see Japanese laid-open patent application No. 9-245667) and a method of correcting by a magnetic field (see Japanese laid-open patent application No. 4-61588).
However, these methods need circuits by which the amperage of the electron beams' currents for displaying red, green and blue are detected and a voltage applied to the auxiliary electrode or current flowing through a coil disposed outside a necked-down portion is adjusted in response to the change of the amperage of each electron beam current.
Because the frequency of a signal applied to a cathode has a tendency to increase as a cathode-ray tube of a television receiver or the like becomes a high-d
Baba Makoto
Ishihara Hirotsugu
Suzuki Yuuichi
Kananen, Esq. Ronald P.
Patel Vip
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