Color display tube

Details Color display tube Color display tube

2001-03-20

2003-05-27

Patel, Ashok (Department: 2879)

Electric lamp and discharge devices

Cathode ray tube

Plural beam generating or control

C313S441000, C313S412000, C313S416000, C313S449000

Reexamination Certificate

active

06570314

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a color display tube. It relates in particular to a color display tube characterized by electrodes that constitute a main lens for focusing three electron beams on a phosphor screen.
2. Description of Related Art
A color display tube device generally has an envelope including a panel and a funnel that is connected integrally therewith. An electron gun is disposed in a neck portion of the funnel. A deflecting device is mounted outside the funnel. A phosphor screen is formed on an inner surface of the panel so as to face a shadow mask. Three electron beams emitted from the electron gun are deflected by horizontal and vertical deflection magnetic fields generated by the deflecting device and excite the phosphor screen while scanning it horizontally and vertically, thereby displaying a color image.
The magnetic fields generated by the deflecting device used in such a color display tube device generally has a self-convergence structure, in which the three electron beams are converged on a screen. For this purpose, the horizontal deflection magnetic field and the vertical deflection magnetic field are distorted to have a pincushion shape and a barrel shape respectively. Accordingly, the three electron beams passing through the deflection magnetic fields are subjected to a diverging effect in a horizontal direction and to a focusing effect in a vertical direction respectively. In the present application, such focusing effect on the electron beams that is larger in the vertical direction than in the horizontal direction due to the diverging effect in the horizontal direction and the focusing effect in the vertical direction is referred to as a negative astigmatism.
When the electron beams come to travel further along with an increase in a deflection angle, because of the self-convergence magnetic field described above, the diverging effect in the horizontal direction and the focusing effect in the vertical direction become distinctive, especially at edge portions of the phosphor screen. Consequently, electron beam spots on the phosphor screen become elongated horizontally such that the major axis is parallel to the horizontal direction, causing a problem of lowering a horizontal resolution. The recent development of flatter panels and larger deflection angle makes the above problem more and more serious.
In general, when the deflecting device has the self-convergence structure, the above-described deformation of the spot shape occurs easily. Thus, in order to display images with high resolution on the phosphor screen, the electron gun has to achieve a smaller spot diameter in the horizontal direction.
Although various factors generally influence the spot diameter in the color display tube device, the spherical aberration of a main lens contributes most to the spot diameter in its relationship with the main lens of the electron gun. In other words, the spot diameter can be reduced with a decrease in the spherical aberration of the main lens of the electron gun. When an incident angle of the electron beam into the main lens is indicated by &agr;, the spot diameter &dgr; is expressed by
&dgr;=(
M·Csp·&agr;
3
)/2
where M is a lens magnification and Csp is a spherical aberration coefficient. When the focusing effect of the main lens is weakened, the lens magnification and the spherical aberration are reduced. One of the methods for weakening the focusing effect of the main lens is to increase an equivalent diameter of the main lens. In other words, by increasing an effective main lens diameter, it is possible to achieve a smaller spot diameter on the phosphor screen.
JP 2(1990)-18540 B discloses a conventional main lens in an electron gun for a color display tube. As shown in
FIG. 11
, the main lens is constituted by a focusing electrode
32
, a final accelerating electrode
33
and a shielding cup
34
connected to the final accelerating electrode
33
. The focusing electrode
32
and the final accelerating electrode
33
are spaced away from each other in a tube axis direction. The focusing electrode
32
and the final accelerating electrode
33
respectively include peripheral electrodes
35
and
36
surrounding three electron beams
8
a
,
8
b
and
8
c
and electrode plates (hereinafter, referred to as “vertical electrode plates”)
37
and
38
. The vertical electrode plates
37
and
38
are retracted with respect to the opposing end faces of the peripheral electrodes
35
and
36
and arranged so as to allow the electron beams to pass substantially perpendicular thereto.
FIG. 12
shows a front view of the vertical electrode plates
37
and
38
. Openings
39
a
,
39
b
,
39
c
in the vertical electrode plate
37
and openings
40
a
,
40
b
and
40
c
in the vertical electrode plate
38
are formed to have a horizontal diameter smaller than their vertical diameter.
The vertical electrode plates
37
and
38
, which are arranged inside the respective peripheral electrodes
35
and
36
of the focusing electrode
32
and the final accelerating electrode
33
constituting the main lens, are retracted as described above, thereby allowing a high electric potential of the final accelerating electrode
33
to enter deeply into the focusing electrode
32
and a low electric potential of the focusing electrode
32
to enter deeply into the final accelerating electrode
33
. This increases the effective main lens diameter, thereby achieving a smaller spot diameter on the phosphor screen.
Also, the negative astigmatism, in which the lens focusing effect is stronger in the vertical direction than in the horizontal direction owing to the peripheral electrodes
35
and
36
having their diameter larger in the horizontal direction, is eliminated by making an opening diameter in the horizontal direction Rh smaller than that in the vertical direction Rv in the vertical electrode plates
37
and
38
so as to prevent the entrance of the electric potential in the horizontal direction.
However, such an electron gun in the conventional color display tube device has had a limitation in increasing the main lens diameter and adjusting the astigmatism concurrently. In this technique, the vertical electrode plates are retracted in order to increase the main lens diameter. On the other hand, however, the horizontal diameter of the openings of the vertical electrode plates is reduced in order to adjust the astigmatism, resulting in a limitation in increasing the main lens diameter especially in the horizontal direction.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a color display tube having a main lens portion structure that can adjust astigmatism easily and achieve a higher horizontal resolution by increasing a main lens diameter in a horizontal direction with a relatively simple configuration.
In order to achieve the object mentioned above, a color display tube of the present invention includes an envelope including a front panel on which a phosphor screen is formed and a funnel, and an in-line electron gun that is provided in a neck portion of the funnel and emits three electron beams. The in-line electron gun has a focusing electrode and a final accelerating electrode that are disposed facing each other in a tube axis direction so as to have a predetermined space therebetween and constitute a main lens, and a shielding cup that has a bottom provided with at least one opening through which the electron beams pass and is connected to a side of the phosphor screen of the final accelerating electrode via the bottom of the cup. A vertical electrode plate having three openings formed in-line through which the three electron beams pass respectively is provided only inside the focusing electrode out of the focusing electrode and the final accelerating electrode. A horizontal electrode plate that is substantially parallel to an in-line plane is formed so as to extend toward the focusing electrode at least one of above and below the opening provided at the bottom of the shielding cup. The h

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