Color cathode ray tube

Details Color cathode ray tube Color cathode ray tube

2000-11-24

2004-04-13

Patel, NimeshKumar D. (Department: 2879)

Electric lamp and discharge devices

Cathode ray tube

Ray generating or control

C313S449000, C313S414000

Reexamination Certificate

active

06720726

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-334599, filed Nov. 25, 1999, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
This invention relates to an electron gun assembly for a color cathode ray tube, and more particularly to a color cathode ray tube provided with an electron gun assembly having a main electron lens of large aperture.
Generally, a color cathode ray tube has an external enclosure or envelope composed of a panel
11
and a funnel
12
integrally joined to the panel
11
as shown in FIG.
1
. On the inside face of the panel
11
, a phosphor screen or target
13
composed of a stripe-like or dot-like three-color phosphor layers that emits blue light rays, green light rays, and red light rays are formed. Inside the phosphor screen
13
, a shadow mask
14
in which a large number of apertures have been made is provided in such a manner that the mask faces the screen. On the other hand, in the neck
15
of the funnel
12
, an electron gun assembly
17
is provided which emits three electron beams
16
B,
16
G, and
16
R. The three electron beams
16
B,
16
G, and
16
R emitted from the electron gun assembly
17
are deflected by a horizontal and a vertical deflecting magnetic field generated by a deflection yoke
18
provided on the outside of the funnel
12
and are directed via the shadow mask
14
toward the phosphor screen
13
. The phosphor screen
13
is scanned horizontally and vertically by the electron beams, thereby displaying a color image.
In recent years, there have been strong demands for higher resolution of color images. The spot diameter of an electron beam formed on the phosphor screen
13
is considered to be a major factor that determines resolution. The electron beam spot diameter is generally determined by the focusing capability of the electron gun assembly.
The focusing capability is generally determined by the diameter of the main electron lens, the hypothetical object point diameter, the magnification, and others. Specifically, the larger the diameter of the main electron lens is, the smaller the hypothetical object point diameter is, or the lower the magnification is, the smaller the electron beam spot diameter becomes, which improves the resolution.
In a conventional electron gun assembly, for example, the electron gun assembly disclosed in, for example, U.S. Pat. No. 4,712,043, Jpn. Pat. Appln. KOKAI Publication No. 8-22780, or Jpn. Pat. Appln. KOKAI Publication No. 9-320485, an intermediate electrode to which about an intermediate potential higher than the focus voltage and lower than the anode voltage is supplied is provided between the focus electrode and anode electrode. In the respective opposite faces, an opening with an elliptical section long in the in-line direction has been made so as to be common to three electron beams.
In the electron gun assembly having such a configuration, the main electron lens of large aperture has been formed by forming not only an expanded electric field in the direction of electron beam advance but also a continuous electric field in the in-line direction. With the electron gun assembly, the main electron lens of large aperture makes smaller the electron beam spot converged on the screen, realizing a high resolution.
In the electron gun assembly with such a configuration, however, the electrode in which an opening with an elliptical section long in the in-line direction has been made so as to be common to three electron beams causes side beams to converge, with great halos occurring in the direction of the center beam as shown in FIG.
2
. To avoid this phenomenon, one approach is to design an electrode structure in designing an electron gun assembly so that side beams may be bent toward the center beam beforehand so as to enter the main electron lens of large aperture at an angle. With the electrode structure thus designed, side beams are caused to enter the main electron lens at an angle, with the result that the side beams pass through the parts where the potential distribution is relatively uniform closer to the center beam within the main electron lens. As a result of the side beams entering the main electron lens at an angle, its spherical aberration is increased, which is balanced against the spherical aberration occurred on the opposite side, with the result that the side beams are prevented from being converged in a state where great halos appear in the direction of the center beam as shown in FIG.
2
.
However, with the electrode structure that causes side beams to enter the main electron lens at an angle, since side beams are bent before they enter the main electron lens, the center of the side beam through-hole, for example, the center of the side beam through-hole in each of the second grid and third grid, or the center of a sub-lens composed of the third, fourth, and fifth grids, has been offset.
In the former case where the center of the side beam through-hole in each of the second and third grids has been offset, when the side beams are bent toward the center beam, an aberration occurs in the side beams between the second grid and third grid, regardless of the great potential difference, because the diameter of the opening is small. This causes the problem of the side beams being distorted seriously. In the latter case where the center of the sub-lens composed of the third, fourth, and fifth grids has been offset, the shape of inner core pins necessary to assemble the electrodes constituting an electron gun assembly must be made complex, which causes the problem of making errors more liable to occur in assembly.
In the main electron lens of large aperture described above, since the shape of the opening between the electrodes is of an elliptical section long in the horizontal direction, the lens diameter in the vertical direction is much smaller than that in the horizontal direction, causing the electron beam spot on the screen to be converged excessively in the vertical direction and insufficiently in the horizontal direction. To overcome this problem, an electric field correcting electrode plate is provided in a position back from the opening in the focus electrode. Each hole corresponding to each of the three electron beams made in the electric field correcting electrode plate is formed into a shape with its height much greater than its width.
As described above, the horizontal diameter of each of the holes corresponding to the respective three electron beams is set small, which corrects insufficient convergence in the horizontal direction and excessive convergence in the vertical direction. However, as a result of the horizontal diameter of each of the holes corresponding to the respective three electron beams being set small, when the electron beams pass through the holes, the holes give local aberrations to the electron beams. This impairs seriously the original effect of the main electron lens of large aperture produced by expanding the lens electric field in the horizontal direction and in the direction of electron beam advance and thereby forming the main electron lens.
Furthermore, there is a limit to the length of the intermediate electrode forming the main electron lens of large aperture. When the intermediate electrode is too long, the lens electric field is divided, practically forming separate electric field lenses between the fifth grid and intermediate electrode GM and between the intermediate electrode GM and the sixth electrode G
6
as shown in
FIG. 3B
, which increases the lens aberration. As a result, the electron beam spot diameter becomes larger, degrading the resolution.
BRIEF SUMMARY OF THE INVENTION
The object of the present invention is to provide an electron gun assembly which not only makes the aperture of the main electron lens larger while alleviating the aberration in the main electron lens, but also assures a high assembly accuracy and a good image characteristic all over the screen.
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