Electric lamp and discharge devices – Cathode ray tube – Plural beam generating or control
Reexamination Certificate
2001-12-26
2003-11-11
Gushi, Ross (Department: 2833)
Electric lamp and discharge devices
Cathode ray tube
Plural beam generating or control
Reexamination Certificate
active
06646370
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-397297, filed Dec. 27, 2000, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a cathode-ray tube apparatus and more particularly to a color cathode-ray tube apparatus capable of improving an oval distortion of a beam spot on a peripheral portion of a phosphor screen and displaying an image with high quality.
2. Description of the Related Art
In general terms, a color cathode-ray tube (CRT) apparatus comprises an inline electron gun assembly for emitting three electron beams, which are horizontally arranged in line, and a deflection yoke for generating non-uniform deflection magnetic fields for horizontally and vertically deflecting the three electron beams. As an electron gun assembly emitting three electron beams, there is known a QPF (Quadru-Potential Focus) double-focus type electron gun assembly, which comprises, as shown in
FIG. 8
, three in-line cathodes K, and six grids G
1
to G
6
successively arranged toward a phosphor screen. Each of the grids G
1
to G
6
has three electron beam passage holes in association with the three in-line cathodes K.
In this electron gun assembly, a voltage of about 150V is applied to the cathodes K. The first grid G
1
is grounded. The second grid G
2
is connected to the fourth grid G
4
within the tube and supplied with a voltage of about 600V. The third grid G
3
is connected to a first segment G
5
-
1
of the fifth grid G
5
within the tube and supplied with a focus voltage of about 6 KV. A second segment G
5
-
2
of the fifth grid G
5
is supplied with a dynamic focus voltage obtained by superimposing upon a reference voltage of about 6 KV an AC component increasing in accordance with an increase in degree of deflection of electron beams. The sixth grid G
6
is supplied with an anode voltage of about 26 KV.
An electron beam generating section is constituted by the cathodes K, first grid G
1
and second grid G
2
and generates electron beams. A prefocus lens is constituted by the second grid G
2
and third grid G
3
and prefocuses the electron beams emitted from the electron beam generating section. A sub-lens is constituted by the third grid G
3
, fourth grid G
4
and first segment G
5
-
1
and further prefocuses the electron beams. A main lens is constituted by the second segment G
5
-
2
and sixth grid G
6
and ultimately focuses the electron beams on the phosphor screen.
In a non-deflection mode in which electron beams are focused on a central portion on the phosphor screen, the electron beams generated from the electron beam generating section are focused on the phosphor screen by the prefocus lens, sub-lens and main lens. At this time, since there is no potential difference between the first segment G
5
-
1
and second segment G
5
-
2
, a quadrupole lens is not created.
On the other hand, in a deflection mode in which electron beams are deflected onto a peripheral portion of the phosphor screen, a higher voltage is applied to the second segment G
5
-
2
and a potential difference occurs between the first segment G
5
-
1
and second segment G
5
-
2
. Thus, a quadrupole lens is created. The quadrupole lens created at this time has such an astigmatism that it has a horizontal focusing function and a vertical diverging function. At the same time, a potential difference between the second segment G
5
-
2
and sixth grid G
6
decreases, and the lens power of the main lens lowers. Thereby, a focus error due to an increase in distance over which the electron beams reach the phosphor screen is corrected, and a deflection aberration due to non-uniform magnetic fields is compensated.
In order to enhance the image quality of the color CRT apparatus, it is necessary to improve focus characteristics on the phosphor screen. In particular, in the case of a color CRT apparatus emitting three in-line electron beams, a beam spot on the phosphor screen may disadvantageously have an elliptically deformed core and blur portion due to deflection aberration, as shown in FIG.
9
A.
In a general double-focus electron gun assembly, a low-voltage side electrode constituting the main lens is composed of a plurality of grids such as the first segment G
5
-
1
and second segment G
5
-
2
. A quadrupole lens is created between these segments in accordance with deflection of the electron beam. Thereby, deflection aberration is compensated, and the problem of blur is improved, as shown in FIG.
9
B.
However, as shown in
FIG. 9B
, oval deformation of the beam spot still remains at an end portion in a horizontal axis H and an end portion in a diagonal axis D on the phosphor screen. The reason is as follows. If the sub-lens, quadrupole lens, main lens and deflection aberration components included in deflection magnetic fields are assumed to be a single lens, the horizontal lens magnification increases and the vertical lens magnification decreases. This results in such oval deformation. Consequently, the vertical dimension of the beam spot becomes too small, and a moire may occur due to interference with the shadow mask. A character, etc., if formed with such a beam spot, could not easily be viewed.
To solve this problem, an electron gun assembly with a double-quadrupole lens structure is proposed. As is shown in
FIG. 10
, the double-quadrupole lens structure has the same basic structure as shown in FIG.
8
. The third grid G
3
comprises a first segment G
3
-
1
and a second segment G
3
-
2
. The second segment G
3
-
2
is connected to the second segment G
5
-
2
and supplied with a dynamic focus voltage at the time of deflection.
In the deflection mode, a first quadrupole lens dynamically varying in synchronism with deflection magnetic fields is created between the first segment G
3
-
1
and second segment G
3
-
2
. The first quadrupole lens has a horizontal diverging function and a vertical focusing function. In short, the first quadrupole lens has an astigmatism with polarities opposite to those of a second quadrupole lens created between the first segment G
5
-
1
and G
5
-
2
.
Thereby, if the first quadrupole lens, sub-lens, second quadrupole lens, main lens and deflection aberration components included in deflection magnetic fields are assumed to be a single lens, a difference between the horizontal lens magnification and the vertical lens magnification can be decreased and the oval deformation of the beam spot improved.
Compared to the conventional double-focus electron gun assembly, this electron gun assembly with the double-quadrupole lens structure requires quadrupole lenses with higher power. In particular, the diameter of each electron beam passing through the first quadrupole lens is small. Thus, in order to obtain sufficient effect of improving oval deformation, the first quadrupole lens must have a very high lens power.
The quadrupole lens is formed by arranging a pair of grids, as shown in
FIGS. 12A and 12B
, such that their electron beam passage holes are opposed to each other. The electron beam passage holes in one of the grids are horizontally elongated ones, and the electron beam passage holes in the other grid are vertically elongated ones. However, a necessary lens power may not be obtained in order to sufficiently improve the oval deformation of the beam spot.
This phenomenon will now be described referring to
FIGS. 13A and 13B
. In
FIGS. 13A and 13B
, an electron beam enters from the left side in the Figures, and exits to the right side. Assume that a voltage V
1
is applied to an incidence-side grid Gin, a voltage V
2
is applied to an emission-side grid Gout, and V
1
<V
2
.
As regards the vertical direction, as shown in
FIG. 13A
, the electron beam entering the quadrupole lens suffers a strong focusing action since the vertical dimension of the incidence-side grid Gin is small. The electron beam exiting the quadrupole lens suffers a weak diverging action since th
Takekawa Tsutomu
Ueno Hirofumi
Gushi Ross
Kabushiki Kaisha Toshiba
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