Electric lamp and discharge devices – Cathode ray tube – Plural beam generating or control
Reexamination Certificate
1999-07-26
2002-01-15
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Plural beam generating or control
C313S449000, C315S382000, C315S015000
Reexamination Certificate
active
06339284
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to a color cathode ray apparatus, and more particularly to a color cathode ray tube apparatus wherein an elliptic distortion of a beam spot at a peripheral portion of a screen is reduced and thereby an image with high quality is displayed.
In general, a color cathode ray tube (CRT) apparatus has a vacuum envelope comprising a panel and a funnel. Three electron beams are emitted from an electron gun assembly disposed in a neck of the funnel. The three electron beams are deflected by horizontal and vertical deflection magnetic fields generated by is a deflection yoke. The deflected beams are then guided through a shadow mask onto a phosphor screen provided on an inner surface of the panel. The phosphor screen is scanned horizontally and vertically by the three electron beams, and thus a color image is displayed on the phosphor screen.
A self-convergence in-line type color cathode ray tube in which an in-line type electron gun assembly is built, in particular, has widely been used as the above color CRT apparatus. In the in-line type electron gun assembly, electron guns are horizontally arranged to emit three in-line electron beams consisting of a center beam and a pair of side beams in the same horizontal plane. In the self-convergence type color CRT, its deflection yoke generates non-uniform magnetic fields, i.e. a pin-cushion-shaped horizontal deflection magnetic field and a barrel-shaped vertical deflection magnetic field, and the in-line three electron beams self-converge on the screen.
Electron gun assemblies for emitting three in-line electron beams may have various structures. There is known an electron gun assembly of a bipotential (BPF) type DACF (Dynamic Astigmatism Correct and Focus) system. The electron gun assembly of the BPF type DAF system, as shown in
FIG. 1
, comprises three in-line cathodes K and first to fourth grids G
1
to G
4
arranged in the named order from the cathode K side toward a phosphor screen. The third grid G
3
is comprised of two divisional segment electrodes G
31
and G
32
. The grids G
1
, G
2
, G
31
, G
32
and G
4
are integrally constructed such that each has three in-line electron beam passage holes for passing electron beams and the positions of these holes correspond in position to three cathodes K.
In this electron gun assembly, a voltage of about 150V is applied to each cathode K. The first grid G
1
is grounded, and a voltage of about 600 to 800V is applied to the second grid G
2
. A voltage of about 6 kV is applied to the first segment electrode G
31
of the third grid G
3
. The second segment electrode G
32
is supplied with a dynamic voltage increasing in synchronism with deflection of an electron beam by the deflection yoke, which dynamic voltage being added to a reference voltage applied to the first segment electrode G
31
. A high voltage of about 26 kV is applied to the fourth grid G
4
.
In the electron gun assembly, with the application of such voltages, the cathodes K and first and second grids G
1
and G
2
generate electron beams and constitute a three-pole (triple-pole) unit for forming an object point on a main lens (described below). The second grid G
2
and the first segment electrode G
31
of the third grid G
3
constitute a prefocus lens for preliminarily focusing the electron beams from the triple-pole unit. The first and second segment electrodes G
31
and G
32
constitute a quadruple-pole lens for horizontally focusing and vertically diverging electron beams when they are deflected. The second segment electrode G
32
and fourth grid G
4
constitute a high-potential (BPF) type main lens for finally focusing the electron beams on the phosphor screen.
In this electron gun assembly, when the electron beams are directed to the center of the screen without deflection, the quadruple-pole lens is not formed between the first and second segment electrodes G
31
and G
32
. The electron beams from the triple-pole unit are preliminarily focused by the prefocus lens and focused on the center of the screen of the main lens.
On the other hand, when the electron beams are deflected toward the periphery of the screen, the voltage of the second segment electrode G
32
is increased in accordance with the amount of deflection of the electron beams and the quadruple-pole lens for horizontally focusing and vertically diverging electron beams is formed between the first and second segment electrodes G
31
and G
32
. At the same time, with the increase in voltage of the second segment electrode G
32
, the power of the main lens formed at the second segment electrode G
32
and fourth grid G
4
is decreased. Thereby, when the electron beams are deflected toward the periphery of the screen, the electro-optical distance between the electron gun assembly and the phosphor screen increases and an image point will form at a long distance. Accordingly, the magnification of the lens varies to cancel a deflection aberration occurring due to the fact that the horizontal deflection field generated by the deflection yoke has a pin-cushion shape and the vertical deflection field has a barrel-shape.
In the meantime, in order to enhance the image quality of the color CRT, it is necessary to enhance the focusing characteristics of the entire screen. However, in an in-line type color CRT having a regular electron gun assembly for emitting three in-line electron beams, as shown in
FIG. 2A
, a beam spot at a peripheral portion of the screen is distorted to a horizontal elliptic shape
1
b
(horizontal deformation) due to a deflection aberration and a vertical blur
2
occurs, although a beam spot
1
b
at a central portion of the screen has a substantially circular shape.
On the other hand, in the in-line type color CRT having the electron gun assembly, as shown in
FIG. 1
, the blur
2
can be eliminated and the focusing characteristics can be enhanced, as shown in FIG.
2
B. This electron gun assembly adopts the DACF system, and the low-voltage side electrode constituting the BPF type main lens is divided into a plurality of segment electrodes and these segment electrodes form the four-pole lens in accordance with the amount of deflection of electron beams, thereby to compensate the deflection aberration. Even in the electron gun assembly with this structure, however, the horizontal deformation of the beam spot
1
b
at the peripheral portion of the screen cannot be eliminated. As a result, a moire occurs due to an interference between the electron beams and the beam passage holes in the shadow mask, and displayed characters, etc. on the screen becomes difficult to view.
In a method of solving the above problem, in the above-described electron gun assembly, as shown in
FIG. 3
, non-circular electron beam passage holes
4
, each having a horizontal long axis, are formed in that surface of the second grid G
2
, which face the first segment electrode G
31
of third grid G
3
. In the electron gun assembly with this structure, the horizontal focusing power of the prefocus lens constituted by the second grid G
2
and the first segment electrode G
31
is weaker than the vertical focusing power thereof, and a horizontal imaginary object point size is reduced and a vertical imaginary object point size is increased. As a result, as shown in
FIG. 2C
, the beam spot la at the central portion of the screen is vertically elongated and the horizontal deformation of the beam spot
1
b
at the peripheral portion of the screen is reduced. Thus, the moiré due to an interference between the electron beams and the beam passage holes in the shadow mask can be prevented.
In this electron gun assembly, as the depth of the non-circular recess
4
with the horizontal long axis, which is formed in the second grid, increases, the horizontal deformation of the beam spot
1
b
at the peripheral portion of the screen can be reduced more effectively. As a result, however, the vertical length of the beam spot
11
at the central portion of the screen is increased and the vertical dimension of the beam spot increases.
Takekawa Tsutomu
Ueno Hirofumi
Kabushiki Kaisha Toshiba
Patel Ashok
Santiago Mariceli
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