Electric lamp and discharge devices: systems – Cathode ray tube circuits – Cathode-ray deflections circuits
Reexamination Certificate
2001-06-28
2003-04-29
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Cathode ray tube circuits
Cathode-ray deflections circuits
C315S368150, C313S414000
Reexamination Certificate
active
06555975
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2000-195897, filed Jun. 29, 2000; and No. 2001-119664, filed Apr. 18, 2001, the entire contents of both of which are incorporated herein by reference.
BACKGROUND 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 shape on a peripheral portion of a phosphor screen and stably providing a high image quality.
A currently dominant self-convergence type inline color cathode-ray tube apparatus comprises an inline electron gun assembly for emitting three in-line electron beams, which travel on a horizontal plane, and a deflection yoke for generating non-uniform deflection magnetic fields for deflecting the electron beams emitted from the electron gun assembly. The deflection magnetic fields comprise a pin-cushion-shaped horizontal deflection magnetic field and a barrel-shaped vertical deflection field. As the degree of deflection of the electron beams increases, the deflection magnetic fields will have a stronger action as an equivalent quadrupole lens for vertically focusing the electron beams and horizontally diverging the electron beams.
The distance between the electron gun assembly and the phosphor screen increases as the location of deflected electron beams shifts from a central portion to a peripheral portion of the phosphor screen. Owing to the difference in this distance, while the electron beams are focused at the central portion of the phosphor screen, the electron beams are defocused at the peripheral portion of the phosphor screen.
Accordingly, the beam spot at the peripheral portion of the phosphor screen is optimally focused in the horizontal direction by virtue of mutual cancellation of the diverging action of the deflection magnetic field and the defocusing due to the difference in distance. However, the beam spot at the peripheral portion of the phosphor screen is over-focused in the vertical direction by the addition of the focusing action of the deflection magnetic field and the defocusing due to the difference in distance. Consequently, the beam spot formed on the central portion of the phosphor screen is substantially circular, while the beam spot formed on the peripheral portion of the phosphor screen includes a horizontally elongated high-luminance portion (core) and a vertically elongated low-luminance portion (halo). Because of this, the resolution at the peripheral portion of the phosphor screen considerably deteriorates.
To solve this problem, Jpn. Pat. Appln. KOKAI Publication No. 61-99249 discloses a DAF (Dynamic Astigmatism and Focus) electron gun assembly. This electron gun assembly is characterized in that a third grid, which functions as a focus electrode, comprises a first segment G
3
-
1
and a second segment G
3
-
2
. An electron beam passage hole formed at the second segment (G
3
-
2
) side surface of the first segment G
3
-
1
has a vertically elongated shape. An electron beam passage hole formed at the first segment (G
3
-
1
) side surface of the second segment G
3
-
2
has a horizontally elongated shape. In addition, a dynamic voltage, which is obtained by superimposition of an AC component varying parabolically in accordance with a variation in the degree of deflection of electron beams, is applied to the second segment G
3
-
2
.
Thus, in accordance with the deflection of the electron beams, a potential difference occurs between the first segment and the second segment. This potential difference creates a quadrupole lens between the first segment and second segment, which horizontally focus the electron beams and vertically diverges the electron beams. The quadrupole lens compensates a deflection aberration occurring due to the deflection of electron beams. In addition, since the second segment is supplied with the dynamic voltage, the focusing action of the main lens is weakened in accordance with the increase in the deflection amount of the electron beams. Thus, the defocusing due to the aforementioned difference in distance is also corrected.
The electron gun assembly, however, has two problems: 1) as the degree of deflection of electron beams increases, the distance between the electron gun assembly and the phosphor screen increases and the beam spot size increases accordingly, and 2) as the degree of deflection of electron beams increases, the beam spot formed on the phosphor screen is horizontally deformed. Owing to these two problems, the beam spot formed at the peripheral portion of the phosphor screen has an increased average size and a deformed shape.
An explanation will now be given of the phenomenon occurring with this electron gun assembly, in which the beam spot size increases at the peripheral portion of the phosphor screen.
FIGS. 8A and 8B
show simplified models for explanation based on only the distance between the electron gun assembly and the phosphor screen, and the power of the main lens. Thus,
FIGS. 8A and 8B
omit illustration of the quadrupole lens component created by the deflection magnetic fields and the quadrupole lens formed in the electron gun assembly.
The size of the beam spot on the phosphor screen depends on a magnification M expressed by the ratio of a divergence angle &agr;o of an electron beam emitted from an electron beam generating section of the electron gun assembly to an incidence angle &agr;i on the phosphor screen. Thus, the magnification M is given by
M=(divergence angle &agr;o/incidence angle &agr;i).
As is shown in
FIG. 8A
, in a case where an electron beam is focused on a central portion of the phosphor screen, the electron beam emitted from an object point O at divergence angles &agr;o in both horizontal and vertical directions is focused by a main lens
20
and made incident on the phosphor screen with incidence angles &agr;i(1) in both the horizontal and vertical directions. A magnification M(1) in this case is expressed by
M(1)=&agr;o/&agr;i(1).
As is shown in
FIG. 8B
, when the electron beam is focused on a peripheral portion of the phosphor screen, the distance between the electron gun assembly and the phosphor screen increases. The electron beam emitted from the object point O at divergence angles &agr;o in both horizontal and vertical directions is focused by the main lens. In the electron gun assembly disclosed in Jpn. Pat. Appln. KOKAI Publication No. 61-99249, the focal distance is increased by weakening the focusing power of the main lens. The electron beam focused by the main lens is made incident on the phosphor screen with incidence angles &agr;i(2) in both the horizontal and vertical directions. A magnification M(2) in this case is expressed by
M(2)=&agr;o/&agr;i(2).
Since the distance between the object point O and the main lens is constant, the magnification &agr;i(2) decreases as the distance (focal distance) between the main lens and the phosphor screen increases. Since &agr;i(1)>&agr;i(2),
M(1)<M(2).
When the focal distance is varied by the main lens power, the magnification M increases and the beam spot size on the phosphor screen increases in accordance with the increase in the focal distance. Thus, in the case of the electron gun assembly disclosed in Jpn. Pat. Appln. KOKAI Publication No. 61-99249, the average spot size of the beam spot formed on the peripheral portion of the phosphor screen is larger than that of the beam spot formed on the central portion of the phosphor screen.
An explanation will now be given of the phenomenon in which the electron beam spot on the peripheral portion of the phosphor screen is horizontally deformed, using an optical lens model as well. A horizontal magnification Mx of the electron beam and a vertical magnification My of the electron beam are expressed by
Mx (horizontal magnification)=&agr;ox (horizontal divergence angle)/&agr;ix (horizontal incidence angle), and
My (vertical magnification)&equals
Ishihara Tomonari
Satou Kazunori
Dinh Trinh Vo
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Wong Don
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