Electric lamp and discharge devices – Cathode ray tube – Plural beam generating or control
Reexamination Certificate
2001-04-25
2004-03-09
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Plural beam generating or control
C313S412000, C313S414000, C313S437000, C313S449000, C313S450000, C315S382100
Reexamination Certificate
active
06703775
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2000-126071, filed Apr. 26, 2000; and No. 2001-081278, filed Mar. 21, 2001, the entire contents of both of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a color cathode ray tube and, more particularly, to a color cathode ray tube apparatus in which the elliptical distortion of electron beam spot shapes on the periphery of a phosphor screen is improved to allow displaying an image of good quality.
Generally, as shown in
FIG. 1
, in a color cathode ray tube, a panel
1
is integrally bonded to a funnel
2
, and a phosphor screen
4
comprised of three color phosphor layers for emitting red, green, and blue light is formed on the inner surface of the faceplate of the panel
1
. A shadow mask
3
having a large number of electron beam holes is mounted inside the panel
1
to oppose the phosphor screen
4
. An electron gun
6
is arranged in a neck
5
of the funnel
2
, and three electron beams
7
B,
7
G, and
7
R emitted from the electron gun
6
are deflected by a magnetic field generated by a deflecting yoke
8
mounted on the outer surface of the funnel
2
and are directed toward the phosphor screen
4
. The phosphor screen
4
is scanned horizontally and vertically by the deflected electron beams
7
B,
7
G, and
7
R, thereby displaying a color image on the phosphor screen
4
.
As a color cathode ray tube of this type, an in-line type color cathode ray tube is available in which the electron gun
6
particularly forms an in-line type electron gun that emits three in-line electron beams made up of a center beam and a pair of side beams traveling on one horizontal plane, while the deflecting yoke generates a non-uniform magnetic field such that the horizontal deflecting magnetic field forms a pincushion type field and the vertical deflecting magnetic field forms a barrel type field, so the three electron beams self-converge.
For the in-line type electron gun for emitting three in-line electron beams, various types and methods are available, and a typical example them is a so-called BPF (Bi-Potential Focus) dynamic focus (Dynamic Astigmatism Correction and Focus) type electron gun. This BPF dynamic focus type electron gun is comprised of first to fourth grids G
1
to G
4
integrated with each other and sequentially arranged from three in-line cathodes K toward a phosphor screen
4
, as shown in FIG.
2
. Each of the grids G
1
to F
4
has three electron beam holes corresponding to the in-line type three cathodes K. In this electron gun, a voltage of about 150 V is applied to the cathodes K, the first grid G
1
is grounded, a voltage of about 600 V is applied to the second grid G
2
, and a voltage of about 6 kV is applied to the (
3
-
1
)th and (
3
-
2
)th grid G
3
-
1
and G
3
-
2
. A high voltage of about 26 kV is applied to the fourth grid G
4
.
In the above electrode structure to which the above voltages are applied, the cathodes K and the first and second grids G
1
and G
2
make up a triode for generating electron beams and forming an object point with respect to a main lens (to be described later). A pre-focus lens is formed between the second and (
3
-
1
)th grids G
2
and G
3
-
1
to pre-focus the electron beams emitted from the triode. The (
3
-
2
)th and fourth grids G
3
-
2
and G
4
form a BPF (Bi-Potential Focus) main lens for finally focusing the pre-focused electron beams onto the phosphor screen. If the deflecting yoke
8
deflects the electron beams to the periphery of the phosphor screen, a preset voltage is applied to the (
3
-
2
)th grid G
3
-
2
in accordance with the deflecting distance. This voltage is lowest when the electron beams are directed toward the center of the phosphor screen and highest when the electron beams are directed toward the periphery of the phosphor screen, thus forming a parabolic wave-shape. As the above electron beams are deflected to the periphery of the phosphor screen, the potential difference between the (
3
-
2
)th and fourth grids G
3
-
2
and G
4
decreases, and the intensity of the main lens described above is decreased. The intensity of the main lens is minimum when the electron beams are directed toward the periphery of the phosphor screen. As the intensity at the main lens changes, the (
3
-
1
)th and (
3
-
2
)th grids G
3
-
1
and G
3
-
2
form a tetrode lens. The tetrode is the most intense when the electron beams are directed toward the corners of the phosphor screen. The tetrode lens has a focusing function in the horizontal direction and a divergent function in the vertical direction. Thus, as the distance between the electron gun and phosphor screen increases and the image point becomes far, the intensity at the main lens decreases accordingly. As a result, a focus error based on a change in distance is compensated for, and deflection astigmatism caused by the pincushion type horizontal deflecting field and barrel type vertical deflecting field of the deflecting yoke is compensated for by the tetrode lens.
To improve the image quality of the color cathode ray tube, the focus characteristics on the phosphor screen must be improved. In particular, in a color cathode ray tube in which an electron gun for emitting three in-line electron beams is sealed, the elliptical distortion and blurring, as shown in
FIG. 3A
, of an electron beam spot which are caused by deflection astigmatism become an issue. In a defection astigmatism compensating method generally called the BPF dynamic focus method (Dynamic Astigmatism Correction Focus method), a low-voltage side electrode which forms the main lens is divided into a plurality of elements such as the (
3
-
1
)th and (
3
-
2
)th grids G
3
-
1
and G
3
-
2
, and a tetrode lens is formed in accordance with the deflection of the electron beams. This method can solve the problem of blurring as shown in FIG.
3
B. As shown in
FIG. 3B
, however, a phenomenon still occurs in which electron beam spots are laterally flattened at the ends of the horizontal axis and the ends of the orthogonal axis of the phosphor screen. This causes a moiré effect due to interference with the shadow mask
3
. If electron beam spots form a character or the like, the character cannot be easily recognized.
The phenomenon in which an electron beam spot is laterally flattened will be described with reference to optical models shown in
FIGS. 4A
,
4
B, and
5
.
FIG. 4A
shows an optical system formed when the electron beams reach the center of the phosphor screen without being deflected, and the loci of the electron beams.
FIG. 4B
shows an optical system formed when the electron beams reach the periphery of the screen after being deflected by the deflecting magnetic fields, and the loci of the electron beams. The size of the electron beam spot on the phosphor screen depends on a magnification (M), and the magnification of the electron beam in the horizontal direction is defined as Mh and that in the vertical direction is defined as Mv. The magnification M can be expressed as (divergent angle &agr;o/incident angle &agr;i) shown in
FIGS. 4A and 4B
.
More specifically,
Mh (horizontal magnification)=&agr;oh (horizontal divergent angle)/&agr;ih (horizontal incident angle)
Mv (vertical magnification)=&agr;ov (vertical divergent angle)/&agr;iv (vertical incident angle)
When the horizontal divergent angle &agr;oh and vertical divergent angle &agr;ov are equal (&agr;oh=&agr;ov), in the non-deflection mode shown in
FIG. 4A
, the horizontal incident angle &agr;ih and vertical incident angle &agr;iv become equal (&agr;ih=&agr;iv) and the horizontal magnification Mh and vertical magnification Mv become equal (Mh=Mv), and in the deflection mode shown in
FIG. 4B
, the horizontal divergent angle &agr;oh becomes smaller than the vertical divergent angle &agr;ov (&agr;oh<&agr;ov), and the vertical magnification Mv becomes smaller than the horizontal magnification Mh (Mv<M
Miyamoto Noriyuki
Takekawa Tsutomu
Ueno Hirofumi
Hodges Matt
Kabushiki Kaisha Toshiba
Patel Nimeshkumar D.
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