Electric lamp and discharge devices: systems – Cathode ray tube circuits – Cathode-ray deflections circuits
Reexamination Certificate
2001-12-21
2002-11-12
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Cathode ray tube circuits
Cathode-ray deflections circuits
Reexamination Certificate
active
06479951
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field 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 elliptic distortion of electron beam spot shapes on the periphery of a phosphor screen is improved to allow displaying an image with a good image quality.
2. Description of the Related Art
Generally, as shown in
FIG. 1
, in a color cathode ray tube, a panel
1
is integrally bonded to a funnel
2
. 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
. 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 the in-line type color cathode ray tube, the electron gun
6
is of an in-line type that emits three in-line electron beams made up of a center beam and a pair of side beams traveling on one horizontal plane. The deflecting yoke
8
generates a nonuniform 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. Thus, 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. A typical example is a so-called BPF (Bi-Potential Focus) dynamic focus (Dynamic Astigmatism Correction and Focus) type electron gun. This BPF dynamic distortion-compensating focus type electron gun is comprised of first to fourth grids G
1
to G
4
. The grids G
1
to G
4
are 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 G
4
has three electron beam holes corresponding to the three in-line 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
. A voltage of about 6 kV is applied to the (3-1)st and (3-2)nd grids 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 prefocus lens is formed between the second and (3-1)st grids G
2
and G
3
-
1
to prefocus the electron beams emitted from the triode. The (3-2)nd and fourth grids G
3
-
2
and G
4
form a BPF (Bi-Potential Focus) main lens for finally focusing the prefocused 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)nd grid G
3
-
2
in accordance with the deflecting distance. This voltage is the lowest when the electron beams are directed toward the center of the phosphor screen and the highest when the electron beams are directed toward the corners of the phosphor screen, thus forming a parabolic waveshape. As the above electron beams are deflected to the corners of the phosphor screen, the potential difference between the (3-2)nd 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 corners of the phosphor screen. As the intensity at the main lens changes, the (3-1)st and (3-2)nd grids G
3
-
1
and G
3
-
2
form a tetrode lens. The tetrode lens 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. 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 elliptic 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 distortion-compensating focus method, a low-voltage side electrode which forms the main lens is divided into a plurality of elements such as the (3-1)st and (3-2)nd grids G
3
-
1
and G
3
-
2
. 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 moire due to interference with the shadow mask
3
. If electron beam spots form a character or the like, the character cannot be recognized easily.
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
4
C.
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)
Assume that 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). In the deflection mode shown in
FIG. 4B
, the horizontal divergent angle &agr;oh becomes smaller than the vertical divergent angle &agr;ov (&agr;ih<&agr;iv), and the vertical magnification Mv becomes smaller than the horizontal magnification Mh (Mv<Mh). In other words, the electron beam spot becomes circular at the center of the phosphor screen but is laterally elongated on the periphery of the phosphor screen.
As a method of moderating the phenomenon in which the electron beam spot becomes laterally elongated on t
Miyamoto Noriyuki
Takekawa Tustomu
Ueno Hirofumi
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Philogene Haissa
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