Tension mask with inner shield structure for cathode ray tube

Details Tension mask with inner shield structure for cathode ray tube Tension mask with inner shield structure for cathode ray tube

2003-01-13

2004-11-30

Patel, Ashok (Department: 2879)

Electric lamp and discharge devices

Cathode ray tube

Shadow mask, support or shield

C313S404000, C313S407000, C313S408000

Reexamination Certificate

active

06825598

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATION
This application claims priority to application No. 2002-0005018, filed in the Korean Intellectual Property Office on Jan. 29, 2002 the disclosure of which is incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to a cathode ray tube with a tension mask wherein tensional strength is applied in uni-axial or bi-axial directions, and more particularly, to a tension mask cathode ray tube with an inner shield capable of minimizing variation in the landing of the electron beam due to geomagnetism.
(b) Description of the Related Art
Generally, a cathode ray tube is a display device wherein three electron beams are scanned on a phosphor screen to thereby display a desired picture image. The route of each electron beam is varied with the axes of north and south poles of the earth, due to geomagnetism. The electron beam is influenced by a purity characteristic, a raster position, and a convergence characteristic.
The geomagnetic field is divided into a vertical force (the vertical geomagnetic field) perpendicular to the earth's surface, and a horizontal force (the horizontal geomagnetic field) parallel to the earth's surface. The geomagnetic field involves different values depending upon the location thereof on the earth. With the cathode ray tube, the movement of the electron beam due to the horizontal geomagnetic field may be shown to be divided into an NS movement factor and an EW movement factor, with respect to the cathode ray tube axis.
The NS movement refers to the movement of the electron beam due to the horizontal geomagnetic field corresponding to the tube axis of the cathode ray tube, while the EW movement refers to the movement of the electron beam due to the horizontal geomagnetic field perpendicular to the tube axis of the cathode ray tube.
The amount of variation in the landing position of the electron beam displayed at the screen under the influence of the geomagnetic field may be shown to be divided into a horizontal component and a vertical component.
Both with a shadow mask for a color picture tube for public use with a longitudinal slot in the vertical direction, and a shadow mask for a color display tube for industrial use with dot-type holes, the electron beam becomes distant from the designated slot or hole due to the horizontal movement component thereof. Accordingly, it is critical to prohibit the horizontal movement component.
Typically an inner shield is mounted within the cathode ray tube to reduce the amount of variation in the landing position of the electron beam due to the geomagnetic field. In the case of a cathode ray tube with a mask formed by way of press-forming (referred to hereinafter as the formed mask cathode ray tube), the inner shield is fabricated using a high magnetic permeable material, thereby reducing the movement scale of the electron beam due to the geomagnetic field.
As shown in
FIG. 1A
, in the case a cylindrical shield
110
is made using two materials
112
bearing the same magnetic permeability, the magnetic force line of the geomagnetic field passes through the inside of the materials
112
as indicated by the arrows. Consequently, the internal magnetic field of the shield
110
is stabilized.
By contrast, as shown in
FIG. 1B
, in the case a cylindrical shield
110
′ is made using a high magnetic permeability material
112
and a low magnetic permeability material
114
(the permeability thereof being {fraction (1/10)} of the high magnetic permeability material), leakage of the magnetic field occurs at the interface
116
between the two materials
112
and
114
as indicated by the arrows. Consequently, the internal magnetic field of the shield
110
′ becomes non-uniform while reducing the shielding effect.
In the case of a formed mask cathode ray tube, the initial magnetic permeability &mgr;
0.35
of the mask is about 600, and the maximum magnetic permeability &mgr;
max
thereof is about 3200. The material for the mask frame used to hold the formed mask has an initial magnetic permeability &mgr;
0.35
of 800 or more, and the maximum magnetic permeability &mgr;
max
thereof is 4000-8000.
The initial magnetic permeability is a value measured at a magnetic flux density of 350 mG.
Therefore, when the material for the inner shield has the same magnetic permeability as the mask frame, as shown in
FIG. 1A
, the magnetic force flows smoothly while increasing the shielding efficiency, as with the case where the two materials
112
bearing the same magnetic permeability are coupled to each other. In this way, the movement scale of the electron beam is reduced.
Korean Patent Publication Nos. 1998-077085 to 077088 and 1999-026171 disclose a method of improving the magnetic permeability of the inner shield material by way of heat treatment.
However, with a cathode ray tube using a tension mask (referred to hereinafter as the tension mask cathode ray tube), as the tension mask and the mask frame for holding the mask must bear a high force, the magnetic permeability of the material for the tension mask and the mask frame is less than that of the formed mask cathode ray tube.
Table 1 illustrates the magnetic characteristics of a usual tension mask and a mask frame. The magnetic characteristics are measured under a condition such that the tension mask and the mask frame are blackened at 460° C., the tension mask is tensioned in the uni-axial or bi-axial directions with a force of 15 kgf/mm
2
or more, and they are mounted within the cathode ray tube. In Table 1, Hc indicates the coercive force, and Br indicates the residual magnetic flux density.
TABLE 1
Remark
Magnetic characteristic
Material
&mgr;
0.35
&mgr;
max
Hc (Oe)
Br (kG)
Br/Hc
name
Tension
120
 970
4.9
9.7
1.98
NSF
mask
Mask
150
1080
5.4
11.6
2.15
SCM415
frame
As illustrated in Table 1, the tension mask and the mask frame exhibit a magnetic permeability &mgr; of about 20% of the relevant parts of the formed mask cathode ray tube.
Accordingly, when a high magnetic permeable inner shield is mounted to a tension mask cathode ray tube, the phenomenon illustrated in
FIG. 1B
occurs. That is, with the use of a high magnetic permeable inner shield, a low magnetic permeable tension mask, and a mask frame, the shielding effect of the inner shield may be satisfactorily produced, but variations in the local magnetism distribution occur due to leakage of the magnetic field at the portion where the inner shield is welded to the frame. Consequently, the movement scale of the electron beam is increased.
For this reason, in the case of a tension mask cathode ray tube, another characteristic value is required in order to select the inner shield material in addition to the magnetic permeability &mgr;, which is the characteristic value used in the formed mask cathode ray tube.
U.S. Pat. No. 5,871,851 discloses that the value of multiplying the coercive force Hc by the residual magnetic flux density Br is taken as the characteristic value for discriminating the desired inner shield material. With the inner shield formed using a material bearing the predetermined specific value (Hc×Br), the movement of the geomagnetic field can be minimized in the tension mask cathode ray tube. Preferably, the specific value of Hc×Br is established to be 28 or more.
However, according to the experiments of the present inventor, as shown in
FIG. 2
, in the case the specific value of Hc×Br is 60 or more, the movement scale of the electron beam is reduced with the increase in the specific value of Hc×Br. By contrast, in the case the specific value of Hc×Br is 80 or more, the movement scale of the electron beam is rather enlarged. In
FIG. 2
, the solid line indicates the value of 0.5(NS+EW), and the dotted line indicates the value of NS+EW.
As indicated in
FIG. 2
by the solid line, the value of 0.5(NS+EW) indicates the value of NS+EW at the location where the target moves by ½ the distance vertically proceeding from the diagonal end

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