JP2002369213A - Cathode ray tube - Google Patents

Abstract

(57) [Problem] To provide a cathode ray tube capable of extremely reducing the mislanding of an electron beam caused by terrestrial magnetism. An internal magnetic shield (4) disposed in a funnel (9) and a geomagnetic correction coil (7) disposed on an outer peripheral portion of the funnel (9) near a fluorescent screen (11), and a pair of upper and lower shield walls of the internal magnetic shield (4) The portion 4a is formed with a magnetic passage portion 13 that allows the magnetic field lines generated by the terrestrial magnetism correction coil 7 to enter the electron beam scanning area inside the tube, and the magnetic passage portion 13 is formed of the shield wall portion. The degree of magnetic passage at the center in the width direction of 4a is large and becomes small at the ends.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cathode ray tube having an internal magnetic shield and a terrestrial magnetism correction coil, and correcting a shift of an electron beam caused by terrestrial magnetism by a magnetic field generated by the terrestrial magnetism correction coil.

[0002]

2. Description of the Related Art A cathode ray tube used for a color television receiver, a display for a computer or the like scans an electron beam emitted from an electron gun along a fluorescent screen by a magnetic field of a deflection yoke. For this reason, the electron beam passing through the electron beam scanning area inside the tube is easily affected by an external magnetic field such as terrestrial magnetism.In order to suppress the effect of the external magnetic field, the cathode ray tube has an internal magnetic shield inside the funnel. Also, a geomagnetic correction coil is provided on the outer peripheral portion of the funnel near the fluorescent screen.

However, when the terrestrial magnetism that does not intersect with the internal magnetic shield, that is, the terrestrial magnetism component in the tube axis direction of the cathode ray tube acts on the cathode ray tube, even if a correction magnetic field is generated from the terrestrial magnetism correction coil, the width in the width direction at the upper and lower portions of the screen is reduced. At the center, as shown by the arrow in FIG. 9, the electron beam receives an extra deflection force leftward or rightward, and as a result, the electron beam reaches a position shifted from a predetermined position on the phosphor screen. In other words, so-called mislanding occurs, and particularly, a shift in the horizontal direction causes deterioration in image quality such as color unevenness.

In general, the displacement of the electron beam caused by the geomagnetic component in the tube axis direction is caused by the upper and lower portions of the screen (hereinafter referred to as “NS section”).
Also called. ), The center of the width direction is larger than the corner side, and conversely, the sensitivity of the correction magnetic field from the terrestrial magnetism correction coil is higher at the corner side in the width direction than at the center side in the screen NS, and the above-mentioned mislanding is considered to occur. ing.

In order to suppress this mislanding, as described in Japanese Patent Application Laid-Open No. 9-233486, the portion of the geomagnetic correction coil corresponding to the center in the width direction of the screen NS portion is curved. A technique has been proposed in which a correction magnetic field generated by a correction coil is strengthened at the center of the screen NS in the width direction. According to this technology, the magnetic force density of the correction magnetic field from the terrestrial magnetism correction coil can be increased at the center in the width direction of the screen NS, so that mislanding of the electron beam generated at the center of the screen NS in the width direction is reduced. can do.

[0006]

However, in recent years,
There is a particularly strong demand for higher definition of images, and it is required that the tolerance of electron beam mislanding is extremely small, and the above technology cannot cope with this high definition. Even if you use
There is a problem that mislanding of the electron beam due to a geomagnetic component in the tube axis direction remains.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has been made in consideration of the above-described problems.
An object of the present invention is to provide a cathode ray tube capable of extremely reducing the mislanding of an electron beam due to terrestrial magnetism without curving a portion corresponding to the center in the width direction of the S portion.

[0008]

In order to achieve the above object, a cathode ray tube according to the present invention comprises an internal magnetic shield disposed in a funnel and a geomagnetic correction disposed in an outer peripheral portion of the funnel near the fluorescent screen. And a coil for correcting a shift of an electron beam caused by terrestrial magnetism by a magnetic field generated from the terrestrial magnetism correction coil, wherein the terrestrial magnetism correction coil is generated on a pair of upper and lower shield wall surfaces of the internal magnetic shield. A magnetic passage is formed to allow the lines of magnetic force to penetrate into the electron beam scanning area inside the tube, and the magnetic passage has a greater degree of magnetic passage at the center in the width direction of the shield wall portion, and smaller at the ends. It is characterized by the following configuration.

According to this configuration, the amount of correction for correcting the deviation of the electron beam caused by the terrestrial magnetism by the magnetic field from the terrestrial magnetism correction coil can be increased at the center in the width direction of the wall surface of the magnetic shield. Further, the magnetic passage portion is a hole formed in each shield wall surface portion, and the degree of magnetic passage is the opening degree of the hole, so that the degree of magnetic passage at the center in the width direction of the shield wall surface portion is larger at the end. The degree of magnetic passage can be small.

[0010] Further, since the magnetic passage portion is formed near the position where the coil is provided along the longitudinal direction of the geomagnetic correction coil, the magnetic field generated from the geomagnetic correction coil can be effectively used. Further, since the magnetic passage portion is made of a material having a lower magnetic permeability than the remaining portion of the internal magnetic shield, the magnetic passage degree at the center in the width direction of the shield wall surface portion is larger, and the magnetic passage degree is larger at the ends. Can be small.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Embodiment) Hereinafter, an embodiment of a cathode ray tube according to the present invention will be described with reference to the drawings. <Overall configuration of cathode ray tube> FIG. 1 shows a cathode ray tube 1 according to the present invention.
FIG. The cathode ray tube 1 is for a color television receiver, and as shown in the figure, includes a hollow glass bulb 2 in which an electron gun 3, an internal magnetic shield 4, a shadow mask 5, and the like are provided. A deflection yoke 6, a geomagnetic correction coil 7, and the like are provided outside the glass bulb 2.

The glass bulb 2 has a substantially rectangular face panel 8, a substantially funnel-shaped funnel 9 having one end joined to the outer peripheral edge of the face panel 8, and a substantially cylindrical shape joined to the other end of the funnel 9. And a neck portion 10 having a vacuum shape. Red, green, and blue phosphors are applied to a face 8 a of the face panel 8 on the funnel 9 side to form a phosphor screen 11.

The electron gun 3 emits an electron beam EB toward the face panel 8 and is provided in the neck 10. The deflection yoke 6 scans the electron beam EB emitted from the electron gun 3 over substantially the entire area from the upper left end to the lower right end of the phosphor screen 11, and is composed of a horizontal deflection coil 6a and a vertical deflection coil 6b. I have. The deflection coils 6a and 6b are attached to the funnel 9 near the neck 10.

Here, the top, bottom, left and right are face panels 8
The vertical direction is also referred to as the vertical direction, and the horizontal direction is also referred to as the horizontal direction. Further, a direction perpendicular to the surface of the face panel 8 (a direction orthogonal to the vertical direction and the horizontal direction) is referred to as a tube axis direction. The shadow mask 5 is a thin metal plate provided substantially in parallel with the face panel 8 at a predetermined interval. The shadow mask 5 has a plurality of apertures that are transmission portions of the electron beam EB.

The internal magnetic shield 4 is for suppressing the influence of geomagnetism or the like on the electron beam EB. The internal magnetic shield 4 surrounds the fluorescent screen 1 so as to surround the electron beam EB emitted from the electron gun 3.
Hollow polygonal pyramid shape (for example, quadrangular pyramid shape) expanding to one side
You are. The details of the internal magnetic shield 4 will be described later. The geomagnetic correction coil 7 corrects the deviation of the electron beam EB due to the geomagnetism, and its coil wire is wound around the outer periphery of the funnel 9 near the fluorescent screen 11 substantially in parallel with the face panel 8, and is substantially rectangular when viewed from the front. It has a shape. Here, in the geomagnetic correction coil 7, a portion located above the funnel 9 is referred to as a coil upper portion 7a,
The part located on the lower side is called the coil lower part 7b.

<Structure of Internal Magnetic Shield> FIG. 2 is a perspective view showing the internal magnetic shield 4 of the cathode ray tube 1 according to the present invention. The internal magnetic shield 4 is assembled from a pair of upper, lower, left and right shield walls as shown in FIG.
Each shield wall extends from the vicinity of the face panel 8 in the funnel 9 to the vicinity of the deflection yoke 6, and is formed of a ferromagnetic material.

On the upper and lower shield wall portions 4a, there are formed magnetic passing portions 13 which allow the lines of magnetic force of the correction magnetic field BC generated by the geomagnetic correction coil 7 to enter the electron beam scanning area inside the cathode ray tube 1. I have. Magnetic passage 13
Are formed near the coil upper part 7a and the coil lower part 7b of the terrestrial magnetism correction coil 7 along the longitudinal direction (left-right direction) of the coil upper part 7a and the coil lower part 7b.

The magnetic passage portion 13 is configured such that the degree of magnetic passage at the central portion in the width direction (lateral direction) of the upper and lower shield wall portions 4a is large, and the degree of magnetic passage is smaller at the ends. The magnetic passage portion 13 is, specifically, a hole formed in the upper and lower shield wall portions 4a, and the degree of magnetic passage increases and decreases according to the degree of opening of the hole. In other words, the magnetic passage portion 13 is formed in a shape elongated in the width direction extending from the center in the width direction of the upper and lower shield wall portions 4a to both left and right sides thereof, and has a degree of opening in the width direction of the upper and lower shield wall portions 4a. It becomes smaller as it moves from the approximate center to both ends.

FIG. 3 is a plan view of the internal magnetic shield 4. This internal magnetic shield 4 is for the cathode ray tube 1 having a 21 inch and a total deflection angle of 90 degrees. As shown in the drawing, the magnetic passage portion 13 is located at the center in the width direction of the upper and lower shield wall portions 4a and has a constant opening degree (substantially rectangular shape) whose dimension in the tube axis direction hardly changes even if it moves in the width direction. Shape) and an opening decreasing portion (the end portion in the width direction is cut off) which is located on the left and right sides of the fixed opening degree portion and whose dimension in the tube axis direction gradually decreases as moving from the center to the end in the width direction. (A right triangle).

(Dimensions of Magnetic Passing Portion) 1) When Aspect Ratio = 4: 3 A width W of the magnetic passing portion 13 in the width direction on the fluorescent screen 11 side.
1 is about 57% of the width W of the internal magnetic shield 4 on the fluorescent screen 11 side, and the width W2 on the electron gun 3 side is about 60% of W1. It is about. In this embodiment, W = 460 mm, W1 =
260 mm and W2 = 160 mm.

The dimension L1 in the tube axis direction at the center in the width direction of the magnetic passage portion 13 is about 1/4 of the dimension L of the internal magnetic shield 4 in the tube axis direction. The dimension L2 in the tube axis direction at both ends in the width direction is about 1/12 of L. In this embodiment, L = 120 mm, L1 = 30 mm, L2 = 10 mm
It has become.

2) When the aspect ratio is 16: 9: The width W of the magnetic passage portion 13 in the width direction on the fluorescent screen 11 side.
1 is preferably about 3/4 of the width W of the internal magnetic shield 4 in the width direction on the fluorescent screen 11 side.
The dimension W2 of the side in the width direction is preferably about 1/2 of W. <Operation of Magnetic Passing Portion> (Regarding Geomagnetic Components in the Tube Axial Direction) FIG.
FIG. 4 is an explanatory view showing a state of a geomagnetic component B1 in the tube axis direction and a correction magnetic field BC generated by a geomagnetic correction coil 7 in a side sectional view of FIG.

1) Regarding Geomagnetic Components in the Tube Axial Direction When the geomagnetic components B1 in the tube axial direction enter the electron beam scanning area inside the tube from the face panel 8 side, they are attracted to the shield walls made of ferromagnetic material. That is, the geomagnetic component B1 near the upper shield wall is attracted to the upper shield wall, and the geomagnetic component B1 near the lower shield wall is attracted to the lower shield wall.

At this time, the vertical component of the terrestrial magnetic component B1 attracted to the upper and lower shield walls is larger than the corner side since the center in the width direction is not affected by the left and right shield walls. Therefore, as shown in FIG. 5A, the electron beam EB is shifted leftward in the NS portion of the screen due to the influence of the geomagnetic component B1 drawn to the upper and lower shield walls, and the amount of the shift is equal to the screen NS portion. Is larger on the center side in the width direction than on the corner side.

2) Correction Magnetic Field of Geomagnetic Correction Coil As shown in FIG. 4, the geomagnetic correction coil 7 has a concentric circle centered on its axis in a direction to cancel the geomagnetic component B1 drawn toward the upper and lower shield walls. A correction magnetic field BC is generated. At this time, since the magnetic passage portion 13 is formed in the upper and lower shield wall portions 4a of the internal magnetic shield 4, the correction magnetic field BC can enter the electron beam scanning area inside the tube. In particular, since the degree of magnetic passage of the magnetic passage portion 13 is large at the center in the width direction of the upper and lower shield wall portions 4a and small at both ends, the number of magnetic force lines of the correction magnetic field BC penetrating into the electron beam scanning area is set. However, the central portion in the width direction of the screen NS becomes larger than the corner portion at the end in the width direction.

For this reason, the electron beam EB is generated by the correction magnetic field BC generated by the geomagnetic correction coil 7 as shown in FIG.
As shown in (1), the image is corrected rightward in the NS section of the screen, and the correction amount is larger than the corner side at the center in the left-right direction. 3) Correction of Geomagnetic Component in Tube Axial Direction The electron beam EB emitted from the electron gun 3 is shifted to the left as shown in FIG. By the correction magnetic field BC of the correction coil 7,
The correction is made to the right as shown in FIG. For this reason, the influence of the geomagnetic component B1 in the pipe axis direction is
Of the electron beam EB can be extremely reduced as a result.

In particular, the amount of deviation of the electron beam EB due to the geomagnetic component B1 in the tube axis direction and the amount of correction of the electron beam EB due to the correction magnetic field BC of the geomagnetic correction coil 7 are the same in both cases. Since it is larger on the center side in the direction than on the corner side, the influence of the geomagnetic component B1 in the tube axis direction can be reliably corrected. As described above, since the magnetic passage portions 13 are formed in the upper and lower shield wall portions 4a, the portions corresponding to the upper and lower shield walls in the geomagnetic correction coil are not curved as in the related art, but inside the internal magnetic shield 4. The number of lines of magnetic force of the entering correction magnetic field BC can be increased at the center in the width direction. Moreover, since the magnetic passage portion 13 is a hole formed in the upper and lower shield wall portions 4a, it can be implemented more easily than a conventional geomagnetic correction coil.

Further, since the magnetic passage portion 13 is disposed along the coil upper portion 7a and the coil lower portion 7b near the coil upper portion 7a and the coil lower portion 7b of the geomagnetic correction coil 7, Can be used effectively, and the degree of magnetic passage can be easily changed by the degree of opening of the hole of the magnetic passage section 13, and the geomagnetic component B1 of the electron beam EB in the tube axis direction can be easily changed. Can be properly and easily corrected.

Further, since the magnetic passage portion 13 has a shape that gradually extends in the width direction, the number of lines of magnetic force of the terrestrial magnetism correction coil 7 that enters the electron beam scanning area can be gradually increased and decreased in the width direction. The beam EB can be corrected in a stable state in the width direction. (About vertical geomagnetic component) Next, by forming a magnetic passage portion 13 on the upper and lower shield wall portions 4a, a vertical geomagnetic component, that is, a vertical geomagnetic component B2 is formed.
Can pass through the magnetic passage section 13, and its effect will be described below.

FIG. 6 is a side cross-sectional view of the internal magnetic shield 4 and is an explanatory diagram showing a state in which a vertical geomagnetic component B2 passes through the internal magnetic shield 4. FIG. 6 shows that the vertical geomagnetic component B2 acts, for example, upward, and the geomagnetic component B2 enters the inner magnetic shield 4 from the lower magnetic passage portion 13 and exits from the upper magnetic passage portion 13. This shows the way to go.

1) Influence of vertical geomagnetic component When the upward geomagnetic component B2 acts on the cathode ray tube 1,
As shown in FIG. 7A, it is known that the electron beam EB shifts to the left at the NS portion of the screen and shifts to the right at the center between the NS portions of the screen. 2) Influence of terrestrial magnetic component passing inside the inner magnetic shield The upward terrestrial magnetic component B2 enters the inner magnetic shield 4 from the lower magnetic passage part 13 and enters the upper terrestrial magnetic passage part 13.
Go out of.

At this time, in the lower magnetic passage portion 13, the upward terrestrial magnetic component B 2 around the lower magnetic passage portion 13 gathers and enters the internal magnetic shield 4 from the magnetic passage portion 13, so that the magnetic flux lines passing through the lower shield wall are removed. Magnetic density increases,
Similarly, in the upper magnetic passage section 13, the inner magnetic shield 4
Since the upward terrestrial magnetism components B2 near the magnetic passage portion 13 in the inside gather and exit from the magnetic passage portion 13, the magnetic force density of the magnetic force lines passing through the upper shield wall increases. Therefore, the Lorentz force received by the electron beam EB near the upper and lower shield walls, that is, near the screen NS, increases.

On the other hand, at the center between the upper and lower shield walls, the terrestrial magnetic component B2 entering the inner magnetic shield 4 is attracted to the upper, lower, left and right shield walls and is dispersed.
The magnetic force density of the line of magnetic force of the upward geomagnetic component B2 decreases. For this reason, the center electron beam EB between the screen NS portions
The Lorentz force it receives is smaller. Therefore, by forming the magnetic passage portions 13 on the upper and lower shield wall portions 4a, the electron beam EB moves rightward as shown in FIG. 7B, and the amount of movement is large in the NS portion of the screen. , And small at the center between the NS portions of the screen.

3) Influence of terrestrial magnetism passing through the inner magnetic shield As described above, by providing the magnetic passage portions 13 on the upper and lower shield wall portions 4a of the inner magnetic shield 4, the influence of the terrestrial magnetism component B2 can be reduced. 7C, the influence of the geomagnetic component B2 passing through the magnetic passage portion 13 cancels out each other, and as shown in FIG. 7C, the shift of the electron beam EB at the center of the screen and the periphery thereof, for example, the NS of the screen, The difference from the shift of the electron beam EB on the center side of the portion becomes small. Therefore, a balance can be maintained between the NS section and the center between the NS sections, and a decrease in image quality of the entire screen can be suppressed.

Note that "balanced" means that the difference between the deviations of the electron beam EB at each position on the entire screen is reduced. Also, FIG. 10 is a schematic diagram showing a horizontal shift amount, a correction amount, and a movement amount of the electron beam EB shown in FIGS. 5, 7, and 9, and the size is relatively shown. (Modifications) Although the present invention has been described based on the embodiments, it is needless to say that the contents of the present invention are not limited to the specific examples shown in the above embodiments. Examples can be implemented.

(1) In the above embodiment, the magnetic passage portions 13 of the upper and lower shield wall portions 4a are long in the width direction of the upper and lower shield wall portions 4a as shown in FIG.
That is, it may be an elliptical shape in which the degree of opening becomes smaller as it moves from the center in the width direction to both ends, or may be a rhombic shape or a substantially isosceles triangle with the fluorescent screen side as the base. Furthermore, when the shape is a rhombus or an isosceles triangle, the boundary between the holes may be curved instead of straight.

(2) In the above embodiment, the magnetic passage portion 13 is formed by a single hole that is long in the width direction. However, as shown in FIG. A plurality of narrow rectangular holes are formed in the tube axis direction, and the width dimension of each hole becomes smaller as it moves toward the electron gun 3, so that the magnetic passage degree of the magnetic passage portion 13 at the center in the width direction is reduced. It may be large and the degree of magnetic passage at the end may be small. Instead of a long rectangular shape, a plurality of holes having a circular shape or a polygonal shape may be provided in the width direction.

(3) In the above embodiment, the magnetic passage portion 13 is constituted by a hole.
For example, the opening of the embodiment may be formed by cutting out the face panel 8 side of the hole. (4) In the above-described embodiment, the magnetic passage portion 13 is constituted by a hole. However, the magnetic passage portion 13 is constituted by a material having a smaller magnetic permeability than the material of the remaining portion of the internal magnetic shield 4, so that geomagnetic The correction magnetic field BC generated from the correction coil 7 may be allowed to pass through the magnetic passage 13 and enter the electron beam scanning area.

In this case, in order to reduce the degree of magnetic passage at the center of the shield wall 4a in the width direction and increase the width at the ends, the area of the magnetic passage 13 is reduced at the center of the shield wall 4a in the width direction. It can be implemented by reducing the thickness at the end in the width direction, or by reducing the thickness of the magnetic passage portion 13 at the center in the width direction of the shield wall portion 4a and decreasing the thickness at the end in the width direction.

[0040]

As described above, according to the present invention, there is provided an internal magnetic shield disposed in a funnel, and a geomagnetic correction coil disposed on an outer peripheral portion of the funnel near the fluorescent screen. Due to the generated magnetic field,
In a cathode ray tube for correcting a displacement of an electron beam caused by terrestrial magnetism, a line of magnetic force generated by the terrestrial magnetism correction coil is allowed to penetrate into an electron beam scanning area inside the tube on a pair of upper and lower shield wall surfaces of the internal magnetic shield. And the magnetic passage portion is configured such that the degree of magnetic passage at the center in the width direction of the shield wall surface portion is large and becomes small at the ends, and thus the magnetic passage portion is formed by the magnetic field from the geomagnetic correction coil. The amount of correction for correcting the displacement of the electron beam caused by terrestrial magnetism can be increased at the center in the width direction of the shield wall portion, and the terrestrial magnetism correction coil does not bend the portions corresponding to the upper and lower shield walls without bending. Can minimize the mislanding of the electron beam caused by the above.

Further, the magnetic passage portion is a hole formed in each of the shield wall surfaces, and the degree of magnetic passage is the opening degree of the hole. Compared to the case where the portion is curved, it can be implemented easily and at low cost. Further, since the magnetic passage portion is formed in the vicinity of the position where the coil is provided along the longitudinal direction of the geomagnetic correction coil, the magnetic field generated by the geomagnetic correction coil can be effectively used. In addition, since the magnetic passage portion is made of a material having a lower magnetic permeability than the remaining portion of the internal magnetic shield, it can be implemented easily and inexpensively.

[Brief description of the drawings]

FIG. 1 is a side view in which a part of a cathode ray tube according to an embodiment of the present invention is broken.

FIG. 2 is a perspective view of an internal magnetic shield according to the embodiment of the present invention.

FIG. 3 is a plan view of the internal magnetic shield according to the embodiment of the present invention.

FIG. 4 is a side cross-sectional view of the cathode ray tube according to the embodiment of the present invention, which is an explanatory diagram for explaining a state of a geomagnetic component in a tube axis direction and a correction magnetic field generated from a geomagnetic correction coil.

5A is a diagram showing a shift amount of an electron beam caused by a geomagnetic component in a tube axis direction, and FIG. 5B is a diagram showing a correction amount of an electron beam by a correction magnetic field from a geomagnetic correction coil. .

FIG. 6 is a side sectional view of the internal magnetic shield according to the embodiment of the present invention, which is an explanatory diagram for explaining a state of a geomagnetic component in a vertical direction.

FIG. 7A is a diagram showing a shift amount of an electron beam caused by a vertical geomagnetic component in a cathode ray tube, and FIG.
FIG. 3C is a diagram showing the amount of movement of an electron beam caused by a geomagnetic component passing vertically through a magnetic passage, and FIG. 3C shows a relationship between a vertical geomagnetic component and a geomagnetic component passing vertically through a magnetic passage. FIG. 9 is a diagram showing the amount of movement of the electron beam when the operation is performed.

FIGS. 8A and 8B are plan views of an internal magnetic shield showing a modification of a magnetic passage portion.

FIG. 9 is a diagram showing a shift amount of an electron beam when a terrestrial magnetism component in a tube axis direction and a correction magnetic field from a terrestrial magnetism correction coil act in the related art.

[Explanation of symbols]

 Reference Signs List 1 cathode ray tube 4 internal magnetic shield 4a shield wall 7 geomagnetic correction coil 7a coil upper part 7b coil lower part 11 phosphor screen 9 funnel 13 magnetic passage part

Claims (4)

[Claims] 1. An internal magnetic shield disposed in a funnel, and a geomagnetic correction coil disposed on an outer peripheral portion of the funnel near the fluorescent screen, wherein a magnetic field generated from the geomagnetic correction coil causes an electron generated by geomagnetism. In a cathode ray tube for correcting a beam shift, on a pair of upper and lower shield wall surfaces of the internal magnetic shield,
A magnetic passage is formed to allow the lines of magnetic force generated by the terrestrial magnetism correction coil to enter the electron beam scanning area inside the tube, and the magnetic passage is provided at the center in the width direction center of the shield wall surface. A cathode ray tube characterized in that the degree of passage is large and becomes small at the ends. 2. The cathode ray tube according to claim 1, wherein the magnetic passage portion is a hole formed in each of the shield wall portions, and the degree of magnetic passage is an opening degree of the hole. 3. The cathode ray tube according to claim 1, wherein the magnetic passage portion is formed near a coil disposition position along a longitudinal direction of the geomagnetic correction coil. 4. The cathode ray tube according to claim 1, wherein the magnetic passage portion is made of a material having a lower magnetic permeability than the remaining portion of the internal magnetic shield.