Color cathode ray tube

Abstract: In a color cathode ray tube in which a shadow mask 1 and an inner magnetic shield 2 are supported by a mask frame 31 and an electron shield 33 is provided in the mask frame 31, at least a part of the electron shield 33 is made to have a smaller anhysteretic magnetic permeability than the shadow mask 1, the mask frame 31 and the inner magnetic shield 2 when an applied magnetic field is 800 A/m (10 Oe). Since the magnetic resistance of the electron shield 33 increases, it is possible to reduce a leakage magnetic field from a tube-axis-side edge of the electron shield 33. Thus, a color cathode ray tube that reduces mis-landing due to a terrestrial magnetism and has no color displacement can be provided.

Description: The present invention relates to a color cathode ray tube. More specifically, the present invention relates to a color cathode ray tube characterized by a configuration of a mask frame in order to improve image quality, especially color uniformity. As shown in FIG. 11, a color cathode ray tube has a glass bulb 13 including a front panel, whose inner surface is provided with a phosphor screen 14, and a funnel. In a neck portion of the glass bulb 13, an electron gun 81 is provided. A shadow mask 1 that is stretched by a mask frame 31 faces the phosphor screen 14. The mask frame 31 has a substantially L-shaped cross-section, and includes a first portion and an inward projecting portion 32; the former stretches the shadow mask 1 and is fixed to the glass bulb 13 and the latter projects toward a tube axis (central axis) side of the glass bulb 13 so as to be substantially in parallel to the shadow mask 1. An inner magnetic shield 2 is fixed to the inward projecting portion 32. Electron beams 5 corresponding to three colors of R (red), G (green) and B (blue) are emitted from the electron gun 81 and pass through the shadow mask 1 that is located immediately in front of the front panel. Based on the incident angle at the time of this passage, positions at which the electron beams 5 strike the front panel can be restricted. According to these impact positions, therefore, the phosphors of R, G and B separately are applied on the inner surface of the front panel, thereby performing a color selection geometrically, so as to form color images on the phosphor screen 14. In a regular color cathode ray tube, images are reproduced by an over scan system so that the images are displayed over an entire screen area of the phosphor screen. The amount of this over scan is about 105 to 110 % in each of horizontal and vertical directions of the phosphor screen. When the phosphor screen is scanned with such an over scan system, a part of the over-scanning electron beams 5 hits the mask frame 31 supporting the shadow mask 1 and is reflected so as to reach the phosphor screen 14 as shown in FIG. 12, so that a phosphor layer other than that in a predetermined position emits light. This lowers color purity and contrast of the image, thus deteriorating image quality. In order to prevent the deterioration of the image quality due to this reflected beam, an electron shield 33 conventionally has been formed at a tube-axis-side edge of the inward projecting portion 32 of the mask frame 31 as shown in FIG. 13. Alternatively, as shown in FIG. 14, an electron shield 33 has been provided between the inner magnetic shield 2 and the inward projecting portion 32 of the mask frame 31 so as to protrude beyond the mask frame 31 toward the tube axis side. However, since the electron shield 33 conventionally has been formed of a magnetic substance, when the cathode ray tube is placed in the presence of a terrestrial magnetism of about 800 A/m (10 Oe), a leakage magnetic field from a front end portion of the electron shield 33 sometimes has caused a phenomenon that the electron beam is subjected to a deflection of its path so as not to strike a desired position of the phosphor layer (mis-landing). US-4931690 discloses a color picture tube with an electron shield. It is an object of the present invention to provide a color cathode ray tube that prevents mis-landing due to a terrestrial magnetism and has no color displacement. According to the present invention, there is provided a colour-cathode ray tube as claimed in claim 1. Since this configuration increases the magnetic resistance of the electron shield, magnetic flux flowing toward a front end portion of the electron shield can be suppressed, thereby reducing a leakage magnetic field from the front end portion of the electron shield. Thus, it is possible to provide a color cathode ray tube that reduces the mis-landing due to the terrestrial magnetism and has no color displacement. Furthermore, it is possible to regulate the magnetic flux flowing from the inner magnetic shield via the mask frame toward the front end portion of the electron shield, thereby reducing the leakage magnetic field from the front end portion of the electron shield. Also, it is preferable that the electron shield is formed so as to elongate a front end portion on an electron beam side of the mask frame. Alternatively, it is preferable that the electron shield is formed of a member different from the mask frame so as to protrude beyond a front end portion on an electron beam side of the mask frame. FIG. 1 shows an enlarged cross-section illustrating a main portion of a color cathode ray tube. FIG. 2 shows a concept illustrating an effect of a magnetic field in a conventional electron shield. FIG. 3 shows a concept illustrating the effect of a magnetic field in an electron shield. FIG. 4 shows an enlarged cross-section illustrating a main portion of a color cathode ray tube. FIG. 5 shows a concept illustrating a state of magnetic flux in the conventional electron shield. FIG. 6 shows a concept illustrating the state of magnetic flux in an electron shield. FIG. 7 shows a concept illustrating the state of magnetic flux in an electron shield according to another example. FIG. 8 shows an enlarged cross-section illustrating a main portion of a color cathode ray tube of a preferred embodiment of the present invention. FIG. 9 shows a concept illustrating a state of magnetic flux in an inward projecting portion of a conventional mask frame. FIG. 10 shows a concept illustrating the state of magnetic flux in an inward projecting portion according to the preferred embodiment of the present invention. FIG. 11 schematically shows a cross-section of a color cathode ray tube (device). FIG. 12 shows a concept illustrating a path of an over-scanning electron beam. FIG. 13 shows an enlarged cross-section illustrating a main portion of a conventional color cathode ray tube in the vicinity of an electron shield. FIG. 14 shows an enlarged cross-section illustrating the main portion of the conventional electron shield as another example. The following is a specific description of the embodiments of the present invention. A cathode ray tube of the present invention is characterized by its configuration in the vicinity of a mask frame. Since a basic configuration of the cathode ray tube is the same as that of the conventional cathode ray tube shown in FIG. 11, the description of the general configuration will be omitted in the following. Instead, a main portion in the vicinity of the mask frame will be described in detail. FIG. 1 shows an enlarged cross-section of the vicinity of a mask frame 31 in a color cathode ray tube. The mask frame 31 has a substantially L-shaped cross-section, and includes a first portion and an inward projecting portion 32; the former stretches a shadow mask 1 and is fixed to a glass bulb 13 (a fixture is not shown in this figure) and the latter projects toward a tube axis (central axis) side of the glass bulb 13 so as to be substantially in parallel to the shadow mask 1. An inner magnetic shield 2 is fixed to the mask frame 31 (a fixture provided in the inward projecting portion 32 is not shown in this figure). The tube-axis-side edge of the inward projecting portion 32 is provided with a belt-like electron shield 33 having substantially the same thickness as the inward projecting portion 32 in such a manner as to extend the inward projecting portion 32 along its entire length. An entirety or a part of the electron shield 33 has a smaller anhysteretic magnetic permeability than the shadow mask 1, the mask frame 31 and the inner magnetic shield 2 when an applied magnetic field is 800 A/m (10 Oe) (corresponding to a terrestrial magnetism). "The anhysteretic magnetic permeability" refers to an effective relative magnetic permeability that can be defined by a magnetic flux density B and a direct current magnetic field H at a convergent point on a hysteresis, which is generated by an anhysteretic magnetization model, when a decaying alternating current magnetic field is reduced to zero. The anhysteretic magnetic permeability is expressed by the following equation. µ µ = (1/ µ 0 ) × (B/ H) where µ 0 represents a magnetic permeability in a vacuum. The anhysteretic magnetic permeability is described, for example, in The Institute of Electronics, Information and Communication Engineers Transactions C-II, Vol. J79-C-II, No. 6, pp.311-319, June 1996. FIGs. 2 and 3 show an effect of a magnetic field in the mask frame 31. FIG. 2 shows a conventional example, which has the electron shield that is formed integrally with the inward projecting portion 32 at the tube-axis-side edge thereof. This electron shield has the same anhysteretic magnetic permeability as the inward projecting portion 32. FIG. 3 shows a configuration of the present arrangement. Arrows 61 and 62 indicate the state of a leakage magnetic field from the electron shield provided in the inward projecting portion 32 of the mask frame 31. The thickness of these arrows corresponds to the intensity of the leakage magnetic field. In the conventional example of FIG. 2, magnetic flux flowing via the inner magnetic shield 2 into the mask frame 31 leaks from the inward projecting portion 32 toward the shadow mask 1 in a vacuum (the leakage magnetic field 61). On the other hand, in the present arrangement shown in FIG. 3, since at least a part of the electron shield 33 provided at the tube-axis-side edge of the inward projecting portion 32 has a smaller anhysteretic magnetic permeability than the shadow mask 1, the mask frame 31 and the inner magnetic shield 2 when the applied magnetic field is 800 A/m (10 Oe), the magnetic resistance between the electron shield 33 and the shadow mask 1 rises, thus reducing the leakage magnetic field 62. Consequently, mis-landing can be reduced. Members having different anhysteretic magnetic permeability can be fixed to each other by welding, screwing or by using a clamping spring. In FIG. 1, the electron shield 33 is fixed at a certain angle with respect to the inward projecting portion 32. With a suitable angle, it is possible to restrict a path of the electron beam that hits the electron shield 33 and is reflected, thereby preventing the generation of halation. When the applied magnetic field is 800 A/m (10 Oe), the anhysteretic magnetic permeability of a material used for the inner magnetic shield 2 was about 12,000 (soft iron), that for the mask frame 31 was about 2,200 (Fe-36Ni, Fe-42Ni or the like), that for the shadow mask 1 was about 2,000 (Fe-36Ni or the like heat-treated at about 570 to 640 °C), and that for the electron shield 33 was about 1,800 (iron). The anhysteretic magnetic permeability of about 1,800 was obtained by heat-treating an iron material (Fe-36Ni) used for the shadow mask at a relatively low temperature (equal to or lower than 450 °C). When the electron shield 33 was formed so as to protrude by 20 mm from the tube-axis-side edge of the inward projecting portion 32, the mis-landing was reduced by 2 µm or more compared with the case of FIG. 2 in which the inward projecting portion 32 was extended by the same amount. Other than the above materials, stainless steel (SUS) or aluminum can be used as the material for the electron shield 33. The anhysteretic magnetic permeability of these materials is about 1 when the applied magnetic field is 800 A/m (10 Oe). As shown in FIG. 4, in another arrangement, an electron shield 33 formed of a sheet with a thickness of about 0.1 to 0.3 mm is provided on an electron-gun-side surface of an inward projecting portion 32 of a mask frame 31. The electron shield 33 extends substantially over the entire length of the inward projecting portion 32 so as to protrude beyond a tube-axis-side edge of the inward projecting portion 32 by about 30 mm toward the tube axis side. The material of the electron shield 33 is soft iron, which is the same as that of the inner magnetic shield 2. The front end portion on the tube axis side of the electron shield 33 is bent slightly toward the electron gun side, thereby preventing the generation of halation. The anhysteretic magnetic permeability when an applied magnetic field is 800 A/m (10 Oe) is not uniform throughout the electron shield 33, that is, the anhysteretic magnetic permeability in one part 8 is smaller than that in the other part. In the present embodiment, instead of providing a member made of a specific material in the one part 8, the one part 8 of the electron shield 33 is formed to be an aperture (a rectangular hole). FIG. 5 shows a state of magnetic flux in the conventional electron shield 33, and FIG. 6 shows that in the electron shield 33 of the present embodiment, both seen from the electron gun side. In the conventional example of FIG. 5, the electron shield 33 has no aperture and an anhysteretic magnetic permeability that is uniform throughout its entire area. FIG. 6 shows the present embodiment, whose configuration is the same as that of FIG. 5 except that the aperture 8 is formed. In FIGs. 5 and 6, the state of the magnetic flux in an upper long side alone is shown for simplification of the figure. In the configuration of the conventional example shown in FIG. 5, the magnetic flux flowing in the electron shield 33 leaks from the electron shield 33 toward the shadow mask 1 in a vacuum. Arrows in the figures indicate the state of the magnetic flux flowing in the electron shield 33 and a leakage magnetic field 61 from the electron shield 33. On the other hand, in the present invention shown in FIG. 6, the magnetic flux flowing from the inner magnetic shield 2 toward a front end of the electron shield 33 (indicated by the arrows in the figure) is regulated by the aperture 8, thereby making it possible to reduce the magnetic flux flowing on the tube axis side (inner side) with respect to the aperture 8 of the electron shield 33. Consequently, a leakage magnetic field 62 from the front end portion of the electron shield 33 can be reduced compared with the conventional configuration (FIG. 5), thus reducing mis-landing. In the present embodiment, when a rectangular aperture 8 having a width of 2 mm and a length of 25 mm was provided at a distance of 5 mm from an inner edge of the electron shield 33 having a width of 40 mm, the mis-landing on the screen was reduced by 2 µm or more. The anhysteretic magnetic permeability of the aperture 8 is about 1. Also, when an L-shaped aperture 8 having a width of 2 mm was provided at a comer of the electron shield 33 as shown in FIG. 7, the mis-landing at the corner of the screen was reduced by 2 µm or more. Instead of leaving the aperture 8 open, the aperture 8 may be sealed with a material with a smaller anhysteretic magnetic permeability than the shadow mask 1, the mask frame 31 and the inner magnetic shield 2 when the applied magnetic field is 800 A/m (10 Oe). For such a material, the material used for the electron shield 33 in the arrangement described in relation to Fig. 1 can be used, for example. The member or the aperture having a small anhysteretic magnetic permeability may be provided in a suitable size and in a suitable number at a place where it is desired to reduce the leakage magnetic field. Although FIGs. 5 to 7 showed the magnetic flux flowing horizontally in the electron shield 33, the present embodiment also produces effects similar to the above with respect to magnetic flux flowing in the other directions. As shown in FIG. 8, according to the preferred embodiment, a belt-like electron shield 33 having substantially the same thickness as an inward projecting portion 32 is provided at a tube-axis-side edge of the inward projecting portion 32. The electron shield 33 extends substantially over the entire length of the inward projecting portion 32 so as to elongate the inward projecting portion 32. The material of the electron shield 33 is Fe-36Ni, Fe-42Ni or the like, which is the same as that of a mask frame 31. One part 9 of the electron shield 33 has a smaller anhysteretic magnetic permeability than the other part of the electron shield 33 when an applied magnetic field is 800 A/m (10 Oe) (corresponding to a terrestrial magnetism). More specifically, the one part 9 is formed to have apertures by providing a plurality of holes. FIG. 9 shows a state of magnetic flux in the inward projecting portion 32 and the electron shield 33 of the conventional example, and FIG. 10 shows that in the inward projecting portion 32 and the electron shield 33 of the present embodiment, both seen from the electron gun side. In the conventional example of FIG. 9, the electron shield 33 has a uniform anhysteretic magnetic permeability in its entire region. FIG. 10 shows a configuration of the present embodiment, which is the same as that of FIG. 9 except that the apertures 9 are formed in the electron shield 33. Although the electron shield 33 provided in an upper long side alone is shown in FIGs. 9 and 10 for a simplification of the figure, the electron shield 33 actually is provided along the entire perimeter of the tube-axis-side edge of the inward projecting portion 32. Also, FIGs. 9 and 10 show the state of the magnetic flux in the upper long side alone. In the configuration of the conventional example shown in FIG. 9, the magnetic flux flowing in the inward projecting portion 32 leaks from the electron shield 33 toward the shadow mask 1 in a vacuum. Arrows in FIG. 9 indicate the magnetic flux flowing in the inward projecting portion 32 and the electron shield 33 and the leakage magnetic field 61 from the electron shield 33. On the other hand, in the present invention shown in FIG. 10, one part on the long side of the electron shield 33 is provided with a plurality of the apertures (holes) 9, which have a smaller anhysteretic magnetic permeability than the other part when the applied magnetic field is 800 A/m (10 Oe). This part having a smaller anhysteretic magnetic permeability (the apertures 9) regulates the magnetic flux flowing from the inner magnetic shield 2 via the mask frame 31 toward a front end of the electron shield 33, thereby reducing the magnetic flux flowing on the tube axis side with respect to the part having a smaller anhysteretic magnetic permeability. Consequently, a leakage magnetic field 62 from the front end portion of the electron shield 33 can be reduced compared with the conventional configuration (FIG. 9), thus reducing mis-landing. In the present embodiment, when a circular aperture 9 having a diameter of 8 mm was provided in four places in the vicinity of the center of the long side of the electron shield 33, the mis-landing on the screen was reduced by 2 µm or more. The number, position and shape of the apertures 9 may be selected suitably according to purposes. Instead of leaving the aperture 9 open, the aperture 9 may be sealed with a material with a smaller anhysteretic magnetic permeability than the shadow mask 1, the mask frame 31 and the inner magnetic shield 2 when the applied magnetic field is 800 A/m (10 Oe). For such a material, the material used for the electron shield 33 in the first embodiment can be used, for example.

Claims: A color cathode ray tube comprising: a mask frame (31); a shadow mask (1) fixed to the mask frame (31); an inner magnetic shield (2) supported by the mask frame (31); and an electron shield (33) provided in the mask frame (31), wherein at least a part of the electron shield (33) has a smaller anhysteretic magnetic permeability than the shadow mask (1), the mask frame (31) and the inner magnetic shield (2) when an applied magnetic field is 800 A/m (10 Oe); characterised in that a part of the electron shield (33) has a region having a smaller anhysteretic magnetic permeability than another part of the electron shield (33) when the applied magnetic field is 800 A/m (10 Oe). The color cathode ray tube according to claim 1, wherein the electron shield (33) is formed so as to elongate a front end portion (32) on an electron beam side of the mask frame (31). The color cathode ray tube according to claim 1, wherein the electron shield (33) is formed of a member different from the mask frame (31) so as to protrude beyond a front end portion (32) on an electron beam side of the mask frame (31).

Inventors: SHIMADA KOJI, WAKASONO HIROMI, SHIMADA, KOJI, WAKASONO, HIROMI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 2001
Main classification: H01J29
IPC codes: H01J29