Patent Publication Number: US-6339282-B2

Title: Color cathode-ray tube having internal magnetic shield

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a continuation of U.S. application Ser. No. 09/466,856, filed Dec. 20, 1999, now U.S. Pat. No. 6,177,758, issued Jan. 23, 2001, which is a continuation of U.S. application Ser. No. 08/950,663, filed Oct. 15, 1997, now U.S. Pat. No. 6,020,678, issued Feb. 1, 2000, the subject matter of which is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a color cathode-ray tube having an internal magnetic shield, and more specifically to a color cathode-ray tube having an internal magnetic shield which is so constructed that an electron beam is less affected by external magnetic field such as terrestrial magnetism from the time it is emitted from an electron gun to the time it strikes a fluorescent layer through a shadow mask so as to provide a display image of high color purity. 
     DESCRIPTION OF THE RELATED ART 
     A color cathode-ray tube generally has an evacuated glass envelope (bulb) comprising a panel portion located at the front and having a face plate of large diameter, a neck portion of small diameter located at the rear, and a substantially funnel-shaped funnel portion connecting the panel portion and the neck portion. In the panel portion, a fluorescent layer is formed on an inner surface of the face plate by coating, and a shadow mask having a large number of electron beam apertures is placed opposite to the fluorescent layer. The neck portion houses an electron gun which emits three electron beams. In the funnel portion, an internal magnetic shield made of a substantially quadrangular pyramid-shaped frame structure is disposed inside the color cathode-ray tube, while a deflection coil is disposed outside the same tube. 
     In this case, the internal magnetic shield is disposed for the purpose that three electron beams emitted from the electron gun are prevented from being affected by terrestrial magnetism. If the internal magnetic shield does not have a sufficient effect of shielding terrestrial magnetism, the three electron beams are affected by terrestrial magnetism to be caused to slightly deviate from the original electron beam path, with the result that the display image of the color cathode-ray tube is deteriorated in color purity and suffered from color contamination. 
     FIGS. 5A to  5 C show an example of construction of a conventional internal magnetic shield used in a known color cathode-ray tube, and FIG. 5A is a perspective view, FIG. 5B is a top view and FIG. 5C is a side view. 
     As shown in FIGS. 5A to  5 C, a known internal magnetic shield is made of a substantially quadrangular pyramid-shaped frame member  40  made up of two long side walls  41 A,  41 B and two short side walls  42 A,  42 B. The internal magnetic shield has a substantially rectangular first opening  43  with a smaller diagonal dimension at one end adjacent to an electron gun and than that of a larger diagonal dimension of a substantially rectangular second opening  44  at the other end adjacent to a shadow mask. The two long side walls  41 A,  41 B are formed in the portions thereof adjacent to the first opening  43  with substantially V-shaped notches  43 A,  43 B having a maximum depth  c ′, respectively. 
     When the frame member  40  is disposed inside the funnel portion, an edge portion  45  of the second opening  44  is fitted to a support frame mounted on the side wall of the panel portion together with the peripheral portion of the shadow mask. In this case, the substantially rectangular first opening  43  of smaller diagonal dimension faces an electron gun and the substantially rectangular second opening  44  of larger diagonal dimension faces the shadow mask so as to allow three electron beams emitted from the electron gun to pass through the inside of the frame member  40  and strike a fluorescent layer through one of electron beam apertures of the shadow mask. 
     In the meantime, the substantially V-shaped notches  43 A,  43 B formed in the two long side walls  41 A,  41 B are provided for regulating the path for the electron beam passing through the inside of the frame member  40 . By selecting the maximum depth  c ′ of the substantially V-shaped notches  43 A,  43 B, the amount of terrestrial magnetism converging on the two long side walls  41 A,  41 B and the two short side walls  42 A,  42 B is controlled. Incidentally, the substantially V-shaped notches  43 A,  43 B may be formed in the two short side walls  42 A,  42 B instead of being formed in the two long side walls  41 A,  41 B, in which case the same performance can be attained as well. 
     In such internal magnetic shield, however, if the maximum depth  c ′ of the substantially V-shaped notches  43 A,  43 B is increased for the purpose of appropriate regulation of the electron beam path, although the electron beam path can be regulated, there arises a problem that the effective area of the two long side walls  41 A,  41 B or the two short side walls  42 A,  42 B is reduced correspondingly to an increment of depth of the substantially V-shaped notches  43 A,  43 B, resulting in deterioration of the total shielding effect of the internal magnetic shield. 
     The present invention aims to solve the above problem. 
     It is an object of the present invention to provide a color cathode-ray tube having an internal magnetic shield which is capable of appropriately regulating an electron beam path even if the maximum depth of a substantially V-shaped notch is made small lest a total shielding effect should be deteriorated. 
     SUMMARY OF THE INVENTION 
     To achieve the above object, there is provided according to the present invention a color cathode-ray tube having an internal magnetic shield, which comprises at least a fluorescent layer formed on an inner surface of a face plate of a panel portion, a shadow mask disposed opposite to the fluorescent layer, an electron gun housed in a neck portion, and the internal magnetic shield disposed in a funnel portion and made of a substantially quadrangular pyramid-shaped frame member which has a substantially rectangular first opening of small diagonal dimension at one end adjacent to the electron gun and a substantially rectangular second opening of large diagonal dimension at the other end adjacent to the shadow mask, and creased lines formed between corresponding corners of the first and second openings, wherein each of the creased lines of the internal magnetic shield is formed in such a manner that an end of an imaginary line extension of the creased line adjacent to the second opening is located on a projected plane parallel to the second opening at a point shifted by a predetermined length from the corresponding corner of the second opening in the direction of a side of the second opening, and a segment is made by connecting a predetermined point on a line connecting between the end of the imaginary line extension and the corresponding corner of the first opening to the corresponding corner of the second opening so as to form a part of the creased line adjacent to the second opening, thereby adjusting the area ratio of side faces of the internal magnetic shield. 
     Preferably, the ends of the imaginary line extensions of the creased lines adjacent to the substantially rectangular second opening are located on the projected plane at the points shifted by a predetermined length from the corners in the direction of long side when the fluorescent layer is made of a large number of phosphor dots. 
     It is also preferred that the ends of the imaginary line extensions of the creased lines adjacent to the substantially rectangular second opening are located on the projected plane at the points shifted by a predetermined length from the corners in the direction of short side when the fluorescent layer is made of a large number of phosphor stripes. 
     According to the present invention, the ends of the imaginary line extensions of the creased lines adjacent to the second opening are located at the points shifted by a predetermined length from the corners in the direction of side for the purpose that the ratio of the effective area of the two long side walls to the effective area of the two short side walls is adjusted by selecting the predetermined length instead of the known means of adjusting the maximum depth of the substantially V-shaped notches formed in the two long side walls or two short side walls. And accordingly, even if the maximum depth of the substantially V-shaped notches is so selected as to become small, it is possible to appropriately regulate the electron beam path, and moreover the total shielding effect is not deteriorated. 
     In the present invention, the ends of the imaginary line extensions of the creased lines adjacent to the second opening are the points located on the sides of the second opening on the projection plane when the internal magnetic shield is projected on a plane parallel to the opening of the magnetic shield. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view showing a schematic structure of a color cathode-ray tube having an internal magnetic shield according to a first embodiment of the present invention; 
     FIGS. 2A to  2 C show the structure of the first embodiment of the internal magnetic shield used in the color cathode-ray tube of FIG. 1 in which substantially V-shaped notches are formed in long side walls and, in which FIG. 2A is a perspective view, FIG. 2B is a top view and FIG. 2C is a side view, FIG. 2B being equivalent to a view projected on a plane parallel to an opening of the internal magnetic shield; 
     FIG. 3 is a characteristic figure showing the relationship between maximum depth of a substantially V-shaped notch and displacement of an electron beam path; 
     FIGS. 4A to  4 C show the structure of a second embodiment of the present invention in which substantially V-shaped notches are formed in short side walls, FIGS. 4A to  4 C corresponding to FIGS. 2A to  2 C, respectively; and 
     FIGS. 5A to  5 C show an example of internal magnetic shield used in a known color cathode-ray tube. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a sectional view showing a schematic structure of a color cathode-ray tube having an internal magnetic shield according to a first embodiment of the present invention. 
     In FIG. 1, reference numeral  1  denotes a panel portion;  2 , a neck portion;  3 , a funnel portion;  4 , a fluorescent layer;  5 , shadow mask;  6 , a support frame;  7 , an internal magnetic shield;  8 , a deflection yoke;  9 , a purity magnet;  10 , a center beam static convergence adjustment magnet;  11 , a side beam static convergence adjustment magnet;  12 , an electron gun; and  13 , an electron beam. 
     An evacuated glass envelope (bulb) constituting the color cathode-ray tube comprises the panel portion  1  located at the front and having the fluorescent layer  4  formed on the inner surface of a face plate, the long and slender neck portion  2  located at the rear and housing the electron gun  12 , and the substantially funnel-shaped funnel portion  3  connecting the panel portion  1  and the neck portion  2 . The shadow mask  5  is attached at the peripheral edge thereof to the support frame  6  mounted on the side wall of the panel portion  1  so as to be disposed and fixed in such a condition that it faces the fluorescent layer  4 . The substantially quadrangular pyramid-shaped internal magnetic shield  7  is mounted at the edge portion thereof on the support frame  6  so that it is disposed inside the evacuated envelope so as to extend from the panel portion  1  to the funnel portion  3 . The deflection yoke  8  is attached to the outside of the evacuated envelope so as to be located at the connecting portion of the funnel portion  3  and the neck portion  2 . The purity magnet  9 , center beam static convergence adjustment magnet  10 , and side beam static convergence adjustment magnet  11  are all placed about the neck portion  2  in side-by-side relation. Three electron beams  13  emitted from the electron gun  12  (only one of them being shown in FIG. 1) are deflected in a predetermined direction by the magnetic field produced by the deflection yoke  8  and then allowed to reach corresponding one of color pixels on the fluorescent layer  4  through one of a large number of electron beam apertures (not shown) formed in the shadow mask  5 . 
     Operation of the color cathode-ray tube having the above construction, that is, image displaying operation is quite the same as that of the known color cathode-ray tube, and therefore description of the image displaying operation of this color cathode-ray tube is omitted. 
     FIGS. 2A to  2 C show the structure of a first embodiment of the internal magnetic shield  7  used in the color cathode-ray tube of the present invention shown in FIG.  1 . FIG. 2A is a perspective view, FIG. 2B is a top view and FIG. 2C is a side view. It is noted that FIG. 2B is equivalent to a view projected on a plane parallel to an opening of the magnetic shield. 
     As shown in FIGS. 2A to  2 C, the internal magnetic shield  7  of this embodiment is made of a substantially quadrangular pyramid-shaped frame structure  14  defined by creased lines and comprising two long side walls  15 A,  15 B, narrow size adjustment side walls  16 A,  16 B connected respectively to the lower portions of the two side walls  15 A,  15 B, two short side walls  17 A,  17 B, narrow size adjustment side walls  18 A,  18 B connected respectively to the lower portions of the two side walls  17 A,  17 B, a creased line  19 A formed between the side walls  15 A and  17 A, a creased line  19 B formed between the side walls  17 A and  15 B, a creased line  19 C formed between the side walls  15 B and  17 B, and a creased line  19 D formed between the side walls  17 B and  15 A. The frame structure  14  has a substantially rectangular first opening  20  with a smaller diagonal dimension at one end adjacent to the electron gun  12  than that of a larger diagonal dimension of a substantially rectangular second opening  21  at the other end adjacent to the shadow mask  5 . The two long side walls  15 A,  15 B are formed in the portions thereof adjacent to the first opening  20  with substantially V-shaped notches  20 A,  20 B having a maximum depth  c , respectively. 
     As shown in FIG. 2B, the creased line  19 A is formed in such a manner that one end adjacent to the first opening  20  coincides with a first corner  20 , of the first opening  20  and the other end which is an imaginary line extension thereof is adjacent to the second opening  21  and does not coincide with a first corner  21   1  of the second opening  21  but is located on a projected plane parallel to the second opening  21  at a point  21   11  shifted by a predetermined length Δ 1  from the first corner  21   1  in the direction of long side. Similarly, the creased line  19 B is formed in such a manner that one end adjacent to the first opening  20  coincide with a second corner  20   2  of the first opening  20  and the other end which is an imaginary line extension thereof is adjacent to the second opening  21  and does not coincide with a second corner  21   2  of the second opening  21  but is located at a point  21   21  shifted by the predetermined length Δ 1  from the second corner  21   2  in the direction of long side. The creased line  19 C is formed in such a manner that one end adjacent to the first opening  20  coincides with a third corner  20   3  of the first opening  20  and the other end which is an imaginary line extension thereof is adjacent to the second opening  21  and does not coincide with a third corner  21   3  of the second opening  21  but is located at a point  21   31  shifted by the predetermined length Δ 1  from the third corner  21   3  in the direction of long side. The creased line  19 D is formed in such a manner that one end adjacent to the first opening  20  coincides with a fourth corner  20   4  of the first opening  20  and the other end which is an imaginary line extension thereof is adjacent to the second opening  21  and does not coincide with a fourth corner  21   4  of the second opening  21  but is located at a point  21   41  shifted by the predetermined length Δ 1  from the fourth corner  21   4  in the direction of long side. 
     The size adjustment side walls  16 A,  16 B and  18 A,  18 B are auxiliary members provided for making the ends of the imaginary line extensions of the creased lines  19 A,  19 B,  19 C,  19 D adjacent to the second opening  21  approximately coincide with their respective physical ends, that is, the corners of the second opening  21 , because the ends of the imaginary line extensions do not coincide with the corners of the second opening  21 . In this case, the size adjustment side walls  16 A,  16 B are so shaped that the creased lines  19 A,  19 B,  19 C,  19 D are bent outward at their respective points close to the second opening  21  in three dimensions so as to make the physical ends of the creased lines  19 A,  19 B,  19 C,  19 D coincide with the corresponding corners of the second opening  21 , respectively. Meanwhile, the size adjustment side walls  18 A,  18 B are so shaped that, in conformity with the fact that the creased lines  19 A,  19 B,  19 C  19 D are bent outward at their respective points close to the second opening  21 , the surfaces of the two short side walls  17 A,  17 B are bent outward in the same manner so as to make the physical ends of the creased lines  19 A,  19 B,  19 C,  19 D coincide with the corresponding corners  21   1 ,  21   2 ,  21   3 ,  21   4  of the second opening  21 , respectively. 
     When the frame structure  14  is disposed inside the funnel portion  3 , the edge portion of the second opening  21  is fitted to the support frame  6  mounted on the side wall of the panel portion  1  together with the peripheral portion of the shadow mask  5 , similarly to the known frame structure  40  (see FIGS. 5A to  5 C). In this case, the substantially rectangular first opening  20  of small diameter is located adjacent to the electron gun  12  and the substantially rectangular second opening  21  is located adjacent to the shadow mask  5 . Three electron beams  13  emitted from the electron gun  12  are allowed to pass through the inside of the frame structure  14  and strike the fluorescent layer  4  through one of electron beam apertures (not shown) of the shadow mask  5 , thereby displaying a required image on the face plate. 
     The substantially V-shaped notches  20 A,  20 B formed in the two long side walls  15 A,  15 B are provided for regulating the path for the electron beam passing through the inside of the frame structure  14 , similarly to the known substantially V-shaped notches  43 A,  43 B (see FIGS. 5A to  5 C). The maximum depth  c  of the substantially V-shaped notches  20 A,  20 B is so selected as to be smaller than the maximum depth  c ′ of the known substantially V-shaped notches  43 A,  43 B (see FIG. 5A to  5 C). 
     According to the internal magnetic shield having the above structure, when forming the creased lines  19 A,  19 B,  19 C,  19 D, the ends thereof adjacent to the first opening  20  are made to coincide respectively with the corresponding corners  20   1 , to  20   4  of the first opening  20 , while the ends of the imaginary line extensions thereof adjacent to the second opening  21  are so selected as to be located on the projected plane at the points  21   11 ,  21   21 ,  21   31 ,  21   41  shifted by the predetermined length Δ 1  from the corresponding corners  21   1  to  21   4  of the second opening  21  in the direction of long side, respectively. Therefore, in comparison with the known internal magnetic shield (see FIGS. 5A to  5 C), as seen from FIGS. 2B and 5B, the effective area of the two long side walls  15 A,  15 B, through which the terrestrial magnetism passes, is reduced and the effective area of the two short side walls  17 A,  17 B is increased. In this case, by suitably selecting the predetermined length Δ 1 , that is, the points  21   11 ,  21   21 ,  21   31 ,  21   41  at which the ends of the imaginary line extensions of the creased lines  19 A,  19 B,  19 C,  19 D adjacent to the second opening  21  are located, the ratio of the effective area of the two long side walls  15 A,  15 B to the effective area of the two short side walls  17 A,  17 B can be adjusted. This makes it possible to appropriately regulate the three electron beam paths passing through the inside of the internal magnetic shield without adjusting the maximum depth  c  of the substantially V-shaped notches  20 A,  20 B. For example, when the predetermined length Δ 1  by which the ends of the imaginary line extensions are shifted from the corners  21   1  to  21   4  in the direction of long side is 18.7 mm, the maximum depth  c  of the substantially V-shaped notches  20 A,  20 B is 44.7 mm. These numerical values, however, are just examples and, needless to say, impose no restrictions on the structure of this embodiment. 
     FIG. 3 is a characteristic figure showing the relationship between the maximum depth of the substantially, V-shaped notch and the displacement of the electron beam path due to terrestrial magnetism, which characteristics are obtained when the color cathode-ray tube is so placed that the center axis thereof lies north and south. 
     In FIG. 3, solid lines show the characteristics obtained by the color cathode-ray tube of this embodiment and broken lines show the characteristics obtained by the known color cathode-ray tube. For both solid and broken lines, a curve  1  shows the characteristics of the color cathode-ray tube in the vertical axis direction (vertical direction, that is, minor axis direction) and a curve  2  show the characteristics of the color cathode-ray tube in the horizontal axis direction (horizontal direction, that is, major axis direction). 
     As is obvious from the characteristic view of FIG. 3, in the known color cathode-ray tube, displacements of electron beam in the vertical axis and horizontal axis directions cannot be made almost equal unless the maximum depth  c ′ of the substantially V-shaped notches is increased to a certain extent, while in the color cathode-ray tube of this embodiment, displacements of electron beam in the vertical axis and horizontal axis directions can be almost equalized without increasing the maximum depth  c  of the substantially V-shaped notches so much. Therefore, the color cathode-ray tube of this embodiment proves to be more excellent in total shielding effect because the maximum depth  c  of the substantially V-shaped notches must not be increased. 
     In the present embodiment, the internal magnetic shield has been described by taking a case that the ends of the imaginary line extensions of the creased lines  19 A,  19 B,  19 C,  19 D are so selected as to be located at the points  21   11 ,  21   21 ,  21   31 ,  21   41  shifted by the predetermined length Δ 1  from the corresponding corners  21   1  to  21   4  of the second opening  21  in the direction of long side, respectively, and the substantially V-shaped notches  20   a ,  20 B are formed in the two long side walls  15 A,  15 B, respectively. However, the internal-magnetic shield according to the present invention is not limited to that having the above structure. As shown in FIGS. 4A to  4 C, it is possible according to a second embodiment to change the structure in such a manner that the ends of the imaginary line extensions of the creased lines  19 A,  19 B,  19 C,  19 D are so selected as to be located at points  21   12 ,  21   22 ,  21   32 ,  21   42  shifted by a predetermined length Δ 1 ′ from the corresponding corners  21   1  to  21   4  of the second opening  21  in the direction of short side, respectively, and substantially V-shaped notches  20 A,  20 B are formed in the two short side walls  17 A,  17 B, respectively. 
     In the second embodiment as well, by suitably selecting the points  21   12 ,  21   22 ,  21   32 ,  21   42  at which the ends of the imaginary line extensions of the creased lines  19 A,  19 B,  19 C,  19 D adjacent to the second opening  21  are located on a projected plane parallel to the second opening  21 , the ratio of the effective area of the two long side walls  15 A,  15 B to the effective area of the two short side walls  17 A,  17 B can be adjusted. This makes it possible to appropriately regulate the three electron beam paths passing through the inside of the internal magnetic shield without adjusting the maximum depth of the substantially V-shaped notches. 
     The first embodiment is suitable for use in the color cathode-ray tube of the type that the fluorescent layer  4  is made of phosphor dots, while the second embodiment is suitable for use in the color cathode-ray tube of the type that the fluorescent layer  4  is made of phosphor stripes. 
     According to the above embodiments, in order to adjust the ratio of the effective area of the two long side walls  15 A,  15 B to the effective area of the two short side walls  17 A,  17 B, the ends of the imaginary line extensions of the creased lines  19 A to  19 D adjacent to the second opening  21  are so selected as to be located at the points  21   11  to  21   41  ( 21   12  to  21   42 ) shifted by the predetermined length Δ 1  (Δ 1 ′) from the corresponding corners  21   1 , to  21   4  in the direction of side without adjusting the maximum depth  c  of the substantially V-shaped notches. Therefore, it is possible to appropriately regulate the electron beam path without deteriorating the overall shielding effect. 
     In the above embodiments, the internal magnetic shield has been described as being formed with V-shaped notches in the side faces. However, even in a shield with no notches, direction of the displacement of electron beam attributed to the terrestrial magnetism, which has been adjusted by forming notches, can be adjusted by making use of the structure of the present invention. 
     As has been described above, according to the present invention, the virtual mean ends of the creased lines adjacent to the second opening are located on a projected plane parallel to the second opening at the points shifted by the predetermined length from the corners in the direction of side for the purpose that the ratio of the effective area of the two long side walls to the effective area of the two short side walls is adjusted by selecting the predetermined length instead of the known means of adjusting the maximum depth of the substantially V-shaped notches formed in the two long side walls or two short side walls. Accordingly, even if the maximum depth of the substantially V-shaped notches is made small, it is possible to appropriately regulate the electron beam path, and moreover the overall shielding effect is not deteriorated.