CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     The present application claims priority from Japanese application JP2004-281851 filed on Sep. 28, 2004, the content of which is hereby incorporated by reference into this application  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a cathode ray tube including an electron gun for generating electron beams to display an image, and more particularly to a cathode ray tube which prevents a getter which is served for increasing a degree of vacuum in the inside of the cathode ray tube and for maintaining the degree of vacuum from being scattered to an electron gun side and deteriorating characteristics of the electron gun.  
         [0004]     2. Description of the Related Art  
         [0005]     For increasing the degree of vacuum in the inside of the cathode ray tube and for maintaining the degree of vacuum over a long period, it is necessary to scatter the getter in the inside of the cathode ray tube. The getter is formed of a metal film made of barium Ba and is scattered over a wide range in the inside of the cathode ray tube to perform a function of absorbing a generated gas. However, when the metal film which originally possesses a low work function is scattered and adheres to electrodes of the electron gun, stray electrons are generated from the metal film. That is, a withstand voltage between electrodes of the electron gun is lowered. Further, phosphors emit light due to the stray electrons thus exerting an adverse influence to an image.  
         [0006]     In Japanese Patent Laid-open 2001-93449, there is disclosed a color cathode ray tube in which a getter is arranged between a funnel and an inner shield arranged close to an inner surface of the funnel, a control plate which controls the scattering of a getter vapor from the getter in the direction other than the electron-gun direction is provided to the getter thus forming a getter film over a wide range without allowing the getter vapor to adhere to a color selection electrode and the electron gun.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     In the cathode ray tube, the getter is mounted on a distal end of an antenna which extends from the electron gun. In the electron gun with a shield cup having a large diameter and a large distance among three electron guns, barium (Ba) enters the inside of a focusing electrode G 5  and hence, the getter is liable to become a source for generating stray electrons through minute flaws or the like.  
         [0008]     In the cathode ray tube with a deflection angle of 100 degree or less, compared to the cathode ray tube with a deflection angle exceeding 100 degree, the direction along which the getter is scattered is relatively directed to the electron gun side. Accordingly, when the getter is scattered, barium Ba is liable to adhere to the inside of the electron gun. As a result, the withstand voltage characteristic of a fifth grid electrode is deteriorated.  
         [0009]     Accordingly, it is an object of the present invention to provide a cathode ray tube which can prevent the deterioration of characteristics of an electron gun attributed to a getter.  
         [0010]     The degree of influence attributed to the scattering of the getter differs depending on a length and an aperture diameter of an electron gun. To prevent the jumping of barium Ba, it is necessary to determine the relationship between the length of the electron gun, an aperture diameter of a focusing electrode G 5  and an aperture diameter of a shield cup SC.  
         [0011]     That is, assuming an aperture diameter of an electron beam aperture formed in a shield cup as Vsc, an aperture diameter of an electron beam aperture formed in an inner electrode as V 5 , and a distance between the electron beam aperture formed in the shield cup and the inner electrode as L, the above-mentioned aperture diameter Vsc, the aperture diameter V 5  and the distance L are determined to satisfy a following formula. 
 
tan θ=( V 5 −Vsc )/2L≧0.08 
 
         [0012]     By determining such a relationship, an area of focusing electrode G 5  which can be observed from the panel side through the electron beam aperture formed in the shield cup SC can be reduced. That is, the adhesion of barium Ba to the focusing electrode G 5  can be reduced.  
         [0013]     According to the present invention, since the adhesion of barium Ba to the focusing electrode G 5  can be reduced, the insufficient withstand voltage and the reproduction steps can be reduced in number whereby a manufacturing cost of the cathode ray tube can be reduced.  
         [0014]     Further, the cathode ray tube according to the present invention is preferably applicable to a television set and a computer display device whose deflection angle is 100 degree or less, a distance between neighboring electron guns is large, the electron guns with electrodes having an elliptical aperture diameter are provided, and an antenna getter is provided to a distal end portion of the electron gun.  
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0015]      FIG. 1  is a schematic view of a 110-degree deflection cathode ray tube showing the getter scattering direction;  
         [0016]      FIG. 2  is a schematic view of a 90-degree deflection cathode ray tube showing the getter scattering direction;  
         [0017]      FIG. 3  is a schematic view of an electron gun showing a state in which barium Ba in the inside of the electron gun is scattered;  
         [0018]      FIG. 4  is a cross-sectional view taken along a line I-I in  FIG. 3 ;  
         [0019]      FIG. 5  is a cross-sectional view taken along a line II-II in  FIG. 3 ;  
         [0020]      FIG. 6  is an explanatory view showing a positional relationship between an aperture diameter formed in a shield cup SC and an aperture diameter formed in an inner electrode  21  (center electron gun);  
         [0021]      FIG. 7  is an explanatory view showing a positional relationship between an aperture diameter formed in a shield cup SC and an aperture diameter formed in an inner electrode  21  (side electron gun);  
         [0022]      FIG. 8  is an explanatory view showing a relationship between an aperture diameter formed in a shield cup SC and an aperture diameter formed in an inner electrode  21 ;  
         [0023]      FIG. 9  is an explanatory view showing a relationship between an aperture diameter formed in a shield cup SC and an aperture diameter formed in an inner electrode  21 ;  
         [0024]      FIG. 10  is an explanatory view showing a relationship between an aperture diameter formed in a shield cup SC and an aperture diameter formed in an inner electrode  21 ; and  
         [0025]      FIG. 11  is an explanatory view showing a relationship between an aperture diameter formed in a shield cup SC and an aperture diameter formed in an inner electrode  21 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]     Preferred embodiments of the present invention are explained hereinafter in conjunction with attached drawings.  
       Embodiment  
       [0027]      FIG. 1  is a conceptual view showing the difference between a deflection angle of a cathode ray tube and a getter scattering direction and also is a schematic view of a 110-degree deflection cathode ray tube whose deflection angle exceeds 100 degree, for example.  FIG. 2  is a schematic view of a 90-degree deflection cathode ray tube having a deflection angle of 100 degree or less, for example, which constitutes an object of the present invention.  
         [0028]     In  FIG. 1  and  FIG. 2 , the cathode ray tube  10  is constituted of a face plate  11 , a funnel  12  and a neck portion  13  and the inside thereof which is surrounded by these parts is held in a vacuum. In the inside of the cathode ray tube  10 , an antenna getter  15  which is mounted on a shield cup SC of an electron gun  14  is provided.  
         [0029]     With respect to barium Ba which is scattered from the antenna getter  15 , in case of the 110-degree deflection cathode ray tube shown in  FIG. 1 , the probability that the barium Ba adheres to the electron gun  14  is small. However, in the case of the 90-degree deflection cathode ray tube shown in  FIG. 2 , as indicated by an arrow, the probability that barium Ba adheres to the electron gun  14  is increased. Accordingly, the smaller an inclination angle, barium Ba is liable to enter the electron gun  14 .  
         [0030]      FIG. 3  is an explanatory view showing a state in which barium Ba adheres to the electrode in the inside of the electron gun  14  and also is an enlarged view of a portion of the electron gun  14 .  FIG. 4  is a view of the shield cup SC shown in  FIG. 3  as viewed from a line I-I, and  FIG. 5  is a view of a focusing electrode G 5  shown in  FIG. 3  as viewed from a line II-II.  
         [0031]     The electron gun shown in  FIG. 3  includes, although not shown in the drawing, an electron beam generating part which is constituted of three cathodes which are disposed in an in-line arrangement, that is, a first grid electrode G 1 , a second grid electrode G 2  and a third grid electrode G 3 . On a stage behind the electron beam generating part, a fourth grid electrode G 4 , the fifth grid electrode (focusing electrode) G 5 , a sixth grid electrode (final electrode) G 6  and the shield cup SC are arranged. These electrodes are arranged at a given interval toward a phosphor screen which is formed on the face plate  11 . Here, these electrodes are supported by a beading glass  23 . In the inside of the fifth grid electrode (focusing electrode) G 5  and the inside of the sixth grid electrode G 6 , inner electrodes  21 ,  22  are respectively provided.  
         [0032]     In  FIG. 3 ,  FIG. 4  and  FIG. 5 , barium Ba which is scattered from the antenna getter is adhered to the focusing electrode G 5 , the inner electrode  21  of the focusing electrode G 5  and the fourth grid electrode G 4  and, at the same time, barium Ba passes through the fourth grid electrode G 4  and also adheres to the third grid electrode G 3  in the electron beam generating part. Here, barium Ba which adheres to the inner electrode  21  of the focusing electrode G 5  is indicated by symbol  24  particularly.  
         [0033]     In  FIG. 4 , an aperture diameter Vscc of the center electron beam aperture formed in the shield cup SC and an aperture diameter Vscs of both side electron beam apertures formed in the shield cup SC are indicated. These aperture diameters are diameters in the vertical direction as viewed from the electron beam scanning direction. In the drawing, since the electron beam apertures have a circular shape, diameters of these electron beam apertures in the horizontal direction are equal to diameters of the apertures in the vertical direction.  
         [0034]     In  FIG. 5 , an aperture diameter V 5   c  of the center electron beam aperture formed in the inner electrode  21  of the focusing electrode G 5  and an aperture diameter V 5   s  of both side electron beam apertures  25  formed in the inner electrode  21  are indicated. These aperture diameters are diameters in the vertical direction as viewed from the electron beam scanning direction. In the drawing, the center electron beam aperture has a circular shape and the side electron beam apertures have an elliptical shape. Here, symbol  26  indicates the electron beam aperture of the focusing electrode G 5 . Further, symbol  24  indicates the adhered barium Ba. Here, Vsc is a general term of the electron beam aperture diameter of the shield cup SC, while V 5  is a general term of the aperture diameter of the inner electrode of the focusing electrode G 5 .  
         [0035]     As can be understood from above, it is important to properly decrease the aperture diameter Vsc of the electron beam aperture formed in the shield cup SC corresponding to the positional relationship L between the shield cup SC and the inner electrode  21  of the focusing electrode G 5 .  
         [0036]      FIG. 6  and  FIG. 7  are explanatory views showing the aperture diameter V 5  of the electron beam apertures formed in the inner electrode  21  of the focusing electrode G 5 , the aperture diameter Vsc of the electron beam aperture formed in the shield cup SC, and the positional relationship between the electron beam aperture formed in the inner electrode  21  of the focusing electrode G 5  and the electron beam aperture formed in the shield cup SC.  FIG. 6  is a cross-sectional view of the center electron gun, while  FIG. 7  is a cross-sectional view of the side electron gun. These drawings are respectively explanatory views showing the positional relationship between panel-side end portions of the electron beam apertures formed in the inner electrode  21  and cathode-side end portions of the electron beam apertures formed in the shield cup.  
         [0037]      FIG. 6  shows the relationship between the electron beam apertures formed in the shield cup SC of the center electron gun and the electron beam apertures formed in the inner electrode  21  of the focusing electrode G 5  using an angle θc. As a result, it is found that the larger the angle θc, the adhesion of barium Ba can be reduced.  
         [0038]     In the same manner as  FIG. 6 ,  FIG. 7  shows the relationship between the electron beam apertures formed in the shield cup SC of the side electron guns and the electron beam apertures formed in the inner electrode  21  of the focusing electrode G 5  using an angle θs. As a result, it is found that the larger the angle θs, the adhesion of barium Ba can be reduced.  
         [0039]     As can be understood from above, by increasing the aperture diameter V 5  of the inner electrode  21  of the focusing electrode G 5 , by decreasing the aperture diameter Vsc of the shield cup SC or by setting the positional relationship L to a small value, the adhesion of the barium Ba can be reduced.  
         [0040]     The aperture diameter V 5  of the inner electrode  21  of the focusing electrode G 5  and the size L are determined to obtain the given focusing characteristic and it is desirable to adjust the aperture diameter V 5  and the size L using the aperture diameter Vsc of the shield cup SC.  
         [0041]      FIG. 8  to  FIG. 11  are explanatory views showing the relationship between the aperture diameter Vsc of the shield cup SC and the aperture diameter V 5  of the inner electrode  21  of the focusing electrode G 5 , and show embodiments when the aperture diameter Vsc of the shield cup SC and the aperture diameter V 5  of the inner electrode  21  of the focusing electrode G 5  are made different from each other.  
         [0042]      FIG. 8  shows one embodiment of the electron gun in which, as shown in an embodiment 1 on a following table 1, the aperture diameter Vscc of the center electron beam aperture formed in the shield cup SC is set to 4.5 mm, the aperture diameter Vscs of both side electron beam apertures formed in the shield cup SC is set to 5.5 mm, the aperture diameter V 5   c  of the center electron beam aperture formed in the inner electrode  21  of the focusing electrode G 5  is set to 6.3 mm, the aperture diameter V 5   s  of both side electron beam apertures formed in the inner electrode  21  of the focusing electrode G 5  is set to 7.8 mm, and the distance L between the shield cup SC and the inner electrode  21  of the focusing electrode G 5  is set to 10.7 mm.  
         [0043]     Provided that tan θ=(V5−Vsc)/2L, to obtain tan θc=(V5 c −Vscc)/2L of the center electron gun and tan θs=(V5 s −Vscs)/2L of both side electron guns respectively from these numerical values, tan θc and tan θs take values shown in the embodiment 1 on following Table 1.  
         [0044]     In the same manner,  FIG. 9  to  FIG. 11  respectively correspond to embodiment 2 to embodiment 4 of following Table 1. However, in  FIG. 10 , the aperture diameter H in the horizontal direction of the shield cup SC is set to 5.0 mm with respect to the center electron beam aperture as well as both side electron beam apertures and shapes of these apertures are formed in an elliptical shape. Further, in  FIG. 11 , the aperture diameter H in the horizontal direction of the center electron beam aperture formed in the shield cup SC is set to 5.0 mm and the shape of the aperture is formed in an elliptical shape.  
                                                         TABLE 1                                   embodiment 1   embodiment 2   embodiment 3   embodiment 4                                    Vscc   4.5   4.0   4.0   4.0       Vscs   5.5   4.0   4.0   4.5       V5c   6.3   6.3   6.2   6.3       V5s   7.8   7.8   7.6   7.8       L   10.7   10.7   10.7   10.7       tanθc   0.08   0.11   0.10   0.11       tanθs   0.11   0.18   0.17   0.16                  
 
         [0045]     As can be understood from Table 1, by setting tan θc to 0.08 or more and by setting tan θs to 0.11 or more, it is possible to freely determine the center electron gun and both side electron guns within a range that the focusing characteristic is not influenced.  
         [0046]     The present invention is effectively applicable to the cathode ray tube in which the distance (hereinafter referred to as “S size”) between the neighboring electron guns is large. In the electron gun adopting the in-line arrangement, when the S size becomes small, the concentration angle between the center electron beam and the side electron beams in the electron beam apertures formed in the shadow mask becomes small.  
         [0047]     Accordingly, when the S size is small, it is necessary to increase the distance between the shadow mask and the phosphor screen. When the distance between the shadow mask and the phosphor screen is increased, the electron beams are liable to be affected by magnetic noises (particularly, earth magnetism) from the outside between the shadow mask and the phosphor screen. As a result, even when the color cathode ray tube is directed in one direction so as to perform the adjustment to allow the electron beams to land on the correct positions, when the color cathode ray tube is directed in other direction, the color cathode ray tube is affected by the different earth magnetism and hence, the electron beams are moved whereby the electron beams cannot be landed at accurate positions thus deteriorating the color purity of the color cathode ray tube.  
         [0048]     On the other hand, when the large S size is set, the concentration angle between the center electron beam and the side electron beams in the electron beam aperture of the shadow mask is increased. Accordingly, even when the distance between the shadow mask and the phosphor screen becomes short, there exists no possibility that the electron beams impinge on the phosphor other than the desired phosphor. Since the distance between the shadow mask and the phosphor screen can be made small, it is possible to reduce the influence of the magnetic noises (particularly, the earth magnetism) from the outside between the shadow mask and the phosphor screen. As the result, it is possible to suppress the deterioration of color purity which occurs when the direction of the color cathode ray tube is changed.  
         [0049]     Further, in the electron gun in which the S size increases, three beam apertures are spaced apart from each other and hence, the aperture diameter of the fifth grid electrode  5  can be increased. By increasing the aperture diameter of the electrode, the electron beams hardly impinge on the electrodes. Along with the increase of the aperture diameter of the fifth grid electrode, the aperture of the shield cup is also increased.  
         [0050]     In case of the cathode ray tube having the neck outer diameter of 29 mm, in the electron gun provided with a cylindrical lens having a diameter of approximately 5.5 mm through which three electron beams pass therethrough independently in the main lens part, the S size is set to 6.6 mm. Further, although there exists a color cathode ray tube in which the S size is set to a value which falls within a range of 4.75 to 5.5 mm, in this embodiment, the S size is set to 6.6 mm.  
         [0051]     In this manner, in the electron gun with the shield cup having the large aperture diameter, the getter is liable to be scattered in the cathode direction and hence, the present invention exhibits the remarkable advantageous effects in controlling the getter.
Assuming an aperture diameter of an electron beam aperture formed in a shield cup as Vsc, an aperture diameter of an electron beam aperture formed in an inner electrode of a focusing electrode as V 5, and a distance between the electron beam aperture formed in the shield cup and the inner electrode as L, the above-mentioned aperture diameter Vsc, the aperture diameter V 5 and the distance L are determined to satisfy tan th=(V 5- Vsc)/2L>=0.08. Due to such a constitution, it is possible to enhance the degree of vacuum in the inside of a cathode ray tube and, it is possible to prevent a getter for elevating the degree of vacuum in the inside of the cathode ray tube and for maintaining the elevated degree of the vacuum from being scattered to the electron gun side thus preventing the deterioration of the characteristics of the electron gun.