Patent Publication Number: US-6909227-B2

Title: Color cathode ray tube

Description:
This application is a Divisional of co-pending application Ser. No. 10/386,515, filed on Mar. 13, 2003, and for which priority is claimed under 35 U.S.C. § 120; and this application claims priority of Application No. 2002-0041381 filed in Korea on Jul. 15, 2002 and Application No. 2002-0043101 filed in Korea on Jul. 23, 2002 under 35 U.S.C. § 119; the entire contents of all are hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a color cathode ray tube, and more particularly, to a shadow mask for a color cathode ray tube using Aluminum Killed (AK) which has improved brightness and bright uniformity at corners, reduced doming effect, enhanced curved surface maintenance strength and lowered production cost. 
   2. Background of the Prior Art 
   In general, the cathode ray tube is the display apparatus that converts electrical signals into electron beams and scans the fluorescent screen with the electron beams to generate visible light and thus display images. Such cathode ray tubes are the most popular display apparatus since they are very excellent in the ratio of display quality to cost. 
     FIG. 1  shows schematically a structure of a conventional color cathode ray tube. Referring to  FIG. 1 , the conventional cathode ray tube includes a panel  1  having its outer surface that is flat or has some curvature, and a funnel  2  coupled sealingly with the panel  1  to form a tube. The panel  1  and the funnel  2  are coupled sealingly with a frit glass. Since the tube including the panel  1  and the funnel  2  keeps its inside in a vacuum, it may explode due to an external impact, which is very dangerous. In order to prevent this, a strengthening band  12  is added to a contact portion of the panel  1  and the funnel  2 . 
   The funnel  2  consists of a neck  10  whose shape is narrow tube and a cone portion that expands at the end of the neck  10 . The neck  10  is provided with electron guns to project electron beams  11 . The cone portion is provided with a deflection yoke  9  to deflect the electron beams  11 . On the other hand, the colors of the electron beams deflected by the deflection yoke  9  are selected by a shadow mask  5  that has fine holes. The electron beams are shot across a fluorescent material coated on the inside of the panel  1 , so that the fluorescent material emits lights of each color, red, green and blue. To achieve this, the shadow mask  5  has a lot of holes and each of the fluorescent materials for red, green and blue is coated on the fluorescent surface corresponding to the holes. The electron beams are shot across the fluorescent material to emit light and images are displayed on the fluorescent surface. The shadow mask is supported by a frame  4  to be spaced with the panel  1 . The support spring installed by the frame  4  is coupled with a stud pin  6  mounted on the panel  1  to be supported firmly in the tube. To prevent the electron beams  11  from shifting due to an external magnetic field, an inner shield  7  made of magnetic material is coupled with the frame  4  to be supported. 
   The operation of the general color cathode ray tube will be described. The electron guns  8  generate electron beams  11  and the electron beans  11  are shot across the fluorescent surface inside the panel  1  by a voltage applied to the cathode ray tube. In this time, the electron beams  11  are deflected by the deflection yoke  9 . Each color of the beams is selected by the shadow mask  5 . The electron beams  11  are properly shot across the fluorescent surfaces of red, green and blue so that the fluorescent surfaces emit lights to display a predetermined image. 
     FIG. 2  is a front view of the conventional shadow mask. Referring to  FIG. 2 , conventional shadow mask  5  is a thin metal plate that has a lot of holes  51 . More specifically, the holes  51  are aligned vertically on the thin metal plate and rows of the holes  51  aligned vertically are aligned horizontally. The electron beams pass through the holes  51 . Invar mask or AK (Aluminum Killed) is used as the material of the shadow mask  5 . The invar mask is trice as expensive as the AK. Both of them are critically different from each other in physical characteristics and are shown in Table 1. 
   
     
       
         
             
             
             
           
             
               TABLE 1 
             
             
                 
             
             
               Material 
               INVAR 
               AK 
             
             
                 
             
           
          
             
               Price 
               High 
               Low 
             
             
               Doming 
               Good 
               Bad 
             
             
               Etching 
               Bad 
               Good 
             
             
               Formability 
               Bad 
               Good 
             
             
               Main component (%) 
               Fe: 64-60 
               Fe: 99.7-99.0 
             
             
                 
               Ni: 35-36 
             
             
               Thermal expansive 
               Equal to or less 
               8 − 20 × 10 −6 /° C. 
             
             
               coefficient 
               than 1.5 × 10 −6 /° C. 
             
             
                 
             
          
         
       
     
   
   Referring to Table 1, AK is a pure iron that contains iron of 99.0%-99.7% and is inexpensive. However, its thermal expansive coefficient is 8-20×10 −6 /° C. and it is easier to be deformed than Invar. AK is as 5.3-13.3 times as Invar in their thermal expansive coefficients. 
   On the other hand, the doming means that the shadow mask  5  bulges due to heat. The heat is almost generated by the electron beams  11  striking the shadow mask  5  while the electron beams pass though the shadow mask  5 . The degree of the doming determines the transmittance and the transmittance determines display quality. 
   The structure of the conventional shadow mask  5  will be described with reference to FIG.  2 . The size and the shape of the conventional holes through which the electron beams pass are described. The structure of the conventional shadow mask satisfies the following relation: 
         B3   P3     ≤     B2   P2         
         where B 1  is a length of a bridge as a distance between holes  51  in a vertical direction at the center portion of the shadow mask  5 ,   B 2  is a length of a bridge as a distance between holes  51  in a vertical direction at four corner portions of the shadow mask  5 ,   B 3  is a length of a bridge as a distance between holes  51  at edge portions of the shadow mask in a direction of a long axis of the shadow mask  5 ,   P 1  is a vertical pitch of the holes at the center portion of the shadow mask  5 ,   P 2  is a vertical pitch of the holes at the four corner portions of the shadow mask  5 , and   P 3  is a vertical pitch of the holes at the edge portions of the shadow mask  5  in a direction of a long axis of the shadow mask  5 .       

   Considering that a pitch Ph is the same as a horizontal width S, the mask transmittance of the four corner portions is reduced. This lowers brightness and uniformity (the ratio of the peripheral portions to the brightness of the center portion). In such a structure, the doming is more serious at the area around the holes of the edge portions in the direction of the long axis than at the corner portions. The reason is as follows. It is known that doming is less as the thermal capacity per unit area is larger. However, the conventional shadow mask that is designed to satisfy the condition 
         B3   P3     ≤     B2   P2         
 
has the bridge B 2  at the diagonal corner portions to be greater than the bridge at the edge portion in the direction of the long axis. In this structure, since the thermal capacity at the corner portions in the direction of the long axis is forced to be comparatively less, the doming due to the electron beams is greater at diagonal corner portions. Because of the problems described above, in order to reduce doming, the Invar mask has been used in the conventional shadow mask  5  even though the Invar mask is expensive but the defect in the structure is still raised as a problem. Accordingly, it is required to suggest the shadow mask that overcomes the structural defect of the conventional shadow mask, is less expensive and has improved doming characteristics.
 
   On the other hand, the outer surface of the effective portion is tended to be flat in order to improve affirmation. This requires for making the effective surface of the shadow mask  5  corresponding to the fluorescent screen installed on the inner surface of the effective portion flat. However, if the curvature radius of the shadow mask  5  is simply increased to make it flat, doming is caused since the shadow mask  5  is locally expanded by heat very greatly due to the collision of electron beams  11  of the high density. The doming makes the color purity degenerated. 
   To overcome this problem, the curvature radius in a direction of a long axis of the shadow mask is reduced to increase the curved sustain strength and suppress the doming. However, the curvature radius of the shadow mask cannot be small limitlessly since the curvature of the shadow mask relates to the curvature of the inner surface of the panel. If the curvature radius of the shadow mask is reduced, the curvature radius of the panel is also reduced. If the peripheral portions are thicker than the center portion by the amount more than some threshold, transmittance of the peripheral portions is reduced to lower the brightness of the peripheral portions and affirmation. 
   In this context, the proper curvature radius of the shadow mask should be suggested. 
   On the other hand, the curved surface strength of the shadow mask  5  is weakened due to the flatness of the shadow mask even though the shadow mask made of INVAR material is used in order to suppress the doming. Color cathode ray tube is easily damaged by an external impact. It is not wondering that the cathode ray tube including the shadow mask made of expensive INVAR material is very expensive. 
   Accordingly, the present invention is directed to a shadow mask for a color cathode tube that substantially obviate one or more problems due to limitations and disadvantages of the related art. 
   The present invention is suggested to overcome the above-mentioned problems. An object of the present invention is to provide a shadow mask for a color cathode ray tube to make the brightness of the corner portions proper, improve the bright uniformity of the entire screen and reduce doming at the edge portions in the direction of a long axis by improving the structure of shadow mask. 
   Another object of the present invention is to provide a shadow mask for a color cathode ray tube capable of reducing superior doming characteristic even though AK material is used for the shadow mask. 
   A further object of the present invention is to provide a shadow mask for a cathode ray tube to reduce doming effect, enhance the maintenance strength of the curved surface and provide the curvature radius of the shadow mask so that the ratio of the thickness of the peripheral portion to the thickness of the center portion is proper. 
   SUMMARY OF THE INVENTION 
   To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, there is provided a color cathode ray tube comprising: a panel having a fluorescent surface therein; a funnel coupled sealingly with the panel; electron guns installed in the funnel, for projecting electron beams; a deflection yoke for deflecting the electron beams; and a shadow mask for selecting colors, wherein the shadow mask has a following relation: 
           B2   P2     ≤     B3   P3       ,       
 
where B 1  is a length of a bridge as a distance between holes in a vertical direction at center portion of the shadow mask; B 2  is a length of a bridge as a distance between holes in a vertical direction at four corner portions of the shadow mask; B 3  is a length of a bridge as a distance between holes at edge portions of the shadow mask in a direction of a long axis thereof;′P 1  is a vertical pitch of the holes at the center portion of the shadow mask; P 2  is a vertical pitch of the holes at the four corner portions of the shadow mask; and P 3  is a vertical pitch of the holes at the edge portions of the shadow mask in a direction of a long axis thereof.
 
   In an aspect of the present invention, there is provided a color cathode ray tube comprising: a panel having a fluorescent surface therein; a funnel coupled sealingly with the panel; electron guns installed in the funnel, for projecting electron beams; a deflection yoke for deflecting the electron beams; and a shadow mask for selecting colors, wherein the shadow mask has a following relation: 
         5.5   &lt;     Rh     (     H   2     )       &lt;   8.0     ,       
 
where Rh is the mean curvature radius in the direction of the long axis of the shadow mask, H is a distance between both ends in a direction of a long axis of an effective surface of the shadow mask, Zh is a recess amount in a direction of a tube axis at an end of the long axis of the effective surface with respect to a center of the effective surface, and a mean curvature radius in the direction of the long axis satisfies the following condition: 
       Rh   =         (     H   2     )     2     +         Zh   2       2   *   Zh       .           
 
   It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the present invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the present invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the present invention and together with the description serve to explain the principle of the present invention. In the drawings: 
       FIG. 1  shows schematically a structure of conventional color cathode ray tube; 
       FIG. 2  is a front view of the conventional shadow mask; 
       FIG. 3  is a front view of a shadow mask according to an embodiment of the present invention; 
       FIG. 4  is a side sectional view showing the mean curvature radius in the long axis direction at effective surface center of the shadow mask according to an embodiment of the present invention; 
       FIG. 5  is an approximation graph of weight at the buckling point of the shadow mask illustrated in table 4; 
       FIG. 6  is an approximation graph of ratio of thickness of peripheral portion to thickness of center portion of the panel illustrated in table 5; and 
       FIG. 7  illustrates variation in size of a bridge that is a gap of through holes for electron beams in the shadow mask according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference will now be made in detail to a preferred embodiment of the present invention with reference to the attached drawings. 
     FIG. 3  is a front view of a shadow mask according to an embodiment of the present invention. Referring to the  FIG. 3 , the front surface of a shadow mask  15  is provided with a plurality of slotted holes  151 . Etching using chemicals may be used in forming the holes  151 . The plurality of holes  151  are aligned in horizontal and vertical directions spaced with one another with a predetermined distance. The vertically neighboring holes  151  are connected to each other via the bridge B. The holes  151  are the openings that electron beams pass through and extend vertically. 
   In the structure of the shadow mask  15  according to the present invention shown in  FIG. 3 , the size and the shape of the holes through which electron beams pass satisfy the following relation: 
         B2   P2     ≤     B3   P3         
         where B 1  is a length of a bridge as a distance between holes  151  in a vertical direction at a center portion of the shadow mask  15 ,   B 2  is a length of a bridge as a distance between holes  151  in a vertical direction at four corner portions of the shadow mask  15 ,   B 3  is a length of a bridge as a distance between holes  151  at edge portions of the shadow mask in a direction of a long axis of the shadow mask  15 ,   P 1  is a vertical pitch of the holes  151  at the center portion of the shadow mask  15 ,   P 2  is a vertical pitch of the holes  151  at the four corner portions of the shadow mask  15 , and   P 3  is a vertical pitch of the holes  151  at the edge portions of the shadow mask in a direction of a long axis of the shadow mask  15 .       

   The structure of the shadow mask  15  according to the present invention satisfying the above relation improves brightness and bright uniformity by enhancing the transmittance at corners comparatively more than that at edge portions in the direction of the long axis. The high transmittance implies low thermal capacity per unit area. In other words, it is designed such that the thermal capacity of the edge portions is relatively larger than that of diagonal corner portions, which is advantageous at doming. 
   Preferably, the shadow mask  15  of the present invention satisfies a relation of 
           B1   P1     ≤     B2   P2     ≤     B3   P3       ,       
 
where B 1  is a length of a bridge as a distance between holes  151  in a vertical direction at a center portion of the shadow mask  15 , B 2  is a length of a bridge as a distance between holes  151  in a vertical direction at four corner portions of the shadow mask  15 , B 3  is a length of a bridge as a distance between holes  151  at edge portions of the shadow mask in a direction of a long axis of the shadow mask  15 , P 1  is a vertical pitch of the holes  151  at the center portion of the shadow mask  15 , P 2  is a vertical pitch of the holes  151  at the four corner portions of the shadow mask  15 , and P 3  is a vertical pitch of the holes  151  at the edge portions of the shadow mask in a direction of a long axis of the shadow mask  15 . If 
           B1   P1     &gt;       B2   P2     ⁢           ⁢   or   ⁢           ⁢     B2   P2       &gt;     B3   P3       ,       
 
then the transmittance of the center portion is lowered and thus it is difficult to realize the absolute brightness at the center portion.
 
   Preferably, the shadow mask  15  of the present invention satisfies a relation of B 1 ≦B 2 ≦B 3 . If B 1 &gt;B 2  or B 1 &gt;B 3 , the transmittance of the center portion is reduced. So it is difficult to realize the absolute brightness at the center portion and the doming at peripheral portions gets worse. 
   Preferably, the shadow mask  15  of the present invention satisfies a relation of P 3 ≦P 2 ≦P 1 . If P 1 &lt;P 2 , P 1 &lt;P 3  or P 2 &lt;P 3 , B 1 ≦B 2 ≦B 3  should be very great to satisfy 
         B1   P1     ≤     B2   P2     ≤       B3   P3     .         
 
In this case, the transmittance of the entire shadow mask is reduced, so that it is difficult to make a proper brightness.
 
   Preferably, the shadow mask  15  of the present invention satisfies the relations of 0.10 mm≦B 1 ≦0.18 mm, 0.10 mm≦B 2 ≦0.18 mm, and 0.10 mm≦B 3 ≦0.18 mm. If relationships are B 1 &lt;0.10 mm, B 2 &lt;0.10 mm and B 3 &lt;0.10 mm, then the formability of the shadow mask degenerates since holes explode during the forming process of the shadow mask. If B 1 &gt;0.18 mm, B 2 &gt;0.18 mm and B 3 &gt;0.18 mm, the brightness is lowered in the structures of the general shadow masks. 
   Preferably, the shadow mask  15  of the present invention satisfies the relations of 0.5 mm≦P 1 ≦0.9 mm, 0.5 mm≦P 2 ≦0.9 mm and 0.5 mm≦P 3 ≦0.9 mm. If the vertical pitches P 1 , P 2  and P 3  are equal to or less than 200% of the thickness of the shadow mask, it is difficult to manufacture the shadow mask since etch is not easy. The vertical pitches P 1 , P 2  and P 3  that are too great allows the transmittance to be great, which is advantageous in the brightness characteristic but allows the area occupied by holes per unit area to be increased, resulting in the decrease in the strength of the shadow mask. In other words, if the vertical pitches are to great, the shadow mask will be torn during the forming process of the shadow mask. According to experiments, it is known that the transmittance per unit area is equal to or less than 20% so as to prevent the shadow mask from being torn. Therefore, it is desirable to allow the vertical pitch to have a relation of P≦0.9 mm so as to make the transmittance per unit area be equal to or less than 20%. 
   Considering that the thickness of the conventional slot type shadow mask  15  is in a range of 0.20-0.25 mm, the vertical pitch (P) is made to have a relation of P≧0.5 mm, which corresponds to 200% or more of the ratio of the vertical pitches P 1 , P 2  and P 3  to the thickness of the shadow mask for the enhancement of the etching property. 
   Thus, the vertical pitches P 1 , P 2  and P 3  preferably have the following relations: 0.5 mm≦P 1 ≦0.9 mm, 0.5 mm≦P 2 ≦0.9 mm and 0.5 mm≦P 3 ≦0.9 mm. 
   Table 2 shows relations between holes and bridges of a shadow mask made of AK material used in 21 inches flat cathode ray tube according to an embodiment of the present invention. 
   
     
       
         
             
             
             
             
           
             
                 
               TABLE 2 
             
           
          
             
                 
                 
             
             
                 
               Embodiment 
               Comparative example 
                 
             
          
         
         
             
             
             
             
          
             
                 
               AK1 
               AK2 
               INVAR 
             
             
                 
                 
             
          
         
         
             
             
             
             
             
          
             
                 
               
                 
                   
                     
                       B1 
                       P1 
                     
                   
                 
               
               018 
               0.15 
               0.15 
             
             
                 
               
                 
                   
                     
                       B2 
                       P2 
                     
                   
                 
               
               0.22 
               0.18 
               0.18 
             
             
                 
               
                 
                   
                     
                       B3 
                       P3 
                     
                   
                 
               
               0.24 
               0.17 
               0.17 
             
             
                 
               P1 
               0.65 
               0.75 
               0.75 
             
             
                 
               P2 
               0.64 
               0.73 
               0.73 
             
             
                 
               P3 
               0.64 
               0.73 
               0.73 
             
             
                 
               B1 
               0.12 
               0.12 
               0.12 
             
             
                 
               B2 
               0.14 
               0.13 
               0.132 
             
             
                 
               B3 
               0.15 
               0.12 
               0.12 
             
             
                 
               Doming 
               42 μm 
               70 μm 
               25 μm 
             
             
                 
               B/U 
               50% 
               48% 
               48% 
             
             
                 
                 
             
          
         
       
     
   
   In Table 2, B 1  is a length of a bridge as a distance between holes  151  in a vertical direction at a center portion of the shadow mask  15 , B 2  is a length of a bridge as a distance between holes  151  in a vertical direction at four corner portions of the shadow mask  15 , B 3  is a length of a bridge as a distance between holes  151  at edge portions of the shadow mask in a direction of a long axis of the shadow mask  15 , P 1  is a vertical pitch of the holes  151  at the center portion of the shadow mask  15 , P 2  is a vertical pitch of the holes  151  at the four corner portions of the shadow mask  15 , and P 3  is a vertical pitch of the holes  151  at the edge portions of the shadow mask in a direction of a long axis of the shadow mask  15 . And, the pitches P 1 , P 2  and P 3  and bridges B 1 , B 2  and B 3  are expressed in millimeter unit and B 1 /P 1 , B 2 /P 2  and B 3 /P 3  is dimensionless. The AK 1  and AK 2  are used for the discrimination of the embodiment according to the present invention and the comparative example. It can be seen that the doming and the bright uniformity are improved. 
   Referring to Table 2, the shadow mask made of AK material according to the present invention has a doming value that is different from the doming value of the conventional cathode ray tube having the shadow mask made of Invar, but is less than 70 μm that is the limit value of the color bleeding margin of the flat cathode ray tube, so that its use is possible. 
   The curvature radius of the shadow mask made of AK material according to the idea of the present invention will be described. 
     FIG. 4  is a side sectional view showing the mean curvature radius in long axis direction at effective surface center of the shadow mask according to an embodiment of the present invention. Referring to  FIG. 4 , the mean curvature radius in the direction of the long axis is defined as follows: 
       Rh   =           (     H   2     )     2     +     Zh   2         2   *   Zh                     where Rh is a mean curvature radius in the direction of the long axis of the shadow mask, H is a distance between both ends in a direction of a long axis of an effective surface  16  of the shadow mask  15 , and Zh is a recess amount in a direction of a tube axis at an end of the long axis of the effective surface  16  with respect to a center of the effective surface  16 .       
   Table 3 shows the curvature radii Rh of the shadow masks  15  of the flat cathode ray tubes that are being mass-produced, including the 21 inches mask made of AK material that is the embodiment of the present invention. 
   
     
       
         
             
             
           
             
                 
               TABLE 3 
             
             
                 
                 
             
             
                 
               
                 
                   
                     
                       Rh 
                       
                         ( 
                         
                           H 
                           2 
                         
                         ) 
                       
                     
                   
                 
               
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
          
             
                 
               21″ AK 
               6.68 
             
             
                 
               21″ INVAR 
               8.90 
             
             
                 
               25″ INVAR 
               9.26 
             
             
                 
               29″ INVAR 
               9.98 
             
             
                 
               28″ INVAR 
               8.29 
             
             
                 
               32″ INVAR 
               8.61 
             
             
                 
                 
             
          
         
       
     
   
   Referring to table 3, it is well known that the curvature radius (Rh) of the conventional shadow mask  3  is designed to be great as a whole. To this end, the conventional shadow mask  3  is weak in the maintenance strength of the curved surface, so that it is easily deformed by an impact in manufacturing process. 
   On the other hand, Table 4 shows the strengths of the shadow masks  3  with respect to curvature radii (Rh) in the direction of a long axis. The data correspond to 21″ shadow masks made of AK material and to relative values of weights at buckling point of the five shadow masks (a), (b), (c), (d) and (e). The data of the table 4 were obtained with a critical value of 60 that is converted into a reference value of 100 at which the shadow mask is easily deformed during the manufacturing process of the color cathode ray tubes. The strength data of the shadow masks  3  are computed by CAE (Computer Aided Engineering) simulation. 
   
     
       
         
             
             
             
           
             
                 
               TABLE 4 
             
             
                 
                 
             
             
                 
               
                 
                   
                     
                       Rh 
                       
                         ( 
                         
                           H 
                           2 
                         
                         ) 
                       
                     
                   
                 
               
               Weight at buckling point 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
          
             
               (a) 
               5.0 
               253 
             
             
               (b) 
               6.0 
               198 
             
             
               (c) 
               7.0 
               151 
             
             
               (d) 
               8.0 
               102 
             
             
               (e) 
               9.0 
               53 
             
             
                 
             
          
         
       
     
   
   In table 4, the weight means an endurable maximum weight. The greater the weight is, the greater the strength is. The bucking point is the time when the shadow mask starts to be deformed while the weight is loaded to the entire surface of the shadow mask. 
     FIG. 5  shows an approximation graph of weights at the buckling point of the shadow mask illustrated in Table 4. Referring to  FIG. 5 , the value of the condition 
       Rh     (     H   2     )           
corresponding to the reference value of 100 is obtained based on the approximation line  14  and is 8.0. Based on the above relation, the relation of 
         Rh     (     H   2     )       &lt;   8.0         
is derived as the reference of the equation 
       Rh     (     H   2     )           
to obtain good maintenance strength of the curved surface. Explaining the above relation again, in order to properly maintain the maintenance strength of the curved surface that is significant in flat cathode ray tubes, it is requested that the value of condition 
       Rh     (     H   2     )           
be less than 0.8, which prevents the deformation of the shadow mask  15  that may occur while manufacturing and carrying flat cathode ray tubes.
 
   On the other hand, when a shadow mask made of AK is employed in a flat cathode ray tube, the smaller curvature radius Rh of the shadow mask is advantageous to the minimizing of the doming effect. Meanwhile, in order to keep a predetermined resolution, the curvature radius of the inner surface of the panel  1  should be also small. In this case, the outer surface of the panel  1  is substantially flat and the inner surface of the panel  1  has the predetermined curvature radius. Such a panel  1  is required to configure a flat cathode ray tube to allow TV watchers to see flat images. 
   However, if the ratio of the thickness Td of peripheral portion of the panel  1  to the thickness Tc of a central potion of the panel  1  exceeds some threshold value, the transmittance of the peripheral portions of the panel  1  is reduced, so that the brightness and the visibility are reduced. 
   Table 5 shows the ratio of the thickness Td of the peripheral portion of the panel  1  to the thickness Tc of the central portion of the panel  1  on condition that the curvature radius (Rh) in a direction of a long axis of the shadow mask is changed and the ratio of the transmittance of peripheral portions to the transmittance of central portion is 40%. 
   
     
       
         
             
             
             
             
           
             
                 
               TABLE 5 
             
             
                 
                 
             
             
                 
               
                 
                   
                     
                       Rh 
                       
                         ( 
                         
                           H 
                           2 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   
                     
                       
                         
                           Thickness 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           of 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           peripheral 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           portion 
                         
                         
                           Thickness 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           of 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           central 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           portion 
                         
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         ( 
                         
                           Td 
                           Tc 
                         
                         ) 
                       
                     
                   
                 
               
               Doming amount 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
          
             
               (a) 
               5.0 
               2.6 
               43 μm 
             
             
               (b) 
               6.0 
               2.4 
               51 μm 
             
             
               (c) 
               7.0 
               2.2 
               60 μm 
             
             
               (d) 
               8.0 
               2.0 
               68 μm 
             
             
               (e) 
               9.0 
               1.8 
               77 μm 
             
             
                 
             
          
         
       
     
   
     FIG. 6  is an approximation graph of the data illustrated in the Table 5. Referring to  FIG. 6 , obtained is an approximation line  17  of ratio of thickness of peripheral portion (corner portions) of the panel to thickness of central portion of the panel corresponding to five kinds of shadow masks represented in table 5. 
   On the other hand, the greater thickness ratio of the panel increases the weight of the panel, and the increased weight of the panel causes a problem in the productivity of the panel, resulting in the rise of the production costs, the lowering in the brightness, and a difficulty in the securing of the visibility. To this end, it is requested that the thickness ratio of the panel be less than 2.5 at most. 
   A value of the condition 
       Rh     (     H   2     )         
 
corresponding to the reference value of 2.5 for the thickness ratio is obtained with reference to the approximation graph  17  of  FIG. 6 , and is 5.5.
 
   In other words, in order to reduce the doming effect followed by the application of the shadow mask made of AK material, it is requested that the curvature radius be designed to be small but the value of the condition 
       Rh     (     H   2     )         
 
should be designed to be greater than 5.5 owing to the lowering of the transmittance of the peripheral portion of the panel and the visibility problem as the ratio of the thickness of the peripheral portion of the panel to the thickness of the center portion of the panel increases.
 
   In the meantime, the ratio of the thickness of the peripheral portion of the panel to the thickness of the center portion of the panel increases such that the outer surface of the panel is approximately flat in order to realize a flat picture and the inner surface of the panel has a predetermined curvature. 
   Tables 5 and 6 also show a doming graph  18  as the doming amount caused from thermal expansion of the shadow mask according to the value of 
         Rh     (     H   2     )       .       
 
As will be seen in  FIG. 6 , the doming amount is ranged from 47 μm to 68 μm in the range 
       5.5   &lt;     Rh     (     H   2     )       &lt;     8.0   .         
 
   When the shadow mask made of AK material is used in the 21″ color cathode ray tube of the embodiment of the present invention and the doming amount is 70 μm or less, there is no color purity problem. In addition, when the shadow mask made of AK material is used in another sized color cathode ray tube according to the embodiment of the present invention and the doming amount is 70 μm or less, there is no color purity problem. 
   Table 6 represents the mean curvature radii in the directions of a long axis, a short axis and a diagonal axis of the 21″ shadow mask of the embodiment of the present invention made of AK material and the shadow masks of the conventional flat cathode ray tube. 
   
     
       
         
             
             
             
             
           
             
               TABLE 6 
             
             
                 
             
             
                 
               Rh 
               Rv 
               Rd 
             
             
                 
               (Mean curvature 
               (Mean curvature 
               (Mean curvature 
             
             
                 
               radius, 
               radius, 
               radius, 
             
             
                 
               X-direction) 
               Y-direction) 
               D-direction) 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
          
             
                 
               21″ AK 
               1249 
               1511 
               1545 
             
             
                 
               25″ INVAR 
               2009 
               1807 
               1637 
             
             
                 
               29″ INVAR 
               2488 
               1855 
               2289 
             
             
                 
               28″ INVAR 
               2189 
               2141 
               1910 
             
             
                 
               32″ INVAR 
               2646 
               2311 
               2399 
             
             
                 
                 
             
          
         
       
     
   
   Referring to Table 6, like the definition of the mean curvature radius Rh in the direction of the long axis of the shadow mask, the mean curvature radius Rh in the direction of the short axis of the shadow mask is defined as: 
       Rv   =           (     V   2     )     2     +     Zv   2         2   *   Zv           
         , where Rv is mean curvature radius, V is a distance between both ends in the direction of the short axis of the shadow mask, and Zv is a recess amount in the direction of a tube axis at an end of the short axis of the effective surface  13  with respect to the center of the effective surface  13 .       

   The mean curvature radius Rd in the direction of a diagonal axis of the shadow mask satisfies the following condition: 
       Rd   =           (     D   2     )     2     +     Zd   2         2   *   Zd           
         , where Rd is mean curvature radius, D is a distance between both ends in the direction of the diagonal axis of the shadow mask, and Zd is a recess amount in the direction of the tube axis at an end of the diagonal axis of the effective surface  13  with respect to the center of the effective surface  13 .       

   As a whole, the mean curvature radius in the direction of the short axis in the conventional shadow mask is designed to be smaller than the mean curvature radius in the direction of the long axis in the conventional shadow mask. However, in the cathode ray tube whose fluorescent surface is in a stripe shape, the factor influencing the color purity is X-direction. In order to reduce the doming effect caused by applying a shadow mask made of AK material, the mean curvature radius in the direction of the long axis of the shadow mask should be designed to be smaller than the mean curvature radii in the directions of the short axis and the diagonal axis of the shadow mask. In other words, the expressions Rh&lt;Rv and Rh&lt;Rd should be satisfied. 
   On the other hand, there may be provided a method for reducing the doming effect caused by applying the shadow mask made of AK to a flat cathode ray tube. The method designs the shadow mask  15  thicker. If the shadow mask  15  is thick, thermal expansion can be compensated by elevating the thermal capacity of the shadow mask  15  even though the shadow mask  15  is expanded by heat due to collision of electron beams. However, if the shadow mask  15  is too thick, etching and formability are problematic and the weight of the shadow mask becomes too heavy. Therefore, the weight of the shadow mask  15  of the present invention is limited to a range of 0.20-0.25 mm. 
   With the same principle as the method for elevating the thermal capacity by making the shadow mask thicker, the transmittance of the shadow mask is made smaller at the same thickness, e.g., the interval between the holes formed in the shadow mask  15  is made larger to thereby obtain the same effect. 
   Referring to  FIG. 7 , in the shadow mask according to the present invention, the bridge that is the interval between the holes which electron beams pass through is increased as it goes to the direction of a long axis (X-direction). This makes the thermal capacity of the shadow mask increase as it travels in the X-direction. As a result, the doming effect influencing the color purity in a stripe-shaped flat cathode ray tube can be effectively reduced. 
   The length of the bridge of the shadow mask  15  is preferably limited to a range of 0.12-0.18 mm. This is because the bridge that is too short in length is cut during the forming process of the shadow mask and thus the shadow mask  15  is broken. A further reason why the length of the bridge of the shadow mask  15  should be less than 0.18 mm is that the value of 0.18 is a critical value (20% of the vertical pitch (Pv) ranged from 0.6 mm to 0.9 mm in the conventional shadow mask  3 ) not to cause the lowering problem in the brightness. If the length of the bridge is 0.18 mm or more, the area through which electron beams pass is decreased, so that the brightness is lowered. 
   If the shadow mask of the present invention is used for a cathode ray tube, the doming phenomena at the end of the portions having holes in the direction of a long axis and the brightness lowering at the end of the portions having holes in the direction of a diagonal axis can be avoided. 
   The present invention can enhance the curved surface sustain strength and reduce the thickness difference between the center portion and the peripheral portions, thereby securing the visibility and effectively improving the doming influencing the color purity. 
   The present invention allows the use of a shadow mask made of AK material that costs one third price of the shadow mask of INVAR material so that the production price can be lowered. 
   The forgoing embodiment is merely exemplary and is not to be construed as limiting the present invention. The present teachings can be readily applied to other types of apparatuses. The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art.