Patent Publication Number: US-6987350-B2

Title: Inner shield and cathode ray tube including the same

Description:
CROSS REFERENCE 
   This application claims priority to and the benefit of Korean Patent Application No. 2002-47554 filed on Aug. 12, 2002 and Korean Patent Application No. 2003-31666 filed on May 19, 2003, the entire disclosures of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   (a) Field of the Invention 
   The present invention relates to an inner shield for a color cathode ray tube, and more particularly, to an inner shield for a color cathode ray tube that effectively blocks external magnetic fields such as the earth&#39;s magnetic field to thereby minimize mis-landing of electron beams caused by fluctuations in the external magnetic fields. The present invention also relates to a cathode ray tube including the inner shield. 
   (b) Description of the Related Art 
   A color cathode ray tube (CRT) is a display device in which a phosphor screen is scanned by three electron beams emitted from an electron gun to realize specific images. A path of the three electron beams is altered by the earth&#39;s magnetic field, which is created by the earth&#39;s north and south magnetic poles, to thereby negatively affect purity, raster position, and convergence characteristics of the CRT. 
   The earth&#39;s magnetic field includes a vertical component that is vertical with respect to the earth&#39;s surface (earth&#39;s vertical magnetic field), and a horizontal component that is horizontal to the earth&#39;s surface (earth&#39;s horizontal magnetic field). Movement of the electron beams by the earth&#39;s horizontal magnetic field may be divided into North-South (N-S) electron beams movement and East-West (E-W) electron beams movement, depending on the direction of the CRT. 
   That is, with reference to  FIG. 1A , N-S movement refers to electron beam movement as a result of the magnetic field (indicated by the arrows) in the vertical direction in the figure (N-S direction) that is parallel to a tube axis Z of the cathode ray tube. Further, with reference to  FIG. 1B , E-W movement refers to electron beam movement as a result of the magnetic field (indicated by the arrows) in the horizontal direction in the figure (E-W direction) that is parallel to the screen of the cathode ray tube. 
   Forces received by the electron beams caused by the earth&#39;s magnetic field include a horizontal component and a vertical component. It is mostly the horizontal component that affects picture characteristics of the CRT. This is because with the shadow mask having elongated vertical slots used mainly in consumer CRTs(also referred to as color picture tube) and the shadow mask having dot-shaped holes used mainly in commercial CRTs(also referred to as color display tube), the horizontal component that moves the electron beams in the horizontal direction (x-axis direction) moves the electron beams away from their designated slots or holes. 
   Therefore, an inner shield is mounted within the CRT to minimize movement of the electron beams caused by the influence of the earth&#39;s magnetic field. A conventional inner shield is shown in  FIG. 2 . 
   An inner shield  100  causes offset or reinforcing interference with the earth&#39;s magnetic field in areas surrounding the path of the electron beams to thereby vary distribution of the earth&#39;s magnetic field (in these areas) in directions that minimize changes in the landing of the electron beams. The inner shield  100  is mounted to a mask frame (not shown) and surrounds a path of the electron beams within a funnel (not shown) of the CRT. The inner shield  100  includes an electron gun opening  102  and a screen opening  104  through which the electron beams pass, and a pair of long sections  106  and a pair of short sections  108 . The long sections  106  and the short sections  108  are interconnected to form the electron gun opening  102  and the screen opening  104 . 
   Formed at the end of each of the short sections  108  forming the electron gun opening  102 , is a V-shaped cutaway section  110  for minimizing N-S electron beams movement. A depth h of the V-shaped cutaway sections  110 , which is measured from an apex of the V-shaped cutaway sections  110  to imaginary lines formed flush with edges of the long sections  106  forming the electron gun opening  102 , is inversely proportional to an amount of N-S electron beams movement and directly proportional to an amount of E-W electron beams movement. That is, the greater the depth h of the V-shaped cutaway sections  110 , the greater the reduction in the amount of N-S electron beams movement and the greater the increase in the amount of E-W electron beams movement. 
   As a result of this adverse affect on E-W electron beams movement while favorably affecting N-S electron beams movement, the depth h of the V-shaped cutaway sections  110  is limited to a predetermined range. This means that N-S electron beams movement may be controlled only up to a point. 
     FIG. 3  shows a one-quarter section of a screen, where the x-axis indicates a distance in the horizontal direction from a center  0  of the screen, and the y-axis a distance in the vertical direction from the center  0  of the screen. 
   With reference to  FIG. 3 , since the cutaway sections are formed as V-shaped elements in the conventional inner shield  100 , although the amount of N-S electron beams movement is effectively reduced in a corner area {circle around (3)}, there is limited reduction in the amount of N-S electron beams movement in the remaining areas of measurement ({circle around (1)}, {circle around (2)}, {circle around (4)}, and {circle around (5)}). In particular, in the area {circle around (4)} that has an insufficient amount of screen margin (also referred to as overspill margin), an effective reduction in N-S electron beams movement is unable to be realized such that the overall quality of the CRT is reduced. 
   Screen margin refers to the amount of margin that exists before the electron beams land on adjacent phosphors of another color when landing errors occur. Screen margin is affected by such factors as pitch, phosphor width, electron beam size (mask hole size), landing errors, and phosphor arrangement errors. 
   In a CRT where the mask and screen are formed in a globe-like shape about a line passing through a tube axis of the electron gun, deviation in the emission angle of the electron beams results in identical amounts of deviation over the entire area of the screen. However, in a CRT where the mask and screen are formed in a flat configuration, the amount of deviation on the screen is not identical over the entire area of the screen. That is, for a flat screen, the same deviation in the emission angle of the electron beams translates into larger amounts of deviation on the screen at peripheral areas thereof. It is for this reason that there is an insufficient screen margin in the area {circle around (4)} as described above. 
   SUMMARY OF THE INVENTION 
   In one embodiment, the present invention provides an inner shield for a color cathode ray tube that effectively reduces N-S electron beams movement, while at the same time preventing increases in the amount of E-W electron beams movement. The present invention also provides a cathode ray tube having the inner shield. 
   An inner shield for a cathode ray tube includes a screen opening and an electron gun opening through which electron beams pass, and a main body including a pair of long sections and a pair of short sections interconnected to form the screen opening and the electron gun opening. Each of the short sections includes a cutaway section having a pair of first cutaway sections formed starting from the electron gun opening and extending at a predetermined angle for a predetermined distance toward the screen opening, and a second cutaway section formed starting from where the first cutaway sections end and extending inwardly toward the screen opening, the second cutaway section being formed to extend past imaginary lines formed from where the first cutaway sections end to a furthermost inward point of the second cutaway section, that is, the point of the second cutaway section closest to the screen opening. 
   In one exemplary embodiment, the second cutaway section is formed as a circular arc or in substantially a circular arc configuration. In this case, if the second cutaway section is extended using imaginary lines to complete the circle started by its circular arc shape, a center of this circle is closer to or alternatively, farther from the screen opening than an imaginary line formed by connecting two points of the first cutaway sections where each of the first cutaway sections start in the electron gun opening. 
   In another exemplary embodiment, the second cutaway section is formed as a predetermined section of an ellipse or in substantially an elliptical form (ellipse). In this case, if the second cutaway section is extended using imaginary lines to complete the ellipse, a center of this ellipse is closer to or alternatively, farther from the screen opening than an imaginary line formed by connecting two points of the first cutaway sections where each of the first cutaway sections start in the electron gun opening. 
   In another exemplary embodiment, if an imaginary line is drawn between two points where the first cutaway sections end, the second cutaway section is formed as a trapezoid with the imaginary line forming the base of the trapezoid. 
   In the case of a consumer CRT(also referred to as color picture tube) having a slot-type phosphor pattern, the present invention is configured with a depth of the cutaway sections being 50% or less of a height of the inner shield, preferably between 44 and 48% of the height of the inner shield. Here, the height of the inner shield is measured as a length between the electron gun opening and the screen opening, and the depth of the cutaway sections is measured in the same direction. Such a configuration ensures that E-W electron beams movement does not abruptly increase. 
   In the case of a commercial CRT(also referred to as color display tube) having a dot-type phosphor pattern, the present invention is configured with the depth of the cutaway sections being equal or greater than 50% of the height of the inner shield. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and together with the description, serve to explain the principles of the invention. 
       FIGS. 1A and 1B  are schematic view used to describe N-S and E-W electron beams movements. 
       FIG. 2  is a perspective view of a conventional inner shield. 
       FIG. 3  shows a one-quarter section of a CRT screen and indicates various points at which N-S movement and E-W movement of electron beams caused by external magnetic fields are measured. 
       FIG. 4  is a rear perspective view of a cathode ray tube having an inner shield, according to an embodiment of the present invention. 
       FIG. 5  is a perspective view of an inner shield according, to a first embodiment of the present invention. 
       FIG. 6  is a side view of the inner shield of  FIG. 5 . 
       FIGS. 7A ,  7 B, and  7 C are side views of first, second, and third modified examples of the inner shield of  FIG. 5  respectively. 
       FIG. 8  is a graph used to compare horizontal forces Fx on electron beams moving toward area {circle around (4)} of  FIG. 4 , when the inner shield of  FIG. 5  and the conventional inner shield are applied to a cathode ray tube. 
       FIG. 9  is a side view of an inner shield, according to a second embodiment of the present invention. 
       FIG. 10  is a side view of an inner shield, according to a third embodiment of the present invention. 
       FIG. 11  is a side view of an inner shield, according to a fourth embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. 
     FIG. 4  is a rear perspective, transparent view of a cathode ray tube having an inner shield according to an embodiment of the present invention. 
   With reference to the drawing, a cathode ray tube (CRT)  10  according to an embodiment of the present invention includes a face panel  12 , a neck  16 , and a funnel  14  interconnecting the face panel  12  and the neck  15 . The face panel  12 , neck  16 , and funnel  14  form a tube assembly  18 , an inside of which is maintained in a high vacuum state. 
   A phosphor screen  12 ′ is formed on an inner surface of the face panel  12 . The phosphor screen  12 ′ is realized through a plurality of red, green, and blue phosphors. An electron gun  20  that emits electron beams toward the phosphor screen  12 ′ is mounted within the neck  16 . Also, a deflection yoke (not shown) is mounted to an outer circumference of the funnel  14 . The deflection yoke generates a deflection magnetic field for deflecting the electron beams emitted from the electron gun  20 . 
   A shadow mask  24  having a plurality of electron beam apertures  22  formed therein is suspended at a predetermined distance from the phosphor screen  12 ′ of the face panel  12  by a mask frame  16 . One end of a magnetic blocking device, that is, an inner shield  28 , is mounted to the mask frame  26 . The inner shield  28  encompasses a section of the path of the electron beams. 
   In the CRT  10  structured as in the above, electron beams (not shown) corresponding to picture signals emitted from the electron gun  20  are deflected by the magnetic field generated by the deflection yoke and pass through the electron beam apertures  22  of the shadow mask  24  to undergo color separation and thereby land on designated phosphors of the phosphor screen  12 ′. 
   In the above process, a path of the electron beams is varied by influence of external magnetic fields. A horizontal force Fx and a vertical force Fy acting on each of the electron beams may be respectively expressed as:
 
 Fx=−e ( VyBz−VzBy )
 
 Fy=−e ( VxBz−VzBx ),  [equation 1]
 
   where e is the electric charge in coulombs (C); Vx, Vy, and Vz are velocities (m/s) of the electron beams in a horizontal direction (x-axis), a vertical direction (y-axis), and a tube axis direction (z-axis) of the CRT  10 ; respectively. Bx, By, and Bz are strengths (T) of the components of the earth&#39;s magnetic field in the horizontal direction (x-axis), vertical direction (y-axis), and the tube axis direction (z-axis) of the CRT  10  respectively. 
   As is evident from equation 1, the force Fx acting on each of the electron beams in the horizontal direction is determined by the strengths of the earth&#39;s magnetic field in the peripheries of each of the electron beams, if the electron beam velocities are constant. That is, the force Fx is proportional to the difference between Bz and By. Similarly, the force Fy acting on the electron beams in the vertical direction is proportional to the difference between Bz and Bx, when it is assumed that the electron beam velocities are constant. 
   Therefore, it is clear that in order to reduce movement of the electron beams caused by external magnetic fields, the component Bz that is parallel to the tube axis (z-axis) of the CRT  10  must be induced toward the component Bx or the component By. The inner shield of the present invention varies the distribution of magnetic fields using a structure described below such that N-S electron beams movement of the electron beams is minimized. In the embodiments described below, the inner shield is by way of example, approximately 184 mm in length and approximately 161 mm in height. 
     FIG. 5  is a perspective view of an inner shield according to a first embodiment of the present invention, and  FIG. 6  is a side view of the inner shield of  FIG. 5 . By way of example, application of the inner shield is made to the CRT  10  of  FIG. 4 . 
   An inner shield  28  of the first embodiment of the present invention includes a pair of long sections  30  provided opposing one another in a vertical direction (y-axis direction), and a pair of short sections  32  provided opposing one another in a horizontal direction (x-axis direction). The long sections  30  and the short sections  32  are interconnected to encompass a section of the path of the electron beams emitted from the electron gun  20 . 
   Flanges  34  are formed along edges of the long sections  30  closest to the phosphor screen  12 ′ of the face panel  12  when the inner shield  28  is mounted within the CRT  10 . The flanges  34  are connected to the mask frame  26  to thereby realize the mounting of the inner shield  28  within the tube assembly  18  of the CRT  10 . An electron gun opening  36  and a screen opening  38  are defined by the long sections  30  and the short sections  32 , and are formed at opposite ends of the inner shield  28 . That is, the inner shield  28  is mounted such that the screen opening  38  is adjacent to the shadow mask  24 , and the electron gun opening  36  is closest to the electron gun  20 . 
   Further, cutaway sections  40  are formed at the ends of the short sections  32  forming the electron gun opening  36 . The cutaway sections  40  act to minimize N-S electron beams movement of the electron beams. Each of the cutaway sections  40  includes two first cutaway sections  42  that are formed each starting from the electron gun opening  36  and extending at a predetermined angle for a predetermined distance toward the screen opening  38 , and a second cutaway section  44  formed starting from where each of the first cutaway sections  42  ends and extending in a predetermined circular arc shape for a predetermined distance toward the screen opening  38 . As shown in  FIG. 6 , the circular arc shape of the second cutaway section  44  is formed such that the circular arc extends outside and encompasses two imaginary lines L 1 . Each of the imaginary line L 1  formed between where a respective first cutaway section  42  ends to a furthermost inward point C 1  of the second cutaway section  44 , that is, the point C 1  of the second cutaway section  44  closest to the screen opening  38 . 
   If the second cutaway section  44  is extended using imaginary lines to complete the circle started by its circular arc shape as shown in  FIG. 6 , a radius r 1  of this circle is approximately 75 mm, and a center C 2  of the circle is closer to the screen opening  38  than an imaginary line L 2  formed connecting points of the first cutaway sections  42  where the first cutaway sections  42  start in the electron gun opening  36 . 
   A height H of the inner shield  28  (i.e., a distance from the screen opening  38  to the electron gun opening  36 ) for use in a 34-inch CRT is approximately 161 mm. Therefore, the cutaway sections  40  structured in this manner have a depth D that is approximately 46% of the height H of the inner shield  28 . 
   Results of measuring the horizontal force Fx acting on the electron beams by external magnetic fields while the electron beams emitted from the electron gun  20  are moving in the area {circle around (4)} of  FIG. 3  are shown in the graph of  FIG. 8 . In  FIG. 8 , the dotted line curve represents the effect with the conventional inner shield having the V-shaped cutaway sections with a depth that is 46% of the height of the inner shield, and the solid line curve represents the effect with the inner shield  28  of the first embodiment of the present invention illustrated in  FIGS. 5 and 6 . 
   The eclipse appearing in  FIG. 8  represents the screen opening of the inner shields. It is clear from the drawing that with the use of the inner shield  28  of the first embodiment of the present invention, the horizontal force Fx acting on the electron beams is greater than when using the conventional inner shield. This indicates that in the present invention, the component Bz of the earth&#39;s magnetic field in the tube axis direction of the CRT is converted to a greater degree to the component By of the earth&#39;s magnetic field in the vertical direction. 
   Amounts of N-S movement and of E-W movement of the electron beams at each area of measurement ({circle around (1)}˜{circle around (5)}) of  FIG. 3  are shown in Table 1 below. 
   
     
       
         
             
             
             
           
             
                 
               TABLE 1 
             
           
          
             
                 
                 
             
             
                 
               N-S Movement (μm) 
               E-W Movement (μm) 
             
          
         
         
             
             
             
             
             
             
             
             
             
             
             
          
             
                 
               {circle around (1)} 
               {circle around (2)} 
               {circle around (3)} 
               {circle around (4)} 
               {circle around (5)} 
               {circle around (1)} 
               {circle around (2)} 
               {circle around (3)} 
               {circle around (4)} 
               {circle around (5)} 
             
             
                 
                 
             
          
         
         
             
             
             
             
             
             
             
             
             
             
             
          
             
               Prior Art 
               43.6 
               46.9 
               43 
               46.3 
               20.6 
               0 
               39.5 
               63.6 
               53.9 
               42 
             
             
               Embodiment 1 
               42.4 
               44.8 
               35.7 
               38.7 
               10.2 
               0 
               41.42 
               67.07 
               58.01 
               49.86 
             
             
               Modified 
               43.25 
               46.2 
               42.24 
               45.08 
               16.52 
               0 
               40.48 
               63.88 
               54.42 
               44.71 
             
             
               example 1 
             
             
               Modified 
               41.42 
               42.4 
               21.3 
               28.75 
               12.24 
               0 
               42.39 
               75.2 
               63.95 
               49.47 
             
             
               example 2 
             
             
                 
             
          
         
       
     
   
   As shown in Table 1, the inner shield  28  of the first embodiment of the present invention significantly reduces N-S movement of the electron beams over the conventional inner shield with the V-shaped cutaway sections of the same depth D as the cutaway sections of the present invention. 
   Hence, the inner shield  28  of the first embodiment of the present invention significantly reduces N-S movement of the electron beams while preventing an increase in E-W movement of the electron beams. Substantial reductions are realized particularly in areas {circle around (3)} and {circle around (4)} indicated in  FIG. 3 . 
     FIG. 7A  is a side view of a first modified example of the inner shield of  FIGS. 5 and 6  of the first embodiment of the present invention. In the first modified example, only the second cutaway sections  44  of the inner shield  28  according to the first embodiment of the present invention are changed in configuration (with a resulting change in the first cutaway sections  42  of the inner shield  28  according to the first embodiment). 
   In more detail, an inner shield  28   a  of the first modified example includes cutaway sections  40   a  that are formed at the ends of the short sections  32  forming the electron gun opening  36  as in the first embodiment. Each of the cutaway sections  40   a  includes two first cutaway sections  42   a  that are formed starting from the electron gun opening  36  and extending at a predetermined angle for a predetermined distance toward the screen opening  38 , and a second cutaway section  44   a  formed starting from where the first cutaway sections  42   a  end and extending in a predetermined arc shape for a predetermined distance toward the screen opening  38 . 
   If each of the second cutaway sections  44   a  is extended using imaginary lines to complete the circle started by its circular arc shape as shown in  FIG. 7A , a radius r 2  of this circle is 65 mm. This reduction in radius over the first embodiment results in the first cutaway sections  42   a  being formed to a greater length than the first cutaway sections  42  of the first embodiment shown in  FIGS. 5 and 6 . The remainder of the configuration of the first modified example is identical to that of the first embodiment. 
     FIG. 7B  is a side view of a second modified example of the inner shield of  FIGS. 5 and 6  of the first embodiment of the present invention. 
   An inner shield  28   b  of the second modified example includes cutaway sections  40   b  that are formed at the ends of the short sections  32  forming the electron gun opening  36  as in the first embodiment. Each of the cutaway sections  40   b  includes two first cutaway sections  42   b  that are formed starting from the electron gun opening  36  and extending at a predetermined angle for a predetermined distance toward the screen opening  38 , and a second cutaway section  44   b  formed starting from where the first cutaway sections  42   b  end and extending in a predetermined circular arc shape for a predetermined distance toward the screen opening  38 . 
   If each of the second cutaway sections  44   b  is extended using imaginary lines to complete the circle started by its circular arc shape as shown in  FIG. 7B , a radius r 3  of this circle is approximately 85 mm. This increase in radius over the first embodiment results in the first cutaway sections  42   b  being formed with a smaller length than the first cutaway sections  42  of the first embodiment shown in  FIGS. 5 and 6 . The remainder of configuration of the second modified example is identical to that of the first embodiment. 
   N-S movement and E-W movement test results for the first and second modified examples of  FIGS. 7A and 7B  also appear in Table 1 respectively. 
   The inner shields  28  and  28   a  of the first embodiment and of the first modified example may be applied to both CPT(color picture tube)s and CDT(color display tube)s. This is because with these inner shields  28  and  28   a , the depths D and D 1  of the cutaway sections  40  and  40   a  are less than 50% of the heights H of the inner shields  28  and  28   a , thereby preventing a reduction in picture quality caused by increases in E-W electron beam movement. 
   The CPTs mentioned above refer to CRTs having slot-shaped phosphors on the phosphor screen  12 ′ that experience a reduction in picture quality with increases in E-W electron beams movement. The CDTs refer to CRTs having dot-shaped phosphors on the phosphor screen  12  in which E-W electron beams movement is limited such that picture quality is not significantly affected by the E-W movement. 
   As described above, the depths D and D 1  of the cutaway sections  40  and  40   a  are less than 50% of the inner shield heights H. Preferably, the depths D and D 1  of the cutaway sections  40  and  40   a  are between 40% and 48% of the heights H of the inner shields  28  and  28   a  to thereby allow application of the inner shield to both CPTs and CDTs. 
   With respect to the second modified example of  FIG. 7B , since the depth D 2  of the cutaway section  40   b  is greater than 50% of the inner shield height H, the inner shield  28   b  of this modified example may be applied to CDTs. 
     FIG. 7C  is a side view of a third modified example of the inner shield of  FIGS. 5 and 6  of the first embodiment of the present invention. 
   An inner shield  28   c  of the second modified example includes cutaway sections  40   c  that are formed at the ends of the short sections  32  forming the electron gun opening  36  as in the first embodiment. Each of the cutaway sections  40   c  includes two first cutaway sections  42   c  that are formed starting from the electron gun opening  36  and extending at a predetermined angle for a predetermined distance toward the screen opening  38 , and a second cutaway section  44   c  formed starting from where the first cutaway sections  42   c  end and extending in a predetermined circular arc shape for a predetermined distance toward the screen opening  38 . 
   If each of the second cutaway section  44   c  is extended using imaginary lines to complete the circle started by its circular arc shape, as shown in  FIG. 7C , a center C 2  of the circle is positioned outside of the area encompassed by the inner shield  28   c . That is, the center C 2  of the circles is positioned farther from the screen opening  38  than an imaginary line L 2  formed by connecting points of the first cutaway sections  42   c  where the first cutaway sections  42   c  start in the electron gun opening  36 . 
   In the above examples, the second cutaway sections  44 ,  44   a ,  44   b , and  44   c  are described as being formed in circular arc shapes. However, as long as the second cutaway section extends outside and encompass imaginary lines formed from where the first cutaway sections end to a furthermost inward point of the second cutaway sections, the second cutaway sections may be formed in a variety of different configurations. Examples of such different configurations are shown in  FIGS. 9 ,  10 , and  11 . 
     FIG. 9  is a side view of an inner shield according to a second embodiment of the present invention. 
   As shown in  FIG. 9 , an inner shield  46  of the second embodiment includes cutaway sections  48  that are formed at the ends of short sections  32  forming an electron gun opening  36 , as in the first embodiment. Each of the cutaway sections  48  includes two first cutaway sections  50  that are formed starting from the electron gun opening  36  and extending at a predetermined angle for a predetermined distance toward the screen opening  38 , and a second cutaway section  52  formed starting from where the first cutaway sections  50  end, and extending in a predetermined configuration for a predetermined distance toward the screen opening  38 . 
   If the second cutaway section  52  is extended using imaginary lines, as shown in  FIG. 9 , the second cutaway section  52  is formed into a multilateral shape such as an octagon. In this case, the second cutaway sections  52  form four sides of the octagon. Depending on CRT type and characteristics, a center C 2  of the octagon may be positioned farther from or closer to the screen opening  38  than an imaginary line L 2  formed by connecting points of the first cutaway sections  50  where the first cutaway sections  50  start in the electron gun opening  36 . Further, a distance r from the center C 2  to one of the corners of the octagon and a depth D of the cutaway sections  48  may be varied as needed. 
     FIG. 10  is a side view of an inner shield according to a third embodiment of the present invention. 
   As shown in  FIG. 10 , an inner shield  54  of the third embodiment includes cutaway sections  56  that are formed at the ends of short sections  32  forming an electron gun opening  36  as in the first embodiment. Each of the cutaway sections  56  includes two first cutaway sections  58  that are formed starting from the electron gun opening  36  and extending at a predetermined angle for a predetermined distance toward the screen opening  38 , and a second cutaway section  60  formed starting from where the first cutaway sections  58  end and extending in a predetermined configuration for a predetermined distance toward the screen opening  38 . 
   If the second cutaway section  60  is extended using imaginary lines as shown in  FIG. 9 , the second cutaway section  60  is formed into an elliptical shape. Although not shown, the second cutaway section  60  may also be formed into a multilateral configuration that has the general overall shape of an ellipse. In the case of the elliptical formation of the second cutaway section  60  shown in the drawing, the second cutaway section  60  may be varied in its dimensions, that is, a distance r from a center C 2  of the ellipse to an end point of the minor axis (i.e., to one of the co-vertices) may be varied as needed. 
     FIG. 11  is a side view of an inner shield according to a fourth embodiment of the present invention. 
   As shown in  FIG. 11 , an inner shield  62  of the fourth embodiment includes cutaway sections  64  that are formed at the ends of short sections  32  forming an electron gun opening  36  as in the first embodiment. Each of the cutaway sections  64  includes two first cutaway sections  66  that are formed starting from the electron gun opening  36  and extending at a predetermined angle for a predetermined distance toward the screen opening  38 , and a second cutaway section  68  formed starting from where the first cutaway sections  66  end and extending in a predetermined configuration for a predetermined distance toward the screen opening  38 . 
   For each of the cutaway sections  64 , if an imaginary line  69  is drawn between two points where the first cutaway sections  66  end (or where the second cutaway section  68  begins), the second cutaway section  68  is formed as a trapezoid with the imaginary line  69  forming the base of the trapezoid. 
   As described above, the inner shield of the present invention having first and second cutaway sections effectively reduces N-S movement of the electron beams at all areas of the screen and that are caused by external magnetic fields, while preventing an increase in E-W movement. Therefore, a reduction in purity, raster distortion, and convergence characteristic variations caused by external magnetic fields such as the earth&#39;s magnetic field are minimized. 
   Although embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that many variations and/or modifications of the basic inventive concepts herein taught which may appear to those skilled in the present art will still fall within the spirit and scope of the present invention, as defined in the appended claims.