Patent Publication Number: US-6700320-B2

Title: Cathode ray tube with structure for preventing electron beam mis-landing caused by geomagnetism

Description:
CLAIM OF PRIORITY 
     This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from an application for CATHODE RAY TUBE HAVING MEANS FOR PREVENTING MIS-LANDING OF ELECTRON BEAMS BY EARTH MAGNETISM earlier filed in the Korean Industrial Property Office on May 18, 2001 and there duly assigned Serial No. 2001-27250. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a cathode ray tube (CRT), and more particularly, to a cathode ray tube with a structure for preventing electron beam mis-landing caused by geomagnetism. 
     2. Related Art 
     Generally, a CRT is designed to realize an image by scanning a phosphor screen deposited with red R, green G, and blue B phosphors with electron-beams emitted from an electron gun. 
     The electron beams are deflected by a deflection yoke and landed on desired phosphors to scan the peripheral portion of the phosphor screen as well as the central portion. 
     However, when the electron beams are deflected, they are affected by outer magnetic fields such as geomagnetism, and the electron beams can be landed on an undesired phosphor. This so-called mis-landing deteriorates the color purity of the cathode ray tube. 
     To solve the above problem, a magnetic field shield member such as an inner shield for shielding the electron beams from geomagnetism has been employed in the CRT. The inner shield is generally mounted on a color selection apparatus composed of a shadow mask and a mask frame, which is disposed inside the cathode ray tube. 
     In recent years, a flat screen panel has been developed to improve the definition of an image realized at a peripheral portion of the large-sized screen. Accordingly, the color selection apparatus employed to realize colors in the CRT has been also flattened and increased in size so that it can be properly associated with the flat screen panel. 
     That is, a color selection apparatus includes a shadow mask provided with plural electron-beam-passing apertures and a frame for supporting the shadow mask applied with a predetermined tension. The frame includes a pair of elastic members and a pair of supporting members coupled to the elastic members, the shadow mask being mounted on the supporting members. 
     Such a color selection apparatus is mounted inside a panel on an inner surface of which a phosphor screen is formed. An inner shield is mounted on the supporting members and the elastic members such that it encloses electron beam emission traces to shield the electron beams from the geomagnetism. 
     Geomagnetism includes a vertical component and a horizontal component. The horizontal component can be classified as a north-south direction component (N-S component) that is in parallel with a tube axis, and an east-west direction component (E-W component) that is perpendicular to the tube axis. In the related art, to shield the electron beams from the horizontal component, a V-shaped notch or a piercing portion is formed on the inner shield. 
     However, the color selection apparatus still has a weakness against the E-W component of the geomagnetism. 
     That is, the E-W component is applied to lateral sides of the panel in a longitudinal direction. Therefore, since a space between the elastic members and the shadow mask and a space between the shadow mask and the phosphor screen are not shielded from the inner shield, the electron beams passing through these spaces are affected by the E-W component. This causes the electron beams to land on undesired phosphors, deteriorating the color purity of the cathode ray tube. 
     To solve the above problems, Japanese unexamined patent application having publication number No. H10-50228 for a  Color Cathode - ray Tube  by Teruhisa discloses a color cathode ray tube having shielding means for shielding the electron beams from outer magnetic fields applied between the color selection apparatus and the phosphor screen at the corners of the frame. However, since the shielding means is designed to enclose the corners of the frame, the amount of horizontal shift of the electron beams may be increased. 
     That is, when the shielding means is designed to cover the corners of the frame, the part of geomagnetism applied to the lateral sides of the frame flows into the longitudinal sides. Accordingly, the electron beams directed toward the corners are affected by the geomagnetism, and as a result, the amount of horizontal shift of the electron beams is increased. This causes the electron beams to land on undesired phosphors, deteriorating the color purity at the corners of the screen of the cathode ray tube. 
     SUMMARY OF THE INVENTION 
     Therefore, the present invention has been made in an effort to solve the above-described and other problems. 
     It is therefore an objective of the present invention to provide a cathode ray tube that is designed to minimize the effect on the electron beams by the geomagnetism, thereby improving the color purity of the cathode ray tube by enhancing the beam-landing accuracy. 
     It is another objective to provide a cathode ray tube that is designed to minimize the effect on the electron beams by geomagnetism and yet be easy and cost efficient to manufacture. 
     It is still another objective to provide an apparatus in a cathode ray tube that is designed to minimize the effect on the electron beams by geomagnetism while avoiding to affect the electrons beams in any other manner by the apparatus. 
     To achieve the above and other objectives, the present invention provides a cathode ray tube, including a panel having a front screen portion on which a phosphor screen is formed and a panel flange formed on an edge of the front screen portion; a funnel connected to the panel flange; a deflection yoke disposed around the funnel; a neck connected to the funnel; an electron gun disposed in the neck; a color selection apparatus for selecting electron beams emitted from the electron gun and allowing the selected electron beams to land on corresponding phosphors, the color selection apparatus including a frame having a pair of supporting members disposed at a predetermined distance from each other in parallel and a pair of elastic members fixed on both ends of the supporting members to correspond to lateral sides of the mask; and a shield apparatus for shielding geomagnetism, the shield apparatus being mounted on a perimeter of the frame of the color selection apparatus and extended toward the neck, wherein the shield apparatus includes disconnection parts defined corresponding to corners of the frame, the shield apparatus being extended toward the phosphor screen over one of longitudinal and lateral sidewalls of the frame. 
     According to an embodiment, the shield apparatus includes a main shield member having a body provided with an electron beam-passing opening and extended toward the neck, and a skirt extended from the body and disposed on longitudinal sides of the frame; and a sub-shielding member disposed on lateral sides of the frame. 
     Preferably, the skirt is fixed on the supporting members while covering a center of the supporting members. The skirt is formed to be asymmetrical with reference to a central portion of the supporting members. 
     Preferably, the main shield member is mounted on the frame while not enclosing the elastic members, and the sub-shield member includes a shielding part disposed along the lateral sides of the frame to cover a space defined between the mask and the elastic members; and coupling parts extended from the shielding part and fixed on the elastic members. 
     Preferably, a width of the shielding part is greater than that of the frame. 
     Further, preferably the skirt is connected to the supporting members while satisfying the following condition: 
     
       
         0.01 mm≦ tk/L≦ 0.15 mm 
       
     
     where t is a thickness of the sub-shield member, k is a length of the skirt extending from the mask toward the phosphor screen over the supporting member, and L is a height of the supporting member. 
     Further, preferably the sub-shield member is fixed on the supporting member under the following condition: 
     
       
         0.01 mm≦ t′k′/L≦ 0.15 mm 
       
     
     where t′ is a thickness of the sub-shield member, k′ is a length of the sub-shield member from the mask toward the phosphor screen, and L′ is a height of the supporting member. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
     FIG. 1 is a perspective view of a color selection apparatus according to a preferred embodiment of the present invention; 
     FIG. 2 is a sectional view of a cathode ray tube where a color selection apparatus according to a preferred embodiment of the present invention is employed; 
     FIG. 3 is a perspective view of a modified example of a color selection apparatus according to the present invention; 
     FIG. 4 is a partial sectional view taken along line III—III of FIG. 2; 
     FIG. 5A is a graph illustrating a distribution curve of a horizontal component of the geomagnetism generated according to the present invention; 
     FIG. 5B is a graph illustrating a distribution curve of a horizontal component of the geomagnetism generated according to the prior art; 
     FIGS. 6A and 6B are graphs illustrating the relation between k 1 /L and the amount of electron beam shift in the present invention; and 
     FIG. 7 is an exploded perspective view of a conventional cathode ray tube. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to the drawings, as shown in FIG. 7, a color selection apparatus  1  includes a shadow mask  3  provided with plural electron-beam-passing apertures  3   a  and a frame  5  for supporting the shadow mask applied with a predetermined tension. The frame  5  includes a pair of elastic members  5   b  and a pair of supporting members  5   a  coupled to the elastic members  5   b , the shadow mask  3  being mounted on the supporting members  5   a.    
     Such a color selection apparatus is mounted inside a panel  9  on an inner surface of which a phosphor screen  7  is formed. An inner shield  11  is mounted on the supporting members  5   a  and the elastic members  5   b  such that it encloses electron beam emission traces to shield the electron beams from the geomagnetism. 
     Geomagnetism includes a vertical component and a horizontal component. The horizontal component can be classified as a north-south direction component (N-S component) that is in parallel with a tube axis, and an east-west direction component (E-W component) that is perpendicular to the tube axis. In the related art, to shield the electron beams from the horizontal component, a V-shaped notch  11   a  or a piercing portion  11   b  is formed on the inner shield  11 . 
     However, the color selection apparatus  1  still has a weakness against the E-W component of the geomagnetism. 
     That is, the E-W component is applied to lateral sides of the panel  9  in a longitudinal direction (see arrows in FIG.  4 ). Therefore, since a space between the elastic members  5   b  and the shadow mask  3  and a space between the shadow mask  3  and the phosphor screen  7  are not shielded from the inner shield  11 , the electron beams passing through these spaces are affected by the E-W component. This causes the electron beams to land on undesired phosphors, deteriorating the color purity of the cathode ray tube. 
     Preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
     FIG. 1 shows a color selection apparatus of the present invention and FIG. 2 shows a cathode ray tube having the color selection apparatus depicted in FIG.  1 . 
     As shown in the drawings, a cathode ray tube includes a panel  20  having a screen portion  20   a , on an inner surface of which a phosphor screen  22  is formed, and a panel flange  20   b  is integrally formed on an edge of the screen portion  20   a . A funnel  23  connected to the panel flange  20   b  of the panel  20 , and a neck  24  is connected to the funnel  23 . A deflection yoke  26  is mounted around the funnel  23 , and an electron gun  28  for emitting electron beams is mounted in the neck  24 . 
     A color selection apparatus is disposed inside the panel  20  so as to select red R, green G, and blue B electron beams emitted from the electron gun  28 . Such a color selection apparatus  30  is designed to employ a tensioned mask  32  provided with plural electron beam-passing apertures  32   a . The tensioned mask  32  is rectangular, having a longitudinal axis X and a lateral axis Y. 
     The mask is tensioned in a direction of the longitudinal axis X or the lateral axis Y, and is mounted on a frame  34 . The frame  34  includes a pair of supporting members  34   a  disposed at a predetermined distance from each other in parallel along the longitudinal axis, and a pair of elastic members  34   b  disposed in parallel along the lateral axis and fixed on both ends of the supporting members  34   a  to define a rectangular frame with the supporting members  34   a.    
     In this embodiment, the mask  32  is tensioned in a direction of the lateral axis Y and is welded on the top surfaces of the supporting members  34   a . Each of the elastic members  34   b  is U-shaped to maintain the tensioned state of the mask  32 . The tension applied to the periphery of the mask  32  is greater than that applied to the center of the mask  32 . 
     The color selection apparatus  30  is disposed inside the panel  20  such that the tensioned mask  32  faces the phosphor screen  22 . That is, the color selection apparatus  30  is mounted on the panel flange  20   b  of the panel  20  by coupling means including a hook  36  and a spring  38 . 
     In this embodiment, a shield apparatus  39  is disposed on the color selection apparatus  30  to shield the electron beams from the geomagnetism affecting the emission trace defined by the frame  34 . The shield apparatus  39  includes a main shield member  40  and a sub-shield member  42 . 
     The main shield member  40  includes a main body  40   b  provided with an electron beam-passing opening  40   a  defining the electron beam emission trace, and it is disposed on the top of the supporting members  34   a  of the frame  34 , with a skirt  40   c  integrally formed with the main body  40   b  extended downward to skirt the longitudinal sidewalls of the frame  34 . 
     The skirt  40   c  is extended toward the phosphor screen  22  over the mask  32  and is fixed on the supporting members  34   a.    
     The sub-shield member  42  is separately formed from the main shield member  40  and is disposed on sidewalls of the elastic member  34   b . The sub-shield member  42  includes a longitudinal shielding part  42   a  disposed in a longitudinal direction to cover the space between the mask  32  and the elastic member  34   b , and bridges  42   b  integrally extended from the shielding part  42   a  and fixed on the elastic members  34   b . The width of the longitudinal shielding part  42   a  is designed to be greater than that of the lateral sides of the frame  34 . 
     FIG. 3 shows an modified example of a sub-shield member of the present invention. 
     In this example, a sub-shield member  52  includes a shielding part for covering a space between the lateral sides of the frame  32  and the elastic member  34   b  and a fixing (securing) part  52   b  bent at both ends of the shielding part  52   a  and fixed on the supporting member  34   a  by for example welding. 
     The skirt  40   c  of the main shield member  40  covers the central portion of the supporting member  34   a  and is asymmetrical with reference to the central portion of the supporting member  34   a . In addition, the main shield member  40  defines the main body  40   b  and is designed not to cover the elastic members  34   b  defining the lateral sides of the frame  34 . 
     Furthermore, as shown in FIG. 4, the skirt  40   c  is preferably connected to the supporting members  34   a  while satisfying the following condition E1: 
     
       
         0.01 mm≦ tk/L≦ 0.15 mm (millimeters)  E1 
       
     
     where t is a thickness of the skirt  40   c , k is a length of the skirt  40   c  extending from the mask  32  toward the phosphor screen  22 , and L is a height of the supporting member  34   a . Further, preferably the skirt  40   c  and the supporting members  34   a  is connected to the supporting members  34   a  while satisfying the following condition: 
     
       
         0.04 mm≦ tk/L≦ 0.12 mm. 
       
     
     Such conditions can be applicable to the sub-shield member  42 . That is, as shown in FIG. 2, the sub-shield member  42  is preferably fixed on the supporting member  34   a  under the following condition E2: 
     
       
         0.01 mm≦ t′k′/L≦ 0.15 mm  E2 
       
     
     where t′ is a thickness of the sub-shield member  42 , k′ is a length of the sub-shield member  42  extending from the mask  32  toward the phosphor screen  22 , and L′ is a height of the supporting member  34   a . Further preferably, the sub-shield member  42  is fixed on the supporting member  34   a  under the following condition: 
     
       
         0.04 mm≦ t′k′/L≦ 0.12 mm. 
       
     
     The ranges of tk/L and t′k′/L are obtained through a couple of tests by the applicants. That is, it has been noted that when the skirt  40   c  and the supporting members  34   a  or the sub-shield member  42  and the supporting members  34   a  are coupled to each other in a state where the tk/L and t′k′/L are maintained less than 0.01 mm, the effectiveness obtained from the main shield member  40  and the sub-shield member cannot be expected, and when the skirt  40   c  and the supporting members  34   a  or the sub-shield member  42  and the supporting members  34   a  are coupled to each other in a state where the tk/L and t′k′/L are maintained greater than 0.15 mm, although the effectiveness obtained from the main shield member  40  and the sub-shield member can be expected, it is difficult to actually manufacture the CRT including such main shield member  40  and the sub-shield member  42 . 
     When the main shield member  40  and the sub-shield member  42  are applied to the color selection apparatus  30 , the body  40   b  of the main shield member  40  is disposed toward the neck  24 , and the skirt  40   c  of the main shield member  40  and the longitudinal shield part  42   a  of sub-shield member  42  are disposed on the perimeter (or circumference) of the frame  34 . At this point, the skirt  40   c  and the shielding part  42   a  are disconnected at the corners of the frame  34  to define disconnection parts  44 . Furthermore, the skirt  40   c  and the shielding part  42   a  are further extended toward the phosphor screen  22  to enclose the perimeter (or circumference) of the frame  34 . 
     Accordingly, in a state where the color selection apparatus  30  employing the inventive shield apparatus  39  is mounted inside the panel  20  as shown in FIG. 2, even when the geomagnetism is applied to the cathode ray tube, the affect of the geomagnetism on the electron beams emitted from the electron gun  28  to scan the phosphor screen  22  can be minimized. 
     Describing in more detail, the electron beams from the electron gun  28  are first deflected by the deflection yoke  26  and directed toward the color selection apparatus  30 . At this point, the main body  40   b  of the main shield member  40  shields the electron beams from the geomagnetism. 
     After passing through the color selection apparatus  30 , the electron beams are directed toward the phosphor screen  22 , during the course of which the skirt  40   c  and the shielding part  42   a  prevent the geomagnetism from applying to the lateral sides  20   b  of the panel. Hence, the skirt  40   c  shields the electron beams from the N-S horizontal component of the geomagnetism and the shielding part  42   a  shields the electron beams from the E-W horizontal component of the geomagnetism. 
     That is, the skirt  40   c  shields the electron beams from the horizontal geomagnetism component applied in the direction in parallel with the tube axis, and the shielding part  42   a  shields the electron beams from the horizontal geomagnetism component applied in the vertical direction with respect to the tube axis. Accordingly, even during the course of passing through the color selection apparatus  30 , the electron beams are not affected by the geomagnetism. Furthermore, since the skirt  40   c  and the shielding part  42   a  are extended toward the phosphor screen  22  while enclosing the frame  34 , the geomagnetism components flowing toward the mask  32  or the supporting members  34   a  can be prevented. 
     In addition, since the skirt  40   c  and the shielding part  42   a  are disconnected from each other by the disconnection parts  44 , the E-W horizontal component of the geomagnetism applied to the shielding part  42  is not directed to the longitudinal sides of the frame  34 , but is interrupted by the shielding part  42 . 
     As described above, the electron beams of the cathode ray tube of the present invention are less affected by the horizontal geomagnetism component when compared with those of the conventional cathode ray tube. This will be described more in detail with reference to FIGS. 5A and 5B. 
     FIG. 5A shows a graph illustrating an N-S direction graph of the geomagnetism distributed from the neck to the panel along the tube axis according to the present invention, and FIG. 5B shows a graph illustrating an N-S direction graph of the geomagnetism distributed from the neck to the panel along the tube axis according to the prior art. In the drawings, FIGS. 5A and 5B, the geomagnetism is expressed in units of gauss (G) and the distance of the tube axis is in millimeters (mm). In the drawings, By and By′ represent magnetic fields in a vertical direction of the panel, which are generated when the N-S component of the geomagnetism passes the shield apparatus (an inner shield in the prior art), and Bz and Bz′ represent magnetic fields in a direction of the tube axis Z, which are generated when the N-S component of the geomagnetism passes the shield apparatus (an inner shield in the prior art). 
     The geomagnetism characteristics graphs are obtained from a CRT with a  34- inch screen having a 3:4 screen ratio. In the present invention, the tk/L and tk′/L are set to be 0.11 mm. 
     As shown in the graphs, the By and Bz from the neck to the funnel of the present invention have distribution curves that are similar to those of By′ and Bz′ of the prior art, while the By and Bz at the panel where the inventive shield apparatus have distribution curves less than those of the By′ and Bz′ at the prior panel. 
     Such distribution curves of the By and Bz shows that the inventive shield apparatus reduces the geomagnetism applied in a direction of the tube axis Z. Accordingly, the electron beams passing through the color selection apparatus  30  and landed on the phosphor screen  22  are less affected by the geomagnetism. That is, the horizontal shift of the electron beams is reduced so that the electron beams can be landed on desired phosphors. 
     Furthermore, it has been noted through a number of tests that when the skirt  40   c  and the shielding part  42   a  are mounted on the frame  34  while satisfying the above condition E1, the mis-landing of the electron beams caused by the N-S component of the geomagnetism is reduced by 29% from the prior art and the mis-landing of the electron beams caused by the E-W component of the geomagnetism is reduced by 16% from the prior art. 
     FIGS. 6A and 6B show relations between the electron beam shift and the kt/L and k′t′/L with respect to the diagonal length of screens (32 inch CRT in FIG. 6A and 34 inch CRT in FIG. 6B) when the kt/L and k′t′/L satisfy the above described conditions. For the reference, the value of the kt/L and k′t′/L are set to be identical to each other. 
     As shown in the drawings, it has been noted the CRT of the present invention can reduce the electron beam shift with respect to the geomagnetism in N-S and E-W directions as the value of the kt/L is increased (i.e., above 0.01 mm). 
     At this point, as described above, it is preferable that the kt/L is maintained less than 0.15 mm. Describing more in detail, in a 32 inch CRT, when the kt/L is 0.07 mm, the most effectiveness can be obtained, and in a 34 inch CRT, when the kt/L is 0.11 mm, the most effectiveness can be obtained. 
     While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.