Abstract:
An electron gun for a cathode ray tube has a cathode structure, a control electrode, a screen electrode, focusing electrodes, and a final accelerating electrode. R, G, and B electron apertures of one pair of the focusing electrodes face each other to form a quadrupole lens unit, to which an AC voltage having a relatively low peak or a static voltage is applied to converge R, G, and B electron beams into one point, even when the electron beams deviate to the corner of a screen. Asymmetrical enlargement portions are included in the rims of each of the R and B electron beam apertures.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electron gun, and more particularly, to an electron gun for a cathode ray tube (CRT) having reshaped electron beam apertures. 
     2. Description of the Related Art 
     In general, an electron gun includes a triode consisting of a cathode structure, a control electrode and a screen electrode, a focusing electrode opposed to the screen electrode to form a pre-focusing lens and a final accelerating electrode opposed to the focusing electrode to form a main focusing lens. 
     If power is applied to a CRT, an electron gun emits electron beams from the cathode structure. The emitted electron beams pass through electron beam apertures of multiple electrodes and are focused and accelerated. The accelerated electron beams are selectively deflected by a deflection yoke installed at the cone portion of a bulb, and excite a phosphor screen on the inner surface of a panel, thereby displaying a picture image. 
     In the above-described CRT, in order to prevent enlargement or distortion of the spot of an electron beam landing on the phosphor screen due to a nonuniform magnetic field of a deflection yoke, a dynamic focusing method using a quadrupole lens, in which the cross section of an electron beam emitted from an electron gun is distorted in the opposite direction of the deflection magnetic field and the focus voltage applied to the electron gun is varied when the electron beam is scanned at the center or periphery of the phosphor screen, has been employed. 
     FIG. 1 shows the first embodiment of parts of electrodes of an electron gun based on the dynamic focusing method, and FIG. 2 is a view in elevation and in section of FIG.  1 . 
     Referring to FIGS. 1 and 2, the focusing electrode of the electron gun includes a static electrode  10  to which a static focusing voltage VF 1  is applied, and a dynamic electrode  100  which faces the static electrode  10  and to which a dynamic voltage DF varying in synchronization with a deflection signal is applied. 
     The electrodes  10  and  100  include outer electrodes  12  and  120  having separate electron beam apertures  11  and  110 , and auxiliary electrodes  14  and  140  inside the outer electrodes  12  and  120  and arranged in-line, respectively. The auxiliary electrodes  14  and  140  have three separate apertures  13   b / 13   a / 13   c  and  130   b / 130   a / 130   c  for R, G and B electron beams so that electrons emitted from cathode structure are focused and accelerated by an electronic lens formed between each of the-respective electrodes according to application of a voltage. 
     Here, the diameters of the G electron beam apertures  13   a  and  130   a  formed in the center, among the three separate apertures  13   b / 13   a / 13   c  and  130   b / 130   a / 130   c , are equal. However, the diameters of the R and B electron beam apertures  13   b / 13   c  and  130   b / 130   c  arranged at opposite sides of the G electron beam apertures  13   a  and  130   a  are different. 
     In other words, whereas the R and B electron beam apertures  13   b  and  13   c  are equal to the G electron beam aperture in diameter in the static electrode  10 , the diameter of the R or B electron beam aperture  130   b  or  130   c  is greater than that of the G electron beam aperture  130   a  in the dynamic electrode  100 . 
     Accordingly, the central axes of the R electron apertures  13   b  and  130   b  are spaced apart by a distance D, and the central axes of the B electron beam apertures  13   c  and  130   c  are also spaced apart by the same distance, as shown in FIG.  2 . As described above, asymmetry in electric fields of the electronic lens formed between each of various electrodes makes it easier to adjust convergence. 
     However, when a dynamic voltage is applied to the final focusing electrode, that is, the dynamic electrode  100 , since the focusing force of the final focusing electrode changes, the focusing force for converging three electron beams onto a phosphor screen changes accordingly. Thus, the capability of correcting convergence at the screen corner is deteriorated, thereby lowering picture quality. 
     In order to manufacture an electron gun having the electrodes  10  and  100 , electrodes are arranged on a zig rod for assembling the electron gun, and spacers for maintaining a gap between each of the respective electrodes are interposed and then assembled. The assembled electrodes are fusion-fixed within the neck portion of a bulb by pressing buried portions at edges of the electrodes when bead glass positioned at both sides of each electrode is semi-fused. 
     However, in the above-described electrodes  10  and  100 , the axis between centers of R electron beam apertures  13   b  and  130   b  and the axis between centers of B electron beam apertures  13   c  and  130   c  are spaced a predetermined distance D apart from each other. Thus, when the electrodes  10  and  100  are inserted into a zig, the R and B electron beam apertures  130   b  and  130   c  having relatively larger diameters become eccentrically disposed from the zig rod, which makes it difficult to attain alignment, resulting in poor assembling efficiency. 
     Although the electrode structure disclosed in U.S. Pat. No. 4,701,678 can easily adjust convergence, it is very difficult to fabricate. 
     In detail, as shown in FIGS. 3 and 4, facing electrodes  30  and  300  according to another conventional example are substantially trapezoidal laterally. In the electrodes  30  and  300 , R electron beam apertures  32  and  320  and B electron beam apertures  33  and  330  are tilted toward the edges of G electron beam apertures  31  and  310  at a predetermined angle. 
     In this case, a problem is encountered in controlling tolerance since the R electron beam apertures  32  and  320  and the B electron beam apertures  33  and  330  are tilted from the top surfaces of the electrodes  30  and  300 . 
     Also, the electrode structure disclosed in U.S. Pat. No. 5,027,043 exhibits deteriorated focusing characteristic. 
     In still another conventional electrode structure shown in FIGS. 5 and 6, outer electrodes  50  and  500  are provided and separate small, R, G and B electron beam apertures  52  and  520  are formed on top surfaces of the outer electrodes  50  and  500 . 
     Here, enlargement portions  530  protruding from the rims of the R and B electron beam apertures  520   b  and  520   c  toward a G electron beam aperture  520   a , are formed in the static electrode  500 . 
     In this case, electron beams converge toward the enlargement portions  530 . Thus, in spite of relatively easy assembling work, electron beam spots are locally distorted, thereby degrading the quality of a picture. Accordingly, the above-described electrode structure is not suitable for a high resolution CRT to which high-current electron beams are applied. 
     SUMMARY OF THE INVENTION 
     To solve the above problems, it is an objective of the present invention to provide an improved electron gun for a cathode ray tube (CRT) which can easily adjust convergence by changing the shape of electron beam apertures of electrodes, and which can reduce a position error when being assembled. 
     Accordingly, to achieve the above objective, there is provided an electron gun for a cathode ray tube having a triode consisting of a cathode structure, a control electrode and a screen electrode, a plurality of focusing electrodes for forming a pre-focusing lens unit for pre-focusing and accelerating R, G and B electron beams emitted from the triode, and a final accelerating electrode facing the focusing electrodes, for forming a main lens unit, wherein among R, G and B electron apertures of one of the focusing electrodes facing each other to form a quadrupole lens unit, to which an AC voltage having a relatively low peak, or a static voltage, is applied, enlargement portions which are asymmetrical with respect to the central axes of the respective electron beam apertures are formed into the rim of each of the R and B electron beams, so that the R, G and B electron beams are converged into one point even when the electron beams deviate to the corner of a screen. 
     Also, first and second vertically elongated polygonal or non-circular enlargement portions, with central axes spaced a predetermined distance from the centers of the R and B electron beam apertures, are formed into the rims of the R and B electron beams on opposite sides of the rims in the lateral direction. 
     Also, a third vertically elongated polygonal or non-circular enlargement portion, with a central axis coinciding with the center of the G electron beam aperture, is formed into the rim of the G electron beam aperture on opposite sides of the rim. 
     Further, the first and second enlargement portions deviate from the centers of the R and B electron beam apertures toward the G electron beam aperture. 
     The distance between each of the centers of the R and B electron beam apertures and the center of the G electron beam aperture is different from the distance from each of the central axes of the first and second enlargement portions to the central axis of the third enlargement portion. 
     Also, the distance between each of the centers of the R and B electron beam apertures and the center of the G electron beam aperture is greater than the distance from each of the central axes of the first and second enlargement portions to the central axis of the third enlargement portion. 
     Further, the sum of each of diameters of the R and B electron beam apertures and lengths of the first and second enlargement portions is different from the sum of the diameter of the G electron beam aperture and the length of the third enlargement portion, in view of the vertical direction of the electrode system. 
     According to another aspect of the present invention, there is provided an electron gun for a cathode ray tube having a triode consisting of a cathode structure, a control electrode and a screen electrode, a plurality of focusing electrodes for forming a pre-focusing lens unit for pre-focusing and accelerating R, G and B electron beams emitted from the triode, and a final accelerating electrode facing the focusing electrodes, for forming a main lens unit, wherein among R, G and B electron apertures of one of the focusing electrodes facing each other to form a quadrupole lens unit, first and second vertically elongated enlargement portions are formed into the rim of the R electron beam aperture on opposite sides of the rim in the lateral direction, and third and fourth vertically elongated enlargement portions are formed into the rim of the B electron beam aperture on opposite sides of the rim in the lateral direction, the respective enlargement portion having predetermined lengths in the normal direction of the horizontal axis of the electron beam apertures, so that the R, G and B electron beams are converged into one point even when the R, G and B electron beams deviate to the corner of a screen 
     Also, fifth and sixth enlargement portions having the same width and length may be formed into the rim of the G electron beam aperture on opposite sides of the rim in the lateral direction. 
     The width of each of the first and second enlargement portions is preferably different from the width of each of the third and fourth enlargement portions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objective and advantages of the present invention will become more apparent by describing in detail a preferred embodiment thereof with reference to the attached drawings in which: 
     FIG. 1 is an exploded perspective view partially illustrating a conventional electrode structure of an electron gun; 
     FIG. 2 is a view in elevation and partially in section of the electrode structure shown in FIG. 1; 
     FIG. 3 is an exploded perspective view partially illustrating another conventional electrode structure of an electron gun; 
     FIG. 4 is a view in elevation and partially in section of the electrode structure shown in FIG. 3; 
     FIG. 5 is an exploded perspective view partially illustrating still another conventional electrode structure of an electron gun; 
     FIG. 6 is a view in elevation and partially in section of the electrode structure shown in FIG. 5; and 
     FIG. 7 an exploded perspective view partially illustrating an electron gun according to a first embodiment of the present invention; 
     FIG. 8 is a plan view of an electrode shown in FIG. 7; 
     FIG. 9 is a view in elevation and partially in section of an electron gun according to a second embodiment of the present invention; and 
     FIG. 10 is a perspective view partially illustrating an electron gun according to a third embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An electron gun according to a first embodiment of the present invention will now be described in detail with reference to the accompanying drawings. 
     FIG. 7 illustrates an electron gun  70  according to a first embodiment of the present invention. 
     Referring to FIG. 7, the electron gun  70  includes a triode with a cathode structure  71  which is an emitting source of thermal electrons, a control electrode  72  for controlling the quantity of electrons emitted from the cathode structure  71  having an external signal, and a screen electrode  73 . 
     Also the electron gun  70  includes first, second, third and fourth focusing electrodes  74 ,  75 ,  76  and  77  aligned with to the screen electrode  73 , for forming an electronic lens portion for focusing and accelerating electron beams, and a final accelerating electrode  78  located in the vicinity of a final focusing electrode, that is, the fourth focusing electrode  77 , to form a main lens portion. 
     In the above-described electron gun  70 , the number of focusing electrodes is not limited to the number described herein and can be increased according to the formation state of the electronic lens portion for focusing electron beams in multiple stages. Three electron beam apertures through which electron beams for exciting R, G and B phosphors are arranged in-line in the respective electrodes. The shapes of the electron beam apertures may be varied according to the sizes of the electronic lenses formed between each electrodes. Alternatively, separate large electron beam apertures may be formed in the electrodes, thereby forming an electronic lens unit through which all of three electron beams pass. These electrodes are fused to bead glass (not shown) installed at both sides of the electron gun  70  at the neck portion of a bulb so they are fixed in position. 
     Here, a static focusing voltage VF 1  is applied to the third focusing electrode  76  constituting a quadrupole lens portion, a dynamic focus voltage VF 2  having a dynamic voltage DF synchronously varying with a deflection signal added thereto, is applied to the fourth focusing electrode  77 , and a high-potential anode voltage VA higher than the voltage applied to any of the electrodes mentioned above, is applied to the final accelerating electrode  78 . 
     Here, an asymmetrical deviating portion is formed on a static electrode, that is, the third focusing electrode  76 , so electron beam apertures  76   a  in plane facing a dynamic electrode, that is, the fourth focusing electrode  77 , compensates for convergence. 
     FIG. 8 is a plan view of an exemplary static electrode  80 . 
     Referring to FIG. 8, the electrode  80  has three separate small apertures  81 ,  82  and  83  through which R, G and B electron beams emitted from a cathode structure ( 71  of FIG. 7) and focused and accelerated by electronic lenses formed between each of the electrodes, pass. Burying portions  84  and  85  to be fused to bead glass are located in the mid portion of the periphery of the electrode  80 . 
     Here, enlargement portions are located along the rim of each of the electron beam apertures  81 ,  82  and  83 . In detail, fifth and sixth enlargement portions  87   a  and  87   b  extended lengthwise, i.e., vertically, in FIG. 8, with respect to the electrode  80 . The fifth and sixth enlargement portions  87   a  and  87   b  extend from the rim of the G electron beam aperture  82  on opposite sides of the rim in the vertical direction. The fifth and sixth enlargement portions  87   a  and  87   b  have polygonal or non-circular shapes. Here, the central axes of the fifth and sixth enlargement portions  87   a  and  87   b  coincide with the center of the G electron beam aperture  82 . 
     In the R and B electron beam apertures  81  and  83 , first and second enlargement portions  86   a  and  86   b  and third and fourth enlargement portions  88   a  and  88   b  extended lengthwise, ie, vertically, in FIG. 8, with respect to the electrode  80 . The first and second enlargement portions  86   a  and  86   b  and the third and fourth enlargement portions  88   a  and  88   b  extended from the rims of the R and B electron beam apertures  81  and  83  on opposite sides of the rims in the vertical direction, respectively. Like the G electron beam aperture  82 , the first and second enlargement portions  86   a  and  86   b  and the third and fourth enlargement portions  88   a  and  88   b  have polygonal or non-circular shapes. 
     Here, the centers of the R and B electron beam apertures  81  and  83  do not coincide with the central axes of the first and second enlargement portions  86   a  and  86   b  and the third and fourth enlargement portions  881  and  88   b . In other words, the first and second enlargement portions  86   a  and  86   b  deviate from the center of the R electron beam aperture  81  toward the G electron beam aperture  82 . Also, the third and fourth enlargement portions  88   a  and  88   b  deviate from the center of the B electron beam aperture  83  toward the G electron beam aperture  82 . 
     Accordingly, an asymmetric electric field is formed at the R, G and B electron beam apertures  81 ,  82  and  83  lengthwise with respect to the electrode  80 . Thus, the capability of correcting electron beam convergence is improved. 
     In more detail, assuming that S 1  represents the distance between the centers of the R and B electron beam apertures  81  and  83  disposed all the left and right sides of the G electron beam aperture  82  and the center of the G electron beam aperture  82  and S 2  represents the distance between the central axes of the fifth and sixth enlargement portions  87   a  and  87   b  and the central axes of the first and second enlargement portions  86   a  and  86   b  or the third and fourth enlargement portions  88   a  and  88   b , S 1  is not equal to S 2 . Instead, when S 1  is greater than S 2 , an asymmetric field is formed, which is advantageous for convergence control. 
     It is assumed that V C  represents the sum of the diameter of the G electron beam aperture  82  and vertical lengths of the fifth and sixth enlargement portions  87   a  and  87   b  and V S  represents the sum of the respective diameters of the R and B electron beam apertures  81  and  83  and vertical lengths of the first and second enlargement portions  86   a  and  86   b  or the third and fourth enlargement portions  88   a  and  88   b . Then, V C  is not equal to V S , and it is advantageous that V S  is greater than V C . 
     Also, it is assumed that A S  represents the horizontal lengths, ie, widths of the first and second enlargement portions  86   a  and  86   b  or the third and fourth enlargement portions  88   a  and  88   b  from the respective centers of the R and B electron beam apertures  81  and  83  toward the periphery of the electrode  80 , B S  represents the horizontal lengths, i.e., widths of the first and second enlargement portions  86   a  and  86   b  or the third and fourth enlargement portions  88   a  and  88   b  from the respective centers of the R and B electron beam apertures  81  and  83  toward the G electron beam aperture  82 , A C  represents the horizontal lengths i.e., width of the fifth and sixth enlargement portions  87   a  and  87   b  from the center of the G electron beam aperture  82  toward the first and second enlargement portions  86   a  and  86   b  , and B C  represents the horizontal lengths, i.e., widths of the fifth and sixth enlargement portions  87   a  and  87   b  from the center of the G electron beam aperture  82  toward the third or fourth enlargement portion  86   a  or  86   b . Then, it is advantageous in forming an asymmetric electric field that the sum of A S  and B S  is not equal to the sum of A C  and B C . Here, A C  equals B C . 
     Likewise, the first and second enlargement portions  86   a  and  86   b  and the third and fourth enlargement portions  88   a  and  88   b  are shaped such that a polygon, e.g., a rectangle or an ellipse, is superposed over each of the R and B electron beam apertures  81  and  83  lengthwise with respect to the electrode  80 . Only the centers of the first and second enlargement portions  86   a  and  86   b  and the third and fourth enlargement portions  88   a  and  88   b  are shifted, without shifting the centers of the R and B electron beam apertures  81  and  83 , to form an asymmetric electric field with respect to the corresponding dynamic electrode, thereby attaining quadrupolar effects. Also, since the asymmetric electric field is horizontally formed, convergence control is easily achieved. 
     Also, the strength of a quadrupole lens is adjusted by varying the vertically elongated length of the R and B electron beam apertures  81  and  83 , inclusive of the superposed first and second enlargement portions  86   a  and  86   b  and the third and fourth enlargement portions  88   a  and  88   b , thereby maximizing the correcting capability of the quadrupole lens for the G electron beam and the R and B electron beams, without affecting convergence. 
     FIG. 9 illustrates an electron gun  90  according to a second embodiment of the present invention. 
     Referring to FIG. 9, the electron gun  90  includes a triode consisting of a cathode structure  91  which is an emission source of thermal electrons, a control electrode  92  for controlling the quantity of electrons emitted from the cathode structure  91  by an external signal, and a screen electrode  93 . 
     Also, the electron gun  90  includes first, second, third, fourth and fifth focusing electrodes  94 ,  95 ,  96 ,  97  and  98  aligned with the screen electrode  93 , for forming an electronic lens portion for focusing and accelerating electron beams, and a final accelerating electrode  99  for forming a main lens portion together with the fifth focusing electrode  98 . 
     Here, a predetermined potential is applied to the respective electrodes. In other words, a static voltage VS is applied to the screen electrode  93  and the second focusing electrode  95 , a static focusing voltage VF 1  is applied to the first focusing electrode  94  and the fourth focusing electrode  97 , and a dynamic focusing voltage VF 2  having a dynamic voltage VD synchronously varying with a deflection signal added thereto, is applied to the third and fifth focusing electrodes  96  and  98 . a high-potential anode voltage VA higher than the voltage applied to any of the electrodes mentioned above, is applied to the final accelerating electrode  98 . 
     Here, since the fourth focusing electrode  97  which is a static electrode has an electron beam aperture asymmetrically deviating and the centers of the respective electron beam apertures are positioned on the same axis, as shown in FIGS. 7 and 8, a detailed explanation thereof will not be given. 
     FIG. 10 illustrates a focusing electrode  100  according to a third embodiment of the present invention, to which a dynamic voltage is applied. 
     Referring to FIG. 10, the electrode  100  has on its top surface three separate small apertures  110  through which electron beams emitted from a cathode structure and focused and accelerated by electronic lens portions located between each of the electrodes, pass. Buried portions  120  to be fused to bead glass in the neck portion of a bulb are formed in the mid portion of the periphery of the electrode  100 . 
     The electron beam apertures  110  are formed in an in-line arrangement so as to share the same central axis. In other words, a G electron beam aperture  111  is located in the center of the electrode  100 , and R and B electron beam apertures  112  and  113  are located at both sides of the G electron beam aperture  111 . 
     Here, enlargement portions are located in the rim of each of the electron beam apertures  110 . In other words, fifth and sixth vertically elongated enlargement portions  111   a  and  111   b  are located in the rim of the G electron beam aperture  111  on opposite sides of the rim in the lateral direction. The fifth and sixth enlargement portions  111   a  and  111   b  have the same width and length. 
     First and second enlargement portions  112   a  and  112   b  and third and fourth enlargement portions  113   a  and  113   b  are also located at the R and B electron beam apertures  112  and  113  lengthwise with respect to the electrode  100 , respectively. In this case, the first and second enlargement portions  112   a  and  112   b  and the third and fourth enlargement portions  113   a  and  113   b  are preferably located asymmetrically in the normal direction from the rims of the R and B electron beam apertures  112  and  113 , unlike the fifth and sixth enlargement portions  111   a  and  111   b  which are symmetrical with respect to the center of the G electron beam aperture  111 , in order to increase the quadrupolar effect and convergence adjusting capability of an electrode to which an AC dynamic voltage having a relatively high peak is applied. 
     In other words, the first and second enlargement portions  112   a  and  112   b  and the third and fourth enlargement portions  113   a  and  113   b  have a predetermined length at the lateral rims of the R and B electron beam apertures  112  and  113 , with respect to the electrode  100 . The first and second enlargement portions  112   a  and  112   b  and the third and fourth enlargement portions  113   a  and  113   b  are integral with the R and B electron beam apertures  112  and  113 . Here, it is advantageous for convergence control to make the widths and lengths of the first and second enlargement portions  112   a  and  112   b  different from each other, and to make the widths and lengths of the third and fourth enlargement portions  113   a  and  113   b  different from each other. 
     When the aforementioned electrode structure of an electron gun is assembled, the respective electrodes are arranged along the zig rod and a spacer having a predetermined thickness is interposed between each two of the respective electrodes in order to maintain a predetermined distance between the respective electrodes. 
     Here, since, the central axes of the R, G and B electron beam apertures are symmetrically located, ecentricity does not occur at the zig rod when the electrodes are inserted into the zig rod, thereby easily attaining alignment. In this state, the respective electrode elements are fused to bead glass disposed at both sides of the electrodes. Accordingly, a proper distance between the respective electrodes are maintained to achieve high precision alignment of electrode elements, thereby showing stable functions. 
     As described above, in the electron gun for a CRT according to the present invention, a quadrupole lens system includes electrodes aligned such that the diameter and center of the electron beam apertures at one of electrodes to which a dynamic focusing voltage is applied, coincide, and asymmetric enlargement portions are located at predetermined portions of rims of the electron beam apertures, thereby facilitating convergence control. Also, since eccentricity does not occur at the zig rod during fabrication, the assembling process is simplified. 
     Having described the exemplary embodiments of the present invention, various changes and equivalent embodiments may be made by those skilled in the art without departing from the spirit and scope of the appended claims. It is therefore contemplated that the true scope of the invention be set forth in the following claims.