Abstract:
Electron gun in a CRT including a main lens electrode having a focus electrode and an anode for focusing electron beams emitted from cathodes onto a screen, an electrostatic field controlling body fitted in each of the focus electrode and the anode each having three electron beam pass through holes, wherein each of outer holes in the electrostatic field controlling body fitted to each of the focus electrode and the anode has a form of a combination of a circle and a rectangle with reference to a vertical axis through a center of the hole in a direction opposite for facing outer holes of the focus electrode and the anode, thereby enlarging a main lens diameter, and providing a spot substantially circular and smaller.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electron gun in a cathode ray tube (CRT), and more particularly, to an electron gun in a CRT, in which electron beam pass through holes in an electrostatic field controlling body provided both to a focusing electrode and anode of a main lens electrode are changed, for improving a screen focusing characteristic. 
     2. Background of the Related Art 
     The CRT, a device for forming a picture by landing electron beams emitted from the electron gun on a screen, is illustrated in FIG. 1, schematically. 
     Referring to FIG. 1, the CRT is provided with a panel  2  fitted to a front for acting as a screen, a fluorescent surface  4  of red R, green G, and blue B fluorescent materials coated on an inside surface of the panel  2 , a shadow mask  8  for selecting a color as electron beams  6  incident on the fluorescent surface  4  pass therethrough, a funnel  10  fitted to rear of the panel  2  for sustaining an inner space of the CRT at a vacuum, and a deflection yoke  12  surrounding an outer circumference of a neck part  10   a  of the funnel  10  for deflecting the electron beams  6 . 
     The electron gun  20  is placed in the neck part  10   a  of the funnel  10  of the CRT, and provided with three independent cathodes  201 , a first electrode  21  spaced a distance away from the cathodes  201 , a second electrode  22 , a third electrode  23 , a fourth electrode  24 , a fifth electrode  25 , and a sixth electrode  26  spaced at fixed intervals from the first electrode  21 , and a shield cup  27  above the sixth electrode  26  having a bulb space contact (B.S.C)  28  fitted thereto for electrical connection of the electron gun  20  to the funnel  10  and fastening the electron gun  20  to the neck part  10   a  of the funnel  10 . 
     The electron gun  20  emits electrons as heaters  203  in the cathodes  201  are heated by a power supplied from respective stem pins  202  at rear end thereof, and the electrons form electron beams, which are controlled by the first electrode  21 , a controlling electrode, and accelerated by the second electrode  22 , an accelerating electrode. Then, the electron beams are partly focused and accelerated by a pre-focus lens formed between the second electrode  22 , the third electrode  23 , the fourth electrode  24 , the fifth electrode  25  (a focus electrode), finally focused and accelerated by the sixth electrode  26  (anode), a final accelerating electrode, pass through the shadow mask  8 , and land on the fluorescent surface  4  on an inside surface of the panel  2 , to make the fluorescent surface to emit a light. 
     The focus electrode  25  and the anode  26  collectively called as a main lens  200 , and a related art main lens  200  will be explained with reference to FIG.  2 . FIG. 2 illustrates a perspective view with a partial cut away view of the focus electrode  25  and the anode  26  in the main lens electrode. 
     Referring to FIG. 2, the focus electrode  25  is provided with a drum formed housing  252  externally, having an fore end facing the anode  26  with an opened central part and a rim  252   a  in a form of a racing track in a periphery, and a plate of electrostatic field controlling body  254  spaced a distance away inward from rim  252   a  with three electron beam pass through holes  254   a,    254   b,  and  254   c  for passing the three electron beams from the cathodes. The anode  26  is also provided with a drum formed housing  262  having a rim  262   a  at one end, and an electrostatic field controlling body  264  with electron beam pass through holes  264   a,    264   b,  and  264   c  inside thereof. 
     FIGS. 3 a  and  3   b  illustrate a plan view of the electrostatic field controlling bodies  254  and  264  of the focus electrode  25  and the anode  26 , respectively. 
     Referring to FIGS. 3 a  and  3   b , it can be known that the electron beam pass through holes  254   a ,  254   b , and  254   c  in the electrostatic field controlling body  254  of the focus electrode  25  are similar, or identical to the electron beam pass through holes  264   a ,  264   b , and  264   c , respectively. 
     One of the most important parameter to be taken into account in design of an electron gun is a spot diameter Dt on a screen. There are three factors that influence the spot diameter on the screen, i.e., a magnification of the lens, a spatial charge repulsive power, and a spherical aberration of the main lens. Since voltage conditions, focal distances, a length of the electron gun, and etc., are already defined basically, the influence of the magnification of the lens to the spot diameter Dx has a small portion for utilizing as design parameter of the electron gun, and minimal effect. 
     The spatial charge repulsive power is a phenomenon in which the collision and repulsion between the electrons in the electron beam enlarge the spot diameter. For reducing enlargement of the spot diameter Dst caused by the spatial charge repulsive power, it is favorable to design an angle (a diverging angle) of the electron beam travel greater. 
     The spherical aberration of the main lens can form the smaller spot diameter on the screen, as the diverging angle of the electron beams is the smaller. In general, the spot diameter Dt on the screen may be expressed as a sum of three factors as follows. 
     
       
           D   t ={square root over (( D   x   +D   st ) 2   +D   ic   2 )} 
       
     
     The best method for reducing the spherical aberration as well as the spatial charge repulsive power is enlargement of the main lens diameter, which reduces enlargement of the spot caused by the spherical aberration even if electron beams with a great diverging angle are incident thereon, and reduces the spatial charge repulsive power after electron beams pass through the main lens, thereby forming a small diametered spot on the screen. However, an enlargement of the rim, and spacing the distance from the rim to the electrostatic field controlling body greater for enlargement of the main lens diameter form the spot to be almost triangular with partial halo as a focusing of the outer main lens is in a 45° direction, with a difference between a central main lens side and an opposite side. 
     FIG. 4 illustrates the foregoing triangular spots S 1  on the screen. 
     The rim  252   a  of the focus electrode  25  focuses the outer beam weak in a 45° direction on the central beam side, and strong in a 45° direction on an opposite side of the central beam side, to form a triangle greater vertically in the 45° direction on the central beam side, and smaller vertically in the 45° direction on an opposite side of the central beam side. There are halos on sides opposite to the central beam in 45° directions. Of course, though forms of the outer spots can be slightly corrected at the anode as the anode acts opposite to the focus electrode, since an action of the main lens is significantly greater at the focusing side than the acceleration side, eventually a state at the focusing side is maintained as it is, due to which, because spot is formed not circular at a periphery of a picture, realization of focusing for meeting requirements of high definition, large sized screen, flat screen, and wide angle is difficult. 
     Moreover, in comparison to the electron gun with circular outer spots, alignments between apertures of electrodes of electron gun and one sided halo inducing characteristic dependent on assembly of electron gun related to a flatness of the electrode are sensitive, assembly of the electron gun is unfavorable. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to an electron gun in a CRT that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     An object of the present invention is to provide an electron gun in a CRT, which can enlarge a main lens diameter and form an excellent spot that is almost circular and has a reduced size. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the electron gun in a CRT includes a main lens electrode having a focus electrode and an anode for focusing electron beams emitted from cathodes onto a screen, an electrostatic field controlling body fitted in each of the focus electrode and the anode each having three electron beam pass through holes, wherein each of outer holes in the electrostatic field controlling body fitted to each of the focus electrode and the anode has a form of a combination of a circle and a rectangle with reference to a vertical axis through a center of the hole in a direction opposite for facing outer holes of the focus electrode and the anode. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention: 
     In the drawings: 
     FIG. 1 illustrates a section showing a related art CRT, schematically; 
     FIG. 2 illustrates a cut away perspective view showing a main lens part having a focus electrode and an anode in an electron gun of a related art CRT, schematically; 
     FIGS. 3 a  and  3   b  illustrate plan views of electrostatic field controlling bodies of a related art focus electrode and an anode; 
     FIG. 4 illustrates a plan view of spots on a screen in an electron gun of a related art CRT; 
     FIGS. 5 a  and  5   b  illustrate plan views of electrostatic field controlling bodies of a focus electrode and an anode in an electron gun of a CRT in accordance with a preferred embodiment of the present invention; 
     FIG. 6 illustrates a plan view showing spots on a screen formed by an electron gun of the present invention; 
     FIG. 7 illustrates plan views of an electrostatic field controlling body and a rim related to the present invention for comparison; and, 
     FIG. 8 illustrates a graph showing a difference of horizontal diameters of main lenses according to a center distance P 1  between a central hole and an outer hole of an electrostatic field controlling electrode of a focus electrode in a case a horizontal distance P 2  between points from a center of the rim of the focus electrode to a point the rim changes from straight to curve is 5.5 mm. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. For reference, parts of the present invention identical to the related art will be given the same reference symbols. FIGS  5   a  and  5   b  illustrate plan views of electrostatic field controlling bodies of a focus electrode and an anode in an electron gun of a CRT in accordance with a preferred embodiment of the present invention. With regard to the related art main lens electrode, FIG. 2 will be referred. 
     An electrostatic field controlling electrode  54  provided to a focus electrode  25  is a planar electrode body fitted in an external housing  252  thereof spaced a distance away from a rim  252   a  having three electron beam pass through holes formed therein inclusive of central hole  54   a,  and outer holes  54   b  and  54   c.  The central hole  54   a  is circular, and the outer hole  54   b  or  54   c  is a combination of a circle and a rectangle. 
     The outer hole  54   b  is a circle combined with a rectangle extended outward from a vertical axis of the circle, a height of which rectangle is the same with the diameter of the circle. The extended portion of the rectangle has a width ‘H’ at least smaller than a radius of the circle. Eventually, the electrostatic field controlling electrode  54  of the focus electrode has the outer hole with a rectangular part  54   e  projected in a direction opposite to the central hole with reference to a vertical axis of the outer hole, for improving the triangular spots in FIG.  4 . 
     Referring to a lower drawing in FIGS  5   a  and  5   b , similar to the electrostatic field controlling electrode  54  of the focus electrode, an electrostatic field controlling body  64  of an anode  26  includes one central hole  64   a  and two outer holes  64   b  and  64   c  each in a form of a combination of a circle and a rectangle, but a rectangular part  64   e  of the outer hole is extended toward the central hole with reference to a vertical axis of the outer hole. That is, the outer holes  54   b  and  54   c  in the electrostatic field controlling electrode  54  of the focus electrode  25  and the outer holes  64   b  and  64   c  in the electrostatic field controlling body  64  of the anode  26  are symmetry with respect to the vertical axis. 
     The operation of the electron gun improved by the present invention will be explained. 
     For compensating for the weak focusing power in a 45° direction on the central beam side at the rim  252   a  of the focus electrode  25 , the rectangular part  54   e  is formed in the outer hole  54   b  or  54   c  in a 45° direction on an opposite side of the central beam side, for weakening a focusing power thereof to compensate for a difference of focusing powers in the rim. 
     Also, in a case it is intended to correct the problem that the outer spots are formed triangular on the screen, the electrostatic filed controlling body  64  in a form as shown in FIGS  5   a  and  5   b  is formed at the anode  26 , and, for compensating a weakened diverging power in a 45° direction on the central beam side at the rim  262   a  of the anode  26 , the rectangular part  64   e  is formed in the outer hole  64   b  or  64   c  in a 45° direction on the central beam side thereof in the electrostatic field controlling body  64 , for compensating for a weakened diverging power. 
     FIG. 6 illustrates a plan view of spots on a screen formed by an electron gun of the present invention. As can be known from the drawing, the electrostatic field controlling body  54  of the focus electrode  25  and the electrostatic field controlling body  64  of the anode  26  compensate for focusing powers of the outer beams, to form substantially circular spots, and to eliminate halos caused by a difference of focusing powers in the related art. 
     FIG. 7 illustrates, when the electrostatic field controlling bodies  54  and  64  in the focus electrode  25  and the anode  26  are formed deeper for increasing diameters of the main lenses, a distance P 1  between centers of the central hole in the electrostatic field controlling body  54  or  64  of the focus electrode or the anode, and a horizontal distance P 2  between points ‘r’ from a center of the rim  252   a  or  262   a  of the focus electrode or anode to a point of the rim changing from a straight line to a curved line. 
     In this instance, for compensation of a horizontal diameter difference of the main lenses in a central beam direction and an opposite direction of the central beam direction of the main lenses of the outer beams, as shown in FIG. 7, P 1  is made greater than P 2  (P 1 &gt;P 2 ). In this instance, after combination of the electrostatic field controlling bodies of the focus electrode and the anode, the spots are made circular. This is because the 45° direction focusing power difference of rims in the focus electrode is required to compensate the 45° direction focusing power difference of the electrostatic field controlling body in the anode as centers of the outer hole of the electrostatic field controlling electrode in the focusing electrode becomes far to weaken a correction power of the rectangular part 45° direction focusing power. 
     FIG. 8 illustrates a graph showing a difference of horizontal diameters of main lenses according to a center distance P 1  between a central hole and an outer hole of an electrostatic field controlling electrode of a focus electrode in a case a horizontal distance P 2  between points from a center of the rim of the focus electrode to a point the rim changes from straight to curve is 5.5 mm. 
     Referring to FIG. 8, as P 1  becomes the greater than P 2 , it can be known that a horizontal lens diameter on the central beam side and an opposite horizontal lens diameter are in conformity. In the case of an embodiment of the electrostatic field controlling bodies of the present invention, by a simple additional step of forming the rectangular parts in the outer holes which are circular in the related art, improved spots can be formed on the screen without any complicated change to a related art process. 
     When an alignment of holes of the electron gun is adjusted in a beading process in which components of the electron gun are fixed with bead glass at preset distances in an electron gun assembly process, a jig called mandrel is used for holding the electrodes of the electron gun. In general, it is the most favorable that the mandrel has a circular section in view of alignment. Since the present invention permits to use the mandrel without change, the present invention is also favorable for the alignment. 
     By changing forms of outer holes in the electrostatic field controlling bodies for improving the outer spots caused by increased main lens diameters that give a great influence to a focus, an excellent focusing performance can be provided for entire screen. Since no change to an electron gun fabrication process is required, and round mandrel can be used, a favorable result can be obtained even in an electron gun alignment characteristic. Compared to a sensitive one sided halo characteristic caused by partial halo occurrence in the related art, a less sensitive result can be obtained even for the one sided halo which may be occurred by defective alignment of the electron gun. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the electron gun in a CRT of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.