Patent Publication Number: US-6713964-B2

Title: Electron gun for cathode ray tube

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 my application ELECTRON GUN FOR CATHODE RAY TUBE filed with the Korean Industrial Property Office on May 30, 2002 and there duly assigned Ser. No. 30328/2002. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to an electron gun for a cathode ray tube, and more particularly, to an electron gun for a cathode ray tube in which an efficiency of a main focus lens is maximized within a limited neck diameter such that high focus performance and resolution characteristics are obtained. 
     2. Related Art 
     A projection system that utilizes cathode ray tubes (CRTs) to realize large screen images typically includes as the main elements three monochrome cathode ray tubes, each for realizing an image of a single color, that is, a green image, a blue image, or a red image; and an optical lens system for enlarging and projecting each of the single color images onto a projection screen to combine the images as a full color image. 
     In the monochrome cathode ray tube, since the screen is scanned using a single electron beam, the focus performance of the electron beam directly affects the resolution of the display device. Further, because the image of each monochrome cathode ray tube is enlarged by approximately ten times before being projected onto the screen, it is necessary to increase screen brightness by emitting an electron beam of a high current density from each of the electron guns. 
     Accordingly, the electron gun provided in the monochrome cathode ray tube uses a unipotential focus or a hi-unipotential focus connecting structure that provides for high focus performance in a high current region, in addition to using an electrode structure that optimizes the performance of a main focus lens. 
     In the unipotential focus or hi-unipotential focus methods, the main focus lens of the electron gun is formed between focus and anode electrodes by a difference between a focus voltage applied to the focus electrode and an anode voltage applied to the anode electrode. The main focus lens focuses an electron beam emitted from a cathode to form an electron beam spot on a phosphor screen. 
     The performance of the main focus lens is affected by equivalent diameter and spherical aberration. Spherical aberration decreases with increases in the equivalent diameter of the main focus lens, and a spot size of an electron beam landing on the phosphor screen increases with increases in spherical aberration. 
     Therefore, there may be an effort to optimize a triode portion to limit the spherical aberration of the main focus lens, or to enlarge the diameter of the main focus lens to increase the efficiency of the same. In particular, to increase the diameter of the main focus lens, it is necessary to physically enlarge the focus electrode and the anode electrode. 
     However, efforts to physically increase the diameter of the focus and anode electrodes are constrained by the standardized diameter of the neck in present commercial use. As a result, there is a need for an electron gun structure that forms the main focus lens to a maximum diameter within the limited diameter of the neck. 
     SUMMARY OF THE INVENTION 
     The present invention provides an electron gun for a cathode ray tube, in which an electrode structure is improved to maximize an equivalent diameter of a main focus lens within a neck of a limited diameter such that exceptional focus performance and resolution characteristics are realized. 
     The present invention provides an electron gun for a cathode ray tube including a single cathode emitting electrons; first and second grid electrodes forming a triode portion with the cathode; a third grid electrode provided subsequent to the second grid electrode; a fourth grid electrode provided subsequent to the third grid electrode and to which a focus voltage is applied, the fourth grid electrode including an input section positioned opposing the third grid electrode and an output section connected to the input section; a fifth grid electrode mounted surrounding part of the fourth grid electrode with a predetermined gap therebetween and to which an anode voltage is applied; and a connector electrically interconnecting the third grid electrode and the fifth grid electrode. The fifth grid electrode is positioned surrounding the fourth grid electrode in such a manner to expose the output section of the fourth grid electrode. 
     Preferably, the fourth grid electrode is cylindrical, and a diameter of the output section Is greater than a diameter of the input section; and the fifth grid electrode is also cylindrical and includes an input section and an output section, the output section having a diameter that is larger than a diameter of the input section. 
     The fourth grid electrode and the fifth grid electrode satisfy the following condition, 
     
       
         1.08 &lt;D   2   /D   1 &lt;2.0  (1) 
       
     
     where D 1  is an outer diameter of the input section of the fourth grid electrode and D 2  is an outer diameter of the input section of the fifth grid electrode, and it is assumed that a thickness of the fifth grid electrode does not exceed 500 micrometers (μm). 
     The fourth grid electrode and the fifth grid electrode satisfy the following condition, 
      1.0 &lt;D   4   /D   3 &lt;1.2  (2) 
     where D 3  is an outer diameter of the output section of the fourth grid electrode and D 4  is an outer diameter of the output section of the fifth grid electrode, and it is assumed the thickness of the fifth grid electrode does not exceed 500 micrometers (μm). 
     The fourth grid electrode is preferably divided into at least two sub-electrodes mounted with a gap therebetween. 
     An angled section is formed between the input and output sections of the fourth grid electrode, the angled section being progressively enlarged in diameter starting from an end connected to the input section of the fourth grid electrode and in a direction toward an end connected to the output section of the fourth grid electrode. 
     As another option, the output section of the fourth grid electrode may be formed such that an end connected to the input section of the fourth grid electrode is substantially identical in diameter to the input section, then is progressively enlarged from this end connected to the input section in a direction away from the cathode. 
     In accordance with the principles of the present invention, as embodied and broadly described, the present invention provides an electron gun for a cathode ray tube, the electron gun comprising: a cathode emitting electrons; first and second grid electrodes forming a triode portion with said cathode; a third grid electrode; a fourth grid electrode receiving a focus voltage, said third grid electrode being disposed between said cathode and said fourth grid electrode, said fourth grid electrode including an input section and an output section, the input section being disposed between the output section and said third grid electrode; a fifth grid electrode encircling a portion of said fourth grid electrode, at least a part of the output section of said fourth grid electrode being not encircled by said fifth grid electrode, said fifth grid electrode being spaced apart from said fourth grid electrode by a predetermined gap, said fifth grid electrode receiving an anode voltage; and a connector electrically connecting said third and fifth grid electrodes. 
     In accordance with the principles of the present invention, as embodied and broadly described, the present invention provides an electron gun for a cathode ray tube, the electron gun comprising: a single cathode emitting electrons; first and second grid electrodes forming a triode portion with said cathode; a third grid electrode; a fourth grid electrode receiving a focus voltage, said third grid electrode being disposed between said cathode and said fourth grid electrode, said fourth grid electrode including an input section and an output section, the input section being disposed between the output section and said third grid electrode, the output section of said fourth grid electrode having an edge facing away from said cathode; a fifth grid electrode receiving an anode voltage, said fifth grid electrode encircling a portion of said fourth grid electrode, at least a part of the output section of said fourth grid electrode being not encircled by said fifth grid electrode, said fifth grid electrode being spaced apart from said fourth grid electrode by a predetermined gap, said fifth grid electrode including an input section and an output section, the output section of said fifth grid electrode having an edge facing away from said cathode; and a connector electrically connecting said third and fifth grid electrodes, the edge of said fourth grid electrode being a first distance from said cathode, the edge of said fifth grid electrode being a second distance from said cathode, the first distance being larger than the second distance. 
     In accordance with the principles of the present invention, as embodied and broadly described, the present invention provides an electron gun for a cathode ray tube, the electron gun comprising: a cathode emitting electrons; a first electrode having an input section and an output section, an input end of the input section of said first electrode separating said cathode from an output end of the output section of said first electrode, said first electrode having a focus voltage applied; and a second electrode having an input section and an output section, an input end of the input section of said second electrode separating said cathode from an output end of the output section of said second electrode, said second electrode having an anode voltage applied, a distance between said cathode and the output end of the output section of said first electrode being greater than a distance between said cathode and the output end of the output section of said second electrode, said second electrode encircling a portion of said first electrode, at least a part of the output section of said first electrode being not encircled by said second electrode, said second electrode being spaced apart from said first electrode by a predetermined gap. 
     In accordance with the principles of the present invention, as embodied and broadly described, the present invention provides a method of operating an electron gun for a cathode ray tube, the method comprising: emitting electrons from a cathode; applying a focus voltage to a first electrode of the electron gun, the first electrode having an input section and an output section, an input end of the input section of the first electrode separating the cathode from an output end of the output section of the first electrode; and applying an anode voltage to a second electrode of the electron gun, the second electrode having an input section and an output section, an input end of the input section of the second electrode separating the cathode from an output end of the output section of the second electrode, a distance between the cathode and the output end of the output section of the first electrode being greater than a distance between the cathode and the output end of the output section of the second electrode, said second electrode encircling a portion of said first electrode, at least a part of the output section of said first electrode being not encircled by said second electrode, said second electrode being spaced apart from said first electrode by a predetermined gap. 
     The present invention is more specifically described in the following paragraphs by reference to the drawings attached only by way of example. Other advantages and features will become apparent from the following description and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings, which are incorporated in and constitute a part of this specification, embodiments of the invention are illustrated, which, together with a general description of the invention given above, and the detailed description given below, serve to exemplify the principles of this invention. 
     FIG. 1 is a perspective view of an electron gun for a cathode ray tube according to a preferred embodiment of the present invention, in accordance with the principles of the present invention; 
     FIG. 2 is a sectional view taken along line I—I of FIG. 1, in accordance with the principles of the present invention; 
     FIG. 3 is a partially enlarged view of a fourth grid electrode and a fifth grid electrode shown in FIG. 2, in accordance with the principles of the present invention; 
     FIG. 4 is a schematic view showing equipotential lines and electron beam traces generated during driving of the electron gun of FIG. 1, in accordance with the principles of the present invention; 
     FIG. 5 is an enlarged view of FIG. 5, in accordance with the principles of the present invention; 
     FIG. 6 is a partially enlarged view of a fourth grid electrode and a fifth grid electrode shown in FIG. 2, in accordance with the principles of the present invention; 
     FIG. 7 is partially enlarged sectional views of a fourth grid electrode and a fifth grid electrode in an exemplary electron gun for cathode ray tubes; 
     FIG. 8 is a graph showing 5% electron beam spot sizes according to variations in electron beam current for the electron gun of FIG.  1  and an exemplary electron gun; 
     FIGS. 9 and 10 are partial sectional views of fourth and fifth grid electrodes according to other preferred embodiments of the present invention, in accordance with the principles of the present invention; 
     FIG. 11 is a graph showing the relation between equivalent diameter and spherical aberration; and 
     FIG. 12 is a graph showing the relation between spherical aberration and electron beam spot size. 
    
    
     DESCRIPTION OF BEST MODE OF CARRYING OUT THE INVENTION 
     While the present invention will be described more fully hereinafter with reference to the accompanying drawings, in which details of the present invention are shown, it is to be understood at the outset of the description which follows that persons of skill in the appropriate arts may modify the invention here described while still achieving the favorable results of this invention. Accordingly, the description of the best mode contemplated of carrying out the invention, which follows, is to be understood as being a broad, teaching disclosure directed to persons of skill in the appropriate arts, and not as limiting upon the present invention. 
     Illustrative embodiments of the best mode of carrying out the invention are described below. In the interest of clarity, not all features of an actual implementation are described. In the following description, well-known functions, constructions, and configurations are not described in detail since they could obscure the invention with unnecessary detail. It will be appreciated that in the development of any actual embodiment numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill having the benefit of this disclosure. Additionally, the embodiments disclosed can be combined to form differently shaped components of the electron gun consistent with the principles of the present invention. 
     The performance of the main focus lens is affected by equivalent diameter and spherical aberration. FIG. 11 is a graph showing the relation between equivalent diameter and spherical aberration, and FIG. 12 is a graph showing the relation between spherical aberration and electron beam spot size. As is evident from the graphs, spherical aberration decreases with increases in the equivalent diameter of the main focus lens, and a spot size of an electron beam landing on the phosphor screen increases with increases in spherical aberration. An example of an effort related to an electron gun for a cathode ray tube is U.S. Pat. No. 4,271,374 entitled ELECTRON GUN FOR CATHODE-RAY TUBE, issued to Kimura on Jun. 2, 1981. 
     The best mode of carrying out the invention will now be described in detail with reference to the accompanying drawings. FIG. 1 is a perspective view of an electron gun for a cathode ray tube according to a preferred embodiment of the present invention, in accordance with the principles of the present invention. FIG. 2 is a sectional view taken along line I—I of FIG. 1, in accordance with the principles of the present invention. 
     With reference to the drawings, the electron gun  2  includes a single cathode  4  for emitting electrons; first and second grid electrodes  6  and  8  forming a triode portion with the cathode  4 , the first and second grid electrodes  6  and  8  controlling the emission of electrons; a third grid electrode  10  being provided adjacent to the second grid electrode  8 ; a fourth grid electrode  12  being provided adjacent to the third grid electrode  10  and to which a focus voltage is applied; a fifth grid electrode  14  mounted surrounding a part of the fourth grid electrode  12  with a predetermined gap therebetween and to which an anode voltage is applied; and a connector  16  for electrically connecting the third grid electrode  10  to the fifth grid electrode  14 . The focus voltage and anode voltage are applied to the fourth grid electrode  12  and fifth grid electrode  14 , respectively. That is, the fourth grid electrode  12  receives the focus voltage, and the fifth grid electrode  14  receives the anode voltage. 
     The above electrodes are fixedly supported by a bead glass  18  in a sequentially aligned manner starting from the cathode  4  and along an axis Z direction (in the drawings). A stem base  20  is fused to an end of a neck  22  such that the electron gun  2  is positioned within the neck  22  with a predetermined gap between the electron gun  2  and an inner surface of the neck  22 . 
     The anode voltage, which is approximately 30˜32 kilovolts (kV), is applied to both the third grid electrode  10  and the fifth grid electrode  14  through the connector  16 . As a result, a pre-focus lens PL is formed between the second and third grid electrodes  8  and  10  by a difference in potential therebetween, and a first main focus lens ML 1  is formed between the third and fourth grid electrodes  10  and  12  by a difference in potential therebetween. 
     In addition, the fourth grid electrode  12  is supplied the focus voltage, which is approximately 7˜10 kilovolts (kV), through a corresponding stem pin (not shown). If a velocity modulator  24  is mounted to an outer circumference of the neck  22 , the fourth grid electrode  12  may be separated into a plurality of sub-electrodes, for example, first, second and third sub-electrodes  12 A,  12 B, and  12 C, with a predetermined gap  26  therebetween. 
     The velocity modulator  24  typically generates a bipolar magnetic field to control a deflection speed. In the case where the fourth grid electrode  12  generates eddy currents by a high frequency current formed by the velocity modulator  24 , a sensitivity of the velocity modulator  24  deteriorates. Therefore, the generation of eddy currents is restrained through the gaps  26 . 
     The first sub-electrode  12 A and the second sub-electrode  12 B of the fourth grid electrode  12  are interconnected by a connector (not shown), and the second sub-electrode  12 B and the third sub-electrode  12 C of the fourth grid electrode  12  are interconnected by a connector (not shown). Accordingly, the same focus voltage is applied to all the sub-electrodes  12 A,  12 B, and  12 C of the fourth grid electrode  12 . 
     Preferably, sub-electrodes  12 A,  12 B, and  12 C forming the fourth grid electrode  12  are cylindrical and hollow to function also as electron beam passageways. The third sub-electrode  12 C, which is the farthest from the cathode  4 , has the largest diameter of the three sub-electrodes  12 A,  12 B, and  12 C. That is, the third sub-electrode  12 C is cylindrical and includes an input section  30  that is identical in diameter to the second sub-electrode  12 B, and an output section  28  having a diameter larger than the diameter of the input section  30 . 
     The fifth grid electrode  14  is also cylindrical and has a diameter larger than that of the fourth grid electrode  12 . The fifth grid electrode  14  is formed surrounding part of the fourth grid electrode  12 . Preferably, the fifth grid electrode  14  includes an input section  32 , which is fixed by the bead glass  18 , and an output section  34 , which has a diameter larger than a diameter of the input section  32 . 
     Two or more bulb spacers  36  are fixed to an outer circumference of the output section  34  of the fifth grid electrode  14 . The bulb spacers  36  contact an inner graphite layer  38  deposited on the inner surface of the neck  22  to transmit an anode voltage applied to the graphite layer  38  to the fifth grid electrode  14 . The bulb spacers  36  also maintain a predetermined gap between the fifth grid electrode  14  and the inner surface of the neck  22  to improve alignment characteristics of the electron gun  2 . 
     In the electron gun  2  of FIG. 1, an end of the output section  28  of the third sub-electrode  12 C, which has the largest diameter out of the three sub-electrodes  12 A,  12 B, and  12 C of the fourth electrode  12 , is not surrounded by the fifth electrode  14  and instead is exposed. Accordingly, a distance of length L 1  from the cathode  4  to the end of the output section  28  of the fourth grid electrode  12  is greater than a distance of length L 2  between the cathode  4  and an end of the output section  34  of the fifth grid electrode  14 . 
     With part of the output section  28  of the fourth grid electrode  12  left exposed and not covered by the fifth grid electrode  14 , the difference in voltage of the fourth grid electrode  12  and the graphite layer  38  results in the formation of a second main focus lens ML 2  of a large diameter within the neck  22  and adjacent to the output section  28  of the fourth grid electrode  12  in a direction toward a phosphor screen. 
     The fifth grid electrode  14  encircles a portion of the fourth grid electrode  12 . At least a part of the output section  28  of the fourth grid electrode  12  is not encircled by the fifth grid electrode  14 , as shown in FIG.  2 . 
     The fifth grid electrode  14  is at least partly cylindrical in shape, and the fourth grid electrode  12  is at least partly cylindrical in shape. The output section  34  of the fifth grid electrode  14  has a larger diameter than the output section  28  of the fourth grid electrode  12 . The output section  34  of the fifth grid electrode  14  encircles or surrounds a portion of the fourth grid electrode  12 , as shown in FIG.  2 . 
     The fifth grid electrode  14  encircles a portion of the third sub-electrode  12 C of the fourth grid electrode  12 . At least a part of the output section  28  of the third sub-electrode  12 C of the fourth grid electrode  12  is not encircled by the fifth grid electrode  14 , as shown in FIG.  2 . 
     As shown in FIG. 2, a part of the output section  28  of the third sub-electrode  12 C of the fourth grid electrode  12  extends beyond the output section  34  of the fifth grid electrode  14 , and thus that part of the output section  28  of the third sub-electrode  12 C of the fourth grid electrode  12  is not encircled by the fifth grid electrode  14 . Also, a part of the input section  30  of the third sub-electrode  12 C of the fourth grid electrode  12  is not encircled by the fifth grid electrode  14 , as shown in FIG.  2 . 
     A connector  16  electrically connects the third grid electrode  10  to the fifth grid electrode  14 , as shown in FIGS. 1 and 2. The fifth grid electrode  14  can have a thickness that does not exceed 500 micrometers. That is, the fifth grid electrode  14  can have a thickness that is equal to or less than 500 micrometers. In other words, the thickness of the fifth grid electrode  14  can be a thickness selected from among a first thickness that is 500 micrometers and a second thickness that is less than 500 micrometers. 
     The output section  28  of the third sub-electrode  12 C of the fourth grid electrode  12  having an edge (or an end) that is facing away from the cathode, and that edge (or end) is a distance L 1  from the cathode as shown in FIG.  2 . The output section  34  of the fifth grid electrode  14  having an edge (or end) facing away from the cathode, and that edge (or end) is a distance L 2  from the cathode as shown in FIG.  2 . The distance L 1  is larger than the distance L 2 , as shown in FIG.  2 . The aforementioned edge of the output section  28  is a distance (L 1 −L 2 ) away from the aforementioned edge of the output section  34 , as shown in FIG.  2 . 
     It can be said that the third sub-electrode  12 C of the fourth grid electrode  12  has an input section  30  and an output section  28 , as shown in FIG.  2 . Also, it can be said that the fourth grid electrode  12  has an input section  30  and an output section  28 . The input section  30  has an input end. The output section  28  has an output end. The output end of the output section  28  is located at the part of the sub-electrode  12 C that is farthest from the cathode  4 , as shown in FIG.  2 . The input end of the input section  30  is located at the part of the sub-electrode  12 C that is closest to the cathode  4 , as shown in FIG.  2 . The output end is separated from the sub-electrode  12 B by the sub-electrode  12 C and one gap  26 , as shown in FIG.  2 . However, the input end is separated from the sub-electrode  12 B only by the one gap  26 , as shown in FIG.  2 . 
     It can be said that the fifth grid electrode  14  has an input section  32  and an output section  34 , as shown in FIG.  2 . The input section  32  has an input end. The output section  34  has an output end. The output end of the output section  34  is located at the part of the fifth grid electrode  13  that is farthest from the cathode  4 , as shown in FIG.  2 . The input end of the input section  32  is located at the part of the fifth grid electrode  14  that is closest to the cathode  4 , as shown in FIG.  2 . 
     A distance between the cathode  4  and the output end of the output section  28  of the fourth grid electrode  12  is greater than a distance between the cathode  4  and the output end of the output section  34  of the fifth grid electrode  14 , as shown in FIG.  2 . 
     As shown in FIG. 2, an electrostatic main focus lens ML 2  is formed by a voltage difference between the focus voltage applied to the fourth grid electrode  12  and the anode voltage applied to the fifth grid electrode  14 . The electrostatic main focus lens ML 2  being formed just beyond the output end of the output section  28  of the fourth grid electrode  12 , as shown in FIG.  2 . Also, the electrostatic main focus lens ML 2  can be said to be formed near to, adjacent to, or at the output end of the output section  28  of the fourth grid electrode  12 , as shown in FIG.  2 . 
     FIG. 3 is a partially enlarged view of a fourth grid electrode and a fifth grid electrode shown in FIG. 2, in accordance with the principles of the present invention. FIG. 3 is a partially enlarged view of the third sub-electrode  12 C and the fifth grid electrode  14 . There is a distance of length A in the axis Z direction between the end of the output section  34  of the fifth grid electrode  14  and the end of the output section  28  of the third sub-electrode  12 C. As a result, part of the output section  28  of the third sub-electrode  12 C is exposed and is not surrounded by the fifth grid electrode  14  such that this exposed portion of the output section  28  of the third sub-electrode  12 C opposes the graphite layer  38  deposited on the inner surface of the neck  22 . 
     To realize this configuration, a length L 3  of the fifth grid electrode  14  in the axis Z direction is smaller than a length L 4  of the third sub-electrode  12 C. Also, all of the fifth grid electrode  14  is positioned surrounding the third sub-electrode  12 C in such a manner that the end of the output section  34  of the fifth grid electrode  14  is distanced from the end of the output section  28  of the third sub-electrode  12 C by the length A as described above. 
     FIG. 4 is a schematic view showing equipotential lines and electron beam traces generated during driving of the electron gun of FIG. 1, in accordance with the principles of the present invention. FIG. 5 is an enlarged view of FIG. 5, in accordance with the principles of the present invention. 
     It can be confirmed from the drawings that the second main focus lens ML 2  is formed starting from the end of the output section  28  of the third sub-electrode  12 C. The second main focus lens ML 2  is formed by the difference between the focus voltage of the third sub-electrode  12 C and the anode voltage of the graphite layer  38 , and acts to converge the electron beam. 
     Accordingly, in the electron gun  2  according to the preferred embodiment of the present invention, the anode voltage applied to the graphite layer  38  and not the anode voltage of the fifth grid electrode  14  is used to form the second main focus lens ML 2  by the potential difference with the fourth electrode  12 . Therefore, a diameter of the second main focus lens ML 2  is maximized within the limited diameter of the neck  22  to thereby improve electron beam focus performance. 
     The fifth grid electrode  14  and the graphite layer  38 , to which a high anode voltage is applied, are designed so that an electrical short does not occur between these elements and the fourth grid electrode  12  to which the focus voltage is applied. That is, so that a short does not occur between these elements and the third sub-electrode  12 C of the fourth grid electrode  12 . The graphite layer  38  is an electrically conductive film. 
     FIG. 6 is a partially enlarged view of a fourth grid electrode and a fifth grid electrode shown in FIG. 2, in accordance with the principles of the present invention. In more detail, with reference to FIG. 6, an inner diameter of the input section  32  of the fifth grid electrode  14  is larger than an outer diameter of the third sub-electrode  12 , and a distance of length B is formed therebetween. Further, a distance of length C is provided in the axis Z direction between a floor portion  40  interconnecting the output section  34  and the input section  32  of the fifth grid electrode  14  and a floor portion  42  interconnecting the output section  28  and the input section  30  of the third sub-electrode  12 C. Also, an inner diameter of the fifth grid electrode  14  is larger than an outer diameter of the output section  28  of the third sub-electrode  12 C, and a distance of length D is formed therebetween. 
     Preferably, the third sub-electrode  12 C and the fifth grid electrode  14  are provided satisfying the conditions outlined below such that withstand voltage characteristics are maintained between the third sub-electrode  12 C and the fifth grid electrode  14 , and to allow for maximum inner and outer diameters of the output section  28  of the third sub-electrode  12 C within the limited size of the neck  22 . 
     
       
         1.08 &lt;D   2   /D   1 &lt;2.0  [Equation 1] 
       
     
     where D 1  is the outer diameter of the input section  30  of the third sub-electrode  12 C, and D 2  is the outer diameter of the input section  32  of the fifth grid electrode  14 . It is assumed that a thickness of the fifth grid electrode  14  does not exceed 500 micrometers (μm). 
     
       
         1.0 &lt;D   4   /D   3 &lt;1.2  [Equation 2] 
       
     
     where D 3  is the outer diameter of the output section  28  of the third sub-electrode  12 C, and D 4  is the outer diameter of the output section  34  of the fifth grid electrode  14 . It is assumed the thickness of the fifth grid electrode  14  does not exceed 500 micrometers (μm). 
     Further, it is preferable that the length C between the floor portion  40  interconnecting the output section  34  and the input section  32  of the fifth grid electrode  14  and the floor portion  42  interconnecting the output section  28  and the input section  30  of the third sub-electrode  12 C is at least 2 millimeters (mm). 
     Also, there is provided a gap of length E between the output section  28  of the third sub-electrode  12 C and the inner diameter of the neck  22 . Preferably, the output section  28  of the third sub-electrode  12 C satisfies the following condition with respect to the inner diameter of the neck  22  such that a maximum diameter is realized while maintaining withstand voltage characteristics of the graphite layer  38 . 
     
       
         1.4 &lt;D   5   /D   3 &lt;1.7  [Equation 3] 
       
     
     where D 3  is the outer diameter of the output section  28  of the third sub-electrode  12 C and D 5  is the inner diameter of the neck  22 . 
     The fifth grid electrode  14  is spaced apart from the fourth grid electrode  12 . The fifth grid electrode  14  is spaced apart from the fourth grid electrode  12  by at least a predetermined gap. As shown in FIG. 6, the gap between electrodes  14  and  12 C includes at least three sections. As shown in FIG. 6, there is a first section of the between output section  28  of third sub-electrode  12 C and the output secton  34  of fifth grid electrode  14 , and that first section has a length D. As shown in FIG. 6, there is a second section of the gap between floor sections of third sub-electrode  12 C and fifth grid electrode  14 , and that second section has a length C. As shown in FIG. 6, there is a third section of the gap between input section  30  of third sub-electrode  12 C and the input secton  32  of fifth grid electrode  14 , and that third section has a length B. 
     FIG. 7 is partially enlarged sectional views of a fourth grid electrode and a fifth grid electrode in an exemplary electron gun for cathode ray tubes. Table 1 below shows various parameters including the equivalent diameter of the second main focus lens ML 2  of the electron gun according to the preferred embodiment of the present invention and of an electron gun of a comparative example (see FIG.  7 ). In the electron gun of the comparative example, the structure between a cathode and a fourth grid electrode  3  is identical to that of the present invention. 
     Also, a fifth grid electrode  1  completely surrounds an output section  5  of the fourth grid electrode  3  such that the second main focus lens ML 2  is formed within the fifth grid electrode  1 . In Table 1 below, the output section of the fourth grid electrode  12  refers to the output section  28  of the third sub-electrode  12 C of the fourth grid electrode  12 . 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 Outer 
                 Outer 
                   
               
               
                   
                   
                   
                 diameter 
                 diameter 
                   
               
               
                   
                   
                   
                 of 
                 of 
                   
               
               
                   
                   
                   
                 fourth grid 
                 fifth grid 
                 Equivalent 
               
               
                   
                 Neck 
                 Neck 
                 electrode 
                 electrode 
                 diameter 
               
               
                   
                 outer 
                 inner 
                 outer 
                 outer 
                 of ML2 
               
               
                   
                 diameter 
                 diameter 
                 diameter 
                 diameter 
                 (mm) 
               
               
                   
               
             
            
               
                 Comparative 
                 29.1 
                 24.0 
                 16.0 
                 22 
                 15.9 
               
               
                 Example 
               
               
                 Preferred 
                 29.1 
                 24.0 
                 20.0 
                 22 
                 22.4 
               
               
                 Embodiment 
               
               
                   
               
            
           
         
       
     
     As shown in Table 1, in comparing the electron gun of the comparative example to the electron gun of the present invention, in which the output section  28  of the fourth grid electrode  12 , that is, the output section  28  of the third sub-electrode  12 C, is enlarged and exposed such that the equivalent diameter of the second main focus lens ML 2  is improved for the present invention by approximately 40.8% over the comparative example. 
     FIG. 8 is a graph showing 5% electron beam spot sizes according to variations in electron beam current for the electron gun of FIG.  1  and an exemplary electron gun. Table 2 below and FIG. 8 show results of measuring 5% electron beam spot sizes according to variations in electron beam current for the electron gun of the present invention and the comparative example. Table 3 shown following Table 2 indicates the different voltages applied to each electrode while taking the measurements of Table 2 and FIG.  8 . 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Electron beam current 
                   
                   
                   
                   
                   
               
               
                 microamps (μA) 
                 1 
                 2 
                 3 
                 4 
                 5 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Electron 
                 Comparative 
                 238.26 
                 223.26 
                 216.88 
                 230.91 
                 260.00 
               
               
                 beam spot 
                 Example 
               
               
                 size (μm) 
                 Present 
                 178.80 
                 170.12 
                 163.24 
                 175.55 
                 190.34 
               
               
                   
                 Invention 
               
            
           
           
               
               
               
               
               
               
            
               
                 Reduction (%) 
                 22.1 
                 19.6 
                 20.6 
                 21.7 
                 26.6 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 First grid 
                 Second grid 
                 Fourth grid 
                 Third and fifth 
               
               
                   
                 electrode 
                 electrode 
                 electrode 
                 grid electrodes 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Comparative 
                 0 volts (V) 
                 350 V 
                 9.8 kV 
                 32 kV 
               
               
                 Example 
               
               
                 Present 
                 0 V 
                 350 V 
                 7.8 kV 
                 32 kV 
               
               
                 Invention 
               
               
                   
               
            
           
         
       
     
     As shown in Table 2 and FIG. 8, the electron beam spot size of the electron beam spot size of the present invention is improved over that of the comparative example by about 20% or more in both low current and high current regions. 
     With the electron gun  2  according to the preferred embodiment of the present invention, in addition to the structure described above, it is also possible for the fourth grid electrode  12  or the fifth grid electrode  14  to be structured in a variety of ways such as in a tapered form. 
     FIGS. 9 and 10 are partial sectional views of fourth and fifth grid electrodes according to other preferred embodiments of the present invention, in accordance with the principles of the present invention. With reference to FIG. 9, a third sub-electrode  12 C′ of a fourth grid electrode  12  has an angled section  44  of a predetermined length formed between an input section  30  and an output section  28 . The angled section  44  interconnects the input section  30  and the output section  28 . An end of the angled section  44  connected to the input section  30  has inner and outer diameters identical to the input section  30 , then the angled section  44  has progressively enlarged inner and outer diameters until reaching the output section  28  where the angled section  44  has inner and outer diameters identical to the output section  28 . 
     With reference to FIG. 9, the third sub-electrode  12 C′ of the fourth grid electrode  12  forms an angled section  44  between the input section  30  and the output section  28 . As shown in FIG. 9, the angled section  44  has progressively larger diameters starting at a portion of the angled section  44  connected to the input section  30 . Thus, a diameter of the angled section  44  at a location where the angled section  44  is connected to the output section  28  is larger than a diameter of the portion of the angled section  44  connected to the input section  30 , as shown in FIG.  9 . 
     With reference to FIG. 10, a third sub-electrode  12 C″ of a fourth grid electrode  12  is formed such that an output section  28 ′ thereof is formed tapered, that is, progressively enlarged from its end connected to an input section  30  in a direction toward the phosphor screen. 
     With reference to FIG. 10, the third sub-electrode  12 C″ of the fourth grid electrode  12  forms an output section  28 ′ that is angled. The angled output section  28 ′ has a first end and has a plurality of different diameters and also has a second end, as shown in FIG.  10 . The second end of the angled output section  28 ′ is connected to the input section  30  of the fourth grid electrode  12 , as shown in FIG.  10 . The second end of the section  28 ′ has a diameter substantially equal to a diameter of the input section  30  of the fourth grid electrode  12 , as shown in FIG.  10 . The first end has a diameter larger than the diameter of the second end, as shown in FIG.  10 . As shown in FIG. 10, the second end of section  28 ′ is disposed between the input section  30  and the first end of section  28 ′. Thus, using these terms, the first end of the section  28 ′ is a first distance away from the cathode  4 , the second end of the section  28 ′ is a second distance away from the cathode  4 , and the first distance is larger than the second distance, these distances of course being measured along a straight line. The first end of the section  28 ′ is farther away from the cathode  4  than is the second end of the section  28 ′. 
     With the above configurations of adding the angled section  44  or tapering the output section  28 ′ itself, the formation of an abrupt angle in the fourth grid electrode  12  is avoided to minimize the possibility of arc discharge occurring. This improves the withstanding voltage characteristics of the electron gun. 
     The fourth grid electrode  12  can alternatively be referred to as a “first electrode  12 ” of the electron gun  2 . The fifth grid electrode  14  can alternatively be referred to as a “second electrode  14 ” of the electron gun  2 . These alternative terms may be useful during detailed discussions of the grid electrodes  12  and  14 , and during other times. 
     In the electron gun for cathode ray tubes of the present invention described above, the diameter of the main focus lens is maximized within the limited neck diameter. Therefore, the spot size of the electron beam landing on the phosphor screen is reduced by about 20% such that exceptional focus performance and resolution characteristics are realized. 
     While the present invention has been illustrated by the description of embodiments thereof, and while the embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative apparatus and method, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit and scope of the applicant&#39;s general inventive concept.