Abstract:
An electron gun for a cathode ray tube is disclosed, which has a simple structure and can prevent the spot on a screen from being degraded. In the electron gun for a cathode ray tube including a cathode that emits electron beams, a first electrode that controls the electron beams emitted from the cathode, a second electrode that accelerates the electron beams emitted from the first electrode, and third to fifth electrodes sequentially arranged in a screen direction to act as pre-focus lenses, the electron gun is characterized in that the third to fifth electrodes have different sized electron beam through holes.

Description:
This application claims the benefit of the Korean Application No. P2001-44873 filed on Jul. 25, 2001 and the Korean Application No. P2002-23428 filed on Apr. 29, 2002, which is hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a cathode ray tube, and more particularly, to an electron gun for a cathode ray tube that can reduce a change in the size of a spot due to a change of a focus voltage and a change of current. 
   2. Discussion of the Related Art 
   Generally, a cathode ray tube includes an in-line electron gun that emits three electron beams, a deflection yoke that deflects the electron beams in a predetermined place of a screen, a shadow mask that selects the electron beams, and a screen that reproduces a picture image by colliding with the electron beams. 
   According to the Japanese Patent Laid-Open No. 60-51775, a typical electron beam spot becomes great if beam current increases. Therefore, to obtain a fine picture image, the beam current should be within the smaller range if possible. However, since a cathode ray tube that requires high current has a great change of current, a uni-bi potential lens structure having an improved pre-focus area in a bi potential main lens structure has been employed to reduce the spot size on the screen. 
   A related art electron gun for a cathode ray tube will be described with reference to FIG.  1 . 
   Referring to  FIG. 1 , the related art electron gun includes a cathode K that emits three electron beams of R, G, and B, a first electrode  1  that controls the electron beams emitted from the cathode K, a second electrode  2  that accelerates a thermal electron emitted from the first electrode  1 , third, fourth and fifth electrodes  3 ,  4 , and  5  that focus the electron beams, and a sixth electrode  6  that acts as an anode. 
   The operation of the aforementioned related art electron gun will now be described. 
   If a heater provided inside the cathode K heats the cathode K, the electron beams are emitted. The emitted electron beams are controlled by the first electrode  1  that acts as a control electrode. Also, the emitted electron beams are accelerated by the second electrode  2  and focused by the third to sixth electrodes  3 ,  4 ,  5 , and  6 . 
   Meanwhile, if high current is generated from the electron gun, the current density of crossover does not increase by an increased value of the beam current due to the space charge repulsion. The current density is uniformly distributed without forming a Gaussian distribution, thereby degrading the crossover. If the crossover is degraded, the spot on the screen is degraded accordingly. 
   To prevent the crossover from being degraded, it is necessary to reduce the potential of the crossover, thereby reducing the space charge repulsion. To increase a voltage of the crossover, the third electrode  3  moves to the second electrode  2 . Thus, the potential of the crossover increases while the space charge repulsion decreases. 
   However, in this case, an emitting angle α increases and thus the size Db of the electron beam in a main lens increases. As shown in  FIGS. 7 and 8 , if the size Db of the electron beam in the main lens increases, spherical aberration increases. In this case, a problem arises in that the size of the spot on the screen increases. 
   To solve such a problem, it is necessary to reduce the emitting angle after the crossover passes. Since the crossover moves over the second electrode  2  under the high current, it is difficult to reduce the emitting angle by means of the third to fifth electrodes  3 ,  4 , and  5 . 
   To reduce the emitting angle α after the crossover passes, another pre-focus lens may be provided between the pre-focus lens by the second and third electrodes  2  and  3  and the main lens. 
   As shown in  FIG. 1 , the pre-focus lens is formed in a uni-potential lens structure by dividing a focus electrode into the third, fourth, and fifth electrodes  3 ,  4 , and  5  and applying the same voltage to the third and fifth electrodes  3  and  5 . At this time, an electron beam through hole  41  of the fourth electrode  4  has the same size as that of an electron beam through hole  51  of the fifth electrode  5 . The electron beam through hole  51  of the fifth electrode  5  is formed in a direction of the fourth electrode. The electron beam through hole  41  of the fourth electrode is greater than an electron beam through hole  31  of the third electrode  3 . 
   In this case, the emitting angle of the electron beams entered into the main lens decreases. This decreases the size Db of the electron beam in the main lens. If the size Db of the electron beam in the main lens decreases, the spherical aberration decreases. As a result, the size of the spot on the screen decreases. 
   However, the aforementioned related art electron gun has several problems. 
   As described above, in the related art electron gun, the fourth electrode is formed in a plate shape in the pre-focus lens at the front of the main lens so that the emitting angle of the electron beams is adjusted. At this time, the fourth electrode is adjacent to the third electrode while the fifth electrode is adjacent to the fourth electrode. In forming the pre-focus lens, the electron beam through hole of the fourth electrode has the same size as that of the fifth electrode. In this case, design factors that can adjust the emitting angle are limited to each thickness of the third, fourth, and fifth electrodes, the distance d 1  between the third electrode and the fourth electrode, the distance d 2  between the fourth electrode and the fifth electrode, the size of the electron beam through hole of the third electrode, the size of the electron beam through hole of the fourth electrode, and the size of the electron beam through hole of the fifth electrode. As a result, to additionally adjust the emitting angle, supplementary electrodes are required among the second electrode, the third electrode, and the fourth electrode. This causes a complicated structure. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention is directed to an electron gun for a cathode ray tube that substantially obviates one or more problems due to limitations and disadvantages of the related art. 
   An object of the present invention is to provide an electron gun for a cathode ray tube that has a simple structure and can prevent the spot on a screen from being degraded. 
   Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may 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 objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, in an electron gun for a cathode ray tube including a cathode that emits electron beams, a first electrode that controls the electron beams emitted from the cathode, a second electrode that accelerates the electron beams emitted from the first electrode, and third to fifth electrodes sequentially arranged in a screen direction to act as pre-focus lenses, the electron gun is characterized in that the third to fifth electrodes have different sized electron beam through holes. 
   Preferably, each electron beam through hole of the third and fourth electrodes is smaller than the electron beam through hole of the fifth electrode. 
   Preferably, the electron beam through hole of the third electrode is smaller than that of the fourth electrode. 
   In another aspect of the present invention, the electron beam through hole of the fourth electrode has a rectangular shape and its vertical length is different from its horizontal length. 
   In another aspect of the present invention, the third electrode has a first through hole opposite to the second electrode and a second through hole opposite to the fourth electrode. 
   Preferably, the size of the first through hole is different from that of the second through hole. More preferably, the size of the first through hole is smaller than that of the second through hole. 
   Therefore, in the present invention, it is possible to effectively prevent the spot from being degraded even if no separate supplementary electrode is provided. 
   It is to be understood that both the foregoing general description and the following detailed description of the present invention 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 application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings: 
       FIG. 1  is a schematic view illustrating a related art electron gun for a cathode ray tube; 
       FIG. 2  is a schematic view illustrating an electron gun for a cathode ray tube according to the first embodiment of the present invention; 
       FIG. 3  is a conceptional view illustrating the potential distribution of an electron gun for a cathode ray tube according to the present invention; 
       FIG. 4  illustrates an optical model of an electron lens of an electron gun for a cathode ray tube according to the present invention; 
       FIG. 5  is a schematic view illustrating an electron gun for a cathode ray tube according to the second embodiment of the present invention; 
       FIG. 6  is a schematic view illustrating an electron gun for a cathode ray tube according to the third embodiment of the present invention; 
       FIG. 7   a  is a graph illustrating the current density distribution on a screen in case of no spherical aberration; 
       FIG. 7   b  is a graph illustrating the current density distribution on a screen in case of spherical aberration; 
       FIG. 8   a  illustrates a shape of a spot on a screen in case of no spherical aberration; and 
       FIG. 8   b  illustrates a shape of a spot on a screen in case of spherical aberration. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
   An electron gun for a cathode ray tube according to the first embodiment of the present invention will be described with reference to FIG.  2 . 
   Similarly to the related art electron gun, an electron gun for a cathode ray tube according to the first embodiment of the present invention includes a cathode K that emits electron beams, a first electrode  11  that controls the electron beams emitted from the cathode K, a second electrode  12  that accelerates the electron beams, a pre-focus lens of third, fourth and fifth electrodes  13 ,  14  and  15  that control an emitting angle of the electron beams, and fifth and sixth electrodes  15  and  16  that constitute a main lens part. 
   In the first embodiment according to the present invention, the third electrode  13  is arranged to oppose and be adjacent to the fourth electrode  14 . The fourth electrode  14  is arranged to oppose and be adjacent to the fifth electrode  15 . The third electrode  13 , the fourth electrode  14 , and the fifth electrode  15  have different electron beam through holes  311 ,  411  and  511 , respectively. 
   For example, the electron beam through hole  411  of the fourth electrode  14  may be greater than the electron beam through hole  311  of the third electrode  13 . The electron beam through hole  511  of the fifth electrode  15  may be greater than the electron beam through hole  411  of the fourth electrode  14 . 
   Alternatively, the electron beam through hole  311  of the third electrode  13  and the electron beam through hole  411  of the fourth electrode  14  may be smaller than the electron beam through hole  511  of the fifth electrode  15 . 
   In this case, an emitting angle α of the electron beam and the size of the electron beam in the main lens part may easily be varied. 
   The principle of the present invention will be described in more detail with reference to  FIGS. 3 and 4 . 
   Referring to  FIGS. 3 and 4 , a lens L 1  denotes an emitting lens by the third and fourth electrodes, a lens L 2  denotes a focus lens by the third, fourth, and fifth electrodes, and a lens L 3  denotes an emitting lens by the fourth and fifth electrodes. 
   In  FIG. 3 , the electron beam through hole  411  of the fourth electrode  14  is greater than the electron beam through hole  311  of the third electrode  13 . The electron beam through hole  511  of the fifth electrode  15  is greater than the electron beam through hole  411  of the fourth electrode  14 . In this case, the intensity of the lens L 2  becomes more robust than that of the related art electron gun (the electron beam through hole  311 &lt;the electron beam through hole  411 =the electron beam through hole  511 ). Thus, the emitting angle α of the electron beam to the main lens and the size Db of the electron beam in the main lens can decrease. A decrease of the emitting angle α of the electron beam and the size Db of the electron beam reduces the spherical aberration, thereby reducing the size of a spot on a screen. 
   If the emitting angle α of the electron beam and the size Db of the electron beam in the main lens depart from an optimal value, the size of the electron beam through hole  411  of the fourth electrode  14  is adjusted appropriately. That is, if the electron beam through hole  411  of the fourth electrode  14  becomes great, the intensity of the lens L 2  is weaker than the intensity of the lenses L 1  and L 3 . Thus, the emitting angle α of the electron beam and the size Db of the electron beam in the main lens become great. On the other hand, if the electron beam through hole  411  of the fourth electrode  14  becomes small, the intensity of the lens L 2  becomes more robust than the intensity of the lenses L 1  and L 3 . Thus, the emitting angle α of the electron beam and the size Db of the electron beam in the main lens become small. 
   Meanwhile, the electron beam through holes  311 ,  411 , and  511  are not limited to shapes suggested in the present invention. That is, the electron beam through holes may have a circular shape, a rectangular shape, or the like. As shown in  FIG. 6 , if the electron beam through hole  411  of the fourth electrode has a rectangular shape, it is preferable that its vertical length  411   h  is different from its horizontal length  411   w.  This is because the emitting angle α of the electron beam in vertical and horizontal directions and the size Db of the electron beam in the main lens can be adjusted. 
   An electron gun for a cathode ray tube according to the second embodiment of the present invention will be described with reference to FIG.  5 . 
   In the above embodiment, while the third and fourth electrodes  13  and  14  have plate shapes, they are not limited to plate shapes. That is, the third electrode  13  and/or the fourth electrode  14  may have a cylindrical shape. The third electrode  13  may have a first through hole  311   a  opposite to the second electrode  12  and a second through hole  311   b  opposite to the fourth electrode  14 . In this case, it is preferable that the size of the first through hole  311   a  is different from the size of the second through hole  311   b.  More preferably, the size of the first through hole  311   a  is smaller than the size of the second through hole  311   b.    
   The fourth electrode  14  may also have a first through hole  411   a  opposite to the third electrode  13  and a second through hole  411   b  opposite to the fifth electrode  15 . 
   Meanwhile, it is preferable to satisfy the relation of {the size of the electron beam through hole  511  of the fifth electrode×0.1}≦the size of the electron beam through hole  311  of the third electrode  13 ≦{the size of the electron beam through hole  511  of the fifth electrode×0.5}. This is because the assembly of the electron gun is not easy if the size of the electron beam through hole  311  of the third electrode is smaller than {the size of the electron beam through hole  511  of the fifth electrode×0.1} while aberration of a tripod increases to increase the size of the spot on the screen if the size of the electron beam through hole  311  of the third electrode is greater than {the size of the electron beam through hole  511  of the fifth electrode×0.5}. 
   Furthermore, it is preferable to satisfy the relation of {the size of the electron beam through hole  511  of the fifth electrode×0.5}≦the size of the electron beam through hole  411  of the fourth electrode  14 ≦{the size of the electron beam through hole  511  of the fifth electrode}. This is because that the emitting angle decreases considerably to depart from an optimal emitting angle, thereby increasing the size of the spot on the screen if the size of the electron beam through hole  411  of the fourth electrode is smaller than {the size of the electron beam through hole  511  of the fifth electrode×0.5} while assembly of the electron gun is not easy if the size of the electron beam through hole  411  of the fourth electrode is greater than {the size of the electron beam through hole  511  of the fifth electrode}. 
   As aforementioned, the electron gun for a cathode ray tube according to the present invention has the following advantages. 
   It is easy to design the emitting angle of the electron beam to the main lens and the size of the electron beam in the main lens. That is, the emitting angle and the size of the electron beam can be reduced by adjusting the respective size of the electron beam through holes of the third to fifth electrodes. This can reduce the spherical aberration and can prevent the spot on the screen from being degraded. 
   It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.