Abstract:
A multi-step focusing type electron gun is disclosed which includes a cathode, electrodes G1 and G2, and electrodes G3, G4, G5, G6, G7, and G8 for forming two unipotential auxiliary lenses and a bipotential major lens in the main lens, characterized in that the electrodes G4, G5, G6, G7 are so constituted as to satisfy some specific formulas. The electron gun can reduce the size of the beam spot and the size of the halo formed around the beam spot. Further the electron gun improves the spherical aberration, and provides the flexibility in designing for different purposes, as well as improving the image quality on the screen.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an electron gun for use in a cathode ray tube, and particularly to a multistep focusing electron gun in which the main lens means consists of two unipotential type auxiliary lenses and a bipotential type major lens. 
     BACKGROUND OF THE INVENTION 
     The image quality of a cathode ray tube greatly depends on the performance of the electron gun, and especially, the focus characteristic of electron beams made by means of an electron gun is an important factor which directly affects the image quality. That is, a good electron gun will form the smallest beam spots as far as possible with low spherical aberration, and such an electron gun having the above mentioned purposes has been developed in various types. Among them, there is one in which, in order to reduce the spherical aberration, the beam passing holes of the main lens are overlapped in such a manner that three beam passing holes should be continuously formed. But in this type, if the beam current is increased, then the spherical aberration may be increased. In order to settle said apprehension, the length of the electrode G3 has to be increased enough for the magnification of the main lens to be reduced. However, such case, there is a problem of practicability. 
     In consideration of these circumstances, Korean Patent publication No. 87-281 discloses an improved electron gun based on a new conception, which is characterized in that the spherical aberration can be reduced to a great degree by providing two unipotential type auxiliary lenses at a position upstream of the bipotential type major lens. That is, the electron beam which had been focused by means of a single main lens in the preceeding technology has come to be focused through two auxiliary lenses and a major lens. Such a dispersion of the focusing steps over three lenses is for reducing the spherical aberration by means of low magnification lenses. That is, while the beam is focused by a multistep mechanism, the distance of the virtual object point (the starting point of the beam) is separated farther from the screen step by step, so that the incident angles of the electron beam relative to the respective lenses should be reduced step by step. 
     The electron gun based on the above described principle provided a satisfactory performance to a certain extent, but in order to obtain the maximum performance, the dimensions of the components and voltages applied to them should be more thoroughly deliberated. 
     SUMMARY OF THE INVENTION 
     Therefore it is an object of the present invention to provide a multistep focusing electron gun for cathode ray tube in which the spherical aberration and the focus characteristics are greatly improved. 
     It is another object of the present invention to provide a multistep focusing electron gun for cathode ray tube in which the size of the beam spot can be easily adjusted, thereby making it possible to design it as a multipurpose electron gun. 
     To achieve the said objects, the present invention provides a multi-step focusing type electron gun comprising an electron beam producing cathode K, a control grid G1, a screen grid G2 and electrodes G3, G4, G5, G6, G7, and G8 for forming two unipotential auxiliary lenses and a bipotential major lens in the main lens means. Further, the electrodes G4, G5, G6 and G7 are designed such that they should satisfy the following formulas: ##EQU1## where D4, D5, D6 and D7 indicate the diameters of the beam passing holes of the electrodes G4, G5, G6 and G7, respectively, and L4, L5, L6 and L7 the lengths of them, respectively. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and other advantages of the present invention will become more apparent by describing in detail the preferred embodiments of the present invention with reference to the attached drawings in which: 
     FIG. 1 is a schematic sectional view of the first embodiment of the present invention; 
     FIG. 2 is a graphical illustration showing the characteristics of the first unipotential lens of the main lens means of the electron gun according to the present invention, the values being obtained by a numeric control method; 
     FIG. 3 is a graphical illustration showing the characteristics of the second unipotential lens of the main lens means of the electron gun according to the present invention, the values being obtained by a numeric control method; 
     FIG. 4 is a graphical illustration comparatively showing the beam spot size-cathode currents for the electron gun of the present invention and that of the conventional technology; 
     FIG. 5 is a graphical illustration comparatively showing the variations of the beam spots in correspondence with the variations of the focusing voltages for the electron gun of the present invention and that of the conventional technology; 
     FIG. 6 is an enlarged view of the beam spot of the electron gun of the present invention and that of the conventional electron gun. 
     FIG. 7 is an enlarged view of the beam spots comparatively showing the characteristics of the beam spots appearing in the peripheral zones of the screen for the electron gun of the present invention and that of the conventional technology; 
     FIG. 8 is a schematic sectional view of the second embodiment of the electron gun according to the present invention; and 
     FIG. 9 is a schematic sectional view of the third embodiment of the electron gun according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1 in which the first embodiment of the electron gun according to the present invention is illustrated, this drawing shows a beam producing cathode K, a control grid G1, a screen grid G2, and electrodes G3, G4, G5, G6, G7, and G8. The electrodes are accelerating and focusing the electron beam from the said cathode, control grid G1, and screen grid G2. All of the electrodes are arranged at proper intervals. 
     Here, the general dimensions of the said electrodes are determined by the relationship between the diameters of the beam passing holes of the respective electrodes and the lengths of their bodies as shown in Table 1 below. The inventor obtained the values of Table 1 through repeated numeric analysis experiments. 
     
                       TABLE 1______________________________________Length of electrodes/diameters of holes        L4/D4    L5/D5    L6/D6  L7/D7______________________________________Ranges       0.18-0.55                 0.36-1.45                          0.07-0.18                                 1.09-1.73______________________________________ 
    
     In the above table, L4, L5, L6 and L7 indicate the lengths of the electrodes G4, G5, G6 and G7, while D4, D5, D6 and D7 indicate the diameters of the beam passing holes of the respective electrodes. 
     Of the said electrodes, the lengths of the electrodes placed in the main lens means may be variable within the allowable ranges as shown in Table 2 below, depending on the application and other conditions of the electron gun, but assuming that all the beam passing holes have a uniform diameter of 5.5 mm. 
     
                       TABLE 2______________________________________  G4    G5          G6      G7______________________________________Lengths  1.0-3.0 2.0-8.0     0.4-1.0                              6.0-9.5______________________________________ 
    
     In the said main lens means, the electrodes G4, G5, G6 and G7 forming an auxiliary lens should preferably have narrow intervals between the adjacent ones, preferably 0.6 mm, while the interval between the electrodes G7, and G8 may be desirably about 1.0 mm. 
     The electrodes G1, G2 form electron beams in cooperation with the cathode which is positioned upstream of the said electrodes G1, G2, forming a three-electrode tube portion, and these electrodes G1, G2 should be designed in the normal type or properly depending on the application of the electron gun. The electrode G3 should also be based on the normal type, but the diameter of the outgoing side of the beam passing hole of this electrode may be formed in a size smaller than that of the electrode G4 properly in different embodiments. 
     That is, as shown in the second embodiment of the present invention illustrated in FIG. 8, the outgoing side beam passing hole 3Hr of the electrode G3 can be formed in a size smaller than the beam passing holes of the electrodes G4, G5, G6, G7, and G8, all of which have holes having the same diameter. 
     Further, referring to FIG. 9 which illustrates the third embodiment of the present invention, the incident side beam passing hole 5Hf of the electrode G5 has a larger diameter, unlike the beam passing hole 4H of the electrode G4 and the outgoing side beam passing hole 5Hr of the electrode G5, both of which have a same diameter. 
     Therefore, the diameters of the beam passing holes of the respective electrodes should satisfy the following formulas: 
     
         D3r&lt;D4&lt;D5f,   D4=D5r 
    
     where D3r indicates the diameter of the outgoing beam passing hole of the electrode G3, D4 the diameter of the beam passing hole of the electrode G4, and D5f the diameter of the incident side beam passing hole of the electrode G5. Of course, the above electrodes should come within the ranges specified in Table 1. 
     In the embodiments of the present invention described above, the proportion of the applied focus voltage Vf to the anode voltage Eb should fall approximately within the range of 23%-34%, and the most desirable focus voltage Vf and anode voltage Eb are, respectively, 7 KV and 25 KV in the case of a 20-inch cathode ray tube. 
     The above specified dimensions of the multistep focusing electron gun according to the present invention are the values obtained as a result of repeated experiments, and should apparently produce a satisfactory result. 
     First, the variations of the performances of the multistep focusing electron gun in correspondence with the variations of the sizes of the electrodes as abtained through the experiments will be described. FIG. 2 shows the variations of the size of the beam spot as against the variations of the lengths L4, L5 of the electrodes G4, G5, and the variations of the ratio of the focus voltage Vf to the anode voltage Eb as against the variations of the lengths of the electrodes G4, G5 for the first embodiment of the present invention. That is, in the case where the electrode G4 has a length of 1.4 mm, and the length of the electrode G5 is varied within the range of 2.0-6.5 mm, Vf/Eb falls within the range of 23%-34%, and the variation of the beam size falls within the range of 1.8-2.1 mm approximately. 
     Further, as shown in FIG. 3, if the length of the electrode G6 is 0.6 mm, and the length of the electrode G7 facing with the said electrode G6 opposingly is varied within the range of 6.0-9.5 mm, then the value of Vf/Eb is varied within the range of 30-32%, and the size of the beam spots will stay in the range of 1.65-1.70 mm. 
     The electron gun according to the present invention having such a characteristic and the conventional electron gun are compared in FIG. 4 which shows that the electron gun according to the present invention forms a smaller beam spot compared with the conventional electron gun Hi-Bi, on the assumption that the cathode current Ib is the same in both of them. Especially, to see into the variations of the beam spot as against the variations of the focus voltage Vf as shown in FIG. 5, the electron gun according to the present invention forms a far smaller beam spot, and its variation range is also smaller under the same focus voltage applied. 
     Further, as shown in FIG. 6, the electron gun according to the present invention forms a smaller beam spot, and shows a smaller variation range compared with the conventional electron gun. 
     Further, as shown in FIG. 7, the focus characteristics on the peripheral areas of the screen is also improved in the electron gun of the present invention compared with the conventional electron gun. The conventional electron gun shows not only a larger beam spot, but also forms a larger halo around the beam spot, whereas the electron gun according to the present invention produces a smaller halo and a smaller beam spot. 
     Meanwhile, according to the second and third embodiments of the present invention which are illustrated in FIGS. 8 and 9, respectively, the characteristics of the electron gun are further improved. That is, not only the spherical aberration is further improved, but also, an excellent focus characteristics is produced. 
     In other words, in the second embodiment illustrated in FIG. 8, an electrostatic unipotential lens is formed by means of the electrodes G3, G4, G5 and a weak diverging section I is formed by the combination of the electrode G3 having an outgoing side beam passing hole 3Hr of a smaller diameter, and the electrode G4 having a beam passing hole 4H of a larger diameter. Accordingly, when the electron beam is passing the section I, the electron beam experiences a weak diverging force, with the result that the electron beam enters with a smaller incident angle into the focusing section II which is formed by the cooperation between the electrodes G4, G5. As a result, the spherical aberration on the screen surface of the cathode ray tube is improved, resulting in that clear and high quality images are produced on the screen. 
     Meanwhile, the third embodiment of the present invention as illustrated in FIG. 9 has a more advanced constitution compared with the second embodiment. Here, the beam passing hole of the electrode G5 forming a part of a beam focusing section II&#39; is formed in a smaller size compared with the beam passing holes of the electrodes G4, and G6, which are disposed at the opposite sides thereof, thereby producing a strong focusing force. Electrode G5 comprises front and rear beam passing sections having beam passing holes 5Hf and 5Hr and corresponding beam passing diameters D5f, D5r and lengths L5f and L5r. 
     Therefore, in this embodiment, the beam diverging force is weakened through the diverging section I&#39;, while the beam focusing force is reinforced through the focusing section II&#39;. Consequently, the spherical aberration is greatly reduced, and the beam focus characteristics is markedly improved compared with the above embodiments. 
     In short, the electron gun according to the present invention has the following advantages. The variations of the size of the image as against the variations of the magnification of the main lens and against the variations of the distance to the screen are reduced, while the spherical aberration most greatly affecting the pattern of the beam is remarkably improved, because the beam divergence angle can be easily adjusted by means of the auxiliary lenses. 
     Thus, the electron gun according to the present invention reduces the size of the beam spot by about 30% compared with the conventional electron guns, thereby making it suitable for use in a cathode ray tube of a high fineness and high resolution. Moreover the astigmatism in the peripheral areas is remarkably improved, thereby making it possible to improve the image quality. 
     Further, the first and second auxiliary lenses are provided in an adjustable form in their magnifications, thereby providing flexibilities in designing them for different purposes. Moreover the voltage differences between the electrodes can be minimized compared with the conventional electron guns, thereby improving the voltage resistance characteristics.