Abstract:
An electron gun for a color cathode ray tube including an electron beam generating unit for generating three electron beams arranged in-line, an auxiliary lens forming unit for forming an auxiliary lens for focusing and accelerating the electron beams generated by the electron beam generating unit, and a main lens forming unit for forming a main lens, for finally focusing and accelerating the electron beams focused and accelerated by the auxiliary lens forming unit, and having first and second outer electrodes which face each other and in each of which a large electron beam aperture through which the three electron beams pass is formed, and first and second inner electrodes each installed inside the first and second outer electrodes and having three electron beam apertures shaped of vertically elongated squares.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a color cathode ray tube (CRT), and more particularly, to an in-line electron gun having improved electrodes for forming a large electronic lens.  
           [0003]    2. Description of the Related Art  
           [0004]    In a general electron gun for a color cathode ray tube, spherical aberration and focusing characteristics are greatly dependent on a main lens. Thus, in order to obtain good focusing characteristics, the diameter of the main lens should be increased.  
           [0005]    However, in an in-line electron gun, since three electron beam apertures are formed at at least two electrodes constituting an electronic lens in an in-line configuration, it is impossible to make the diameter of an electron beam aperture larger than a distance between the centers of two adjacent electron beam apertures, which will be called an “eccentric distance” hereinafter.  
           [0006]    An electron gun for improving spherical aberration in a conventional main lens is disclosed in U.S. Pat. No. 4,370,592, and is shown in FIG. 1.  
           [0007]    As shown in FIG. 1, burring portions  1   b  and  2   b  are formed at edges of an electron outlet surface  1   a  of a focusing electrode  1  and an electron inlet surface  1   a  of a final accelerating electrode  2 , and large electron beam apertures  1 H and  2 H having a predetermined depth are formed in the center. Also, small electron beam apertures  1 H′ and  2 H′ through which R, G and B electron beams pass independently, are formed in the large electron beam apertures  1 H and  2 H.  
           [0008]    When electron beams pass through a main lens formed by the focusing electrode  1  and the final accelerating electrode  2 , since vertical and horizontal focusing field components acting on the electron beams passing through the central small electron beam aperture and the side small electron beam apertures are different due to horizontal elongation of the large electron beam apertures  1 H and  2 H, it is not possible to form symmetric electron beam spots landing on a fluorescent surface. In other words, as shown in FIG. 2, side electron beams RB and BB having passed through the large electron beam apertures  1 H and  2 H of the focusing electrode  1  or the final accelerating electrode  2 , are close to the burring portions  1   b  and  2   b  across which a low voltage or a high voltage is horizontally applied, and the central electron beam GB is relatively far from the burring portions  1   b  and  2   b.  Thus, the side electron beams RB and BB are relatively strongly focused and the central electron beam GB is weakly focused.  
           [0009]    Also, since the distances between the side electron beams RB and BB and the burring portions  1   b  and  2   b  are different according to the direction, the horizontal focusing power and vertical focusing power for the electron beams are different. Further, the vertical distances between the central electron beam GB and the burring portions  1   b  and  2   b  are shorter than the horizontal distances, so the electron beam is subjected to relatively strong vertical focusing power. Also, the central electron beam GB is subjected to divergent power in a diagonal direction of the large electron beam apertures  1 H and  2 H. Thus, the cross sections of the side electron beams RB and BB having passed through the main lens are substantially triangular and the cross section of the central electron beam GB is radially projected, thereby preventing the attainment of uniform cross sections of electron beams throughout the entire fluorescent plane.  
           [0010]    In particular, the sizes of the small electron beam apertures  1 H′ and  2 H′ are limited by the diameter of a neck portion of a CRT. This sets a limit on increasing the eccentric distance between the small electron beam apertures  1 H′ and  2 H′. Further, since the recent trend is toward reduction of the diameter of the neck portion in order to reduce deflection power, the distance between the small electron beam apertures  1 H′ and  2 H′ is reduced, thereby increasing spherical aberration and degrading focusing characteristics.  
           [0011]    Technologies for solving the above-described problems are disclosed in U.S. Pat. Nos. 5,481,560, 4,599,534 and 4,412,149, in which independent small electron beam apertures are vertically elongated in inner electrodes installed inside an outer electrode for an electron beam, or side electron beam apertures are formed using the inner electrode and the inner circumferences of the outer electrode.  
           [0012]    In the above-described electrode system for forming a main lens, electron beams passing through side electron beam apertures are made vertically elongated to reduce a focus voltage difference due to the large diameter of the outer electrode. However, the electron beam landing on a phosphor layer is severely distorted due to the high current density of upper and lower parts of the vertically elongated electron beam.  
           [0013]    Another arrangement of electrodes of an electron gun for solving the above-described problems is disclosed in U.S. Pat. No. 5,414,323. As shown in FIG. 3, this arrangement of electrodes of an electron gun includes an electrode plate member  12  in the center of an outer electrode  11  having a large electron beam apertures. A vertically elongated small electron beam aperture  13  is formed in the center of the electrode plate member  16 . Side edge portions of the electrode plate member  12  are recessed in a half-elliptical shape to form side electron beam apertures  14  and  15 .  
           [0014]    Since the central small electron beam aperture is vertically elongated, the astigmatism generated by the large electron beam aperture can be offset. However, according to this electrode arrangement, 8-pole coma aberration of the central electron beam aperture and 8-pole coma aberration of side electron beam apertures cannot easily be corrected.  
           [0015]    Another arrangement of large electrodes is disclosed in U.S. Pat. No. 4,626,738. As shown in FIG. 4, this arrangement of electrodes includes an outer electrode  21  having a large electron beam aperture, and an inner electrode  22  installed inside the outer electrode  21  and having polygonal small electron beam apertures  22 R,  22 G and  22 B. Here, although the aberration generated by the large electron beam aperture can be corrected by the polygonal small electron beam apertures  22 R,  22 G and  22 B, it is not easy to fabricate the small electron beam apertures  22 R,  22 G and  22 B.  
         SUMMARY OF THE INVENTION  
         [0016]    To solve the above problems, it is an object of the present invention to provide an electrode system of an electron gun for a color cathode ray tube, which can easily correct aberration of an electronic lens formed by a large electron beam aperture, and can improve focusing characteristics.  
           [0017]    It is another object of the present invention to provide an electron gun for a color cathode ray tube, which can reduce astigmatism by compensating for distortion of an electron beam due to a difference in the voltage applied across the space between each of three apertures disposed in an in-line arrangement.  
           [0018]    To accomplish the first object of the present invention, there is provided an electron gun for a color cathode ray tube including means for generating three electron beams arranged in-line, means for forming an auxiliary lens for focusing and accelerating the electron beams generated by the electron beam generating means, and means for forming a main lens, for finally focusing and accelerating the electron beams focused and accelerated by the auxiliary lens forming means, and having first and second outer electrodes which face each other and in each of which a large electron beam aperture through which the three electron beams pass is formed, and first and second inner electrodes each installed inside the first and second outer electrodes and having three electron beam apertures shaped of vertically elongated squares.  
           [0019]    In the present invention, recessed portions having different depths are formed at facing vertical peripheries of side electron beam apertures among the three electron beams of the first inner electrode, the recessed portions formed at the outer vertical peripheries being deeper than those formed at the vertical peripheries positioned near the central electron beam aperture. Also, recessed portions having the same depth are formed at facing vertical peripheries of side electron beam apertures among three electron beam apertures of the second inner electrode. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    The above object and advantages of the present invention will become more apparent by describing in detail a preferred embodiment thereof with reference to the attached drawings in which:  
         [0021]    [0021]FIG. 1 is a cross-sectional view illustrating a conventional electrode system of an electron gun for a color cathode ray tube;  
         [0022]    [0022]FIG. 2 is a front view of a conventional electrode system of an electron gun, illustrating the cross sections of electron beams;  
         [0023]    [0023]FIGS. 3 and 4 are front views of other examples of a conventional electrode system of an electron gun;  
         [0024]    [0024]FIG. 5 is a cross-sectional view of an electron gun for a color cathode ray tube according to the present invention, in which application of voltages is shown;  
         [0025]    [0025]FIG. 6 is an exploded perspective view of electrodes which constitute a main lens unit according to the present invention; and  
         [0026]    [0026]FIG. 9 is a graph showing the relationship between a focus voltage and an electron beam size in an electron gun according to the present invention and a conventional electron gun. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0027]    [0027]FIG. 5 shows an electron gun for a color cathode ray tube (CRT) employing an electrode system according to an embodiment of the present invention.  
         [0028]    As shown, the electron gun for a color CRT includes a triode having three cathodes  11 ,  12  and  13  formed in an in-line arrangement, for emitting electron beams for exciting a phosphor layer, a control electrode  14  having electron beam apertures at positions facing the respective cathodes  11 ,  12  and  13  and a screen electrode  15 , first, second and third focusing electrodes  21 ,  22  and  23  sequentially installed from the screen electrode  15  and having electron beam apertures to form auxiliary lens forming unit  20  for focusing and accelerating electron beams, and first and second main electrodes  31  and  35  sequentially installed from the third focusing electrode  23  of the auxiliary lens forming unit  20  to form a main lens forming unit  30 . Here, electron beam apertures  14 H formed in the control electrode  14  are circular or vertically elongated, electron beam apertures  15 H formed in the screen electrode  15  are formed stepwise such that they have a smaller diameter at the cathode side and a large diameter at the first focusing electrode side. Also the electron beam apertures  21 H,  22 H and  23 H formed in the inlet surface of the first, second and third focusing electrodes  21 ,  22  and  23  may be formed to be circular or vertically elongated.  
         [0029]    The first and second main electrodes  31  and  35  forming the main lens forming unit  30  include first and second outer electrodes  32  and  36  having large electron beam apertures  32 H and  37 H through which three electron beams pass, on facing surfaces, and first and second inner electrodes  33  and  37  installed inside the first and second outer electrodes  32  and  36  and having separate small electron beam apertures  33 R/ 33 G/ 33 B and  37 R/ 37 G/ 37 B, respectively.  
         [0030]    The three separate small electron beam apertures  33 R,  33 G and  33 B formed in the first inner electrode  33 , as shown in FIG. 6, are shaped of vertically elongated squares, and recessed portions  34   a / 34   a ′,  34   b / 34   b ′ and  34   c / 34   c ′ are formed in the central portions of facing vertical peripheries. The recessed portions  34   a  and  34   c′  positioned at the vertical peripheries of the outer sides of the side electron beam apertures  33 R and  33 B, that is, far from the central electron beam aperture  33 G, are recessed deeper than the recessed portions  34   a′  and  34   c  positioned at the vertical peripheries facing the recessed portions  34   a  and  34   c′ , that is, at the central electron beam aperture sides.  
         [0031]    The three separate small electron beam apertures  37 R,  37 G and  37 B formed in the second inner electrode  37  positioned inside the second outer electrode  35  are shaped of vertically elongated squares. Recessed portions  38   a / 38   a′ ,  38   b / 38   b′  and  38   c / 38   c′ are formed in the central portions of the facing vertical peripheries. Here, the horizontal widths Hi of the central small electron beam apertures  33 G and  37 G of the first and second inner electrodes  33  and  37  are narrower than the horizontal widths of the side small electron beam apertures  33 R/ 33 B and  37 R/ 37 B.  
         [0032]    As described above, predetermined voltages are applied to the respective electrodes constituting an electron gun. In one embodiment of the present invention, a voltage VS1 of 0 to 200 V may be applied to the control electrode  14  and a voltage VS2 of 200 to 700 V may be applied to the screen electrode  15  and the second focusing electrode  22 .  
         [0033]    Also, a voltage VF is applied to the first and third focusing electrodes  21  and  23  and the first main electrode  31 , the voltage VF being 28 to 30% of the voltage applied to the second main electrode  35  to which the same voltage as that applied to the internal conductive layer of the CRT is applied.  
         [0034]    Here, the voltage applied to the first and third focusing electrodes  21  and  23  and the first main electrode  31  may be a dynamic focus voltage synchronized with a deflection signal. A voltage VE of 25 to 30 KV is applied to the second main electrode  35 .  
         [0035]    The operation of the above-described electron gun for a color CRT according to the present invention will now be described.  
         [0036]    A predetermined voltage is applied to the cathodes  11 ,  12  and  13  and various electrodes constituting the electron gun in the above-described manner. If the voltage is applied, a pre-focus lens is formed between the control electrode  14  and the screen electrode  15 , and a unipotential auxiliary lens and a bipotential auxiliary lens are formed between the first, second and third focusing electrodes  21 ,  22  and  23  and the main lens  31 . A main lens is formed between the first and second main electrodes  31  and  35 .  
         [0037]    The main lens formed between the first and second main electrodes  31  and  35  is formed by equipotential lines normal to the electric field formed between the first and second focusing electrodes  31  and  35 .  
         [0038]    Here, as described above, since the large electron beam aperture  32 H ( 36 H) is horizontally elongated, there is a difference in the focus voltage acting on the central electron beam aperture and the side electron beam apertures. Thus, vertically convergent components and horizontally convergent components of electron beams having passed through the central small electron beam aperture  33 G ( 37 G) and the side small electron beam apertures  33 R and  33 B ( 37 R and  37 B), are different, respectively, so that the electron beams are subjected to different convergent and divergent forces. In other words, since the horizontal and diagonal distances across the large electron beam aperture  32 H ( 36 H), in which a low voltage and a high voltage are distributed, are relatively long, large divergent forces are applied to the electron beams horizontally and diagonally. This action causes a difference in the focusing voltage among three electron beams, thereby lowering focusing characteristics of three electron beams.  
         [0039]    In the aforementioned electron gun, the side separate small electron beam apertures  33 R/ 33 B and  37 R/ 37 B of the first and second focusing electrodes  31  and  35  forming a main lens and the central electron beam apertures  33 G and  37 G are vertically elongated and have recessed portions having different depths formed at facing vertical peripheries, thereby compensating for distortion of the cross section of an electron beam and enlarging the electron beam aperture.  
         [0040]    In more detail, in the central electron beam apertures  33 G and  37 G among the separate small electron beam apertures  33 R/ 33 G/ 33 B and  37 R/ 37 G/ 37 B of the first and second main electrodes  31  and  35  forming the main lens, and the central electron beam passing through an electronic lens formed by the large electron beam aperture  32 H ( 36 H), the horizontally diverging power is larger than the horizontally converging power and vertically diverging power is larger than the vertically converging power.  
         [0041]    Also, the side electron beam apertures  33 R/ 33 B and  37 R/ 37 B and the central electron beam passing through an electronic lens formed by the large electron beam aperture  32 H ( 36 H) are subjected to larger horizontally convergent power than the horizontally divergent power and larger vertically divergent power than the vertically convergent power. This divergent power is smaller than the divergent power acting on the central electron beam passing through the central electron beam apertures  33 G and  37 G and the electronic lens formed by the large lens.  
         [0042]    Therefore, it is possible to prevent the distortion of the cross section of the electron beam due to a difference in the focus voltage between the central and peripheral portions of the electronic lens formed by the large electron beam aperture.  
         [0043]    [0043]FIG. 7 shows the relationship between the focus voltage and the electron beam size, in an electron gun according to the present invention, and a conventional electron gun having separate small electron beam apertures of electrodes forming a main lens which are simply vertically elongated.  
         [0044]    Referring to FIG. 7, in the electron gun according to the present invention, the horizontal and vertical sizes of electron beams landing on a phosphor layer are not considerably varied according to the change in the focus voltage (see plots PH and PV). However, in the conventional electron gun, when the focus voltages are 6,000 V and 6,400 V, the change in the horizontal and vertical beam sizes is large (see plots CH and CV).  
         [0045]    In the electron gun according to the present invention, among the recessed portions  34   a / 34   a ′,  34   c / 34   c ′,  38   a / 38   a′  and  38   c / 38   c′  formed at the peripheries of the side separate small electron beam apertures  33 R/ 33 B and  37 R/ 37 B, the outer recessed portions  34   a / 34   c  and  38   a / 38   c  are recessed deeper than the central aperture side recessed portions  34   a′ / 34   c′ and  38   a′ / 38   c ′. Thus, the electronic lens formed by the side separate small electron beam apertures  33 R/ 33 B and  37 R/ 37 B are asymmetrically formed, thereby making the side electron beams among three electron beams disposed in an in-line arrangement converge toward the central electron beam.  
         [0046]    In the electrodes of an electron gun for a color CRT according to the present invention, aberration of an electron beam caused by large electron beam apertures can be reduced, and the cross section of an electron beam can be changed into a desirable shape. In particular, focusing characteristics of electron beams can be improved by reducing a difference in the focusing forces on electron beams passing through a large electron beam aperture.  
         [0047]    While the present invention has been described in conjunction with the preferred embodiments disclosed, it will be apparent to those skilled in the art that various modifications and variations can be made within the spirit or scope of the invention defined in the appended claims.