Patent Publication Number: US-6903501-B2

Title: Electron gun assembly for cathode ray tube

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority to and the benefit of Korea Patent Application No. 2002-0036668 filed on Jun. 28, 2002 in the Korean Intellectual Property Office, the entire disclosure of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   (a) Field of the Invention 
   The present invention relates to an electron gun assembly for a cathode ray tube (CRT) and, more particularly, to an electron gun well adapted for a monochrome CRT to be mounted within a projection display device to realize a monochrome image. 
   (b) Description of the Related Art 
   Generally, a CRT-based projection display device mainly has three monochrome CRTs for realizing red (R), green (G) and blue (B) monochrome images, and an optical system for amplifying the monochrome images made at the three CRTs and projecting the amplified images to a projection screen to produce color images. 
   As the monochrome CRT scans the display screen with one stream of electron beams, and the respective monochrome CRT screen images are projected to the projection screen while being amplified by about ten times, the brightness of the display screen related to the monochrome CRT is lower than that of the display screen related to the usual CRT. Therefore, compared to the usual CRT, relatively high electric currents need to be applied to the electron gun for the monochrome CRT to heighten the brightness of the display screen. 
   Usually, the electron gun for the monochrome CRT emits electron beams with the application of electric currents ranging from 0.5 mA to 3 mA, which are two or three times more than those applied to the electron gun for a usual color CRT ranging from 0.2 mA to 1 mA. A high unipotential focus (Hi-UPF) type exhibiting an excellent focus characteristic in the range of higher currents is commonly used for the monochrome CRT electron gun. 
   With the Hi-UPF type electron gun, the second electrode receives the screen voltage, and the fourth electrode receives the focus voltage. A third electrode is placed between the second electrode and the fourth electrode to receive a high anode voltage (roughly, 32 kV). A strong pre-focus lens is formed between the second and the third electrodes due to the high potential difference between the second and the third electrodes, and reduces the spot size of electron beams in the range of higher electric currents. 
   Further, the monochrome CRT electron gun serves to make formation of monochrome images practically under the application of electric currents of 2 mA or less. With the available electric current range of 0.5-3 mA, the Hi-UPF type electron gun exhibits an excellent focus characteristic in the higher current range of more than 2 mA. By contrast, in the relatively lower current range of 2 mA or less, it turns out that the spot size of electron beams is increased. 
   The increase in the beam spot size occurs because when the electron 
   The increase in the beam spot size occurs because when the electron beam current is lowered, the crossover point of the electron beams formed at the triode portion moves from the second electrode to the third electrode, and the emission power of the electron beams incident upon the pre-focus lens is weakened. Consequently, with the electric current range of 2 mA or less, the spot size of the electron beams is increased while deteriorating the resolution, resulting in unclear display images. 
   In order to reduce the beam spot size with the current range of 2 mA or less, it has been proposed that the size of the beam-guide hole formed at the first electrode be reduced. However, this reduction makes the area of electron emission for the cathode so small that the life span of the electron gun with the cathode is reduced. 
   U.S. Pat. No. 4,271,374 discloses a CRT electron gun with a structure where the equivalent diameter of the main-focus lens (formed between the fourth electrode receiving the focus voltage and the fifth electrode receiving the anode voltage) is enlarged to increase the capacity thereof. 
   However, with the above structure, as the CRT neck portion mounting the electron gun thereon is limited in its diameter, there is a limit in mechanically enlarging the opening diameter of the fourth and the fifth electrodes forming the main-focus lens. Such a limit is made because the electron gun formation electrodes need to be spaced apart from the inner surface of the neck portion by a predetermined distance to grant the withstand voltage characteristic to the CRT. Accordingly, as the opening diameter of the fourth and the fifth electrodes is established in a predetermined manner, it is difficult to achieve the desired electrode capacity in an effective manner. 
   SUMMARY OF THE INVENTION 
   It is an aspect of the present invention to provide a monochrome CRT electron gun for a projection display device which optimizes the spot size of electron beams emitted under the application of electric currents ranged from 0.5 mA to 2 mA. 
   It is another aspect of the present invention to provide a monochrome CRT electron gun for a projection display device which minimizes the degree of deterioration in the focus characteristic of electron beams scanning the periphery of the display screen. 
   According to one aspect of the present invention, the electron gun includes a cathode for emitting thermal electrons, a first electrode adjacent the cathode, and a second electrode adjacent the first electrode to receive a screen voltage and control the emission of thermal electrons from the cathode. A third electrode is adjacent the second electrode, and a fourth electrode is adjacent the third electrode to receive a focus voltage. A fifth electrode partially surrounds the fourth electrode while being adjacent the fourth electrode to receive an anode voltage together with the third electrode. The second electrode has a bottom portion with a stepped portion surrounding a hole for guiding the electron beams while being protruded toward the first electrode, and a sidewall portion extended from the periphery of the bottom portion toward the third electrode. The first and the second electrodes are structured to satisfy the following condition: 0.54≦T/G≦1.50 where T(mm) indicates the thickness of the bottom portion of the second electrode, and G(mm) indicates the distance between the first and the second electrodes. 
   According to another aspect of the present invention, the electron gun includes a cathode for emitting thermal electrons, a first electrode adjacent the cathode, and a second electrode adjacent the first electrode to receive a screen voltage and control the emission of thermal electrons from the cathode. A third electrode is adjacent the second electrode, and a fourth electrode is adjacent the third electrode to receive a focus voltage. A fifth electrode partially surrounds the fourth electrode while being adjacent the fourth electrode to receive an anode voltage together with the third electrode. The second electrode has a bottom portion with a stepped portion surrounding a hole for guiding the electron beams while being protruded toward the first electrode, and a sidewall portion extended from the periphery of the bottom portion toward the third electrode. The first and the second electrodes are structured to satisfy the following condition: 0.15≦T(mm)≦0.3, 0.20≦G(mm)≦0.28 wherein T(mm) indicates the thickness of the bottom portion of the second electrode, and G(mm) indicates the distance between the first and the second electrodes. 
   The bottom portion and the stepped portion of the second electrode are preferably shaped with a circle while satisfying the following conditions: 0.08≦D 1 /D 2 ≦0.30, 1.0≦D 1 (mm)≦3.0 wherein D 1 (mm) indicates the diameter of the stepped portion of the second electrode, and D 2 (mm) indicates the diameter of the bottom portion of the second electrode. 
   The second electrode is structured to satisfy the following conditions: 0.02≦H 1 /H 2 ≦0.17, 0.05≦H 1 (mm)≦0.30 wherein H 1 (mm) indicates the height of the stepped portion of the second electrode, and H 2 (mm) indicates the height of the sidewall portion of the second electrode. 
   The stepped portion of the second electrode may be of non-circular shape. 
   More specifically, the stepped portion is rectangular-shaped with a long side proceeding in the vertical direction of the screen, and a short side proceeding in the horizontal direction. Alternatively, the stepped portion may be rectangular-shaped with a long side proceeding in the horizontal direction of the screen, and a short side proceeding in the vertical direction. Furthermore, the stepped portion may be oval-shaped with a long side proceeding in the vertical direction of the screen and a short side proceeding in the horizontal direction, or with a long side proceeding in the horizontal direction of the screen and a short side proceeding in the vertical direction. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a front view of an electron gun for a CRT according to an embodiment of the present invention. 
       FIG. 2  is a cross sectional view of the electron gun taken along the I—I line of FIG.  1 . 
       FIG. 3  is a partially elevated perspective view of a second electrode for the electron gun shown in FIG.  1 . 
       FIG. 4  is a partially amplified view of the electron gun shown in FIG.  2 . 
       FIG. 5  schematically illustrates the equi-potential lines and the electron beam locus formed at the triode portion with the driving of an electron gun according to a prior art. 
       FIG. 6  schematically illustrates the equi-potential lines and the electron beam locus formed at the triode portion with the driving of an electron gun according to an embodiment of the present invention. 
       FIG. 7  is a graph illustrating the spot size of 5% of the electron beams as a function of the variation in the applied electric currents with an electron gun according to an embodiment of the present invention, and an electron gun according to a prior art. 
       FIG. 8  is a graph illustrating the relation of the bottom thickness of a second electrode to the cut-off voltage. 
       FIG. 9  is a partially amplified view of the electron gun shown in FIG.  2 . 
       FIG. 10  is an amplified view of a second electrode for the electron gun shown in FIG.  2 . 
       FIGS. 11  to  14  schematically illustrate variations of a stepped portion of a second electrode for an electron gun according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a front view of an electron gun for a CRT according to an embodiment of the present invention.  FIG. 2  is a cross sectional view of the electron gun taken along the I—I line of FIG.  1 . As shown in  FIGS. 1 and 2 , electron gun  2  includes cathode  4  for emitting thermal electrons, and first electrode  6  and second electrode  8  for forming a triode portion together with cathode  4  to control the emission of thermal electrons from cathode  4 . Electron gun  2  further includes third electrode  10  adjacent second electrode  8 , fourth electrode  12  adjacent third electrode  10  to receive the focus voltage, fifth electrode  14  partially surrounding fourth electrode  12  while being adjacent fourth electrode  12  to receive the anode voltage, and first connector  16  electrically connecting third electrode  10  and fifth electrode  14  to each other. 
   The afore-mentioned electrodes are fixed to bead glass  18 , and arranged in the Z direction while proceeding from cathode  4 . Stem base  20  mounting electron gun  2  thereon is fixed to the end of neck portion  22  such that electron gun  2  is placed within neck portion  22  while being spaced apart from the inner wall of neck portion  22  by a predetermined distance. 
   In operation, cathode  4  receives voltages of 50-190V. First electrode  6  is grounded such that it can make a predetermined voltage difference with respect to cathode  4 . Second electrode  8  receives the screen voltage (approximately, several hundred volts) operated as a cut-off voltage, and controls the amount of electrons emitted from cathode  4 . 
   Third electrode  10  commonly shares the anode voltage (approximately, 30-32 kV) together with fifth electrode  14  by way of first connector  16 . Fourth electrode  12  receives the focus voltage, and particularly, the dynamic focus voltage of 7-10 kV. Fifth electrode  14  is electrically connected to graphite film  26  coated on the inner surface of neck portion  22  by way of bulb spacer  24  to receive the anode voltage from graphite film  26 . 
   Accordingly, as seen in  FIG. 2 , pre-focus lens PL is formed between second electrode  8  and third electrode  10  due to the potential difference thereof. First main-focus lens ML 1  is formed between third electrode  10  and fourth electrode  12  due to the potential difference thereof. Second main-focus lens ML 2  is formed within fifth electrode  14  due to the potential difference between fourth electrode  12  fifth electrode  14 . 
   When velocity modulator  28  is installed on the outer periphery of neck portion  22  to control the deflection velocity of the electron beams, fourth electrode  12  is partitioned into a plurality of sub-electrodes, including first sub-electrode  12 A, second sub-electrode  12 B and third sub-electrode  12 C. In this case, the so-called VM gap is made between the respective sub-electrodes to enhance the sensitivity of velocity modulator  28 . 
   When fourth electrode  12  is partitioned into a plurality of sub-electrodes, a pair of second connectors  30  electrically connect first sub-electrode  12 A and second sub-electrode  12 B as well as the second sub-electrode  12 B and third sub-electrode  12 C to each other such that the three sub-electrodes  12 A,  12 B and  12 C commonly share the focus voltage. 
   Preferably, outermost third sub-electrode  12 C of fourth electrode  12  distant from cathode  4  has outlet portion  12 D with largest inner and outer diameters amongst the first to the third sub-electrodes  12 A,  12 B and  12 C. Fifth electrode  14  surrounds outlet portion  12 D of third sub-electrode  12 C while being spaced apart from third sub-electrode  12 C such that the equivalent diameter of second main-focus lens ML 2  formed within fifth electrode  14  can be maximized. 
   With electron gun  2  according to the embodiment of the present invention, pre-focusing lens PL is controlled to realize an optimum electron beam spot size in the range of electric currents of 0.5-2 mA, usually applied to the monochrome CRT electron gun. For this purpose, the electron gun has a triode structure with an optimized electrode outline and an optimized inter-electrode distance. 
     FIG. 3  is a partially elevated perspective view of the second electrode.  FIG. 4  is a partially amplified view of the electron gun shown in FIG.  2 . As shown in  FIGS. 3 and 4 , second electrode  8  is shaped with a cup. That is, second electrode  8  has bottom portion  32  with beam-guide hole  8   a , and sidewall portion  34  extended from the periphery of bottom portion  32  toward third electrode  10 . Stepped portion  36  is formed at bottom portion  32  while surrounding beam-guide hole  8   a . Stepped portion  36  is protruded from bottom portion  32  toward first electrode  6  by a predetermined height. For instance, stepped portion  36  may be shaped with a circle having a predetermined diameter. 
   Particularly with second electrode  8 , the thickness of bottom portion  32  with stepped portion  36  is established to be smaller than the thickness of sidewall portion  34 . The reduction in the thickness of bottom portion  32  makes the spot size of electron beams change more sensitively to the variation in the electron beam currents. In other words, with the current range of 0.5-3 mA applied to the monochrome CRT electron gun, bottom portion  32  reduced in the thickness makes the spot size of electron beams in the lower current range of 0.5-2 mA be reduced. 
   Furthermore, as second electrode  8  has stepped portion  36  protruding toward first electrode  6 , the distance between second electrode  8  and first electrode  6  is reduced. The reduction in the distance between first electrode  6  and second electrode  8  makes the crossover point of the electron beams move toward cathode  4 , compared to the conventional electron gun. The movement in the crossover point of the electron beam, and the operation pursuant thereto will be explained with reference to  FIGS. 5 and 6 . 
   With electron gun  2  having the above-structured second electrode  8 , the triode portion is established to satisfy the mathematical formula 1.
 
0.54 ≦T/G≦ 1.50  (1)
 
wherein T indicates the thickness of bottom portion  32  of second electrode  8 , and G indicates the distance between first electrode  6  and second electrode  8 .
 
   Particularly in this embodiment, second electrode  8  is structured such that the thickness T of bottom portion  32  satisfies the mathematical formula 2.
 
0.15 ≦T (mm)≦0.30  (2)
 
   Sidewall portion  34  of second electrode  8  is formed with a thickness of about 0.4 mm. Even though the thickness of bottom portion  32  is smaller than that of sidewall portion  34 , stepped portion  36  reinforces bottom portion  32  to give a predetermined structural strength thereto. 
   Furthermore, the distance G between second electrode  8  and first electrode  6  is reduced by way of stepped portion  36  while satisfying the mathematical formula 3.
 
0.20 ≦G (mm)≦0.28  (3)
 
     FIGS. 5 and 6  schematically illustrate the equi-potential lines and the electron beam locus formed at a triode portion with the driving of an electron gun according to a prior art (Comparative Example 1), and an electron gun according to an embodiment of the present invention (Example 1). In the drawings, reference numeral  1  indicates the cathode, reference numeral  3  indicates the first electrode, reference numeral  5  indicates the bottom portion of the second electrode, and reference numeral  7  indicates the third electrode. For the purpose of explanatory convenience, only stepped portion  36  of the second electrode is specifically illustrated in FIG.  6 . 
   With the electron guns according to the embodiment of the present invention and according to the prior art, only the triode portion is differentiated from each other. The case illustrated in the drawings is made such that the first electrode is grounded, a voltage of 500V is applied to the second electrode, and a voltage of 32 kV is applied to the third electrode. The structural characteristics of the triode portions for the electron guns are listed in Table 1. 
   
     
       
         
             
             
             
             
             
           
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
                 
               Thickness 
                 
                 
             
             
                 
                 
               (mm) of 
                 
                 
             
             
                 
               Distance (mm) 
               bottom 
               Diameter 
                 
             
             
                 
               between first 
               portion of 
               (mm) of 
               Height (mm) 
             
             
                 
               and second 
               second 
               stepped 
               of stepped 
             
             
                 
               electrodes 
               electrode 
               portion 
               portion 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
          
             
               Example 
               0.25 
               0.20 
               2.00 
               0.20 
             
             
               Comparative 
               0.30 
               0.40 
               — 
               — 
             
             
               Example 
             
             
                 
             
          
         
       
     
   
   It can be seen from  FIG. 5  that with the electron gun according to the prior art, the crossover point (COP) of the electron beams focused at the triode portion by way of the pre-focus lens is positioned at the location distant from cathode  1  by 0.58 mm. The electron beams passing the crossover point (COP) proceed toward third electrode  7  while being diffused at an angle of lower degrees. 
   In contrast, it can be seen from  FIG. 6  that with the electron gun according to the embodiment of the present invention, the crossover point (COP) of the electron beams focused at the triode portion by way of a pre-focus lens is positioned at the location distant from cathode  4  by 0.42 mm. The electron beams passing the crossover point (COP) proceed toward the third electrode  10  while being diffused at an angle of higher degrees. 
   As described above, the crossover point of the electron beams with electron gun  2  according to the embodiment of the present invention comes closer to cathode  4 , compared to the electron gun according to the prior art. This strengthens the emission force of the electron beams toward first main-focus lens ML 1 , and makes the spot size of the electron beams landing on the phosphor screen be reduced. 
   Table 2 and  FIG. 7  illustrate the results of measuring the spot size of 5% of the electron beams as a function of the variation in the electron beam currents with an electron gun according to an embodiment of the present invention (Example), and an electron gun according to a prior art (Comparative Example). 
   
     
       
         
             
             
             
             
             
             
           
             
                 
               TABLE 2 
             
             
                 
                 
             
           
          
             
                 
               Electron beam current (mA) 
               0.5 
               1.0 
               2.0 
               3.0 
             
          
         
         
             
             
             
             
             
             
             
          
             
                 
               5% electron 
               Comparative 
               240.0 
               225.0 
               220.0 
               235.0 
             
             
                 
               beam spot size 
               Example 
             
             
                 
               (μm) 
               Example 
               205.0 
               207.5 
               220.0 
               238.0 
             
          
         
         
             
             
             
             
             
             
          
             
                 
               Electron beam spot size 
               14.6 
               7.8 
               0 
               −1.3 
             
             
                 
               reduction rate (%) 
             
             
                 
                 
             
          
         
       
     
   
   With the practical range of electric currents of 0.5-3 mA applied to the monochrome CRT electron gun, the electron gun according to the embodiment of the present invention involves reduction in the spot size of the electron beams in the lower current range of 0.5-2 mA, and the beam spot size reduction rate maximally reaches 14.6%. 
   Further, as an additional effect pursuant to the above structural modification, the electron gun according to the embodiment of the present invention involves a variation in the cut-off characteristic. The focus characteristic in the higher current range of more than 2 mA and in the condition of scanning the periphery of the display screen due to the deflection of the electron beams can be prevented from being deteriorated. 
   First, the variation in the cut-off characteristic will be now explained. 
     FIG. 8  is a graph illustrating the relation of the bottom thickness of the second electrode to the cut-off voltage. It can be seen from  FIG. 8  that as the thickness of bottom portion  32  of second electrode  8  is reduced, the cut-off voltage required for emitting the thermal electrons is decreased. As the thickness of bottom portion  32  of second electrode  8  is determined to be in the range of 0.15-0.30 mm, the cut-off voltage is lowered to be in the range of 300-400V, compared to the conventional one reaching up to 500V. 
   The decrease in the cut-off voltage is made because the high anode voltage applied to third electrode  10  influences the inside of second electrode  8 , and more strongly focuses the electron beams emitted from cathode  4 . Accordingly, electron gun  2  according to the embodiment of the present invention is operated with a low screen voltage, thereby serving to reduce the production cost of the CRT, and improve the display quality thereof. 
   Electron gun  2  with the above-described triode structure is given with a new cut-off formulation expressed by mathematical formula 4. 
               Cut   ⁢     -     ⁢   off   ⁢           ⁢   voltage   ⁢           ⁢     (   V   )       =     k   ⁢           ⁢         ϕ   ⁡     (   G1   )       ×     ϕ   ⁡     (   G3   )             g   ⁡     (     G1   ·   G2     )       ×     g   ⁡     (     K   ·   G1     )       ×   t   ⁢           ⁢   G1   ×     2   tG2         ×   Ec2   ×     Eb   .               (   4   )             
 
wherein k indicates a constant, φ(G 1 ) indicates the diameter of beam-guide hole  6   a  of first electrode  6 , φ(G 3 ) indicates the diameter of beam-guide hole  10   a  of third electrode  10 , g(G 1 ·G 2 ) indicates the distance between first electrode  6  and second electrode  8 , g(K·G 1 ) indicates the distance between cathode  4  and first electrode  6 , tG 1  indicates the thickness of first electrode  6 , tG 2  indicates the thickness of bottom portion  32  of second electrode  8 , Ec 2  indicates the screen voltage applied to second electrode  8 , and Eb indicates the anode voltage applied to third electrode  10  (referring to FIG.  9 ).
 
   The focus characteristic of the electron beams in the higher current range of more than 2 mA and in the condition of scanning the periphery of the display screen by way of the deflected electron beams will be now explained. 
   As shown in  FIG. 9 , stepped portion  36  of second electrode  8  is protruded from bottom portion  32  of second electrode  8  toward first electrode  6  by a predetermined height, and hence, takes a role of enlarging the distance between the center of bottom portion  32  of second electrode  8  surrounding beam-guide hole  8   a , and third electrode  10 . 
   With the above structure, the electron beams in the lower current range of 2 mA or less are largely diffused toward pre-focus lens PL while being reduced in their spot size. In the higher current range of more than 2 mA and in the condition of deflecting the electron beams, pre-focus lens PL is reinforced so that the electron beams move along the paraxial trace, and hence, the focus characteristic is prevented from being deteriorated. 
   Accordingly, with electron gun  2  according to the embodiment of the present invention, the reasonable focus characteristic over the center and the periphery of the display screen is maintained in the current range of 0.5-3 mA applied to the monochrome CRT electron gun, thereby improving the resolution of the projection screen. 
     FIG. 10  is a cross sectional view of second electrode  8 . In this embodiment, stepped portion  36  of second electrode  8  is structured to satisfy the mathematical formula 5 or 6. Consequently, stepped portion  36  of second electrode  8  bears a sufficient influential power with respect to pre-focus lens PL.
 0.08 ≦D   1 / D   2 ≦0.30  (5) 1.0 ≦D   1 (mm)≦3.0  (6) 
   In the mathematical formulas 5 and 6, D 1  indicates the diameter of stepped portion  36  of second electrode  8 , and D 2  indicates the diameter of bottom portion  32  of second electrode  8 . 
   Furthermore, stepped portion  36  is established to satisfy the mathematical formula 7 or 8 such that the withstand voltage characteristic between first electrode  6  and second electrode  8  can be maintained. Consequently, the formation of second electrode  8  is made in an easy manner, and pre-focus lens PL in the higher current range of more than 2 mA and in the condition of deflecting the electron beams is reinforced, thereby improving the focus characteristic of the electron beams.
 
0.02 ≦H   1   /H   2 ≦0.17  (7)
 
0.05 ≦H   1 (mm)≦0.30  (8)
 
   In the mathematical formulas 7 and 8, H 1  indicates the height of stepped portion  36  of second electrode  8  in the Z direction, and H 2  the height of sidewall portion  34  of second electrode  8 . 
   Further, with the usual monochrome CRT electron gun, circular electrode and lens structures are commonly used. In recent times, studies on the electron guns adapted for a wide CRT where the ratio of the horizontal length to the vertical length of the screen is 16:9 have been made. Particularly, studies on the electron gun structure where the beam spot sizes in the horizontal axis direction and in the vertical axis direction are non-symmetrically determined have been made to improve the focus characteristic at the periphery of the display screen. 
   In this connection, electron gun  2  according to the embodiment of the present invention makes beam-guide hole  8   a  of second electrode  8  be shaped with a circle while altering the shape of stepped portion  36  of second electrode  8  in various manners, such as a circle, an oval, and a rectangle. Consequently, the inter-electrode alignment of the beam-guide holes is made in an easy manner while being well adapted for the wide screening and the non-symmetrical beam spot outlining. 
     FIGS. 11  to  14  schematically illustrate variations of the stepped portion of the second electrode. 
   As shown in  FIGS. 11 and 12 , stepped portions  36 A and  36 B may be shaped with a rectangle. That is, as shown in  FIG. 11 , stepped portion  36 A of second electrode  8  is rectangular-shaped with a long side proceeding in the vertical direction (in the Y direction) of the screen, and a short side proceeding in the horizontal direction (in the X direction). Alternatively, as shown in  FIG. 12 , stepped portion  36 B of second electrode  8  may be rectangular-shaped with a long side proceeding in the horizontal direction (in the X direction), and a short side proceeding in the vertical direction (in the Y direction). 
   As shown in  FIGS. 13 and 14 , stepped portions  36 C and  36 D may be shaped with an oval. That is, as shown in  FIG. 13 , stepped portion  36 C of second electrode  8  is oval-shaped with a long side proceeding in the vertical direction (in the Y direction) of the screen, and a short side proceeding in the horizontal direction (in the X direction). Alternatively, as shown in  FIG. 14 , stepped portion  36 D of second electrode  8  may be oval-shaped with a long side proceeding in the horizontal direction (in the X direction), and a short side proceeding in the vertical direction (in the Y direction). 
   As described above, the shape of second electrode  8  and the distance between first electrode  6  and second electrode  8  are altered to effectively reduce the beam spot size in the current range of 0.5-2 mA, where the monochrome CRT is practically operated. Furthermore, stepped portion  36  formed at second electrode  8  prevents the focus characteristic of the electron beams in the higher current region of more than 2 mA and in the condition of deflecting the electron beams from being deteriorated. 
   While the present invention has been described in detail with reference to certain embodiments, those skilled in the art will appreciate that various modifications and substitutions can be made thereto without departing from the spirit and scope of the present invention as set forth in the appended claims.