Abstract:
A cathode ray tube (CRT) has an electron gun including a cathode for emitting electron beams, a control electrode for controlling emission of the electron beams from the cathode, and a screen electrode for accelerating the flow of the electron beams passing the control electrode are arranged in series. In the CRT, during a scanning period, a voltage applied to at least one of the control electrode and the screen electrode changes in response to a voltage of a data signal applied to the cathode. The control electrode and screen electrode each include three mutually electrically insulated sections for independently controlling each of three electron beams passing through the electrodes.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a cathode ray tube (CRT), and, more particularly, to a CRT having an electron gun in which a cathode for emitting electron beams, a control electrode for controlling emission of the electron beams from the cathode, and a screen electrode for accelerating the flow of the electron beams passing the control electrode are arranged in series.  
           [0003]    2. Description of the Related Art  
           [0004]    Referring to FIG. 1, a conventional CRT includes a panel  12 , a funnel  13 , an electron gun  11 , and a deflection yoke  15 . A fluorescent film  14  in which fluorescent substances for producing red (R), green (G), and blue (B) light are aligned in a dot or strip pattern is installed on the inner surface of the panel  12 . The funnel  13  having a neck portion  13   a  and a cone portion  13   b  is sealed to the panel  12 . The electron gun  11  is installed in the neck portion  13   a  of the funnel  13 . The deflection yoke  15  is installed on and surrounding the cone portion  13   b  of the funnel  13  for deflecting the electron beams emitted from the electron gun  11 .  
           [0005]    The performance of the CRT  1  is determined according to a state of the electron beams emitted from the electron gun  11  and landing on the fluorescent film  14 . To make the electron beams emitted from the electron gun  11  accurately land on the fluorescent film  14 , a number of technologies improving focus characteristics and reducing aberration of electron lenses have been developed.  
           [0006]    In particular, the shapes of the electron beams landing on the fluorescent film  14  are horizontally elongated when the electron beams emitted from the electron gun  11  are deflected by the deflection yoke  15 , due to a difference between barrel and pincushion magnetic fields. To prevent the elongation, a dynamic focus electron gun is used. The dynamic focus electron gun synchronizes the electron beams emitted from the electron gun  11  with horizontal and vertical deflection periods so that the shapes of the electron beams are vertically elongated.  
           [0007]    However, in the dynamic focus electron gun, as the size of the screen of the CRT increases, horizontal line deformation at the peripheral portion of the screen becomes severe. To solve that problem, a double focus CRT is used.  
           [0008]    [0008]FIG. 2 shows a conventional double dynamic focus CRT. Referring to the drawing, a video signal processing portion  21  processes a composite video signal Sc and outputs a horizontal synchronizing signal, a vertical synchronizing signal, a data signal, and a horizontal/vertical blanking signal. The data signal including red (R), green (G), and blue (B) brightness signals, is amplified by a data signal amplifier  27 . The amplified data signal Sd is biased by a voltage supplied by a first bias supplier  31  and applied to a cathode K of the electron gun  11 .  
           [0009]    A vertical deflecting signal generator  22  generates a vertical deflecting signal corresponding to the vertical synchronizing signal output from the video signal processor  21  and supplies the vertical deflecting signal to a vertical deflecting signal amplifier  24 . A horizontal deflecting signal generator  23  generates a horizontal deflecting signal corresponding to the horizontal synchronizing signal output from the video signal processor  21  and supplies the generated horizontal deflecting signal to a horizontal deflecting signal amplifier  25 . The vertical and horizontal deflecting signals amplified by the vertical and horizontal deflecting signal amplifiers  24  and  25  are respectively applied to vertical and horizontal deflecting yokes  15  on the CRT  1 .  
           [0010]    The horizontal/vertical blanking signal output from the video signal processor  21  is amplified by a blanking signal amplifier  26 . A horizontal/vertical blanking signal Sb output from the blanking signal amplifier  26  is applied to the cathode K of the electron gun  11 . A control signal Vc from a fifth bias supplier  37  is supplied to a control electrode C of the electron gun  11 . A heater power supplier  36  supplies electric power to a heater (not shown) of the cathode K of the electron gun  11 . A second bias supplier  32  applies a screen voltage Vec to a screen electrode S and a second focus electrode F 2  of the electron gun  11 . A third bias supplier  33  applies a static focus voltage Vfs having a positive polarity to first, third, and fifth focus electrodes F 1 , F 3 , and F 5  of the electron gun  11 . The static focus voltage Vfs has a positive polarity and a magnitude higher than the screen voltage Vec, which also has a positive polarity, to enhance acceleration and focus of the electron beams. A dynamic focus driver  35  applies a dynamic focus voltage Vfd, which changes periodically within a range above and below the static focus voltage Vfs, to fourth and sixth focus electrodes F 4  and F 6  so that the electron beams emitted from the electron gun  11  are made relatively oval. A fourth bias driver  34  applies an anode voltage Veb having the highest positive polarity to a final acceleration electrode A of the electron gun  11 .  
           [0011]    [0011]FIG. 3 shows the structure of the electron gun in the CRT of FIG. 2. In FIG. 3, the same reference numerals denote the same elements shown FIG. 2. In FIG. 3, reference characters K R , K G , and K B  denote respective cathodes for producing electron beams that generate red, green, and blue light when the electron beams land on the fluorescent screen. Reference character Sd R /Sb R  denotes data and blanking signals for red light, reference character Sd G /Sb G  denotes data and blanking signals for green light, and reference character Sd B /Sb B  denotes data and blanking signals for blue light respectively applied to cathodes K R , K G , and K B .  
           [0012]    [0012]FIG. 4 shows the relationship between driving voltages in a conventional double dynamic focus method. In FIG. 4, reference character T HS  denotes horizontal scanning period, reference character V pl  denotes the minimum voltage of the dynamic focus voltage Vfd, and reference character V ph  denotes the maximum voltage of the dynamic focus voltage Vfd.  
           [0013]    [0013]FIG. 5A shows electron lenses formed in the electron gun of FIG. 3 during the period t 1 -t 3 , when the static focus voltage Vfs is higher than the dynamic focus voltage Vfd. FIG. 5B shows electron lenses formed in the electron gun of FIG. 3 during the periods  0 -t 1  and t 3 -t 4 , when the static focus voltage Vfs is lower than the dynamic focus voltage Vfd. In FIGS. 5A and 5B, reference character A V  denotes the vertical direction in the electron gun, reference character A H  denotes the horizontal direction in the electron gun, reference character P B  denotes direction of movement of the electron beams, reference character F V  denotes the vector force in the vertical direction A V  applied to the electron beams, and F H  denotes the vector force in the horizontal direction A H  applied to the electron beams.  
           [0014]    Referring to FIGS. 3, 4,  5 A, and  5 B, electron beams are generated according to the data signals S dR , S dG , and S dB  corresponding to the respective cathodes K R , K G , and K B . The electron beams are emitted in response to the control voltage Vc applied to the control electrode C. The electron beams emitted through openings of the control electrode C are accelerated by the screen voltage Vec applied to the screen electrode S.  
           [0015]    The static focus voltage Vfs applied to the first focus electrode F 1  is higher than the screen voltage Vec applied to the screen electrode S. The shapes of an outlet of the screen electrode S and an inlet of the first focus F 1  are circular, but the outlet of the screen electrode S is smaller than the inlet of the first focus F 1 . Thus, a focus lens is formed between the screen electrode S and the first focus electrode F 1 . The shapes of the inlets of the first focus electrode F 1  to which the static focus voltage Vfs is applied, the inlets and outlets of the second focus electrode F 2  to which the screen voltage Vec is applied, and the inlets of the third focus electrode F 3  to which the static focus voltage Vfs is applied are all circular. Therefore, a focus lens SL is formed as a pre-focus lens (S L  of FIG. 5A or  5 B) among the first, second, and third focus electrodes F 1 , F 2 , and F 3 . The electron beams emitted from the third focus electrode F 3  are focused by the focus lens S L .  
           [0016]    The shapes of the outlets of the third focus electrode F 3  are horizontally elongated while the shapes of the inlets of the fourth focus electrode F 4  are vertically elongated. The shapes of the outlets of the fifth focus electrode F 5  are vertically elongated while the shapes of the inlets of the sixth focus electrode F 6  are circular. The static focus voltage Vfs is applied to the third and fifth focus electrodes F 3  and F 5  while the dynamic focus voltage Vfd is applied to the fourth and sixth focus electrodes F 4  and F 6 . The anode voltage Veb is applied to the final acceleration electrode A.  
           [0017]    The double dynamic focus CRT is driven as follows.  
           [0018]    In the periods  0 -t 1  and t 3 -t 4  in which the static focus voltage Vfs is lower than the dynamic focus voltage Vfd, a first dynamic quadrupole lens acting as a focusing lens (Q L1V  of FIG. 5B) in the vertical direction and as a diverging lens (Q L1H  of FIG. 5B) in the horizontal direction is formed between the third and fourth focus electrodes F 3  and F 4 . A second dynamic quadrupole lens acting as a diverging lens (Q L2V  of FIG. 5B) in the vertical direction and a focusing lens (Q L2H  of FIG. 5B) in the horizontal direction is formed between the fifth and sixth focus electrodes F 5  and F 6 . After passing through the second dynamic quadrupole lens, the electron beams pass through a main lens ML between the sixth focus electrode F 6  and the final acceleration electrode A. Then, electron beams having oval shapes corresponding to the vertical and horizontal deflecting voltages are output from the main lens M L .  
           [0019]    In the period t 1 -t 3  in which the static focus voltage Vfs is higher than the dynamic focus voltage Vfd, a first dynamic quadrupole lens acting as a diverging lens (Q L1V  of FIG. 5A) in the vertical direction and as a focusing lens (Q L1H  of FIG. 5A) in the horizontal direction is formed between the third and fourth focus electrodes F 3  and F 4 . Also, a second dynamic quadrupole lens acting as a focusing lens (Q L2V  of FIG. 5A) in the vertical direction and a diverging lens (Q L2H  of FIG. 5A) in the horizontal direction is formed between the fifth and sixth focus electrodes F 5  and F 6 . After passing through the second dynamic quadrupole lens, the electron beams pass through a main lens M L  between the sixth focus electrode F 6  and the final acceleration electrode A. Therefore, electron beams have oval shapes corresponding to the vertical and horizontal deflecting voltages are output from the main lens M L .  
           [0020]    In the electron gun for a CRT operating as described, if the CRT has a large screen, the deflecting frequency needs to be increased. Also, to increase the maximum brightness of the CRT, the range of the voltage change of the data signal applied to the electron gun should be increased. However, as the range of a voltage change of the data signal applied to the electron gun increases, the quality of the image deteriorates due to distortion of the data signal.  
           [0021]    Accordingly, a method of efficiently driving an electron gun producing increased current density electron beams without increasing the range of a voltage change of the data signal applied to the electron gun is needed.  
           [0022]    Referring to Japanese Unexamined Patent Application Publication No. 11-224,618, an additional modulation electrode is provided between a second grid electrode (a screen electrode) and a third grid electrode (a focus electrode). Since a voltage having a negative polarity is applied to the modulation electrode, electron beams having a low current density are cut off and electron beams having a high density current can pass through the modulation electrode. That is, the cathode current can be increased.  
           [0023]    However, in the conventional CRT, a leakage current flows through the second grid electrode (the screen electrode) to which a voltage having a positive polarity is applied and between the first grid (the control electrode) and the modulation electrode, so that the life span of the electron gun is reduced.  
         SUMMARY OF THE INVENTION  
         [0024]    To solve the above-described problems, it is an object of the present invention to provide a CRT which can efficiently increase cathode current density without increasing the range over which the voltage of a data signal applied to the electron gun changes.  
           [0025]    To achieve the above object, there is provided a CRT having an electron gun including, arranged in series, a cathode for an emitting electron beam, a control electrode for controlling emission of the electron beam from the cathode, and a screen electrode for accelerating the electron beam passing through the control electrode, wherein, during a scanning period, a voltage applied to at least one of the control electrode and the screen electrode changes in response to voltage of a data signal applied to the cathode.  
           [0026]    In this CRT, the cathode includes a cathode for emitting an electron beam for producing red light, a cathode for emitting an electron beam for producing green light, and a cathode for emitting an electron beam for producing blue light, and the control electrode is divided into a control electrode for red light, a control electrode for green light, and a control electrode for blue light, the control electrodes for red light, for green light, and for blue light being mutually electrically insulated from each other. Further, a voltage is applied to the control electrode for red light during the scanning period changes in response to voltage of a data signal applied to the cathode for producing red light, a voltage is applied to the control electrode for green light during the scanning period changes in response to voltage of a data signal applied to the cathode for producing green light, and a voltage is applied to the control electrode for blue light during the scanning period changes in response to voltage of a data signal applied to the cathode for producing blue light.  
           [0027]    Yet another CRT according to the invention includes a cathode for emitting electron beams, a control electrode for controlling emission of the electron beams from the cathode, and a screen electrode for accelerating the electron beams passing through the control electrode arranged in series, wherein, the cathode includes a cathode for emitting an electron beam for producing red light, a cathode for emitting an electron beam for producing green light, and a cathode for emitting an electron beam for producing blue light, and the control electrode is divided into a control electrode for red light, a control electrode for green light, and a control electrode for blue light, the control electrodes for red light, for green light, and for blue light being mutually electrically insulated from each other. In this CRT, the control electrode for red light includes a first beam passing aperture for passing both of the electron beams from the cathodes for producing green light and blue light and a second beam passing aperture for passing the electron beam from the cathode for producing red light and the first beam passing aperture is larger than the second beam passing aperture, the control electrode for green light includes a first beam passing aperture for passing both of the electron beams from the cathodes for producing red light and blue light and a second beam passing aperture for passing the electron beam from the cathode for producing green light and the first beam passing aperture is larger than the second beam passing aperture, and the control electrode for blue light includes a first beam passing aperture for passing both of the electron beams from the cathodes for producing red light and green light and a second beam passing aperture for passing the electron beam from the cathode for producing blue light and the first beam passing aperture is larger than the second beam passing aperture.  
           [0028]    A still further CRT according to the invention includes a cathode for emitting electron beams, a screen electrode for screening emission of the electron beams from the cathode, and a screen electrode for accelerating the electron beams passing through the screen electrode arranged in series, wherein, the cathode includes a cathode for emitting an electron beam for producing red light, a cathode for emitting an electron beam for producing green light, and a cathode for emitting an electron beam for producing blue light, and the screen electrode is divided into a screen electrode for red light, a screen electrode for green light, and a screen electrode for blue light, the screen electrodes for red light, for green light, and for blue light being mutually electrically insulated from each other. In this CRT, the screen electrode for red light includes a first beam passing aperture for passing both of the electron beams from the cathodes for producing green light and blue light and a second beam passing aperture for passing the electron beam from the cathode for producing red light and the first beam passing aperture is larger than the second beam passing aperture, the screen electrode for green light includes a first beam passing aperture for passing both of the electron beams from the cathodes for producing red light and blue light and a second beam passing aperture for passing the electron beam from the cathode for producing green light and the first beam passing aperture is larger than the second beam passing aperture, and the screen electrode for blue light includes a first beam passing aperture for passing both of the electron beams from the cathodes for producing red light and green light and a second beam passing aperture for passing the electron beam from the cathode for producing blue light and the first beam passing aperture is larger than the second beam passing aperture.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]    The above object and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which:  
         [0030]    [0030]FIG. 1 is a sectional view showing the structure of a conventional light CRT;  
         [0031]    [0031]FIG. 2 is a block diagram illustrating the driving of a conventional dynamic focus CRT;  
         [0032]    [0032]FIG. 3 is a perspective view showing the internal structure of the electron gun of the conventional CRT driven as illustrated in FIG. 2;  
         [0033]    [0033]FIG. 4 is a graph showing the driving voltage of the conventional dynamic focus CRT as a function of time for one horizontal scan;  
         [0034]    [0034]FIG. 5A is a view showing the electron lenses formed when the static focus voltage is higher than the dynamic focus voltage in the electron gun of FIG. 3;  
         [0035]    [0035]FIG. 5B is a view showing the electron lenses formed when the static focus voltage is lower than the dynamic focus voltage in the electron gun of FIG. 3;  
         [0036]    [0036]FIG. 6 is a block diagram illustrating the driving of a double dynamic focus CRT according to the present invention;  
         [0037]    [0037]FIG. 7 is a perspective view showing the internal structure of the electron gun of the CRT driven as illustrated in FIG. 6;  
         [0038]    [0038]FIG. 8A is a perspective view showing the structure of cathodes and control electrodes of the electron gun of FIG. 7;  
         [0039]    [0039]FIG. 8B is a sectional view showing the assembled cathodes and control electrodes of FIG. 8A;  
         [0040]    [0040]FIG. 9 is a timing diagram showing the data signal for red light applied to the cathode producing an electron beam producing red light and the control signal applied to the control electrode for controlling red light, for the CRT and electron gun shown in FIGS. 7 through 8B;  
         [0041]    [0041]FIG. 10 is a timing diagram showing the data signal for red light applied to the cathode for producing an electron beam producing red light and the driving signal applied to the screen electrode for red light, for the CRT and electron gun shown in FIGS. 7 through 8B; and  
         [0042]    [0042]FIG. 11 is a graph showing measured cathode current with respect to the voltage of the data signal. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0043]    [0043]FIG. 6 shows the structure of a double dynamic focus CRT according to the present invention. Referring to FIG. 6, the video signal processor  21  processes a composite video signal Sc input from outside and outputs a horizontal synchronizing signal, a vertical synchronizing signal, a data signal, and a horizontal/vertical blanking signal.  
         [0044]    The data signal including red (R), green (G), and blue (B) brightness signals is amplified by the data signal amplifier  27 . The amplified data signal Sd is biased by a voltage supplied by the first bias supplier  31  and applied to the cathode K of the electron gun  11 .  
         [0045]    The vertical deflecting signal generator  22  generates a vertical deflecting signal corresponding to the vertical synchronizing signal output from the video signal processor  21  and supplies the vertical deflecting signal generated to the vertical deflecting signal amplifier  24 . The horizontal deflecting signal generator  23  generates a horizontal deflecting signal corresponding to the horizontal synchronizing signal output from the video signal processor  21  and supplies the horizontal deflecting signal generated to the horizontal deflecting signal amplifier  25 . The vertical and horizontal deflecting signals amplified by the vertical and horizontal deflecting signal amplifiers  24  and  25  are respectively applied to the vertical and horizontal deflecting yokes  15  of the CRT  1 .  
         [0046]    The horizontal/vertical blanking signal output from the video signal processor  21  is amplified by a blanking signal amplifier  26 . The horizontal/vertical blanking signal Sb output from the blanking signal amplifier  26  is applied to the cathode K of the electron gun  11 .  
         [0047]    A control electrode driver  28  operated in response to the data signal output from the video signal processor  21  generates a control signal Sc. The control signal Sc is applied to the control electrode C. The voltage applied to the control electrode C during the scanning period changes in response to a voltage of the data signal Sd applied to the cathode K. Accordingly, the voltage applied to the control electrode C increases only when electron beams are emitted from the cathode K in response to the data signal Sd, so that electron beams having high current density can be emitted.  
         [0048]    A screen electrode driver  32   a  operated by the data signal output from the video signal processor  21  generates a driving signal of the screen electrode S. The voltage applied to the screen electrode S changes in response to the voltage of the data signal Sd applied to the cathode K. Accordingly, the voltage applied to the screen electrode S increases only when the electron beams are emitted from the cathode K in response to the data signal Sd, so that electron beams having current high density can be emitted.  
         [0049]    The heater power supplier  36  supplies electric power to a heater (not shown) of the cathode K of the electron gun  11 . The second bias supplier  32  applies a constant voltage having a positive polarity to the second focus electrode F 2  of the electron gun  11 . The third bias supplier  33  applies a static focus voltage Vfs having a positive polarity to first, third, and fifth focus electrodes F 1 , F 3 , and F 5  of the electron gun  11 . The static focus voltage Vfs having a positive polarity has a magnitude higher than the screen voltage Vec, which also has a positive polarity, to enhance acceleration and focus of the electron beams. The dynamic focus driver  35  applies a dynamic focus voltage Vfd, which changes periodically within a range above and below the static focus voltage Vfs, to fourth and sixth focus electrodes F 4  and F 6  so that the electron beams emitted from the electron gun  11  are relatively oval. The fourth bias driver  34  applies an anode voltage Veb having the highest magnitude of the applied voltages and a positive polarity to the final acceleration electrode A of the electron gun  11 .  
         [0050]    [0050]FIG. 7 shows the internal structure of the electron gun for a CRT of FIG. 6. In FIG. 7, the same reference numerals as those in FIG. 6 indicate the same elements having the same functions. In FIG. 7, reference characters K R , K G , and K B  denote cathodes for producing respective electron beams that produce red green, and blue light when the respective electron beams land on the fluorescent screen of the CRT. Reference character Sd R /Sb R  denotes a data signal for producing red light and a horizontal/vertical blanking signal, reference character Sd G /Sb G  denotes a data signal for producing green light and a horizontal/vertical blanking signal, and reference character Sd B /Sb B  denotes a data signal for producing blue light and a horizontal/vertical blanking signal respectively applied to the cathodes K R , K G , and K B .  
         [0051]    The control electrode C is divided by insulating portions AI 1  and AI 2  into a control electrode C R  for red light, a control electrode C G  for green light, and a control electrode C B  for blue light. Accordingly, a control signal Sc R  for red light, a control signal Sc G  for green light, and a control signal Sc B  for blue light are respectively applied to a control electrode C R , for red light, a control electrode C G , for green light, and a control electrode C B , for blue light.  
         [0052]    Likewise, the screen electrode S is divided by insulating portions AI 3  and AI 4  into a screen electrode S R  for red light, a screen electrode S G  for green light, and a screen electrode S B  for blue light. Accordingly, a screen signal Ss R  for red light, a screen signal Ss G  for green light, and a screen signal Ss B  for blue light are respectively applied to a screen electrode S R  for red light, a screen electrode S G  for green light, and a screen electrode S B  for blue light.  
         [0053]    [0053]FIG. 8A shows the detailed structure of the cathodes K R , K G , and K B  and the control electrodes C R , C G , and C B  of the electron gun of FIG. 7. FIG. 8B shows the assembled cathodes K R , K G , and K B  and the control electrodes C R , C G , and C B  of FIG. 8A. In FIGS. 8A and 8B, the same reference characters as those in FIG. 7 indicate the same elements having the same functions.  
         [0054]    Referring to FIGS. 8A and 8B, in the control electrode C B  for blue light, a large beam passing area is provided for passing both of the electron beams for producing green and red light. However, only a relatively small beam passing hole is provided for the electron beam for producing blue light. Thus, the electron beam for producing blue light is affected by the control signal Sc B  for blue light applied to the control electrode C B  for blue light while the electron beams for producing green and red light are not influenced by the control signal Sc B . Also, in the control electrode C G  for green light, a large beam passing area is provided for passing both of the electron beams for producing blue and red light. However, only a relatively small beam passing hole is provided for the electron beam for producing green light. Thus, the electron beam for green light is affected by the control signal Sc G  for green light applied to the control electrode C G  for green light while the electron beams for producing blue and red light are not influenced by the control signal Sc G . Likewise, in the control electrode C R  for red light, a large beam passing area is provided for passing both of the electron beams for producing green and blue light. However, only a relatively small beam passing hole is provided for the electron beam for producing red light. Thus, the electron beam for producing red light is affected by the control signal Sc R  for red light applied to the control electrode C R  for red light while the electron beams for producing green and blue light are not influenced by the control signal Sc R . The positions of the respective cathodes K R , K G , and K B  are adjusted such that the distance between the cathode K R  for producing an electron beam for producing red light and the control electrode C R  for red light, the distance between the cathode K G  for producing an electron beam for producing green light and the control electrode C G  for green light, and the distance between the cathode K B  for producing an electron beam for blue light and the control electrode C B  for blue light are constant. Accordingly, uniform operating conditions are obtained. The same structure of the control electrodes of FIGS. 8A and 8B can be used for the screen electrodes S R , S G , and S B  of FIG. 7.  
         [0055]    Referring to FIGS. 4, 5A,  5 B, and  7  through  8 B, the electron beams are generated according to the data signals Sd R , Sd G , and Sd B  corresponding to the respective cathodes K R , K G , and K B . The voltage of the control signal Sc R  applied to the control electrode C R  for red light changes in response to the voltage of the data signal Sd R  for red light. The voltage of the control signal Sc G  applied to the control electrode C G  for green light changes in response to the voltage of the data signal Sd G  for green light. Likewise, the voltage of the control signal Sc B  applied to the control electrode C B  for blue light changes in response to the voltage of the data signal Sd B  for blue light. Accordingly, since the voltage applied to the control electrodes C R , C G , and C B  increase only when the electron beams are emitted from the respective cathodes K R , K G , and K B  in response to the respective data signals Sd R , Sd G , and Sd B , electron beams having high current density can be emitted.  
         [0056]    The electron beams emitted through apertures of the respective electrodes C R , C G , and C B  during the period of scanning are accelerated by the screen signals Ss R , Ss G , and Ss B  applied to the respective screen electrodes S R , S G , and S B . The voltage of the screen signal Ss R  applied to the screen electrode S R  for red light changes in response to the voltage of the data signal Sd R  for red light. The voltage of the screen signal Ss G  applied to the screen electrode S G  for green light changes in response to the voltage of the data signal SdG for green light. Likewise, the voltage of the screen signal Ss B  applied to the screen electrode S B  for blue light changes in response to the voltage of the data signal Sd B  for blue light. Accordingly, since the voltage applied to the screen electrodes S R , S G , and S B  increases only when the electron beams are emitted from the respective cathodes K R , K G , and K B  in response to the respective data signals Sd R , Sd G , and Sd B , electron beams having high density current can be emitted.  
         [0057]    The static focus voltage Vfs applied to the first focus electrode F 1  is higher than the maximum voltage of the screen signals Ss R , Ss G , and Ss B  applied to the respective screen electrodes S R , S G , and S B . The shapes of the outlets of the respective screen electrodes S R , S G , and S B  and the inlets of the first focus electrode F 1  are all circular. However, the outlets of the respective screen electrodes S R , S G , and S B  are smaller than the inlets of the first focus electrode F 1 . Thus, a focus lens is formed between each of the screen electrodes S R , S G , and S B  and the first focus electrode F 1 . The shapes of the inlets of the first focus electrode F 1  to which the static focus voltage Vfs is applied, the inlets and outlets of the second focus electrode F 2  to which the screen voltage Vec is applied, and the inlets of the third focus electrode F 3  to which the static focus voltage Vfs is applied are all circular. Therefore, a focus lens SL is formed as a pre-focus lens (SL of FIG. 5A or  5 B) among the first, second, and third focus electrodes F 1 , F 2 , and F 3 . The electron beams emitted from the third focus electrode F 3  are focused by the focus lens S L .  
         [0058]    The shapes of the outlets of the third focus electrode F 3  are horizontally elongated while the shapes of the inlets of the fourth focus electrode F 4  are vertically elongated. The shapes of the outlets of the fifth focus electrode F 5  are vertically elongated while the shapes of the inlets of the sixth focus electrode F 6  are circular. The static focus voltage Vfs is applied to the third and fifth focus electrodes F 3  and F 5  while the dynamic focus voltage Vfd is applied to the fourth and sixth focus electrodes F 4  and F 6 . The anode voltage Veb is applied to the final acceleration electrode A.  
         [0059]    The driving of the double dynamic focus CRT is now described.  
         [0060]    In the periods  0 -t 1  and t 3 -t 4  in which the static focus voltage Vfs is lower than the dynamic focus voltage Vfd, a first dynamic quadrupole lens acting as a focusing lens (Q L1V  of FIG. 5B) in a vertical direction and diverging lens (Q L1H  of FIG. 5B) in a horizontal direction is formed between the third and fourth focus electrodes F 3  and F 4 . A second dynamic quadrupole lens acting as a diverging lens (Q L2V  of FIG. 5B) in a vertical direction and a focusing lens (Q L2H  of FIG. 5B) in a horizontal direction is formed between the fifth and sixth focus electrodes F 5  and F 6 . After passing through the second dynamic quadrupole lens, the electron beams pass through the main lens ML between the sixth focus electrode F 6  and the final acceleration electrode A. Thus, electron beams having oval shapes corresponding to the vertical and horizontal deflecting voltages are output from the main lens M L .  
         [0061]    In the period t 1 -t 3  in which the static focus voltage Vfs is higher than the dynamic focus voltage Vfd, a first dynamic quadrupole lens acting as a diverging lens (Q L1V  of FIG. 5A) in a vertical direction and a focusing lens (Q L1H  of FIG. 5A) in a horizontal direction is formed between the third and fourth focus electrodes F 3  and F 4 . Also, a second dynamic quadrupole lens acting as a focusing lens (Q L2V  of FIG. 5A) in a vertical direction and a diverging lens (Q L2H  of FIG. 5A) in a horizontal direction is formed between the fifth and sixth focus electrodes F 5  and F 6 . After passing through the second dynamic quadrupole lens, the electron beams pass through the main lens M L  between the sixth focus electrode F 6  and the final acceleration electrode A. Thus, electron beams have oval shapes, in cross-section, corresponding to the vertical and horizontal deflecting voltages are output from the main lens M L .  
         [0062]    [0062]FIG. 9 shows the data signal S dR  for red light applied to the cathode K R  for producing red light and the control signal Sc R  applied to the control electrode C R  for red light, which are shown in FIGS. 7 through 8B. Referring to FIG. 9, in the conventional CRT, a constant voltage +VC 1  is applied to the control electrode C R  during a scanning period T HS  and a blanking period T HB  of a horizontal driving period T HD . However, in the CRT according to the present invention, during the scanning period T HS  of the horizontal driving period T HD , the voltage of the control signal Sc R  increases to +VC 3  when the voltage of the data signal Sd R  is lowered to +VK 1  for the emission of the electron beams. When the voltage of the data signal Sd R  increases to +VK 2 , to reduce the emission of the electron beams, the voltage of the control signal Sc R  decreases to +VC 1 . Thus, the density of the cathode current can be efficiently increased without increasing the range of the change in the voltage of the data signal Sd R  applied to the cathode K R  for producing red light. During the blanking period T HB  of the horizontal driving period T HD , the constant voltage +VC 1  is applied to the control electrode C R  as in the conventional CRT.  
         [0063]    [0063]FIG. 10 shows the data signal Sd R  for red light applied to the cathode K R  for producing red light and the driving signal Ss R  applied to the screen electrode S R  for red light which are shown in FIGS. 7 through 8B. In FIG. 10, the same reference numerals as those of FIG. 9 indicate the same elements having the same functions. Referring to FIG. 10, in the conventional CRT, a constant voltage +VS 1  is applied to the screen electrode S R  during the scanning period T HS  and the blanking period T HB  of the horizontal driving period T HD . However, in the CRT according to the present invention, during the scanning period T HS  of the horizontal driving period T HD , the voltage of the screen signal Ss R  increases to +VS 3  when the voltage of the data signal Sd R  is lowered to +VK 1  for the emission of the electron beams. When the voltage of the data signal S dR  increases to +VK 2 , to reduce the emission of the electron beams, the voltage of the screen signal Ss R  decreases to +VS 1 . Thus, the density of the cathode current can be efficiently increased without increasing the range of the change in the voltage of the data signal Sd R  applied to the cathode electrode K R . During the blanking period T HB  of the horizontal driving period T HD , the constant voltage +VS 1  is applied to the screen electrode S R .  
         [0064]    [0064]FIG. 11 shows the measured characteristic cathode current I R  with respect to the voltage V AD  of a data signal. In FIG. 11, reference character C OLD  denotes a characteristic curve of a conventional CRT and reference character C NEW  denotes a characteristic curve of a CRT according to a preferred embodiment of the present invention. Referring to FIG. 11, it can be seen that the cathode current I K  increases without increasing the range of the change in the voltage V AD  of a data signal applied to the electron gun in the CRT according to the present invention.  
         [0065]    The described operation of the CRT according to the present invention may be performed only when the electron beams are scanned onto the periphery portion of the screen. That is, the horizontal scanning period (T HS  of FIGS. 4, 9, and  10 ) may be divided into early, middle, and late scanning periods and the present driving method can be performed only during the early and late scanning periods ( 0 -t 1  and t 3 -t 4  of FIG. 4). Accordingly, display performance at the peripheral portion of the screen can be much improved.  
         [0066]    As described above, in the CRT according to the present invention, since the voltage applied to at least one of the control electrode and the screen electrode increases only when the electron beams are emitted from the corresponding cathode in response to the respective data signals, electron beams having high current density can be emitted. Thus, the density of the cathode current can be efficiently increased without increasing the range of the change, i.e., amplitude, of the voltage of the data signal applied to the cathode.  
         [0067]    While this invention has been particularly shown and described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.