Abstract:
A projection type cathode ray tube includes a phosphor screen, a cathode, G1, G2 and a main lens composed of first, second and third electrodes. The first and third electrodes are supplied with an anode voltage of the phosphor screen, and the second electrode is supplied with a focus voltage lower than the anode voltage. An inside diameter of an opening in a phosphor screen side end of the second electrode is from 14 mm to 18 mm, and the phosphor screen side end of the second electrode is disposed within the third electrode. An aperture diameter D mm in the G1 electrode and an axial length L mm of the second electrode satisfy the following inequalities: L mm≧60×D mm+27.6 mm, L mm≦−646×D mm+396.3 mm, D mm≧0.44 mm, and L mm≦75 mm.

Description:
CLAIM OF PRIORITY 
   The present application claims priority from Japanese application Serial No. 2003-350467, filed on Oct. 9, 2003, the content of which is hereby incorporated by reference into this application. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a cathode ray tube, and in particular, to a cathode ray tube for use in a projection type image display apparatus such as a projection type TV receiver and a video projector. 
   The projection type image display apparatus employs three projection type cathode ray tubes for emitting red, green and blue lights, respectively. Three images on the respective panel portions of the three projection type cathode ray tubes are enlarged by a projection lens and are combined on a screen. In a projection type image display apparatus, since images on a panel portion of 127 mm (5 inches) to 178 mm (7 inches) in diagonal dimension of a projection type cathode ray tube are enlarged and projected onto a screen of 1,016 mm (40 inches), for example, in diagonal dimension, the images formed on the panel portion of the projection type cathode ray tube are required to be highly bright and good in focus characteristics. That is to say, it is necessary to limit degradation in focus characteristics to acceptable amounts even when a beam current is increased to produce a highly bright image on the panel portion. 
   In the present, a horizontal deflection frequency in the projection type image display apparatus is changed from 15 kHz for the conventional NTSC signals to 30 kHz for the Hivision (the Japanese high-definition television format) signals, and the use of signals capable of higher-resolution display has become the most common in the projection type image display apparatus. Consequently, there is a demand for improvement on focus characteristics of projection type cathode ray tubes so that the projection type image display apparatus can produce higher-resolution images. Focus characteristics can be improved by increasing a diameter of a neck portion of a projection type cathode ray tube from 29 mm to 36 mm, and thereby increasing a diameter of a main lens of an electron gun of the projection type cathode ray tube, but, in view of use in place of conventional projection type cathode ray tubes, it is demanded that focus characteristics should be improved by using projection type cathode ray tubes having a 29 mm-diameter neck portion. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a projection type cathode ray tube having improved focus characteristics without a substantial increase in a diameter of its neck portion. 
   The following will explain the summary of a representative one of the inventions disclosed in this specification. 
   In accordance with an embodiment of the present invention there is provided a projection type cathode ray tube comprising: a glass envelope comprising a panel portion, a neck portion, and a funnel portion connecting said panel portion and said neck portion; a phosphor screen formed on an inner surface of said panel portion; and an electron gun housed within said neck portion and emitting an electron beam toward said phosphor screen, wherein said electron gun is provided with an electron beam generating section which comprises a cathode provided with an electron-emissive material, a G1 electrode for controlling said electron beam and a G2 electrode for accelerating said electron beam arranged in the order named, and a main lens which comprises a first cylinder electrode, a second cylinder electrode and a third cylinder electrode arranged in the order named from a cathode side of said main lens and focuses said electron beam from said electron beam generating section on said phosphor screen, wherein said first cylinder electrode and said third cylinder electrode are configured so as to be supplied with a voltage equal to an anode voltage applied to said phosphor screen, and said second cylinder electrode is supplied with a focus voltage lower than said anode voltage, wherein an inside diameter of an opening in an end of said second cylinder electrode on a phosphor screen side thereof is in a range of from 14 mm to 18 mm, and said end of said second cylinder electrode on said phosphor screen side thereof is disposed within said third cylinder electrode, and wherein an aperture diameter D mm in said G1 electrode and an axial length L mm of said second cylinder electrode satisfy the following inequalities: L mm≧60×D mm+27.6 mm, L mm≦−646×D mm+396.3 mm, D mm≧0.44 mm, and L mm≦75 mm. 
   With the above configuration, the present invention provides an advantage of improving focus characteristics of projection type cathode ray tubes without a substantial increase in a diameter of their neck portion. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings, in which like reference numerals designate similar components throughout the figures, and in which: 
       FIG. 1  is a graph showing results obtained by simulation on a relationship between a diameter of an aperture in a G1 electrode and a 5%-beam spot diameter measured at the 5% point of the beam current density profile; 
       FIG. 2  is a graph showing a relationship between a diameter of an aperture in a G1 electrode and lifetime in cases where an oxide cathode containing barium scandate is employed; 
       FIG. 3  is a graph showing results obtained by simulation on a relationship between an axial length of a G4 electrode and a beam spot diameter with a diameter of an aperture in a G1 electrode as a parameter, in the case of a cathode current of 1.0 mA; 
       FIG. 4  is a graph showing results obtained from  FIG. 3  on a relationship between an axial length of a G4 electrode of a cathode ray tube and a diameter of an aperture in a G1 electrode for a beam spot diameter to be 0.089 mm or smaller at a cathode current of 1.0 mA; 
       FIG. 5  is a graph showing results obtained by simulation on a relationship between an axial length of a G4 electrode and a beam spot diameter with a diameter of an aperture in a G1 electrode as a parameter, in the case of a cathode current of 4 mA; 
       FIG. 6  is a graph showing results obtained from  FIG. 5  on a relationship between an axial length of a G4 electrode of a cathode ray tube and a diameter of an aperture in a G1 electrode, for a beam spot diameter to be 0.256 mm or smaller at a cathode current of 4.0 mA; 
       FIG. 7  is a graph showing a relationship between a diameter of an aperture in a G1 electrode and an axial length of a G4 electrode and a beam spot diameter for providing a perceptible improvement in resolution at cathode currents of both 1 mA and 4 mA; 
       FIG. 8  is a graph showing a relationship between an axial length of a G4 electrode and a dynamic focus voltage; 
       FIG. 9  is a schematic cross-sectional view of a projection type cathode ray tube; 
       FIG. 10  is a schematic cross-sectional view of an electron gun used in a projection type cathode ray tube; and 
       FIG. 11  is a waveform diagram for explaining a dynamic focus voltage dVf applied on a G4 electrode by being superposed on a focus voltage Vf, in synchronism with deflection of an electron beam. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The following will explain representative embodiments in accordance with the present invention in detail by reference to the drawings by comparing those embodiments with a conventional electron gun. The same reference numerals or characters designate functionally similar parts or portions throughout the figures, and repetition of their explanations is omitted. 
     FIG. 9  is a schematic cross-sectional view of a projection type cathode ray tube (hereinafter referred to as a PRT) to which the present invention is directed. The PRT is used for a projection type TV receiver (hereinafter referred to as a PTV) and the like. In  FIG. 9 , a vacuum envelope is formed by coupling a panel portion  1  and one end of a neck portion  3  with a funnel portion  2  and sealing the other end of the neck portion  3  with a stem  5 . An electron gun  6  housed within the neck portion  3  comprises a cathode  62  provided with an electron-emissive material  62 A for emitting an electron beam  8 , a heater  63  for heating the electron-emissive material  62 A, a G1 electrode  64  for controlling the amount of the electron beam  8 , a G2 electrode  65  for accelerating the electron beam  8 , a G3 electrode  66  for forming a prefocus lens between it and the G2 electrode  65 , and a G4 electrode  67  serving as a focus electrode forming a main lens in cooperation with a G5 electrode  68  which serves as an anode electrode. The stem  5  has pins  51  embedded therein for supplying voltages to the respective electrodes of the electron gun  6 , and a base  4  is provided for protecting the stem  5  and the pins  51 . An anode button  21  is embedded in the funnel portion  2 , and an internal graphite film  22  is coated on an inner surface of the funnel portion  2 . An anode voltage supplied to the anode button  21  is applied to the G5 electrode  68  of the electron gun  6  via the internal graphite film  22 . A generally rectangular phosphor screen  11  for generating one of primary colors is formed on an inner surface of the generally rectangular panel portion  1 . A single electron beam  8  is emitted from the electron gun  6 , and then the electron beam  8  is deflected in horizontal and vertical directions by a deflection yoke  7  to scan the phosphor screen  11  and thereby to form an image on the phosphor screen  11 . 
   Usually, the outside diameter of the neck portion  3  is selected to be 29 mm, considering deflection sensitivity of the deflection yoke  7 , sensitivity of a convergence yoke (not shown) for correcting distortions of rasters projected onto a screen (not shown) and errors in registration among three color rasters projected onto the screen, and use of other standard components. The overall length of a PRT is selected to be in a range of from 240 mm to 290 mm since the PRT is incorporated into an ordinary PTV, and consequently, usually a distance Lg 4   p  from an open end of the G4 electrode on its panel portion  1  side to the center of the phosphor screen  11  is selected to be in a range of from 120 mm to 150 mm for the purpose of preventing interference of magnetic fields of the deflection yoke  7 . In an example of a conventional electron gun, the distance Lg 4   p  is selected to be 140 mm. 
     FIG. 10  is a schematic illustration of the electron gun  6  for the PRT. The cathode  62  emits an electron beam by being heated in a range of from 600° C. to 1200° C. The G1 electrode  64  is supplied with a voltage Ec 1  of approximately 0 V for controlling the amount of the electron beam to be emitted from the cathode  62 . The G2 electrode  65  is supplied with a voltage Ec 2  in a range of from 200 V to 1,000 V for accelerating the electron beam. The G3 electrode  66  is supplied with a voltage of 25,000 V to 35,000 V, equal to a voltage Eb applied to the G5 electrode  68  which serves as an anode, for the purpose of forming a strong prefocus lens between the G2 electrode  65  and the G3 electrode  66 . The G4 electrode  67  is supplied with a focus voltage Vf of 5,000 V to 12,000 V to form a main lens which focuses the electron beam on the phosphor screen  11  (see  FIG. 9 ) formed on the inner surface of the panel portion  1 , between the G4 electrode  67  and the G5 electrode  68 . As described above, the G5 electrode  68  serving as the anode electrode is supplied with the voltage Eb of 25,000 V to 35,000 V, equal to the voltage applied to the G3 electrode  66 . 
   As shown in  FIG. 11 , it is often that the G4 electrode  67  is supplied with a voltage having a waveform of the focus voltage Vf superposed with a dynamic focus voltage dVf varying in synchronism with scanning deflection of the electron beam, in order to obtain optimum focus of the electron beam over the entire area of the raster. 
   Conventionally, a diameter of an aperture in the G1 electrode  64  was in a range of from 0.54 mm to 0.60 mm, a diameter of an aperture in the G2 electrode  65  was approximately equal to that of the G1 electrode  64 , and in a range of from 0.54 mm to 0.60 mm. A thickness of the G1 electrode  64  was in a range of from 0.05 mm to 0.15 mm, and that of the G2 electrode  65  was in a range of from 0.2 mm to 0.7 mm. A diameter of an aperture in the G3 electrode  66  on its G2 electrode  65  side was in a range of from 1.0 mm to 3.0 mm, and a length of the G3 electrode  66  in a direction of the axis of the PRT was selected to be in a range of from 15 mm to 25 mm, considering breakdown voltage. 
   In this electron gun, the open end of the G4 electrode  67  on its panel portion  1  side is disposed within the G5 electrode  68  so that the diameter of the main lens is increased, and so that the focus condition does not change even if a potential of the neck portion  3  varies which is produced by charging up of the inner wall of the neck portion  3 . The inside diameter of the G5 electrode  68  was selected to be in a range of from 20 mm to 22.5 mm since the wall thickness of the G5 electrode  68  was in a range of from 0.2 mm to 0.5 mm, considering physical tolerance between the inner wall of the neck portion  3  and the G5 electrode  68  in the manufacture of the PRT. The inside diameter of the open end of the G4 electrode  67  on its panel portion  1  side was selected in a range of from 14 mm to 18 mm since the wall thickness of the G4 electrode  67  was in a range of from 0.2 mm to 0.5 mm, to provide a spacing between the G4 electrode  67  and the G5 electrode  68  which ensures a satisfactory withstand voltage therebetween. 
   An example of the currently-used system employing the PRT of 29 mm in neck portion diameter is described as a comparison example in Table 1 of “A 16-cm Dual Neck Diameter, Integrated Component, Projection CRT,” the Journal of the Institute of Image Information and Television Engineers, Vol. 57, No. 8, pp. 983–988 (2003). The present invention will be explained in comparison with this example. For the purpose of improving resolution of the PTV, improvements need to be made on focus characteristics of the current electron gun employed in the above-mentioned currently-used system. In the current electron gun, a diameter of an aperture in the G1 electrode  64  is 0.54 mm, a thickness of the G1 electrode  64  is 0.07 mm, a diameter of an aperture in the G2 electrode  65  is 0.55 mm, and a thickness of the G2 electrode  65  is 0.36 mm. In the G3 electrode  66 , a diameter of an aperture in the G3 electrode  66  on its G2 electrode  65  side is 2.0 mm, a length of the G3 electrode  66  in a direction of the PRT tube axis is 20 mm. In the G4 electrode  67 , a diameter Dg 4  of an aperture in the G4 electrode  67  on its panel portion  1  side is 16 mm, a length of the G4 electrode  67  in the direction of the PRT tube axis is 48.7 mm. An inside diameter of the G5 electrode  68  is 22 mm. A distance Lg 4   p  (see  FIG. 9 ) between the open end of the G4 electrode  67  on its panel portion  1  side and the center of the phosphor screen  11  is 140 mm. 
   The average cathode current of 1 mA flows in the PRT incorporated in the PTV. Therefore, for the purpose of improving focus characteristics, it is necessary to reduce the beam spot diameter at a cathode current Ik=1 mA. Further, it is difficult to perceive improvement in resolution on the PTV screen if the beam spot diameter is not reduced by 10% or more. Therefore, to achieve the improvement on resolution on the PTV screen, it is necessary to reduce the beam spot diameter Ds 1  at the cathode current Ik=1 mA by 10% or more compared with that obtained by the current electron gun. 
   The beam spot diameter is reduced by using a method of reducing the diameter of an aperture in the G1 electrode as described in Japanese Patent Application Laid-Open No. 2000-250491 Publication. Therefore, the beam spot diameters Ds 1  obtained by the diameter of the aperture in the G1 electrode of the current electron gun and the further reduced diameters of the aperture in the G1 electrode are calculated by simulation. In this simulation, the electrode structures other than the aperture diameter in the G1 electrode are the same as those in the above-mentioned specifications of the current electron gun. 
   First, calculated was a 5%-beam spot diameter Ds 1  (a beam spot diameter measured at the 5% point of the beam current density profile, and hereinafter referred to merely as a beam spot diameter also) for a cathode current Ik=1 mA, and the results are shown in  FIG. 1 . When the aperture diameter D in the G1 electrode  64  is 0.54 mm as in the case of the current PRT, the 5%-beam spot diameter Ds 1  turned out to be 0.099 mm. This shows that the beam spot diameter for realizing improvement in focus characteristics is equal to or smaller than 0.089 mm which reduces the current beam spot diameter Ds 1  by 10% or more.  FIG. 1  shows that the aperture diameter D in the G1 electrode  64  needs to be selected to be equal to or smaller than 0.39 mm, to make the beam spot diameter Ds 1  equal to or smaller than 0.089 mm. 
   However, a problem arises in that the reduction in the aperture diameter D in the G1 electrode  64  increases load on a cathode and degrades lifetime characteristics. It is thought that lifetime of the PRT needs to be 20,000 hours or more, as described in “Barium-Scandate Dispersed Oxide Cathode for CRT with High Beam Current Density,” IDW &#39;02 CRT5-2, pp. 631–634. 
     FIG. 2  shows a relationship between lifetime and the aperture diameter D in the G1 electrode  64  in a case where an oxide cathode is employed which contains barium scandate and is the most common at present.  FIG. 2  shows that the aperture diameter D in the G1 electrode  64  needs to be equal to or larger than 0.44 mm for obtaining lifetime of 20,000 hours or more. On the other hand, the aperture diameter D in the G1 electrode  64  needs to be 0.39 mm or smaller for improving focus characteristics, and this dimension of the aperture diameter D in the G1 electrode  64  causes a great problem with lifetime characteristics. Consequently, the improvement in focus characteristics can not be achieved by reducing of the aperture diameter D in the G1 electrode  64  only. Therefore, improvement in focus characteristics were studied by varying the axial length L of the G4 electrode  67 .  FIG. 3  shows results of simulation on a relationship between the axial length L of the G4 electrode  67  and the beam spot diameter Ds 1  with the aperture diameter D in the G1 electrode  64  as a parameter. In this simulation, the electrode structures other than the aperture diameter D in the G1 electrode and the axial length L of the G4 electrode  67  are the same as those in the above-mentioned specifications of the current electron gun. 
     FIG. 3  shows that the beam spot diameter Ds 1  is made smaller by lengthening the axial length L of the G4 electrode  67 . When the axial length L of the G4 electrode  67  is increased, the object point is made further from the main lens, and therefore the incidence angle of electrons entering the main lens is made smaller. Since aberration characteristics of a lens deteriorate in proportion to the third power of the angle of incidence, by increasing the axial length L of the G4 electrode  67  and thereby decreasing the angle of incidence of electron beams entering the main lens the aberration characteristics of the lens are improved and thereby the beam spot diameter on the phosphor screen is reduced. 
     FIG. 4  shows a relationship obtained from  FIG. 3 , between the aperture diameter D in the G1 electrode and the axial length L of the G4 electrode, required to make the beam spot diameter Ds 1  equal to or smaller than 0.089 mm at a cathode current Ik=1.0 mA, the value of the beam spot diameter Ds 1  being necessary for improvement in resolution on the screen of the PTV.  FIG. 4  shows that, if the aperture diameter D in the G1 electrode  64  and the axial length L of the G4 electrode  67  satisfy the following inequality:
   L  mm≧60× D  mm+27.6 mm  (1), 
the beam spot diameter Ds 1  at a cathode current Ik=1.0 mA can be made equal to or smaller than 0.089 mm.
 
   However, it was found that the following problem arises. When the PTV produces a peak-brightness image, a cathode current Ik=4 mA flows in the PRT. If focus characteristics is degraded at the cathode current Ik=4 mA, resolution of images viewed on the PTV is deteriorated even when focus characteristics at the cathode current Ik=1 mA have been improved. In view of this, the 5%-beam spot diameter Ds 4  at the cathode current Ik=4 mA was calculated by simulation on the current electron gun using the above-described specifications for the current electron gun. The beam spot diameter Ds 4  for the current electron gun turned out to be 0.256 mm. This indicates that the beam spot diameter Ds 4  at the cathode current Ik=4 mA needs to be made equal to or smaller than 0.256 mm. 
     FIG. 5  shows results of simulation on a relationship between the beam spot diameter Ds 4  and the axial length L of the G4 electrode  67  with the aperture diameter D in the G1 electrode  64  as a parameter.  FIG. 5  indicates that, if the axial length L of the G4 electrode  67  is made longer than 48.7 mm which is the corresponding dimension of the current gun in a case where the aperture diameter D in the G1 electrode  64  is 0.54 mm as in the case of the current electron gun, the beam spot diameter Ds 4  turned out to be increased. Further, in a case where the aperture diameter D in the G1 electrode  64  is 0.50 mm, when the axial length L of the G4 electrode  67  is made longer than 48.7 mm which is the corresponding dimension in the current electron gun, the beam spot diameter Ds 4  is somewhat reduced, but begins to increase where the axial length L of the G4 electrode  67  is approximately 60 mm. This can be explained as follows. When the axial length L of the G4 electrode  67  is increased, aberration characteristics of the main lens are improved as explained above, but at the cathode current Ik=4 mA, an optimum diameter of the electron beam entering the main lens cannot be secured, thereby increasing the amount of deviation of the diameter of the electron beam incident into the main lens from its optimum diameter, and consequently, at some value of the axial length L of the G4 electrode  67 , the deviation of the electron beam diameter from its optimum diameter exerts greater influences than the improvement on the aberration of the main lens, and thereby the beam spot diameter Ds 4  is increased. As is apparent from  FIG. 5 , this relationship between the axial length L of the G4 electrode  67  and the beam spot diameter Ds 4  varies with the aperture diameter D in the G1 electrode  64 . 
     FIG. 6  shows a relationship between the aperture diameter D in the G1 electrode  64  and the axial length L of the G4 electrode  67 , obtained from  FIG. 5 , required to make the beam spot diameter Ds 4  at the cathode current Ik=4 mA equal to or smaller than 0.256 mm which is the beam spot diameter Ds 4  obtainable by the current electron gun.  FIG. 6  indicates that if the aperture diameter D in the G1 electrode  64  and the axial length L of the G4 electrode  67  satisfy the following relationship:
   L  mm≦−646× D  mm+396.3 mm  (2), 
the beam spot diameter Ds 4  at the cathode current Ik=4 mA can be made equal to or smaller than 0.256 mm which is the beam spot diameter obtainable by the current electron gun.
 
   For the purpose of improving resolution of the PTV, it is necessary that both the inequalities (1) and (2) are satisfied at the same time.  FIG. 7  shows the relationships between the aperture diameter D in the G1 electrode  64  and the axial length L of the G4 electrode  67  represented by the inequalities (1) and (2).  FIG. 7  indicates that if the aperture diameter D in the G1 electrode  64  and the axial length L of the G4 electrode  67  lie in an area defined by the following inequalities:
 
 L  mm≦60× D  mm+27.6 mm  (1), and
 
 L  mm≦−646× D  mm+396.3 mm  (2),
 
resolution of the PTV can be improved.
 
   By way of example, a 29 mm-diameter neck PRT will be explained which employs a Hi-UPF type electron gun (which incorporates a unipotential electron lens in which a focus electrode is supplied with a high voltage). In this PRT, the aperture diameter D in the G1 electrode  64  is selected to be 0.5 mm, and the axial length L of the G4 electrode  67  is selected to be 59 mm. In this case, an aperture diameter in the G2 electrode  65  is 0.5 mm, an axial length of the G3 electrode  66  is 20 mm, and a diameter Dg 4  of an opening in an end of the G4 electrode  67  on its panel portion  1  side is 16 mm. In this example, the beam spot diameter Ds 1  at the cathode current Ik=1 mA is 0.088 mm, the beam spot diameter Ds 4  at the cathode current Ik=4 mA is 0.232 mm, these values satisfy the criteria on which to judge whether the improvement in focus characteristics has been achieved, that is, the beam spot diameter Ds 1  at the cathode current Ik=1 mA being 0.089 mm or smaller, the beam spot diameter Ds 4  at the cathode current Ik=4 mA being 0.256 mm or smaller, and therefore the improvement in resolution of the PTV has been achieved. 
   The following will explain the cathode  62 . As shown in  FIG. 10 , the cathode  62  has the electron-emissive material  62 A on its top surface. The electron-emissive material  62 A can be composed chiefly of an oxide of alkaline earth metals including at least Ba, for example, an oxide composed of barium, strontium and calcium, (Ba.Sr.Ca)O, and can also contain a composite oxide of barium and scandium, for example, barium scandate Ba 2 Sc 2 O 5 . Here, the amount of scandium can be selected to be in a range of from 0.01 wt % to 5.0 wt %. 
   As an example of an electron gun using the above-described electron-emissive material  62 A, by selecting the aperture diameter D in the G1 electrode  64  to be 0.44 mm or larger and selecting the aperture diameter D in the G1 electrode  64  and the axial length L of the G4 electrode  67  so as to satisfy the above-described inequalities (1) and (2), lifetime of 20,000 hours or more is secured, and the improvement in resolution of the PTV can be achieved. 
   Further, as a more specific example of an electron gun using the above-described electron-emissive material  62 A, the following will explain an electron gun of the Hi-UPF type used in a 29-mm diameter neck PRT in which the aperture diameter D in the G1 electrode  64  is selected to be 0.5 mm, the axial length L of the G4 electrode  67  is selected to be 59 mm, and the dimensions of the remainder of the electrodes are selected to be the same as those in the above-described current electron gun. The beam spot diameter Ds 1  at the cathode current Ik=1 mA is 0.088 mm, the beam spot diameter Ds 4  at the cathode current Ik=4 mA is 0.232 mm, and lifetime of 30,000 hours can be secured. These results achieves the above-described target of improvement in focus characteristics, in which the beam spot diameter Ds 1  at the cathode current Ik=1 mA is 0.089 mm or smaller, the beam spot diameter Ds 4  at the cathode current Ik=4 mA is 0.256 mm or smaller, and lifetime is 20,000 hours or longer, and this indicates that the improvement in resolution of the PTV can be accomplished. 
   Further, the axial length L of the G4 electrode  67  is also related to the dynamic focus voltage dVf explained in connection with  FIG. 11 . The dynamic focus voltage dVf increases as the axial length L of the G4 electrode  67  is increased, and  FIG. 8  shows this relationship between the axial length L of the G4 electrode  67  and the dynamic focus voltage dVf. If the dynamic focus voltage dVf becomes 1,200 V or higher, fabrication of a circuit for generating the dynamic focus voltage dVf becomes difficult due to withstanding voltage characteristics of ICs (Integrated Circuits) forming the circuit. Therefore it is desirable to limit the dynamic focus voltage dVf to 1,200 V.  FIG. 8  shows that it is necessary to select the axial length L of the G4 electrode  67  to be 75 mm or shorter so as to limit the dynamic focus voltage dVf to 1,200 V. Therefore, when the above-described inequalities (1) and (2) are satisfied and the axial length L of the G4 electrode  67  is equal to or shorter than 75 mm, the resolution of the PTV can be improved without causing problems in fabrication of a circuit for generating the dynamic focus voltage dVf. 
   Here, in an electron gun of the Hi-UPF type used in a 29-mm diameter neck PRT in which the aperture diameter D in the G1 electrode  64  is selected to be 0.5 mm, the axial length L of the G4 electrode  67  is selected to be 59 mm, and the dimensions of the remainder of the electrodes are selected to be the same as those in the above-described current electron gun, the dynamic focus voltage dVf is 990 V, the beam spot diameter Ds 1  at the cathode current Ik=1 mA is 0.088 mm, the beam spot diameter Ds 4  at the cathode current Ik=4 mA is 0.232 mm. Therefore, the dynamic focus voltage dVf is not higher than 1,200 V, and the above results satisfy the conditions for providing a perceptible improvement in focus characteristics that the beam spot diameter Ds 1  at the cathode current Ik=1 mA is equal to or smaller than 0.089 mm, the beam spot diameter Ds 4  at the cathode current Ik=4 mA is equal to or smaller than 0.256 mm. Consequently, the improvement in resolution of the PTV can be achieved without causing problems with fabrication of a circuit for generating the dynamic focus voltage dVf. 
   The above explanation has been made only about representative examples of the electrode configurations for the electron gun, but the same advantages as in the case of the above-described representative examples of the electrode configurations can also be obtained in a case where the inside diameter Dg 4  of the opening in the end of the G4 electrode  67  on its phosphor screen  11  side is in a range of from 14 mm to 18 mm, the axial length of the G3 electrode  66  is in a range of from 15 mm to 25 mm, the inside diameter of the G5 electrode  68  is in a range of from 20 mm to 22.5 mm, the distance between the end of the G4 electrode  67  on its phosphor screen  11  side and the center of the phosphor screen  11  is in a range of from 120 mm to 150 mm, and the thickness of the G1 electrode  64  is in a range of from 0.05 mm to 0.15 mm. 
   By adopting the above-described dimensions of the electrodes for an electron gun, the overall length of the projection type cathode ray tube can be limited to a range of from 240 mm to 290 mm.