ELECTRON GUN FOR COLOR-PICTURE TUBE

Abstract: An electron gun (100) for a color-picture tube includes adjacent low and high potential electrodes (130, 140) forming an electron lens, which electrodes have electron beam path holes (135B, 135G, 135R; 143B, 143G, 143R) horizontally formed with electric-field correcting members (160, 161; 170, 171) or raised portions. According to this construction, vertical equipotential lines are projected into the electrode so that the low potential electrode (130) adds to electron beams a vertical focusing effect more strongly than a horizontal one and the high potential electrode (140) adds to electron beams a vertical divergent effect more strongly than a horizontal one. The section of the electron beam in a deflection region has an ellipse form whose major axis extends horizontally, resulting in suppressing a halo portion on the screen. The electron beam is properly focused horizontally and vertically so that the electron beam spot has a circular form on the center of a screen.

Description: The present invention relates to an electron gun used for a color-picture tube. Recently, a normal electron gun for color-picture tube is an inline three-beam type tube. The inline three-beam type tube comprises three cathodes disposed on one plane, a first grid and a second one common to these cathodes, and a focusing electrode having two or more electrodes respectively with a plurality of holes and being disposed at given intervals in the axial direction of the tube. The three cathodes and the first and the second grids serve to generate three electron beams, and then the focusing electrode allows the three electron beams to pass through the holes for focusing these beams. And, the inline type triple-gun color-picture tube normally provides a deflection yoke, which generates an inhomogeneous magnetic field consisting of a pin-cushion type horizontally deflecting magnetic field as shown in Fig.1(a) and a barrel type vertically deflecting magnetic field as shown in Fig.1(b). The deflection yoke thus allows the three electron beams to self-converge on a fluorescent screen. In Fig.1, B1, B2, and B3 respectively denote electron beams emitted from the inline electron gun. Curves show magnetic fields. This type of self-convergence deflection system does not require an additional device for converging the three electron beams such as a dynamic convergence device, which means it is less costly and allows easier convergence control. Hence, the color-picture tube employing the inline type triple-electron gun greatly contributes to the quality and performance of a color-picture tube. The inhomogeneous magnetic field brings about an adverse effect of lowering resolution on the peripheral part of the screen of the color-picture tube. The adverse effect is more distinguished as the deflection angle increases from 90° to 110°. This effect results from the fact that the inhomogeneous magnetic field of the deflection yoke as shown in Figs.1(a) and (b) weakens horizontal focusing level of the electron beams and strengthens vertical focusing level of them to the contrary. As a result, a beam spot 1, which is located on the center of the screen, is substantially circular, but a beam spot 2, which is located on the pripheral part of the screen, is formed to have an elliptic high brightness core portion 3 extending horizontally and a low brightness halo portion 4 extending vertically. This phenomenon will be explained with reference to Figs.3 and 4. As shown in Fig.3, the electron beam spot on the center of the screen is assumed to have a circular form 5 in section as a result of being converged and diverged while the electron beams pass through a low potential region I and a high potential region II of a main lens. That is, a focusing angle α₂ is assumed to allow the electron beams through a deflection region 6 to be substantially circular. As shown in Fig.4, the electron beam 7 receives as a vertical force the vertical force components 10 and 11 serving to over-focus the vertical components of an electron beam. After being deflected, therefore, the electron beam spot section is formed to be an ellipse 13 whose major axis extends horizontally and a halo 12. To improve the deflection distortion described above, it is possible to employ a system having a pre-focusing lens for focusing an electron beam strongly and reducing the diameter of an electron beam passing through a main lens section and in a deflecting magnetic field, in which system the vertical force components of the force in the deflecting area are made smaller so that the deflection distortion is reduced. This system, however, must have an increased crossover diameter so that the electron beam spot diameter on the center of the screen is made larger, resulting in lowering the resolution on the center of the screen. Another system for reducing the deflection distortion is a system providing an asymmetric pre-focusing lens or locating an asymmetric main lens for under-focusing the vertical components of the electron beam (the latter is disclosed in the U.S. Patent No.4086513). Reference will be directed to the latter system. As shown in Fig.5, the low potential region III and the high potential region IV of the main lens are respectively assumed to set the vertical divergence level (line segment A-B-C and a-b-c) being stronger than the horizontal divergence level (line segment A-D-E and a-d-e). And a vertical focusing angle α₁ and a horizontal focusing angle α₂ are assumed to allow the sectional form of the electron beam on the center of the screen to be an ellipse 14 whose major axis extends in the vertical direction, that is, allow the electron beam diameter in the deflection region 15 to be an ellipse 22 whose major axis extends horizontally and a halo 21. When the electron beam spot on the center of the screen is formed to be an ellipse whose major axis extends vertically, the vertical focusing angle α₁ of the electron beam is smaller than α₂ assumed when it is formed to be a substantial circle (as shown in Figs.3 and 4). Hence, the vertical force components 19 and 20 shown in Fig.6 are made smaller than those 10 and 11 shown in Fig.4, so that the halo portion 21 is made smaller than the halo portion 12. By assuming the vertical divergent effect to be larger than the horizontal divergent effect, therefore, it is possible to improve resolution on the peripheral part of the screen. In the foregoing system, however, the electron beam spot on the center of the screen is formed to be an ellipse whose major axis extends vertically, which brings about a shortcoming that the resolution on the center of the screen is made lower. The other system of providing an asymmetric main lens or an asymmetric pre-focusing lens also has the same shortcoming. As set forth above, the self-convergence color-picture tube employing an inline type triple-gun greatly contributes to the quality and performance of the color-picture tube, but it has a shortcoming that the resolution on the peripheral part of the screen is inferior and, for improving it, the resolution on the center of the screen is forced to be lower. It is known per se from EP-A-0 192 436 to provide an in-line CRT electron gun with a bipotential lens having different converging angles in the vertical and horizontal directions, for improving beam spot quality. This document relates to a CRT electron gun with a crossover of the beams in the main lens. To further improve the picture quality given by the inline type triple-gun color-picture tube while keeping the advantages of the self-convergence system employing the above gun, accordingly, it is necessary to improve the resolution on the peripheral part of the screen without having to lower the resolution on the center of the screen. It is an object of the present invention to provide an electron gun for color-picture tube which offers improved resolution onto the peripheral part of the screen without having to lower the resolution on the center of the screen and excellent resolution onto the overall screen. It is another object of this invention to provide an electron-gun for color-picture tube which suppresses a halo portion generated on the peripheral part of the screen or completely eliminates it. The electron gun for color-picture tube according to this invention is defined in Claim 1. Several factors such as form, size and position of the electric field correcting members are variable depending on the size or deflection angle of a picture tube and strength or form of the magnetic field caused by a deflection yoke. The position of the electric-field correcting members is preferably such that the distance between the electric-field correcting members of the low potential electrode to the baseplate is larger than the distance between those members of the high potential electrode to its baseplate. Moreover, by changing the form of the electron beam path holes formed on the high potential electrode side of the low potential electrode or the low potential electrode side of the high potential electrode, it is possible to adjust the focusing effect and the divergent effect. Preferably, a thin plate having a plurality of electron beam path holes should be attached on the high potential electrode side of the low potential electrode, because it is possible to promote a lens effect of a small electron lens caused near each electron beam path hole as well as to control the main lens function by changing the form of each electron beam path hole formed on the thin plate. For properly achieving the above focusing or divergent effect, it is also possible to vertically mount raised portions in the electron beam path holes formed on the high and low potential electrodes. The electron beam path holes may provide electric-field correcting members as raised portions horizontally formed inside of the low potential electrode and the high potential electrode. The equipotential lines extending in the electrodes, therefore, serve to vertically offer the focusing effect around the low potential electrode or the divergent effect around the high potential electrode, so that both effects are stressed vertically. The vertical size of an electron beam section in the deflection region is shorter than the horizontal size thereof so that the sectional shape of the electron beam is an ellipse extending horizontally. The deflection distortion, therefore, is reduced, because the vertical components given by the horizontally deflecting magnetic field within the inhomogeneous magnetic field are reduced. The vertical focusing angle is smaller than in the prior art so that the halo portion caused by the deflection may be suppressed. The electron beams are properly focused on the fluorescent screen of the color-picture tube through the weak horizontal focusing and divergent effects and strong vertical focusing and divergent effects. The electron beam spot on the center of the screen is formed to be circular. Consequently, without lowering the resolution on the center of the screen, the resolution on the peripheral part of the screen can be improved. Fig.1(a) is a view showing a pin cushion type magnetic field, and Fig.1(b) is a view showing a barrel type magnetic field; Fig.2 is a view showing forms of electron beam spots on the center and the peripheral part of the screen of the conventional color-picture tube; Fig.3 is a view showing the functioning of a conventional main lens; Fig.4 is an explanatory view for illustrating how a horizontally deflecting magnetic field influences the electron beam focused by the main lens shown in Fig.3; Fig.5 is a view showing the function of an other conventional main lens; Fig.6 is an explanatory view for illustrating how the horizontally deflecting magnetic field influences the electron beam focused by the main lens shown in Fig.5; Fig.7(a) is a schematic plan section showing one embodiment of an electron gun for a color-picture tube according to this invention, and Fig.7(b) is a schematic vertical section showing an electron gun for a color-picture tube shown in Fig.7(a); Fig.8(a) is a vertical section showing an equipotential distribution around a main lens, and Fig.8(b) is a horizontal section showing the equipotential distribution around the main lens; Fig.9 is a view for illustrating the functioning of the main lens; Fig.10 is an explanatory view for illustrating how the horizontally deflecting magnetic field influences an electron beam focused by the main lens shown in Fig.9; Fig.11 is a view showing the form of electron beam spots on the center and the peripheral part of the screen of the color-picture tube; Fig.12(a) is a schematic horizontal view showing an other embodiment of an electron gun for a color-picture tube according to the invention, and Fig.12(b) is a schematic vertical section showing the electron gun shown in Fig.12(a); Fig.13 is a perspective view showing a burring portion employed for the electron gun for a color-picture tube according to the invention; Fig.14 is a view showing the position of mounting an electric-field correcting member employed for the electron gun for a color-picture tube according to the invention; Fig.15 is a view showing an example of electron beam path holes employed for the electron gun for a color-picture tube according to the invention; Fig.16 is a view showing an other example of electron beam path holes employed for the electron gun for a color-picture tube according to the invention; Fig.17 is a perspective view showing an example of the electric-field correcting member employed for the electron gun for a color-picture tube according to the invention; and Fig.18 is a perspective view showing an other example of the electric-field correcting member employed for the electron gun for a color-picture tube according to the invention. Hereinafter, an embodiment of this invention will be described with reference to the drawings. Fig.7(a) is a schematic plan section showing an embodiment of an electron gun for a color-picture tube according to the invention, and Fig.7(b) is a schematic side section showing the above. In Fig.7(a), an electron gun 100 comprises a heater (not shown) inside of itself and three cathodes KR, KG, and KB disposed in a line, a first electrode 110, a second electrode 120, a third electrode 130, a fourth electrode 140, and a convergence cup 150 disposed in the axial direction of the tube. The electron gun 100 is supported and secured by an insulating supporting rod (not shown). The first electrode 110 is plate-like and its thickness is as thin as 0.2 mm. The electrode 110 includes three electron beam path holes 111R, 111G, and 111B formed therein. The diameter of the holes 111R, 111G, 111B of the electrode 110 is as small as about 0.7 mm, and each distance between the centers of the holes is 6.6 mm. The third electrode 130 consists of two cup-like electrodes 131, 132 whose opening ends are mounted to each other, and a thin plate 133 whose thickness is about 0.6 mm. On the second electrode 120 side of the cup-like electrode 131 are formed three electron beam path holes 134R, 134G, and 134B, each diameter of which is 1.3 mm. The fourth electrode 140 side of the cup-like electrode 132 is substantially tabular with no burring portion. On this side are formed three substantially circular electron beam path holes 135R, 135G, and 135B, the maximum diameter of which is 6.2 mm. On the thin plate 133 are formed three substantially circular electron beam path holes 136R, 136G, and 136B, which are identical to the electron beam path holes 135R, 135G, and 135B of the cup-like electrode 132. On the inner wall of the cup-like electrode 132 are formed electric-field correcting members 160 and 161 respectively consisting of tabular plates whose thickness is about 1.2 mm, length is about 3.0 mm, and width is 19.0 mm. The electric-field correcting members 160 and 161 are located in a horizontal manner to a trajectory surface of each electron beam and as if they pinch the trajectory surface. These members keep an axial distance (L₁) of 3.0 mm from the surface containing the electron beam path holes 135R, 135G, and 135B. A fourth electrode 140 consists of two cup-like electrodes 141 and 142 whose opening ends are closely mounted to each other. The third electrode 130 side of the cup-like electrode 141 is substantially tabular with no burring portion. On this fourth electrode 141 are formed substantially circular electron beam path holes 143R, 143G, and 143B which are similar to the electron beam path holes 135R, 135G, and 135B of the cup-like electrode 132. On the inner wall of the cup-like electrode 141 are formed electric-field correcting members 170 and 171 respectively consisting of tabular plates whose thickness is about 1.5 mm, length is about 3.0 mm, and width is 19.0 mm. The electric-field correcting members 170 and 171 are located in a horizontal manner to a trajectory surface of each electron beam and as if they pinch the trajectory surface. These members keep an axial distance (L₂) of 2.0 mm from the surface containing the electron beam path holes 143R, 143G, and 143B. On the convergence cup 150 side of the cup-like electrode 142 are formed three substantially circular electron beam path holes 144R, 144G, and 144B, respectively, with large diameters. The convergence cup 150 is in contact with these holes. On the cup-like electrode 142 side of the convergence cup 150 are formed substantially circular electron beam path holes 151R, 151G, and 151B respectively with large diameters. A spring 180 is fixed to the lower portion of the convergence cup 150. It is applied on a conductive film (not shown) coated on the neck inner wall. A d.c. voltage of about 150 V and a modulation signal corresponding to an image to be displayed on a screen are applied on the cathodes KR, KG, and KB of the electron gun 100. The first electrode 110 is grounded. A potential of about 600 V is applied to the second electrode 120. A voltage of about 7 KV is applied to the third electrode 130 and a high voltage of about 25 kV is applied to the fourth electrode 140 through the conductive film, the spring 180, and the convergence cup 150. The cathodes KR, KG, KB, the first electrode 110, and the second electrode 120 compose a triode, which serves to emit an electron beam and form a crossover. The interval between the second electrode 120 and the third electrode 130 composes a pre-focusing lens for preliminarily focusing an electron beam emitted from the triode. The interval between the third electrode 130 and the fourth electrode 140 composes a main lens for finally focusing the electron beams on the fluorescent screen. The main lens affords a focusing effect on the third electrode 130 side to which the relatively low voltage is applied and a divergent effect on the fourth electrode 140 side to which the relatively high voltage is applied. Since the electron beam is greatly influenced by the low voltage side effect, at the last stage, the electron beam is focused on the fluorescent screen. The electric field correcting plates 160, 161, 170, and 171 are provided inside of the third electrode 130 and the fourth electrode 140, so that the horizontal curvature for electric field penetration is different from the vertical one near the electron beam path holes 135R, 135G, 135B, 136R, 136G, 136B, 143R, 143G, and 143B. Herein, the equipotential distribution near the main lens will be described with reference to Fig.8. Fig.8(a) is a vertical section showing the equipotential distribution near the main lens, and Fig.8(b) is a horizontal section showing the above. As shown in Fig.8(a), the vertical equipotential distribution located inside of the cup-like electrodes 132 and 141 is designed so that the central portions of the equipotential lines are projected within the electrode through the effect of the electric-field correcting members 160, 161, 170, and 171. This effect is very large in the cup-like electrode 141 where the distance L₂ is short. As shown in Fig.8(b), on the other hand, the horizontal equipotential distribution is designed so that no equipotential lines are projected as shown in Fig.8(a) because of the absence of the horizontal electric field correcting plates. The vertical curvature of the equipotential lines is designed to be larger than the horizontal curvature. In other words, the vertical focusing and divergent effects are relatively stronger, and the horizontal focusing and divergent effects are relatively weaker. Figs.9 and 10 conceptually show the function of the main lens. In Fig.9, the electron beam is shown by a real line. When the electron beam passes through the third electrode area V, the vertical focusing effect has a stronger influence over the electron beam as shown in lines F-G and f-g, and the horizontal focusing effect has a weaker influence over it as shown in lines F-H and f-h. In the fourth electrode area VI of the main lens, the vertical divergent effect has stronger influence over the electron beam as shown in lines G-I and g-i, and the horizontal divergent effect has weaker influence over it as shown in lines H-J and h-j. As set forth above, the main lens affords respective functions to the electron beam according to the vertical or horizontal direction. αv is a focusing angle in the vertical direction, and αH is a focusing angle in the horizontal direction. The sectional shape of the electron beam in the deflection region 200 has a smaller vertical diameter than the horizontal one. That is, the electron beam has an elliptic shape in section, the major axis of which extends horizontally. The electron beam spot shape 201 is substantially circular. As shown in Fig.10, since the electron beam 300 receives small vertical force components 303 and 304 of the forces 301 and 302 afforded by the horizontally deflecting magnetic field when it is deflected, the deflected beam is hardly distorted. The focusing angle αv in the vertical direction is small. Hence, the electron beam spot shape deflected on the peripheral part of the screen is an ellipse 305 with a suppressed halo portion, the major axis of which ellipse extends horizontally. The central electron beam spot 400 has a substantially circular shape, and the peripheral part electron beam spot 401 has an elliptic shape with a suppressed or no halo portion. Thus, the resolution on the peripheral part of the screen can be improved without having to lower the resolution on the center of the screen. Fig.12 shows another embodiment of an electron gun for a color-picture tube according to this invention. Fig.12(a) is a schematic plan section showing the embodiment, and Fig.12(b) is a schematic side section showing it. The electron gun 500 shown in Fig.12 is identical to the electron gun 100 shown in Fig.7 except that the thin plate 133 is removed. When employing the electron gun 500, it is possible to obtain the similar effect as when using the electron gun 100. In Fig.12, like reference numbers are given to the members common to those shown in Fig.7 In place of the electric-field correcting members 160, 161, 170, and 171 shown in Figs.7 and 12, it is possible to locate a burring portion 600 with no horizontal raised portion inside of the low potential electrode face opposite to the high potential electrode and the high potential electrode face opposite to the low potential electrode, both the faces composing the main lens, for the purpose of obtaining a similar effect as when using the electric-field correcting members, as is shown in Fig.13. The shape of the electron beam spot is variable depending on the size or deflection angle of a color-picture tube or the strength, shape or change rate of the deflection field. For optimizing the functioning of an asymmetric lens, it is necessary to set variable parameters such as the form, length, or mounting position of a electric-field correcting member or the shape of each electron beam path hole. If the deflection yoke generates a stronger magnetic field than that in the foregoing embodiment, for optimizing the function of the asymmetric lens, it is possible to fix the distances L₁ and L₂ between the electric-field correcting members 160, 161, 170, and 171 and the electron beam path holes 135G and 143G as being smaller than those in the foregoing embodiment or to fix L₁=0 and L₂=0, as shown in Fig.14 wherein the electron gun 500 shown in Fig.12 is employed. In Fig.14, the members common to those in Fig.12 has similar reference numbers as those in Fig.12. As an optimizing method, there exist the following methods. (1) At least one group of electron beam path holes is selected out of the electron beam path holes formed on the electron lens side of the low potential electrode or the thin plate closely located on the low potential electrode side and those holes formed on the electron lens side of the high potential electrode, and the selected electron beam path holes respectively should have oval forms with the height X of each hole being set as a parameter, as shown in Fig.15. (2) The method described in (1) should be combined with the conditions of the distances L₁ and L₂ between the foregoing electric-field correcting members and the electron beam path holes. Furthermore, for optimizing the form of a central beam and a side beam using the above (1) and (2) methods, there exist the following methods. First, at least one group of electron beam path holes are selected out of the electron beam path holes formed on the electron lens side of the low potential electrode or the thin plate closely located on the low potential electrode side or those holes formed on the electron lens side of the high potential electrode, and the openings of the selected group of electron lens path holes should consist of the combination of a circular opening 900 and oval ones 901 as shown in Fig.16. Second, it is possible to employ the method of varying the thickness t₁ of the center beam portion on the electric-field correcting member and the thickness t₂ of the side beam portion thereof, as shown in Fig.17. Third, it is also possible to employ the method of varying the length ℓ₁ of the center beam portion of the electric-field correcting member and the length ℓ₂ of the side beam portion thereof, as shown in Fig.18. The foregoing methods allow the functioning of the asymmetric lens to be optimized, thus making it possible to achieve excellent resolution over the whole screen of the color-picture tube. Although the embodiments of this invention have been described with reference to a bi-potential type electron gun, the function and the effect of this invention may be applied to another type electron gun such as a uni-potential type electron gun or quadru-potential type electron gun.

Claims: An electron gun (100) for a color-picture tube having a plurality of cathodes (KR, KG, KB) disposed with their axes substantially in a single plane for generating a corresponding plurality of electron beams, first (130) and second (140) coaxial cylindrical electrodes having pate holes (135R, 135G, 135B; 143R, 143G, 143B) through which the electron beams pass, the first and second cylindrical electrodes being each cup-shaped with a cylindrical wall and a base plate, the base plates being mutually opposed with a predetermined axial separation, low voltage being applied to said first electrode and relatively high voltage being applied to said second electrode, said first and second electrodes constituting a main lens for focusing the beams horizontally and vertically on a screen, said electron gun further comprising a first electric field correcting member (160, 161; 600) extending axially on said first cylindrical electrode to produce a field correcting effect such that the vertical convergence of the beam is stronger than its horizontal convergence at said main lens and (140) a second electric field correcting member (170, 171; 600) extending axially on said second cylindrical electrode to produce a field correcting effect such that the vertical divergence of the beam is stronger than its horizontal divergence at said main lens. An electron gun according to Claim 1, characterized in that it further comprises a thin plate (133) adjacent to the base plate of the first cup-shaped electrode (130). An electron gun according to Claim 1 or 2, characterized in that at least one group (800, 900) of the electron path holes comprises a central electron beam path hole and side electron beam path holes located on respective sides of the central electron beam path hole, the ratios of vertical to horizontal dimensions of the central electron beam path hole and of the side electron beam path holes being different. An electron gun according to Claim 3, characterized in that the distance between the central electron beam path hole and said electric field correcting member of at least one of said first and second cup-shaped electrodes (130, 140) is different (l₂-l₁,) from the distance between the side electron beam path hole and said electric field correcting member of the same electrode. An electron gun according to Claim 4, characterized in that the distance between the central electron beam path hole (135G, 143G) and said electric field correcting member of at least one of said first and second cup-shaped electrodes (130, 140) is larger (l₂-l₁,) than the distance between the side electron beam path hole and said electric field correcting member of the same electrode. An electron gun according to Claim 1 or 2, characterized in that each base plate includes a surface having a center beam portion and a side beam portion, the distance in the said axial direction of the color-picture tube between the electric correcting member of at least one of said first and second cup-shaped electrodes and said center beam portion being different from the corresponding distance between said electric field correcting member and said side beam portion.

Inventors: KOSHIGOE, SHINPEI C/O PATENT DIVISION, MURAI, RYUICHI, SHIMOMA, TAKETOSHI C/O PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1989
Main classification: H01J29
IPC codes: H01J29