CATHODE RAY TUBE

Abstract: In a cathode ray tube comprising an electron gun having a cathode 1 and five grids (G1, G2, G3, G4 and G5), the first grid (G1) being a control grid, and the fifth grid (G5) being a final acceleration grid and the second grid (G2) being an acceleration grid, the third grid (G3) is impressed with a higher potential than the neighboring second grid (G2) and the fourth grid (G4), thereby producing an axial potential distribution profile having a maximum point at the region of the third grid (G3), to provide a steeply rising electric field in the beam forming part near the cathode, and on the other hand producing a gently changing potential distribution after the third grid (G3).

Description: Title of the Invention Cathode ray tube Background of the Invention 1. Field of the Invention: The present invention generally relates to an improvement of a cathode ray tube and particularly to a cathode ray tube of high resolution. 2. Description of the Prior Art: Generally speaking, resolution of a cathode ray tube depends on the size of beam spot which forms picture element on a fluorescent screen. That is, as the diameter of the beam spot becomes smaller, the resolution of the reproduced picture becomes higher. On the other hand, since the diameter of the beam spot increases as beam current increases, when a relatively large beam current flows to produce a high luminance spot, a blooming is produced thereby lowering resolution. The above-mentioned is elucidated with reference to attached FIGUREs 1, 2, 3 and 4. FIG. 1 shows sectional view along the axis showing configuration of a bipotential type electron gun of prior-art, and FIG. 2 is a graph showing potential distribution along the axis of the electron gun. Thermal electronsemitted'from the cathode lis focused by means of an electrostatic lens 4 called cathode lens which is constituted with a first grid (Gl) as a control grid 2 and a second grid (G2) as an acceleration electrode 3, to produce a crossover 5, which is then preliminarily focused by a pre-focus lens 7 produced between the second grid (G2) 3 and the a third grid (G3) 6 as a focusing grid. Then, the pre-focused electron beam is finally focused by a main lens 9 constituted with the third grid (G3) 6 and a fourth grid (G4) 8 as a final acceleration grid, thereby to produce a beam spot 11 on a fluorescent screen 10 by impinging thereto. That the beam spot 11 is an image of the crossover 5 projected by the pre-focus lens 7 and the main lens 9. As shown in FIG. 1 and FIG. 2 which shows distribution of potential along axial position of the electron gun, the potential gradually rises from the cathode 1 to the rear end (inlet end) of the third grid (G3) 6, thereby forming the cathode lens 4 and the pre-forcus lens 7. The cathode lens 4 and the pre-focus lens 7 can not be clearly distinguished, hereinafter these two lenses together are comprehensively called a beam forming part. Though the axial distribution in the third grid (G3) 6 is substantially constant at a value of Vfoc , thereafter the axial potential distribution rapidly rises from the outlet end of the third grid (G3) 6 to the fourth grid (G4) 8 to a high potential Va, thereby forming the main lens 9. Though the behavior of thermal electrons in the aforementioned beam forming part is very much complicated when the beam current is largenthe electron paths become as represented by several curves 12, 13, 14, 15, 16 and 17 as shown in FIG. 3. If all the electron beam paths 12 to 17 cross at one point, then an ideal crossover of a small diameter and a small beam spot are produced, but in actual cathode ray tube, paths 12, 13 emitted from the central part of the cathode 1 cross at a point 18 which is the farest crossing fran the cathode 1, and on the contrary thermal electron paths 16, 17 emitted from peripheral parts cross at a nearer point 19. This is because the lens of the above-mentioned beam forming part has spherical aberration, and accordingly the effective crossover diameter and beam spot diameter are considerably larger than their theoretical limit values. Since there is the pre-focus lens 7 the actual beam spot 11 projected on the fluorescent screen 10 by the main lens 9 is the image of virtual image of the crossover S having a diameter d. This virtual image of the crossover 5 can be obtained by extending the straight line part of the electron beam paths 12-17 towards the direction of the cathode 1. The crossing part of the straight lines gives the location of the virtual image. Now, provided that the virtual image of the crossover has a diameter d , then the diameter d of o s the beam spot 11 on the fluorescent screen 10 is represented by the following equation (1). d =d x M + 1 D3 ................... (1), 0 S wherein M is a magnification of the main lens 9, Cs is spherical aberration coefficient of the main lens 9, D is diameter of the electron beam at the main lens 9.. The magnification M is represented by the following equation (2). EMI4.1 wherein a is a maximum divergence angle of electron beam emanated from the beam forming part, Ii is distance between the center of the main lens 9 and the fluorescent screen 10, Vg3 is potential of the third grid (G3) 6, V is potential of the fourth grid (G4) 8. a Then, by substituting the equation (2) into the equation (1), the following equation (3) is obtained: EMI4.2 In the above-mentioned equation (3), the value of the bracketted part is determined by the-size and operation conditions of the cathode ray tube and Cs is determined by the diameter of lens used. ¯ The beam diameter D exists in the form of D in the leftest part of the right side, and in the form of D3 in the rightest part, accordingly there is a value that will make d5 minimum, and usually such value is selected. Under such condition, when d5 is intended to be small, it is necessary to decrease a-dO as small as possible. According to the study of the inventors, it has been found that when potential gradient in the part between the second grid (G2) and the third grid (G3) is made steep, a-dO can be decreased. FIG. 4 is a graph showing relations between the gap between the second grid (G2) and the third grid (G3) ' versus the values adO , a and do as such. When the gap between the second grid (G2) and third grid (G3) is decreased, do drastically decreases and a increases contrarily, but as a result, a-dO decreases. The reason that d drastically decreases as shown in FIG. 4 0 is regarded as owing to peripheral aberration at the beam forming part decreases as the potential gradation increases. The effect of the decrease in product a-d o becomes prominent for potential gradient above about 105 V/cm, and as the product a-dO becomes smaller, the minimizing of beam spot diameter at a large current becomes easier. The diameter of beam spot, however, becomes larger at a small beam current under such high potential gradient. Besides, when the potential gradient is excessively large, electron emission due to field emission is liable to take place from the surface of the second grid (G2), and the emitted electrons make undesirable light at iitinging on the fluorescent screen 10. As has been described, practically admissible value of the potential gradient has an upper limit which is, according 5 to the inventors' experiments, about 5 x 10 V/cm. Accordingly, when potential differende-between the second grid (G2)-and the third grid (G3) is selected 8 KV, taking account the above-mentioned admissible potential ¯gradient, the gap betweefl the second grid (G2) and the third grid (G3) becomes 0.8 mm to 0.16 m=. Even though the above-mentioned potential gradient is applied to the beam forming part of an electron gun of the conventional cathode ray tube, diameter of the beam spot on the fluorescent screen can not be decreased as desired. This is because that beam divergence angle a increases when the potential gradient between the second grid (G2) and the third grid (G3) is raised. When the beam divergence angle a increases, diameter D of the electron beam in the main lens 9 increases, thereby undesirably increasing such part of the rightest part of the equation (3) which is influenced by the aberra 'tion of the main lens 9. Since the leftest part is prapotional to third power of D, even a small increase of the diameter D of the electron beam prominently increases the rightest part. Accordingly, even though the value a-d in the first term of the right 0 side of the equation (3) decreases, the diameter d of S the beam spot on the fluorescent screen increases, on the contrary. The decrease of the beam diameter D of the electron beam can be prevented by decreasing length of the third grid (G3), but then it becomes necessary that the focal length of the main lens 9 must be shortened in order to satisfy the focusing condition. Accordingly, the potential of the third grid (G3) must be lowered. Such lowering of the third grid potential decreases potential gradient between the second grid (G2) and the third grid (G3), thereby wastly diminishing the effect of decrease of a-dO in spite of decreasing the gap between the second grid (G2) and the third grid (G3). Summary of the Invention The present invention intends to dissolve the above-mentioned problems and eliminates the above-mentioned shortcomings of the conventional cathode ray tube, to provide an improved cathode ray tube having uniform small diameter of beam spot even for a wide range of brightness from a low brightness range to a high brightness range of operation. The cathode ray apparatus in accordance with the present invention comprises an electron gun, a fluoresent screen and an evacuated enclosure enclosing the electron gun and the fluorescent screen therein, the electron gun comprising a cathode, a first grid (G1) as a control grid, a. second grid (G2) on which an accelerating potential is to be applied, a third grid (G3), a fourth grid (G4) and a fifth grid (G5) as a final acceleration grid, which are disposed in this sequential order, wherein the third grid (G3) is impressed with a higher potential than a potential impressed on the second grid (G2) and the fourth grid (G4) is impressed with a lower potential than the potential impressed on the third grid (G3), thereby forming such an axial potential distribution of the cathode ray tube that a maximum potential is produced at an axial position of the third grid (G3), and thereafter the axial.potential gradually decreases towards a minsrm potential at an-axial position of the fourth grid (G4) and further - increases gradually towards an axial region of the fifth grid (G5), thereby forming substantially a single main lens at a region ranging from the third grid (G3) to the fifth grid (G5). According to the cathode ray tube of the present invention, by increasing potential gradation at the beam forming part, spherical aberration as well as diameter of virtual crossover can be decreased, and focal length of the main lens is shortened by adoption of a novel potential gradient profile in the main lens part formed by a third grid (G3), a fourth grid (G4) and a fifth grid (G5), thereby enabling to limit the diameter of the electron beam in the part of the main lens even under an increase of the beam divergence angle. This invention attains smallness of diameter of the beam spot on the fluorescent screen even when a large beam current flows for high brightness, thereby attaining high resolution characteristic. Brief Description of the Drawing FIG. 1 is the sectional view along the axis of conventional electron gun)with schematically shown fluorescent screen. FIG. 2 is the graph showing the distribution of potential gradient along the axial position of the electron gun shown in relation to the positions in FIG. 1. FIG. 3 is a schematical chart showing electron paths in the electron gun of the prior art of FIG. 1. FIG. 4 is a graph showing relations between the gap between the second grid (G2) and the third grid (G3) and value do, a and a-do. FIG. 5 is a sectional view along the axis of electron gun; with schematically shown fluorescent screen) as a first embodiment in accordance with the present invention. FIG. 6 is a graph showing the distribution of potential along the axial position of the electron gun shown in relation to the positions in FIG. 5. FIG. 7 is a sectional view along the axis of electron gun7with schematically shown fluorescent screen, as a second embodiment in accordance with the present invention. FIG. 8 is a graph showing the distribution of potential along the axial position of the electron gun shown in relation to the positions in FIG. 7. FIG. 9 is a sectional view along the axis of electron gunZwith schematically shown fluorescent screen as a third embodiment in accordance with the present invention. FIG. 10 is a graph showing the distribution of potential -along the axial position of the electron gun shown in relation to the positions in FIG. 9. Description of the Preferred Embodiments A cathode ray tube in accordance with the present invention is described on preferred embodiments shown hereinafter with reference to FIG. 5 and thereafter. FIG. 5 shows a first embodiment wherein the electron gun comprises a first grid (Gl) 2, a second grid (G2) 3, a third grid (G3) 22, a fourth grid (G4) 23 and a fifth grid (G5) 21, in this order from the cathode side to the fluorescent screen side. The first grid (G1) 2 works as a known control grid, the second grid (G2) 3 has the same configuration as a known acceleration grid, and a third grid (G3) 22 is shaped a bored disk and is disposed close to the second grid (G2) 3 in order to make a large potential gradient of 105 V/cm--- 5 x 105 V/cm. The fourth grid (G4) 23 and the fifth grid (G5) 21 are both in simple cylindrical shape, and the third grid (G3) 22 is impressed with a constant voltage Vg3 of +10 KV, and the fourth grid (G4) 23 is impressed with a variable focus voltage Vfoc which is lower than the constant voltage Vg3, and the fifth grid (G5) 21 is impressed with a positive high voltage Va of about 30 KV. By such configuration, axial potential distribution at the beam forming part steeply rises from the cathode side towards the fluorescent screen, and accordingly the value a-d 0 prominently decreases, but on the other hank beam divergence angle a increases. That is, as shown in FIG. 6, the axial potential distribution rises in the electron beam aperture 24 of the third grid (G3) 22 up to the potential Vg3, and gradually decreases towards the central part of the fourth grid (G4) 23. The potential again increases from the central part of the fourth grid (G4) 23 towards the central part of the fifth grid (G5) 21, and at the central part of the fifth grid (G5) 21 reaches the high potential Va and rennns alraost constant thereafter. Accordingly, by the relative congiguration of the third grid (G3) 22, the fourth grid (G4) 23 and the fifth grid (G5) 21, and their potential distributions, a complex lens field is produced, thereby substantially forming a single thick main lens 25. The axial potential distribution in the fourth grid (G4) 23 is lower than the constant potential Vg3 of the third grid (G3) 22, therefore the focal length of the main lens 25 is reduced in comparison with the conventional electron gun configuration. From the above-mentioned configuration, even with retaining the potential rise in the beam-forming-part very steep, mutual distance between the virtual image crossover and the main lens can be shortened, thereby enabling the diameter of diverged beam at the part of the main lens 25 to decrease even when the beam divergence angle a increases. In view of the equation (3), this means that the value a-dO can be decreased without increase of the diverged beam diameter, and therefore the beam spot diameter d5 can be decreased. Nextly, a second preferred embodiment is described with reference to FIG. 7 and FIG. 8. In the example of FIG. 7, the mechanical configurations and their relative arrangements are substantially equal to the first embodiment of FIG. 5 and FIG. 6, but their potential distribution profile is modified. That is, the third grid (G3) 22 is electrically connected to the fifth grid (G5) 21, and the second grid (G2) 3 is impressed with a constant voltage so as to produce a potential gradient of 10 V/cm-5X 10 V/cm. The substantially cylindrical fourth grid (G4) 23 is impressed with a variable potential Vfocl variable from almost 0 V to several KV. The fifth grid (G5) 21 is impressed with a constant potential of about 30 KV. That is, in this embodiment the third grid (G3) 22 and the fifth grid (G5) 21 are impressed with high potentials, and the fourth grid (G4) 23 is impressed with a lower potential of several KV or lower, so that the potential distribution profile as shown in FIG. 8 is produced. Though the fourth grid (G4) 23 and the fifth grid (G5) 21 are drawn to have substantially the same diameter, these may be of different diameters. Especially when the variable potential Vfoc is used at a voltage near 0 volt, the diameter of the fourth grid (G4) 23 should be preferably larger than the diameter of the fifth grid (G5) 21. As a result of the above-mentioned configuration and electric connection, axial potential distribution in the fourth grid (G4) 23 can be lower than the potential of the third grid (G3) 22 of the potential Veg3, accordingly the focal distance can be shortened. Furthermore, since the electron gun can be shortened the overall cathode ray tube length can be shortened. FIG. 9 shows a third embodiment. In this embodiment, the components corresponding to the first embodiment of FIG. 5 are designated by the corresponding numerals as marks, and their redundant superposed descriptions are omitted for simplicity. The principal difference of the third embodiment of FIG. 9 from the first embodiment of FIG. 5 is that between the fourth grid (G4) 23 and the fifth grid (G5) 21, an auxiliary grid (G4-5) 26 is added. The fourth grid (G4) 23, the auxiliary grid (G4-5) 26 and the fifth grid (G5) 21 are equally cylindrical-shaped, and the third grid (G3) 22 and the auxiliary grid (G4-5) 26 are each other electrically connected in the cathode ray tube, and they are to be impressed with a focusing potential Vfoc of about 6 KV10 KV. The fourth grid (G4) 23 is impressed with a potential V . which is'lower than the focusing potential Vfocl and g4 the fifth grid (G5) 21 is impressed with a high potential V a of about 30 KV. As a result of the above-mentioned mechanical configuration and electric arrangement, the axial potential distribution profile in the beam forming part steeply rises from the cathode 1 towards the fluorescent screen 10, thereby drastically decreasing the value a-dO when the beam current is large, and on the other hand the beam divergence angle a increases. That is, as shown in FIG. 10 which is a graph showing axial potential distribution profile drawn in relation to the electrode disposition of FIG. 9, the potential rises from the cathode 1 to the electron beam aperture 24 of the third grid (G3) 22 upto the focusing potential Vfocl and thereafter gently decreases towards the central part of the fourth grid (G4) 23. Then the potential gently rises from the central part of the fourth grid (G4) 23 through the auxiliary grid (G4-5) 26 and to the central part of the fifth grid (G5) 24 as one continuous region, reaching up to the high potential of Va Accordingly, as a result of configurations, spatial dispositions and relative potential distributions of the third grid (G3) 22, the fourth grid (G4) 23, the auxiliary grid (G4.5) 26 and the fifth grid (G5) 21, an broad region electric field is produced, thereby forming a substantially single very thick main lens 27 shown in FIG. 9. In this embodiment, the axial potential of the fourth grid (G4) 23 is lower than the focusing potential VfoC, and therefore the main lens 27 has a shorter focal distance in comparison with the conventional electron gun configuration. Furthermore, since the thick main lens 27 has electric field distribution which gently changes in a very broad range, the spherical aberration of the main lens is small, -thereby making the aberration coefficient Cs of the rightest term.of the equation (3) very small, a-ccordingly minimizing the diameter d of the beam spot. S

Claims: What is claimed is 1. A cathode ray tube comprising an electron gun (2), a fluorescent screen (10) and an evacuated enclosure (not shown) enclosing said electron gun and said fluorescent screen therein, said electron gun comprising a cathode (1), a first grid (G1) (2) as a control grid, a second grid lG2) (31 on which an accelerating potential is to be applied, a third grid (G3) (22), fourth grid (G4) (23) and a fifth grid (G5) (21) as a final acceleration electrode, which are disposed in this sequential order, wherein said third grid (G3) is impressed with a higher potential than a potential impressed on said second grid (G2) and said fourth grid (G4) is impressed with a lower potential than said potential impressed on said -third grid (G3), thereby forming such an axial potential distribution of said cathode ray tube that a maximum potential is produced at an axial position of said third grid (G3), and thereafter the axial potential gradually decreases towards a minimum potential at an axial position of said fourth grid (G4), and further increases gradually towards an axial region of said fifth grid (G5), thereby forming substantially a single main lens at a region ranging from said third grid (G3) to said fifth grid (G5). 2. A cathode ray tube in accordance with claim 1, wherein electric field between said second grid (G2) and said third grid (G3) is selected to be 105 V/cm-S X l0 V/cm. 3. A cathode ray tube in accordance with claim z or claim 2, wherein potential of said third grid (G3) is 10 KV or lower. 4. A cathode ray tube in accordance with claim 1 or one of the other preceeding claims, wherein said third electrode (G3) and said fifth grid (G5) are each other connected in said cathode ray tube. 5. A cathode ray tube in accordance with claim 1 or one of the other preceding claims, which further comprises an auxiliary grid (G4-5) between said fourth and fifth grid (G4, G5), which is connected to said third grid (G3).

Inventors: SUZUKI, HIROSHI
Applicants: MATSUSHITA ELECTRONICS CORPORATION
Application year: 1983
Main classification: H01J29
IPC codes: H01J29