Color picture tube having an inline electron gun with an astigmatic prefocusing lens

Abstract: An improved color picture tube includes an inline electron gun (26) for generating and directing three inline electron beams (28) along coplanar beam paths toward a screen (22). The gun includes a plurality of electrodes (44,46, 48,50,52,56) which form a beam-forming region (L1), a prefocusing lens (L2), and a main focusing lens (L3) for the electron beams. The improvement resides within the prefocusing lens, which includes four active surfaces. At least one of the active surfaces has asymmetric prefocusing recesses formed therein.

Description: The invention is concerned with a color cathode-ray tube including the features as mentioned in the preamble of claim 1. From EP-0 300 705 A2 a color picture tube is known which includes an electron gun for generating and directing three inline electron beams, a center beam and two side beams, along initially coplanar paths toward a screen of the tube. The gun includes three spaced electrodes which form an einzel lens in the path of each electron beam to form a main focus lens for focusing the electron beams. A first einzel lens electrode includes a first portion having three inline apertures that are set back from a second portion forming a single large aperture through which all three electron beams pass. A second einzel lens electrode includes a first portion having three inline apertures that are set back from second portion forming a single large aperture through which all three electron beams pass. The second portion of the first einzel lens electrode faces the second portion of the second einzel lens electrode. The second einzel lens electrode also includes a third portion having three inline apertures that are set back from a fourth portion forming a single large aperture through which all three electron beams pass. A third einzel lens electrode includes a first portion having three inline apertures set back from a second portion forming a single large aperture through which all three electron beams pass. The fourth portion of the second einzel lens electrode faces the second portion of the third einzel lens electrode. An inline electron gun for a color cathode-ray tube, as known from GB-21 01 804 A, includes a main focus lens formed by two spaced electrodes each having three separate inline apertures and a peripheral rim, and the apertured portion of each electrode is within a recess set back from the rim, wherein the width of the rim of one of the electrodes is narrower at the side beam paths than at the center beam path, measured perpendicular to the plane containing the electron beam paths. Also the recess in electrode has a width at the sidebeam paths that is wider than its width at the centre beam paths. By thus modifying the peripheral rim geometry the focus fields acting on the electron beams is balanced. In a further arrangement, the electrode is fabricated from two elements. Further, from EP-0 301 648 A3, a color cathode-ray tube is known, which has an integrated electrode gun structure including mirrored main focusing and accelerating electrodes. The electron gun comprises a triode section formed by three inline arranged cathodes and a first and second grid electrodes whose apertures are symmetrically disposed about respective central and outer axes which pass through their respective cathodes. A prefocusing electrode having eccentric outer apertures is positioned next to the second electrode. The field produced by the prefocusing electrode serves to converge the electron beams produced in the triode section. At least one, but more conveniently two, further electrodes are provided between the prefocusing electrode and the main focusing lens electrode. The outer apertures of at least one of the further electrodes are elongated to provide an asymmetrical lens field which is used to neutralise spot errors, beam displacement and beam asymmetry. The shapes of the elongated apertures (44) are such that at a portion of their peripheries are concentric about the axes. An electron gun, such as a six electrode gun, designed for use in a large screen entertainment-type color picture tube must be capable of generating small-sized high-current electron beam spots over the entire screen. A conventional television receiver utilizes a color picture tube with an inline electron gun and a self-converging deflection yoke, for providing a horizontal deflection field having a pincushion-shaped distortion and a vertical deflection field having a barrel-shaped distortion. The fringe fields of such a yoke introduce into the tube strong astigmatism and deflection defocusing caused, primarily, by vertical overfocusing and, secondarily, by horizontal underfocusing of the deflected electron beams. Beam spots formed by the electron beams passing through such distorted horizontal and vertical deflection fields are asymmetrically-shaped when deflected to the periphery of the screen. Additionally, many inline electron guns exhibit a misconvergence of the outer electron beams due to a change in the strength of the electron lens caused by changes in the focus voltage. Such a misconvergence results in a variation in beam landing position with changes in focus voltage. The object of this invention is to provide for an inline electron gun generating electron beams with improved convergence even with changes of the focus voltage. This object is being achieved by the features of claim 1. Preferred embodiments of the invention are being defined by the subclaims. The invention addresses the above indicated problems in an expeditious and cost effective manner without sacrificing performance. The present invention provides an improvement in a color picture tube having an inline electron gun for generating and directing three inline electron beams along coplanar beam paths toward a screen. The gun includes a plurality of electrodes which form a beam-forming region, a prefocusing lens and a main focusing lens for the electron beams. The improvement resides within the prefocusing lens,which includes four active surfaces. At least one of the active surfaces has asymmetric prefocusing means formed therein. In the drawings: Fig. 1 is a plan view, partially in axial section, of a shadow mask color picture tube embodying the invention. Figs. 2 and 3 are schematic axial section side views of electron guns with which the invention may be employed. Fig. 4 is an axial section top view of a novel electron gun in accordance with the present invention. Fig. 5 is a partial section top view of a first embodiment of the prefocusing lens of the present invention. Fig. 6 is a section view of an electrode of the prefocusing lens of Fig. 5, taken along line 6-6. Fig. 7 is a graph of the beam current density contour at the center of the screen for an electron gun utilizing the prefocusing lens electrode of Fig. 5. Figs. 8 and 9 are section views of the electron gun shown in Fig. 4, taken along lines 8-8 and 9-9. Fig. 10 is a partial section top view of a second embodiment of the prefocusing lens of the present invention. Fig. 11 is a section view of an electrode of the prefocusing lens of Fig. 10, taken along line 11-11. Fig. 12 is a graph of the beam current density contour at the center of the screen for an electron gun utilizing the prefocusing lens of Fig. 10. Fig. 13 is a partial section top view of a third embodiment of the prefocusing lens of the present invention. Fig. 14 is a graph of the beam current density contour at the center of the screen for an electron gun utilizing the prefocusing lens of Fig. 13. Fig. 15 is a partial section top view of a fourth embodiment of the prefocusing lens of the present invention. Fig. 16 is a graph of the beam current density contour at the center of the screen for an electron gun utilizing the prefocusing lens of Fig. 15. Fig. 17 is a section view of a prior embodiment of an electrode of the prefocusing lens. Fig. 18 is a graph of the beam current density contour at the center of the screen for an electron gun using the prior prefocusing lens electrode of Fig. 17. Fig. 1 shows a rectangular color picture tube 10 having a glass envelope 11 comprising a rectangular faceplate panel 12 and a tubular neck 14 connected by a rectangular funnel 16. The panel 12 comprises a viewing faceplate 18 and a peripheral flange or sidewall 20 which is sealed to the funnel 16 with a frit seal 21. A mosaic three-color phosphor screen 22 is located on the inner surface of the faceplate 18. The screen is preferably a line screen with the phosphor lines extending substantially perpendicular to the high frequency raster line scan of the tube (normal to the plane of Fig. 1). Alternatively, the screen could be a dot screen. A multiapertured color selection electrode or shadow mask 24 is removably mounted, by conventional means, in predetermined, spaced relation to the screen 22. An improved inline electron gun 26, shown schematically by dashed lines in Fig. 1, is centrally mounted within the neck 14 to generate and direct three electron beams 28, along coplanar convergent beam paths, through the mask 24 to the screen 22. The tube of Fig. 1 is designed to be used with an external magnetic deflection yoke, such as the yoke 30, located in the neighborhood of the funnel-to-neck junction. When activated, the yoke 30 subjects the three beams 28 to magnetic fields which cause the beams to scan horizontally and vertically, in a rectangular raster, over the screen 22. The initial plane of deflection (at zero deflection) is shown by the line P-P in Fig. 1, at about the middle of the yoke 30. Because of fringe fields, the zone of deflection of the tube extends axially from the yoke 30 into the region of the gun 26. For simplicity, the actual curvature of the deflection beam paths in the deflection zone is not shown in Fig. 1. The inline electron gun 26 includes six electrodes, G1 through G6, in addition to the cathodes, K. The gun may be of a first type 26', shown in Fig. 2, in which the G2 and G4 electrodes are interconnected and operated at a first potential, and the G3 and G5 electrodes are interconnected and operated at a second potential; or the gun may be of a second type 26'', shown in Fig. 3, in which the G3 and G5 electrodes are interconnected and operated at a third potential, and the G4 and G6 electrodes are interconnected and operated at a fourth potential. In each of the electron guns 26' and 26'', three electron lenses, L 1 , L 2 , and L 3 , are formed by the above-identified electrodes. The present invention relates primarily to the second or prefocusing lens, L 2 . The details of a first embodiment of the novel electron gun 26' are shown in Figs. 4 through 9. With reference to Fig. 4, the gun 26' comprises three equally spaced coplanar cathodes 42 (one for each beam); a control grid 44 (G1); a screen grid 46 (G2); a third electrode 48 (G3); a fourth electrode 50 (G4); a fifth electrode 52 (G5), the G5 electrode including a portion G5' identified as element 54, for a purpose described below; and a sixth electrode 56 (G6). The electrodes are spaced from the cathodes, in the order named, and are attached to a pair of support rods (not shown). The G1 electrode 44, the G2 electrode 46 and a first portion 72 of the G3 electrode 48, facing the G2 electrode 46, comprise a beam-forming region of the electron gun 26' and form the first electron lens, L1. Another portion 74 of the G3 electrode 48, the G4 electrode 50 and the G5 electrode 52 comprise an asymmetric prefocusing or second electron lens, L2, one embodiment of which is shown in Fig. 5. The element (G5' electrode portion) 54 and the G6 electrode 56 comprise a third or main focusing lens L3. Each cathode 42 comprises a cathode sleeve 58 closed at its forward end by a cap 60 having an end coating 62 of an electron emissive material thereon, as is known in the art. Each cathode 42 is indirectly heated by a heater coil (not shown) positioned within the sleeve 58. The G1 and G2 electrodes, 44 and 46, are two closely spaced, substantially flat, plates each having three inline apertures, 64 and 66, respectively, therethrough. The apertures 64 and 66 are centered with the cathode coating 62 to initiate three equally-spaced coplanar electron beams 28 (shown in Fig. 1), which are directed towards the screen 22. Preferably, the initial electron beam paths are substantially parallel, with the middle path coinciding with the central axis, A-A, of the electron gun. The G3 electrode 48 includes a substantially flat outer plate portion 68 having three inline apertures 70 therethrough, which are aligned with the apertures 66 and 64 in the G2 and G1 electrodes, 46 and 44, respectively. The G3 electrode 48 also includes a pair of cup-shaped first and second portions, 72 and 74, respectively, which are joined together at their open ends. The first portion 72 has three inline apertures 76, formed through the botton of the cup, which are aligned with the apertures 70 in the plate 68. The second portion 74 of the G3 electrode has three apertures 78 formed through its bottom, which are aligned with the apertures 76 in the first portion 72. Extrusions 79 surround the apertures 78. Alternatively, the plate portion 68, with its inline apertures 70, may be formed as an internal part of the first portion 72. As shown in Fig. 5, the G4 electrode 50 comprises a plate having identically-shaped recesses 51a and 51b formed in the opposed major surfaces thereof. Three inline apertures 80 are formed through the body of the electrode 50, within the recesses 51a and 51b, and aligned with the apertures 78 in the G3 electrode 48. Again with respect to Fig. 4, the G5 electrode 52 is a deep-drawn, cup-shaped member having three apertures 82, surrounded by extrusions 83, formed in the bottom end thereof. A substantially flat plate member 84, having three apertures 86, aligned with the apertures 82, is attached to and closes the open end of the G5 electrode 52. A first plate portion 88, having a plurality of openings 90 therein, is attached to the opposite surface of the plate member 84. The G5' electrode portion 54 comprises a deep-drawn, cup-shaped member having a recess 92 formed in the bottom end thereto, with three inline apertures 94 extending therethrough. Extrusions 95 surround the apertures 94. The opposite open end of the G5' electrode portion 54 is closed by a second plate portion 96 having three openings 98 formed therethrough. The openings 98 are aligned and cooperate with the openings 90, in the first plate portion 88, in a manner described below. The G6 electrode 56 is a cup-shaped, deep-drawn member having a large opening 100 at one end through which all three electron beams pass, and an open end which is attached to and closed by a plate member 102 that has three apertures 104 therethrough which are aligned with the apertures 94 in the G5' electrode portion 54. Extrusions 105 surround the apertures 104. The shape of the recess 51b, formed in the G4 electrode 50, is shown in Fig. 6. The recesses 51a and 51b have a uniform vertical height at each of the apertures 80 and have rounded ends. Such a shape has been referred to as the "race-track" shape. The recess 92, formed in the bottom end of the G5' electrode portion 54, is also race-track shaped, but it differs dimensionally from the recesses 51a and 51b in the G4 electrode 50 as described below. The shape of the large opening 100 in the G6 electrode 54 is shown in Fig. 8. The opening 100 is vertically higher at the outside apertures 104 than it is at the center aperture. Such a shape has been referred to as the "dog-bone" or "barbell" shape. With respect to Fig. 4, the first plate portion 88 of the G5 electrode 52 faces the second plate portion 96 of the G5' electrode portion 54. The apertures 90 in the first plate portion 88 have extrusions extending from the plate Portion that have been divided into two segments, 106 and 108, for each aperture. The apertures 98 in the second plate portion 96 of the G5' electrode portion 54 also have extrusions extending from the plate portion 96 that have been divided into two segments, 110 and 112, for each aperture. As shown in Fig. 9, the segments 106 and 108 are interleaved with the segments 110 and 112. These segments are used to create quadrupole lenses in the paths of each electron beam when different potentials are applied to the G5 and G5' electrode and electrode portion 52 and 54,respectively. By proper application of a dynamic voltage differential to either the G5 electrode 52 or the G5' electrode portion 54, it is possible to use the quadrupole lenses established by the segments 106,108,110 and 112 to provide an astigmatic correction to the electron beams,which compensates for astigmatism occurring in either the electron gun or the deflection yoke. Such a quadrupole lens structure is described in U.S. Pat. No. 4,731,563, issued to Bloom et al. on March 15, 1988. The novel second lens, L2, of the present invention does not require the use of a quadrupole lens formed by the above-described G5 and G5' electrodeand electrode portion, 52 and 54,respectively. A unitized G5 electrode, fabricated by eliminating the first and second plate portions 88 and 96 and attaching together the open ends of elements 52 and 54, may be used; however, such a gun structure would not provide an optimized deflected electron beam shape, although it might be useful where a tradeoff between performance and cost is permissible. Specific dimensions of a computer modeled electron gun for the first preferred embodiment are presented in TABLE I. In the embodiment presented in TABLE I, the electron gun is electrically connected as shown in Fig. 2. Typically, the cathode operates at about 150V, the G1 electrode at ground potential, the G2 and G4 electrodes are electrically interconnected and operate within the range of about 300V to 1000V, the G3 and G5 electrodes also are electrically interconnected and operate at about 7650V, and the G6 electrode operates at an anode potential of about 25kV. In the electron gun 26', the first lens, L1 (Fig. 2), provides a symmetrically-shaped, high quality electron beam into the second lens, L2. The first lens, L1, comprises the beam forming region of the gun and includes the G1 electrode 44, the G2 electrode 46, and the first portion of the G3 electrode 48 adjacent the G2 electrode. The second lens, L2, is the novel asymmetric prefocusing lens and comprises the G4 electrode 50 and the adjacent portions of the G3 electrode 48 and the G5 electrode 52. In the first embodiment, the identical pair of recesses, 51a and 51b, are formed in the opposed, major, active surfaces of the G4 electrode 50 (see, e.g., Figs. 5 and 6). While the recesses are preferably race-track shaped, other shapes, e.g., rectangular, which produce the effect described below may be used. The active, facing surfaces of the G3 and G5 electrodes, 48 and 52, respectively, are substantially flat. The combination of the above-described active elements produces quadrupole fields which form the asymmetric or astigmatic prefocusing lens which provides a horizontally-elongated electron beam (not shown) into the third or main focusing lens, L3. By providing the astigmatic focusing correction in the prefocusing lens, L2, beyond the electron beam cross-over point which occurs within the first lens, L1, the effectiveness of each quadrupole field is substantially independent of changes in the beam current. Additionally, the race-track-shaped recesses, 51a and 51b,provide a preconverging action which eliminates misconvergence of the outer beams at the screen, due to changes in the focus voltage,by providing a compensating change in the strength of the prefocusing lens, L2. While the invention is described in terms of two recesses, it is possible to achieve the same results by forming only one recess in either surface of the G4 electrode 50. The single recess would have a greater depth than either of the recesses 51a or 51b,and the lateral dimensions,i.e., vertical height and horizontal width, would be less than those of either of the recesses, to provide equivalent asymmetric and convergence corrections to the beams. The dimensions of the single recess would depend upon the extent of beam corrections required. The main focusing lens, L3, formed between the G5' electrode portion 54 and the G6 electrode 56, also is an asymmetric lens, having low aberration, which provides a vertically elongated, or asymmetrically-shaped, electron beam spot at the center of the screen. The spacing between adjacent apertures 94 in the G5' electrode portion 54 and the apertures 104 in the G6 electrode 56 is 6.22mm, rather than the 6.60mm aperture-to-aperture spacing that exists from the cathodes to the apertures 82 in the bottom G5 electrode 52. This reduced main lens aperture-to-aperture spacing ensures that the preconverged outer beams pass through low-aberration regions of the main lens, L3, to minimize coma distortions. A graph of a computer simulation of the electron beam spot at the center of the screen of a 27V110 o tube, operated at a cathode drive voltage of 103.2V, a G3/G5 focus voltage of 7650V, an ultor voltage of 25kV and 4mA beam current, is shown in Fig. 7. The beam spot is elliptically-shaped along the vertical axis to reduce the overfocusing action of the yoke when the beam is deflected. The undeflected, center beam spot includes a substantially rectangularly-shaped 90% peak beam current density portion, which is circumscribed by larger elliptically-shaped 50% and 5% peak beam current density portions. The size of the 5% peak beam current density spot is about 2.5mm x 4.2mm (HxV). With the width of the G4 recesses 51a and 51b as specified in TABLE I, and the overall length of the gun from the G3 bottom to the top of the G5' electrode portion adjusted to 35.05mm, the focus voltage is kept below 7700V, and the misconvergence of the outer beam is reduced to substantially zero. By utilizing the multipole lens described with respect to Fig. 4, and applying to the G5' electrode portion 54 a dynamic differential focus voltage that ranges from the potential on the G5 electrode 52, with no deflection, to about 1000 volts more positive at maximum deflection, the beam current density spot size can be optimized when the beams are deflected to the periphery of the screen. This mode of operation is discussed in U.S. Pat. No. 4,764,704, issued to New et al. on Aug. 16, 1988. A second embodiment of the present invention is obtained by increasing the length of the G3 electrode 148 to 5.84mm, from the value of 5.08mm shown in TABLE I, and modifying the asymmetric prefocusing lens, L2, as shown in Fig. 10. In the second embodiment of the lens L2, the G4 electrode 150 comprises a substantially flat plate having a thickness of about 0.025 inch (0.64mm), with circular apertures 180 formed through the oppositely disposed, active, major surfaces thereof. The active surfaces of the facing G3 and G5 electrodes,148 and 152, respectively, have rectangular slots enclosing the electron beam apertures. As shown in Fig. 11, each of the slots 149, in the G3 electrode 148, has a slot width, W, of 5.82mm, and a slot height, H, of 10.16mm. Also, each of the slots 149 has a depth, d, of 0.76mm, shown in Fig. 10. The slot-to-slot spacing, S, shown in Fig. 11, is 7.11mm. Inasmuch as the aperture-to-aperture spacing, s, within the prefocusing lens, L2, is 6.60mm, and the slot-to-slot spacing, S, is 7.11mm, it can be seen,in Fig. 11, that the two outer slots 149 in the G3 electrode 148 are displaced outwardly relative to the outer apertures 178 formed therein. This displacement of the slots 149 in the G3 electrode, and a similar displacement of the identically-dimensioned slots 153 in the G5 electrode 152, cooperate to form an asymmetric prefocusing lens, L2, which provides a horizontally-elongated electron beam (not shown) into the third lens, L3. The novel slot configuration in the G3 and G5 electrodes 148 and 152, respectively, also provides a preconverging action to eliminate misconvergence of the outer beams at the screen,in a manner similar to that described for the first embodiment. A computer simulation of the resultant vertically-elongated beam spot at the center of the screen is graphically shown in Fig. 12. When operated at an ultor voltage of 25kV and 4mA beam current in a 27V110 o tube, the beam sizes at 90% and 50% peak current density are comparable to those of the first embodiment, shown in Fig. 7, and the beam size at 5% peak current density is about 2.26mm x 3.68mm (HxV), at a cathode drive voltage of 103.2V and a G3/G5 focus voltage of 7650V. All other gun parameters are as listed in TABLE I. Equivalent performance can be achieved by forming the slots in only one of the active surfaces, i.e., in either the G3 electrode 148 or the G5 electrode 152. Slots formed in only one active surface must be deeper than the slots described above, and the small dimension of each slot must be reduced, while the amount of outer slot offset must be increased. A third embodiment of the present invention is achieved by modifying the electron gun to provide the electrical configuration shown in Fig. 3. The asymmetric prefocusing lens, L2, of the gun 26'' is shown in Fig. 13. The length of the G3 electrode 248 is maintained at 5.84mm, the same dimension utilized in the second embodiment, and a race-track-shaped recess 249 is formed in the active, major surface of the G3 electrode facing the G4 electrode 250. The recess 249 has a horizontal width of 19.43mm, a vertical height of 5.84mm and a depth of 0.76 mm. An identically-shaped and dimensioned race-track recess 253 is formed in the active surface of the G5 electrode 252, facing the substantially flat G4 electrode 250. While the race-track shape is preferred, other geometric shapes which provide an asymmetric lens with a preconvergence correction may be used. In the third embodiment, the G4 electrode 250 has a thickness of about 0.64mm, with circular apertures 280 formed therethrough. The asymmetric prefocusing lens, L2, of the third embodiment provides the preconverging action, and forms horizontally-elongated electron beams (not shown), as previously described, into the third lens, L3. A computer simulation of the resultant vertically-elongated beam spot at the center of the screen is graphically shown in Fig. 14. When operated at an ultor/G4 voltage of 25kV and 4mA beam current in a 27V110 o tube, the beam size and shape at 90% peak beam current density is larger and more elliptical than in the first and second embodiments, while at 50% peak beam current density the elliptically-shaped spot is more vertically elongated than in the first two embodiments. At 5% peak beam current density, the beam spot size is about 1.94mm x 3.44mm (HxV). The cathode drive voltage in this embodiment is 103.2V, the G3/G5 focus voltage is 7650V and the G2 voltage is typically about 400V. All other gun parameters are as listed in TABLE I. As described above, a single recess can be formed in either the active surface of the G3 or G5 electrodes, 248 or 252, respectively, if the depth is increased and the lateral dimensions are suitably reduced to provide equivalent performance. A fourth embodiment of the asymmetric prefocusing lens, L2, is shown in Fig. 15. The length of the G3 electrode 348 is 5.08mm, and the active surface facing the G4 electrode 350 is substantially flat,with three circular apertures 378 formed therethrough. The apertures 378 have a diameter of 4.01mm. The G4 electrode 350 has rectangular slots 350a and 350b formed in the opposed major active surfaces thereof, with the slots 350a facing the G3 electrode 348 and the slots 350b facing the G5 electrode 352. Each of the slots 350a and 350b has a width of 5.79mm, a height of 10.16mm and a depth of 0.76mm. The slot-to-slot spacing is 7.01mm. The circular apertures 380, formed through the G4 electrode 350, have a diameter of 4.01mm and are enclosed within the rectangular slots 350a and 350b, in the same manner as discussed with respect to the slots shown in Fig. 11. The active major surface of the G5 electrode 352 facing the G4 electrode 350 also is substantially flat, with three circular apertures 382 formed therethrough. The apertures 382 also have a diameter of 4.01mm. Inasmuch as the aperture-to-aperture spacing within the prefocusing lens, L2, is 6.60mm and the slot-to-slot spacing of the slots 350a and 350b of the G4 electrode 350 is 7.01mm, the two outer slots are displaced outwardly relative to the outer apertures 380 formed within the slots. The configuration and displacement of the G4 slots form an asymmetric lens which provides the preconverging action and horizontally-elongated electron beams (not shown), as previously described, into the third lens, L3. A computer simulation of the resultant vertically-elongated beam spot at the center of the screen is graphically shown in Fig. 16. The beam spot shape is similar to that shown in Fig. 14. When operated at an ultor/G4 voltage of 25kV and 4mA beam current in a 27V110 o tube, the beam size at 5% peak beam current density is about 1.96mm x 3.49mm (HxV), at a cathode drive voltage of 103.2V and a G3/G5 focus voltage of 7700V. The G2 voltage in this embodiment is typically about 400V. All other gun parameters are as listed in TABLE I. Alternatively, slots can be formed in only one of the active surfaces of the G4 electrode 350. The depth of the slots must be increased,and the small dimension of each slot must be decreased,from the respective dimensions described immediately above. Additionally, the amount of offset of the outer slots must be increased to obtain performance equivalent to that of the fourth embodiment. The novel electron gun of the present invention is to be contrasted to an electron gun of the type described in U.S. Pat. No. 4,764,704, referenced above. In that patent, a G4 electrode, similar to the G4 electrode 450 of the prefocusing, or second, lens shown in Fig. 17 herein, has rectangularly-shaped apertures 480 therethrough. Specific dimensions of a computer model of an embodiment of that prior electron gun are presented in TABLE II. That embodiment has the electrical configuration shown in Fig. 2 herein, and is similar in construction to the electron gun shown in Fig. 4 herein, with similar gun elements being identified with corresponding numbers, prefixed by the number "4". In the prior electron gun described in TABLE II, the cathode operates at a drive voltage of about 103.2V, the G1 electrode is at ground potential, the G2 and G4 are electrically interconnected and operate within the range of 300V to 1000V, the G3 and G5 electrodes also are interconnected and operate at about 6600V,and the G6 electrode operates at an anode potential of about 25kV. The prefocusing lens, L2, of the prior electron gun, with the rectangular apertures 480 formed through the substantially flat G4 electrode 450, provides a horizontally-elongated electron beam (not shown) into the main focusing lens, L3. A computer simulation of the resultant vertically-elongated beam spot at the center of the screen is graphically shown in Fig. 18. The beam size at 5% peak current density is about 2.30mm x 3.49mm (HxV) at the previously described operating parameters. CONCLUSION The performance of the present prefocusing lens, L2, of embodiments 1 through 4, as measured by the resultant electron beam spot size on the screen, is comparable to that of the prior electron gun described in U.S. Pat. No. 4,764,704, which utilizes a prefocusing lens having rectangularly-shaped apertures in the G4 electrode thereof. A comparison of results is contained in TABLE III. EMBODIMENT Beam Spot Size on Screen Horizontal (mm) Vertical (mm) 1 2.50 4.20 2 2.26 3.68 3 1.94 3.44 4 1.96 3.49 Prior 2.30 3.49 The four embodiments of the present electron gun structure provide ease of manufacturing, because the use of circular apertures throughout the electron gun reduces the misalignment problems posed by the rectangularly-shaped G4 apertures of the prior gun. Additionally, the prior gun requires a slight increase in the G3 aperture-to-aperture spacing (from 6.60mm to 6.69mm) to eliminate the misconvergence of the outer electron beams with changes in focus voltage. The present device achieves comparable performance by controlling either the horizontal width of the race-track-shaped recesses within the prefocusing lens, L2, in embodiments 1 and 3, or the slot-to-slot spacing of the rectangular slots formed within prefocusing lens, L2, in embodiments 2 and 4. In each of the four embodiments, the aperture-to-aperture spacing from the cathode 42 to the bottom of the G5 electrode 52 is maintained at a constant value of 6.60mm, thereby simplifying the assembly and alignment of the gun components.

Claims: A color cathode-ray tube including an envelope having therein an inline electron gun for generating and directing three inline electron beams, a center beam and two outer beams, along initially coplanar beam paths toward a screen on an interior portion of said envelope, said gun having six electrodes which form a beam-forming region, a prefocusing lens and a main focusing lens, said prefocusing lens including four active surfaces, at least one of which has one or more recesses formed therein; characterized in that said active surfaces provide quadrupole fields which provide astigmatic prefocusing, and said one or more recesses (51a, 51b; 149, 153; 249, 253; 350a, 350b) provide a preconverging action on said outer electron beams, three circular apertures (80; 178, 182; 278, 282; 380) being provided within said one or more recesses. The tube as described in claim 1, wherein said beam-forming region includes a first electrode, a second electrode and a first portion of a third electrode for providing substantially symmetrically-shaped beams to said prefocusing lens, said prefocusing lens comprises a second portion of said third electrode, a fourth electrode and a first portion of a fifth electrode and provides asymmetrically-shaped beams to said main focusing lens, and said main focusing lens comprises a second portion of said fifth electrode and a sixth electrode and is a low aberration lens; characterized in that said fourth electrode (50; 350) has substantially identical asymmetric beam-focusing recesses (51a, 51b; 350a, 350b) formed in the oppositely disposed active surfaces thereof, three separate inline circular apertures (80; 380) being disposed within each of said recesses. The tube as described in claim 2, characterized in that a single recess (51a, 51b) is formed in each active surface of said fourth electrode (50). The tube as described in claim 2, characterized in that three separate, substantially rectangular recesses (350a, 350b), comprising two outer recesses and a center recess, are formed in each active surface of said fourth electrode (350). The tube as described in claim 4, characterized in that each of said outer recesses (350a, 350b) has an outer aperture (380) therethrough and is displaced outwardly relative to said outer aperture. The tube as described in claim 1, wherein said beam-forming region includes a first electrode, a second electrode and a first portion of a third electrode for providing substantially symmetrically-shaped beams to said prefocusing lens, said prefocusing lens comprises a second portion of said third electrode, a fourth electrode and a first portion of a fifth electrode and provides asymmetrically-shaped beams to said main focusing lens, and said main focusing lens comprises a second portion of said fifth electrode and a sixth electrode and is a low aberration lens; characterized in that said second portion of said third electrode (148; 248) and said first portion of said fifth electrode (152; 252) have substantially identical, asymmetric beam-focusing recesses (149, 153; 249, 253) formed in the active surfaces thereof, three separate inline circular apertures (178, 182; 278, 282) being disposed within each of said recesses. The tube as described in claim 6, characterized in that a single recess (249, 253) is formed in each of said second portion of said third electrode (248) and said first portion of said fifth electrode (252). The tube as described in claim 6, characterized in that three separate, substantially rectangular recesses (149, 153), comprising two outer recesses and a center recess, are formed in said active surfaces of said second portion of said third electrode (148) and of said first portion of said fifth electrode (152). The tube as described in claim 8, characterized in that each of said outer recesses (149, 153) has a circular aperture (178, 182) therethrough, said outer recesses being displaced outwardly relative to the outer circular apertures.

Inventors: NEW DAVID ARTHUR, NEW, DAVID ARTHUR
Applicants: RCA THOMSON LICENSING CORP, RCA THOMSON LICENSING CORPORATION
Application year: 1991
Main classification: H01J29
IPC codes: H01J29, H04N9