IMPROVED MESH SCAN EXPANSION CRT LENS

Abstract: Mesh lens (12) for PDA-type cathode-­ ray tubes (10) is constructed in a manner that permits deformation into a concavo-convex shape with a substantially shorter radius of curvature than heretofore obtainable with prior art devices. The mesh lens (12) particularly comprises a multitude of interconnected webs (58) forming an array of apertures (60). Each web (58) has opposing ends and a midline (62) extending between those ends. The mesh (12) is configured so that an individual aperture (60) of the array is formed by a set of webs (58) interconnected at their ends. The midline (62) of each web in the undeformed mesh defines a bent line. The mesh can be deformed into a concavo-convex shape having a relatively short radius of curvature. This is so because the individual webs of the mesh respond to the application of deformation forces by initially straightening, thereby effectively delaying the development of tensile stresses in the webs.

Description: Background of the Invention The present invention relates generally to electrostatic scan expansion lens systems for electron discharge tubes, such as cathode-ray tubes, and more particularly to an improved mesh-type scan expansion lens for such tubes. Cathode-ray tubes (CRT's) include an evacuated envelope comprising a tubular neck and a generally frustum-shaped funnel portion that is contiguous with one end of the neck and diverges outwardly therefrom. The outer end of the funnel portion is sealed against a face plate that carries a phosphorescent display screen. An electron gun is positioned within the neck at the end opposite the funnel portion. The electron gun produces a beam of electrons that passes through the neck and funnel portion and illuminates a spot on the phosphorescent display screen. The beam also passes between two pairs of electrostatically charged deflection plates that are located in the neck between the electron gun and the display screen. The direction of the beam is deflected (hence, the position of the spot on the screen is changed) whenever a deflection voltage is applied to at least one of the pairs of deflection plates. The deflection voltage is continuously altered to deflect the beam so that, for example, a particular waveform is illuminated on the display screen. The amount of deflection of the beam for a given deflection voltage is known as deflection sensitivity. To increase the deflection sensitivity of a CRT is to increase the beam deflection without increasing the deflection voltage applied across the deflection plates. The brightness of the spot illuminated on the display screen by the beam is characterized as the display luminance. The display luminance is increased by increasing the velocity of the beam, which is accomplished by applying an accelerating voltage to the beam. It is usually desirable to construct a CRT with high display luminance and high deflection sensitivity. However, with conventional CRT designs, these performance goals usually conflict. Specifically, if the beam accelerating voltage is increased to raise the velocity of the beam and thereby increase the display luminance before the beam passes the deflection plates, the deflection sensitivity of the beam decreases. That is, the beam is more resistant to deflection. This is so because deflection sensitivity is inversely proportional to the accelerating voltage. This conflict traditionally has been resolved by deflecting the beam in a region of low potential, then increasing the beam velocity by means of a high-voltage field after the beam exits the deflection region. This technique is commonly known as post-deflection acceleration, or PDA. One type of PDA CRT creates the high-voltage field by placing an anode within the funnel portion of the CRT. Specifically, the anode comprises an electron-transparent conductive target layer overlying the display screen and an electrically connected continuous conductive film applied to the interior surface of the funnel portion. The electric field resulting from the presence of such an anode has increasing potential in the direction of beam travel and is, therefore, effective in accelerating the beam and increasing the display luminance. To enhance the deflection sensitivity of the CRT while simultaneously preventing penetration of the high-voltage field into the low-voltage deflection region, a field-forming mesh electrode is positioned within the tube between the deflection plates and the anode. The mesh comprises a multitude of interconnected elongate elements (referred to herein as webs) forming an array of apertures. When incorporated into the CRT, the mesh has a concavo-convex configuration and is positioned with its convex surface facing the display screen, as viewed in the direction opposite the beam propogation direction. As a result, the equipotential surfaces of the high-voltage field generally conform to the convex shape of the mesh. Since the forces created by the high-voltage field direct the electron beam to pass in a direction that is normal to the equipotential surfaces, the above-described force field created by the anode and mesh combination represents that of a diverging electron lens. That is, a beam passing through this field tends to diverge from the central longitudinal axis of the CRT. Accordingly, the beam divergence produced by this electron lens increases the deflection sensitivity of the CRT. If the radius of curvature of the mesh is reduced, the resulting curvature of the equipotential surfaces of the high-voltage field will cause correspondingly greater divergence of the beam. It can be readily appreciated that to achieve high deflection sensitivity, it is desirable to produce a mesh that is deformable into a concavo-convex shape having as short a radius of curvature as possible. A mesh is typically formed by electrodeposition of metal (for example, nickel) onto a planar mandrel. The resulting planar mesh is then annealed. The concavo-convex shape is formed by deforming the mesh within a curved mold. In the past, the mesh could be deformed by only a limited amount because too much deformation resulted in breakage of the fragile metal webs. Breakage results from tensile stresses that develop over the entire cross section of each web when the mesh is deformed. Metal, such as nickel, will strain ( i.e. , stretch) somewhat in response to the tensile stress but quickly reaches its tensile strength and breaks. As a consequence, the limited amount of curvature that could be formed into the mesh correspondingly limited the deflection sensitivity of the CRT into which the mesh was incorporated. U.S. Patent No. 3,240,972 is directed to a post-deflection acceleration (PDA) type cathode-ray tube having a mesh-like scan expansion electrode. The electrode is disposed in the path of the tube's electron beam, immediately adjacent the primary beam deflection region of the tube and between this region and the display screen to provide a radial electrostatic deflection field that increases the amount of beam deflection produced by an external deflection yoke. The mesh electrode preferably has a dome shape, as shown, for example, in Fig. 8 of the patent. This drawing figure shows the mesh electrode apertures as having a circular shape. The specification itself indicates that the electrode may consist of a woven mesh, an electro-formed integral cross grid structure, an apertured plate or various other constructions. French Patent Application 2,358,011 discloses a process for producing a hexagonal type honeycomb grid for an electron tube. The process includes the steps of forming a stack of thin, refractory metal foil sheets. Narrow strips of a second metal are deposited on one side of each sheet. These strips, which have a melting point slightly lower than that of the refractory metal, are of uniform length and are arranged parallel with and equidistant from each other, with the strips on alternate sheets being shifted laterally by one-half of the strip spacing distance. The resulting assembly is then compacted and soldered together along the strips by diffusion or sintering, after which it is expanded in a direction normal to the plane of the sheets to produce a honeycomb-like structure. Electrode elements are produced by filling the honeycomb structure with a resin, cutting it into thin slices, and then forming it to a desired curved profile, after which the resin is removed. Summary of the Invention This invention enables the provision of an improved mesh-type electron lens element that is constructed in a manner that permits deformation into a concavo-convex shape with a substantially shorter radius of curvature than heretofore obtainable with prior art devices. In accordance with this invention there is provided a planar deformable mesh for use in a scan expansion lens of an electron discharge tube, said mesh comprising a multitude of interconnected elongate elements disposed in a plane and forming an array of apertures, each elongate element having opposing ends and a midline extending between the ends, the mesh being configured so that each aperture of the array is formed by a set of elongate elements interconnected at their ends, and the mesh being further configured so that the midlines of at least some of the interconnected elongate elements are bent, e.g. the midline is curved or has a sharp corner. The invention also extends to a method of forming such a mesh, and a cathode-ray tube incorporating such a mesh. A mesh formed as just described can be deformed into a concavo-convex shape having a relatively short radius of curvature. This is so because the individual bent elongate elements or webs of the mesh respond to the application of deformation forces initially by straightening, thereby effectively delaying the development of tensile stresses that tend to break the web. A mesh formed in accordance with this invention, once deformed and incorporated into a CRT, creates an electron lens that has high beam deflection sensitivity. Brief Description of the Drawings FIG. 1 is a schematic longitudinal section view of a cathode-ray tube incorporating a mesh PDA scan expansion lens formed in accordance with this invention. FIG. 2 is a greatly enlarged plan view showing a portion of a conventional rectangular shaped mesh. FIG. 3 is a greatly enlarged plan view showing a portion of a mesh lens formed in accordance with this invention prior to deformation into a concavo-convex shape. FIG. 4 is a greatly enlarged plan view of a portion of a mesh lens formed in accordance with this invention after deformation into a concavo-convex shape. Detailed Description of a Preferred Embodiment A PDA-type CRT 10 incorporating a curved or dome-shaped mesh scan expansion lens 12 formed in accordance with this invention is shown in FIG. 1. The CRT 10 includes an evacuated envelope comprising a tubular glass neck 14 with a generally frustum-shaped ceramic funnel portion 16 attached to one end of the neck. The outer end of the funnel portion is sealed against a transparent glass face plate 18, which carries a phosphorescent display screen 20. An electron gun 22 is mounted within the neck at the end opposite the funnel portion 16. The electron gun is of conventional design, including a cathode and control grid unit 24, a first anode 26, a focus electrode 28 and a second anode 30. The gun produces an electron beam 32 that passes through the neck 14 and funnel portion 16 to illuminate a spot on the phosphorescent display screen. Prior to striking the display screen 20, the beam passes between adjacent pairs of vertical deflection plates 34 and horizontal deflection plates 36, respectively. The vertical and horizontal deflection plates are separated by a shield electrode 38. The beam passes through a cylindrical metal tube 40 that is positioned between the horizontal deflection plates 36 and the display screen 20. The end of tube 40 nearest the display screen carries the metal mesh 12 of the present invention. The mesh 12 is of concavo-convex shape, and positioned with the convex side of the mesh facing the display screen, as viewed in the direction opposite the beam propagation direction. The mesh is electrically connected through tube 40 and spring contacts 42 to a conductive band 44 that lines the inside surface of the neck adjacent to the funnel portion 16. Band 44 is maintained at the approximate mean potential of horizontal deflection plates 36, typically at or near ground potential, thereby establishing an essentially field-free region between the mesh 12 and the outer ends of the horizontal deflection plates 36. A high-voltage field present between the mesh 12 and the display screen 20 accelerates the beam after it exits the deflection region. The high voltage field is created by constructing an anode within the funnel portion 16 of the tube. More particularly, a thin electron-transparent aluminum target layer (not shown) is mounted to overlie display screen 20. The target layer is electrically connected to a conductive coating 46 that covers the inner surface of funnel portion 16. Coating 46, which is connected in a known manner to an external high-voltage source at +HV (typically in the range of about 13-20 KV), terminates near the outer perimeter of mesh 12 as shown in FIG. 1. The strong electrostatic field created within the funnel portion 16 accelerates the electrons of the beam 32 as they exit the curved mesh. The mesh 12 is a field-forming electrode imparting within the funnel portion a field curvature that corresponds to the shape of the mesh. This field is represented by a family of equipotential surfaces E in FIG. 1. The shape of the equipotential surfaces creates an electron lens that magnifies the beam deflection imparted by the deflection plates. This increase in beam deflection occurs in the absence of a corresponding increase in deflection voltage. Thus, the curved mesh 12 increases the deflection sensitivity of the CRT. It is apparent that the amount of beam deflection that occurs after the beam exits the deflection region depends to a great extent on the shape of the mesh 12 or, more particularly, the shape of the electrostatic field formed by the mesh. Thus, for a curved mesh 12 with a radius of curvature r₁ (FIG. 1), the radii of curvature of the equipotential surfaces E will generally correspond to radius of curvature r₁. However, if the mesh is constructed so that it can be deformed to have only a radius of curvature that is relatively longer than the first-mentioned radius of curvature r₁, the resulting equipotential surfaces will be correspondingly flatter. Such an alternatively shaped mesh is shown as 12′ in FIG. 1. Also illustrated is a single representative equipotential surface E′ resulting from the alternatively shaped mesh 12. It is clear that the electrostatic field conforming to the flatter ( i.e. , longer radius of curvature) mesh 12′ will result in relatively less deflection in the beam (illustrated as 32′ in FIG. 1) compared to deflection achieved with a mesh 12 having a relatively shorter radius of curvature. It is noteworthy that the mesh need not have a constant radius of curvature over the entire concave surface. The mesh may be formed into a curve having radii of curvature changing over the concave surface ( e.g. , a parabolic curve). In any event, the radii of curvature referred to herein are as measured along the central longitudinal or beam axis 48 of the CRT, since that axis is typically coincident with the shortest radius of curvature of the mesh. FIG. 2 shows, greatly enlarged, a portion of a prior art mesh 50 having square apertures 52 arranged in a repeating row/column pattern. The apertures 52 are defined by interconnected webs 54. A domed mesh of this type, designed for a modern, 100 MHz oscilloscope CRT, may have a thickness of 8 to 10 microns and a pitch of about 295 lines/cm (750 lines per inch). The webs 54 of the mesh are approximately seven microns wide. The mesh is formed by electrodeposition of a metal, such as nickel, on a planar mandrel. The mesh is then annealed. The planar mesh is then deformed into a smooth curve. This deformation is typically accomplished by securing the edge of the mesh and then forcing the central portion of the mesh into a concave mold. It is the planar version of a prior art mesh 50 that is depicted in FIG. 2. As shown in FIG. 2, the central axis or midline 56 of each web 54 defines a straight line. When the mesh is forced out of the planar shape and into the curved mold, tensile stresses immediately develop over the cross section of each web. To achieve the shortest possible radius of curvature in the mesh, the mold is designed so that the mesh will be deformed as much as possible without exceeding the tensile strength of the webs. The mesh of the present invention is configured so that when deformed, the development of tensile stresses tending to break the webs is delayed, thereby permitting a greater amount of deformation than would otherwise be possible. Specifically, with reference to FIG. 3, the mesh 12 comprises a multitude of interconnected webs 58 forming an array of apertures 60. Each aperture 60 is defined by a set of webs 58 that are interconnected at their ends. The mesh 12 is formed by a conventional electrodeposition process. Mesh 12 may have a thickness of 8 to 9.5 microns and a predeformation pitch of about 325 lines/cm (830 lines per inch). The webs 58 of the mesh are about 8 to 12 microns wide. Each of the webs 58 is bent such that the midline 62 of each web defines a bent line, e.g., a curved line or a line with a sharp corner. Each midline 62 defines a line that extends along the length of and bisects a web 58. In the preferred embodiment, the webs 58 are formed so that every other web in an even-numbered set of webs ( e.g. , six webs) defining a single aperture 60 is bent outwardly from the center 64 of the aperture. The intervening webs are bent inwardly toward the center 64 of the aperture. This scheme results in a regular array of substantially identically shaped apertures. The centers 64 are spaced apart by 30-52 microns. It is contemplated that the webs can be bent in any uniform or random fashion (for example, zigzag) as long as the midlines of the webs are bent, that is, not straight. In one exemplary embodiment, each of the webs 58 has a bend (e.g., a sharp corner) and a length that provides an array of apertures 60 having between the centers 64 a distance of 30 microns before deformation and a distance of 33 microns after deformation. For instance, webs shaped to define an array of circular apertures would not be suitable, since the midlines of those webs are essentially straight. When a mesh formed in accordance with this invention is deformed from a planar to a curved shape, the deformation forces initially straighten the bent webs. The straightening of the webs effectively delays the development of tensile stresses acting over the cross section of each web that would otherwise tend to break the webs apart. Accordingly, the mesh is deformable into a concavo-convex shape having a relatively short radius of curvature before the tensile strength of the metal mesh is reached. FIG. 4 depicts the resulting configuration of the mesh 12 of FIG. 2 after deformation into a concavo-convex shape. The webs 58 are deformed until they are substantially straight. Further, the shape of each aperture 60 after deformation is substantially hexagonal. This shape is a consequence of the predeformed configuration of the mesh 12 shown in FIG. 3. A mesh formed in accordance with the present invention can have a radius of curvature of about 0.825 cm, which is about 46.4% less than that achievable with a prior art mesh with webs whose midlines are essentially straight. While a preferred embodiment of this invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the scope of the invention as defined in the claims. For instance, it is not necessary that every web of the mesh be configured to have a midline that defines a bent line. A few webs, such as those near the edge of the mesh, may have straight midlines without substantially affecting the deformation of the mesh.

Claims: A planar deformable mesh for use in a scan expansion lens of an electron discharge tube, said mesh comprising a multitude of interconnected elongate elements disposed in a plane and forming an array of apertures, each elongate element having opposing ends and a midline extending between the ends, the mesh being configured so that each aperture of the array is formed by a set of elongate elements interconnected at their ends, and the mesh being further configured so that the midlines of at least some of the interconnected elongate elements are bent. The mesh of claim 1, wherein the mesh is formed of electrically conductive material. The mesh of claim 1 or 2, wherein each set of elongate elements is comprised of an even number of elongate elements; and wherein every other elongate element in the set is bent outwardly from the center of the aperture defined by that set, and wherein each elongate element interconnected between each said every other elongate element is bent inwardly toward the center of the aperture defined by that set. The mesh of claim 3, wherein each set of elongate elements comprises six elongate elements that are configured so that when deformed into a concavo-convex shape the aperture defined by each deformed set of elongate elements is substantially hexagonal. The mesh of claim 1 or 2, configured so that when deformed into a concavo-convex shape, the resulting shape of the apertures is substantially hexagonal. A method of constructing a curved mesh for a scan expansion lens, the method comprising constructing a planar mesh as claimed in any preceding claim, and deforming the mesh into a concavo-convex shape in such a manner that said midlines have, when the mesh is viewed in plan, a straighter configuration than they had prior to deformation. The method of claim 6 in which the elongate elements are formed by electrodepositing lines of metallic material on a planar mandrel. A mesh formed by a method according to claim 6 or 7. A cathode-ray tube, comprising: electron gun means positioned at one end of the tube for producing a beam of electrons directed along a beam axis in the tube; deflection means for deflecting the electron beam; and post-deflection acceleration means positioned adjacent the deflection means along the beam axis for accelerating the electron beam, the post-deflection acceleration means including an electrically conductive mesh as claimed in claim 8, the mesh being secured within the tube.

Inventors: MCGLOTHLAN, J. KIRK
Applicants: TEKTRONIX, INC.
Application year: 1988
Main classification: H01J29
IPC codes: H01J29, H01J9