Cathode ray tube comprising a non-rotationally symmetrical element
United States Patent [1 1 Scheele Apr. 29, 1975 CATHODE RAY TUBE COMPRISING A NON-ROTATIONALLY SYMMETRICAL ELEMENT [75] lnventor: Edial Francois Scheele. Emmusingel.  
 Eindhoven. Netherlands [73] Assignee: U.S. Philips Corporation, New  
 York. NY.  
 [22] Filed: Mar. 12, 1973 [2|] Appl. No.: 340,051  
 [30] Foreign Application Priority Data Mar. 24, 1972 Netherlands 7203931 [52] U.S. Cl. 313/453 [5 1] Int. Cl. H0lj 29/56 [58] Field of Search 313/86, 453  
 [56] References Cited UNITED STATES PATENTS 2.840.754 6/l958 Linder et ul. 313/86 2.884.559 4/1959 Cooper et al. 3 l 3/86 3.524.094 8/1970 Husker et al 3 l 3/86 3.579.010 5/l97l Jones 313/453 Primary E.\&#39;aminerVincent P. McGraw Assistant Examiner-Richard A. Rosenberger Attorney, Agent, or Firm-Frank R. Trifari; George B. Berka [57] ABSTRACT In a cathode ray tube comprising a non-rotationally symmetrical electron-optical element, the image ratio of the main lens is adapted to the degree of astigmatism of the electron beam such that an electron spot having a predetermined axial ratio is produced on a target. Such a cathode ray tube is notably an index colour television tube.  
 9 Claims, 5 Drawing Figures CATI-IODE RAY TUBE COMPRISING A NON-ROTATIONALLY SYMMETRICAL ELEMENT The invention relates to a cathode ray tube, comprising an image screen and an electron gun which is provided with a cathode, a control grid, an acceleration anode, a non-rotationally symmetrical electron-optical element, and a main lens for the formation of a target of an electron beam to be emitted by the cathode on the image plane.  
  A cathode ray tube of this kind is known, for example,from US. Pat. No. 2,058,482. An electron gun described therein comprises a control grid having a nonrotationally symmetrical aperture. Thereby, an electron beam to be emitted by a cathode is deformed into a non-rotationally symmetrical beam. Requirements are imposed as regards the relationship between the distance from the cathode to the control grid and the largest transverse dimension ofthe aperture in the control grid. Theseconditions must be satisfied so as to enable to realization of an electron beam of sufficient current intensity in the case of electron gunscomprising an acceleration anode having a comparatively large aperture.  
  The invention has for its object to&#39;provide a-cathode ray tube in which an accurately defined target having a selected axial ratio can be focussed on the image screen by means of a non-rotationally symmetrical electron-optical element in the electron gun. To this end. a cathode ray tube of the kind set forth according to the invention is characterized in that the nonrotationally symmetrical electron-optical element produces an emissive cathode surface having an oval contour and an astigmatic electron beam, the position of the main lens being adapted to the degree of astigmatism of the beam such that an image of the electron beam in the image plane constitutes an accurately defined target having a predetermined axial ratio.  
  In a cathode ray tube according to the invention, a target having a comparatively small width can be realized by a suitable choicelof the said quantities. The length of the target can thenbe exactly maintained within a predetermined value. The position of the main lens in a cathode ray tube is determined by other conditions in most cases. For example, the distance from the main lens to the image screen is usually determined by the type of tube. Because a maximum spot width may not be exceeded, the magnification factor of the main lens will have to remain below a given value. The position of the main lens between an object point of the electron beam and the image screen is determined by the given distance between the main lens and the image screen and an imposed magnification factor. By imparting a degree of astigmatism to the beam which corresponds to this position of the main lens, according to the invention a target can be realized which satisfies the requirements imposed. In particular, in a preferred embodiment according to the invention an oval target can be realized in an index colour tube by means of which a pure colour image of adequate resolution can be formed. The structure of the image screen in a tube of this kind allows an axial ratio of approximately for the target. By choosing the grid aperture of the electron gun as the non-rotationally symmetrical element, the cathode load and the space charge in the electron beam are reduced with respect to a corresponding rotationally symmetrical electron beam.  
  Some preferred embodiments of cathode ray tubes according to the invention will be described in detail hereinafter with reference to the drawings. In the drawmgs:  
  FIG. 1 is a diagrammatic sectional view of an electron gun which is suitable for a cathode ray tube according to the invention,  
  FIG. 2 shows a preferred embodiment of an assembly of a grid aperture, a first anode aperture and a diaphragm of a cathode. ray tube according to the invention,  
  FIG. 3 is a diagrammatic view of a beam path of the electron beam, measured in two symmetry planes of the electron gun,  
  FIG. 4 shows a graph in which a measure for the astigmatism of the electron beam is given as a function of the axial ratio of the grid aperture, and  
  FIG. 5 shows sectional views of an index colour tube according to this invention.  
  An electron gun as shown in FIG. 1 comprises a cathode l, a control grid 2, a first anode 3, a high voltage anode 4, a main lens electrode 5, and a second high voltage anode 6. In practical electron guns, the cylindrical electrodes are assembled to form one unit by means of mounting pins 7 and, for example, three glass rods 8. The cathode is preferably mounted in the control grid tube by means of ceramic rings. The mounting rings fix the distance between the cathode and the control grid. A filament 9 is mounted in the grid tube. The cathode consists, for example, of a dispenser cathode having a plate 10 of porous material such as sintered tungsten. An electron gun of this kind can have the following dimensions. Distance between the cathode and the limitation of the control grid facing the cathode microns. Thickness of the control grid 75 microns. Distance between the control grid and the first anode microns. Thickness of the first anode 150 microns. Distance between the first anode and the first high voltage anode 4 mm. Distances between the high voltage anodes and the main lens anode 2.5 mm. Length of the first high voltage anode 38 mm. Length of the main lens anode 30 mm. Length of the second high voltage anode 15 mm. Inner diameter of the anode sleeves 20 mm. On its side facing the control grid, the first high voltage anode is tapered off to 4.5 mm. However, for the invention it is irrelevant whether the gun is a tetrode gun as described or a triode gun in which is the first anode is omitted. The main lens can also be formed by an electromagnetic lens or an accelerating electrostatic lens instead of the described unipotential lens.  
  In a preferred embodiment, the control grid has an aperture 11 of the shape shown in FIG. 2, to be referred to hereinafter as a diamond shape. A diamond shape of this kind is composed of a central rectangle or square 12, in this case a square having a side of 250 microns, adjoined by two triangles 13 which are proportioned such that the overall length of the control grid aperture is 1250 microns. The first anode 3 comprises an aperture 14 which again has the shape of a diamond, the side of a corresponding square 15 having a length of 450 microns. Including the adjacent triangles 16, the overall length of the first anode aperture is 1350 microns. FIG. 2 also shows a diaphragm aperture 17. In this case the diaphragm aperture is circular and has a diameter of 1,000 microns. A diaphragm plate 18 in which the aperture 17 is situated is arranged in the first high voltage anode 4 as is shown in FIG. 1. The distance between the control grid and the diaphragm amounts to, for example mm. The position of the diaphragm in the anode sleeve is determined notably in that the diaphragm must be situated in a unipotential region. Lens-action on the diaphragm is thus prevented. A lens-action at this area would cause high spheric aberration because the electron beam fills the diaphragm substantially or even completely. The dimensions of the diaphragm aperture, which may also be elongated, can be adapted to the optimum position in the anode sleeve. The diaphragm determines a maximum value of the transverse dimension of the beam in the main lens and intercepts stray radiation, for example, caused by grid emission. In an active gun, the high voltage anodes carry a voltage of, for example. KV, the main lens anode carries approximately 7.5 KV and the first anode carries approximately 500 V. On the basis&#39;of measurements and calculations it can be demonstrated that the optical properties of the lens can be adequately described by means of a single main plane 19 in front of the main lens and situated in the centre of the electrode 5.  
  A broken line 20 in FIG. 2 denotes a contour of an emissive surface. The emissive surface of all grid apertures described here has an approximately elliptical limitation. The major axis of the emissive surface, to be measured along the line I-&#39;-I in FIG. 2, is directed along the major axis of the grid aperture, and the minor axis, to be measured along the line IIII, is perpendicular thereto. The electron beam comprises two symmetry planes which are given by the said lines I] and II-II and the optical axis of the system. In normal circumstances the target on the image screen has the same direction as the emissive surface. Hereinafter, the beam dimensions measured in the symmetry plane through 1-1 will be referred to as major axis, and those situated in the symmetry plane through IIII as minor axis. The ratio of the axes thus measured will be referred to as the ellipticity e, even if the axes are measured at a location where the beam section is not an ellipse. The axial ratio of the various grid-anode apertures will also be denoted by e. By means of the main lens, an object 21, situated at a distance p in front of the main plane 19, is imaged in a plane 22 which is situated at a distance q behind the main plane 19. In FIG. 3 this image is denoted by a construction beam 23 for an electron beam. The object 21 is in this case a cross-over of the electron beam, viewed in the symmetry plane through the minor axis. In fact, in this case the virtual object is meant, that is to say the object which is found by linearly extending the defining beams of the electron beam in the unipotential region into the object space as far as the intersection with an optical axis 24 in FIG. 3. In the described gun, J has a value of 60 mm and, if the gun is used in a 90 21 inch index colour tube, q has a value of 400 mm. As an additional condition it is assumed, by way of example, that the electron beam extends parallel to the optical axis behind the main plane in the symmetry plane through the major axis. The reference 25 in FIG. 3 denotes a construction beam for the major axis. Consequently, in this Figure the part of the drawing above the optical axis is situated in the symmetry plane through the minor axis, and the part of the drawing below the optical axis is situated in the symmetry plane through the major axis. So the two planes shown in one plane are actually perpendicular to each other. The imposed requirement can be satisfied by a suitable choice of the ellipticity e of the emissive cathode surface. The ellipticity thereof can be determined by the shape of the control grid aperture. So as to obtain more insight into this matter, FIG. 4 shows an empirically determined series of curves, in which the degreeof astigmatism of the electron beam, expressed in a length A s (a sagital distance 26 as shown in FIG. 3), is given as a function of the axial ratio e of the emissive cathode surface, and as a function of the axial ratio of the control grid aperture. It was found that the ellipticity of an elliptical control grid aperture is taken over by the emissive cathode surface. The dependency on current intensity and on the aperture in the first anode or the voltage of the first anode will not be dealt with in this context. The condition for a parallel, in any case nondiverging, beam in the direction of the major axis can now be simply derived from FIG. 3. This is given by p z A s(q p).  
  For p 60 mm and p+q 460 mm, this results in A s s 8 in the described embodiment. According to FIG. 4, the ellipticity of the emissive surface must then be 4. This can be realized by means of a control grid aperture in the shape of an ellipse with e s 4. It is then advantageous to approximate the maximum permissible value of e as closely as possible. This benefits both the beam diameter and the dimension of the minor axis of the target. An electron gun comprising an ellipse having axes of 350 and 1400 microns was in principle found to be satisfactory for a 21 inch tube. In a preferred embodiment, however, a tetrode gun having a grid aperture of a different shape will preferably be used because the adjusting facilities are then increased and less aberration occurs.  
  If an electron gun of the described kind performs satisfactorily in a cathode ray tube, an electron gun for a different type of tube can be directly derived therefrom. For example, in 1 10 23 inch index colour tube, in which the distance between the main plane of the main lens and the screen q 315 mm, a magnification factor of again maximum approximately 6.5 results in an object distance p 50 mm. It follows from the formula p As (q-l-p) that A s s 6. In accordance with FIG. 4, this corresponds to a value of 3.5 of the ellipticity of the emissive surface. This can be realized by means of a grid aperture in the form of an ellipse having this ellipticity. Measurements have shown that aberrations occur when the target is formed, and that these aberrations can be reduced by using grid apertures having the shape of a diamond or a shape which is limited by two arcs of a circle, to be referred to hereinafter as a circle peak. The emissive surface is again an ellipse, but its ellipticity is smaller than the axial ratio of the grid aperture. This is denoted in FIG. 4 which shows a curve b for a circle peak shape and a curve 0 for a diamond shape in addition to a curve a for an ellipse. As result, the advantage of a comparatively long grid aperture is coupled to a small value of A s. The emissive surface can be calculated for any configuration and potential of the control grid and any further electrodes by means of potential calculations for the cathode surface.  
  A practical advantage of the diamond shape over the ellipse or the circle peak is that the apertures of the control grid and of the first anode can be very accurately arranged with respect to each-other when a cathode gun is assembled, as is shown in the FIGS. 1 and 2. A slanted orientation cannot only be more readily detected visually in the case of linear limitations, but a jig can also be used with more accuracy for assembly. The apertures are&#39;provided in the various plates. for example, by spark erosion. ln the case of linear limitations. a higher accuracy can then again be obtained. The grid aperture and&#39; the first anode aperture can also be orientated to be parallel. This can be used. for example, in cathode raytub&#39;esin which a line focus is desired such as the target of an electron beam generating X-rays in an X-ray tube comprising line focus.  
  The circumstancesbeing the same, an electron beam can generally be focussed to a smaller target at its length is permitted to be longer. This is due to the fact that the influence of the space charge on the formation ofthe target is reduced because the electron flow is distributed over a larger (elongated) surface. By using an oval emissive surface, this gain not only occurs between the main plane and the image screen, i.e. mainly near the image screen, but also at the formation of the crossover which is now linear instead ofcircular. By suitable proportioning, to be established at a given astigmatism, imaging and current intensity by calculations, a compensating action can be obtained between the space charge effect and the effect of spheric aberration of the main lens on the electron beam. As a result, a target can be realized which is narrower than would be possible on the basis of the two quantities individually which increase the relevant axis of the target.  
  A preferred embodiment according to the invention in the form of a 90 1 1 inch index colour tube will be described hereinafter with reference to FIG. 5. The figures show a sectional view in the line direction, coinciding with the symmetry plane through the minor axis of an electron gun 31, a sectional view 32 in the image direction, coinciding with the symmetry plane through the major axis of the electron gun, and a sectional view 33 through a diagonal ofa 90 11 inch envelope. The screen 34 in the tube accommodates three colour phosphors which are provided in the form of lines having a width of approximately 100 microns, transverse to the line direction of the television image, i.e. in the direction of the major axis. Present between the phosphors, denoted by R, G and B for red, green and blue, respectively, are lines 35 of an inert dark material. These black lines preferably have a width which is equal to that of the colour lines and they are provided so as to ensure proper colour purity of the image. Provided on this line pattern is a thin aluminum layer 36 which keeps the entire screen at the same potential and which also serves for light separation. On this aluminum layer, the metal backing, phosphor lines 37 are provided behind every second black line, the phosphor lines consisting of a phosphor having a shortwave luminescence, to be referred to hereinafter as u.v. phosphor. Radiation pulses to be emitted by the u.v. phos-&#39; phor when passing the target, are captured by a photomultiplier 39 via a window 38. By means of starting lines (not shown) for establishing the correct phase, an index system is realized by means of which the electron gun can be controlled in a colour-dependent manner. For the writing of the television image, an electromagnetic deflection unit 40 is provided about the neck of the envelope. In addition to the cathode l, the control grid 2. the first anode 3, the high voltage anodes 4 and 6, and the main lens electrode 5, the diagrammatically shown electron gun comprises an electrically conductive connection 41 between the electrodes 4 and 6, and an electrical conductor 42 between electrode 6 and an electrically conductive layer (not shown) on the inner wall of the envelope. This conductive layer forms one conductor with the layer 36. For the control of the electron gun, passage pins 43 are provided to which the main lens electrode, the first anode, the control grid. the cathode etc. are connected in an electrically conductive manner. The distance between the main plane 19 of the main lens and the image screen amounts to 180 mm in this case. Using a magnification factor 6, p 30 mm, q= l mm, and p +q 210 mm. This results in a value of approximately 4.3 for A s. An ellipse with e 3 would satisfy this. However, for the previously stated reasons preference is given to a diamond shape, in this case a diamond having an axial ratio of approximately 4.5. The diamond shape consists of a rectangle of 150 X 200 square microns, the value 150 microns relating to the minor axial direction. The 150- micron sides are adjoined by triangles having a height of 225 microns, so that the overall length of the aperture amounts to 650 microns. Particularly the dimension of the triangles is not very critical, provided that they are sufficiently high. Using such a control grid aperture and an adapted first anode aperture of approximately 250 X 800 square microns with a square centre part, an electron gun is realized by means of which a good image can be formed in the described tube. A l 1 inch tube is then exceptionally suitable for use in a portable television receiver. The high voltage must then be adjusted to approximately 15 KV. The degree of astigmatism of the beam can be changed by varying the voltage on the first anode, without the current characteristic of the gun being excessively changed. This can be advantageous, for example, if a different astigmatism is desired in view of errors caused by the deflection. In principle, the astigmatism can be dynamically controlled by coupling the first anode voltage to the deflection.  
  For a 1 1023 inch index colour tube, similar considerations lead to an electron gun having a diamond shape with a minor axis of 300 microns and a major axis of 1300 microns, in which the centre piece has a length of 400 microns. The first anode then has the shape of a diamond with a square of 550 microns and an overall length of 1650 microns. In all described guns, the diaphragm does not intercept more than 5 to 10 percent of the beam current at peak current.  
 What is claimed is:  
  l. A cathode ray tube comprising an image screen and an electron gun containing a cathode, a control grid, an acceleration anode, a non-rotationally symmetrical electron-optical element, a main lens for the formation on an image plane of a target of an electron beam to be emitted by the cathode and an apertured diaphragm plate disposed at said main lens, said nonrotationally symmetrical element producing an emissive cathode surface having an ovall contour and an astigmatic electron beam, the position of the main lens being adapted to the degree of astigmatism of the beam such that an image of the electron beam in the image plane constitutes an accurately defined target having a predetermined axial ratio.  
  2. A cathode ray tube as in claim 1, wherein said nonrotationally symmetrical electron-optical element consists of a control grid aperture having a non-rotationally symmetrical limitation.  
 BEST AVAILABLE COPY 3. A cathode ray tube as in claim 2. wherein said grid aperture has a smaller surface area than an ellipse having substantially the same axial ratio and width.  
  4. A cathode ray tube as in claim I, wherein said grid aperture has the configuration of an ellipse.  
  S. A cathode ray tube as in claim I, wherein said nonrotationally symmetrical element is bounded by straight lines.  
  6. A cathode rat tube as in claim 1. further comprising a second non-rotationally symmetrical electronoptical element.  
  7. A cathode ray tube as in claim 6. wherein said second non-rotationally symmetrical element forms the ode ray tube is an index color television tube.