Color display tube system with reduced spot growth

Abstract

A color display tube system comprising an electron gun for producing three co-planar electron beams, and including a main deflection system which in operation generates deflection fields of the self-convergent type, and an auxiliary deflection system which in operation generates a 45° magnetic 4-pole field facing the electron gun and moving the outer electron beams away from each other, and a 45° magnetic 4-pole field facing the display screen and moving the outer beams towards each other.

We claim: 
     
       1. A colour display tube system comprising (a) an evacuated envelope having a neck, a cone and a display window,   (b) an electron gun in the neck, which gun has a beam-forming part for generating a central electron beam and first and second outer electron beams whose axes are co-planar, and a first and a second electrode system which in operation jointly constitute a main lens and are connectable to means for supplying an energizing voltage, and   (c) a deflection unit for generating first and second deflection fields for deflecting the electron beams in the horizontal and vertical directions and for scanning the display window by means of convergent beams, characterized in that a first convergence influencing element is arranged between the beam-forming part of the electron gun and a side of the deflection unit facing the display window for generating a magnetic field exerting a force on each outer electron beam having a component in the plane of the electron beams directed towards the central electron beam, and in that a second convergence influencing element is arranged between the first convergence influencing element and the beam-forming part of the electron gun for generating a magnetic field exerting a force on each outer electron beam having a component in the plane of the electron beams directed away from the central electron beam.     
     
     
       2. A colour display tube system as claimed in claim 1, characterized in that each element influencing convergence is adapted to generate a 45° magnetic 4-pole field. 
     
     
       3. A colour display tube system as claimed in claim 1 or 2, characterized in that a direct current is used for generating the magnetic fields. 
     
     
       4. A colour display tube system as claimed in claim 1 or 2, characterized in that it has means for dynamically varying the strength of the magnetic fields generated by the convergence influencing elements with the strength of the first deflection field. 
     
     
       5. A colour display tube system as claimed in claim 4, characterized in that the means for dynamically varying the strength of the magnetic fields generated by the convergence influencing elements with the strength of the first deflection field comprise means for applying a dynamically varying control voltage having a component which varies synchronously with the first deflection field. 
     
     
       6. A colour display tube system as claimed in claim 5, characterized in that the component is parabolic. 
     
     
       7. A colour display tube system as claimed in claim 1 or 2, characterized in that the electron gun comprises means for producing a quadrupole field lens which is energizable for compensating astigmatic defocusing. 
     
     
       8. A colour display tube system as claimed in claim 1 or 2, characterized in that the display tube has a window with an aspect ratio of approximately 9:16. .Iadd. 
     
     
       9.  A deflection apparatus, comprising: a cathode ray tube including an evacuated glass envelope, a display screen disposed at one end of said envelope, and an electron gun assembly disposed at a second end in said envelope, said electron gun assembly, in operation, producing central and first and second outer electron beams that form respective beam spots at electron beam landing locations on said screen;   a plurality of deflection windings for producing a main deflection field in a main deflection region of a deflection yoke that deflects the electron beams to said beam landing locations;   a first quadrupole arrangement for producing in a first region traversed by said outer electron beams a first quadrupole magnetic field that varies in accordance with said beam landing locations and produces forces acting to diverge said outer electron beams from each other; and   a second quadrupole arrangement for producing in a second region traversed by said diverged outer electron beams a second quadrupole magnetic field that varies in accordance with the beam landing locations and produces forces acting to converse said outer electron beams toward each other..Iaddend.