Color display tube with reduced deflection defocussing

Abstract

Colour display tube comprising an electron gun 5 of the in-line type. The electron gun 5 comprises a main lens which is constituted by a first focussing electrode 25 and a second focussing electrode 26. The first focussing electrode comprises sub-electrodes 27, 28 placed at a distance from each other between which an auxiliary electrode constituting an astigmatic element GAST is positioned. The auxiliary electrode GAST is connected during operation to means for applying a constant voltage, whilst at least the sub-electrode 28 forming part of the main lens is connected during operation to means for applying a control voltage. The control voltage may be a static voltage or a dynamically varying voltage, for example, a parabolic voltage which is in synchronism with the line deflection.

What is claimed is: 
     
       1. A color display tube comprising an envelope containing a luminescent screen, and an electron gun for producing deflectable central and first and second outer in-line electron beams directed toward said screen, characterized in that said electron gun comprises, in succession: a. a plurality of in-line cathodes for emitting respective ones of the electron beams;   b. first electrode means including a first prefocusing electrode having central and first and second outer apertures for passing the respective electron beams;   c. second electrode means including a plurality of discrete electrodes each having central and first and second outer apertures for passing respective ones of the electron beams, said electrodes including: (1) a second prefocusing electrode disposed adjacent the first prefocusing electrode, upon application of respective voltages thereto said prefocusing electrodes producing therebetween a prefocusing lens field for prefocusing the electron beams;   (2) a first main-focusing electrode disposed remotely from the first electrode means; and   (3) an auxiliary electrode disposed between the second prefocusing electrode and the first main-focusing electrode, the central and first and second outer apertures in the auxiliary electrode being shaped to produce a quadripolar field for astigmatically affecting the cross-sectional shapes of the electron beams; and     d. third electrode means including a second main-focusing electrode disposed adjacent the first main-focusing electrode and having central and first and second outer apertures for passing respective ones of the electron beams, upon application of respective voltages thereto said main focusing electrodes producing therebetween a main focusing lens field for focusing the electron beams onto the screen; during operation, said auxiliary electrode having applied thereto a constant voltage and at least said first main-focusing electrode having applied thereto a variable control voltage, variation of said control voltage affecting the astigmatism of the electron beams in a first direction substantially independently of said astigmatism in a direction perpendicular to said first direction.     
     
     
       2. A color display tube as .Iadd.set forth .Iaddend.in claim 1 where the variable .[.color.]. .Iadd.control .Iaddend.voltage is set at a constant value. 
     
     
       3. A color display tube as in claim 1 where the control voltage varies dynamically with the deflection of the beams across the screen. 
     
     
       4. A color display tube as in claim 3 where the control voltage has a parabolic component which is in synchronism with a line deflection voltage for deflecting the electron beams. 
     
     
       5. A color display tube as in claim 3 where the control voltage includes a combination of parabolic components which are in synchronism with line deflection and field deflection of the electron beams. 
     
     
       6. A color display tube as in claim 1 where, during operation, the variable control voltage is applied to both the second prefocusing electrode and the first main-focusing electrode. 
     
     
       7. A color display tube as .Iadd.claimed .Iaddend.in claim 1 where, during operation, the variable .[.color.]. .Iadd.control .Iaddend.voltage is applied to only the first main-focusing electrode. 
     
     
       8. A color display tube as in claim 7 where, during operation, the constant voltage is applied to the second prefocusing electrode and the auxiliary electrode. 
     
     
       9. A color display tube as in claim 8 where the auxiliary electrode is affixed to the second prefocusing electrode. .Iadd. 
     
     
       10.  A color display tube comprising an envelope containing a luminescent screen, and an electron gun for producing deflectable central and first and second outer in-line electron beams directed toward said screen, characterized in that said electron gun comprises, in succession: a. a plurality of in-line cathodes for emitting respective ones of the electron beams;   b. first electrode means including a first prefocusing electrode having central and first and second outer apertures for passing the respective electron beams;   c. second electrode means including: (1) a second prefocusing electrode, having central and first and second outer apertures for passing respective ones of the electron beams, disposed adjacent the first prefocusing electrode, upon application of respective voltages thereto said prefocusing electrodes producing therebetween a prefocusing lens field for prefocusing the electron beams;   (2) a first main-focusing electrode having central and first and second outer apertures for passing respective ones of the electron beams, disposed remotely from the first electrode means; and   (3) four pole field producing means disposed in the vicinity of the second prefocusing electrode and the first main-focusing electrode which, upon application of at least a first voltage, produces a quadripolar field in the region between the second prefocusing electrode and the first main-focusing electrode which quadripolar field astigmatically affects the cross sectional shapes of the electron beams;     d. third electrode means including a second main-focusing electrode disposed adjacent the first main-focusing electrode and having central and first and second outer apertures for passing respective ones of the electron beams, upon application of respective voltages thereto said main focusing electrodes producing therebetween a main focusing lens field for focusing the electron beams onto the screen; and   e. means for applying a second voltage on at least said first main-focusing electrode so that variation of said second voltage affects the astigmatism of the electron beams in a first direction substantially independent of said astigmatism in a direction perpendicular to said first direction. .Iaddend. .Iadd.11. A color display tube as set forth in claim 10 where the second voltage has a constant value. .Iaddend. .Iadd.12. A color display tube as in claim 10 where the second voltage varies dynamically with the deflection of the beams across the screen. .Iaddend. .Iadd.13. A color display tube as claimed in claim 12 where the second voltage has a parabolic component which is in synchronism with a line deflection voltage   
     
     
        for deflecting the electron beams. .Iaddend. .Iadd.14.  A color display tube as in claim 12 where the second voltage includes a combination of parabolic components which are in synchronism with the line deflection and field deflection of the electron beams. .Iaddend. .Iadd.15. A color display tube as in claim 10 wherein the means for applying the second voltage applies the second voltage on both the second prefocusing electrode and the first main-focusing electrode. .Iaddend. .Iadd.16. A color display tube as claimed in claim 10 wherein the means for applying the second voltage applies the second voltage only to the first main-focusing electrode. .Iaddend. .Iadd.17. A color display tube comprising: an evacuated envelope containing a luminescent screen, and an electron gun for producing deflectable central and first and second outer in-line electron beams directed towards said screen;   wherein said electron gun comprises:   focusing lens means disposed along paths of said electron beams which include a first focusing electrode remote from the display screen and a second focusing electrode disposed between the first focusing electrode and the display screen, said focusing lens means, upon application of a focusing voltage between said focussing electrodes, producing a focussing lens field therebetween;   four-pole field means disposed adjacent the focusing lens means, said four-pole field means, upon application of a voltage thereto, producing a quadripolar electric field within at least a region adjacent the focusing lens field;   said electron gun, upon application of a control voltage to at least one of said first and second focusing electrodes which is nearest to said four-pole field means, affecting the astigmatism of the electron beams in a first direction substantially independently of said astigmatism in a   
     
     
        direction perpendicular to said first direction. .Iaddend. .Iadd.18.  A color display tube as set forth in claim 17 where the control voltage has a constant value. .Iaddend. .Iadd.19. A color display tube as in claim 18 where the control voltage varies dynamically with the deflection of the beams across the screen. .Iaddend. .Iadd.20. A color display tube as in claim 18 where the control voltage has a parabolic component which is in synchronism with a line deflection voltage for deflecting the electron beams. .Iaddend. .Iadd.21. A color display tube as in claim 18 where the control voltage includes a combination of parabolic components which are in synchronism with line deflection and field deflection of the electron 
     
     
        beams. .Iaddend. .Iadd.22.  A color display tube as in claim 17 comprising a prefocussing electrode wherein the control voltage is applied both to the prefocusing electrode and to the first focusing electrode. .Iaddend.