EP0192336A1

EP0192336A1 - Focusing an electron beam in a cathode ray tube

Info

Publication number: EP0192336A1
Application number: EP86300411A
Authority: EP
Inventors: Kern Ko Nan Chang
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1985-01-29
Filing date: 1986-01-21
Publication date: 1986-08-27
Also published as: JPS61176038A; DD241667A5

Abstract

The cathode-ray tube (10) includes a faceplate panel (12) and an electron gun (14), adapted to emit a beam of electrons for striking a cathodoluminescent screen (16) disposed on the panel. It has a conductive loop (28) disposed around the perimeter of the screen for locally altering the trajectory of the electron beam adjacent the perimeter, thereby achieving a focusing effect for reducing the size of cathodoluminescent spots adjacent the perimeter of the screen. The conductive loop has a significantly lower voltage applied thereto than to the screen.

Description

This invention relates to a cathode ray tube and is concerned with focusing means for altering the trajectory of an electron beam within the tube.
A standard cathode-ray tube comprises a faceplate panel with a cathodoluminescent screen, a funnel-shaped back section having a protruding neck, and a mount containing an electron gun adapted to emit a beam of electrons for striking the screen. A deflection yoke alters the trajectory of the electron beam so that it scans the entire cathodoluminescent screen. The maximum angle that the beam is deflected to diagonal corners of the tube screen is known as the deflection angle. Typical values for the deflection angle are 70°, 90° and 110°, which represent the total corner-to-opposite corner angle through which the electron beam is deflected. For example, a deflection angle of 110° means that the electron beam is deflected 55° from center to corner.
Efforts to decrease the overall depth of cathode-ray tubes have resulted in larger deflection angles. However, tubes with a deflection angle larger than 110° have exhibited poor image size and reduced transmission current at the perimeter of the cathodoluminescent screen, due to the larger angle of incidence for the electron beam trajectory.
According to one aspect of the invention, there is provided a cathode-ray tube having a faceplate panel and an electron gun adapted to emit a beam of electrons for striking a cathodoluminescent screen disposed on said panel, characterized by conductive focusing means disposed around the perimeter of said screen and insulated therefrom, for locally altering the trajectory of said electron beam adjacent said perimeter.
According to another aspect, there is provided a method of focusing a beam of electrons emanating from an electron gun of a cathode-ray tube and striking a cathodoluminescent screen disposed on a faceplate panel of said tube, characterized by the step of applying a significantly lower voltage to a conductive loop disposed around the perimeter of said screen than to said screen, in order to locally alter the trajectory of the electron beam adjacent said perimeter.
In accordance with an embodiment of the present invention, a cathode-ray tube employs localized focusing means whereby the trajectory of a beam of electrons, emanating from an electron gun of the tube and striking a cathodoluminescent screen, is locally altered adjacent the perimeter of the screen. Conductive focusing means is disposed around the perimeter of the screen for altering the electron beam trajectory, and has a significantly different voltage applied thereto than to the screen.
A preferred embodiment of the present invention provides a localized retarding field in the vicinity of large deflection in order to achieve a more favorable landing angle of the electron beam on the cathodoluminescent screen.
In the drawings:

FIGURE 1 is a front elevation view, partially cut away, illustrating a cathode-ray tube with localized focusing means in accordance with the present invention.
FIGURE 2 is a cross-sectional view taken along line 2-2 of FIGURE 1.
FIGURE 3 is a cross-sectional view of an insulating post used in a preferred embodiment of the present invention.
FIGURE 4 is a diagrammatic view illustrating a focusing effect achieved by the present invention.
FIGURES 1 and 2 show a thin cathode-ray tube 10 comprising a faceplate panel 12 and an electron gun 14 adapted to emit beams of electrons for striking a cathodoluminescent screen 16 disposed on the panel 12.

The electron gun 14 is part of a mount 18 which is frit sealed to a back section 20 of the tube 10_.
The back section 20 is substantially symmetrical in shape to that of the faceplate panel 12 in order to form a thin cathode-ray tube 10.
Such a thin tube 10 has a large deflection angle which may be as high as 150°. An apertured shadow mask 22 is positioned near the screen 16 and attached to a mask frame 24, which is supported by mounting studs 26 that extend inwardly from the faceplate panel 12.
Conductive focusing means is disposed around the perimeter of the cathodoluminescent screen 16 and insulated therefrom. Preferably, the conductive focusing means comprises a metallic loop 28 which is mounted near the edge of and in back of the shadow mask 22. The loop 28 is attached to insulating posts 30 by screws. The insulating posts 30 may be made of a ceramic material, and are attached to the shadow-mask frame 24 by partially-imbedded bolts (not shown). The posts 30 have a cylindrical-shaped outer surface 32 with a plurality of spaced circular grooves 34 adjacent thereto, as illustrated in FIGURE 3. The purpose of the grooves 34 is to prevent getter material, deposited thereon when a getter 36 within the tube 10 is subsequently flashed, from creating a conductive path between the metallic loop 28 and the screen 16 which is at a voltage significantly different from that of the loop 28.
Preferably, the metallic loop 28 comprises a rectangular-shaped band having a width of about 12 millimeters and a thickness of about 1 millimeter. The band should be attached to the insulating posts 30 in a manner wherein the major surface area of the band is positioned in a plane substantially parallel to the trajectory of the electron beams, for reasons explained further below. The loop 28 may alternatively comprise a circular wire having a diameter of about 1.5 millimeters. Preferably, the metallic loop 28 is made of either nonmagnetic stainless steel or a nonmagnetic alloy of nickel and chromium. In order to be able to apply a voltage to the loop 28, a conductive lead 38 is welded to the loop 28 and passes through apertures in both the frame 24 and the faceplate panel 12. Since the aperture in the panel 12 must be sealed around the lead 38, the lead 38 should be made of an alloy compatible with the glass material of the panel 12. Preferably, the portion of the lead 38 adjacent the inside surface of the faceplate panel 12 is surrounded by an insulating tube 40 in order to prevent getter material from creating a conductive path between the lead 38 and the anode (not shown) which is at a significantly higher voltage.
In operation, a lower voltage is applied to the conductive loop 28 than to the cathodoluminescent screen 16 or anode. Preferably, the voltage difference is about 15kV. In the present example, the voltage applied to the loop 28 is about 10kV, while the voltage on the screen is about 25kV. This voltage difference creates a localized retarding field in the vicinity of large deflection which occurs adjacent the perimeter of the screen 16.
The significance of this retarding field is shown diagrammatically in FIGURE 4. As an electron beam 42 approaches the cathodoluminescent screen 16 in the vicinity of the conductive loop 28, the beam 42 is bent away from the loop 28 and toward the screen 16, causing a smaller angle of incidence for the electron beam trajectory adjacent the screen 16. This more favorable landing angle for the electron beam 42 results in a better cathodoluminescent spot size adjacent the perimeter of the screen 16, i.e., one that is smaller in diameter and, thus, better focused. In other words, a better spot size is obtained at large deflection angles because of the retarding field. Due to the different angles of incidence, the diameter of a spot luminesced by unretarded electron beams, labeled as A in FIGURE 4, is larger than the diameter of the spot luminesced by retarded electron beams, labeled as B. In actual experiments, where the ultor or screen voltage was about 25kV and the voltage applied to the conductive loop 28 was about lOkV, it has been demonstrated that spot size can be reduced from 2mm in diameter to about 1.3mm by using the conductive loop 28, which represents a reduction in spot size of approximately 35%. As a result, a 150° angle of deflection, which has not been previously feasible, is within reach with good spot sizes across the entire display area.
This realization of a more favorable landing angle for the electron beam 42 on the cathodoluminescent screen 16, in the vicinity of large deflection, has significant application in the fabrication of a thin cathode-ray tube 10 where the spacing between the electron gun 14 and the screen 16 is limited. The focusing loop 28 enables a tube 10 with a deflection angle larger than 110° to achieve good image size and increased transmission current (brightness) at the perimeter of the screen 16, due to the larger angle of incidence for the electron beam 42 caused by the local retarding field. By using a band for the loop 28 wherein the major surface thereof is substantially parallel to the trajectory of the electron beam 42, the retarding field and resulting focusing effect are able to be applied to a plurality of beam trajectories which strike the screen 16 at distances further away from the perimeter thereof, thereby achieving the desirable focusing benefits over a greater area of the screen 16 adjacent the perimeter. The present invention may also be utilized in cathode-ray tubes having deflection angles of 110° or less in order to further improve their performance by enhancing picture resolution and brightness at the corners of the screen.

Claims

1. A cathode-ray tube having a faceplate panel and anelectron gun adapted to emit a beam of electrons for striking a cathodoluminescent screen disposed on said panel, characterized by conductive focusing means disposed around the perimeter of said screen (16) and insulated therefrom, for locally altering the trajectory of said electron beam (42) adjacent said perimeter.

2. A cathode-ray tube as defined in claim 1, wherein said conductive focusing means comprises a conductive loop (28) attached to insulating posts (30) supported by said faceplate panel (12).

3. A tube as defined in claim 2, wherein the conductive loop is metallic.

4. A cathode-ray tube as defined in claim 2 or 3 wherein said loop (28) comprises a circular wire having a diameter of about 1.5 millimeters.

5. A cathode-ray tube as defined in claim 2 or 3 wherein said loop (28) comprises a rectangular-shaped band having a width of about 12 millimeters and a thickness of about 1 millimeter.

6. A cathode-ray tube as defined in claim 5, wherein the major surface area of said band (28) is positioned in a plane substantially parallel to the trajectory of said electron beam (42).

7. A cathode-ray as defined in claim 2, 3, 4, 5 or 6 wherein said loop (28) comprises nonmagnetic stainless steel.

8. A cathode-ray tube as defined in claim 2, 3, 4, 5 or 6 wherein said loop (28) comprises a nonmagnetic alloy of nickel and chromium.

9. A cathode-ray tube as defined in anyone of claims 2 to 8 wherein each of said insulating posts (30) has a cylindrical-shaped outer surface (32) with a plurality of spaced circular grooves (34) adjacent thereto.

10. A cathode-ray tube as defined in claim 9 wherein said insulating posts (30) are attached to a shadow-mask frame (24) supported by said faceplate panel (12).

11. A tube according to anyone of claims 2 to 10 further comprising means arranged to apply significantly lower voltage to the said loop (28) than to said screen (16) in order to locally alter the said trajectory.

12. A method of focusing a beam of electrons emanating from an electron gun of a cathode-ray tube and striking a cathodoluminescent screen disposed on a faceplate panel of said tube, characterized by the step of applying a significantly lower voltage to a conductive loop (28) disposed around the perimeter of said screen (16) than to said screen, in order to locally alter the trajectory of the electron beam (42) adjacent said perimeter.

13. A method as recited in claim 12 or a tube according to claim 11, wherein the voltage difference between said conductive loop (28) and said cathodoluminescent screen (16) is about 15kV.

14. A method as recited in claim 13, or a tube according to claim 13, wherein the voltage applied to said conductive loop (28) is about 10kV and the voltage on said cathodoluminescent screen (16) is about 25kV.