Color Picture Tube Having an Improved Expanded Focus Lens Type Inline Electron Gun

The present invention relates to color picture tubes having improved inline electron guns, and particularly to such guns having an improved expanded focus lens for reduced spherical aberration.

An inline electron gun is one designed to generate or initiate preferably three electron beams in a common plane and direct those beams along convergent paths in that plane to a point or small area of convergence near the tube screen. In one type of inline electron gun shown in U.S. Pat. No. 3,873,879, issued to R. H. Hughes on Mar. 25, 1975, the main electrostatic focusing lenses for focusing the electron beams are formed between two electrodes referred to as the first and second accelerating and focusing electrodes. These electrodes include two cup-shaped members having bottoms facing each other. Three apertures are included in each cup bottom to permit passage of three electron beams and to form three separate main focus lenses, one for each electron beam. In a preferred embodiment, the overall diameter of the electron gun is such that the gun will fit into a 29 mm tube neck. Because of this size requirement, the three focusing lenses are very closely spaced from each other, thereby providing a severe limitation on focus lens design. It is known in the art that the larger the focus lens diameter, the less will be the spherical aberration which restricts the focus quality.

In addition to the focus lens diameter, the spacing between focus lens electrode surfaces is important, because greater spacing provides a more gentle voltage gradient in the lens which also reduces spherical aberration. Unfortunately, greater spacing between electrodes beyond a particular limit (typically 1.27 mm) generally is not permissible because of beam bending from electrostatic charges on the neck glass penetrating into the space between the electrodes, which causes electron beam misconvergence.

In copending U.S. patent application Ser. No. 201,692, filed Oct. 29, 1980 by R. H. Hughes and B. G. Marks, now Pat. No. 4,370,592, an electron gun is described wherein the main focus lens is formed by two spaced electrodes. Each electrode includes a plurality of apertures therein equal to the number of electron beams and also a peripheral rim, with the peripheral rims of the two electrodes facing each other. The apertured portion of each electrode is located within a recess set back from the rim. The effect of this main focus lens is to provide the gentle voltage gradient sought to reduce spherical aberration. Because of the asymmetrical shape of the peripheral rims of the two electrodes, described in patent application Ser. No. 201,692, horizontal and vertical focus voltage components for the inner and outer guns are not the same. In the vertical direction, the center electron beam sees more a slot, and experiences more focusing action, than the sides, where the focusing geometry is bounded in part by a circular arc. This is because the field penetrates the slot in the vertical direction more easily than an inscribed circular boundary. Likewise, the horizontal focusing component at the outer electron beams may be more active than at the center beam, because the field in the horizontal direction falls away more rapidly at the side ends of the peripheral rims than within the center of the recessed cavity. Therefore, there is a need to modify the peripheral rim geometry to unitize the focus voltages.

An improved color picture tube has an inline electron gun for generating and directing three electron beams, a center beam and two side beams, along coplanar paths toward a screen of the tube. The gun includes a main focus lens for focusing the electron beams. The main focus lens is formed by two spaced electrode members each having three separate inline apertures therein. Each electrode also includes a peripheral rim. The peripheral rims of the two electrodes face each other. The apertured portion of each electrode is within a recess set back from the rim. The width of the recess in at least one of the electrodes is wider at the side beam paths than at the center beam path, measured perpendicular to the plane containing the electron beam paths.

FIG. 1 is a plan view partly in axial section, of a shadow mask color picture tube embodying the invention.

FIG. 2 is a partial axial section view of the electron gun shown in dashed lines in FIG. 1.

FIG. 3 is an axial sectional view of the G3 and G4 electrodes of the electron gun of FIG. 2.

FIG. 4 is a front view of the G4 electrode taken at line 4--4 of FIG. 3.

FIG. 5 is a plan view of the stigmators on the G4 electrode taken at line 5--5 of FIG. 2.

FIG. 1 is a plan view of a rectangular color picture tube having a glass envelope 10 comprising a rectangular faceplate panel or cap 12 and a tubular neck 14 connected by a rectangular funnel 16. The panel comprises a viewing faceplate 18 and peripheral flange or sidewall 20 which is sealed to the funnel 16. A mosaic three-color phosphor screen 22 is carried by the inner surface of the faceplate 18. The screen is preferably a line screen with the phosphor lines extending substantially perpendicular to the high frequency raster line scan of the tube (normal to the plane of FIG. 1). A multiapertured color selection electrode or shadow mask 24 is removably mounted, by conventional means, in predetermined spaced relation to the screen 22. An improved inline electron gun 26, shown schematically by dotted lines in FIG. 1, is centrally mounted within the neck 14 to generate and direct three electron beams 28 along coplanar convergent paths through the mask 24 to the screen 22.

The tube of FIG. 1 is designed to be used with an external magnetic deflection yoke, such as the yoke 30 in the neighborhood of their junction. When activated, the yoke 30 subjects the three beams 28 to magnetic fields which cause the beams to scan horizontally and vertically in a rectangular raster over the screen 22. The initial plane of deflection (at zero deflection) is shown by the line P-P in FIG. 1 at about the middle of the yoke 30. Because of fringe fields, the zone of deflection of the tube extends axially, from the yoke 30 into the region of the gun 26. For simplicity, the actual curvature of the deflection beam paths in the deflection zone is not shown in FIG. 1.

The details of the gun 26 are shown in FIGS. 2 through 5. The gun comprises two glass support rods 32 on which the various electrodes are mounted. These electrodes include three equally spaced coplanar cathodes 34 (one for each beam), a control grid electrode 36 (G1), a screen grid electrode 38 (G2), a first accelerating and focusing electrode 40 (G3), and a second accelerating and focusing electrode 42 (G4), spaced along the glass rods 32 in the order named. Each of the G1 through G4 electrodes has three inline apertures therein to permit passage of three coplanar electron beams. The main electrostatic focusing lens in the gun 26 is formed between the G3 electrode 40 and the G4 electrode 42. The G3 electrode 40 is formed with four cup-shaped elements 44, 46, 48 and 50. The open ends of two of these elements, 44 and 46, are attached to each other, and the open ends of the other two elements, 48 and 50, are also attached to each other. The closed end of the third element 48 is attached to the closed end of the second element 46. Although the G3 electrode 40 is shown as a four-piece structure, it could be fabricated from any number of elements, including a single element of the same length. The G4 electrode 42 also is cup-shaped, but has its open end closed with an apertured plate 52.

The facing closed ends of the G3 electrode 40 and the G4 electrode 42 have large recesses 54 and 56, respectively, therein. The recesses 54 and 56 set back the portion of the closed end of the G3 electrode 40 that contains three apertures, 58, 60 and 62, from the portion of the closed end of the G4 electrode 42 that contains three apertures, 64, 66 and 68. The remaining portions of the closed ends of the G3 electrode 40 and the G4 electrode 42 form rims 70 and 72, respectively, that extend peripherally around the recesses 54 and 56. The rims 70 and 72 are the closest portions of the two electrodes 40 and 42. It has been found that the vertical focusing action on the center electron beam can be decreased by reducing the width of the rim 72 on the G4 electrode 42, the divergent side of the electrostatic lens formed in and between the recesses 54 and 56. As shown in FIG. 4, the recess 56 in the G4 electrode 42 is wider at the side beam path than at the center beam paths, the width being measured perpendicular to the plane containing the electron beam paths. It also has been found that the horizontal focusing action on the two outer beams can be decreased by decreasing the length of the recess 56 in the G4 electrode.

The electron gun 26 of FIG. 2 provides a main focusing lens having substantially reduced spherical aberration compared to that of prior guns discussed above. The reduction in spherical aberration is caused by an increase in the size of the main focus lens. This increase in lens size results from recessing the electrode apertures. In most prior inline guns, the strongest equipotential lines of the electrostatic field are concentrated at each opposing pair of apertures. However, in the gun 26 of FIG. 2, the strongest equipotential lines extend continuously from between the rims 70 and 72, so that the predominant portion of the main focus lens appears to be a single large lens extending through the three electron beam paths. The remaining portion of the main focus lens is formed by weaker equipotential lines located at the apertures in the electrodes. The performance and advantages of an electron gun similar to the electron gun 26 are discussed in previously cited copending U.S. patent application Ser. No. 201,692.

There is a slot effect astigmatism formed by the main focusing lens as a result of penetration of the vertical focusing field through the open areas of the recesses. This effect is caused by the greater compression of vertical equipotential lines than of horizontal equipotential lines. The field penetration causes the focus lens to have greater vertical lens strength than horizontal lens strength. A correction is made for this astigmatism in the electron gun 26 of FIG. 2 by the inclusion of a horizontal slot opening at the exit of the G4 electrode 42. One particular embodiment has the slot width one-half the lens diameter and is spaced from the opposite surface of the G4 electrode at 86 percent of the lens diameter. This slot is formed by two strips 96 and 98, shown in FIGS. 2 and 5, welded to the apertured plate 52 of the G4 electrode 42 so as to extend across the three apertures therein in the plate 52.

To statically converge the two outer beams with the center beam, the length "E" of the recess 56 in the G4 electrode 42 is slightly greater than the length "F" of the recess 54 in the G3 electrode 40 (FIG. 3). The effect of the greater recess length in the G4 electrode 42 is the same as that discussed with respect to the offset apertures in U.S. Pat. No. 3,772,554, issued to Hughes on November 13, 1973.

Some typical dimensions for an electron gun such as the electron gun 26 of FIG. 2, but without the slot formed by strips 96 and 98, are presented in the following table.

In various other inline electron gun embodiments, the depth "G" of the recesses in the electrodes 40 and 42 may vary from 1.30 mm to 2.80 mm and the depth of the recesses in the two electrodes 40 and 42 may vary from each other.