In-Line Type Electron Gun

The present invention relates to an in-line electron gun built in a color picture tube.

Generally, in a color picture tube comprising an in-line type electron gun from which three electron-beams are emitted into a plane, as shown in FIG. 1, a center beam 1a and side beams 1b, 1c pass through main lenses 2a, 2b, 2c, respectively, to be focussed. In order that the side beams 1b, 1c are converged to a point 3 at the center of the phosphor screen together with the center beam 1a, a beam convergence angle .theta. is given between each of the side beams 1b, 1c and the center beam 1a. Also, a self-convergence deflection magnetic field is provided so that the convergence of the three beams 1a, 1b, 1c is performed automatically even at deflection to the peripheries of the screen. In the picture tube system thus constructed, the beam convergence angle .theta. affects the beam convergence characteristics over the entire phosphor screen.

The beam convergence angle .theta. can be given by arranging three electron guns obliquely. But, in that method, the beam convergence angle .theta. is liable to be varied by assembly errors that occur when the three independent electron guns are integrated into an assembled gun. Generally, therefore, a unitized electron gun structure in which the relative displacement of the three electron beams is expected to be small is employed as shown in FIG. 2. A unitized electron gun is described in detail in Japanese Patent Publication No. 4905/77 and others. The center axes 6b, 6c of the side beam apertures 5b, 5c among the beam apertures 5a, 5b, 5c at the end of a focussing electrode 4 on the side of an anode electrode, and the center axes 9b, 9c of the side beam apertures 8b, 8c among the beam apertures 8a, 8b, 8c at the end of the anode electrode 7 on the side of the focussing electrode are displaced or offset to each other to obtain axially asymmetric side main lenses 10b, 10c, so that the side beams are electrostatically deflected by the beam convergence angle .theta..

Incidentally, in this unitized electron gun structure, the beam convergence angle .theta. is determined by the relative positions of the side beam apertures 5b, 5c of the focussing electrode 4 and the side beams apertures 8b, 8c of the anode electrode 7, and therefore a very severe manufacturing accuracy is required for the electrodes 4 and 7.

In the in-line type electron gun according to the present invention, the center axis common to the respective side beam apertures of the control electrode and the accelerating electrode is offset toward the tube axis from the center axis of the side beam aperture of the focussing electrode end surface on the side of the accelerating electrode, and at the same time, the center axis common to the respective side beam apertures at the opposite side surfaces of the focussing electrode and the anode electrode is offset toward the tube axis from the center axis common to the respective side beam apertures of the control electrode and the accelerating electrode.

FIG. 1 is a diagram for explaining the convergence of three electron beams by a conventional in-line type electron gun,

FIG. 2 is a side sectional view showing the electrode configuration of a part of the same electron gun,

FIG. 3 is a side sectional view of an in-line type electron gun embodying the present invention, and

FIG. 4 is a diagram for explaining the convergence of three electron beams from the electron gun shown in FIG. 3.

In the preferred embodiment of the invention shown in FIG. 3, three cathode electrodes 11a, 11b, 11c, arranged on a horizontal straight line, a control electrode 12, an accelerating electrode 13, a focussing electrode 14 and an anode electrode 15 make up a unitized in-line electron gun. A center beam aperture 16a and side beam apertures 16b, 16c of the control electrode 12 share common central axes 18a, 18b, 18c respectively with a center beam aperture 17a and side beam apertures 17b, 17c of the accelerating electrode 13. The center axis 18a common to the center beam apertures 16a and 17a is coaxial with the tube axis 19.

As shown in FIG. 3, the anode electrode 15 preferably is a cup shaped member provided in its end surface with a center beam aperture 23a and side beam apertures 23b and 23c, and the focusing electrode 14 preferably is a composite member formed of two axially joined cup-shaped members, each provided in its end surface with respective center and side beam apertures 20a, 20b, 20c (which are disposed opposite the beam apertures 17a, 17b and 17c, respectively, of the accelerating electrode 13) and 22a, 22b, 22c (which are disposed opposite the beam apertures 22a, 22b, 22c, respectively, of the anode electrode 15). As shown, the one of the the cup-shaped members of the focusing electrode 14 whose end surface faces, i.e. is opposite, the end surface of the anode electrode 15 is of the same size and shape as the anode electrode 15.

The center axis 21a of the center beam aperture of the focussing electrode 14 on the side of the accelerating electrode 13, is coaxial with the tube axis 19. However, the center axes 21b, 21c of the side beam apertures 20b, 20c respectively are displaced from the above-mentioned common center axes 18b, 18c respectively. In other words, the common center axes 18b, 18c for the side beam apertures 16b, 16c, 17b, 17c of the control electrode 12 and the accelerating electrode 13 respectively are offset toward the tube axis from the center axes 21b, 21c of the side beam apertures 20b, 20c of the focussing electrode 14 on the side of the accelerating electrode 13.

Further, the center beam aperture 22a and the side beam apertures 22b, 22c of the focussing electrode 14 on the side of the final accelerating electrode 15 share common center axes 24a, 24b, 24c respectively with the center beam aperture 23a, and the side beam apertures 23b, 23c of the final accelerating electrode 15 on the side of the focussing electrode. The common center axis 24a for the center beam apertures 22a, 23a is coaxial with the tube axis 19, while the common center axes 24b, 24c for the side beam apertures 22b, 22c, 23b, 23c respectively are offset toward the tube axis from the common center axes 18b, 18c respectively.

In the in-line electron gun constructed in this way, an axially symmetric prefocus lens electric field is formed between the center beam aperture 17a of the accelerating electrode 13 and the center beam aperture 20a of the focussing electrode 14, while axially-asymmetric prefocus lens electric fields are formed between the side beam apertures 17b, 17c of the accelerating electrode 13 and the side beam apertures 20b, 20c of the focussing electrode 14 respectively. As a result, the three electron beams generated from the three cathode electrodes 11a, 11b, 11c and passed through the center beam aperture 16a and the side beam apertures 16b, 16c of the control electrode 12 are pre-focussed by said prefocus lens electric fields. Since the both side prefocus lens electric fields are axially asymmetric, the side beams are deflected slightly toward the tube axis.

FIG. 4 shows three prefocus lens sections as equivalent electron sources 25a, 25b, 25c. The equivalent electron sources 25b, 25c on the both sides are displaced from the above-mentioned common center axes 24b, 24c respectively by .DELTA.x. The center beam 26a advances straight along the tube axis 19 and enters the axially-symmetric center main lens 27a on the tube axis 19, while the side beams 26b, 26c advance obliquely at an angle of .alpha. and enter the axially-symmetric side main lenses 27b, 27c.

The center beam 26a and the side beams 26b, 26c are focussed respectively by the main lenses 27a, 27b, 27c, and in the absence of the deflection magnetic field acting thereon, the side beams 26b, 26c are biased by .DELTA.x.multidot.M from the center axes 24b, 24c on the phosphor screen 28. M indicates the lens magnification.

Therefore, when the center displacement .DELTA.x is set so that the bias amount (.DELTA.x.multidot.M) is equal to the distance S between the center axes 24b, 24c and the tube axis 19 (.DELTA.x.multidot.M=S), the center beam 26a and the side beams 26b, 26c can be converged to a point at the center on the phosphor screen 28.

The prefocus lenses on both sides 26b, 26c are axially asymmetric. If the respective amounts of displacement of the center axes 18b, 18c from the center axes 21b, 21c are appropriately set to provide an appropriate inclination angle .alpha., the beam spot (bright spot) on the phosphor screen 28 can be made a true circle. Also, since the center axes of the associated beam apertures 22 and 23 on the respective opposed or facing side ends of the focussing electrode 14 and on the final accelerating electrode 15 are not required to be displaced from each other, the half of the focussing electrode 14 facing the final accelerating electrode or anode 15 can be formed in the same press die used for forming the final accelerating electrode 15. Thus convergence failures caused by variations in the shape of the electrodes 14, 15 can be reduced. Further, even when a satisfactory roundness of the beam apertures cannot be obtained due to the natures inherent to the press die, at least the opposite side ends of the electrodes 14, 15 can be reversely combined in their upper and lower relation, and therefore a superior beam spot shape with a high uniformity of convergence can be obtained.

As explained above, the in-line electron gun according to the present invention facilitates the manufacture, management and assembly of the focussing electrode and final accelerating electrode of comparatively complicated construction, thus producing a superior beam spot shape, that is, a high-resolution characteristic.