More particularly, the present invention relates to an electron gun for color cathode ray tube wherein the penetrating holes for electron beams at the focusing electrode are formed in the shape of a concave lens. The spots of electron beams from the electron gun are uniform on the central part and peripheral part of the fluorescent screen and the resolution is improved.

Background Art A general constitution of a dynamic focusing electron gun for the color cathode ray tube (hereinafter referred to as"color CRT") is shown in Figure 1. In this figure, the electrode assembly comprises the first grid electrode (11), the second grid electrode (12), the third grid electrode (13), the forth grid electrode (14), the fifth grid electrode (15), the sixth grid electrode (16) and the seventh grid electrode (17), and cathode supporter (18), which are assembled with predetermined intervals on the bead glass (19). A shielding cup (20) is welded on the seventh grid electrode (17) by spot-welding, and Bulb Space Contact (21) is welded on the shielding cup (20) by spot-welding so that the electron gun is located on a neck part (not shown) of the CRT with predetermined intervals. Stem (22) is welded on Stem Lug (23) by spot- welding. The Stem (22) is used to apply a voltage to the first grid electrode (11), the second grid electrode (12), the third grid electrode (13), the forth grid electrode (14), the fifth grid electrode (15), the sixth grid electrode (16), and the seventh grid electrode (17), and the cathode (not shown) of the cathode supporter (18).

The wiring scheme for applying voltage in the electron gun (10) is shown in Figure 2. A ground voltage (V1) is applied to the first grid electrode (11). A positive screen voltage (V2) is applied to the second grid electrode (12) and the forth grid electrode (14). A focusing voltage (Vfs) is applied to the third grid electrode (13) and the fifth grid electrode (15). A dynamic focusing voltage (Vd), which is synchronously modulated with deflection signals of the deflection yoke and whose base voltage is the focusing voltage (Vfs), is applied to the sixth grid electrode (16).

An anode voltage (Va) which is higher than the focusing voltage is applied to the seventh grid electrode (17). Three cathodes (18R), (18G), (18B) are configured with horizontal in-line arrangement so that the electron beams emitted from the cathodes respectively make the red, green or blue phosphors to be luminous. Each of grid electrodes may have three penetrating holes for electron beams.

The above-mentioned electron gun is sealed in the CRT (not shown).

Heated electrons which are heated by heater are emitted from the three cathodes (18R), (18G), (18B). The heated electrons are accelerated by the voltage (V2) applied to the second grid electrode (12), and form electron beams. The amount of the electron beams is controlled by the voltage applied to the cathode supporter (18).

The electron beams are more accelerated after they passed the second grid electrode (12). As the electron beams pass the third grid electrode (13), the forth grid electrode (14), and the fifth grid electrode (15), the path and the amount of the electron beams are adjusted, and then, the electron beams are incident into the sixth grid electrode (16) and the seventh grid electrode (17). An electrostatic lens formed between the sixth grid electrode (16) and the seventh grid electrode (17) forms electron beam spots on the screen of CRT. The electron beams should be scanned on the full screen with deflection field which is the electromagnetic field generated by the deflection yoke. The electron gun emits the plural electron beams and makes the cathode ray and phosphors to be luminous. Images on the screen are formed by the

combination of three colors. It is important to concentrate plural electron beams to a certain point on the screen with accuracy.

The electron gun emits plural electron beams in horizontal in-line mode, and adopts a self convergence mode wherein the deflection yoke system modify the horizontal deflection system to Pin Cushion type, and modify the vertical deflection system to Barrel type non-uniformly. In the deflection yoke, there is a quadrupole lens as shown in Figure 3, which affects the electron beams. The electron beams which pass through a deflection region are formed in the shape of oblong with longer horizontal line (31). As a result, the deterioration of focus and the moire phenomenon is occurred. Therefore, the resolution of the peripheral part of the screen is inferior to that of the central part.

Disclosure of Invention Recently, a dynamic focus electron gun is proposed. In the dynamic focus electron gun, the quadrupole lens is formed between the fifth grid electrode (15) and the sixth grid electrode (16) in order to correct the shape of electron beam spots into the round shape at the peripheral part of the screen. A dynamic focus voltage (Vd), which is synchronized with the horizontal deflection frequency and the vertical deflection frequency and which is a modulation voltage higher in the peripheral part of screen, is superposed on the focusing voltage (Vfd) which is applied to sixth grid electrode (16) (which is a divergent electrode), so that the elliptical shape of electron beam spots in the peripheral part of screen may modified into a circular shape shown in the center of screen (41) in Figure 4.

In the above-mentioned dynamic focus electron gun, penetrating holes whose shapes are oblong with longer vertical line and oblong with longer horizontal line are formed in the reciprocally facing surface of dynamic electrodes. A Focusing voltage

(Vfs) is applied to the fifth grid electrode (15) which is a focusing electrode. An anode voltage (Va) is applied to the seventh grid electrode (17) which is an anode electrode. A superposition voltage of the dynamic voltage (Vd) and focusing voltage (Vfd) is applied to the sixth grid electrode (16) which is a dynamic electrode as shown in Figure 5. The dynamic voltage (Vd) is varied, and its wave forms the shape of curve, such as a parabola depending on the vertical synchronizing signal period (V).

The dynamic voltage is also varied depending on the horizontal synchronizing signal period (H).

If the electron beams are not deflected in the above-mentioned dynamic <BR> <BR> focusing electron gun (i. e. , if the electron beams are scanned on the center of the fluorescent screen), the dynamic voltage (Vd) is 0 [V] and no quadrupole lens are formed between the fifth grid electrode (15) (which is a focusing electrode) and the sixth grid electrode (16) (which is a dynamic electrode) since no dynamic voltage (Vd) is applied to the dynamic electrode. Therefore, the landing shape of the electron beams on the center of fluorescent screen through the main lens is optimally a circular shape as shown in Figure 6.

On the other hand, if the electron beams are deflected to the peripheral part of screen by the electromagnetic of deflection yoke, the quadrupole lens are formed between the fifth grid electrode (15) (which is a focusing electrode) and the sixth grid electrode (16) (which is a dynamic electrode) as shown in Figure 3 since a dynamic voltage (Vd), which is varied depending on the vertical synchronizing signals and the horizontal synchronizing signals, is applied to the sixth grid electrode (16) (which is a dynamic electrode). A weaker focusing lens and a stronger divergent lens in the vertical direction in comparison with the horizontal direction are formed respectively.

Therefore, the electron beams which pass through these two lens are received the force of focusing in the horizontal direction and the force of divergence in the vertical direction. Thus, the shape of oblong with longer vertical line is formed.

Since the beam spots, whose shape is oblong with longer horizontal line, formed in the peripheral part of screen are modified in the shape of oblong with longer vertical line, the distortion of electron beams resulted from non-uniform electromagnetic field of deflection yoke is corrected. Thus, the electron beam spots are formed in the circular shape on the peripheral part of screen as shown in Figure 7.

In addition, since the superposition voltage of focusing voltage (Vfd) and dynamic voltage (Vd) is applied to the sixth grid electrode (16) (which is a dynamic electrode), the strength of main lens becomes weaker and the focal distance becomes longer. Therefore, the optimal focus may be obtained as shown in Figure 8 even though the electron beams are deflected to the peripheral part of screen.

If the shape of penetrating holes for electron beams at the fifth grid electrode (15) (which is a focusing electrode) and the sixth grid electrode (16) (which is a dynamic electrode), which are forming the quadrupole lens, and the dynamic voltage (Vd) which is applied to the dynamic electrode are adjusted properly, the focusing force and the divergent force are changed. Thus, the optimal focus and the improved resolution may be obtained on all over the fluorescent screen.

In order to accomplish such a proposal, many kinds of penetrating holes in the shape of oblong with longer vertical line for electron beams are introduced, such as key-hole shaped penetrating holes. However, there is a limitation to improve the resolution, and the electron beam spots are not formed uniformly on the peripheral part and central part of the screen.

Therefore, the object of the present invention is to provide an electron gun for color CRT which is able to form electron beam spots uniformly over the peripheral part of the screen and the central part of the screen, which may bring an improved resolution.

Another object of the present invention is to provide an electron gun for color

CRT wherein the shape of penetrating holes for electron beams in the focusing electrode and the dynamic electrode are modified, which may bring an improved resolution.

To accomplish such objects, the electron gun for CRT of the present invention comprises: three electrode part consisting of the plural cathode emitting electron beams, control electrode for controlling the emission amount of the electron beams, and acceleration electrode; one or more pre-focusing lens consisting of two or more electrodes which are focusing the predetermined amount of electron beams; focusing electrode and anode electrode forming main lens to focus the electron beams on the screen; and quadrupole lens forming part in the facing surface of two electrodes, of which the first electrode is one among the cathode and the plural electrodes to which the predetermined voltages are applied, and of which the second electrode is one among the electrodes to which the dynamic voltage is applied, and in the electron gun, penetrating holes in the shape of concave lens are formed at the focusing electrode facing to the divergent electrode to which the dynamic voltage is applied.

Preferably, the penetrating holes in the shape of concave lens may be formed at the divergent electrode instead of the focusing electrode to obtain electron beam spots in the shape of oblong with longer horizontal line.

Preferably, the dynamic voltage is applied to the focusing electrode, and the predetermined voltage is applied to the divergent electrode.

Preferably, the right rounded side and the left rounded side of the penetrating holes for electron beams in the shape of concave lens may be symmetric or asymmetric.

Brief Description of Drawings Figure 1 illustrates the structure of the conventional electron gun for color CRT to which the present invention may apply.

Figure 2 is the voltage applying scheme for the electron gun of Figure 1.

Figure 3 illustrate a function of a quadrupole lens due to the non-uniform electromagnetic field of the deflection yoke.

Figure 4 illustrates the electron beam spots formed on the screen.

Figure 5 illustrates the wave of voltage which is applied to the dynamic electrode of the dynamic focusing electron gun.

Figure 6 illustrates the astigmatism due to the non-uniform electromagnetic field of the deflection yoke.

Figure 7 illustrates the state wherein the astigmatism of Figure 6 is corrected.

Figure 8 illustrates the state wherein the focus is corrected.

Figure 9 illustrates penetrating holes for electron beams in the shape of concave lens at the focusing electrode for quadrupole electrostatic lens electrode of electron gun for color CRT according to the present invention.

Figure 10 illustrates the curvature of the penetrating holes at the focusing electrode and the divergent electrode of the electron gun for color CRT according to the present invention.

Figure 11 illustrates the strength of quadrupole lens depending on the radius of the curvature of the right rounded side and the left rounded side of the penetrating holes for electron beams at the focusing lens of Figure 10.

Figure 12 illustrates the strength of quadrupole lens depending on the radius of the curvature of the upper rounded side and the lower rounded side of the penetrating holes for electron beams at the divergent lens of Figure 10.

Best Mode for Carrying Out the Invention Reference will now be made in detail to the present invention as illustrated in the accompanying drawings.

Figure 9 illustrates penetrating holes for electron beams formed at the focusing electrode and the dynamic electrode of the electron gun for color CRT according to the present invention.

As shown in Figure 9, the first plate unit (150) is provided in cylindrical body of the fifth grid electrode (15) (which is a focusing electrode shown in Figure 1), and three penetrating holes (151), (152), (153) are formed in the first plate unit (150) at a predetermined intervals separately. In addition, the second plate unit (160) is provided in cylindrical body of the sixth grid electrode (16) (which is a dynamic electrode), and three penetrating holes (161), (162), (163) are formed in the second plate unit (160) at a predetermined intervals separately. The three penetrating holes (161), (162), (163) are located on the same axis with the axis of penetrating holes (151), (152), (153). The actual distance between the first plate unit (150) and the second plate unit (160) is conventionally 0. 5mm even though it might look like separate with a distance more than 0. 5mm in the drawing.

The penetrating holes (151), (152), (153) are in the shape of rectangle with

longer vertical line, wherein the vertical line is longer than the horizontal line.

Especially, the right side and the left side of the rectangular shape are rounded toward the inner center, as the shape of concave lens. The both sides are rounded in a radius of curvature (R). The penetrating holes (161), (162), (163) are in the conventional shape of rectangle with longer horizontal line, wherein the horizontal line is longer than the vertical line.

A simulation is performed to measure the ratio (V/H) of the vertical size to the horizontal size of the electron beam spots depending on the variation of the curvature radius (R1) of right rounded side and the left rounded side of penetrating holes (151), (152), (153) at a distance wherein the effect of the lens does not appear, and the strength of electron beams is 5%, 10% or 25% relative to the strength of electron beams in the center wherein the strength of electron beams is most strong, under the condition that the focusing voltage (Vfs) is applied to the fifth grid electrode (15) having the penetrating holes (151), (152), (153), and that the focus voltage (Vfd+Vfs) is applied to the sixth grid electrode (16) when the electron beams are incident through the penetrating holes (151), (152), (153).

In addition, a simulation is also performed to measure the ratio (V/H) of the vertical size to the horizontal size of the electron beam spots depending on the variation of the curvature radius (R2) of the upper rounded side and the lower rounded side of penetrating holes (161), (162), (163) at a distance wherein the effect of the lens does appear, and the strength of electron beams is 5%, 10% or 25% relative to the strength of electron beams in the center wherein the strength of electron beams is most strong, under the condition that the focusing voltage is applied to the fifth grid electrode (15), and that the superposition voltage of focusing voltage (Vfd) and the dynamic voltage (Vd) is applied to the sixth grid electrode (16) (which is a convergent voltage). The incident electron beams are ones which pass the penetrating holes (151), (152), (153). The radius (Rl), (R2) are shown in Figure 10.

Finally, a similar simulation is also performed to the electron gun wherein the penetrating holes in the conventional key-hole shape are adopted at the fifth grid electrode.

As the result of the above simulations, the effect of the quadrupole lens is changed depending on the variation of the radius (R1) of the right rounded side and the left rounded side of the fifth grid electrode (15) (which is focusing electrode) as shown in Figure 11. The effect of the quadrupole lens is also changed depending on the variation of the radius (R2) of upper rounded side and the lower rounded side of the sixth grid electrode (16) (which is a divergent electrode) as shown in Figure 12.

As the radius (Rl) of the right rounded side and the left rounded side of the penetrating holes (151), (152), (153) in the fifth grid electrode (15) is shorter, the effect of the quadrupole lens is greater as shown in Figure 11. As shown in Figure 12, as the radius (R2) of the upper rounded side and the lower rounded side of the penetrating holes (161), (162), (163) in the sixth grid electrode (16) is longer, the effect of quadrupole lens is as great as the effect of the quadrupole lens in the penetrating holes (161), (162), (163) in the shape of key-hole.

For the convenience of the explanation of the present invention, it was assumed that the right rounded side and the left rounded side of the penetrating holes in the shape of concave lens are symmetric. However, it is possible that the right rounded side and the left rounded side of the penetrating holes are asymmetric.

Therefore, in the electron gun according to the present invention which has penetrating holes in the shape of key-hole at the divergent electrode and has penetrating holes in the shape of concave lens at the focusing electrode, the effect of the quadrupole lens is greater than in the conventional electron gun which has penetrating holes in the shape of key-hole at both of the focusing electrode and the divergent electrode.

In contrast to the method of applying voltage shown in Figure 9, if the superposition voltage (Vfd+Vd) is applied to the fifth grid electrode (15) and the focus voltage (Vfs) is applied to the sixth grid electrode (16), electron beam spots may be formed in the shape of oblong with longer horizontal line.

Industrial Applicability As explained in the above, the electron gun for color CRT according to the present invention has penetrating holes in the shape of conventional key-hole at the sixth grid electrode (which is a divergent electrode), and penetrating holes in the shape of concave lens or oblong with longer vertical line, wherein the right side and left side are rounded toward the inner center, at the fifth grid electrode (which is a focusing electrode).

Therefore, as the radius of the right rounded side and the left rounded side of the penetrating holes at the focusing electrode are shorter, the effect of the quadrupole lens is greater. As a result, uniform electron beam spots may be formed on the peripheral part and the central part of the screen and the resolution is improved as well.

The present invention is not limited to the attached drawings and detailed description of the present invention set forth above. Rather, it is apparent to the persons with ordinary knowledge in the relevant field that the present invention may be modified and changed in various manners within the extent not exceeding the essence of the present invention claimed in the following claims.