CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2000-126071, filed Apr. 26, 2000; and No. 2001-081278, filed Mar. 21, 2001, the entire contents of both of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a color cathode ray tube and, more particularly, to a color cathode ray tube apparatus in which the elliptical distortion of electron beam spot shapes on the periphery of a phosphor screen is improved to allow displaying an image of good quality.

[0003] Generally, as shown in FIG. 1 , in a color cathode ray tube, a panel 1 is integrally bonded to a funnel 2 , and a phosphor screen 4 comprised of three color phosphor layers for emitting red, green, and blue light is formed on the inner surface of the faceplate of the panel 1 . A shadow mask 3 having a large number of electron beam holes is mounted inside the panel 1 to oppose the phosphor screen 4 . An electron gun 6 is arranged in a neck 5 of the funnel 2 , and three electron beams 7 B, 7 G, and 7 R emitted from the electron gun 6 are deflected by a magnetic field generated by a deflecting yoke 8 mounted on the outer surface of the funnel 2 and are directed toward the phosphor screen 4 . The phosphor screen 4 is scanned horizontally and vertically by the deflected electron beams 7 B, 7 G, and 7 R, thereby displaying a color image on the phosphor screen 4 .

[0004] As a color cathode ray tube of this type, an in-line type color cathode ray tube is available in which the electron gun 6 particularly forms an in-line type electron gun that emits three in-line electron beams made up of a center beam and a pair of side beams traveling on one horizontal plane, while the deflecting yoke generates a non-uniform magnetic field such that the horizontal deflecting magnetic field forms a pincushion type field and the vertical deflecting magnetic field forms a barrel type field, so the three electron beams self-converge.

[0005] For the in-line type electron gun for emitting three in-line electron beams, various types and methods are available, and a typical example them is a so-called BPF (Bi-Potential Focus) dynamic focus (Dynamic Astigmatism Correction and Focus) type electron gun. This BPF dynamic focus type electron gun is comprised of first to fourth grids G 1 to G 4 integrated with each other and sequentially arranged from three in-line cathodes K toward a phosphor screen 4 , as shown in FIG. 2 . Each of the grids G 1 to F 4 has three electron beam holes corresponding to the in-line type three cathodes K. In this electron gun, a voltage of about 150 V is applied to the cathodes K, the first grid G 1 is grounded, a voltage of about 600 V is applied to the second grid G 2 , and a voltage of about 6 kV is applied to the (3-1)th and (3-2)th grid G 3 - 1 and G 3 - 2 . A high voltage of about 26 kV is applied to the fourth grid G 4 .

[0006] In the above electrode structure to which the above voltages are applied, the cathodes K and the first and second grids G 1 and G 2 make up a triode for generating electron beams and forming an object point with respect to a main lens (to be described later). A pre-focus lens is formed between the second and (3-1)th grids G 2 and G 3 - 1 to pre-focus the electron beams emitted from the triode. The (3-2)th and fourth grids G 3 - 2 and G 4 form a BPF (Bi-Potential Focus) main lens for finally focusing the pre-focused electron beams onto the phosphor screen. If the deflecting yoke 8 deflects the electron beams to the periphery of the phosphor screen, a preset voltage is applied to the (3-2)th grid G 3 - 2 in accordance with the deflecting distance. This voltage is lowest when the electron beams are directed toward the center of the phosphor screen and highest when the electron beams are directed toward the periphery of the phosphor screen, thus forming a parabolic wave-shape. As the above electron beams are deflected to the periphery of the phosphor screen, the potential difference between the (3-2)th and fourth grids G 3 - 2 and G 4 decreases, and the intensity of the main lens described above is decreased. The intensity of the main lens is minimum when the electron beams are directed toward the periphery of the phosphor screen. As the intensity at the main lens changes, the (3-1)th and (3-2)th grids G 3 - 1 and G 3 - 2 form a tetrode lens. The tetrode is the most intense when the electron beams are directed toward the corners of the phosphor screen. The tetrode lens has a focusing function in the horizontal direction and a divergent function in the vertical direction. Thus, as the distance between the electron gun and phosphor screen increases and the image point becomes far, the intensity at the main lens decreases accordingly. As a result, a focus error based on a change in distance is compensated for, and deflection astigmatism caused by the pincushion type horizontal deflecting field and barrel type vertical deflecting field of the deflecting yoke is compensated for by the tetrode lens.

[0007] To improve the image quality of the color cathode ray tube, the focus characteristics on the phosphor screen must be improved. In particular, in a color cathode ray tube in which an electron gun for emitting three in-line electron beams is sealed, the elliptical distortion and blurring, as shown in FIG. 3 A, of an electron beam spot which are caused by deflection astigmatism become an issue. In a defection astigmatism compensating method generally called the BPF dynamic focus method (Dynamic Astigmatism Correction Focus method), a low-voltage side electrode which forms the main lens is divided into a plurality of elements such as the (3-1)th and (3-2)th grids G 3 - 1 and G 3 - 2 , and a tetrode lens is formed in accordance with the deflection of the electron beams. This method can solve the problem of blurring as shown in FIG. 3B . As shown in FIG. 3 B, however, a phenomenon still occurs in which electron beam spots are laterally flattened at the ends of the horizontal axis and the ends of the orthogonal axis of the phosphor screen. This causes a moiré effect due to interference with the shadow mask 3 . If electron beam spots form a character or the like, the character cannot be easily recognized.

[0008] The phenomenon in which an electron beam spot is laterally flattened will be described with reference to optical models shown in FIGS. 4A, 4B , and 5 .

[0009] FIG. 4A shows an optical system formed when the electron beams reach the center of the phosphor screen without being deflected, and the loci of the electron beams. FIG. 4B shows an optical system formed when the electron beams reach the periphery of the screen after being deflected by the deflecting magnetic fields, and the loci of the electron beams. The size of the electron beam spot on the phosphor screen depends on a magnification (M), and the magnification of the electron beam in the horizontal direction is defined as Mh and that in the vertical direction is defined as Mv. The magnification M can be expressed as (divergent angle αo/incident angle αi) shown in FIGS. 4A and 4B . More specifically,

Mh (horizontal magnification)=α oh (horizontal divergent angle)/α ih (horizontal incident angle)

Mv (vertical magnification)=α ov (vertical divergent angle)/α iv (vertical incident angle)

[0010] When the horizontal divergent angle αoh and vertical divergent angle αov are equal (αoh=αov), in the non-deflection mode shown in FIG. 4 A, the horizontal incident angle αih and vertical incident angle αiv become equal (αih=αiv) and the horizontal magnification Mh and vertical magnification Mv become equal (Mh=Mv), and in the deflection mode shown in FIG. 4 B, the horizontal divergent angle αoh becomes smaller than the vertical divergent angle αov (αoh<αov), and the vertical magnification Mv becomes smaller than the horizontal magnification Mh (Mv<Mh). In other words, the electron beam spot becomes circular at the center of the phosphor screen but is laterally elongated on the periphery of the phosphor screen.

[0011] As described above, in order to improve the image quality of the color cathode ray tube, a good focusing state must be maintained on the entire surface of the phosphor screen, and the elliptic distortion of the electron beam spot must be decreased. In the conventional BPF type dynamic focus electron gun, an appropriate dynamic voltage is applied to the low voltage side of the main lens in order to change the intensity of the main lens, and simultaneously to form a tetrode lens that changes dynamically, so the blur of the electron beam in the vertical direction, which is caused by the deflection aberration, can be eliminated. As a result, focusing can be performed on the entire surface of the phosphor screen. On the periphery of the phosphor screen, however, lateral flattening of the electron beam spot is apparent. This phenomenon occurs because, when the electron beam scans the periphery of the phosphor screen, the horizontal magnification Mh and vertical magnification Mv maintain a relationship Mv<Mh due to the electron lens formed by the electron lens and the astigmatism of the deflecting magnetic field.

[0012] As a prior art, a method of adjusting an induced dynamic voltage by newly adding an electrode or capacitor member outside an electron gun assembly is known as in, e.g., Jpn. Pat. Appln. KOKAI Publication No. 6-124633 or Jpn. Pat. Appln. No. 2000-73854. According to this method, when an additional component is to be attached to the electrode, the electrode may deform, thus rendering the focus performance unstable. When the additional component is placed near the neck of the cathode ray tube or another electrode, the breakdown voltage decreases. In addition, welding and addition of a component lead to an increase in the unit price of the electron gun.

[0013] According to the method known in Jpn. Pat. Appln. KOKAI Publication No. 2000-260349, dielectric portions are arranged between a plurality of divided focus electrodes, thereby adjusting a dynamic voltage induced in the electrodes connected to a resistor. In this method, since a tetrode lens and dielectric portions are arranged on a side closer to the cathode than the center of the main lens, the difference between the horizontal magnification and vertical magnification cannot be moderated, and improvement of the lateral flattening of the beam spot on the periphery of the screen, which is the object of the present invention, cannot be achieved.

BRIEF SUMMARY OF THE INVENTION

[0014] It is an object of the present invention to provide a color cathode ray tube with a good performance on the entire surface of the phosphor screen, in which the elliptic distortion of an electron beam spot is decreased on the entire surface of a phosphor screen.

[0015] According to the present invention, there is provided a color cathode ray tube apparatus comprising an electron gun in which a plurality of electron lenses including a main lens for accelerating and focusing an electron beam onto a screen are formed, and a deflecting yoke for deflecting the electron beam emitted from the electron gun in order to scan the screen in horizontal and vertical directions with the deflected electron beam, the main lens of the electron gun being comprised of at least a focus electrode and a final acceleration electrode along at least a traveling direction of the electron beam, wherein the electron gun has at least one intermediate electrode arranged between the final acceleration electrode and the focus electrode that make up the main lens, a voltage divided by a voltage dividing resistor for dividing a voltage to be applied to the final acceleration electrode is applied to the intermediate electrode, a dynamic voltage which increases along with an increase in deflecting amount of the electron beam is applied to the focus electrode, and a dielectric portion is formed between the electrodes that make up the main lens, the dielectric portion being formed on either one of the electrodes.

[0016] According to the present invention, there is provided a color cathode ray tube apparatus with the above arrangement, wherein the dielectric portion is provided between the electrode to which the dynamic voltage is applied and the intermediate electrode and is formed on either one of the electrodes, and the intermediate electrode is formed into a disk-like shape and has a non-circular electron beam hole with a major axis in a direction parallel to a horizontal direction of the screen.

[0017] According to the present invention, there is also provided a color cathode ray tube apparatus with either one of the arrangements described above, wherein the dielectric portion is provided between the intermediate electrode and the final acceleration electrode and is formed on either one of the electrodes by plating, and the intermediate electrode is formed into a disk-like shape and has a non-circular beam hole with a major axis in a direction parallel to a vertical direction of the screen.

[0018] Furthermore, according to the present invention, there is provided a color cathode ray tube apparatus with either one of the arrangements described above, wherein the dielectric portion is made of at least one ceramic or glass material selected from the group consisting of Al ₂ O ₃ , AlN, Si ₃ N ₂ , BaTiO ₃ , soda lime glass, SiO ₂ , borosilicate glass, and optical glass.

[0019] Furthermore, according to the present invention, there is also provided a color cathode ray tube apparatus with either one of the arrangements described above, wherein a relationship in characteristic curve of thermal expansion between the dielectric portion and a material that forms the electrode on which the dielectric portion is to be formed is set such that a difference in thermal expansion coefficient is not less than continuous 70% of a segment in a range of not less than room temperature and not more than 500° C. is between not less than 5×10 ⁻⁷ /° C. and not more than 15×10 ⁻⁷ /° C.

[0020] As a method of moderating the difference between the horizontal magnification Mh and vertical magnification Mv, a tetrode lens arranged on the preceding stage of the main lens is formed at the center of the electrode that forms the main lens.

[0021] This will be described by using optical models. As described above, FIG. 4B shows a case in a conventional electron gun wherein electron beams reach the periphery of a screen due to a deflecting magnetic field. In FIG. 4 B,

Mh (horizontal magnification)=α oh (horizontal divergent angle)/α ih (horizontal incident angle)

Mv (vertical magnification)=α ov (vertical divergent angle)/α iv (vertical incident angle)

[0022] It is apparent that Mh>Mv occurs because αih<αiv. More specifically, the above problem is moderated by increasing αih and decreasing αiv.

[0023] FIG. 5 shows an optical model in which a tetrode lens is formed at substantially the center of the main lens. In this optical lens, in the same manner as in the models shown in FIGS. 4A and 4B ,

Mh ′ (horizontal magnification)=α oh ′ (horizontal divergent angle)/α ih ′ (horizontal incident angle)

Mv ′ (vertical magnification)=α ov ′ (vertical divergent angle)/α iv ′ (vertical incident angle)

[0024] As is apparent from comparison of FIGS. 4B and 5 , when the tetrode lens becomes closer to the tetrode formed by the deflecting magnetic field,

αoh (horizontal divergent angle)=αoh′ (horizontal divergent angle)

αov (vertical divergent angle)=αov′ (vertical divergent angle)

[0025] αih (horizontal incident angle)>αih′ (horizontal incident angle)

αiv (vertical incident angle)>αiv′ (vertical incident angle)

[0026] In other words,

Mh′<Mh

Mv′>Mv

[0027] are obtained, and the elliptic ratio of the electron beam spot on the periphery of the screen is moderated as shown in FIG. 6 .

[0028] With the above arrangement, a tetrode lens is formed in the main lens. When a dielectric portion is formed on some of the electrodes that make up the main lens, an electrode that opposes the electrode having the dielectric portion forms a capacitor with an electrostatic capacitance necessary for forming the tetrode lens.

[0029] The operation of an electron gun in which a dielectric portion is formed between an electrode to which a dynamic voltage is applied and an intermediate electrode and non-circular electron beam holes with major axes in the horizontal direction are formed in the intermediate electrode will be described.

[0030] When the electron beams are not deflected, a voltage is supplied from a voltage dividing resistor to the intermediate electrode such that the potential distribution on the central axis of the electron beam hole from the focus electrode to the final acceleration electrode becomes similar to that of a bi-potential type main lens. For example, when the voltage of the focus electrode is 6 kV, the voltage of the final acceleration electrode is 26 kV, and the intermediate electrode is arranged at the mechanical center of the main lens, the voltage to be supplied to the intermediate electrode is 16 kV, which is an intermediate value between the voltage of the focus electrode and the voltage of the final acceleration electrode. Hence, the field strength from the focus electrode to the intermediate electrode and that from the intermediate electrode to the final acceleration electrode are equal, and potential penetration does not occur near the electron beam holes of the intermediate electrode. Therefore, the main lens constituted by components ranging from focus lens to the final acceleration electrode is equivalent to a bi-potential type electron lens, and the focusing power in the horizontal power and that in the vertical direction become equal.

[0031] When the electron beams are deflected, an AC voltage component of the dynamic voltage is induced in the intermediate electrode by the electrostatic capacitance of the capacitor formed with respect to the focus electrode, and the voltage of the intermediate electrode is increased. Hence, the potential distribution on the central axis of the electron beam hole from the focus electrode to the final acceleration electrode becomes different from that of the bi-potential type main lens, and the field strength between the focus electrode and the intermediate electrode becomes higher than that between the intermediate electrode and the final acceleration electrode. Consequently, potential penetration occurs in the final acceleration electrode side through the non-circular electron beam holes formed in the intermediate electrode and with the major axes in the horizontal direction. A tetrode lens with a divergent function in the vertical direction and a focusing function in the horizontal direction is formed in the main lens, and astigmatism occurs in the main lens. Therefore, the blur of electron beam spots on the periphery of the screen is solved, and since the tetrode lens is formed in the main lens, the difference between the horizontal magnification Mh and vertical magnification Mv is decreased, so that the elliptic distortion of the electron beam spots can be moderated.

[0032] To sufficiently increase the intensity of the tetrode lens, a higher AC voltage component must be induced in the intermediate electrode. A voltage V 1 induced in the intermediate voltage is expressed by the following equation: 1 $\mathrm{V1}=\frac{\mathrm{C1}}{\mathrm{C1}+\mathrm{C2}}\mathrm{Vd}$

[0033] where C 1 is the electrostatic capacitance of a capacitor formed between the focus electrode and intermediate electrode, C 2 is the electrostatic capacitance between the final acceleration electrode and intermediate electrode, and Vd is the AC voltage component of the dynamic voltage to be applied to the focus electrode, as shown in FIG. 7 .

[0034] Therefore, to obtain a sufficiently high intensity for a tetrode lens, the electrostatic capacitance C 1 of the capacitor may be increased. Then, the dynamic voltage V 1 induced in the intermediate electrode increases so a large difference is produced between the field strength between the focus electrode and intermediate electrode and the field strength between the intermediate electrode and the final acceleration electrode, thereby increasing the intensity at the tetrode lens in the main lens. In other words, the dynamic voltage necessary for obtaining a tetrode lens with a desired intensity can be decreased.

[0035] Generally, in a cathode ray tube, the gap between the electron gun and neck is small, and a space for placing a capacitor with a sufficiently large electrostatic capacitance cannot be ensured.

[0036] According to the present invention, the capacitor can be set within the electrode gap of the electron gun assembly. Thus, a capacitor with several 10 pF to several 1,000 pF or more can be obtained by appropriately selecting the material type of the dielectric portion, which is larger than that obtained when the electrostatic capacitance of an arbitrary portion is formed of only a vacuum state. An appropriate combination of dielectric portion materials can make a tetrode lens with a sufficiently high intensity.

[0037] If C 1 is 18.0 pF and C 2 is 2.5 pF, the AC voltage component V 1 induced in the intermediate electrode is as follows: 2 $\mathrm{V1}=\frac{18.0\mathrm{pF}}{18.0\mathrm{pF}+2.5\mathrm{pF}}\approx 0.88\mathrm{Vd}$

[0038] In other words, about 88% of Vd can be induced in the intermediate electrode, so the intensity at the tetrode lens can be increased.

[0039] Component deformation of the intermediate electrode directly influences the focus performance and thus must be prevented as much as possible. Formation of the dielectric portion increases the mechanical strength of the electrode itself. In addition, if the intermediate electrode is fixed to another electrode through the dielectric portion, when the intermediate electrode is to be built in the electron gun assembly, a deforming force may not act on the intermediate electrode itself. As a result, a focus performance can be stably obtained with an inexpensive, simple structure.

[0040] With the above operation, the elliptic distortion of the electron beams can be moderated more efficiently, and a stable focus performance can be obtained.

[0041] So far a case has been described wherein a dielectric portion is formed between an intermediate electrode and an electrode to which a dynamic voltage is to be applied and non-circular electron beam holes with major axes in the horizontal direction are formed in the intermediate electrode. The same operation can be obtained when a dielectric portion is formed between the intermediate electrode and final acceleration electrode and non-circular electron beam holes with major axes in the vertical direction are formed in the intermediate electrode. The latter case is different from the former case in that the voltage induced in the intermediate electrode is suppressed as much as possible.

[0042] In the latter case, a voltage V 2 induced in the intermediate electrode is expressed by the following equation: 3 $\mathrm{V2}=\frac{\mathrm{C1}}{\mathrm{C1}+\mathrm{C2}}\mathrm{Vd}$

[0043] Therefore, if an electrostatic capacitance C 2 of a capacitor formed by the dielectric portion formed between the intermediate electrode and final acceleration electrode is set sufficiently larger than an electrostatic capacitance C 1 between a focus electrode and the intermediate electrode, the dynamic voltage V 2 induced in the intermediate electrode becomes close to zero, and a change in voltage becomes very small.

[0044] Similarly, if C 1 =2.5 pF and C 2 =18.0 pF, V 2 becomes as follows: 4 $\mathrm{V2}=\frac{2.5\mathrm{pF}}{2.5\mathrm{pF}+18.0\mathrm{pF}}\mathrm{Vd}\approx 0.12\mathrm{Vd}$

[0045] In other words, the dynamic voltage induced in the intermediate electrode can be suppressed to about 12% of Vd.

[0046] As a result, the potential difference with respect to the focus electrode to which the dynamic voltage is applied can be decreased, so a large difference is produced between the field strength between the focus electrode and intermediate electrode and the field strength between the intermediate electrode and final acceleration electrode. Consequently, the intensity at the tetrode lens in the main lens can be further increased, and accordingly the same operation as that described above can be obtained.

[0047] Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.