This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-365927, filed Nov. 30, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates generally to a cathode ray tube (CRT) apparatus, and more particularly to a color cathode ray tube apparatus equipped with an in-line electron gun assembly for emitting three electron beams travelling in the same horizontal plane.
2. Description of the Related Art
In recent years, a self-convergence in-line type color cathode ray tube apparatus has widely been put to practical use. This CRT apparatus is characterized in that three in-line electron beams are self-converged on the entire area of a phosphor screen.
In this type of color CRT apparatus, a method of increasing a lens aperture of a main lens section created by an electron gun assembly is effective as means for obtaining good image characteristics. Typical means for increasing the lens aperture of the main lens section are an overlapping field type lens and an extended field type lens.
As is shown in FIG. 1, an overlapping field type lens 52 is created between two adjacent electrodes 50a and 50b, which have outer peripheral electrodes 51a and 51b at their opposing faces. The overlapping field type lens 52 is an electric field lens acting commonly on three electron beams passing through three electron beam passage holes formed in each of the electrodes 50a and 50b. Thereby, the lens diameter of the main electric filed is substantially increased.
As is shown in FIG. 2, an extended field type lens 65 is created by disposing an intermediate electrode 62 between a focus electrode 61 and an anode 63. A focus voltage is applied to the focus electrode 61, an anode voltage higher than the focus voltage is applied to the anode 63, and a voltage of an intermediate level between the focus voltage and anode voltage is applied to the intermediate electrode 62. In general, in consideration of breakdown voltage characteristics, a voltage obtained by resistor-dividing the anode voltage via a resistor 64 is applied to the intermediate electrode 62. The extended field type lens 65 increases the lens diameter by extending the lens region in the tube axis direction.
Jpn. Pat. Appln. KOKAI Publication No. 9-320485, for instance, discloses that two lenses are combined to obtain more improved image characteristics.
On the other hand, the effect of deflection magnetic fields upon electron beams cannot be ignored. In the color CRT apparatus, electron beams, which have passed through non-uniform magnetic fields, are affected by deflection aberration components included in the deflection magnetic fields. Consequently, a beam spot deforms on a peripheral portion of the phosphor screen, and the resolution considerably deteriorates.
An electron beam 12 deflected onto a peripheral portion of the phosphor screen is affected by a force exerted by a pincushion type horizontal deflection magnetic field 11 in the direction of arrows 13, as shown in, e.g. FIG. 3A. As a result, as shown in FIG. 3B, the beam spot on the peripheral portion of the phosphor screen horizontally deforms, and the resolution greatly deteriorates.
The electron beam affected by the deflection aberration components is horizontally enlarged and vertically over-focused. The beam spot formed on the peripheral portion of the phosphor screen thus produces a high-luminance core portion 14 deformed in a horizontal direction X and a low-luminance halo portion 15 enlarged in a vertical direction Y.
Jpn. Pat. Appln. KOKAI Publication No. 61-99249, for instance, discloses structural means for solving the problem of deterioration in resolution. The electron gun assembly in this structural means basically comprises first to fifth grids. The electron gun assembly also includes an electron beam generating section, a quadrupole lens and a main lens, which are disposed in the direction of travel of electron beams. The third and fifth grids disposed adjacent to each other to create the quadrupole lens have, respectively, vertically and horizontally elongated non-circular electron beam passage holes in their mutually opposing faces.
The lens function of the quadrupole lens dynamically is varied by applying a dynamic focus voltage that varies in synchronism with deflection magnetic fields to the fourth grid. Thus, the quadrupole lens corrects the deformation due to deflection aberration of the electron beam deflected on the peripheral portion of the phosphor screen.
If the quadrupole lens is combined with the above-mentioned two lenses (overlapping field type lens and extended field type lens), good image characteristics can be obtained over the entire area of the screen.
The overlapping field type lens can increase the horizontal lens diameter relative to the electron beams, but it cannot increase the vertical lens diameter as much as the horizontal lens aperture. This results in a difference in lens diameter between the horizontal and vertical directions, and the focal distance in the vertical direction becomes shorter than that in the horizontal direction. Thus, this overlapping field type lens has a negative astigmatism. The electron beam, which has passed through the overlapping field type lens, is horizontally under-focused and vertically over-focused. In order to compensate the negative astigmatism, one of the electrodes which is arranged back from the overlapping field type lens is generally provided with vertically elongated electron beam passage holes.
However, this electrode structure makes the horizontal dimension of the electron beam passage hole less than the vertical dimension thereof. Consequently, the distance between the electron beam and the horizontal end portions of the electron beam passage hole in the electrode decreases, and local aberration occurs. In practice, even if the length of the outer peripheral electrode is to be extended in the tube axis direction to realize a large-aperture lens, the above-mentioned horizontal local aberration restricts the length of the outer peripheral electrode and makes it difficult to obtain a desired lens aperture.
The combination of the above-mentioned quadrupole lens and the extended field type lens will now be considered.
In an electron gun assembly as shown in FIG. 4, a quadrupole lens is formed between a first focus electrode 803 and a second focus electrode 804 to which a dynamic focus voltage is applied. The first focus electrode 803, second focus electrode 804, an intermediate electrode 805 and an anode 806 constitute an extended field type main lens. The intermediate electrode 805 is supplied with a voltage from the anode 806 via a resistor 807.
In this structure, if a dynamic focus voltage is applied to the second focus electrode 804, part of the AC component of the dynamic focus voltage is superimposed on the intermediate electrode 805 due to the electrostatic capacitance created among the second focus electrode 804, intermediate electrode 805 and anode 806. Thus, the potential of the intermediate electrode 805 increases.
As is shown in FIG. 5, a potential Vf of the second focus electrode, a potential Vgm of the intermediate electrode and a potential Eb of the anode are set to become higher in the named order. When an AC component of the dynamic focus voltage is not applied to the second focus electrode, the extended field type main lens has a potential distribution 904. When an AC component of the dynamic focus voltage has been applied to the second focus electrode and a part of the AC component of the dynamic focus voltage is not superimposed on the intermediate electrode, the extended field type main lens has a potential distribution 905. When an AC component of the dynamic focus voltage has been applied to the second focus electrode and a part of the AC component of the dynamic focus voltage has been superimposed on the intermediate electrode, the extended field type main lens has a potential distribution 906.
Let us consider the position of the principal plane of the main lens, that is, the position of the center of the lens, in the respective potential distributions.
The principal plane of the main lens having the potential distributions 904 and 906 is at a position 907. On the other hand, the principal plane of the main lens having the potential distribution 905 is at a position 908 and slightly shifts to the phosphor screen side. Specifically, when a part of the AC component of the dynamic focus voltage is not applied to the intermediate electrode, the position of the principal plane of the main lens gradually moves to the phosphor screen side as the electron beam is deflected from the screen center toward the peripheral portion of the screen. On the other hand, when a part of the AC component of the dynamic focus voltage has been applied to the intermediate electrode, the position of the principal plane of the main lens remains substantially unchanged, even as the electron beam is deflected from the screen center toward the peripheral portion of the screen.
This behavior will now be considered referring to a simplified optical system shown in FIG. 6.
Assume that a position of a principal plane of the electron lens is S, a distance between an electron beam generating section I and the principal plane S is P, and a distance between the principal plane S and a phosphor screen O is Q. In this case, a magnification M of the electron lens is expressed by
M=Q/Pxe2x80x83xe2x80x83(1)
In general terms, in the case of a color CRT apparatus, the distance between the electron beam generating section and the phosphor screen is longer at the peripheral portion of the screen than at the central portion of the screen. Assuming that the difference in distance between the peripheral portion of the screen and the central portion of the screen is xcex1 and the magnification M of the electron lens at the central portion of the screen is given by equation (1), a magnification M1 of the electron lens at the time the position of the principal plane is unchanged (at the time part of the AC component of the dynamic focus voltage has been superimposed on the intermediate electrode) is expressed, based on equation (1), by
M1=(Q+xcex1)/Pxe2x80x83xe2x80x83(2)
It is thus understood that the lens magnification is greater at the peripheral portion of the screen than at the central portion of the screen and the electron beam diameter increases. On the other hand, a magnification M2 of the electron lens at the time the position of the principal plane has shifted to the phosphor screen side by xcex2 (at the time part of the AC component of the dynamic focus voltage is not superimposed on the intermediate electrode) is expressed by
M2=(Q+xcex1xe2x88x92xcex2)/(P+xcex2)xe2x80x83xe2x80x83(3)
Compared to equation (2), the magnification decreases. It is thus understood that at the peripheral portion of the screen, the magnification decreases as the position of the principal plane shifts to the phosphor screen side and the electron beam diameter decreases.
In the electron gun assembly formed by combining the extended field type lens and quadrupole lens, the main lens diameter can be increased and the electron beam diameter at the peripheral portion of the screen can be improved. In practice, however, part of the AC component of the dynamic voltage is superimposed on the intermediate electrode due to the electrostatic capacitance among the electrodes and it is difficult to shift the principal plane of the main lens to the phosphor screen side.
The present invention has been made in consideration of the above problem, and its object is to provide a cathode ray tube apparatus having an electron gun assembly capable of obtaining good image characteristics over the entire area of a phosphor screen.
In order to achieve the object, the present invention may provide a cathode ray tube apparatus comprising: an electron gun assembly having an electron beam generating section which generates a plurality of electron beams and a main lens section which focuses the electron beams generated from the electron beam generating section on a phosphor screen; and a deflection yoke which produces a deflection magnetic field that deflects the electron beams emitted from the electron gun assembly in a horizontal direction and a vertical direction, wherein the main lens section comprises a focus electrode supplied with a focus voltage of a first level, at least one intermediate electrode supplied with a voltage of a second level equal to or higher than the first level, and an anode supplied with an anode voltage of a third level higher than the second level, the focus electrode, at least one intermediate electrode and the anode being arranged in a direction of travel of the electron beams, and the main lens section includes an electric field lens acting commonly on the electron beams on a focus region side of the main lens section, which is formed by the focus electrode and at least one intermediate electrode, and a plurality of electric field lenses acting respectively on the electron beams on a divergence region side of the main lens section, which is formed by at least one intermediate electrode and the anode.
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.