Color cathode ray tube

Details Color cathode ray tube Color cathode ray tube

2001-04-12

2003-02-18

Patel, Vip (Department: 2879)

Electric lamp and discharge devices

Cathode ray tube

Plural beam generating or control

C313S412000

Reexamination Certificate

active

06522058

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a cathode ray tube, and more particularly to a color cathode ray tube having an electron gun which is capable of obtaining a favorable focusing in a wide phosphor screen without increasing a focus voltage which controls the correction of astigmatism associated with the deflection of electron beams and the correction of image curvature.
2. Related Art
In a cathode ray tube such as a television picture tube, a monitor tube of an information terminal equipment, other display tube or the like, electron beams emitted from an electron gun scan a phosphor screen on which a phosphor is formed (hereinafter, sometimes simply called “screen”) in two directions consisting of a horizontal direction and a vertical direction to form given images.
With respect to an electron gun used in this type of color cathode ray tube, to obtain the favorable focus characteristics on the entire region of the phosphor screen, it is necessary to perform the control of shape of beam spots landed on the phosphor screen corresponding to the deflection angle of emitted electron beams.
Recently, a monitor or a television picture tube which mounts a flat tube having an outer surface of a panel thereof flattened (flat-panel type color cathode ray tube) has been commercialized. Particularly, with respect to a flat tube having a large screen which has an effective diameter of 51 cm or the like in the diagonal direction, the focusing difference between the central portion and the peripheral portion of the screen becomes large.
As a countermeasure to decrease this focusing difference, there has been known a method in which a focus electrode which constitutes an electron gun is divided into a plurality of electrode members and a focus voltage of a fixed voltage and other focus voltage which is produced by superposing a dynamic voltage which is changed in synchronism with a deflection quantity to the fixed voltage are applied to the focus electrode to form an electrostatic quadrupole lens and a curvature-of-image-field correction lens whereby the deterioration of focusing in the periphery of the screen derived from the increase of the deflection angle can be reduced.
FIG. 19
is a schematic view for explaining a general lens constitution of an electron gun which is applied to a color cathode ray tube. In the drawing, BS indicates a beam generating part, PFL indicates a prefocus lens, FL indicates a front-stage main focus lens, IL indicates a curvature-of-image-field correction lens, ML indicates a rear-stage main focus lens (also called “final-stage main focus lens), and SC indicates a phosphor screen.
Respective lenses described above are arranged in the direction of the phosphor screen SC from the beam generating part BS side along a tube axis Z—Z These lenses focus electron beams B generated by the beam generating part BS, then accelerate the electron beams B and finally make the electron beams B impinge on the phosphor screen SC so as to form electron beam spots (simply called “beam spots” hereinafter).
To be more specific, the above-mentioned electron gun is constituted by the beam generating part (triode part) which is constituted by a cathode (usually called “K”), a control electrode (usually called “G
1
”) and an accelerating electrode (usually called “G
2
”) and generates a plurality of electron beams, and a main lens part which is made of focus electrodes (usually called “G
3
”, “G
4
” “G
5
”) and an anode (usually called “G
6
”) and focus the electron beams generated by the beam generating part toward the phosphor screen.
Here, the electron gun adopts a multi-stage dynamic focusing (MDF) system where the focus electrode (G
5
) is divided into a plurality of electrode members. By applying a fixed focus voltage and a dynamic correction voltage which is produced by superposing a dynamic voltage which is changed in synchronism with a deflection quantity to the divided electrode members, an electrostatic quadrupole lens and a curvature-of-image-field correction lens which are provided for ensuring desired focusing characteristics in a wide range of the phosphor screen are formed. Most of the conventional electron guns adopt the non-multi-stage dynamic focusing.
FIG. 20
is an explanatory view of the focus voltage applied to the focus electrode divided into a plurality of electrode members. Further,
FIG. 21
is an explanatory view of an output voltage of a flyback transformer which generates two focus voltages.
As shown in
FIG. 20
, the focus electrode G
5
of the electron gun is divided in multi-stages (here, three stages consisting of electrode members A, B and C) so as to constitute an electron gun of a composite lens type and the electrostatic quadrupole lens and the curvature-of-image-field correction lens are formed among the electrode members A, B and C. The curvature-of-image-field correction lens is provided for correcting the difference of distance from the center of deflection to the phosphor screen and is usually arranged at a position closer to the phosphor screen than the electrostatic quadrupole lens.
The electrostatic quadrupole lens controls the cross section of the beam spots which pass through the electrostatic quadrupole lens so as to reduce the shape of the beam spot on a phosphor screen into a shape similar to a circle.
The first fixed voltage Vf
1
is applied to the electrode member B and other focus voltage (Vf
2
+dVf) which is produced by superposing a dynamic voltage dVf which is changed in synchronism with a deflection quantity to the second fixed voltage Vf
2
is applied to the electrode members A and C.
The above-mentioned focus voltages Vf
1
, Vf
2
+dVf are generated by the flyback transformer FBT shown in FIG.
21
. Here, Eb indicates an anode voltage (maximum voltage) which is applied to the anode G
6
, Ec
2
indicates a prefocus voltage of approximately 600V applied to other electrodes (G
2
, G
4
) of the electron gun.
FIG. 22
is an explanatory view of the focus voltage applied to the electrode members of the divided focus electrode, wherein 1V indicates 1 vertical deflection cycle (1 frame cycle or 1 field cycle) and 1H indicates 1 horizontal deflection cycle.
When the dynamic voltage dVf is increased, that is, when the deflection quantity of the electron beams is large (at the time of deflecting the electron beams toward the peripheral portion of the screen), the potential difference at the curvature-of-image-field correction lens becomes small so that the intensity of the lens is decreased. Accordingly, the force to focus the electron beams becomes weak at the time of deflecting the electron beams so that the image curvature is corrected.
This type of conventional technique is, for example, disclosed in Japanese Laid-open Patent Publication 43532/1992 and Japanese Laid-open Patent Publication 161309/1995.
With respect to the conventional technique, particularly Japanese Laid-open Patent Publication 43532/1992, a focus electrode disposed close to an anode is divided into a plurality of first electrode members and a plurality of second electrode members, wherein the first electrode member and the second electrode member are alternately arranged in the electron beam advancing direction. Then, the first electrode member and the second electrode member form a curvature-of-image-field correction lens in the state that the first electrode member and the second electrode member are made electrically independent from each other to form an electron lens which changes the intensity thereof in synchronism with the deflection of the beams between the first electrode member and the second electrode member.
Further, a non-axially-symmetric electron lens for correcting astigmatism which deforms the cross-sectional shape of the electron beams due to the above-mentioned fluctuating dynamic voltage is formed adjacent to a main lens so that even when the fluctuation of the focus voltages is suppressed at a low level, a favorable image can be obtained on the whole screen.
SUMMARY OF THE INVENTION
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