Color cathode ray tube

Details Color cathode ray tube Color cathode ray tube

2001-05-24

2003-05-06

O'Shea, Sandra (Department: 2875)

Electric lamp and discharge devices

Cathode ray tube

Shadow mask, support or shield

C313S404000, C313S407000

Reexamination Certificate

active

06559585

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-157061, filed May 26, 2000, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a color cathode ray tube provided with a shadow mask.
In general, a color cathode ray tube comprises a vacuum envelope that includes a panel and a funnel. The panel includes a substantially rectangular effective portion having curved inner and outer surfaces and a skirt portion that is set up on the periphery of the effective portion. The funnel is bonded to the skirt portion. A substantially rectangular phosphor screen is provided on the inner surface of the panel effective portion. The phosphor screen includes black non-luminous layers and three-color phosphor layers that are embedded in gaps between the non-luminous layers and glow blue, green, and red, individually. Further, a substantially rectangular shadow mask is opposed to the phosphor screen. On the other hand, an electron gun for accelerating, focusing, and emitting three electron beams is located in a neck of the funnel.
In the color cathode ray tube constructed in this manner, the three electron beams emitted from the electron gun are deflected by means of magnetic fields, that are generated by a deflecting yoke mounted on the outside of the funnel, so as to horizontally and vertically scan the phosphor screen through the shadow mask, whereupon a color image is displayed.
The shadow mask is provided with a mask body and a mask frame. The mask body includes a substantially rectangular principal mask surface that is formed of a curved surface opposed to the phosphor screen and a skirt portion that is formed by bending the peripheral edge portion of the principal surface. A large number of electron beam passage apertures are formed in the principal mask surface and arranged at given pitches. The mask frame is welded to the skirt portion of the mask body. The electron beam passage apertures of the mask body serve to screen the electron beams that are landed on the three-color phosphor layers.
In order to display an image free from color drift on the phosphor screen, the electron beam passage apertures of the mask body and their corresponding phosphor layers must be kept in specific relative positions. During the operation of the color cathode ray tube, the positional relations between the electron beam passage apertures and the phosphor layers, especially the distance (value q) between the principal mask surface of the mask body and the inner surface of the panel, must be kept within a given allowable range.
On the other hand, the electron beams that pass through their corresponding electron beam passage apertures and reach the phosphor screen during the operation of the color cathode ray tube account for ⅓ or less of all the electron beams that are emitted from the electron gun. The remaining electron beams run against some other portions of the mask body than the electron beam passage apertures and are converted into thermal energy, thereby heating the mask body. As the mask body is heated in this manner, it undergoes thermal expansion, whereupon so-called doming occurs such that the mask body bulges toward the phosphor screen. If this doming causes the distance between the principal mask surface of the mask body and the inner surface of the panel to exceed the given allowable range, the positions in which the electron beams are landed on the phosphor layers shift, so that the color purity lowers. This shift of landing of the electron beams on the phosphor layers substantially varies depending on the luminance or duration of image patterns.
As modern color cathode ray tubes become larger in size, their deflection angles are widened, while their screens are made flatter. In the color cathode ray tubes of this type, the color purity is lowered more drastically by the thermal expansion of the mask body. In many of flat-screen wide-type color cathode ray tubes, Invar (36%—Ni—Fe alloy plate) with a low coefficient of thermal expansion is used for their mask body. In order to avoid an increase in cost of the shadow mask, in this case, a cold-rolled steel plate that is less expensive than Invar is frequently used for the mask frame.
If the mask body and the mask frame are formed of Invar and the cold-rolled steel plate, respectively, however, heat of the mask body that is generated by prolonged operation of the color cathode ray tube is transmitted to the mask frame, so that the mask frame undergoes thermal expansion. Owing to the difference in thermal expansion between the low-expansion mask body and the high-expansion mask frame causes the mask body, in this case, the mask body is pulled and deformed by the mask frame. In a manufacturing process for the color cathode ray tube, moreover, the shadow mask is heated to a higher temperature than by heating that is attributable to the electron beam collision. Accordingly, the difference in thermal expansion between the mask body and the mask frame increases, so that the mask body is deformed considerably.
In order to reduce the deformation of the mask body that is attributable to the difference in thermal expansion between the mask body and the mask frame, there is provided a shadow mask designed so that the skirt portion of the mask body is located inside the mask frame, and tongue portions on the skirt portion are welded to the mask frame. If the mask frame pulls the mask body, with this arrangement, the tongue portions are elastically deformed to absorb the pulling force of the mask frame, thereby reducing the deformation of the principal mask surface.
Although the shadow mask is constructed in this manner, however, it is inevitably large-sized if it is used in a large-sized color cathode ray tube. Thus, the mask body and the mask frame are subject to a substantial difference in elongation that is attributable to the difference in thermal expansion. The pulling force of the mask frame and the extent of the pull increase in proportion to the difference in elongation. In a color cathode ray tube with a flat screen, moreover, the curvature of the principal surface of the mask body is smaller, and its curvature retention strength is smaller than that of a conventional shadow mask. Accordingly, the principal mask surface is inevitably deformed even if the pulling force is small. In the shadow mask of this type, therefore, the pulling force that acts on the principal mask surface should be minimized.
BRIEF SUMMARY OF THE INVENTION
The present invention has been contrived in consideration of these circumstances, and its object is to provide a color cathode ray tube designed so that lowering of the color purity that is attributable to deformation and dislocation of a shadow mask is reduced to improve the image quality.
In order to restrain deformation that is attributable to the difference in thermal expansion between a mask body and a mask frame, it is to be desired that tongue portions of a skirt portion of the mask body should be shaped so that they are as deformable as possible. These tongue portions should be narrow and long.
In this case, however, the tongue portions are liable to be deformed in the crosswise direction, so that the mask body can be dislocated in a direction perpendicular to the tube axis inside the mask frame when it is subjected to a crosswise shock or vibration. In consequence, landing on a phosphor screen shifts, and the color purity lowers. In a phosphor screen forming process, moreover, phosphor layers are exposed with use of a shadow mask as a photo mask, so that attachment to and detachment of the shadow mask from a panel are repeated several times. These attachment and detachment operations cause the mask body to be dislocated, so that the resulting phosphor layers are dislocated, and a desired phosphor screen cannot be formed.
Recently, in particular, there has been an increasing demand for images of higher quality, and the arrangemen

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