Electric lamp and discharge devices – Cathode ray tube – Screen
Reexamination Certificate
1998-12-24
2001-12-25
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Screen
C313S402000, C313S407000, C313S408000, C313S47700R, C220S00210A
Reexamination Certificate
active
06333594
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a color cathode ray tube with a panel having a substantially flat outer surface, and more particularly, to a color cathode ray tube improved in visibility and in strength for the maintenance of a curved surface of a shadow mask.
Generally, a color cathode ray tube has a vacuum envelope that is composed of a glass panel and a glass funnel. In the color cathode ray tube, three electron beams emitted from an electron gun assembly in a neck of the funnel are deflected by a magnetic field that is generated by a deflection yoke attached to the outside of the funnel. A phosphor screen, which is formed of three-color phosphor layers on the inner surface of an effective portion of the panel, is scanned horizontally and vertically with the electron beams transmitted through the shadow mask, whereupon a color image is displayed on the screen.
In the color cathode ray tube of this type, in general, a side wall or skirt portion is attached to the peripheral portion of a faceplate that has a substantially rectangular effective portion. In order to obtain strength high enough to stand the load of the atmospheric pressure that acts on the vacuum envelope, the panel is formed having different curved surfaces as its inner and outer surfaces so that the central portion of the effective portion is thinner than the peripheral portion. Moreover, the outer surface of the effective portion is a curved surface such that its height above sealed surfaces of the panel and the funnel is greatest in the central portion and is reduced with distance from the central portion. More specifically, the outer surface of the effective portion is a spherical surface, a cylindrical surface having an infinite curvature radius in the minor-axis direction and a curvature in the major-axis direction, or a curved surface that can be represented by a high-order polynominal expression.
Flattening the outer surface of the effective portion of the panel has recently been promoted to improve the visibility of images. Described in Jpn. Pat. Appln. KOKAI Publication No. 9-245685 is an example of an improved panel in which the outer surface of the effective portion is a substantially flat surface and the inner surface is a substantially cylindrical curved surface having a substantially infinite curvature radius in the major-axis direction and a curvature in the minor-axis direction.
The visibility of an image formed on the phosphor screen is influenced by the shape of the inner surface of the effective portion, which is fitted with the screen, as well as by the shape of the outer surface of the effective portion. In the case where the outer and inner surfaces of the effective portion are different, the difference in thickness between the central and peripheral portions of the effective portion has no substantial influence on the visibility if it is small. If the outer surface is flattened so that the difference in thickness between the central and peripheral portions increases, however, the influence of the difference in thickness is remarkable.
In order to improve the visibility, both the inner and outer surfaces of the effective portion should preferably be flat. In the case of a panel, such as the one described in Jpn. Pat. Appln. KOKAI Publication No. 9-245685, in which the outer surface of the effective portion is a substantially flat surface and the inner surface is a substantially cylindrical curved surface having a substantially infinite curvature radius in the major-axis direction and a curvature in the minor-axis direction, however, the curvature of the short-side inner surface of the effective portion sometimes may be seen sideways from the short sides, although the effective portion frontally looks rectangular. Accordingly, the panel is expected to be further improved for the enhancement of the visibility or the display of flat images, which is a primary object of the flattening of the outer surface of the effective portion.
In the process of molding this glass panel, moreover, the cooling speed of glass varies due to the great difference in thickness between the central and peripheral portions, so that accurate curved surfaces cannot be obtained, in some cases.
On the other hand, the shadow mask is composed of a substantially rectangular mask body with a thickness of about 0.1 to 0.3 mm and a substantially rectangular mask frame attached to the peripheral portion of the mask body. A large number of beam apertures are formed in an effective surface of the mask body that faces the phosphor screen on the inner surface of the effective portion of the panel.
In general, the effective surface of the mask body must sustain a predetermined relation to the inner surface of the effective portion of the panel, so that these surfaces are similar curved surfaces. The shadow mask of the conventional color cathode ray tube has a curved surface whose profile is highest in the central portion and is reduced with distance from the central portion when the panel is turned upward. More specifically, the curved surface of the shadow mask, like that of the panel, is a spherical surface, a cylindrical surface having an infinite curvature radius in the minor-axis direction and a curvature in the major-axis direction, or a curved surface that can be represented by a high-order polynominal expression.
In the case of the panel in which the outer surface of the effective portion is a substantially flat surface and the inner surface is a substantially cylindrical curved surface having a substantially infinite curvature radius in the major-axis direction and a curvature in the minor-axis direction, the effective surface of the mask body of the corresponding shadow mask, like the inner surface of the effective portion of the panel, must be a cylindrical surface having a substantially infinite curvature radius in the major-axis direction and a curvature radius in the minor-axis direction. One such shadow mask is described in Jpn. Pat. Appln. No. 9-3632.
In general, the strength for the maintenance of the curved surface of the shadow mask is settled depending on the shape of the curved surface of the effective surface of the mask body, the thickness of the mask body, and the configuration, size, and arrangement of the electron beam apertures. If the thickness of the mask body and the configuration, size, and arrangement of the electron beam apertures are fixed, therefore, the curved surface maintenance strength of the shadow mask is settled depending on the shape of the curved surface of the effective surface.
If the maximum and minimum curvature radii, among curvature radii in all directions with respect to an optional point on the effective surface, are Rmax and Rmin, respectively, as indexes for the curved surface maintenance strength of the shadow mask, there is an average curvature that can be defined by the sum (1/Rmax+1/Rmin) of the minimum and maximum curvatures 1/Rmax and 1/Rmin, the respective reciprocals of the maximum and minimum curvature radii.
If the depth of depression (difference in distance along the tube axis between the center and each diagonal portion of the mask) of each diagonal portion of the effective surface of the conventional shadow mask at which the depression has its maximum is fixed, the curved surface maintenance strength can be enhanced by forming the effective surface in the shape of a cylindrical surface that has the highest average curvature, as in the case of the shadow mask described in Jpn. Pat. Appln. No. 9-3632.
In a shadow mask that is combined with a panel in which the outer surface of the effective portion is a substantially flat surface, however, the depth of depression of each diagonal portion of the effective surface is so small that the curved surface maintenance strength cannot be great enough.
The reduced curved surface maintenance strength of the shadow mask has various bad influences on the performance of the color cathode ray tube.
If any external force or impact is applied to the color cathode ray tube, the curved effective surfac
Inoue Masatsugu
Nakagawa Shinichiro
Shimizu Norio
Haynes Mack
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Patel Vip
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