Electric lamp and discharge devices – Cathode ray tube – Shadow mask – support or shield
Reexamination Certificate
2002-02-27
2003-09-30
O'Shea, Sandra (Department: 2875)
Electric lamp and discharge devices
Cathode ray tube
Shadow mask, support or shield
Reexamination Certificate
active
06628060
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-054701, filed Feb. 28, 2001, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a color cathode ray tube having a shadow mask.
In general, a color cathode ray tube comprises a vacuum envelope that includes a substantially rectangular face panel and a funnel. The face panel includes an effective portion formed of a curved surface and a skirt portion set up on the peripheral part of the effective portion. The funnel is fixed to the skirt portion. A phosphor screen is formed on the inner surface of the effective portion of the face panel. The phosphor screen includes black non-luminous layers and three-color phosphor layers that are embedded individually in gaps between the non-luminous layers. Inside the face panel, moreover, a substantially rectangular shadow mask is opposed to the phosphor screen.
An electron gun that emits three electron beams is located in a neck of the funnel. The color cathode ray tube displays a color image in a manner such that the three electron beams are deflected by means of magnetic fields that are generated by means of a deflection yoke on the outside of the funnel, and that the phosphor screen is scanned horizontally and vertically with the electron beams that are passed through the shadow mask.
The shadow mask includes a substantially rectangular press-molded mask body and a substantially rectangular mask frame that is attached to the peripheral part of the mask body. The mask body has an effective surface formed of a curved surface that is opposed to the phosphor screen, and a large number of electron beam passage apertures are formed in the effective surface. The shadow mask is detachably supported on the inside of the face panel in a manner such that elastic supports attached to the respective middle portions of the side faces of the mask frame or to the outside of its corner portions are anchored to stud pins on the skirt portion of the face panel.
In order to display an image without a color purity drift on the phosphor screen, the three electron beams that pass through the electron beam passage apertures of the mask body must be landed correctly on the three-color phosphor layers. To attain this, it is necessary to keep the relative positions of the face panel and the shadow mask correct, and in particular, to keep the space (q-value) between the inner surface of the effective portion of the face panel and the effective surface of the mask body within a given tolerance.
In a modern color cathode ray tube, the outer surface of the effective portion of the face panel is expected to be formed into a flat surface or a curved surface that, having a radius of curvature of 10 m or more, is as flat as possible. Corresponding to this, the respective radii of curvature of the inner surface of the effective portion and the effective surface of the mask body must be increased.
If the mask body is press-molded so that the radius of curvature of its effective surface is increased, however, its curved surface retention lowers. In consequence, the mask body may be deformed substantially to lower the color purity of the color cathode ray tube during manufacturing processes for the tube.
If the radius of curvature of the inner surface of the face panel and the radius of curvature of the shadow mask are lessened to avoid this, the difference in thickness between the central and peripheral parts of the panel becomes so great that the manufacture of the face panel itself is rendered difficult. Further, this situation results in lowering of visibility, such as diminution of the visual angle, distortion of an image reflected by the inner surface of the face panel, etc. Preferably, therefore, it is desirable that the radius of curvature of the inner surface of the face panel is enlarged.
According to a color cathode ray tube described in Jpn. Pat. Appln. KOKAI Publication No. 11-242940, for example, the inner surface of the effective portion of the face panel is shaped so that it has a given curvature in the minor-axis direction, a substantially infinite radius of curvature in the major-axis direction near its center, and a given curvature near its peripheral edge. According to this configuration, the curved surface strength of the mask body can be improved, and besides, the atmospheric-pressure strength of the vacuum envelope can be enhanced.
In the color cathode ray tube described in this publication, however, the strength of the mask body near the center and the minor-axis end is still lower than the strength at the peripheral part. This phenomenon is particularly conspicuous in a color cathode ray tube with an aspect ratio of 16:9.
In flattening the face panel, its curved surface changing rate, that is, a ratio of a sag of the effective surface of the panel at a position distant from the center of the effective surface to the distance of the position from the center of the effective surface, must be adjusted to at least 0.02 or more, in consideration of the atmospheric-pressure strength and implosion-proof performance. In this application, a sag represents the deference between the level at a position of the face panel or the shadow mask and the level at the center of the face panel or the shadow mask in the tube axis direction.
Further, the color cathode ray tube described in the aforesaid publication has a problem on the quality level of the phosphor screen. In general, the phosphor screen of a color cathode ray tube is formed by photolithography. In this method, phosphor slurry that consists mainly of a phosphor material and a photosensitive resin is spread over the inner surface of the face panel and dried, whereupon a phosphor slurry layer is formed. This phosphor slurry layer is exposed through a shadow mask and then developed, whereupon a phosphor layer is formed. The phosphor screen is formed by repeating these processes for each of three color phosphor layers.
In the exposure process, light from an exposure light source is approximated to the respective trajectories of the three electron beams from the electron gun by means of an optical lens system, and the phosphor slurry layer is exposed so that the phosphor layer is formed in a predetermined position relative to the electron beam passage apertures of the shadow mask.
In an in-line color cathode ray tube, the three-color phosphor layers or black non-luminous layers are formed as elongate stripes that extend in the minor-axis direction of the face panel. The shadow mask has a large number of slit-shaped electron beam passage apertures, which are arranged so that a large number of electron beam passage aperture lines extend along the minor axis.
In the case where the phosphor screen of the in-line color cathode ray tube with the configuration described in the aforementioned publication is formed in the aforesaid method, a meandering phenomenon called light source bending occurs, since the middle portion of the inner surface of the face panel in the major-axis direction has a substantially infinite radii of curvature in the major-axis direction and a given curvature only in the minor-axis direction. In consequence, the phosphor layers fail to be straight and meander especially near the middle portion of each long side of the phosphor screen. The image quality level lowers in this case.
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, of which the shadow mask strength and the display quality level can be improved without failing to ensure the flatness of a face panel.
In order to achieve the above object, a color cathode ray tube according to an aspect of the invention comprises: an envelope including a face panel having a substantially rectangular effective portion with a substantially flat outer surface; a phosphor screen provided on the inner surface o
Inoue Masatsugu
Shimizu Norio
Kabushiki Kaisha Toshiba
Krishnan Sumati
O'Shea Sandra
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